Geometrically frustrated triangular lattice system NaxVO2:: superparamagnetism in x=1 and trimerization in x≈0.7

The structural and electronic properties of the triangular lattice system Na(x)VO(2) with x = 1 and approximate to 0.7 are explored by x-ray powder diffraction and through transport and magnetic measurements. For x = 1, the structure at 293 K is modelled by three oxygen layers with an octahedral oxygen environment for Na. This composition, which is classified as being an insulator phase, exhibits superparamagnetism below the transition temperature T(ms) similar or equal to 98 K to a tripled unit cell. Above T(ms), the temperature dependence of magnetic susceptibility is understood in terms of the pseudotrimer model, using the exchange coupling constant determined from a high-temperature series expansion. This model is also applied to the paramagnetic phase for LiVO(2), which indicates spin-singlet trimerization at T(c1) approximate to 500 K on heating and T(c2) approximate to 440 K on cooling. The compound with x approximate to 0.7 basically has a two-layer structure with a prismatic oxygen surrounding Na and it may exhibit a first-order spin-1/2 trimerization at T(c1) approximate to 360 K and T(c2) approximate to 290 K.