Z.S. Teweldemedhin

Magnetotransport Properties in the Charge Density Wave State of the Quasi-Two-Dimensional Compounds (PO2)4(WO3)2m

Magnetotransport has been studied on the quasi-two-dimensional monophosphate tungsten bronzes (PO2)4(WO3)2m for m = 4, 6 and 7. These series provide a model system where the low-dimensional character and the electron density can be changed with m. In P4W8O32 (m = 4) and P4W12O44 (m = 6), both Hall effect and magnetoresistance data corroborate the existence of two successive charge density wave transitions. Large anisotropic magneto-resistance in the low-temperature state is well accounted for by the presence of small electron and hole pockets with large mobilities. In P4W14O50 (m = 7), the high-temperature transition, close to first order, may be associated with strong electron-phonon coupling or strong electron-electron interactions. The low-temperature transition is mainly structural.

Antiferro-to-ferromagnetic transition in metallic TlCo2SxSe2-x (0 ⪕ x ⪕ 2.0) with the ThCr2Si2 type structure

Abstract Solid solutions with the composition: TlCo 2 S x Se 2-x (0.0 ⪕ x ⪕ 2.0) , were synthesized by high temperature solid state reactions. Resistivity measurements on sintered pellets demonstrated that all of the compositions were highly metallic (ϱ = 10 -4 - 10 -5 Ω-cm) . The magnetic phase diagram was determined by magnetic susceptibility measurements. The Se-rich phases (0.0 ⪕ x ⩽ 0.75) order antiferromagnetically, the S-rich phases (0.75 ⩽ x ⪕ 2.0) order ferromagnetically, and the intermediate composition (x = 0.75) shows competition between antiferromagnetic and ferromagnetic behavior.

Magnetic and electronic transport properties of the monophosphate tungsten bronze (PO2)4(WO3)2m, 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 2 O 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 .

New tungsten pyrophosphate with cubic zirconium pyrophosphate-type structure

Abstract Tungsten pyrophosphate, WP2O7, was prepared by reacting mixtures of (NH4)2HPO4, WO3 and W at 900 °C in an evacuated quartz tube. Powder X-ray diffraction study shows the compound to be isostructural with cubic zirconium pyrophosphate, ZrP2O7, with a = 7.947 (1) A . The electrical resistivity of the compound at room temperature is in the Mega-Ohm region indicating insulating behavior. The magnetization study made on a pelletized sample shows a paramagnetic behavior similar to MoP2O7. The effective magnetic moment calculated in the temperature range 100–300 K is 2.4 BM.

Growth and physical properties of metallic In2O3 single crystals

Abstract Single crystals of metallic indium oxide were grown for the first time by molten salt electrolysis and their electrical and magnetic properties were investigated. The crystals are the cubic form of indium oxide with a C-type rare earth oxide structure as inferred by X-ray powder diffraction. The crystals are highly reflective and have colors ranging from dark green to green-yellow depending on the crucible used for preparation and on the location of the crystals at the cathode surface. The resistivity as a function of temperature, on all the crystals, showed metallic behavior up to about 100 K and almost temperature-independent behavior down to ∼2 K. The room temperature resistivity of the dark green sample is in the range of 10−4 Ω-cm. Qualitative Seebeck measurements indicate that electrons are the primary charge carriers. The temperature variation of the magnetic susceptibility is Pauli paramagnetic, consistent with the observed metallic property. The crystals were found to be stable up to ∼1200 K upon exposure to oxidizing atmospheres. The measured physical properties are supportive of the presence of a small concentration of charge carriers with high mobilities.

CDW state and superconductivity in the quasi-two-dimensional monophosphate tungsten bronze P4W14O50

Abstract The monophosphate tungsten bronzes (PO 2 ) 4 (WO 3 ) 2 m are quasi two-dimensional conductors which show electronic instabilities of the Peierls type. The m = 7 member of the series P 4 W 14 O 50 , undergoes two successive Peierls transitions at T P 1 = 188 K and T P 2 = 60 K. Weak localisation effects are observed in the low temperature charge density wave state. Resistivity, magnetoresistance and a.c. susceptibility measurements establish that a superconducting transition takes place at temperatures lower than approximately T c = 0.3 K. This indicates that superconductivity is related to parts of the Fermi surface left over after the Peierls gap openings.