HIGH TEMPERATURE PHASE TRANSITION OF Sr RuO₃ AND Sr Hf O₃: X-RAY DIFFRACTION AND PAC SPECTROSCOPY. J. A. Guevara, S. L. Cuffini, Y. P. Mascarenhas, Instituto de Física de São Carlos, Universidade de São Paulo, cp 369 CEP 13560-970, São Carlos, SP, Brazil, P. de la Presa, A. Ayala, A. Lopez Garcia, D. F., Universidad Nacional de La Plata, C. C. 67 1900, La Plata, Argentina

Recently ruthenate compounds are being used as substrates for the growth of High-Tc superconductors because of their structural and electrical properties (Science, Vol.258 1766, 1992 - Appl. Phys. Lett., 62(2) 1993). Although the SrRuO₃ (SR) and SrHfO₃ (SH), as other perovskites oxides, have been known for well over 30 years, their crystal structures are still not well established. In the case of SR, some reports assigned a cubic symmetry with space group, P-3m (Solid State Ion. 43, 171 (1990)) and the other one reported an orthorhombic symmetry, Pnma (Acta Cryst. (1989 C45, 365 - Physica C 252 (1995) 303). SH has been less studied and the latest report was in 1935 assigned a cubic symmetry (Pm3m) (Z. Phys. Chem. 28, 74 (1935)). In order to study the structure at room temperature and its behavior until higher temperatures, X-ray diffraction and PAC spectroscopy data were collected from room temperature up to 1000deg. C. The structure were refined by the Rietveld Method using the DB W S9411 program. The refinements of X-ray diffraction patterns indicate that both, SR and SH, have an orthorhombic Pnma symmetry at room temperature. The lattice parameters are a=5.535(5)Å, b=7.845(1)Å, c=5.569(3)Å and a=5.7834(2)Å, b=8.1673(1)Å, c=5.7737(1)Å for SR and SH respectively. Changes of structural parameters with temperature such as unit cell dimensions, atomic positions, interatomic distances and angles, octahedral tilting, etc. were calculated. The structure analysis and PAC spectroscopy measurements of SR and SH compounds show that around 700deg.C the structure has a phase transition from orthorhombic to cubic symmetry 700deg.C. Acknowledgement: FAPESP, CAPES, FINEP, CNPq.