Study of the ferroelastic phase transition in the tetraethylammonium compound [N(C2H5)4]2ZnBr4 by magic-angle spinning and static NMR

Ae Ran Lim

doi:10.1063/1.4943978

Abstract

The ferroelastic phase transition of tetraethylammonium compound [N(C2H5)4]2ZnBr4 at the phase transition temperature (TC) = 283 K was characterized by magic-angle spinning (MAS) and static nuclear magnetic resonance (NMR), and confirmed by optical polarizing spectroscopy. The structural geometry near TC was studied in terms of the chemical shifts and the spin-lattice relaxation times T1ρ in the rotating frame for 1H MAS NMR and 13C cross-polarization (CP)/MAS NMR. The two inequivalent ethyl groups were distinguishable in the 13C NMR spectrum, and the T1ρ results indicate that they undergo tumbling motion above TC in a coupled manner. From the 14N NMR results, the two nitrogen nuclei in the N(C2H5)4+ ions were distinguishable above TC, and the splitting in the spectra below TC was related to the ferroelastic domains with different orientations.

Highlights

IntroductionThe crystal structure of [N(C2H5)4]2ZnBr4 is very interesting with good optical property but relatively poorly understood
The ferroelastic phase transition of tetraethylammonium compound [N(C2H5)4]2ZnBr4 at the phase transition temperature (TC) = 283 K was characterized by magic-angle spinning (MAS) and static nuclear magnetic resonance (NMR), and confirmed by optical polarizing spectroscopy
The two inequivalent ethyl groups were distinguishable in the 13C NMR spectrum, and the T1ρ results indicate that they undergo tumbling motion above TC in a coupled manner

Summary

Introduction

The crystal structure of [N(C2H5)4]2ZnBr4 is very interesting with good optical property but relatively poorly understood. It becomes optically biaxial below the phase transition temperature TC = 282.6 K14 with the two phases designated I and II in order of decreasing temperature, where the 180◦ domains appear. Ferroelasticity has long been recognized as a crystal property.[15,16] A crystal is ferroelastic when it has two or more orientation states in the absence of mechanical stress, and can be shifted from one to another by mechanical stress Any pair of these states is identical or enantiomorphous in crystal structure, but different in their mechanical strain tensors at null mechanical stress. The crystal is divided into domains possessing different spontaneous deformations

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