STRUCTURAL PHASE-TRANSITION AND T-C DISTRIBUTION IN HF-DOPED LAMNO3 INVESTIGATED USING PERTURBED-ANGULAR-CORRELATION SPECTROSCOPY

Using perturbed-angular-correlation (PAC) spectroscopy, via the Hf-181 -->Ta-181 probe, we have measured Mn-site electric-field gradients (EF G's) at Ta nuclei in ceramic samples of LaMnO3. Two crystallographic p hases coexist over a temperature interval of approximate to 16 K near the orthorhombic-to-rhombohedral transition at approximate to 724 K, w hich shows a thermal hysteresis of approximate to 1.7+/-0.2 K. Concurr ently, in the two phases, we determined the temperature dependence of the EFG parameters, V-zz, eta, and delta, and the ratio of the probe c oncentrations A(1)/A(2) To explain the apparent coexistence of two pha ses in this weakly first-order transition, we present a model that ass umes a spatial distribution of T-c values. This distribution could ari se from a spatially nonuniform distribution of Mn4+ ions. We show the PAC technique to be a uniquely powerful probe of local symmetries that reflect the effects of a local-distribution of valences, which drive the phase transition.