Phase transitions in La1-xCaxMnO3 (0.2<x<0.33) single crystals

The study of the concentration dependence of the Curie temperature and the type of the magnetic transition in La1-xCaxMnO3 (0.2 < x < 0.33) single crystals prepared under the same conditions was performed. It has been shown that the point (x(t)=0.25,T-t=205 K) on the (x,T) plane is the tricritical point, so that if T-C < T-t, the ferromagnetic-to-paramagnetic transition is second order, while at T-C > T-t, the transition is first order. The longitudinal sound velocity and the internal friction have been measured. The analysis of the magnetization and elastic property data has revealed that the magnetic transition is not accompanied by a structural transformation. The structural transition between the low temperature orthorhombic phase and the high temperature orthorhombic phase has been shown to occur in the paramagnetic state.