MAGNETIC AND ELECTRONIC TRANSPORT-PROPERTIES OF THE MONOPHOSPHATE TUNGSTEN BRONZE (PO2)4(WO3)2M, M=2

Large plate-like dark-brown crystals of monophosphate tungsten bronze (PO2)4(WO3)2m, m = 2 or PWO5 were prepared by reacting stoichiometric mixtures of P2O5, WO3, and W at 1200°C. The temperature dependence of electrical resistivity along each of the three unique crystallographic axes of a single crystal shows semiconducting behavior down to 50 K with an activation energy of ∼0.084 eV. The room temperature resistivitity along the direction of corner sharing WO6 octahedra is 5 × 10-3 Ω · cm and about one to two orders of magnitude lower than along other unique directions, which implies quasi one-dimensional behavior. The magnetization study made on a batch of crystals in the temperature range of 2 to 300 K is indicative of antiferromagnetic ordering with a maximum at 15 K. An earlier theoretical study on the band electronic structure of (PO2)4(WO3)4 predicted both localized and delocalized electrons in narrow and dispersive bands, respectively. The observed magnetic moment of PWO5 is consistent with the theoretical prediction, but the observed semiconductivity behavior is not. The difference in the observed electronic transport properties of PWO5 from that of theoretically predicted behavior, as well as the anomalous magnetic and transport properties compared to the higher members of the series of the monophosphate tungsten bronzes {(PO2)4(WO3)2m, m = 4, 6}, is discussed in terms of the unique structure of PWO5. © 1991.