Magnetic and structural phase transitions in systems with spin crossover under pressure

The effect of the exchange interaction between excited high-spin terms of transition metal ions in magnetic Mott–Hubbard insulators on the thermodynamics of the system with singlet terms of ions in the ground state near the crossover of singlet and high-spin terms with the increase in the pressure is studied with the effective Hamiltonian. It is shown that the crossover at the temperature below the critical T* < TN is a first-order phase transition and is accompanied by a volume jump. The crossover at the temperature above T* is accompanied by a smooth change in the crystal volume.