Lattice instability and low-temperature phase transition in Pr2NiO4.

Abstract

Some layered perovskites with general formulas {ital R}{sub 2}{ital M}O{sub 4} ({ital R}=rare earth; {ital M}=Cu, Ni or Co) undergo a sequence of structural phase transitions. The high-temperature tetragonal phase (HTT) is transformed to the low-temperature orthorhombic phase (LTO) at {ital T}{sub {ital c}2}, after further cooling, there is a new low-temperature tetragonal (LTT) phase at {ital T}{sub {ital c}1}. The soft mode behavior at {ital T}{sub {ital c}1} is observed with inelastic neutron scattering in this family of compounds for Pr{sub 2}NiO{sub 4}. One of the two branches, of the doubly degenerate {ital X}-point phonon of the HTT phase, becomes softer at {ital T}{sub {ital c}1}. In the LTT phase, this phonon branch is stiffer and appears at higher energy. The orthorhombic strain and the soft-mode frequency undergo a discontinuous jump at the transition temperature, indicating a clear first-order character of this phase transformation. From these data, the dynamics of the LTO{r arrow}LTT phase transformation in the insulator Pr{sub 2}NiO{sub 4+{delta}} ({delta}=0) can be well characterized. For La{sub 2{minus}{ital x}}Ba{sub {ital x}}CuO{sub 4} ({ital x}=1/8) the same kind of transition is observed, which coincides with the suppression of the superconducting state, but the presence of free charge carriers maymore » complicate the interpretation of the results in this case.« less