Local probing CaMnO₃ structural phase transitions

Full text: Perovskite-type oxides exhibit rich and varying physical properties, making them a meaningful object of study, both for fundamental physics and applied related issues. CaMnO₃ is an interesting material as it is the parent perovskite of the novel multiferroic material Ca₃Mn₂O₇, sharing similar octahedral MnO₆ distortions, and CaMnO₃-δ based manganite are known as one of the best n-type materials for thermoelectric conversion applications at high temperatures conditions. Previous powder CaMnO₃ X-ray diffraction measurements performed at high temperatures showed that CaMnO₃ undergoes two consecutive structural transitions. At 1170K, for increasing temperature, CaMnO₃ transits from the orthorhombic Pnma space group to the tetragonal I4/mcm structure. At 1190K a second structural transition occurs and the system lattice adopts the undistorted cubic structure with a space group Pm-3m. However, first-principles density functional theory calculations and ab initio molecular dynamics simulations have proposed recently that the nature of the observed structural transitions may be understood as order-disorder type, where the high symmetric cubic (Pm-3m) structural phase can be understood as a dynamic average as the system hops between equivalent orthorhombic (Pnma) configurations. In this context, local probe studies such as perturbed angular correlation (PAC) spectroscopy, may provide for the first time experimental evidence for these order-disorder dynamics as well the means to measure their characteristic times as function of the temperature. At ISOLDE-CERN PAC measurements were performed in a wide range of temperatures (1240K - 11K) by using metastable 111mCd isotopes as radioactive probes. In this work the characterization of the measured electric field gradient in the vicinity of the 111mCd probe isotope within the CaMnO₃ crystal is studied and its temperature dependence is reported under the light of previously predicted order-disorder dynamics. (author)