A. Trueba

Spectrochemical Series and the Dependence of Racah and 10Dq Parameters on the Metal−Ligand Distance: Microscopic Origin

The origin of the spectrochemical series and the different dependence of crystal-field splitting (10Dq) and Racah parameters on the metal-ligand distance, R, is explored through ab initio calculations on Cr(3+)-doped K2NaScF6, Cs2NaYCl6, Cs2NaYBr6, and Cs2NaYI6 lattices. For this purpose both periodic and cluster calculations have been performed. An analysis of ab initio results proves that 10Dq values mostly come from the small admixture of deep nLs ligand orbitals present in the antibonding eg(∼ x(2)-y(2),3z(2)-r(2)) level and not from the dominant covalency with valence nLp ligand orbitals, which is actually responsible for the reduction of Racah parameters. This study thus reveals the microscopic origin of the stronger dependence upon R of 10Dq when compared to that observed for Racah parameters, thus explaining why electronic transitions which are 10Dq-independent give rise to sharp optical bands. As a salient feature, while the covalency with nLp levels increases significantly on passing from CrF6(3-) to CrI6(3-), the nLs admixture in eg is found to be practically unmodified. This fact helps to understand the progressive decrease of 10Dq through the series of CrF6(3-), CrCl6(3-), CrBr6(3-), and CrI6(3-) complexes embedded in the corresponding host lattices when compared at the corresponding equilibrium distance at zero pressure. The growing importance of the nLs admixture is well-depicted using deformation density diagrams on passing from the ground state (4)A2(t2g(3)) to the (4)T2(t2g(2)eg) excited state depicted at several R values.

Quantifying Local and Cooperative Components in the Ferroelectric Distortion of BaTiO3: Learning from the Off-Center Motion in the MnCl65– Complex Formed in KCl:Mn+

The delicate balance between cooperative and local contributions in the ferroelectric distortions of BaTiO3 is explored by means of ab initio calculations. As a salient feature, it is found that a single Ti(4+) ion in BaTiO3 is not allowed to move off-center at ambient pressure, while this is no longer true if the lattice is expanded by only ∼5%, stressing the high sensitivity of the local contribution to chemical and hydrostatic pressures. In order to further understand the effect of local contributions on the phase transition mechanism of ferroelectrics, we have investigated the surprising C3v → C4v → Oh local transformations occurring in the 10-50 K temperature range for the MnCl6(5-) complex formed in KCl:Mn(+) that mimic the behavior of BaTiO3. From Boltzmann analysis of the vibronic levels derived from ab initio calculations and considering decoherence introduced by random strains, the present calculations reproduce the experimental phase sequence and transition temperatures. Furthermore, our calculations show that the off-center instability in KCl:Mn(+) would be suppressed by reducing by only 1% the lattice parameter, a situation that then becomes comparable to that found for BaTiO3 at ambient pressure. The present results thus stress the deep link between the structural phase transitions of ferroelectric materials and local phase transitions displayed by transition-metal impurities in insulators.

Instabilities in Doped Materials Driven by Pseudo Jahn–Teller Mechanisms

Substitution of a host lattice ion by an impurity can produce a spontaneous instability lowering the initial local symmetry of the site. In some cases the impurity remains on-centre and the distortion only involves the motion of the nearest ions while in other cases the impurity moves off-centre along particular directions of the lattice. In this article experimental and theoretical work on spontaneous instabilities of impurities in ionic solids driven by pseudo Jahn–Teller vibronic coupling mechanisms is reviewed. Particular attention is paid to the results of density functional theory calculations addressed to understand the microscopic origin of the instability and also to quantify the involved distortion. Particularly, we aim to help to overcome a paradigm taken root among many researchers of physics and chemistry of solids: that the instabilities of atoms and ions in pure and doped solids are due to difference of atomic sizes. On the contrary, we present a great quantity of experimental evidences and theoretical results showing that it is an effect of the vibronic coupling.

Nature of the Fe 4 2 center in KTaO 3 : A density functional theory study

This work is aimed at clearing out the nature of the axial

Influence of internal electric fields on bonding and properties of impurities in insulators : Mn2+ in LiBaF3 and normal perovskites

\text{Fe}\frac{4}{2}

Jahn–Teller effect in Ag2+ doped KCl and NaCl: Is there any influence of the host lattice?

center detected by electron-paramagnetic resonance in iron-doped

Insulators containing CuCl4X2 2− (X = H2O, NH3) units: Origin of the orthorhombic distortion observed only for CuCl4(H2O)2 2−

{\text{KTaO}}_{3}

Impurities bound to vacancies in insulators: Electronic relaxation and physical properties of the Cr 3 + − V M model center in K M F 3 ( M = Mg, Zn)

for which two different models have been put forward. While some authors ascribe such a center to a

High magnetic anisotropy of Fe + ions in KTaO 3 and SrCl 2

{\text{Fe}}^{+}

Pressure-induced changes in Cr 3 + -doped elpasolites and LiCaAlF 6 : Interpretation of macroscopic data

impurity at a