Structural analysis and physical properties of Sr2−xLaxVO4−δ

The effects of the oxygen deficiency, δ, on the electrical and magnetic properties of Sr2−xLaxVO4−δ were investigated. For this objective, single-phase polycrystalline materials of Sr2−xLaxVO4−δ (x = 0, 0.04, 0.1, 0.2, 0.8 and 1) were synthesized. The value of δ was determined by thermo-gravimetric-analysis. The δ of Sr2−xLaxVO4−δ (x ⩽ 0.2) was almost the same value, ∼0.2. The δ of x = 0.8 was 0.05 and that of x = 1 was ∼0. Electrical resistivity measurement showed that all the samples were semiconductive. For the low magnetic field (∼0.002 T) measurement, the spin-glass-like behavior was clearly observed for larger δ samples (x = 0.04, 0.1 and 0.2). This behavior became weak for smaller δ samples (x = 0.8 and 1). It is considered that the random distribution of the oxygen vacancies in the two-dimensional VO2 plane lead to this result. From the high magnetic field (1 T) measurement, it was found that the Curie-Weiss term, χcw, dominated in the magnetic susceptibility, χ, of Sr2−xLaxVO4−δ (x ⩽ 0.2, δ ∼ 0.2). The χcw considerably reduced for x = 0.8 (δ = 0.05) and almost disappeared for x = 1 (δ ∼ 0) i.e., SrLaVO4. Thus, the χcw is considered to be originated from oxygen vacancies. Hence, it is revealed that the δ is critical to the properties of Sr2−xLaxVO4−δ, in addition to x. It is worthy to note that almost temperature independent χ of SrLaVO4 is anomalous, because in this material, all the V are V3+ with S = 1. The almost temperature independent susceptibility may be due to the spin fluctuation enhanced by the strong two-dimensionality of the crystal structure, quantum fluctuation or the spin frustration in VO2 square lattice.