Raman Spectra of the Solid-Solution between Rb

298 Bull. Korean Chem. Soc. 2001, Vol. 22, No. 3 Hee Jin Kim et al.

Raman Spectra of the Solid-Solution between Rb

La

Ti

O

and RbCa

Nb

O

College of Environment and Applied Chemistry, KyungHee University, KyungKi 449-701, Korea

‘Department of Chemisty, AjouUniversity, Kyung Ki442-749, Korea Received October 9, 2000

A site preference of niobium atom in Rb2-xLa2Ti3-xNbxO^io (0.0V xV 1.0) and RbLa2-xCaxTi2-xNbi+xO^iQ (0.0V x V 2.0), which are the solid-solutionsbetween Rb2La2Ti3Oⁱ⁰ and RbCa2Nb3Oⁱ⁰,hasbeen investigatedbyRa­ man spectroscopy. The Raman spectra of Rb2-xLa2Ti3-xNbxOi0 (0.0V < i.0)gaveanevidence that niobium atoms substituted for titaniumatomspreferably occupythehighly distorted outeroctahedral sites rather than the central ones in triple-octahedral perovskite layers. Incontrast, the Raman spectra of RbLa2-xCaxTi2- xNbi+xOi0 (0.0 V xV 2.0) showed no clearinformation for thecationicarrangement in perovskiteslabs.This differenceindicated that asitepreference of niobium atoms is observed only when thelinearRb-O-Ti linkage canbe replaced bymuch strongerterminal Nb-Obond with double bond character. From comparison with the Raman spectroscopicbehaviorof CsLa2-xA’xTi2-xNbi+xOi0 (A’= Ca and Ba; 0.0 V x V 2.0), it isalso proposed thata localdifferenceinarrangement of interlayer atomscauses asignificantly different solidacidity and pho- tocatalyticactivity of thelayered perovskite oxides, despite theircrystallographically similar structures.

Kywords : Layered structure, Oxides,Raman spectra, Site preference.

Introduction

The oxides containing Tiand Nbhave potential applica­ tions in the photocatalytic decomposition of H2O. Some examples of them could be referred to SrTiO3, KNb⁶Oi7, Na2Ti6Oi3, BaTiQ% KzTiQ% K2La2Ti3Oi0,Na2Ti3O7, La2- Ti2。％ and AzNbzO? (A=Ca, Sr).I-8 Many of such examples have a layered structure. In particular, a typical layered perovskite A^2-xLa2Ti^3-xNbxOⁱ⁰ (A=K, Rb, Cs) solid-solution showedhigh photocatalyticactivity for thewatersplitting9,i0 Theremarkable shape selectivity and dependenceof the cat­

alyticactivityonxvalue suggested animportantroleof the interlayerspace.

An arrangement of niobium and titanium atoms in the octahedralperovskite slabs of ACa2-xLaxNb3-xTixOi0 (A=alkali- metals) were also investigated by two research groups. A significant change in the solid acidity of their protonated phases, HCa2-xLaxNb3-xTixOi0, induced a followingsugges­

tion: The corner sharedtriple-octahedra are ordered in the sequence of NbO6-TiO6-NbO6-along the c axis inACaLa- Nb2TiOⁱ⁰ (x=i), but the ordering ofniobium and titanium atoms isreversed in ALa2NbTi2Oi0 (x=2),the niobiumatoms occupyingcentral octahedra.11 In contrast,thestructuralana­ lyses ofthe same series gave a completely different con­

clusion such that the cationic arrangements are in the sequence of -(Ti0.i5Nb0.85)O6-(Ti0.7Nb0.3)O6-(Ti0.i5Nb0.85)O6- and -(Ti0.5Nb0.5)O6-TiO6-(Ti0.5Nb0.5)O6- along the c axis in ACaLaNb2TiOi0 and ALa2NbTi²Oⁱ⁰, respectively.i2

TheRamanspectroscopicapproachmaygive a supporting evidence for a site preference ofniobiumatoms inthe octa­

hedral slabs of layered perovskite structure. Recent study has been focused on the relationship between interlayer structure and Raman spectra ofion-exchangeable layered perovskites." An interestingfeature observed in suchworks

was that the correlation between triple-octahedral layer and interlayer structure is significantly different depending on their connection mode.Anexistenceor absence of thelinear linkage between them couldbeconfirmedby thecharacter­ istic band around 850-950 cm-1 without knowledge ofthe exactcrystalstructure.

In this work, the Raman spectra of Rb2-xLa2Ti3-xNbxOi0

(0.0Mx^M i.0) andALa^2-xA’^xTi2-xNbi+xOi0 (0.0^三 2.0; A= Rb, Cs;A’=Ca, Ba)solid-solutionswereexamined to inves­

tigate a cationicarrangement and bondingnature in octahe­ dral layer and interlayer space. The spectral interpretation was based on the previous reference Raman spectra of Rb2La2Ti3Oi0 andACa²Nb3Oi0 (A=Rb and Cs), which are the end members of such solid-solutions. Interestingly, a good evidence was given for a site preference ofniobium atoms in triple-octahedral perovskite slabs as well as a sig­ nificant localchangeofinterlayer structure in thesolid-solu­ tion range.

Experiment^지Section

Rb2-xLa2Ti3-xNbxOi0 (0.0 M < i.0) and ALa2-xA’xTi2-x- Nbi+xOi0 (0.0 M x M 2.0;A=Rb,Cs; A’=Ca,Ba)solid-solu­ tions were prepared by heating appropriate mixture of A²CO3, La2O³, A’CO3, Nb2O⁵, and TiO2 in airat I000-II50 oC for 2 days with two intermittent grindings.3-7 Anexcess (~20mol%)of A2CO3 wasadded tocompensate for theloss ofvolatile alkali-metal components. Afterthe reaction, the products were washed with distilledwater and dried at i20 oC.

A stoichiometric composition was confirmedby the ele­

mental analysisof alkali-metal using theinductivelycoupled plasma (ICP) and the energy-dispersive X-ray emission (EDX)techniques. Theformation ofsinglephasewas con­

firmed by the powder X-ray diffraction (XRD). All the pat­ terns ofprepared oxides in this work were indexed onthe basis of tetragonal or orthorhombic cell andinwellagree­

ment with the literature data.11,14-17

Raman spectra were obtained with a Bruker RFS 100/S FT-Raman spectrometercoupledto thehigh sensitivity Raman detector(ModelD418-S),whichwas cooled down to liquid nitrogen temperature. Powdered samples packed into small aluminum cups were excited by the 1064-nm line of the Nd:YAG laser with 10 mWof power. The laser beam was focused on the sample and the scatteredlight wascollimated into the spectrometerby 180o angle configuration. Raman spectra were collected at room temperature. The overall spectral resolution ofthespectra wasdetermined tobeabout 2 cm-1.

ResultsandDiscussion

Thestructures of end members,Rb2La2Ti³O10 and RbCa2- Nb3O10, for solid-solution are schematically compared in Figure 1. In the structure of Rb2La2Ti3O10, the perovskite slab is composed ofcorner sharedtriple-TiO6 octahedra and the adjacent slabshavethe staggeredconformationleading tothe caxisdoubling. TheRbatomsoccupyallof9-coordi- nate sites in the interlayerspace as shown inFigure 1a. A displacementoftitanium atom from thecenter of its octahe­

dron toward rubidium atom layers gives highly distorted outeroctahedrallayers and slightly distorted central one.13,18 The corner shared TiO6 octahedra are relatively rotated

(〜12.5o) and tilted aboutthe c axis. Althoughoverall feature oftriple-NbO6 octahedral layer is also true for RbCa2Nb3O10, onthe contrary, the interlayerrubidium atoms occupy dis­

torted cubic sites which are given by the eclipsed conforma­ tionof adjacent perovskite slabs (Figure 1b). Thus, it should be notedthatthestructures ofend members have an impor­ tant difference inbonding natureof rubidium atom.There is a practically linear Rb-O-Ti linkage parallel to thec axisin Rb2La2Ti3O10. Due tothis structuralfeature, the competing Ti-O bond vibration was sensitively changed depending on theinterlayerstructure. In contrast, no linear Rb-O-Nb con­

nection is there in the structure of RbCa2Nb3O10. Accord­ ingly, theinterlayerstructuredidnot greatly affecttheNb-O terminalbond with double bond character.

Based onthe previous report,13 thelocal structural differ­

ence between two end members is reflected quite well in their Raman spectra when the interlayeratoms are changed.

Figure 2 shows the Ramanspectra of A²La2Ti3O10 (A=Na, K, and Rb). The bands around 900-860 cm^-1 and around 580-540 and 520-510 cm^-1 are assigned to the symmetric stretching mode and the asymmetric modes of highly dis­

torted outer TiO6 octahedra, respectively. The band around 700-670cm-1 is relatedtothe slightly distorted centralTiO6

octahedra. The bands below 400 cm-1 are assignedto exter­ nal modes located in the alkali-metal layer. A considerable shift of the band at 902 cm-1 (Na2La2Ti3O10), which is assigned tothe stretching mode of short Ti-O bond along the c axis, to 875 (K2La2Ti3O10) and 867 cm-1 (Rb2La2Ti3O10) reflects anexistenceof linear A-O-Ti linkage.In contrast, no significant shift of this bandis observedin spite of a large

Figure 1. Schematic illustrations of the structure for (a) RbzLa?- Ti3O10 and (b) RbCa2Nb3O10 perpendicular to the c direction of the tetragonal or orthorhombic unit cell. The triple-octahedral unit is composed of the highly distorted outer octahedra and the slightly distorted central octahedra. Small shaded spheres, large shaded spheres, black spheres, and white spheres are La or Ca, Rb, Ti or Nb, and O atoms, respectively.

294

1000 800 600 400 200

(cm-1)

Figure 2. The Raman spectra of (a) Na2La2Ti3O10, (b) K2La2Ti3- O10, and (c) Rb2La2Ti3O10.

300 Bull. Korean Chem. Soc. 2001, Vol. 22, No. 3 Hee Jin Kim et al.

1000 800 600 400 200

(cm'1)

9

Illi 39

(a) 226

58, :

36 (b)

250 238

i— 一 义 " r 쯔*

38 230

I

! i

l—— 낏 — — 八任拒으/ J <

234

36 A

(d) 1

1

A 494

'J 一 丿

Illi

Figure 4. The Raman spectra of Rb2-^La2Ti3-ANbA；O10 with (a) x=

0.0, (b) x=0.25, (c) x=0.5, (d) x=0.75, and (e) x=1.0.

Figure 3. The Raman spectra of (a) NaCazNbaOio, (b) KCa2Nb3- O10, (c) RbCa2Nb3On), and (d) CsCa2Nb3O10.

latticeexpansionifsuch a linear connection is absent.

Figure 3 shows the Raman spectra of ACa²Nb3O10 (A=

Na, K, Rb, and Cs)whose structure containsnolinearA-O- Nb linkage.A sharpstrong bandaround936cm^-1 is assigned tothevibrationalmodeof theNb-O terminal bondof highly distorted NbO⁶ octahedra. The bands around 580 and 490 cm^-1 arealsoattributedtothehighly distorted NbO6 octahe­ dra. The band around 760 cm^-1 is due to the slightly dis­

torted central NbO⁶ octahedra. Essentially constant Raman bands inthe region400-1000cm-1regardless of the change ofalkali-metallayers are associated with the absence oflin­ ear A-O-Nb connection in the structure of ACa2Nb3O10.

In attemptstoprovide an information for a cationic arran­

gement and bonding nature in perovskitelayer and interlayer space, the Raman spectra of Rb2-xLa2Ti3-xNbxO1Q (0.0 M x <

1.0) were examined. This solid-solution corresponds to a bridgingsystemofRb2La2Ti3O10 and RbLa2Ti2NbO10.Their Raman spectra arecompared as afunction of xvalue in Fig­

ure 4. A remarkable shift and broadeningofthe band at 867 cm^-1 is induced with increasingx. Itsrelative intensity also largely decreases and thisbandultimately disappears inthe spectrum of RbLa²Ti2NbO10 (x=1.0). A new weak band at 957 cm-1 appears when x=0.25 and becomes more intense with increasing x. Thisdramaticchange is explainedasfol­ lows: The niobium atoms substituted for the titaniumones may preferablyoccupy thehighlydistortedouteroctahedral layer oftriple-perovskite slab to form the terminal Nb-O

bond, giving riseto the band at957cm^-1. Due to theinduced vacancies inthe interlayer spaces, the Rb-O-Ti bond devi­ ates from alinear connection with increasing x. A strength­ eningofTi-Obondthenleads the band at867cm^-1 to shift toward higher frequency. Such an assumption is supported by the fact thattheintensity of the band at957cm^-1 is pro­ portional to the amount of replaced niobium atomsbut no shiftof this band is observed regardless ofx. The site prefer­

ence ofniobium atom is also consistent with a considerable broadening and weakeningofthe band at547cm-1 which is attributed to thehighly distorted outer TiO6 octahedra. Adis­

appearance of this band is accompanied by an increased intensity ofthe band at 610 cm^-1 which appears from x=0.5, indicating thatthis new bandis associated with the highly distorted outer (Ti, Nb)O6 octahedra. Although its relative intensity is notsignificant, a weakband appeared at ~760 cm-1 suggests a formationof small amountof centralNbO6

octahedra. Noobservation of thebands at 867 and 547cm^-1 when x=1 (RbLa2Ti2NbO10) thereforereflectsanabsence of linear Rb-O-Ti connection despite that a number of outer octahedral sites are still occupied by titanium atoms. The central TiO6-related mode (695 cm-1-band) shows only a small and gradual shifttoward lower frequency. This indi­ catesthat the increasedoccupancy ofniobium atoms for the outeroctahedralsitesdecreasesthecovalentcharacterofthe central TiO6 octahedra. This explanation is relatively in agreement with previous structural data having proposed an

957 938

226A

230

1000 800 600 400 200

(cm'1)

Figure 5. The Raman spectra of RbLa2-xCaxTi2一xNbi+xOio with (a) x=0.0, (b) x=0.5, (c) x=1.0, (d) x=1.2, (e) x=1.5, (f) x=1.8, and (g) x

二2.0.

Figure 6. The Raman spectra of CsLa2-xCaxTi2-xNb1+xO10 with (a) x=0.0, (b) x=0.5, (c) x=1.0, (d) x=1.2, (e) x=1.5, (f) x=1.8, and (g) x

=2.0.

1

57 936

(a) 676

(b)

681

(C)

689

、(d)

697

J

(e)

705

T

⑴

797 734

1 (g)

764

octahedral arrangement -(Ti0.5Nb0.5)O6-TiO6-(Ti0.5Nb0.5)O6- alongthe caxis for RbLa^zTi^zNbO^］。.12As evidenced by an observation of weak band at ~760 cm-1, however, an exist­

ence of central NbO6 octahedra should not be completely ignored for thisphase.

Further replacement of titanium atoms by niobium ones wasextended to a RbLa2-xCaxTi^2-xNb1+xO10 (0.0 M < 2.0) series. This systemcorrespondstoa solid-solutionbetween RbLa2Ti2NbO10 and RbCa2Nb3O10. The preference of nio­

bium atoms for the outer octahedral siteswas however not evident for this system in which there is no linear Rb—O—Ti bond. As shown in Figure5,the band at 957cm-1 related to theterminal Nb-O bondgraduallynarrows and shiftstoward 938 cm^-1 with increasing x. Similarshift toward lower fre­ quency is observed for the band at 610 cm-1. On the con­

trary, a gradual weakening and shift toward higher frequency ofthe central octahedra-related band around 672 cm^-1 is inducedasxincreases. Hence, it is most likely thatthe cen­ tral TiO6 octahedra are replaced by more covalent NbO6

ones inproportion to x.Thisindicatesthat therelative occu­ pancy ofniobium atom becomes gradually higher for the central octahedral sites rather than for the outeroctahedral ones with increasingx. Theenhanced averagecovalent char­

acter of thecentral (Ti, Nb)O6 octahedra reduces inturn the covalent character of highly distorted outer octahedra, resulting in a gradual shiftofthe bands at957 and 610 cm^-1 toward lower frequency.

The Raman spectral characteristics ofCsLa2-xCaxTi2-xNb1+x- O10 shown in Figure 6 are quite similartothose of RbLa2-x- CaxTi2-xNb1+xO10 solid-solution, indicating that they are iso- structural. All ofRamanbehaviorsare alsotrue for CsLa2-x- BaxTi2-xNb1+xO10 systemwhich has much largerlattice vol­

ume(Figure 7). In particular,thebandsattributed tothecen­ tral(Ti, Nb)O6 octahedra(699 and705 cm-1 inFigures 5(c) and 6(c), respectively) are observed until x=1.5 (ALa0.5- Ca1.5Ti0.5Nb2.5O10). Thus, both the central and outer TiO⁶ octahedra are likely to exist over all solid-solution range.

This would be in agreement with a proposed octahedral arrangement -(Ti^0.15Nb0.85)O6-(Ti^0.7Nb0.3)O6-(Ti^0.15Nb0.85)O6- along the c axis forCsLaCaTiNb²O10 (x=1).12

As aconsequence,it can be proposed thata site preference is observed when thelinearA-O-Tilinkagecan be replaced by much stronger terminal Nb-O bond with double bond character and therefrom a considerable decrease of lattice energycan beachieved.Onemoreevidencesupportingsuch a conclusioncould bereferred to the cationic arrangement in triple-octahedrallayer of Li2La1.78(Nb0.66Ti2.34)O10.19 The struc­

tureofthisoxide canbederivedfromLi2La2Ti3O10 which is isostructural withA2La2Ti3O10 (A=KandRb).15 When tita­ niumatomsarepartially replaced byniobiumatoms,most of them occupy the outer octahedral sites (in the sequence of -(Ti^0.69Nb0.31)O6-(Ti^0.96Nb0.04)O6-(Ti^0.69Nb0.31)O6-). The lith­ iumatomsoccupythe distorted tetrahedral sites in the inter­

layer space, giving terminal Nb-O bonds.

302 Bull. Korean Chem. Soc. 2001, Vol. 22, No. 3 Hee Jin Kim et al.

957 940

1000 800 600 400 200

(cm」)

Figure 7. The Raman spectra of CsLa2-^Ba¥Ti2-A；Nbi+A；Oi0 with (a) x=0.0, (b) x=0.5, (c) x=1.0, and (d) x=2.0.

However, oneof theimportantaspects to be pointed out is the considerable changeof thebands at 200-400cm^-1 which are relatedto theinterlayerstructure.Althoughthecoordina­ tionenvironment for cesium or rubidium atoms wasregard­ edas the same in the structural analysis of ALa2Ti2NbO10, ALaCaTiNb2O10,and ACa²Nb3O10,12 present Ramanspectra clearly shows that rubidium or cesium layers are continu­

ously rearrangeduntilall oftitanium atoms are replaced by niobiumones. Thus,the arrangement of interlayeratoms is closely dependent of the NbO⁶/TiO6 ratioin theouter octa­

hedral layer oftriple-perovskite slab. A local difference in theinterlayer structure would beaccordinglyresponsiblefor the quite differentphotocatalyticactivities9of A²La2Ti3O10, A1.5La2Ti2.5Nb0.5O10, and ALa2Ti2NbO10 (A=K,Rb,Cs) and solid acidity11 ofH2La2Ti3O10, HLa2Ti2NbO10, and HCa2- Nb3O10.

In conclusion, although they have crystallographically

similar structure, the layered perovskite oxides can give a significantlydifferentinterlayerchemistry depending onthe local structureof interlayer space.In this pointof view, the Raman spectroscopycould beanefficient tool for the confir­

mation oflocalstructuralchangeduring a chemicalreaction in theinterlayerspaceof layered perovskite.

Acknowledgment. H. J.KimthankstheMinistry of Edu­

cation for thestudentship through BK21 program.Thiswork was supported by grant No. 97-05-01-03-01-3 from the BasicResearch Programof theKorea Science & Engineer­

ing Foundation.

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