Structural properties in incommensurately modulated spinel compound CuV₂S₄

We have studied the incommensurate crystal structure of the spinel compound CuV₂S₄ at low temperatures using the synchrotron powder diffraction technique. The powder diffraction pattern at 70K shows that the splitting of the fundamental 800 reflection peak and the weak superlattice reflections with a modulation vector q ̃ 3/4[110] due to the structural phase transition at 90K were observed. It is found that the crystal structure at 70K has an orthorhombic unit cell with the superspace-group Imm2(0β0) with a modulation vector q = 0.7391(5)b*. By Rietveld analysis including the incommensurate modulation along the b-axis, large static atomic displacements were found at the positions of V atoms, suggesting that V atoms play an important role in a charge-density wave (CDW) transition. The atomic displacements indicate that V1 atoms at the 4c site form dimers along the a-axis, suggesting that a commensurate CDW forms along the a-axis. Furthermore, the V2 atoms at the 4d site tend to approach V1 dimers, giving rise to the formation of V clusters such as trimers and tetramer-like clusters. The arrangements of V clusters form a pattern along the b-axis with a period of nearly four fundamental cells, which may induce the formation of an incommensurate CDW along the b-axis.