Molecular Dynamics and Phase Transitions in Solids Studied by ESR
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).