The specific heat, magnetic susceptibility and electron spin resonance signals for Na-deficient vanadate samples, V2NaxO5 (where x ranged from 1 to 0.9), were studied at 0.07 to 10K. This was well below the transition to a spin-gap state. The contribution made by Na vacancies to the specific heat was determined. This contribution comprised a low-temperature part, which did not tend to zero until at least 0.3K, and a high-temperature power-like tail which appeared above 2K. Such a dependence was suggested to correspond to the existence of local modes and correlations between defects in V-O layers. The magnetic measurements and electron spin resonance data revealed S = ½ degrees of freedom for the defects; with their effective number increasing with temperature and in a magnetic field. The latter resulted in non-saturating magnetization at low temperatures. No long-range magnetic ordering in the system of defects was found. A model for the defects was proposed, in order to explain the observed effects, which was based upon electron jumps near to vacancies. The concept of a frustrated 2-dimensional correlated magnet induced by the defects was considered to be responsible for the absence of magnetic ordering.

Thermal and Magnetic Properties of Defects in the Spin-Gap Compound NaV2O5. A.I.Smirnov, S.S.Sosin, R.Calemczuk, V.Villar, C.Paulsen, M.Isobe, Y.Ueda: Physical Review B, 2001, 63[1], 014412 (7pp)