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 t hat 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 pseudohexagona l 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].