The vibrational spectrum of Nd2BaZnO5

T The vibrational spectrum of Nd2BaZn05 E. J. B A R A N * , C. O. D E L L A V E D O V A Qufrnica Inorg~nica (QUINOR), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962, 1900-La Plata, Argentina A. E. L A V A T , M. T R E Z Z A Area de Qufmica, Facultad de Ingenierfa, Universidad Nacional del Centro de la Provincia de Buenos Aires, 7400-Olavarria, Argentina Materials of stoichiometry Ln2BaMIIO5 (M II -- Co, Ni, Cu, Zn) belong to four different structural types, depending on the Ln(iii) and/or MOO cations present in the lattice [1,2]. In the case of MOO = Zn, the phases containing Y and Ln(m) = Sm to Ho adopt the orthohombic Pbnm structural type, in which the Zn(II) ions are present as isolated square-pyramidal units [3]. Phases with Ln(iii) - La or Nd adopt a tetragonal structure, space group I4/mcm and Z = 4, with isolated tetrahedral ZnO 4 moieties present in the lattice [4, 5]. In a previous paper we presented a general analysis of the infrared spectra of the different Ln2BaMIIO5 structural types [2]. As the tetragonal I4/mcm structure is only present in La2BaZnO5 and Nd2BaZnO5 we have now attempted to obtain a deeper insight into its vibrational behaviour by careful analysis of the Raman and infrared spectra of Nd2BaZnOs, for which a detailed structural analysis has been published recently [5]. Samples of Nd2BaZnO5 were obtained by solid state reactions starting from 1:1:1 mixtures of Nd203, BaCO 3 and ZnO. The well mixed powders were treated in air, progressively at 700, 800 and 900 °C, with several intermediate grindings, and finally heated for 24 h at 950-1000 °C. The formation of a single phased material was confirmed by X-ray powder diffractometry. The infrared spectra were recorded using a Perkin-Elmer 580B spectrophotometer, using the KBr pellet technique. Raman spectra were obtained using a Jovin-Yvon U-1000 instrument, using the 488.0 nm line of an Ar ÷ laser (Spectra Physics, model 165) for excitation. The obtained spectra are shown in Fig. 1. The investigated material is built up by eightfold coordinated Nd(iIi) ions and BaOi0 bicapped antiprisms, whereas the Zn(ii) ions are present in the form of isolated tetrahedral units [5]. Therefore, it seems possible to attempt an approximate assignment of these ZnO 4 vibrations on the basis of a factor group analysis of the lattice [6, 7]. The correlation between the point group of the "free" ion (Td), its site-symmetry (D2d) and its factor group (Dgh) is shown in Table I, and Table II presents the proposed assignment based on this analysis. *Author to whomall correspondenceshouldbe addressed. 0261-8028 © 1994 Chapman & Hall J I I I I I [ I I I I i 700 6 0 500 4 0 300 200 >, ,m ( o• 0~ # (b) cm-1 Figure I (a) Raman and (b) infrared spectraof Nd2BaZnO~. The general spectral pattern points to important coupling effects, probably related to the weakness of the Z n - O bonds. This is also probably one of the reasons for the relatively low intensity and broadening of the Raman line assigned to the Vl(Alg) mode. It is also evident that in the region below 350 cm -1, coupling between ZnO4-deformational modes and N d - O motions becomes important, as some L n - O modes are predicted to lie in this region [2, 8, 9]. A weak Raman line located at 434 cm -1 cannot be assigned with certainty. A comparison of the infrared spectrum of Nd2BaZnO 5 with that of the respective lanthanum compound, La2BaZnO5 [2], clearly shows that all of the bands are displaced to higher wavenumbers in 577 T A B L E I Factor group analysis of the internal vibrations of the ZnO 4 groups in the NdzBaZnO5 lattice (I4/mcm and Z = 4/2) Free ion (Td) Site-symmetry (D2d) Factor group (D4h) vl A1 v2E v3F2 v4Fz A1 AI+B~ B2+E B2+E Alg + Alg + Big + Big + Bzu B2u + B2g + Axu A2u + Eg -1- E a A2u + Eg + Eu Activity of the factor group modes: Alg , Big , B2g, Eg = Raman active Acknowledgements A2u, Eu = infrared active B2u, A ~ = inactive T A B L E II Assignment of the vibrational spectrum of Nd2BaZnOs (values in cm -1) Infrared Raman Assignment 636 602 v3(Eg) v3(Blg) v3(Eu q- A2H) 488 Vl(Aig) 582 406 363 333 277 288] 160~ 143j bonds and found a value slightly below unity, a result which supports the presumption of weak Z n - O bonds in this material and shows again that the tetrahedral ZnO4 units are subjected to great couplings with the motions of the other building units present in the lattice. vg(A2~) v4(Eg + Big ) v4(E~) + v(Nd-0) v(Nd-0) External modes This work was supported by CONICET (Argentina). C.O.D.V. thanks Professor O. Sala valuable contributions and the Universidade de S~o Paulo for financial support. References 1. 2. 3. 4. 5. 6. the neodymium material, in agreement with its smaller unit cell volume, and following a trend usually observed in isostructural lanthanide compounds [2, 10-13]. It was also interesting to compare the spectra of NdzBaZnO5 with those of NdzBaCuOs, in which square planar CuO4 units are present [9]. Both materials show very similar infrared spectra but the Raman spectra are totally different. The overall comparison indicates that the square-planar CuO4 units may present slightly stronger metal-oxygen bonds than the tetrahedral ZnO 4 moieties. Finally, we have made a rough estimation of the force constant of the Z n - O bonds, using a modified valence force field [14] and the Raman values for the two stretching vibrations. A force constant of 250N/m can be determined for the metaloxygen bonds, whereas the bond/bond interaction constant lies around 8 N/re. Using the value of the principal force constant we have also estimated the bond order (according to Siebert [15]) for the Z n - O 578 7. 8. 9. 10. 11. 12. 13. 14. 15. J . K . B U R D E T T and J. F. MITCHELL, J. Amer. Chem. Soc. 112 (1990) 6571. A. E. L A V A T , E. J. B A R A N , R. SAEZ P U C H E , A. SALINAS S A N C H E Z and J. M. M A R T I N - L L O R E N T E , Vibrat. Spectrosc. 3 (1992) 291. C. M I C H E L and B. R A V E A U , J. Solid St. Chem. 49 (1983) 150. C. MICHEL, L. E R - R A K H O and B. R A V E A U , ibid. 42 (1982) 176. M. TAIBE, J. A R I D E , J. D A R R I E T , A. M O Q U I N E and A. B O U K H A R I , ibid. 86 (1990) 233. S. D. ROSS, "Inorganic infrared and raman spectra" (McGraw-Hill, London, 1972). A. M U L L E R , E. J. B A R A N and R. O. C A R T E R , Struct. Bonding 26 (1976) 81. S. L. H E R R , K. K A M A R A S , D. B. 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