CHARGE-DENSITY-WAVE INSTABILITIES IN THE FAMILY OF MONOPHOSPHATE TUNGSTEN BRONZES - (PO2)(4)(WO3)(2M)

The structural instabilities towards the formation of a charge density wave (CDW) ground state exhibited by the members of the family of mon ophosphate tungsten bronzes with pentagonal tunnels (MPTB(p)) of gener al formula (PO2)(4)(WO3)(2m) are studied for m=8,9. In the m=8 member two incommensurate CDW modulations are detected, but no long range ord er (LRO) is achieved. In the m=9 member two commensurate CDW modulatio ns towards a LRO occurs. Evidence for a superstructure formation is al so found in other members of large values of m. We emphasize the speci al role of the m=8 member which marks the borderline between incommens urate and commensurate CDW phase transitions.