The solid solution series Tl(V1−xCrx)5Se8: crystal structure, magnetic and thermoelectric properties

Abstract

The crystal structure of single crystalline members of the solid solution series Tl(V_1−xCr_x)₅Se₈ (x = 0–1 and Δx = 0.2) was determined and the magnetic and thermoelectric properties of bulk TlV₅Se₈ were investigated. All compounds crystallize in the pseudo-hollandite-type structure (C2/m) with a nonlinear increase in the unit cell volume due to a simultaneous increase/decrease in the transition metal distances across the edge/face-sharing octahedra. A slight Tl deficiency was found for single crystalline TlV₅Se₈ and Tl(V_0.8Cr_0.2)₅Se₈ as well as for bulk TlV₅Se₈ according to Rietveld and single crystal structure refinements. The metallic character of bulk TlV₅Se₈ and its low electrical resistivity compared to poly- and single-crystalline TlCr₅Se₈ can probably be associated with this probable Tl deficiency. In TlV₅Se₈, anomalies in the low temperature Seebeck coefficient, i.e. a maximum at T ∼ 14 K and two sign changes at T ∼ 47 K and T ∼ 167 K, were found. Bulk TlV₅Se₈ is a highly frustrated (f = |θ|/T_N ≈ 42 K) Curie–Weiss paramagnet ordering antiferromagnetically (θ = −1287 K) below T_N ≈ 31 K with an effective magnetic moment of μ = 2.68 μ_B corresponding to V³⁺. Low-field M(H) measurements revealed the existence of a small ferromagnetic component below T_N in bulk TlV₅Se₈ and, despite the observed spin frustration, no spin glass phase was found in this compound.