HIGH-TEMPERATURE PHASE-TRANSITIONS AND LOW-TEMPERATURE MAGNETIC-ORDERING IN SRRUO3 AND CARUO3 CERAMICS STUDIED USING PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY

Perturbed-angular-correlation (PAC) spectroscopy was used to measure nuclear-electric-quadrupole interactions in the orthorhombically-distorted perovskites SrRuO3 and CaRuO3 over temperatures ranging from laboratory to very high temperatures. At 77 K, PAC spectroscopy was used to measure combined electric-quadrupole and magnetic-dipole interactions in magnetically ordered SrRuO3 and pure electric-quadrupole interactions in paramagnetic CaRuO3. The In-111 --> Cd-111 PAC probe was used for these measurements, and it substituted into the Ru site in SrRuO3 and very likely into the Ru site in CaRuO3. The temperature dependence of the electric-field-gradient (EFG) parameters for SrRuO3 indicates the onset of a structural phase transition at approximately 800 K. The presence of this transition indicates that the laboratory-temperature structure of SrRuO3 has lower-than-cubic symmetry. At very high temperatures > 1600 K, the structure of CaRuO3, as given by the EFG parameters, becomes very similar to the laboratory-temperature structure of SrRuO3. At 77 K in SrRuO3, the measured Ru-site supertransferred hyperfine field is 39 +/- 3 kOe. Using Ru-99 and Fe-57 Mossbauer-effect information and other I-111 --> Cd-111 PAC measurements, the magnetic hyperfine fields at the Ru site in SrRuO3 and at the Fe site in PrFeO3 are compared.