Electric transport investigations on the trivalent-ion doped tungsten bronzes YxWO3 and LaxWO3 near the metal-insulator transition

Abstract

Electric transport investigations were performed on the trivalent–ion doped tungsten bronzes YxWO3 (0.05 ≤ x ≤ 0.20) and LaxWO3 (0.05 ≤ x ≤ 0.23). The range of dopant concentrations includes the cubic phase, as well as the predicted metal–insulator transition occurring in these systems. Polycrystalline samples were prepared by solid state reaction by sintering compressed powder pellets in evacuated quartz ampoules at 1100°C for 72 hr. Characterisation of the samples included powder x–ray diffraction, diffuse reflection spectroscopy, specific heat and magnetization measurements. The results are similar to those found in other tungsten bronzes. An electrical transport station was assembled to measure the transport properties as a function of temperature (1.6 K ≤ T ≤ 300 K) and magnetic field (–1.2 T ≤ B ≤ +1.2 T). The standard four–wire method for bar samples and the van der Pauw technique for pellets were both used for dc conductivity and Hall effect measurements. The conductivity σ versus temperature T plots for a range of x values show a family of curves typical of metal–insulator transition systems and comparable to results for NaxWO3 and NaxTayW1-yO3. A change from metallic–type (dσ/dT < 0) to semiconducting–type (dσ/dT > 0) behaviour is observed as x is reduced. The metal–insulator transition is found to occur for nominal concentrations between x = 0.05 and x = 0.06 in both the YxWO3 and LaxWO3 samples. This corresponds to a critical electron concentration nC ~ 3×1021 cm–3. Room temperature Hall effect results support the fully donor model for the tungsten bronzes. The Hall concentration decreases on cooling, indicating a partial “freeze–out” of charge carriers, observed previously in cubic single–crystal NaxWO3. No evidence is found for local moment formation from magnetization and specific heat measurements at low temperatures. Magnetoresistance effects appeared at low temperatures with small resistivity changes ≲ 0.1% in 1 T fields. For lower x values the YxWO3 and LaxWO3 samples show very similar behaviour in all their properties, confirming that the donor valence and not donor type is the dominant factor in determining the properties of the tungsten bronzes.