14N NMR study and Landau theory of phase transitions in [N(CH3)4]2ZnI4

Abstract

The14N NMR spectra and spin-lattice relaxation timeT 1 of [N(CH3)4]2ZnI4 have been studied between room temperature and 200 K. Two phase transitions atT c 1=255 K and atT c 1=217 K are observed. The14N NMR lineshape andT 1 data suggest that the intermediate phase is commensurate rather than incommensurate in spite of the presence of a Lifshitz invariant in the expansion of the free energy density in powers of the order parameter. We also discuss the phenomenological theory of structural phase transitions in [N(CH3)4]2ZnI4.