MAGNETIC-PROPERTIES OF PHASES POSSESSING BLOCK TYPE STRUCTURES IN THE NB2O5-2X SYSTEM, WITH 0 LESS-THAN-OR-EQUAL-TO X LESS-THAN-OR-EQUAL-TO 0083

The magnetic susceptibility of phases with block type structures in the Nb2O5-2x system, with 0 less-than-or-equal-to x less-than-or-equal-to 0.083, has been determined by the Faraday method in the temperature region 77-400 K. The samples consisted of crystals of composition Nb28O70 (Nb2O5), Nb53O132(NbO2.491), Nb25O62(NbO2.480), Nb47O116(NbO2.468), Nb22O54(NbO2.454) and Nb12O29(NbO2.417). It is shown that the observed paramagnetic behaviour cannot be explained within the framework of classical Debye-Langevin behaviour or by Pauli paramagnetism. Instead, a Van Vleck type analysis of the magnetic susceptibility data gives evidence for the occurrence of bipolaron/single-polaron states at lower degrees of reduction (x < 0.05), in agreement with previous interpretations of electrical and optical properties. The quasi-metallic state for x greater-than-or-equal-to 0.05 reveals localized electrons, as suggested earlier.