Session CE - Condensed Matter Physics.
MIXED session, Saturday morning, November 02
Room 222, Morton Hall
The \beta \rightarrow \gamma displacive phase transformation in Ca_2SiO_4 is accompanied by an increase in specific volume of about 12%. Moreover, the transformation is controlled by a particle size effect which suppresses the transformation for particles of less than a critical size of order 1 \mum. Volume change and particle size effect make it possible to use this phase transformation for transformation toughening, a scheme to overcome the brittleness of ceramic materials. We are using time-differential perturbed angular correlation spectroscopy (PAC) to study the electric field gradient (EFG) at the site of ^181Hf/Ta probe nuclei in Ca_2SiO_4. Both symmetry and magnitude of the EFG are affected by a phase transition. PAC spectra were recorded in-situ between room temperature and 960^oC. Three polymorphs are stable in this temperature range; the orthorhombic \gamma phase (RT - 850^oC), the orthorhombic \alpha_L^\prime phase (above 850^oC), and the monoclinic \beta phase (675^oC - 490^oC, on cooling only). PAC spectra show broad linewidths indicating considerable structural disorder or interstitial probe location. PAC spectra for the \gamma, \beta, and \alpha_L^\prime phases of Ca_2SiO_4 will be discussed.
