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.
The structural phase change arising from the cooperative Jahn-Teller effect has been studied by Raman scattering and optical birefringence methods in the mixed crystals TbpGd1-pVO4 and DypY1-pVO4 with p varying from one to zero. The introduction of Gd and Y dilutes the active components causing the phase change, resulting in a reduction of the transition temperature TD(p). Varying p varies the splitting of Eg phonons below Tp(p) and also the energy of low-lying collective electronic excitations. The Tb system shows unique behaviour with p=0.365; a transition from tetragonal to orthorhombic symmetry occurs at 8.5K but the crystal reverts to tetragonal symmetry below 2.3K. The effect in the Tb system can be explained by mean-field theory. The Dy compounds are explained using series expansion and modified mean-field methods.
Measurements have been made of the Raman scattering and infrared absorption peaks arising from excitation of magnon pairs in the system (Mn1-cZnc)F2. Single crystals were investigated with c varying from 0 to 1. The measurements are compared with predictions of both a mean field model and an Ising cluster model. Approximate mean field values are obtained for the first, second and third moments of the two-magnon peak. The Ising model has been applied in a way that takes account of the proximity in pairs of the manganese atoms responsible for the two-magnon scattering and absorption and it gives a mean frequency that is higher than the mean field result although not quite as high as the experimental results.
The authors have measured the effects of phase transitions in PrAlO3 on the frequencies of Raman lines below 100 cm-1. Particular attention has been paid to the transition at 151K and results are compared with the predictions of a theory for the cooperative Jahn-Teller effect.
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