Molecular Dynamics and Phase Transitions in Solids Studied by ESR

Abstract

Paramagnetic centers can be used as a monitor of the crystalline -Field local symmetry related to a lattice de-Formation and dynamics of the paramagnetic complexes. This dynamics can be also related to the number of ligands o-F the paramagnetic ion. It is well illustrated by the Cu(NH₃)_n complex, which depending on the value of n, is either strictly localized or undergoes a free reorientation. In the mono- crystal CU(NH₄)_N(C10₄)₄ the CU(NH₄)₄ complex is immobilized while the Cu(NH₃)₅ complex can freely change its orientation. The first crystal does not undertake the phase transition up to the lowest temperatures, whereas in the Cu(NH₃)₅ (C10₄)₂ crystal the phase transition related to a decay of the free reorientation is observed. The latter crystal exhibits a typical liquid-like EPR spectrum which is a consequence of a dynamic equilibrium of the two configurations: square pyramid (ps) and trigonal bipyramid (bpt).