Pressure-induced cubic-to-orthorhombic phase transformation in the negative thermal expansion material HfW2O8

The effect of pressure on the crystal structure of HfW2O8 has been investig ated by neutron powder diffraction. At a hydrostatic pressure of 0.62 GPa a t room temperature the cubic material transforms, with a 5% reduction in vo lume, to the same orthorhombic phase that is seen in the isostructural comp ound ZrW2O8 above 0.21 GPa. The transformation is sluggish, requiring about 24 h to complete at constant pressure. Once formed, the orthorhombic phase is retained upon release of pressure. Upon heating to 360 K, the metastabl e orthorhombic phase transforms back to the cubic phase. The substantially higher pressure for the cubic-to-orthorhombic transition in HfW2O8, compare d to ZrW2O8, may be important for the application of this material in compo sites with controlled thermal expansion because rather large local pressure s can occur in such composites. (C) 2001 American Institute of Physics.