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 ¹¹¹→¹¹¹Cd 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 ⁹⁹Ru and ⁵⁷Fe Mössbauer-effect information and other ¹¹¹→¹¹¹Cd PAC measurements, the magnetic hyperfine fields at the Ru site in SrRuO₃ and at the Fe site in PrFeO₃ are compared.