Order-disorder transitions of t2g-orbitals of V3+ ions and incommensurate structural deformations in the metallic spinel CuV2S4

Structural transitions of the spinel CuV2S4 have been studied by means of X-ray diffraction using synchrotron radiation. The temperature dependence of reflections forbidden in a cubic spinel structure of Fd3m suggests that there are order-disorder transitions of 3d2(t2g) orbitals of V3+ ions at about 75 and 210 K. An incommensurate superlattice reflection of (2+3/4,2,2+3/4) shows the structural phase transition occurring at about 90 K, in addition to the superlattice structural modulation at about 30-35 and 55 K. A reduced wavevector q=(3/4-δ,0,3/4-δ) shows the temperature dependence of the δ value from 0.015 to -0.025. A lock-in of δ=0.0 and a crossover of the δ value occur in the region of 40 to 50 K. The incommensurate structural modulation along [110], [011], and [101] directions appearing below about 90 K affects a paramagnetic spin order of V3+ ions at about 30-35 and 55 K, in addition to the 90 K structural transition. The paramagnetic behavior is interpreted mainly by the structural deformation and the (dxy,dyz,dzx) orbitals in the VS 6 octahedral chains sited along [110], [011], and [101] directions. (copyright 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)