LATTICE INSTABILITY AND LOW-TEMPERATURE PHASE-TRANSITION IN PR2NIO4

Some layered perovskites with general formulas R2MO4 (R = rare earth; 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 T(c2), and after further cooling there is a new low-temperature tetragonal (LTT) phase at T(c1). The soft mode behavior at T(c1) is observed with inelastic neutron scattering in this family of compounds for Pr2NiO4. One of the two branches, of the doubly degenerate X-point phonon of the HTT phase, becomes softer at T(c1). 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 --> LTT phase transformation in the insulator Pr2NiO4+delta (delta = 0) can be well characterized. For La(2-x)Ba(x)CuO4 (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 may complicate the interpretation of the results in this case.