Phase transformations in opals under thermal and thermobaric actions

The ultrasmall- and wide-angle neutron diffraction methods are used to study multiscale structures and phase transformations in synthetic opals at different temperatures and pressures (as high as 1500°C and 10 GPa, respectively). Monodisperse colloidal particles of amorphous silica dioxide (a-SiO2) with an average diameter of 150‒1700 nm whose deviation from the mean is less than 5% are synthesized to fabricate the samples. Opal matrices up to 3 cm thick are obtained via the natural sedimentation of a SiO2-globule suspension followed by drying and heat treatment. Neutron-diffraction processes are investigated using a DISK multidetector superposition diffractometer mounted at the IR-8 reactor of the National Research Centre Kurchatov Institute at a neutron wavelength of 1.668 A. Ultrasmall-angle diffraction experiments are performed in the two-crystal mode of a STOIK spectrometer.