Specific heat of Na_1-xV₂O₅ single crystals

Temperature dependences of the specific heat C and the magnetic susceptibility χ of Na_1-xV₂O₅ single crystals (x = 0, 0.01, 0.02, 0.03, and 0.04) are studied. In NaV₂O₅, the transition to the spin-gap state (T_c = 34 K) is accompanied by a sharp decrease in χ, while C exhibits a λ-shaped anomaly. At low temperatures, the specific heat of NaV₂O₅ is approximated by the sum of phonon ∼T³ and magnon ∼exp(-Δ/T) contributions, which makes it possible to estimate the Debye temperature Θ_D = 336 K and the gap in the magnetic excitation spectrum Δ = 112 K. With the departure from stoichiometry, the anomalies observed in the behavior of χ and C are spread and shifted to lower temperatures. The low-temperature specific heat of nonstoichiometric samples is determined by the sum of phonon and magnon components and the contribution due to the presence of defects. The values of magnetic entropy characterizing the phase transitions in Na_1-xV₂O₅ are calculated.