Thermal properties at the low-temperature structural and magnetic phase transitions in Pr2NiO4 crystals.

Abstract

The coefficients of the thermal expansion and the specific heat of ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$ single crystals have been measured in order to study the thermodynamic and structural properties of the low-temperature structural and magnetic phase transitions. We find large, strongly anisotropic length changes and first-order specific heat anomalies at the low-temperature tetragonal (LTT) to low-temperature orthorhombic (LTO) transition at ${\mathit{T}}_{\mathrm{LT}}$. These anomalies imply huge anisotropic uniaxial pressure dependences of ${\mathit{T}}_{\mathrm{LT}}$. The anisotropy and size of the length changes indicate that the LTT phase occurs in order to minimize the effects of the ionic size mismatch. The discontinuities in both properties at ${\mathit{T}}_{\mathrm{LT}}$ in ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$ are orders of magnitude larger than those in the high-${\mathit{T}}_{\mathrm{c}}$ superconducting cuprates and the pressure dependences differ even qualitatively. Besides the anomalies at ${\mathit{T}}_{\mathrm{LT}}$ we find two first-order phase transitions in the thermal expansion of ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$ at T\ensuremath{\approxeq}90 K due to the reorientation of the Ni spins in the quasi-two-dimensional antiferromagnet. No corresponding anomalies could be resolved in the specific heat, which implies a very large and anisotropic pressure dependence of these magnetic transitions and thus shows an intimate coupling between structural and magnetic degrees of freedom in ${\mathrm{Pr}}_{2}$${\mathrm{NiO}}_{4}$. \textcopyright{} 1996 The American Physical Society.