Infrared study of the low-temperature phase transitions in incommensurate Cs2HgBr4

In this work we present a complete infrared reflectivity study of the five low-temperature phases of Cs2HgBr4, including the incommensurate structure. Contrarily to the predictions by group theory, no extra modes are observed at the incommensurate (T-I) and lock-in (T-c) phase transitions, showing that the structural changes associated with the modulation does not distort significantly the HgBr4 tetrahedra. The absence of predicted modes and the presence of a low-frequency relaxation mechanism are discussed in terms of the orientational disorder of the HgBr4 tetrahedra about the pseudohexagonal axis. An important increase of the infrared dielectric constant epsilon(y), observed in the incommensurate phase, is ascribed to the existence of a layered structure of polar domain walls below T-I. [S0163-1829(99)00917-0].