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 KCuF3. 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. Moreover, based on the work of Öpik and Pryce, we show that the coupling between lattice (homogeneous strain) and motif (phonons) distortions is essential to understand the main stable configurations of the lattice. Using this information, we predict a new low-energy phase in KCuF3 that could strongly alter its properties and provide guidance on how to stabilize it through strain engineering.

Chemical Bonding, 10Dq Parameter and Superexchange in the Model Compound KNiF3.

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. Seeking to clarify that crucial issue, this work is focused on KNiF3, a reference system among insulating transition metal compounds. By means of first principles calculations we show that, contrary to what is usually thought, the R-dependence of 10Dq arises neither from the crystal field contribution nor from the covalent admixture of 3d(Ni) with valence 2p(F) orbitals. Indeed, we prove that it is mainly due to the residual covalency with deep 2s(F) orbitals, highly sensitive to R variations. As a salient feature the present calculations show that the 3d-2pσ and 3d-2pπ admixtures raise practically equal the energy of antibonding eg and t2g orbitals of NiF6 4- units in KNiF3 thus leading to a null contribution to 10Dq. This conclusion is not significantly altered when considering the change of covalency on passing from the ground state 3A2(t2g 6eg 2) to the excited state 3T2(t2g 5eg 3). The different influence of chemical bonding on the superexchange constant, J, and 10Dq is also discussed in a second step. It is pointed out that the strong dependence of J upon R can hardly be explained through the behavior of the 3d-2pσ covalency derived for a single NiF6 4- unit. For the sake of clarity, the meaning of 10Dq is also briefly analyzed.

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 CuCl6 4- units is explored through first-principles calculations and the analysis of available experimental data. The results on Cu2+ -doped (C2 H5 NH3 )2 CdCl4 , 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 CuCl6 4- units but with a hole displaying a dominant 3z2 -r2 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 0.21 Šthe longest Cu2+ -Cl- distance, lying in the layer plane, while leaving unmodified the two other metal-ligand distances. Owing to this fact, it is shown that the lowest d-d transition would experience a red shift of 0.34 eV while the first allowed charge transfer transition is also found to be red shifted but only by 0.11 eV that reasonably concurs with the experimental value. The parallel study on Jahn-Teller systems CdCl2 :Cu2+ and NaCl:Cu2+ involving tetragonal elongated CuCl6 4- units shows that the reduction of the long axis by a pressure of 3 GPa is three times smaller than that for the layered (C2 H5 NH3 )2 CdCl4 :Cu2+ compound. Accordingly, the optical transitions of such systems, which involve a positive force constant, are much less sensitive to pressure than in layered compounds. The origin of the red shift under pressure undergone by the lowest d-d and charge transfer transitions of (C2 H5 NH3 )2 CdCl4 :Cu2+ is discussed in detail.

Pressure Effects on 3dn (n=4, 9) Insulating Compounds: Long Axis Switch in Na3 MnF6 not Due to the Jahn-Teller Effect.

The pressure-induced switch of the long axis of MnF6 3- units in the monoclinic Na3 MnF6 compound and Mn3+ -doped Na3 FeF6 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 3dn (n=4, 9) systems displaying a static Jahn-Teller effect. By contrast, we prove that in Na3 MnF6 the switch arises from the anisotropic response of the low symmetry lattice to hydrostatic pressure. Indeed, while the long axis of a MnF6 3- unit at ambient pressure corresponds to the Mn3+ -F3 - direction, close to the crystal c axis, at 2.79 GPa the c axis is reduced by 0.29 Šwhile b is unmodified. This fact is shown to force a change of the HOMO wavefunction favoring that the long axis becomes the Mn3+ -F2 - direction, not far from crystal b axis, after the subsequent relaxation process. The origin of the different d-d transitions observed for Na3 MnF6 and CrF2 at ambient pressure is also discussed together with changes induced by pressure in Na3 MnF6 . The present work opens a window for understanding the pressure effects upon low symmetry insulating compounds containing d4 or d9 ions.

New Ideas for Understanding the Structure and Magnetism in AgF2 : Prediction of Ferroelasticity.

In the search for new high-temperature superconductors, it has been proposed that there are strong similarities between the fluoroargentate AgF2 and the cuprate La2 CuO4 . We explored the origin of the possible layered structure of AgF2 by studying its parent high-symmetry phase and comparing these results with those of a seemingly analogous cuprate, CuF2 . Our findings first stress the large differences between CuF2 and AgF2 . Indeed, the parent structure of AgF2 is found to be cubic, naturally devoid of any layering, even though Ag2+ ions occupy trigonal sites that, nevertheless, allow the existence of a Jahn-Teller effect. The observed Pbca orthorhombic phase is found when the system is cooperatively distorted by a local E⊗e trigonal Jahn-Teller effect around the silver sites that creates both geometrical and magnetic layering. While the distortion implies that two Ag2+ -F- bonds increase their distance by 15 % and become softer, our simulations indicate that covalent bonding and interlayer electron hopping is strong, unlike the situation in cuprate superconductors, and that, in fact, exfoliation of individual planes might be a harder task than previously suggested. As a salient feature, these results prove that the actual magnetic structure in AgF2 is a direct consequence of vibronic contributions involved in the Jahn-Teller effect. Finally, our findings show that, due to the multiple minima intrinsic to the Jahn-Teller energy surface, the system is ferroelastic, a property that is strongly coupled to magnetism in this argentate.