The phase transitions of PrAlO₃

Abstract

PrAlO3 has the cubic perovskite structure at high temperatures but is trigonally distorted at room temperature. At about 200 K it undergoes a first-order phase transition to an orthogonal phase, and there is a further phase transition to a triclinic phase at about 150 K. The authors have studied this system by Raman scattering, and optical fluorescence spectroscopy, and by observing the EPR of Gd3+ impurities. A further transition to a tetragonal phase at 99+or-5 K has been established. From the observed behaviour of the electronic energy levels of the Pr3+ ion, it is found that the main features of the phase transitions can be explained by consideration of the coupling of the Pr3+ ion to the soft Gamma 25 R-point phonon of the high-temperature cubic phase. A molecular-field model for this coupling which qualitatively accounts for the existence of the observed phases has been developed.