Crystal structures together with physical properties under nonambient conditions are significant subjects in the effort to understand geophysical phenomena or solid-state physics. Miniature diamond anvil pressure cell (DAC) and multianvil high-pressure apparatus have become effective tools for the observation of pressure effects on crystalline materials, not only for X-ray diffraction measurements but also for physical property studies such as electrical conductivity and magnetism. These high-pressure studies have been made at high temperatures by electric resistance heater or laser and at low temperatures by cryostat.

For the last twenty years, synchrotron radiation facilities have accelerated the study of high-pressure crystallography because of their great advantages for diffraction studies at nonambient conditions. Application of synchrotron radiation enhances structure analyses as a function of pressure. Pressure dependence of electron-density distributions around atoms is elucidated by single-crystal diffraction study using deformation electron-density analysis. In this study, compression mechanisms were investigated through structure analyses. The maximum entropy method (MEM) based on the observed structure F_obs(hkl) of reflection hkl was applied to reveal an electron-density map, and the results were compared with difference Fourier synthesis based on F_obs(hkl) – F_calc(hkl). Radial electron distribution revealed the localization or delocalization of electrons around atomic positions together with bonding electron densities. The diffraction intensity measurements of FeTiO₃ ilmenite and γ-SiO₂ stishovite single crystals were made at high pressures. In both cases, the valence electrons became more localized around the cations with increasing pressure. This is consistent with molecular orbital calculations—both methods show that the bonding electron density becomes smaller with pressure. The thermal displacement parameters of both samples were reduced with increasing pressure.