The metal-insulator transition in LaxWO3 and YxWO3

The metal-insulator (MI) transition in the trivalent-ion doped cubic tungsten bronzes, LaxWO3 and YxWO3 ( 0.05 $<$ x $<$ 0.20), has been investigated using low temperature electrical transport, NMR and magnetization measurements on sintered samples.The critical concentration is estimated to be xc = 0.07 with some uncertainty because a mixed phase appears below x = 0.085. The trivalent ions donate their electrons into the WO3 (t2g) conduction band. Hall effect measurements as a function of T indicate that partial ‘freeze-out’ of carriers occurs at low temperatures for x $<$ 0.12. However, no evidence for local moments is obtained. The results suggest that for 0.07 $<$ x $<$ 0.12 an intermediate just-metallic phase, with properties that evolve with x, is important. The findings are compared with available theory for the MI transition in tungsten - bronzes.