Specific heat anomalies in the quasi two-dimensional monophosphate tungsten bronzes KxP4W8O32

The monophosphate tungsten bronzes KxP4W8O32 (0.75<x<2) are quasi-two-dimensional conductors which show electronic transitions at a critical temperature Tc depending on the concentration of the alkali metal. The phase diagram shows a maximum at Tc=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.