Low temperature crystallographic phase transitions in DyVO4 and TbVO are attributed to a co-operative Jahn-Teller effect. A preliminary report of the theory is given, based on a harmonic oscillator Hamiltonian for the crystal and an electron lattice coupling which is small compared to typical phonon zero point energies. An Ising Hamiltonian is shown to describe the co-operative ordering.

https://iopscience.iop.org/article/10.1088/0022-3719/4/9/013/pdf

Theory of co-operative Jahn-Teller distortions in DyVO4 and TbVO4 (Phase transitions)

High-resolution powder neutron diffraction data have been collected at $\ensuremath{\sim}50$ temperatures between 4.2 and 300 K for perovskites ${\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{AlO}}_{3}$ with $x=0$, 0.05, 0.1, and 0.25. Structural and lattice parameter data extracted by Rietveld refinement have then been used to follow details of octahedral tilting and spontaneous strains associated with the sequence of phase transitions $Pm\overline{3}m\ensuremath{\leftrightarrow}R\overline{3}c\ensuremath{\leftrightarrow}Imma\ensuremath{\leftrightarrow}C2∕m$. These are interpreted in terms of strain/order parameter coupling using a single Landau free-energy expansion for a $Pm\overline{3}m$ reference structure with two instabilities (${\mathrm{R}}_{4}^{+}$ and ${\ensuremath{\Gamma}}_{3}^{+}$ active). Data from the literature relating to the $Pm\overline{3}m\ensuremath{\leftrightarrow}R\overline{3}c$ transition are consistent with second-order character and an extrapolated transition temperature of $1864\ifmmode\pm\else\textpm\fi{}31\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The $R\overline{3}c\ensuremath{\leftrightarrow}Imma$ transition is first order in character and can be understood as occurring because coupling takes place between tilting and electronic order-parameter components via a common tetragonal strain. Strains for the $Imma\ensuremath{\leftrightarrow}C2∕m$ transition conform closely to the Landau solution for a proper ferroelastic transition with second-order character and low-temperature saturation. The acoustic anomaly reported at 118 K appears to be associated with a metrically tetragonal structure which develops, as an accidental strain degeneracy, at a temperature between $\ensuremath{\sim}110$ and $\ensuremath{\sim}120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Differences in saturation temperatures for the order parameters associated with tilting and electronic ordering are probably responsible for small additional anomalies in the evolution of strains below $\ensuremath{\sim}110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Strain mechanism for order-parameter coupling through successive phase transitions in PrAlO 3

Publisher Summary This chapter deals with rare earth (R) aluminates and gallates, which adopt the perovskite-type structure. Experimental data on crystal structures, their thermal evolution and transformations of over hundred of RAlO3 and RGaO3 compounds, and their solid solutions are reviewed. Crystal chemistry has been applied to study and analyze perovskite structures and distortions, which take place in R aluminates and gallates over a wide temperature range. Crystal structures of RAlO3 and RGaO3 compounds and their solid solutions have been analyzed in terms of structure deformation parameters, for example bond-length distortion, tilting angles between octahedra, polyhedral volume ratio, tolerance factors in different coordinations, and individual and average cation–cation distances. Experimental results have been supplemented by bond–valence calculations. The influence of isovalent substitution in the rare earth sublattice on the average structure, its thermal behavior, and phase transformations has been established for R aluminates and gallates. Based on experimental results and available reference data, phase diagrams of RAlO3–R′AlO3 and RGaO3–R′GaO3 systems versus average radii of R-cations are proposed. Common features of the thermal expansion of rare earth aluminates and gallates, as well as the observed anomalies, including the negative thermal expansion observed in RGaO3 containing praseodymium, are analyzed and discussed. Reference data on dielectric properties of R aluminates and gallates with perovskite structure are reviewed.

Chapter 242 Perovskite-Type Aluminates and Gallates

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.

https://iopscience.iop.org/article/10.1088/0022-3719/6/14/018/pdf

L. A. Risberg

Papers

Structural Phase Transitions in PrAlO 3