Low-temperature polymorphism in tungsten trioxide powders and its dependence on mechanical treatments

The polymorphism of WO3 powder samples, resulting from mild mechanical treatments and from temperature changes between 30 K and room temperature, has been investigated by using Raman spectroscopy and X-ray di¤raction. A transition from the monoclinic (I) c-phase to the triclinic d-phase after moderate mechanical treatments has been observed for untreated powder, just what happens when the same samples are rapidly cooled to low-temperature. Evidences of the low temperature monoclinic (II) polar e-phase have been found at room temperature in samples after a stronger milling treatment. The sequence of the low-temperature phase transitions appears to be strongly dependent on the mechanical history of the powders. A new lowtemperature N-phase has been observed below about 200 K in di¤erent samples: it is the main phase in commercial untreated powders, having the monoclinic (I) c-phase at room temperature, but constitutes only a small fraction in moderately treated powders, having the triclinic d-phase at room temperature.