Thermodynamics of the low-temperature structural transition in rare-earth-doped La_(2-x)Sr_xCuO₄

The Sr concentration dependence of the structural transition from the orthorhombic into the tetragonal low-temperature phase in rare-earth-doped (R = Nd, Eu) La_{2-x-y}Sr_xR_yCuO_4 has been studied in detail. Independently of the rare earth concentration the transition temperature is strongly reduced at x ~ 0.05. We give a qualitative argument that the effect can be attributed to the Coulomb repulsion between doped carriers. We find the enthalpy jump at the low-temperature transition to scale transition temperature between the high-temperature tetragonal and the orthorhombic phase. This effect can be understood in terms of a Landau expansion.