The spin-gap state and the phase transition in the δ-phase TlxV2O5 polaronic bronze

The crystal structure and electronic properties of delta-phase bronze TlxV2O5, where 0.47 less than or equal to x less than or equal to 0.496, have been explored by means of x-ray four-circle diffraction and through measurements of lattice constants, electrical resistivity, thermoelectric power, magnetization, and electron paramagnetic resonance. The structure of Tl0.479V2O5 redetermined with residual factors of R = 0.052 and R-w = 0.041 indicates that the trellis-layer model is applicable to this system as in the case of delta-Ag2/3V2O5 in the low-temperature phase. A spin-gap behaviour derived from that model and the hopping conduction of negative small polarons are found. A phase transition takes place at around 150 K accompanied by significant changes of the superexchange coupling constants (or spin-gap energies) and the carrier-creation energies.