▪ Abstract The synthesis of the two currently used superhard materials, diamond and cubic boron nitride, is briefly described with indications of the factors influencing the quality of the crystals obtained. The physics of hardness is discussed and the importance of covalent bonding and fixed atomic positions in the crystal structure, which determine high hardness values, is outlined. The materials investigated to date are described and new potentially superhard materials are presented. No material that is thermodynamically stable under ambient conditions and composed of light (small) atoms will have a hardness greater than that of diamond. Materials with hardness values similar to that of cubic boron nitride (cBN) can be obtained. However, increasing the capabilities of the high-pressure devices could lead to the production of better quality cBN compacts without binders.

${\mathrm{SnO}}_2$ was investigated at pressures up to 49 GPa by angle-dispersive x-ray diffraction using an imaging plate. Three phase transitions were observed on compression. Rutile-type ${\mathrm{SnO}}_2$ underwent a second-order transition to a ${\mathrm{CaCl}}_2$-type phase at 11.8 GPa under hydrostatic conditions, as determined from the pressure dependence of the spontaneous strain. This transition was observed at significantly lower pressures under nonhydrostatic conditions. A second transition to an α-${\mathrm{PbO}}_2$-type phase was observed to begin above 12 GPa under nonhydrostatic conditions; however, only a small amount of this phase was obtained. Both the α-${\mathrm{PbO}}_2$-type and the ${\mathrm{CaCl}}_2$-type phases transformed to a modified fluorite-type phase, space group Pa3-bar, above 21 GPa. Upon decompression, retransformation was observed and the sample recovered under ambient conditions consisted of a mixture of the rutile-type and α-${\mathrm{PbO}}_2$-type phases. The structures of the rutile, ${\mathrm{CaCl}}_2$ and modified fluorite type phases were refined in situ by the Rietveld method allowing the structural evolution of ${\mathrm{SnO}}_2$ to be followed as a function of pressure. The relationships between the high-pressure structures of ${\mathrm{SnO}}_2$ are discussed using group theory and potential transformation pathways identified. The transition sequence observed for tin dioxide has important implications for the high-pressure behavior of other rutile-structured compounds.

The potentially hard oxides ${\mathrm{ZrO}}_2$ and ${\mathrm{HfO}}_2$ are investigated using ab initio electronic structure calculations and structural properties of the lowest phases compared. In general there are strong similarities between some phases apart from the softer monoclinic baddeyelite phase, which in ${\mathrm{HfO}}_2$ is the lowest energy phase with a bulk modulus almost twice that of ${\mathrm{ZrO}}_2.$ Other differences relate to the formation of the first orthorhombic phase, especially the inter-relation between $\mathrm{Pbc}21\mathrm{}$ and $\mathrm{Pbca}$ structures. The bulk moduli of the high-pressure, cotunnite-type phases are in good agreement with experimental results. The present calculations confirm that these phases are highly incompressible and are thus good candidates for hard materials.

A joint experimental–modelling study has demonstrated a large flexibility of the MIL-47(V^IV) upon mechanical pressure which strongly deviates from its rigid behaviour in presence of guest molecules. A structural transition suspected by mercury intrusion and further confirmed by X-ray powder diffraction and molecular dynamics simulations, leads to a closed MIL-47(V^IV) form never observed so far corresponding to a cell contraction of up to 43%. The microscopic key features that govern this transition are then elucidated from complementary Raman experiments.