Quantum chemical simulations of hole self-trapping in corundum

Abstract

Microscopic quantum chemical calculations and simulations based on atom-atom potentials have been undertaken for hole self-trapping in pure corundum ( alpha -Al2O3) crystals. A comparison of different modes of ionic relaxation during hole trapping has shown that the inward Jahn-Teller 40% displacement of two O ions accompanied by the 20% outward displacement of the two nearest Al ions is energetically the most favourable. Eighty per cent of the hole density is concentrated on these two O ions, thus confirming that a small-radius two-site polaron model similar to that for alkali halides (the VK centre), is applicable here. The calculated absorption energy of the STH (2.9 eV) is close to that observed experimentally.

Hole polarons in pure BaTiO3 studied by computer modeling

“…c Calculated as a difference between the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), i.e., only short-range correlation effects have been taken into account through the parametrization. to a reduction of the bandgap width for the oxide crystals by approximately 2-4 eV [37,38]. This estimated quantity is reasonably close to our "error" of 2.44 eV.…”

“…Thereby optimized parameters are given in Tables III and IV. We have to note that the utilized computer program has been proved to be very reliable in a number of applications, including defect studies in such complex oxides as α-Al 2 O 3 [37,39,40], TiO 2 [41,42], and WO 3 [43]. It also has been used before very successfully to study STH or hole polarons in a number of alkali halide crystals, MgO, α-Al 2 O 3 , and TiO 2 .…”

“…E 2 pol includes both atomic and electronic polarization terms. It was estimated in a number of works [37,46] that E 1 pol and E 2 pol do not differ much in the absolute value but have the opposite signs, so one can neglect these two terms in the first approximation.…”

Cation Substitution for BaTiO3 Phase in Low-Fired Glass-Ceramics for Tunable Microwave Applications

“…the peripheral boundary atoms were allowed to relax to the equilibrium geometry (minimum Et) in order to take into account the effect of their broken bonds with 'atoms' outside the cluster. This point is discussed in more detail in a previous paper (Jacobs et al 1992) but the boundary relaxations were typically several per cent of the corresponding perfect crystallographic distance. Then a hole was inserted into a cluster (perfect, or containing an AI vacancy or an Mg substitutional) and different ionic relaxations were imposed in order to find the minimum in E, without making any apriori assumptions about the electron (hole) density distribution, which indeed is a result of our calculations.…”

Role of silicon impurities in the trapping of holes in KTiOPO4 crystals

“…No self-trapped hole center has yet been demonstrated to exist as a stable defect in an oxide crystal. 11 Second, their reported hyperfine interaction with the primary phosphorus nucleus is very anisotropic, varying from 1.5 to 9 G. This appears to be too small an interaction to support their proposed model of a (PO 4 ) 2Ϫ center. We bring attention to an analogous (PO 4 ) 2Ϫ trapped hole center in KH 2 PO 4 crystals 12 which is associated with a hydrogen vacancy and has a predominantly isotropic 32 G hyperfine interaction with the primary phosphorus nucleus.…”