Pressure-dependent phase transformation of solid helium confined within a nanoporous material

Transmission x-ray diffraction experiments have been carried out on solid helium grown in porous Vycor glass. Measurements were made at temperatures near 0.7 K and at pressures up to 162 bars. The crystalline phases of solid helium in Vycor are found to differ significantly from the bulk. At pressures from 70 bars through 98 bars the helium is polycrystalline and displays a single size broadened scattering peak. Above 98 bars the peak splits into three close peaks. No higher order peaks are seen at any pressure, indicating significant reduction in intensity due to disorder. A broad peak is present at all pressures, which may indicate the presence of amorphous solid. We tentatively identify the low-pressure phase as bcc and the high-pressure phase as coexistence between bcc and hcp. Size broadening indicates an average grain size of approximately 5 nm, comparable to the Vycor pore size.