Angular-dispersive x-ray in situ powder-diffraction experiments have been performed on pure zirconia, ${\mathrm{ZrO}}_2$, at room temperature under high pressure up to 50 GPa. Under increasing pressure four phases were successively encountered: baddeleyite (monoclinic, P$2_1$/c) from normal pressure up to about 10 GPa, orthorhombic-I (Pbca) to 25 GPa, orthorhombic-II to 42 GPa, and orthorhombic-III above 42 GPa. The unit-cell parameters and the volume have been determined as a function of pressure. The bulk moduli of the two lower pressure phases have been calculated using Birch’s equation of state. The bulk modulus of baddeleyite, 95 GPa, is much lower than expected from bulk modulus-volume systematics, 195 GPa, while for the orthorhombic-I phase, the experimental and calculated values are almost identical. A generalized P-T diagram for ${\mathrm{ZrO}}_2$, including an orthorhombic-IV phase, is proposed and discussed. The phase transitions to orthorhombic-II and orthorhombic-III phases can be described by a simple rotation of the unit cell of the orthorhombic-I phase about either the b axis to form the orthorhombic-II phase or a axis to form the orthorhombic-III phase. All high-pressure cells (orthorhombic-I, -II, and -III) have eight formula units (Z=8). The orthorhombic-II phase was found not to have the cotunnite ${\mathrm{PbCl}}_2$-type structure which was proposed previously. There is no longer any example of a compound which transforms to such a cotunnite-type structure under high pressure. The behavior of zirconia and hafnia under high pressure is different although they have very close chemical properties at ambient pressure and identical structures in the two lower-pressure phases.

The database on structural phase transitions is presented in the simple and easy accessible form of table. The symmetry changes and temperature for 3446 phase transitions and the references for each of 2242 registered crystalline materials with well defined stoichoimetry are the main data in this table. The rules for constructing and reading the database table are also presented.

The phase transformations and pressure-volume dependence of ${\mathrm{HfO}}_2$ have been investigated at room temperature by angle-dispersive powder x-ray diffraction under high pressure to 50 GPa in a diamond anvil cell. The phase transformation from the monoclinic I (baddeleyite) to orthorhombic phase II was observed around 10 GPa. This phase is stable up to 26 GPa where it transforms to a new phase III with another orthorhombic unit cell. At about 42 GPa, a third phase transition occurs to phase IV of tetragonal symmetry. The pressure dependences of the cell parameters and volume have been determined. The successive volume discontinuities are 2.5%, 2.5%, and 5%, respectively. The bulk moduli of all the phases have been calculated from Birch’s equation of state and are discussed. The high-pressure phases were found to be metastable at normal pressure. No orthorhombic cotunnite-type structure was observed under pressure at room temperature. Although the structural properties of ${\mathrm{HfO}}_2$ and ${\mathrm{ZrO}}_2$ are similar at lower pressures, their evolutions are different above 20 GPa.

Lattice parameters of LiKSO₄ have been measured in the temperature range 155–300 K with an X-ray single-crystal Bond diffractometer. Below 180 K LiKSO₄ transforms on cooling into an orthorhombic phase. Above this temperature there is a broad range of an intermediate hexagonal phase which depends on the real structure of the sample and differs considerably on cooling and on heating. The anomalies in the physical properties of LiKSO₄ reported in the literature have been explained. The crystal structure of the low-temperature and intermediate phase, as well as the mechanism of the phase transitions, are suggested and discussed.