VO Epr studies on lattice strains and the ferroelectric phase transition in tris-sarcosine calcium chloride crystals

Abstract

The molecular origin for the spontaneous strains and the ferroelectric phase transition in tris-sarcosine calcium chloride (TSCC) crystals was investigated by means of VO++ EPR probes. VO++ impurities substituted for Ca++ ions in the TSCC lattice exhibited six spectra, corresponding to the interaction with individual ligand sarcosine molecules. We have considered that the pseudo-trigonal lattice symmetry is represented by the asymmetrical Ca++-sarcosine complex, which was found to approach a symmetrical octahedron with increasing temperature.At high temperatures, the impurity ion acquires another stable direction under a low strain condition, reorienting its axis between this and the original directions. While VO++ ions at Ca++ sites are perturbed by the sarcosine mode, this new direction is characterized by the absence of local strains, thereby the nature of lattice strains can be deduced from annealed TSCC crystals.In the ferroelectric phase, the VO++ lines are displaced by the internal electric field due to the spontaneous polarization, suggesting that a u-mode composed of the sarcosine cluster is frozen, shifting the dipolar directions toward the ferroelectric b-axis.