Spectroscopic and structural studies of the hexagonal perovskite Ba2CoTeO6

0584-6539/85s3.00 + 0.00 0 1985Pergamon Press Ltd.

SpectrochimicaAcra, Vol. 41A, No. 4, pp. 523-529, 1985 Printed in Great Britain.

Spectroscopic and structural studies of the hexagonal perovskite Ba2CoTe0, M. University

LIEGEOIS-DUYCKAERTS

of Liege, Institute of Chemistry, Department of General Chemistry, 4000 Sart Tilman par Liege 1, Belgium 20 September 1984)

(Received for publication

Abstract-In a previous study of the hexagonal 12-layer perovskites Ba,B*rTeO, with Bt* = Ni and Zn, we observed a somewhat different X-ray powder diagram and infrared (ir.) spectrum for the cobalt compound. In the present paper, we point out how vibrational spectroscopy and X-ray powder diffraction can be used to propose an ordered hexagonal six-layer perovskite structure for BarCoTeO,.

INTRODUCTION

tion by Raman spectroscopy and we prepared less coloured compounds with the same structure as Ba$oTeO,.

According

to BAYER [l] the three tellurates Ba,BnTeO, with Bn = Ni, Co and Zn have the same structure. KOHL et al. [2] described the hexagonal 12layer structure of Ba2NiTe06. The crystal structure was solved by single crystal X-ray diffraction. In a previous study of these tellurates by vibrational spectroscopy [3] we found very similar i.r. and Raman spectra for the Ni and Zn compounds. The Ba,CoTeO, specimen was too dark to record a Raman spectrum; however, we noted a few differences in the i.r. spectrum in the Te-0 stretching vibration region. Likewise, the X-ray powder pattern suggested a somewhat different structure. Consequently, to solve this structural problem, we tried to get more informa-

EXPERIMENTAL Synthesis

of the compounds

In addition to BarCoTeOe, we prepared systems of different compositions from cubic C Ba,MgTeO, to hexagonal phase 12H, Ba,NiTeO, and BasZnTeOe, respectively. According to the compositions given in Table 1, appropriate amounts of tellurium dioxide (TeO,), carbonates (BaCO,, NiC03), oxalate (CoCrO.,) or oxides (MgO, ZnO) ofdivalent cations were heated progressively to 600°C and kept at this temperature to ensure tellurium oxidation. Samples were treated, increasing the temperature stepwise up to above 1OOOC.X-Ray diffraction was used to follow the evolution of

Table 1. Thermal treatment of the system Bas(B, _*Mg,)TeO, Composition of the mixture B = Ni x=0 0.1 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Nature of the phases obtained at T= 1100°C T= 1300°C 72 h 24 h

T=96O’=C

12H J2&+

Q&+6H

6H

12H +&H

Nf6H

6H 6H 6H 6H 6H &H + C 6H+C 6H+C C

6H 12H+6H 6H m+6H+C 6H+c

LH+C 6H+C 6H+g

T=

B = Zn x=0 0.3 0.4 0.5

T = 1200°C

1100°C 24 h

48 h

12H 6H 6H 6H

0.7

6H+C

1 The major phases are underlined. 523

12H 6H 6H 6H C

6H+C_ C

M.

524

LIEGEOIS-DUYCKAERTS

the reaction. Indeed, each phase, C, 12H or 6 H, has characteristic peaks which can be used to identify it in a mixture. In the case of BazCoTeOb, the isotopic species were synthesized by the same technique using lz2Te (96.21 %) and “‘Te (99.49%) purchased from the Oak Ridge National Laboratory.

Table 2. X-Ray powder diffraction data for Ba,CoTeO, (Co K, radiation) Hexagonal unit cell dimensions: a = 5.802 A, c = 14.23,,A d obh d uk. I hkl 4.142 4.103 3.454 2.904

Spectra

The i.r. spectra of powders were recorded by the conventional pressed disc technique on Beckman IR 12 (2m250cm-r: KBr discs) and on Cameca SI 36 (25(r5Ocm-t: polythene discs) spectrometers. For the sharp bands the estimated error is f 1 cm-‘. The error may be larger for high-frequency broader bands. Special care has been taken in the investigation of the isotopic species; the isotopic shifts are only significant for the sharp bands. The Raman spectra were obtained on a Coderg PHO double monochromator equipped with a 50mW Spectra Physics He-Ne laser.

2.481 2.317 2.370 2.247 2.221 2.053 1.887

RESULTS 1.835

New 6H phases were isolated (Table 1) in the case of mixed Mg and Zn tellurates of formula Ba,(Zn,_,Mg,)TeO, with a Mg content ranging from 0.3 to 0.5. In the case of gradual replacement of Ni by Mg, our results are slightly different from those given by K~HL[~]. We obtained 6H phases with a wider composition range, i.e. with a Mg content ranging from 0.25 to 0.6. STRUCTURAL

ANALYSIS

VIBRATIONAL

BY X-RAY DIFFRACTION

AND

SPECTROSCOPY

We were able to index the X-ray pattern of Ba,CoTe06 in a hexagonal cell. Cell parameters appear in Table 2 together with the hkl indices of planes. The c axis in the cell is halved when nickel is replaced by cobalt. The 1Zlayer structure for Ba,NiTeO, becomes a six-layer one of BaTi03 type for Ba,CoTeO, and for the solid solutions Ba,(Bi*_.Mg,)TeOb with Bn = Zn or Ni (Fig. 1). If the BaTiO, space group P63/mmc (D&) is assumed, one must conclude that there is a random distribution of the Co and Te ions on the 4foctahedral

1.764 1.675 1.581

4.738 4.104 3.449 2.903 2.901 2.416 2.415 2.474 2.372 2.369 2.248 2.220 2.052 1.883 1.885 1.882 1.836 1.835 1.763 1.675 1.675 1.580 1.580 1.581

14 11 4 > 100 30 15 15 3 36 83 36 3 12 80 28

101 102 103 104 110 105 113 201 006 202 114 203 204 205 107 211 116 212 213 214 300 207 215 009

sites or even on both 2a and 4f sites. However, the similarity of vibrational behavior observed in the Raman spectrum for the 1Zlayer and the six-layer compounds suggests some ordering in the distribution of the Co and Te cations as will be seen later. This order occurs in one of the isotranslational subgroups of P6Jmmc namely PJml (D&). This space group PTml has already been proposed for Ba,NiReO,[S], BazCoOsO, and Ba2NiOs0, [6], BaJnRuO, [7] compounds, on the basis of X-ray diffraction intensities. In order to analyse the various structural assumptions for Ba,CoTeO,, we have summarized in Table 3 the structural characteristics of the 6H phases,

lb)

Fig. 1. Network of BO, octahedra in perovskite polytypes (redrawn from KOHL et al. [2]). (a) Ba,MgTeO, cubic unit cell with three layers stacking, (b) BaTiO, hexagonal unit cell with six layers stacking, (c) Ba,NiTeO, hexagonal unit cell with twelve layers staking.

Tbe hexagonal perovskite Ba2CoTeOs

525

Table 3. Structural data on 6H and 12H phases Bonding of isolated Space group

P63I-

Atom

I co

RTm

Site symmetry

III (Co, Te) (Co, Te)

D 36 C 3” Du C 3” Dad C 3”

Te(1) Te(2)

D 36 C 3”

co co

D 3d C 30

WU W2)

Da, D 36

Ni

C 30

E+ep)) (Co, Te)

P7ml

Wyckoff sites

Groups (Te)

Bonds B*wTe

Phase 6H

Te,O, + TeO, TeG(l) TeO, condensed

6A+(3A+F) 6A

- Colinear

TeO, condensed TeO6(1) TeOh(2)

and P%l, and of the 12H phase Rh The possibility ofa P63 Jmmc structure as in BaTiO, would imply the existence of TeO, condensed groups (hypotheses II and III of Table 3) or TezO, discrete groups (hypothesis I); whereas in the P7ml space group, there are TeO, isolated groups, such in the 12-layer R3m structure (see also Fig. 1). We have thus chosen to study the vibrational behavior of phases containing Sbz09 groups [S, 91 (hypothesis II, Table 3), i.e. Ba,BnSbzO, six-layer perovskites with Bn = Ni Co Zn . . . , in parallel with the 12-layer phases of Ba,‘NiTeOi type already studied in a previous paper [3]. Thus, we shall only report the i.r. and Raman spectra of the six-layer phases; they are represented in Fig. 2(a,b) and Fig. 3(a,b). No specific influence of the divalent cation can be observed in the 800_350cm-’ region, and we can P6,/mmc

:;p$’ 6 (apex A, face F)

6A 3A+F

- Colinear Bent and asymmetrical

6H

6A 2F

Colinear Bent and symmetrical

12H

assign these bands to the internal vibrations of TeO, or SbrO, groups. There is a remarkable analogy between the Raman spectra of the new compounds and that of the 12H phases. This analogy is especially noticeable in the valence modes domain of higher frequency. In the case of 12H phases, two peaks are observed [3]: one at a higher frequency is rather broad, while the other is sharper and of greater intensity. The broad and highest frequency peak is assigned to the symmetric stretch of the isolated octahedron TeO,(l) (Fig. l), sharing apexes with Ni06 octahedra, vr (A,,) TeO,(l); the sharper and lower frequency peak is assigned to the same motion of the Te0,(2) octahedron, sharing faces with Ni06 octahedra, vr (A,,) TeO,(2). In the solid solutions Ba,(Ni, -XMg,.)TeO, (Fig. 3b), we notice again a sharp and strong band, and

Frequency

Y (cm-‘)

Fig. 2a. Infrared spectrum of Ba,CoSbzO,.

526

M. LIEGEOIS-DUYCKAERTS

0

200

400

6cO

800

Frequency Y (cm-‘)

Fig. 2b. Infrared spectrum of J&CoTeO,.

a broad and less intense band. These two peaks (Table 4) are located in the same frequency region as those of the two Al, bands of the 1Zlayer phase Ba,NiTeOs (743-749 and 677-683 cm-t). These analogies suggest that there are two types of TeOd isolated octahedra in the Bal(Ni,Mg)TeO, structure, and hence in the isostructural compound Ba&oTeO,. These findings are supported by the peculiar vibrational behavior of Ba,(Zn, _*Mg,)TeO, solid solutions. The high-frequency band is split into two components which are located at nearly the same frequency as in the corresponding pure end-members (about 765 and 736cm-r in the 6H solid solutions, against 769 cm-’ for pure 12H BatZnTeO,, and 724cm-’ for pure C BazMgTeO, [lo]). Since we have shown previously [3] the great influence of the divalent B cation on the higher frequency mode, this two-mode behavior results necessarily from the influence of the B cation (Zn or Mg) and can be explained only by an ordered distribution of Te and B cations over the available octahedral sites of a 6H structure. This

splitting is not observed in the solid solutions BaJNi, _XMgX)TeO, where we find only a significant broadening of the high-frequency band; but in this case, the frequency difference between the pure 12H compounds and C Ba,MgTeO, Ba,NiTeO, (749-724cm-‘) is not large enough to permit a splitting of the mode. One observes only some broadening of the band (Fig. 3b) corresponding to an overlapping of the two AI, modes. This behavior of the highest frequency mode in the solid solutions emphasizes the influence of the nature of the divalent cation on the vibrational frequency of the (A,,) mode v,TeO,(l). We can therefore conclude that there is an ordering of Bn and Te ions and we retain the P%l space group for the BarCoTeO, and related solid solution structures. Assuming the existence of the above described structure (Table 3) we may now consider the effects of descent in symmetry on the TeO, groups internal modes when passing from R%r to P&d space group.

Table 4. Raman frequencies and general assignments %Wl-xMgx)Te06 0.3 0.4 0.5

0.6

0.3

743

744

744

744

744

765 736

766 738

678

678 -610

681 -611

681

681

682

TeO&)

677 -608

L,(2)

-560

-560

-561

MO&S TeOdl) L,(Z)

“5

x = 0.25

W

408

416

414

395 380

396 384

-400 Sh

-402 Sh

Sh: shoulder, w: weak.

12H Zn

767 739

749

769

684

683 614

ccl01 MR

724

W

W

-550

- 550

404

408

410

-581 470 408

395 386

Sh Sh

Sh -392

394 378

- 547

409

12H Ni

691 620

The hexagonal

(a)

perovskite

521

Ba2CoTe0,

-779

615

I

I

700

I

I

600

500 Frequency

Y

I

1

400

300

200

(cm-‘)

Fig. 3a. Raman spectrum of BaaZnSb20,.

(b) 679 f I:

i

I

I

I

600

700

600 Frequency

Fig. 3b. Ran .n spectra of Ba2(Ni,.,Mg,,,)Te06

I 500

I 400

Y (cm-‘)

(dotted line) and Ba,(Zn,.,Mg,.,)TeO,

(fun line).

M. LIEGEOIS-DUYCKAERTS

528

each internal mode gives rise to two components, one is i.r. active, the other is Raman active. One might thus expect to observe an i.r. component for the v, , vz, v5 active modes and a Raman component for the vj, v4, v6 active modes. Moreover, the stretching modes (vi and v2) involve an important displacement of the oxygen atoms, the other deformation modes may be described as complex motions of oxygen and Te atoms. Indeed, in the i.r. spectra we observe a “new” band in the 565-580 range (see Table 6). This band exhibits no frequency shift if we compare the i.r. spectra of the isotopic “‘Te and 13’Te species: it is thus assumed to be the stretching mode vz(E,). Since thecorresponding

These results are collected in Table 5. For each structure, we have two types of octahedral sites. However, in the R%I space group of the 12H polytype, both TeO, groups have the same DJd site symmetry. On the contrary, in the P%tl space group assumed for 6H the symmetry of one site, namely Ba&oTeO,, TeO,(l), remains unchanged (I&), whereas the site group symmetry of TeO,(2) is reduced from D3dto Csv. This descent in symmetry of Te0,(2) is reduced from D,, to C,,. This descent in symmetry will not produce a site group splitting but will lead to a unit cell group splitting due to the existence of two equivalent site group in the Bravais unit cell. Each component of

Table 5. Site group Molecular

group

analysis

--+Site

GM 0,

D3d

D 34

Vl Vt V3 V.4 y5 V6

A I#

A 19

A I#

E, AZ.+-% Azu + Eu AI,+E, A,u+E,

4 Azu + E, Az,+E, AI,+& A,,+Eu Inact.

J4 T 1” T I” T 2e T InaZive 0, + Site group+

Ca, -+ Unit cell +

VI

E A,+E A,+E A,+E A,+E

V3 V4 V5

v6

Table 6. Infrared

Ba,CoTeO, 750 - 695 (Te)* 645 broad

External modes

AI,+&

A,

v2

frequencies

of 6H perovskites

E,+& A,,+Azu+Eg+E, A,,+Azu+E,+E, Aig+Aau+Eg+E, A,,+Aag+Eg+Eu Inact. Inact.

and general assignments

Baa (Nii _= Mg,)TeOS 0.4 0.5

X = 0.3 750 Sh 663 Sh

750 w - 705 665 Sh

D,,

splitting

splitting

Modes

Factor group GF

group --+ GS

Mode

750 w - 705 663 Sh

WZni 0.6 750 w Sh 664

-,Mg,)TeO 0.3

764w Sh _ 667 - 645

0.4 765 w 667 - 650

567

573

574

574

575

572

572

479 (Te)* 407 385

484 418 398

492 419 399

487 422 399

487 425 403

482 415 393

482 415 395

363 (Te)*

370

374

376

382

- 374

- 375

284 220 167 150 133 127 112

308 280 228 211

311 283 225 210

314 284 - 225 212

314 285 Sh 211

300 275 245 198

305 278 241 199

159 140 117 97

160 140 119 97

160 140 118 96

155 133

155 133

91

97

(Te)*: isotopic shift obtained Sh: shoulder, w: weak.

Sh

for Ba,Co ‘a2Te06

and Ba2Co’s0Te0,

160 140 118 98 (4cm-‘).

529

The hexagonal perovskite BarCoTeO, Raman mode vZ(Ep) is observed in the same frequency region, we conclude that the coupling between the TeO,(2) molecular groups is rather weak. The vr (A,) mode and vs(A. and E,) modes might be observed in the v3 stretching modes and v4 deformation modes frequency range. A detailed assignment in these regions is difficult since there are some overlappings and mixing of various modes. Our discussion of Raman spectra is restricted to the more intense TeO, stretching modes. The results obtained are fully consistent with the I%1 space group for Ba,CoTeO,. Acknowledgements-The author wishes to thank Professor P. TARTE, A. RULMONT~~~ M. AURAYfor helpful discussion, and A. M. FRANSOLET for performing the X-ray measurements.

REFERENCES

G. BAYER, U.S. Pat. 3, 309, 168 (1967). P. KOHL, U. MULLER and D. REINEN, 2. anorg. al/g. Chem. 392, 124 (1972). M. LIEGEOWDUYCKAERT~, Spectrochim. Acta 31A, 1585 (1975). P. KOHL, 2. anorg. al/g. Chem. 427, 205 (1976). C. P. KHAI-~AKand D. E. Cox, Magn. Lett. 1,23 (1976). U. TREIB~R and S. KEMMLER-SACK,Z. anorg. allg. Chem. 470,95 (1980). c71 H. U. SCHALLERand S. KEMMLER-SACK,Z. anorg. al/g. Chem. 473, 178 (1981). PI A. J. JACOBSENand A. J. CALVERT,.I. inorg. nucl. Chem. 40, 447 (1978). 191 U. TREIIIER and S. KEMMLER-SACK,Z. anorg. alig. Chem. 487, 161 (1982). Cl01M. LIEGEOIEDUYCKAERTSand P. TARTE, Spectrochim. Acta 3OA, 1771 (1974).