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.

Strain-Phonon Cooperation as a Necessary Ingredient to Understand the Jahn-Teller Effect in Solids.

Spatial degeneracy is the cause of the complex electronic, geometrical, and magnetic structures found in a number of materials whose more representative example is KCuF. In the literature the properties of this lattice are usually explained through the Kugel--Khomskii model, based on superexchange interactions. Here we provide rigorous theoretical and computational arguments against this view proving that structural and magnetic properties essentially arise from electron-vibration (vibronic) interactions.

Chemical Bonding, 10Dq Parameter and Superexchange in the Model Compound KNiF.

The cubic field splitting parameter, 10Dq, plays a central role in the ligand field theory on insulating transition metal compounds. Experimental data obtained in the last 50 years prove that 10Dq is highly dependent on changes of the metal-ligand distance, R, induced by chemical or applied pressures. Despite this fact has important consequences on optical and magnetic properties of such compounds, its actual origin is still controversial.

Red Shift in Optical Excitations on Layered Copper Perovskites under Pressure: Role of the Orthorhombic Instability.

The red shift under pressure in optical transitions of layered compounds with CuCl units is explored through first-principles calculations and the analysis of available experimental data. The results on Cu -doped (C H NH ) CdCl , that is taken as a guide, show the existence of a highly anisotropic response to pressure related to a structural instability, driven by a negative force constant, that leads to an orthorhombic geometry of CuCl units but with a hole displaying a dominant 3z -r character (z being the direction perpendicular to the layer plane). As a result of such an instability, a pressure of only 3 GPa reduces by 0.

Pressure Effects on 3d (n=4, 9) Insulating Compounds: Long Axis Switch in Na MnF not Due to the Jahn-Teller Effect.

The pressure-induced switch of the long axis of MnF units in the monoclinic Na MnF compound and Mn -doped Na FeF is explored with the help of first principles calculations. Although the switch phenomenon is usually related to the Jahn-Teller effect, we show that, due to symmetry reasons, it cannot take place in 3d (n=4, 9) systems displaying a static Jahn-Teller effect. By contrast, we prove that in Na MnF the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure.

New Ideas for Understanding the Structure and Magnetism in AgF : Prediction of Ferroelasticity.

In the search for new high-temperature superconductors, it has been proposed that there are strong similarities between the fluoroargentate AgF and the cuprate La CuO . We explored the origin of the possible layered structure of AgF by studying its parent high-symmetry phase and comparing these results with those of a seemingly analogous cuprate, CuF . Our findings first stress the large differences between CuF and AgF .