Low-Temperature Polymorphism in Tungsten Trioxide Powders and Its Dependence on Mechanical Treatments

The polymorphism of WO3powder 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 diffraction. A transition from the monoclinic (I)-phase to the triclinic-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-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 low-temperature N-phase has been observed below about 200 K in different samples: it is the main phase in commercial untreated powders, having the monoclinic (I)-phase at room temperature, but constitutes only a small fraction in moderately treated powders, having the triclinic-phase at room temperature.