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

The polymorphism of WO, powder samples, resulting from mild mechanical trea tments and from temperature changes between 30 K and room temperature, has been investigated by using Raman spectroscopy and X-ray diffraction. A tran sition from the monoclinic (I) gamma-phase to the triclinic delta-phase aft er moderate mechanical treatments has been observed for untreated powder, j ust what happens when the same samples are rapidly cooled to low-temperatur e. Evidences of the low temperature monoclinic (II) polar epsilon-phase hav e been found at room temperature in samples after a stronger milling treatm ent. The sequence of the low-temperature phase transitions appears to be st rongly dependent on the mechanical history of the powders. A new low-temper ature 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 room temperature, but constitutes only a small fraction in moderately treated powders, having the triclinic delta-phase at room tempe rature. (C) 1999 Academic Press.