TEM Observation of Pattern Formation in the Incommensurate Phase of Quartz

Abstract

Incommensurate (IC) phases have been found in many kinds of materials such as transition metal dichalcogenides, ferroelectrics, alloys and polymers. In the IC phase, a fundamental structure varies spatially by a modulation wave such as an atom displacement wave or an atom occupation wave. The modulation wave changes to an array of ‘discommensurations (DC’s)’ at near the transition temperature (Tc) between an IC and commensurate (C) phases as predicted by McMillan1). They can be directly observed by transmission electron microscopy (TEM). The existence of the DC’s in 2H—TaSe2 was first confirmed by TEM 2,3)$ an(j characteristic defects of the DC lattice were also revealed in the TEM images. Recently the time evolution of DC patterns has been studied theoretically by several authors 4). However, there are only a little experiment to be compared with the theories so far. Here we present the recent TEM observations for the pattern formation of the DC lattice appearing in the IC phase of quartz. The observation has been carried out using a lMeV high voltage TEM in Tokyo Institute of Technology, which is equiped with a TV camera system. The pattern evolutions observed in the microscope were recorded on VTR and were analyzed after the observation.5)