Cascading transitions toward unconventional charge density wave states in the quasi-two-dimensional monophosphate tungsten bronze P4W16O56

Single crystals of the m = 8 member of the low-dimensional monophosphate tungsten bronzes (PO2)(4)(WO3)(2m) family were grown by chemical vapour transport technique and the high crystalline quality obtained allowed a reinvestigation of the physical and structural properties. Resistivity measurements revealed three anomalies at T-C1 = 258 K, T-C2 = 245 K and T-C3 = 140 K, never observed until now. Parallel X-ray diffraction investigations showed a specific signature associated with three structural transitions, i.e. the appearance of different sets of satellite reflections below T-C1, T-C2 and T-C3. Several harmonics of intense satellite reflections were observed, reflecting the non-sinusoidal nature of the structural modulations and a strong electron-phonon coupling in the material. These transitions could be associated with the formation of three successive unconventional charge density wave states.