Structural Instabilities of Ammonium Hexahalometallates Studied by the Calculation of Quantum Chemistry

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Abstract
For the first time, the lattice instabilities and the natures of the lattice instabilities of ammonium hexahalometallates are studied by calculations of quantum chemistry and compared with our experimental results. It is found that, the barrier height to the reorientation of the ammonium ion group is very sensitive to the structural parameters of the crystals. There are two different tendencies of the mechanism for the structural instabilities with a change of the structural parameters. It is revealed that there exists a strong connection between the motions of the ammonium ions and the natures of the lattice instabilities of the crystals. A common existence of the trifurcated hydrogen bonds in the ammonium hexahalometallate crystals is supposed.