Single-crystal growth, crystallography, magnetic susceptibility, heat capacity, and thermal expansion of the antiferromagnetic S=1 chain compound CaV2 O4

The compound CaV2 O4 contains V+3 cations with spin S=1 and has an orthorhombic structure at room temperature containing zigzag chains of V atoms running along the c axis. We have grown single crystals of CaV2 O4 and report crystallography, static magnetization, magnetic susceptibility χ, ac magnetic susceptibility, heat capacity Cp, and thermal expansion measurements in the temperature T range of 1.8-350 K on the single crystals and on polycrystalline samples. An orthorhombic-to-monoclinic structural distortion and a long-range antiferromagnetic (AF) transition were found at sample-dependent temperatures TS 108-145 K and TN 51-76 K, respectively. In two annealed single crystals, another transition was found at 200 K. In one of the crystals, this transition is mostly due to V2 O3 impurity phase that grows coherently in the crystals during annealing. However, in the other crystal the origin of this transition at 200 K is unknown. The χ (T) shows a broad maximum at 300 K associated with short-range AF ordering and the anisotropy of χ above TN is small. The anisotropic χ (T→0) data below TN show that the (average) easy axis of the AF magnetic structure is the b axis. The Cp (T) data indicate strong short-range AF ordering above TN, consistent with the χ (T) data. We fitted our χ data by a J1 - J2 S=1 Heisenberg chain model, where J1 (J2) is the (next)-nearest-neighbor exchange interaction. We find J1 230 K and surprisingly, J2 / J1 0 (or J1 / J2 0). The interaction J between these S=1 chains leading to long-range AF ordering at TN is estimated to be J / J1 0.04.