Understanding Pressure Effects on Structural, Optical, and Magnetic Properties of CsMnF₄ and Other 3d Compounds

The pressure dependence of structural, optical, and magnetic properties of the layered compound CsMnF₄ are explored through first-principles calculations. The structure at ambient pressure does not arise from a Jahn-Teller effect but from an orthorhombic instability on MnF₆^3- units in the tetragonal parent phase, while there is a P4/n → P4 structural phase transition at P = 40 GPa discarding a spin crossover transition from S = 2 to S = 1. The present results reasonably explain the evolution of spin-allowed d-d transitions under pressure, showing that the first transition undergoes a red-shift under pressure following the orthorhombic distortion in the layer plane. The energy of such a transition at zero pressure is nearly twice that observed in Na₃MnF₆ due to the internal electric field and the orthorhombic distortion also involved in K₂CuF₄. The reasons for the lack of orthorhombic distortion in K₂MF₄ (M = Ni, Mn) or CsFeF₄ are also discussed in detail. The present calculations confirm the ferromagnetic ordering of layers in CsMnF₄ at zero pressure and predict a shift to an antiferromagnetic phase for pressures above 15 GPa consistent with the reduction of the orthorhombicity of the MnF₆^3- units. This study underlines the usefulness of first-principles calculations for a right interpretation of experimental findings.