Impurities Jahn-Teller distortion

Multicentre vibronic interactions are found in a wide number of systems such as impurity Jahn-Teller centres, molecular clusters and Jahn-Teller crystals [1], In these compounds the localized electrons in orbitally degenerate states interact directly only with active distortions of the local environment of corresponding vibronic centres. The distortions of different centres interact via the common vibrational modes of the system, which thus mediate the indirect interaction between Jahn-Teller centres [2,3]. As a result the corresponding vibronic problem becomes rather involved, with many electronic states mixed by many vibrational modes. 

Recently, approximate MO theories have been applied to a wide range of solid-state phenomena in addition to those reviewed in this paper. A short review of some of these problems indicates its versatility. Messmer and Watkins (3) have used EH to predict the position of N impurity levels in diamond using a 35-atom C lattice. Their calculations indicated the presence of a Jahn-Teller effect in accordance with electron paramagnetic resonance (EPR) experiments. The calculation was successful in explaining the deepening of the N donor level as due to Jahn-Teller distortion. 

V3+ cations have been incorporated along with Mg or Ni as the divalent cations [51, 52], Although CuM3+-LDHs have been reported, including CuAl-, CuMgAl- and CuCr-LDHs, in general they are poorly crystallized and contain impurity phases (such as malachite, Cu20H2C03) [53-55]. The instability of Cu2+ (d9) towards LDH formation may be attributed to a Jahn-Teller distortion... 

Also BaF2 Eu does not show Eu emission, but impurity-bound exciton emission. The. same holds for NaF Cu where the excited state consists of a Jahn-Teller distorted Cu ion which binds an election. The absence of luminescence in La203 Ce has also been ascribed to quenching by photoionizaiion [15]. 

We consider a lattice of crystal-field split ions in which some of the ions are replaced by impurities. These impurities can be either other RE-ions with different crystal-field splitting and/or different coupling parameter to the neighbouring ions or they can be ions such as La which act like holes in the lattice due to the absence of 4f electrons. In the following we want to discuss the influence of such impurities on the properties of the otherwise perfect lattice system. We shall divide the problem into two parts. First we shall consider the one impurity problem. Here the changes of the excitation spectrum are of interest (local modes, resonant modes) as well as the possible occurrence of such phenomena as giant moments or giant Jahn-Teller distortions. After this we consider finite impurity concentrations and their effect on the transition temperature. 

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