Magnetovolume effects and the p-T magnetic phase diagram of UNi₂Si₂

The thermal expansion of single-crystalline UNi₂Si₂ along the a and c axes has been measured with high accuracy. The linear thermal expansion is found to be highly anisotropic and to show a spontaneous magnetostriction. In the ground state, the spontaneous volume magnetostriction reaches 4 × 10^-4. The lattice volume changes abruptly at T₁ = 43 K and T₂ = 103 K, where UNi₂Si₂ undergoes the magnetic phase transitions. When increasing temperature, a considerable volume expansion (by 3.6 × 10^-8 m³ mol^-1) at T₁ and tiny shrinkage (by 6.5 × 10^-10 m³ mol^-1) at T₂ have been observed. On the other hand, no apparent volume effect is observed around the magnetic ordering temperature T_N = 124 K. Thermal expansion data are analyzed using thermodynamic relations in order to elucidate the previously published pressure dependences of temperatures of magnetic phase transitions T₁, T₂, and T_N.