Vanadate Family as Spin-Gap Systems

The structural and magnetic properties of the vanadate family with the chemical formula AV2O5 (A = Li, Na, Cs, Mg, and Ca) are reviewed in terms of low-dimensional quantum-spin systems. These compounds have layered structures with A cations lying between layers. A structural element common to them is a VO5 square pyramid formed by five oxygens surrounding the V ion. Each compound has a structure with a characteristic arrangement of VO5 square pyramids formed by the sharing of edges and/or corners. All of these compounds are low-dimensional, spin-1/2 magnets and are spin-gap systems except γ-LiV2O5. α‘-NaV2O5 is a spin-Peierls system with the transition temperature TSP = 35 K and the energy gap Δ = 114 K. The parameter 2Δ/kBTSP = 3.53 holds well for the all organic and inorganic spin-Peierls compounds except α‘-NaV2O5 for which the measure is 6.44. This difference suggests that the transition observed in α‘-NaV2O5 is not a conventional spin-Peierls transition but an exotic one. Both MgV2O5 and CaV2O5 are spin-ladder systems with a similar two-leg ladder structure but the gap energy (∼17 K) observed in MgV2O5 is very small compared with that (∼600 K) in CaV2O5. CsV2O5 is a dimer system with a spin gap of ∼160 K. γ-LiV2O5, which has one-dimensional zigzag chains in the structure, shows neither spin gap nor magnetic order down to 0.5 K.