OBSERVATION OF A HIGH-TEMPERATURE INCOMMENSURATE PHASE-TRANSITION AND DISCOMMENSURATIONS IN TATE4

The tetrachalcogenides TaTe4 and NbTe4 provide an excellent model system for the study of charge-density wave (CDW) phenomena in quasi-one-dimensional materials. TaTe4 is commensurately modulated at room-temperature and, upon heating, has been observed to undergo a commensurate-to-commensurate phase transition at about 450 K. In this paper, we report an additional commensurate-to-incommensurate transition occurring above 550 K. Satellite dark-field electron microscopy reveals the presence of defects in the microstructure of the high-temperature phases including discommensuration arrays in the incommensurate state. The evolution of the discommensuration arrays during heating and cooling through the 550 K transition is reported. These results are discussed in relation to the sequence of phase transitions previously observed for the isostructural incommensurate compound NbTe4 on cooling below room-temperature.