Specific heat anomalies in the quasi two-dimensional monophosphate tungsten bronzes K_xP_4W_8O_(32)

摘要

The monophosphate tungsten bronzes K_xP_4W_8O_(32) (0.75 < x < 2) are quasi-two-dimensional conductors which show electronic transitions at a critical temperature T_c depending on the concentration of the alkali metal. The phase diagram shows a maximum at T_c = 170 K for x = 1.30. We report specific heat measurements in the range 120-190 K. The thermal anomalies found at the transition temperature are larger than in conventional charge density wave materials. This corroborates that the transition is not a 'pure' charge density wave transition and that a structural transition dominates the instability.