Structural Properties in Incommensurately Modulated Spinel Compound CuV₂S₄

Abstract

We have studied the incommensurate crystal structure of the spinel compound CuV 2 S 4 at low temperatures using the synchrotron powder diffraction technique. The powder diffraction pattern at 70 K shows that the splitting of the fundamental 800 reflection peak and the weak superlattice reflections with a modulation vector q $ 3=4½110 due to the structural phase transition at 90 K were observed. It is found that the crystal structure at 70 K has an orthorhombic unit cell with the superspace-group Imm2ð00Þ with a modulation vector q ¼ 0:7391ð5Þb à . 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.

Charge density wave and lock-in transitions of CuV2S4

“…) with δ = −0.0109 (negative) [26]. An orthorhombic lattice distortion is in agreement with the number of six domains in the CDW phase as observed by electron diffraction [27].…”

“…An orthorhombic lattice distortion is in agreement with the number of six domains in the CDW phase as observed by electron diffraction [27]. However, Kawaguchi et al [26] report the diffraction to be equal at 20 and 70 K, thus failing to see any structural effects of the phase transition at 50 K.…”

“…Specific-heat data resulted in T CDW = 92 K and T c2 = 58 K [25]. Based on x-ray powder diffraction (XRPD), Okada et al [24] discovered a tetragonal lattice distortion at all temperatures below 90 K. More recently, again using XRPD, Kawaguchi et al [26] determined the modulated structure of the incommensurate CDW at 70 K, employing the orthorhombic superspace group Imm2(0 σ o 0) and modulation wave vector q =…”

The three-dimensional charge-density-wave compound CuV2S4

“…Examples of three-dimensional (3D) CDW materials include RNiC2, R5Ir4Si10 and R2Ir3Si5 (R = rare earth element) [1,2]. Here, we will discuss 3D CDW systems, with emphasis on the incommensurate CDW in the spinel compound CuV2S4 [3]. CuV2S4 has cubic symmetry Fd3m, and thus lacks a manifest direction in its crystal structure, while the electrical conductivity is isotropic.…”

“…CuV2S4 has cubic symmetry Fd3m, and thus lacks a manifest direction in its crystal structure, while the electrical conductivity is isotropic. At TCDW = 90 K a phase transition commences towards a 1D CDW state with orthorhombic symmetry and modulation wave vector q = (3/4+, 3/4+, 0) [3]. We will present an accurate structure model for the CDW state and provide a discussion of the origin of CDW formation in CuV2S4.…”