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

The pressure dependence of structural, optical, and magnetic properties of the layered compound CsMnF 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 MnF units in the tetragonal parent phase, while there is a 4/ → 4 structural phase transition at = 40 GPa discarding a spin crossover transition from = 2 to = 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 NaMnF due to the internal electric field and the orthorhombic distortion also involved in KCuF. The reasons for the lack of orthorhombic distortion in KMF (M = Ni, Mn) or CsFeF are also discussed in detail. The present calculations confirm the ferromagnetic ordering of layers in CsMnF 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 MnF units. This study underlines the usefulness of first-principles calculations for a right interpretation of experimental findings.