Microscopic insight into properties and electronic instabilities of impurities in cubic and lower symmetry insulators: the influence of pressure

Abstract

This article reviews the microscopic origin of properties due to transition-metal (TM) impurities, M, in insulator materials. Particular attention is paid to the influence of pressure upon impurity properties. Basic concepts such as the electronic localization in an MX(N) complex, the electrostatic potential, V(R), arising from the rest of the lattice ions or the elastic coupling of the complex to the host lattice are initially exposed. The dependence of optical and magnetic parameters on the impurity-ligand distance, R, in cubic lattices is discussed in a first step. Emphasis is put on the actual origin of the R dependence of 10Dq. Examples revealing that laws for strict cubic symmetry cannot in general be transferred to lower symmetries are later given. This relevant fact is shown to come from allowed hybridizations like nd-(n+1)s as well as the influence of V(R). As a salient feature the different colour in ruby and emerald is stressed to arise from distinct V(R) potentials in Al(2)O(3) and Be(3)Si(6)Al(2)O(18). The last part of this review deals with electronic instabilities. The phenomena associated with the Jahn-Teller (JT) effect in cubic lattices, the origin of the energy barrier among equivalent minima and the existence of coherent tunnelling in systems like MgO:Cu(2+) are discussed. An increase of elastic coupling is pointed out to favour a transition from an elongated to a compressed equilibrium conformation. Interestingly the equilibrium geometry of JT ions in non-cubic lattices is shown to be controlled by mechanisms different to those in cubic systems, V(R) playing again a key role. The relevance of first principles calculations for clarifying the subtle mechanisms behind off-centre instabilities is also pointed out. Examples concern monovalent and divalent TM impurities in lattices with the CaF(2) structure. The instability due to the transition from the ground to an excited state is finally considered. For complexes with significant elastic coupling vibrational frequencies and the Stokes shift are expected to undergo bigger changes through a chemical rather than a hydrostatic pressure. The reduction of Huang-Rhys factors upon increasing the pressure is discussed as well.

Insulators containing CuCl4X22−(X=HGarcía‐Fernández1, Garcı́a-Lastra2, Trueba3 et al. 2012Phys. Rev. B175430View full textAdd to dashboardCite

“…, it is not correct to treat the axial and equatorial fragments as being independent. 29 Indeed in an a 1g bonding or antibonding orbital, a change of the covalency in the equatorial plane modifies that of the axial fragment and vice versa.…”

First-principles calculations of electronic, optical and elastic properties of ZnAl2S4 and ZnGa2O4

“…Usually, band gaps increase with pressure, which also can be interpreted in terms of a blue shift of charge transfer transitions (which are, in other words, band-to-band transitions) with applied hydrostatic pressure [34]. It is also interesting to follow dependence of the Mulliken charges on pressure, which is represented in Table 4 The results of calculation of the elastic constants can be used further to estimate one of the most important thermal characteristics of the studied compounds, the Debye temperature D θ .…”

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“…ruby, emerald, red spinel) requires to know the structural environment of the coloring impurity. 4,6,7,8 The ionic radius of a substitutional impurity being usually different from that of the substituted ion, the accommodation of the mismatch imposes a structural relaxation of the crystal structure. Vegard's law states that there is a linear relationship between the concentration of a substitutional impurity and the lattice parameters, provided that the substituted cation and impurity have similar bonding properties.…”