Magnetic and electronic transport properties of the monophosphate tungsten bronze (PO 2) 4(WO 3) 2 m, m = 2

Abstract

Large plate-like dark-brown crystals of monophosphate tungsten bronze (PO 2) 4(WO 3) 2 m , m = 2 or PWO 5 were prepared by reacting stoichiometric mixtures of P 2O 5, WO 3, 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 WO 6 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 (PO 2) 4(WO 3) 4 predicted both localized and delocalized electrons in narrow and dispersive bands, respectively. The observed magnetic moment of PWO 5 is consistent with the theoretical prediction, but the observed semiconductivity behavior is not. The difference in the observed electronic transport properties of PWO 5 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 {(PO 2) 4(WO 3) 2 m , m = 4, 6}, is discussed in terms of the unique structure of PWO 5.