artigo raman para nbo6

7/30/2019 Artigo Raman Para NbO6

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100 Chem. Ma ter. 1991,3, 100-107Presumably it was some GaSb-Ga mixtu re.

SummaryTh e pyrolysis of TV Sb h as been investigated in a flowtube reactor using Dz nd He carr ier gases. For TVS balone, the m ost likely pyrolysis reaction involves an Sb -centered reductive eliminatio n pathway. A less likelypossibility is pyrolysis via homolysis of the Sb-C bon ds,yielding vinyl radicals. Unfo rtunate ly, examination of theorganic byproduc ts in both He and D, yields insufficientinform ation to form a definit ive hypothesis. However, inHe the pyrolysis rate for TVSb is more rapid than forTM Sb. Since vinyl radicals form stronger bonds thanmethyl radicals, this da tum contradicts the Sb-C bondhomolysis mechanism. Again, the activation energy forpyrolysis is less tha n th e expected Sb-vinyl bond stre ngth.Finally, the addition of C7D, produces no CH,=CHD,

indicative of th e absence of vinyl radicals. T o elucidateour understanding of GaSb growth by using TM Ga a ndTV Sb, the pyrolysis rates for this com bination of reac tantswere also studied. CH, radicals from (CH3N ), pyrolysiswere found to enhance TVSb pyrolysis in He. TM Ga alsoincreases the TVSb pyrolysis rate, mainly due to themethyl radicals produced. A heterogeneous pyrolysis re-action appears at high surface area. At V /III ratios nor-mally used for OMVPE growth, carbonaceous depositswere formed. Thu s, TVS b may be a useful precursor forOMVPE only at V/III ratios less than unity.Acknowledgment. We acknowledge financial supportfrom th e Office of N aval Research, th e Office of N avalTechnology, and the Air Force Office of Scientific Re-search.

Registry No. TVSb, 5613-68-3; TMGa, 1445-79-0.

Structural Chemistry and Raman Spectra of NiobiumOxidesJih-Mirn Jehng an d Israel E. Wachs*

Zettlem oyer Center for Surface Studies, D epartm ent of Chemical Engineering, LehighUniversity, Bethlehem, Pennsylv ania 18015Received Ma y 29, 1990. Revised M anus cript Received November 5 , 1990

A series of niobium oxide reference compounds w ere investigated by Raman spectroscopy in order todetermine th e relationship between niobium oxide structures a nd their corresponding Raman spectra. Th eassignments of the Ra man band s were based on the known niobium oxide structures. Th e Raman studie sindicate that the R aman frequencies strongly depend on the niobium oxide structures. For the slightlydistorted octahedral NbOs structures (KNb03,NaNbO,, and L iNb 03) , he major Raman frequencies appearin the 500-700-cm-' region. For the highly distorted octahedral N b0 6 structures (KsNbsO lg,AlNbO,, andN b( H Cz 04 )J , he major Raman frequencies shift from the 500-700- to the 850-1000-cm-' region. Th edistortions in the niobium oxide compounds are caused by the corner- or edge-shared Nb0 6 octahedra.Both slightly distorted and highly distorte d octahedral N b0 6 sites coexist in the KCazN a,-3Nb,03,+1, n=3-5, layered compounds. Most of the niobium oxide compounds possess an octahed rally coordinatedNbOs structu re with different ex tents of di stortion , and only a few rare-eart h ANbO, (A =Y, Yb, Sm,and La) compounds possess a tetrahedrally coordinated NbO, structure. For the tetrahedral N b0 4structureof YbNb0, the major Raman frequency appears at -813 cm-'. In situ Ram an studies assisted in thediscrimination between bulk and surface functionalities in th e niobium oxide reference compounds possessinghigh surface areas (Nb205.nH20 nd HCazNb3010).Introduction

Niobium oxide, Nbz05 , has been reported to exist indifferent polymorphic an d the phase transfor-mations of niobium oxide strongly depend on the heattreatmen t. Upon heat treatments between 300 a n d 1000"C, amorphous niobium oxide increases in degree ofcrysta l lin i ty and forms more s tab le N bz 05 phases .Amorphous niobium oxide, Nb z0 5-n Hz 0, ossesses dis-to r ted N b0 6 octahedra , N b0 7 pentahedra , and NbOshexahedra as s t ruc tura l un its .5 The TT-N b2 05phase,300-500 "C, possesses a pseudohexagon al unit cell, witha constitutional defect of an oxygen atom per u nit cell, andforms tetragonal and pentagonal bipyramids' with six or

(1 ) Ikeya, T.; Senna , M. J.N o n - C r y s t . Solids 1988, 105, 243.(2 ) Izumi, F.; Kodama, H. 2. Anorg. Al lg. Chem. 1978 , 440 , 155.(3 ) McConnell, A. A.; Anderson, J . S.;Rao, C. N . R. Spectrochimica( 4 ) Weissman, J. G. ; KO ,E. I.; Wynblatt, P.; Howe, J. M. Chem. Mater .(5 ) Aleshina, L. A.; Malnenko, V. P.; Phouphanov, A. D.; Jakovleva,

A cta 1976 , 32A , 1067.1989, 1 , 187.N. M. J . Non- Csys t . Solids 1 9 8 6 , 8 7 , 350.

seven oxygen atoms coordinated to the N b atom. Th eT-NbZO5 hase , 700-800 "C, possesses an orthorhombicunit cell and forms distorted tetragonal or pentagonalbipyramids with six or seven oxygen atoms coordinatedto the N b atom. One out of seventeen Nb ato ms occupiesthe intersti t ial si tes between two unit cells and is sur-rounde d by eight oxygen atoms.6 The se polyhed ra arejoined by corner or edge sharing in the a b plane and bycorner sharing along the c axis. Th e H-Nb2O5phase, above1000 "C , is the most thermodynamically st able form of theN b z 0 5polymorphs. Th e structure of H-NbZO 5 ontainstwo diffe rent sizes of Re o3-ty pe blocks: 3 X 4 nd 3 X 5blocks composed of corner- or edge-shared N b 06 octahe-dra. Only 1 out of 28 Nb si tes is a t e t r a h e d r ~ n . ~Th e niobium oxide struc ture can be m odified by cationsubsti tution into the crystall ine latt ice to form differentkinds of niobium oxide compounds : perovskite struc-ture ,8 -1 0 aye red s t r ~ c t u r e , ~ ' - ' ~nd N b601g8-cluster^.'^-'^( 6 ) Kato, K.; Tamu ra, S. Acta Crys tal logr . 1975, B31 ,673 .(7 ) Gatehouse, B. M.; Wad sley, A. Acta Crystallogr. 1964, 17 , 1545.

0897-4756/91/2803-OlOO$O2.50/0 0 991 American Chemical Society


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Niobium OxidesThe se modifications enhance th e physical a nd chemicalproperties of niobium oxide. Th e perovskite niobium oxidecompounds are used in various fields of materials sciencesu ch as g l a ~ s e s ' ~ J ~n d ceramic^,'^^^^ and th e layered nio-bium oxide compounds undergo intercalation reactionswith organic amines.21 N b2 05 lso reacts with oxalic acidto form niobium oxalate complexes which can exist inaqueous solution.22 T he above niobium oxide compoundspossess an octahedrally coordinated NbO, structure w ithdifferent extents of distortion. Th e tetrahedrally coor-dinated N b0 4 structure is not a typical structure for nio-bium oxide because the Nb5+ atom is too large to fit intoan oxygen-anion tetrah edron . Only a few rare-earthA N b 0 4 (A =Y, Yb, Sm, and La) compounds have beenfo un d t o po ss ess te tr ah e dr al c ~ o r d i n a t i o n . ~ ~ - ~ ~Ram an spectroscopy is very sensit ive to th e stru ctureand bond order of metal oxides, especially in the regionof metal-oxygen stre tchi ng modes, because many of th eRaman frequencies depend on the bond order in the~ tr u c tu r e .~ h igher meta l-oxygen bond order, corre-sponding to a shorter bond distance, shif ts the Ramanbond to higher wavenumbers. In this stu dy, various nio-bium oxide reference compounds are characterized byRam an spectroscopy and divided into two main categories:the octahedrally coordinated a nd th e tetrahedrally coor-dinated niobium oxide compounds. Th e octahedrally co-ordinated niobium oxide compounds consist of slightlydistorted niobium oxides, highly distorted niobium oxidestructu res, layered structures, and pure niobium oxide.Tetrahedra l ly coordinated AN b04 (A =Y, Yb, Sm, andLa) comp ou n ds have been r ep or ted b y B l a s ~ e , ~ ~ooksbyet al.,24 nd Yoshida et al.25 o possess a slightly distortedscheelite structure with NbOl units, and only YN b04 haspreviously been characterized by Ram an spectroscopy. Inthe present study Y bN b0 4 was used as the niobium oxidereference compound containing a tetrahed rally coordinatedN b0 4 structure. Th e niobium oxide structure-Ramanspectroscopy relationships developed for the referencecompounds will subsequently be used to assign the mo-lecular structures of the su rface niobium oxide phases insupported niobium oxide catalysts.

Experimental SectionMaterials. Hydrated niobium pentoxide, Nbz 05-n Hz0 ,wa s

~

(8) Megaw, H. D., Acta Crys tal logr . 1968, A24, 589.(9 ) Sakowski-Cowley, A. C.; Lukaszwicz, K.; Megaw, H. D. Acta(10) Katz , L.; Megaw, H. D. Acta Crys tal logr . 1967, 22 , 269.(11) Dion, M.; Ganne, M.; Tournoux, M. Mater . Res . Bull . 1981,16,(12) Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. Inorg. C hem.(13) Jacobson, A. J.; Lewandowski, J. T.; Johnson, J. W. J . Less-(14) Farrell, F. J. ; Maroni, V. A ,; Spiro, T. G. Inorg. Chem . 1969, 8,(15) Tobias, R. S. Can. J. C h e m . 1965, 43 , 1222.(16) Rocchiccioli-Deltcheff, C.; Thouveno t, R.; D abbabi, M. Spectro-(17) Glass, A. M.; Nassau, K.; Negran, J. T. J . A p p l . P h y s . 1978,49,(18) El Jazouli, A.; Viala, J. C.; Parent, C.; Hagenmuller, P. J. olid(19) Jang, S. J.; Uchino, K. ; Nomura, S.; Cross, L. E. Ferroelectrics(20) Swartz, S. L.; Shrout, T. R. Mater . Res . Bull . 1982, 17, 1245.(21) Jacobson, A. J.; Johnson, J. W.; Lewandowski, J. T. Mater . R es .(22) Jehng, J. M.; Wachs, I. E. ACS Diu. Petrol . Chem . Prepr . 1989,

Crys tal logr . 1969, R25, 851.1429.1985, 24 , 3727.C ommon Met . 1986, 116, 137.2638.ch im. A cta 1977,33A, 143.1075.S t a t e C h e m . 1988, 73, 433.1980, 27, 31.Bull . 1987, 22 , 45..?A SA6.,(23) Blasse, G. J. Solid S t a t e C h e m . 1973, 7, 169.(24) Rooksby, H. P.; White, E. A. D. Acta . Crys tal logr . 1963,16,888.(25) Yoshida, S.; Nishimura, Y.; Tanaka, T.; Kanai, H.; Funabiki, T.

Chem. Mater . , Vol. 3 , N o. 1, 1991 1011 n

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1200 1000 600 600 400 200Raman Shift (cm-1)Figure 1. Ram an spec tra of the BN b0 3 (B =Li, Na, K) com-pounds.provided by Niobium Pr odu cts Co. (Pittsburgh, PA) with aminimum p urity of 99.0%. Th e major impurities after calcininga t 800 "C are 0.02% T a and 0.01% C1. N iobiu m oxa late was alsoprovided by Niobium P roducts Co. with the chemical analysisof 20.5% Nbz0 5, 790 ppm Fe , 680 ppm S i , and 0.1% insolublesolid.T h e B N b 0 3 ( B =Li, Na, and K) compounds were obtainedfrom Johnson M atthey Chemicals Co. with a purity greater than99.99%. P otassium niobate, K8Nb6019,was obtained from Pfaltzand Bauer, Inc . The KCazNa,_3Nbn03n+ln=3-5) compoundswere provided by A. Jacobson of Exxon Research a nd EngineeringCo.; th e detailed synthe sis procedures of these layered oxides weredescribed by Jacobson et a l . I 3 The hydrated HCazNb3010 ayeredoxide was also provided by A. Jacobson. Th e dehydrated H -CazNb3010 ayered oxide was prepared by drying the h ydratedHCa2N b3010 ayered oxide at 120 "C fo r 16 h.BET Surface Area Measurement. The BE T surface areasof bulk niobium oxide samples were obtained with a Quantsorbsurface area analyzer (Quantac hrome Corp. Model OS-9) usinga 3:7 ratio of Nz /H e mixture as a probe gas. Typically, 0. 20 0. 30 0g of sample was used for the m easureme nt, and th e sample wasoutgassed a t 250 "C prior t o N2 adsorption.Raman Spectroscopy. Rama n spectra were obtained witha Spex triplemate spectrometer (Model 1877)coupled to an EG&Gintensified photodiode array detector, which were cooled ther-moelectrically to -35 "C, an d interfaced with an EG&G OMA I11optical multichannel analyzer (M odel 1463). Th e samples wereexcited by the 514.5-nm line of the Ar' laser with 10mW of power.Th e laser beam w as focused on the sample illuminator, where thesample typically spins a t about 2000 rpm to avoid local heating,an d was reflected in to the spectrom eter by a 90' angle with th eincident light. Th e scattered Ram an light was collected by thespectromet,er at room temperatu re and analyzed with an OM AI11 software package. T he overall spectral resolution of th e spectrawas determined to be abou t 2 cm-'. Th e detailed schematicdiagram of th e Ra man spe ctrometer is described elsewhere.26An in situ qu artz cell was designed in orde r to investiga te th eRam an changes upon dehyd ration of the niobium oxide samplesabove room temperature. Th e sample holder was made from aquartz glass, and t he sam ple disk was held by a stationary slo tin the sample holder. Th e sample was heated by a cylindricalheating coil surrounding the q uartz cell, an d the temp erature wasmeasured with an internal thermocouple. Th e cell was capableof operating u p to 600 "C. Reaction gas mixtures were introducedinto the cell from a manifold at a r ate of 50-500 cm3/min witha delivery pressure of 150-200 Torr.

Results(a) Octahedrally Coordinated Niobium Oxide Com-pounds. Slightly distorted niobium oxides: Th e R a-man spectra of the BN b0 3 ( B =Li, Na, and K) referencecompounds a r e shown in Figure 1. The major Raman(26) Wachs, I. E.; Hardcastle , F. D.; Chan, S. S. Spectroscopy 1986,1, 30.ata l . T oday 1990, 8, 67 .


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102 Chem. Ma ter., Vol. 3 , No. I , 1991 Jehng and Wachs

1 , 1 1 11200 1000 800 600 400 200Raman Shlft (cm-1)Figure 2. Raman spectra of the highly distorted niobium oxidecompounds.

1200 1000 BO O 600 400 200Raman Shift (cm-1)

Figure 3. Raman spectra of the KCa2Nan-3Nbn03n+ln =3-5)layered oxide compounds .band of these reference compounds appears in the 620-630-cm-* egion, and a s houlder ap pears in the 520-580-cm-' region. Th e Li N b0 3 compound possesses additionalRa ma n bands a t -870, -430, -375, and -325 cm-'.However, the intensity of these Raman bands is signifi-cantly decreased for the NaNb03 compound an d are es-sen tia lly absen t fo r the KN b0 3 compound. For theK N b 0 3 compound, an additional Raman band is observeda t -840 cm-'. Multiple an d strong Ram an bands in thelow-wavenumber region, between 100 and 300 cm-', arealso observed for these reference compounds.Highly distorted niobium oxides: The Raman spectraof the K8Nb6019,AlNbO,, and Nb (H C2 04)5 eferencecompounds are shown in Figure 2. In the high-wave-number region (800-1000 cm-'), KaN b601g ossesses Ra-man bands a t -900, -880, an d -840 cm-', A1Nb04 hasa Ram an band at -930 cm-', a nd Nb( HC 204),possessesa Ram an band at -960 cm-'. In the intermediate-wave-number region (400-800 cm-'), the major Raman band ofthese niobium oxide compounds appears at -540 (Ka-Nb6 019), 420 (AlNbO,), an d -570 cm-' (Nb(HC,O,),).Additional Raman bands at - 80 , - 30, and - 80 cm-'are observed for AlNbO,. In the low-wavenumber region(100-300 cm-') , KaN b601 g nd Nb(HC,O,) , have Ram anband s at -230 and -290 cm-' and A1Nb04 has Ram anbands a t -275, -240, -210, and -180 cm-'.Layered oxides: Th e R aman sp ec t r a o f t h eKCazNa,-3Nb,0s,+l (n = 3-5) layere d oxide referencecompounds are presented in Figure 3. Th e major Ramanban d of these layered oxides appea rs at -930 cm-'. Ad-ditional Ram an bands a t -760, -575, and 300-500 cm-'

1100 900 700Raman Shift ( c m ' )

Figure 4. In situ Raman pectra of the HCa2Nb3010ayered oxidecompound (a) hydrated state; (b) heated a t 100 C under flowingoxygen; (c) heated at 175 "C nder flowing oxygen; (d ) cooled to50 "C under flowing oxygen.

6o01z0100 300 500 700 900

Ca lcination Temperature ('C)Figure 5. Surface area of bulk niobium oxide as a function ofcalcination te mpera ture.are also present for the KCa2Nb3010 ompound. A shif tof the Rama n band from -760 to -825 cm-' is observedupon increasing the num ber of layers from three to four,and this band further shif ts to -840 cm-' as the numberof layers is increased to five. Th e Ram an bands a t -450and -375 cm-' d isappe ar for the compounds possessingthree and four layers.Th e dehydrated stat e of th e HCa,Nb3010 layered oxidewas investigated by in situ Raman spectroscopy underflowing oxygen, and th e results are shown in Figure 4. T he- 30-cm-' Ram an band for the KC a2Nb 3010 ompound,Figure 3, shifts to -965 cm-' for the hydrated H Ca2Nb3010compound. Upon dehydration, the strong Ram an banda t -965 cm-' splits into two bands at -980 and -960cm-l, and the weak Ram an band at -840 cm-I disappears.Bulk niobium oxide (Nb205): he surface area of bulkniob ium oxide af ter d i f feren t thermal t rea tments inpresented in Figure 5. Th e results show that the niobiumoxide surface area dramatically decreases with increasingcalcination tempera ture due t o the formation of largerNb 20 5 rystall i tes. Th e Raman spectra of bulk niobiumoxide after thermal treatm ents from 120 to 1000 "C areshown in Figure 6. For Nb20 5.nH 20 r ied at 120 "C for16 h, a broad and weak Ram an band is observed a t -900cm-' as well as a broad and strong Rama n band a t -650


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Niobium OxidesI I

LEb;4

1200 1000 800 60 0 400 200Figure 6. Raman spectra of bulk niobium oxide as a functionof calcination tem peratures.

Raman Shift (cm-1)

4009:200 :

1100 1000 900 800 70 0Raman Shift ( cm ' )

Figure 7. In situ R a m a n spectra of Nb20gn H20: a) hydratedstate; (b) heated at 200 "C under flowing oxygen; (c ) heated at400 "C under flowing oxygen; (d) cooled to 50 "C under flowingoxygen; (e ) readsorbed water molecules under ambient conditions.cm-'. Whe n the niobium oxide sample is heat ed for 2 hat 500 "C in air , the broad Raman band at -900 cm-'becomes even weaker and the strong Ram an band at - 50cm-' sh if ts to -690 cm-l. Th e Nb 20 5 500 " C ) Ramanband s in the low -wavenum ber region (200-300 cm-') be-come more intense and better resolved than those ofN b 2 0 g n H 2 0 1 20 "C ) in this region. T he Rama n featuresof the Nb 20 5 ample t rea ted a t 800 "C for 2 h are similarto those of the N b2 05 500 " C ) sample. However, addi-tional weak Rama n band s in th e 400-500-cm-' region areobserved in the N b2 05 800 " C ) sample. For t h e N b 2 0 5sample t rea ted a t 1000 "C fo r 2 h, the Raman resu l tsindicate tha t a phase transformation of the N b2 05 am plehas occurred and additional Raman bands are observedin the spectrum (particularly in the high-wavenumberregion between 900 and 1000 cm-'). T he different Nb 20 5p h ases a r e a l so co n f i r med b y X- r ay d i f f r a~ t io n .~ 'N b 2 0 5 . n H 2 0 1 20 " C ) is found to be poorly crystalline,showing a diffuse pattern of either monoclinic Nb205orNb 0 2 ,4 3 2oxygen-deficient Nb 205 ). N b2 05 500 "C) andNb 2 0 5 8 00 " C ) are s imilar, matching well-crystlline or-thorhombic Nb205 . Nb 205 1000 " C ) is very crystalline aswell, indicating a mixture of monoclinic phases of Nb205.

(27) Je hng, J. M.; McCaslin, P. C.; Wachs, I. E., to be published.

Chem. Mater . , Vol. 3, No. 1, 1991 1031 I

1200 1000 800 600 400 200

Figure 8. Ram an spectrum of the YbNbOl compound.Table I . Relationships between Niobium Oxide Structures

Raman Shift (cm-1)

and Raman FrequenciesRamanbands,struct cm-' comDounds

~ ~

0 790-830 YNbO4, YbNbO4\ PNbo/ '0

500-700 N b 2 0 5 amorphous, TT, T, and H),LiNb03,N aN b03 , K N b03 , layeredoxides850-1000 A1NbO4,K8Nb6Ol9,Nb(HC204)5,ayeredoxides

I6In s i tu Raman s tud ies o f the h igh sur face areaNb 2 0 5 .n H2 0 1 20 "C ) sample were also undertaken inorder to discrimin ate between bulk an d surface function-alit ies,26 nd t he sp ectra in the 700-1100-cm-' region arepresented in Figure 7. For the untreated Nb2 05.nH 20(120 "C), a broad Raman band is observed in the 800-900-cm-' region. For the Nb 205. nH 20 120 "C) treat ed at200 "C for 1h, the Ram an features remain broad and aresomewhat similar to the untreated N b205 .nH2 0 120 "C).Two new Raman bands, however, are observed at -930and -980 cm-' af ter thermal treatm ent at 400 "C fo r 1h. These two Rama n bands are still present after coolingto 50 "C. However, these two Raman b ands disappearafter the sample is rehydrated by exposure to ambientwater vapor overnight. Th e major niobium oxide Ramanband remained at -650 cm-I during this thermal treat-ment and ind ica tes th a t the N b205-n H20 ample d id no tundergo phase transformations.(b) Tetrahedrally Coordinated Niobium OxideCompound. Th e Raman spect rum of Y bN b0 4 s shown

in Figure 8. Raman s tud ies reveal tha t Y bN b04 pos-sessing Ram an ban ds a t -813, -331, -717, an d -435cm-*. Ra ma n band s a t -992, -678, -630, and -237 cm-'indicate tha t Y bN b0 4 also possesses a s tructure similarto th a t of' H-N b20 5 see Figure 6). Th e R aman b an d sbelow 200 cm-' are du e to lat tice vibrations.DiscussionMost of the niobium oxide compounds contain an oc-tahedral N b0 6 structure with different extents of distor-tions present in the structures. Niobium oxide compoundscontaining a tetrahedral Nb 04 structure are extremely rare.The major Raman frequencies of the niobium oxide com-pounds with the corresponding structures are classified inTable I. A higher niobium-oxygen bond order, corre-


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104 Chem. Mater . , Vol. 3, No. 1, 1991sponding to a shorter bond distance, shif ts the Ramanband t o higher wavenumbers. Every niobium oxide com-pound in this study has a unique Raman sp ectrum tha tis related to the sym metry an d bond order of its structu re.The Raman band assignments are based on the corre-sponding Nb-0 bond order and known structure.The tetrahe dral Nb0,3- species does not exist in aqueoussolutions because of the high electronegativity and smallradius of the oxygen atom.28 In addition, the tetrahe dralNbO, stru ctu re is rarely found in niobium oxide com-pound s because th e Nb5 + atom is usually too large to f itinto an oxygen-anion etrahedron.*, Th e rare-earth ANb 04(A =Y, Sm , and La) compounds have been investigatedby Bla se z3 nd Rooksby et al.,24and they determined thatthese compounds possess Nb0 4 etrahedra that appear tobe isomorphous with the scheeli te CaWO, structure.Bl ase z3 tud ied the YNb0, compound by Raman spec-troscopy and pre dicted t he vibrationa l modes of a regularNbO, te trahedro n with no interactions and distortions tooccur at 816 ( u l ) , 650 (u,), 420 (u,), and 340 cm-' ( v 2 ) .Yoshida et al.25 nvestigated the YbNbO, compound byXA NES and EXAFS techniques and determined that theYbN b0, compound possesses a Nb 04 te t rahedron . TheA N b 0 4 (A =Y, Yb, Sm, and L a) compounds were furtherstudied by Kinzh ibalo et aLZ9 n d Tr uno v et al.,,O whodetermined t ha t these compounds possess a fergusonitestruct ure a t room temperature. In the fergusonite struc-ture, the coordination number of the N b atom containsfour oxygen atoms with distances of 1.83-1.93 A and twofu rt he r oxygen atom s with distan ces of 2.40-2.52 A as aresult of th e connection between two adjacent tetrahed ra.The transit ion from the octahedral fergusonite to thetetrahe dral scheelite depen ds on tempera ture. An increasein temp erature results in bonds breaking between theniobium atom a nd th e two further oxygen atoms, and theN b atom coordination becomes tetrahedral.For the ANb0, (A =Y, Yb, Sm, and L a) compounds,the transition temperature from the octahedral fergusoniteto the te trahedral sheelite depends on the ionic radius ofthe rare-earth metal . Th e rare-earth metal possessing asmall ionic radius has a high phase tran sition tem peratur ebecause of the relatively sh ort bond distances between theniobium ato m and the two further oxygen atoms. Thus ,the rare-earth AN b04 (A =Y, Yb, Sm, and L a) compoundspossess NbO, octahedra a t room temperature, an d tran-sition from a NbO, octahedron to a NbO, tetrahedronoccurs at high temperature. For example, the phaset ransi t ions of the Y Nb 04 and th e Yb Nb0, compoundsoccur above 825 O C 3 0 Blasse2, did not address the Y Nb 04struct ures as a function of temperature. Th e multipleRam an ba nds appe aring in the 400-800-cm-' region for theYNbO, compound reported by Blasse2, are probably dueto the simultaneous presence of NbO, tetrahedra andN b0 6 octahedra in the s t ruc ture .Raman studies of the annealed YbN b0, compound re-veal bands a t -813, -717, -435, an d -331 cm-'. Th eRam an bands appearing at -813 and -331 cm-' are dueto the Nb-0 symmetric modes of the NbO, tetrahedralstructure, and Raman bands appearing at -717 and -435cm-' are due to the N b-0 antisymmetric modes of theNbO, tetrah edral structure. Th is is consistent withBlase 's results that the vibrational modes of a regularNbO, tetrahedra, with no interactions and distortions,

Jehng and Wachsshould occur at 816 ( u l ) , 650 (u,), 420 ( v J , and 340 cm-'Additional weak Raman bands appearing at -992,-678, -630, an d -237 cm-' indicate th at the Yb Nb 04compound also contains an impurity of H-N b20 5d u e toa small excess of niobium oxide in the sample. Th us ,tetrahedrally coordinated niobium oxide reference com-pounds possess their major Raman bands in the 790-830-cm-l region.I t appears th at the Nb5+ is also too small to form aregular octahedral NbO, structure,,l and th e oxygens ina regular octahedral NbO, stru ctur e do not obey Pauling'selectrostatic valence rule.32 Consequently, the niobiumoxide compounds predominantly contain NbO, octahedrawith different extents of distortion due to corner oredge-shared NbO, polyhedra.The BNbO, (B =Na and K ) compounds belong to theperovskite struc ture family. Th e Nb atom lies at eachcorner of a cubic cell, and the oxygen atom lies at thecenter of each cubic edge. Each N b atom is surround edby six oxygen atoms to form a N b0 6 octahedron an d twoadjacent NbO, octahedra are connected by a shared corner.Th e cation ( Na or K) lies in the center of the cubic cell.In an ideal perovskite struct ure all NbO, oc tahed ra wouldbe perfectly regular with the cation (N a or K) surroundedby 1 2 oxygens and Nb by 6 oxygens. However, such anideal perovskite structure does not exist in the BNbO, (B=Na and K) comp ounds because of the tilting of the NbO,octahedra as well as the displacements of the Nb atomsfrom the center which is induced by the cation (Na or K)and the unbalanced interatomic forces present in theperovskite structures.8Th e perovskite BNbO, ( B =Na and K) compoundspossess a major Raman ba nd a t 620-630 cm-' and ashou lder in th e 520-580-cm-' region (Figure 1) . Thesebands correspond to slightly different Nb-0 bond dis-tances and are assigned to the symmetric stretching modeof the NbO, octahedra of these perovskite compounds.Th e LiNbO, compound contains a hexagonal close-packedstruc ture in which the regular NbO, octahedra are con-nected by shar ed corners with a 30' tilting angle betweentwo adjacent NbO, octahedra and a 0.26 A off-centerdisplacement of the N b atom.8 T he LiNbO, compoundpossesses similar Ram an features as he perovskite BNb 0,( B =Na a nd K ) compounds in the 500-700-cm-' region.For the LiNbO, (hexagonal close packing) Ra ma n spec-trum , the additional Raman band a t -870 cm-' is assignedto the antisymmetric stretching mode of the Nb-0-Nblinkage, and the associated bending m odes of th e Nb-O-Nb linkage appear a t -430 and -375 cm-l. For theNaNbO, (perovskite) compound, however, the intensityof the Ram an bands at -870, -430, and -375 cm-' sig-nificantly decrease. This is probably due t o the lower tiltang le , 18" , be tween the adjacen t NbO, ~ c t a h e d r a . ~ur -thermore, the disappearance of these Raman bands in theKNbO, (perovskite) Ram an spectrum suggests tha t notilting occurs between the adjacent NbO, octahedra in thisstr uct ure and is consistent with K atz e t al. 's results.'O Anew Ram an band is observed a t -840 cm-' for KN bO B,which is characteristic of th e symm etric stretching modeof the Nb-0-Nb collinear bond present in the structure.Th e distortions of the BNb0 , (B =Li , Na, and K ) com-pounds d epen d on the size of the A cations. For the largersize cation such as K t he cation is coordinated to 1 2 oxy-gens, and free space is unavailable for the N b0 6 octahedra

(28) Muller, M.; Dehan d, J. Bull. S O C . h i m . Fr . 1971, 8, 2837.(29) Kinzhibalo, L. A.; Trunov , V. K.; Evdokimov, A. A.; Krongauz,V. G . Kristalloerafiva 1982. 27. 43 .(30) runov;V.'K.; Efremov; V. A.; Velikopodny, Yu. A,; Averina, I.M . Kristolhgrajiya 1981, 26, 67.(31) Orgel, L. E. An Introduction to Transition Metal Ch emistry;(32) Pauling, L. The Nature of the Chemical Bond; Oxford UniversityWiley: N ew York , 1960.Press: New York, 1952.


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Niobium Oxides Chem. Mater. , Vol. 3, No. 1, 1991 105

c 2

to t i l t re lat ive to one another. Consequently, the NbO,octahedra maintain the parallel orientation. However, thesmaller size Li cation only has six coordin ated oxygens andforms an octahedral LiO, structure . Th e orientat ions ofthe NbO, and L i0 6 octahedra induce the t i l ting betweenthe two adjacent N b0 6 oc tahedra .aIt is known th at niobium oxide can exist as Nb601g8-clusters in a lkaline aqueous solut ions and N b complexesin acidic aqueous solutions as well as precipitates from suchsolut ions. Th e Nb,01,8- un it is a well-characterizedstruc ture th at consists of three different types of Nb-0bonds at each niobium center (see Figure 2) : a shortNb=O terminal double bond, a longer Nb-0-Nb bridgingbond, an d a very long and weak Nb- - -0 bond connectedto the center of the cagelike octahedra l ~tructure .'"'~ Fromthe known stru ctur e of K8Nb6Ol9, he m ain frequenciesof the K8Nb6ol9Ram an spectrum in Figure 2 can be as-signed: Nb=O terminal stre tching mode (-900, -880,and - 40 cm-'1, edge-shared octahedral N b0 6 stretchingmode (-730, -540, an d -460 cm-l), Nb- - -0 t re tchingmode (-290 cm-') , and Nb-0-Nb bending mode (-230cm-'). Th e multiple terminal stretching modes present inthe h igh-wavenumber region a re due t o unequa l N b=Obond lengths tha t are present in the K8Nb6Ol9 tructure .T he niobium oxalate struc ture consists of a [NbO(O-H) (C 20 4), H2 0] r [NbO(C,O,),] unit and water moleculesconnected by hydrogen bond^.^^-^, Th e presence of dif-feren t niobium oxalate units is dep end ent on the prepa-rat ion methods. Th e [NbO(OH )(C,O4),H2O]uni t has apentagonal bipyramidal stru cture with a short Nb=Oterminal double bond and a long Nb-OH2 bond nearlyperpendicular t o the pentagon al equatorial plane whichconsists of two bid entate ox alato l igands and a hydroxylgroup. Th e [NbO (C204 )3] n i t a l so has a pentagona l b i -pyramidal structure with a short Nb=O terminal doublebond and th ree bide ntate oxalato l igands coordinated tothe pentagonal equa tor ia l p lane . Th e sharp and s t rongRam an band at -960 cm-' in the niobium oxalate Ram anspectrum is assigned to the N b= O terminal double bond,and the Raman ban d a t -290 cm-' is assigned to the longand weak Nb-OH, bond. Th e Ram an band at -570 cm-'arises from th e bid entate oxalato l igands coordinated tothe niobium atom which is characteristic of th e breathingmode of the Nb-O2-C2 b ridging bond.Th e str uc tur e of A1NbO4 consists of highly distortedoctahedral NbO, and AlO,, two NbO, units an d two AlO,units, sharing edges together to form a unit cell and linkedto the adjacent u nit cel l by sharing two corners.37 Th eNb -O bond which links to the adjacent unit cell by sharingtwo corners possesses the shortest bond length, and theRam an band a t -930 cm-l is characteristic of the sym-metric stre tching mode of this shortest Nb=O terminaldou ble bond. Ra ma n band s in th e 400-800-cm-' wave-numb er region a re assigned to th e symm etric and anti-symm etric stretching mode of the Nb-0-Nb linkage, andthe associated bending modes appear in the low-wave-numb er region (200-300 cm-I). Th us, the K8Nb6Olg,A1NbO4, an d Nb(HC,O,), compo unds possess a highlydistorted octahedral NbO, s truc ture with Raman band sapp earing in th e high-wavenum ber region, 850-1000 cm-',which are characteristic of the symmetric stretching modeof the Nb=O terminal double bond.

~ ~(33) Kojic-Prodic, B. ; Limingu, R.; Scavnicar, S., Act a Crystallogr.1973. B29. 864.- - - . - - - , - - -(34) Galesic, N.; M atkovic, B.; Herceg, M.; Sljukic, M. J . Less-Com-(35) Brnicevic, N.; Djordjevic, C. J . Less-Common M e t . 1971,23, 107.(36) Mathern Par , G.; Weiss, R. Acta Crystallogr. 971, B27, 1610.(37) Pedersen, B. F . Act a Che m. Sc and. 1962, 16 , 421.

m o n M e t . 1971, 25, 234.

- . n.3 n = 4 n.5KICa,Nan.,Nb,0a,.71

Figure 9. R a m a n band intensity ratios for t h eKCa2Na,-3Nb,03,+l (n=3-5) layered oxide compounds.Th e str uctu re of the K[Ca2Na,-3Nb,03,+ll (n=3-5)layered oxides contains corner-shared NbO, octahedra witha large cation occupying 12-coordinated sites in t he centerof each cube as found in the perovskite structure .I2 T hethickness of each perovskite layer is determined by thenumber of corner-shared Nb 06 octahedra connected along

the direct ion perpendicular to t he layers (n=3-5). TheKCa2Nb3010 tructure has been determined by Dion et al."to possess two types of N b0 6 sites connected t o th e cations(K and Ca): one is the capping sites between two adjacentlayers and the other is the internal si tes in the layer.Ram an studies also detect th e two types of NbO, sitespresent in the layered oxide compounds: a highly distortedoc tahedra l s t ructure , Raman band a t -930 cm-l, and asl ightly distorted octahedral structure , Raman band at-580 cm-'. Th e relative concentrations of the two NbO,octahedra in the layered oxide compounds can be d eter-mined by taking the rat io of these Raman bands (theintegrated Ram an intensi ty of the bands at -930 and-580 cm-') . Th e Ram an I(-9 30 cm-' )/I(-580 cm-I)rat ios for th e different layered compounds are shown inFigure 9 and d ecrease with increasing num ber of niobiumoxide layers. For example, the R am an measuremen ts ofthe layered oxide KCa2Nb3010, n = 3, has 1(-930cm-')/I(-580 cm-') = 2, which reflects the ratio of thehighly distorted octahedral si tes to the sl ightly distortedoctahedral sites. This is consistent with the layered oxidestructu re described by Dion et al." Upon decreasing thenumber of the niobium oxide layers, the 1(-930 cm-l)/I(-58 0 cm-') ratio fu rth er decreases to 1 or K(Ca,Na)-Nb&, n =4, and 0.8 for K(Ca2Na,)Nb,01,, n = 5, asexpected from the known structures.12 Th e quanti ta t iveagreement between the Raman ratios and the actual ratiosin these layered stru cture s reveals that the R aman crosssections of th e slightly distorted and highly distorted N bO,oc tahedra a re essent ia l ly the same. Th e Ram an I ( -800cm-')/1(-580 cm-') ratios in Figure 9, however, do notchange with increasing number of niobium oxide layers.This indicates tha t the Ram an band a t -800 cm-' is as-soc ia ted wi th th e s l ight ly d i s tor ted N b0 6 s i te ban d a t-580 cm-' and corresponds to different Nb -0 bondlengths. Increasing the number of niobium oxide layersalso shifts the band from -760 to -840 cm-l and elimi-na tes the Raman bands a t -450 and -375 cm-'. TheseRaman features reflect th e different extents of distortionstha t exist in the layered oxide compounds and are probablydue to th e different sizes between th e Ca and N a cations.Many of these layered oxide compounds can also un-dergo an alkali-metal ion exchange reaction with theprotons present in aqueous acidic solut ions du e to their


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106high ionic conductivity. Th e potassium cation in thelayered oxide KC a2Nb 3010 xchanges with th e hydrogenproton in an aqueo us acidic solution to form the layeredoxide HCa2N b3010.Th e layered oxide HCazN b3010 ps-sesses a tetragonal unit cell with an interlayer separa tion14.38 A, which is contracted relative to the values of 14.73A observed for the layered oxide KC a2Nb3010.'3 Wh enhydrogen replaces potassium, the interlayer hydrogenstabilizes the terminal oxygen atoms by forming a termina l-OH group interacting with a termina l oxygen in the ad-jacent layer. Th e stoichiometry corresponds to one protonfor every pair of term ina l oxygens from th e adjacen t layers.The hydra ted HCa2NbHOloompound has a compositionHCazN b3Ol0~1.5H,O ith an inter layer separation 16.23A. The interlayer hydrogen bonding between the twoadjac ent termi nal oxygens creates some free volume in theHCa zNb3 010 tructure t o accomm odate 1.5 water mole-cules.13Th e in si tu Ram an s tudies of the layered oxide HCa,-Nb3OIo ndicate tha t the Nb=O terminal double bond isaffected by the adsorbed water since the Raman band a t-965 cm-' broadens and spli ts into two bands at -980and -960 cm-l upon dehydration (see Figure 4). T heRaman band a t -840 cm-' is assigned to the Nb=O an-tisym metric stretching mo de, which is better resolved inthe hydra ted s ta te . Thus, the in s itu Raman s tud iesdemonstrate that the water molecules in the layered oxideHCa2Nb3010 tructure associate with all the interlayeredtermina l oxygens to form Nb=O- - -H bonds. Th e resultsare in agreement with the structural studies of Jacobsonet al.13 The in situ Raman studies of the layered oxideHCa zNb 3010 lso reveal t ha t hydrogen bonding via watermolecule addi tion results in a decrease in the Nb==O bondorder a nd i ts corresponding vibrational frequency.Bulk Nbz05undergoes the following phase transforma-tions during heatamo rp h o us Nb 2 0 5 .n H 2 0 3oo-500 oc *

Chem. Mater . , Vol. 3, No. 1, 1991

TT-Nb205 ,oo-*oooc* T-" - -Nb205Th e amorphous N b,05.nH 20 contains sl ightly distortedNbO,, NbO,, and NbO, polyhedra.' T he strong an d broadRam an band at -650 cm-' is assigned to the symmetricstretching mode of the niobia polyhedra. Th e broad Ra-ma n feature reflects the distrib ution of th e NbO,, NbO,,an d NbO, sites in the stru cture of the amorpho usNb 2 0 5 - n H2 0 .Th e weak and broad Raman band a t -900cm-' IS assigned to the s ymm etric stretchin g mode of theNb=O surface sites because of th e response of this bandto surface dehydration during the in si tu Ram an studies(see following paragraph). He at tr eatm ent a t 500 "C for2 h leads to crystallization into the TT -N b2 05 hase. Th ecorresponding shift of the Ram an band from -650 to-690 cm-' is due to th e increasing bond order of th e niobiapolyhedra, and the s harpening of Rama n band at -690cm-' is due to the more ordered structure present in theTT-NbzOS hase. The TT -Nb 205 hase conta ins oc tahe-dral and pentagonal bipyramid structures, and t he weakR aman b an d a t -900 cm-' is due to the small concen-tration of Nb==O surface sites after the th erma l treatm ent.Additional Ram an ban ds in the low-wavenumber region,200-300 cm-', are also observed in the TT -N b20 5 pectrum,which are characteristic of the bending modes of the Nb-0-N b linkages. Raman features of the T-N b2 05 800"C)phase are similar to those of th e TT -N b2 05 500 "C) phase

Jeh n g a nd Wa ch s(see Figure 6) . This indicates that the T -N b2 05 800 "C)phase possesses a struct ure similar to that of the TT- Nb2 05(500 "C) phase and is consistent with Weissman e t al .' sco nc lu sion s th a t t h e TT- Nb 2 0 5 an d T - N b z0 5 p h asespossess nearly identical structure^.^T h e H - N b Z 0 ,phase contains 3 X 4 nd 3 X 5 blocks ofcorner- or edge-shared octahedral N b0 6 as well as 1 et-rahedral site per 28 N b sites. Bhide et al.41have assignedthe Raman band in the high-wavenumber region, 900-1200cm-' , to the Nb=O terminal bond. Therefore, Ram anbands a t -997 and -900 cm-' for the H- Nb z0 5 hase inFigure 6 are c haracteristic of th e sym metric and antisym -metric stretching modes of th e Nb=O termin al bond.Bl ase z3 tud ied the Y Nb 04 compound by Raman spec-troscopy and determined that the tetrahedra l N b0 4 pos-sesses a major Ram an band a t 830 cm-'. McConnell et al.3and Iwasawa et ale4, eferred to Blasse's result and de-t e rmin ed th a t t h e R aman b an d a t 840 cm- ' in H-N bz0 5is due to the tetrahedral N b0 4 structure. This assignmentis not correct because Blasse did not address that thetransition of YN b0 4 from the octahedral fergusonite to th et e tr ah e dr a l s ch ee li te d e pe n ds o n t e m p e r a t ~ r e . ~ ~n ad-dition, the N b-0 bond distances of the tetrahedra l site inH-Nb,O, have been dete rmined t o be 1.65 and 1.68 A byGatehouse et al .7 Th e predicted Raman frequency withcorresponding Nb-0 bond distance of the H -N b2 0, tet-rahedral si te appears at 99 3 cm-' by using Hardcastle'sbond dist ance/freq uency correlation with th e overall un-certainty of 30 Thus, the Raman band a t 840 cm-'for H-NbZ O5 an not be assigned to the N b0 4 etrahedron.Th e corner-shared octahedral NbO, in the H -Nb,0 5 phaseforms a Nb-0-Nb collinear bond with a correspondingRaman band appearing at -840 cm-'.' Ram an ban ds inthe 500-800-cm-' region are characteristic of the stretch ingmodes of th e slightly distorted octahedral NbO, structu res.At the low-temperature treatme nts (e800 " C ) , he struc-ture of niobium oxide possesses the slightly distorted oc-tahedra l niobia polyhedra. Th e structure of the niobiumoxide treated a t high temp eratures, greater tha n 1000 OC,possesses a highly d istorted octahedral N b0 6 struc ture inaddition to the sl ightly distorted octahedral N b0 6 struc-ture. Thu s, the phase transformations of niobium oxideare strongly dependent on the heat treatm ents, an d thisis reflected in th e Raman spectra.During in si tu Raman experiment the samples areheated to desorb the adsorbed moisture, and those Ram anfeatures tha t respond to th e dehydrat ion t rea tment areiden ti fi ed a s su rf ace f u n ~ t io n a l i t i e s .~ ~his process isreversed by readsorbing water vapor on th e surface func-tionalities. Th e in situ Raman studies of Nb2 05 .nH z0 seeFigure 7) demonstrate that the broad and weak Ramanban ds in the 800-900-cm-' region shift to higher freque n-cies due to therma l desorption of the adsorbed moisture.After dehydration of Nbz05vzH20 t 400 "C in air for 1h, two new Raman bands are observed a t -930 and -980cm-' which are ch aracteristi c of two different Nb=Osurface si tes. No phase transformations occurred duringthis thermal treatm ent since the R aman features of bulkniobium oxide remained the same. Th e concentration ofthe Nb=O terminal sites of bulk Nb z0 5 pretreated at 500"C) is too small to be observed due to the much lowersurface area. Thus, the amorphous Nb205.nH,0 material

(38) Brauer, G. 2. norg. Allg. Chem. 1941, 248, 1.(39) Schafer, H.; Breil, G. Z. Anorg. Allg. Chem. 1952, 267, 265.(40) Schafer, H.; Grueh n, R.; Schulte, F. Angew. Chem. 1966, 78, 28.

(41) Bhide, V.; Husson, E. Mater . R e s . Bull. 1980, 15,1339.(42) Nishimura, M.; Asakura, K. ; Iwasawa, Y . J . Chem. SOC., h em.C o m m u n . 1986, 1660.(43) Hardcastle , F. D. Dissertation, Lehigh University; UniversityMicrofilms Internation al: Ann Arbor, MI , 1990.(44) Chan, S. S.; Wachs, I. E. ; Murrell , L. L.; Wang, L. ; Hall, W. K .J . Phys. C h e m . 1984,88, 5831.


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Chem. Mate r. 1991, 3, 107-115 107possesses terminal Nb=O sites on the surface, an d thenumb er of terminal Nb=O sites can be eliminated byhigh-temperature calcinations which dramatically de-creased the surface area . Th e in s i tu Raman s tud iesconfirm th e presence of th e terminal Nb=O surface siteson amorphous N b205.nH2 0 and t he ass ignment of th eRaman band a t -900 cm-' to the terminal Nb=O surfacesites.

ConclusionsTh e relationships between niobium oxide structu res andtheir co rresponding Rama n spectra were systematicallystudied for various types of niobium oxide compounds.T he Ram an frequencies strongly depend on the bond orderof the niobium oxide structure. A higher niobium-oxygenbond o rder , corresponding to a shorter bond d istance,shifts the Rama n frequency to higher wavenumbers. Mostof the niobium oxide compou nds possess an octahedrallycoordinated N b0 6 structure, sl ightly or highly distorted.Only a few niobium oxide compounds (such as YN b0 4,YbN b04, LaNbO,, and SmN bO,) can possess a tetrahe-dra lly coord inated N b0 4 s t ruc ture t ha t i s s imilar to thescheeli te- like structure. For the tetrahed ral N b0 4 struc-

tu re , the major R aman f requency appears in the 790-830-cm-' region. For the slightly distorted octa hedralNb 06 s t ruc ture , the major Ram an f requencies appear inth e 500-700-cm-' wavenum ber region. For the highlydistorted octahedral Nb06 tructure, the Raman frequencyshifts from the 500-700- to th e 85Ckl000-cm-' region. Bo thslightly distorted and highly distorted octahedral N b0 6sites coexist in the KCa2N an-3Nbn03n+l,=3-5, layeredcompounds. Th e distortions in the niobium oxide com-pounds are caused by the formation of corner- or edge-shared N b0 6 octahedra .Acknowledgment. Financial support for this work byNiobium Products Company Inc. is gratefully acknowl-edged. We wish to thank A. Jacobson of Exxon Researchand Engineering Co. for providing the layered niobiumoxide compounds a nd S. Yoshida of Kyoto University,Japan , for p rov iding the Yb Nb 04 compound.KNbO,, 12030-85-2; NaNb 03, 12034-09-2;LiNb03,12031-63-9;K8Nb,0,,, 12031-11-7;AlNbO,, 12258-25-2;Nb(CH204)5,2404-95-4;K(Ca2Na2)Nb5OI6,8820-43-0;K(Ca2-Na)Nb4013,8820-40-7;KCA2Nb3Ol0, 0487-87-2; HCa2 Nb3OI0,98820-36-1;Nb2 05, 313-96-8;NbzOgnH20,12326-08-8;YbNbO,,

Registry No.

12034-62-7.

Phase-Transfer Palladium(0)-Catalyzed PolymerizationReactions. 6. Synthesis and Thermotropic Behavior ofMono- and Difluorinated1,2 Bis 4-n alkoxyphenyl)acetylene Monomers?Coleen Pugh and Virgil Percec*

Dep ar tmen t o f Macromolecular Science, Case Western Reserve U niversity,Cleveland, Ohio 44106Received May 30, 1990

Symmetrically difluorinated 1,2-bis(3-fluoro-4-n-alkoxyphenyl)acetylenen =4-12) and asymmetric,monofluorinated 1-(3-fluoro-4-n-alkoxyphenyl)-2-(4-n-alkoxyphenyl)acetylenen =5-12) monomers wereprepared by a one-pot, phase-transfer Pd(O)/Cu(I)-catalyzed three-s tep coupling of th e appro priate arylhalides with 2-methyl-3-butyn-2-01. All odd mem bers of the 1,2-bis(3-fluoro-4-n-alkoxyphenyl)acetyleneseries are crystalline with a virtual nematic mesophase. All even members of th e series present m onotropicnematic mesophases. The thermotropic behavior of th e l-(3-fluoro-4-n-alkoxyphenyl)-2-(4-n-alkoxy-pheny1)acetylenes changes continuously with n. The n =5 derivative is crystalline. T he n =6-10 derivativeseach have an enantiotropic nematic mesophase. In addition, the n =7,sderivatives exhibit an enantiotropicsmectic mesophase, and t he n =9, 10 derivatives exhibit monotropic smectic C mesophases. Both thenematic an d smectic C mesophases of th e n =11,12 derivatives are monotropic.

IntroductionIn a re cent publication,' we d escribed a one pot, solid-liquid phase-transfer Pd(O)/Cu(I)-catalyzedsynthe sis of1,2-bis(4-alkoxyaryl)acetylenes rom aryl halides deacti-vated by alkoxy substi tuents . Th e three-step, one-potsynthesis outlined in Scheme I was adapte d from Carpita 2e t a1.k liquid-liquid phase -transfe r catalyzed synth esis ofdiheteroarylacetylenes. In th e first step, an aromatic halideis coupled with a monoprotected acetylene. Th e resultingcarbinol derivative is deprotec ted in the second step with' P a r t 5: Pugh, C.; T a r n s t r o m , C.; Percec , V. Mol. C r y s t . L i q .*T o whom cor respondence sho u ld be sen t .Crys t . , in press.

forma tion of an aryl acetylene, which is then coupled w itha second aryl halide in the final step. In this phase-transfercatalyzed (P TC) , one-pot procedure, only th e f inal di-arylacetylene product is isolated.The subsequent papers in this series described thethermotropic behavior of both symmetrically and asym-metrically substitute d 1,2-bis(4-alkoxyaryl)acetylenemo-n o m e r ~ . ~ - ~he linear l,Bbis(4-n-alkoxyphenyl)acetylenes(1)Pugh , C.; ercec, V. J.Polym. Sci., Polym. Chem. E d . 1990, 28,(2 ) Carpita, A. ; Lessi, A. ; Rossi, R. Syn thes is 1984,571.(3) Pugh, C.; Percec, V. Mol . Cryst. Liq . Crys t . 1990,178, 193.(4) ugh , C.; ercec, V. Polym. Bull. 1990, 23, 177.(5 ) Pugh , C.; Tarnstrom, C.; Percec, V. Mol. Crys t . Li q . Cryst., in

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press.0897-4756/91/2803-0107$02.50/0 0 1991 American Chemical S ociety

artigo raman para nbo6

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