Vanadate family as spin-gap systems

The structural and magnetic properties of the vanadate family with the chemical formula AV(2)O(5) (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 gamma-LiV2O5. alpha'-NaV2O5, is a spin-Peierls system with the transition temperature T-SP = 35 K and the energy gap Delta = 114 K. The parameter 2 Delta/k(B)T(SP) = 3.53 holds well for the all organic and inorganic spin-Peierls compounds except alpha'-NaV2O5 for which the measure is 6.44. This difference suggests that the transition observed in alpha'-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 (similar to 17 K) observed in MgV2O5 is very small compared with that (similar to 600 K) in CaV2O5. CsV2O5 is a dimer system with a spin gap of similar to 160 K. gamma-LiV2O5, which has one-dimensional zigzag chains in the structure, shows neither spin gap nor magnetic order down to 0.5 K.