LOW-TEMPERATURE POLYMORPHISM IN TUNGSTEN TRIOXIDE POWDERS AND ITS DEPENDENCE ON MECHANICAL TREATMENTS

摘要

The polymorphism of WO3 powder samples, resulting from mild mechanical treatments and from temperature changes between 30 K and RT, has been investigated by using Raman spectroscopy and XRD. A transition from the monoclinic (I) gamma-phase to the triclinic delta-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 RT 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) gamma-phase at RT, but constitutes only a small fraction in moderately treated powders, having the triclinic delta-phase at RT. 29 refs.