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0038-1098/93 $6 00+.00 Pergamon Press Ltd

{;olid State Communications, Vo|. 86, No 6, PP. 369-371, 1993 Printed in Great Britain.

RAMAN SPECTROSCOPY OF BaRuO 3 J. Quilty, H.J. Trodahl and A. Edgar Physics Department, Victoria University of Wellington PO Box 600, Wellington, New Zealand (Received 21 December 1992 by J. Tauc)

BaRuO3 exists in two polytypes (4H and 9R) based on different stacking sequences of close-packed BaO3 layers. We have performed Raman measurements on ceramic samples of both structures and on a 9R single crystal. The symmetries of most of the 9R modes have been determined from polarisation-resolved measurements on the single crystal. A factor group analysis predicts nine Raman modes in the 9R and eight in the 4H structure, and we have observed all but one mode in each polytype. Donohue et al2. A mixture of 20:1 flux to barium mthenate was allowed to soak in a platinum crucible at 1150°C before being slowly cooled at rates of between 0.5°C/hour and 2°C/ hour to 940°C. The resulting crystals were for the most part black hexagonal platelets; the largest were approximately 0.3mm thick and lmm x lmm in lateral dimensions. The chemical composition was checked using an electron microprobe, and the structure confh-med as the 9R polytype using X ray diffraction. No evidence for the 4H polytype was found. The sample used for Raman measurements was approximately 0.5 mm on a side, and showed natural growth faces corresponding to the (001) and (110) crystallographic planes. Raman measurements were performed in the backscatteringgeometry using the 514.5 nm and 476.5 nm lines of an Ar+ laser and the 647.1 nm line of a Kr + laser. The exciting illumination was focussed onto a line of dimensions approximately 2mm x .04 mm for the polycrystalline samples, and onto a point (.05 mm diameter) for the single crystal experiments. The power was typically 200 mW in the line focussed configuration and 75 mW or less for a point focus. The scattered light was analysed at a resolution of approximately 5 cm -1 with a Jobin Yvon U 1000 monochromator.

1. Introduction The transition metal oxides form a large class of materials, spanning the entire spectrum from insulators, through semiconductors to metals, including the recent high Tc superconductors. The ruthenates, in particular, show high conductivity, with in many cases a temperature coefficient that resembles that in metals. Barium ruthenate, BaRuO3, shares several common features with the high T¢ superconductors - a large polarisable closed-shell cation, a smaller variable-valence cation, and a perovskite-related structure. Its room temperature resistivity lies in the range of 1-10 mf~-cm for polycrystalline material, and shows temperature coefficients which vary between metallic- to semiconducting-like, depending on the preparation conditionst. The material is not superconducting at temperatures above 4.2 K. In this note we report Raman measurements on this material, and compare the results with the mode characters expected by a factor group analysis. BaRuO 3 can be prepared as polycrystaUine material in the two structures2"5 shown in figure 1, which both incorporate stacks of face-sharing RuO 6 octahedra. These stacks comprise either two (41-1)or three (9R) octahedra, and each stack is connected to the next sequence of stacks by a shared corner. There are thus chains of multiply-connected stacks running along the c-axis. Together with the fact that the Ru ions within each stack are closer together than in Ru metal, this has prompted the speculation that the conductivity is higher along the e-axisL The stacks in a given basal plane are separated by Ba atoms.

3. Results A typical set of polarisation-resolved single crystal 9R spectra, collected from both the (001) and the (110) faces, with 514.5 nm exciting light, is shown in figure 2. Six modes are seen in the polarised (with the scattered and incoming polarisations parallel) configurations, and three appear in each of the depolarised (polarisations perpendicular) traces. For both faces it is expected that both Al.g and Eg modes will contribute to the polarised signal, but only the E~ modes will provide a depolarised signal0, so that based on t]aese data alone one can assign the symmetries of the modes as given by the first six entries in Table 1.

2. Experimental Details Ceramic samples of both polytypes of BaRuO3 were prepared by grinding together dried BaO or BaCO 3 and RuO2 and firing at temperatures in excess of 1000°C for several hours in flowing oxygen, followed by quenching to room temperature. X-ray diffraction showed that the resulting material contained both polytypes; for preparation temperatures above 1100°C the ratio of 4H to 9R was typically around 8%. Ceramic samples which were predominantly the 4H polytype could be made by replacing a fraction (-20%) of the barium in the starting ingredients by the equivalent strontium2 or potassium1 compounds. The sample used for the Raman measurements reported in this note was St-substituted. Single crystals of the 9R polytype were produced using a BaCI2 flux, following the method initiated by

Table 1. Raman modes, symmetries and assignments, 9R. Frequency (cm-l~ Symmetry Ion 89 - At s Ba 104 Es Ba 198 Es Ru 276 Es O 309 s O 616 "~tg 0 462 O 511 O 369

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Vol. 86, No. 6

RAMAN SPECTROSCOPY OF BaRuO3 I

In figure 3 we compare data taken with different exciting lines, which shows that the lines at 89, 309, and 616 cm -1 are all strongly resonant in green-blue light. There are two additional weak features at 462 and 511 cm -1 appearing consistently in spectra taken with 476.5 nm illumination, for which we have been unable to obtain

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Fig. 1.Structure of the two polytypes of BaRuO 3. The oxygen positionsare denoted by open circles, and ruthenium and barium ions are denoted by filled and crosshatched circles, respectively. (a) The 9R structure. The O ions are connected by lines to emphasise their arrangements on octahedra, except for those surrounding one stack of Ru ions in the second level, which are omitted entirely for clarity. The vertical lines show the alternating sets of three Ru followed by three Ba ions. The central ions, labelled by I, are at centres of inversion symmetry. (b) The 4H polytype. The Ba ions labelled II lie directly above and in line with the two Ru ions. The Ba ions along the edges of the cell, two of which are labelled I, lie at centres of inversion symmetry.

convincing polarisation data. These two modes have been included as the final two entries in Table 1, with their symmetries left unspecified. A Raman spectrum from polycrystalline samples of the 4H polytype is shown in figure 4. The frequencies for the 4H material are listed in Table 2, without, of course, any symmetry assignments. Again we find that the high frequency (622 cm -1) line shows a resonant enhancement towards the blue. Table 2. Raman modes and assignments, 4H. Frequency (cm'l] Ion 91 Ba 186 Ru 245 Ru 267 Ru 335 O 425 O 622 O

4. Interpretation The space group for the 9R polytype is D3d5, with three molecular units in the primitive cell 2-5. A site group

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Fig. 3.Raman spectra for the 9R polytype, taken with red (647.1 nm), green (514.5 nm) and blue (476.5 nm) excitation.

Fig. 4.Raman spectrum from a ceramic (polycrystalline) sample of the 4H polytype, 514.5 nm excitation.

analysis7 predicts nine Raman active modes ( 4 A i r + 5E~), sixteen IR modes (7A2u + 9Eu), and three silent optic moc[es (2Alu + A2,). The 4Hpolytype has D6h4 symmetry2-5, and four molecular units in the primitive cell, with eight Raman modes (2Als + 2El~ + 4F~g), twelve IR modes (5A2u + 7Elu) and eighteen siIent opta~ modes (A2s + Alu + 3B12 + 2Blu + 5B2u + 6F_~). Clearly there is one mode missfng from each of the 9R and 4H spectra. We next suggest assignments for the ions involved in the observed normal modes. The process is simplified in the 9R polytype by the fact that one third of each ionic species lies at eentres of inversion symmetry, and these ions do not vibrate in any Raman active mode. We expect that the Ba ions, which are the heaviest in the structure, will be responsible for the lowest frequency modes. They are found in linear groups of three aligned with the c-axis, the central of which is at a point of inversion symmetry. The other two ions contribute one A 1 and one E s frequency, which we "assign to the modes at ~ and 103 cm -1, respectively. There are analogous modes involving predominantly ruthenium motion, which again involve only those ions at the ends of the chains of three, and we suggest that the line at 198 (Es) cm -1 is one of these. The remaining lines, as well as those not observed, then involve primarily motion of the oxygen ions. It is only the oxygens in the shared faces which are not at centres of inversion symmetry, and it is of course only vibrations that are symmetric about the central Ru ion which

will be Raman active. We note that with these assignments it is vibrations which involve ions in the same (BaO3) plane which show resonance. In the 4H structure one half of the Ba ions are at centres of inversion symmetry, and the others contribute only one (E2E) Raman mode. The spectra in this case show only one mode below 150 cm -l, in the re~ion of the 9R Ba modes. We therefore assign this 91 em -x mode as the one (E2g) involving predominantly Ba vibration. We expect that the next group of three, at 186, 246 and 276 em -1 are associated with three modes (A]g, EI~ and E2~) which involve predominantly Ru motion, ~[ndthat the remmnder are predominantly oxygen modes. It is interesting to compare these assignments with the Raman dam 8 for RuO2, a rutile structure consisting of edgeand corner-sharing RuO 6 octahedra. Only four Raman modes exist in the rutile structure, and none of them involve motion of the Ru ion, which is at a centre of inversion symmetry in the rutile structure. The RuO 2 spectra show lines at 97 (BIE), 528 (E~), 646 (Als) and 716 (B2,)cm-L With the exception of the'97 cm-] line, which we suggest is associated with a bond-bending rotation permitted by the weaker constraints in the edge-sharing configuration, these O-ion modes all lie in the region of those in BaRuO 3. Acknowledgement - This work was supported by grants from the Victoria University Internal Grants Committee.

References 1. B Szymanik and A Edgar, Solid State Commun. 7 9, 355 (1991) 2. P C Donohue, L Katz, and R Ward, Inorganic Chemistry 4,306 (1965) 3. P C Donohue, L Katz, and R Ward, Inorganic Chemistry 5,335 (1966) 4. P L Gai and C N R Rao, Pramana 5,274 (1975) 5. B Szymanik, MSc thesis, Victoria University of

Wellington, 1990 6. M Cardona in Light Scattering in Solids II, ed. by M Cardona and G Giintherodt (Spinger-Verlag, Berlin, Heidelberg, New York, 1982) p. 19 7. D L Rousseau, R P Bauman and S Porto, J. Raman Spectr. 10, 253 (1981) 8. Y S Huang and F H Pollak, Solid State Commun. 4 3, 921 (1982)