Cascading transitions toward unconventional charge density wave states in the quasi-two-dimensional monophosphate tungsten bronze P 4 W 16 O 56

Single crystals of the = 8 member of the low-dimensional monophosphate tungsten bronzes (PO ) (WO ) 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 = 258 K, = 245 K and = 140 K, never observed until now. Parallel X-ray diffraction investigations showed a specific signature associated with three structural transitions, the appearance of different sets of satellite reflections below , and . 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.