Electric, Magnetic, and Magnetic Resonance Properties of the Nonstoichiometric Sodium—Vanadium Bronzes

Abstract

Electric and magnetic studies have been carried out on the system NaxV2O5 (0.24<x<0.33). Single-crystal electrical-conductivity measurements on Na0.33V2O5 show semiconducting behavior from 77° to 500°K. A room-temperature conductivity of 230 Ω−1·cm−1 with an activation energy of about 0.02 eV is observed. Thermoelectric power amounts to −116 μV/K° and is constant from 273° to 500°K. The Hall voltage is too small to be observed, indicating >1019 carriers/cc. Gouy-susceptibility measurements have been carried out on a series of bronzes of variable Na/V ratio in the range 1.5° to 300°K. All specimens are highly paramagnetic and follow a Curie—Weiss law between 63° and 300°K. Weiss constants vary from −139° to −157°K; Curie constants from 0.119 to 0.159. The effective magnetic moments range from 1.94 to 2.01 Bohr magnetons. Below 63°K, the reciprocal susceptibility decreases more rapidly than linearly with temperature. No evidence of an antiferromagnetic transition is observed. The results, together with other electrical, ESR, and NMR data, cannot be explained by a hopping model but are consistent with a 3d t2g band model in which a majority of V4+ donor sites remain un-ionized over the temperature range investigated.