THE LOW-TEMPERATURE STRUCTURE OF NBTE4

The incommensurate room-temperature (RT) structure of NbTe4 undergoes a phase transition into an intermediate incommensurate low-temperature (LT) phase at low temperatures, before the lock-in transition into the commensurate low-temperature (C) phase occurs at ca 50 K. In an X-ray experiment, the additional satellite reflections of the intermediate low-temperature phase are diffuse at room temperature, but remain diffuse in one dimension (along c*) also at very low temperatures. A structure determination has been carried out for the LT phase with the diffuse satellite reflections based on a data set which was measured at 100 K. The incommensurate LT structure is described as a (2 x 2 x 3)-fold superstructure approximation. In a supercell, the formation of triplets of Nb atoms with phase shifts of 2/3 pi between neighbouring chains are found. The final R value in the space group P4 is R = 0.060.