In Situ Observation of the Humidity Controlled Polymorphic Phase Transformation in Glycine Microcrystals

Abstract

Glycine is a model crystal exhibiting three polymorphic phases and important functional properties such as piezoelectricity and ferroelectricity. We report here in situ observation of the irreversible transformation of the solution-grown glycine crystals from a beta phase into a gamma phase. The slow transformation process was monitored by piezoresponse force microscopy at room temperature. The process of beta to gamma conversion was entirely controlled by the variation of relative humidity in the sample chamber. The results show that the rate of phase transformation in glycine is humidity dependent with a threshold of about 25% RH. It is demonstrated that the phase boundary is highly rugged and the transformation front propagates inhomogeneously along the polar axis of the beta phase. The mechanism of the phase transformation is discussed.