Julian Haines

The search for superhard materials

Superhard materials such as diamond or cubic boron nitride are widely used in industry for fast machining and drilling. A great challenge to scientists is to make a material which could be harder than diamond. A short review of the compounds predicted to be superhard is made. A new approach based on recent experimental high-pressure results is to consider heavier-atom compounds crystallizing in dense structures; possible new materials are proposed.

Second-order rutile-type to CaCl2-type phase transition in β-MnO2 at high pressure

Abstract MnO2 was studied in a diamond anvil cell by angle-dispersive X-ray diffraction up to 46 GPa using both an imaging plate and film. A phase transition from the tetragonal rutile-type phase to an orthorhombic phase was observed at below 1 GPa. The absence of discontinuity in the lattice constants and the zero volume change are indicative of a second order transition and the pressure dependence of the spontaneous strain indicates a transition pressure of 0.3 GPa. Rietveld refinements of the structure at high pressure confirmed that the orthorhombic phase has a CaCl2-type structure, space group Pnnm, a = 4.437(3), b = 4.312(3), c = 2.862(2) A , x(O) = 0.339(1), y(O) = 0.278(2) with Z = 2 at 7.3 GPa . The angle of rotation of the MnO6 octahedra about their two-fold axes parallel to c was found to increase from 3° at 1.0 GPa to 12° at 9.4 GPa. The p-V data for the orthorhombic phase were fitted using a Birch-Murnaghan equation of state yielding a bulk modulus of 328(18) GPa with a first pressure derivative of 4(2). The changes observed at this ferroelastic transition in MnO2 present a model for the corresponding phase transition in stishovite-SiO2 at high pressure.

A moganite-type phase in the silica analog phosphorus oxynitride

Abstract A new polymorph of phosphorus oxynitride (PON) a silica analog has been recovered at ambient pressure by quenching after a treatment at 850 °C under a pressure of 2.5 GPa using cristobalite- or quartz-type phases as starting materials. This PON polymorph is a thermodynamically stable phase with its own stability field in a P-T diagram. The structure of this PON phase was refined by the Rietveld method from an X-ray powder diffractogram. It is isostructural with “moganite” (SiO2). The discovery of this PON polymorph should stimulate a renewed interest in the occurrence of this phase in the silica system, because “moganite” may have a small but defined P-T stability field. These results confirm the structure of “moganite” as a new structure-type in AX2 compounds.

Hardness and elasticity in cubic ruthenium dioxide

The Knoop hardness of the highly incompressible cubic phase of ruthenium dioxide was found to be 19–20 GPa from indentation tests. This value scales well with the shear modulus approximated by the elastic constant C44 of 144 GPa obtained from Brillouin scattering measurements. This work provides evidence that the shear modulus is a better indicator of hardness than the bulk modulus for ionic and covalent materials.

Structural characterisation of the pa3̄-type, high pressure phase of ruthenium dioxide

Cubic Pa3-type RuO2 was prepared from the ambient pressure, rutile-structured phase at 20 GPa and 1100 °C in a multianvil device. The structure of this Pa3 phase was refined by time-of-flight, neutron powder diffraction on the quenched sample yielding a cell constant a = 4.85892(3) A and an oxygen positional parameter u = 0.35115(8). The ruthenium cation is rhombohedrally coordinated with six anions at 1.9893(4) A and two more distant anions at 2.9552(4) A. The minimum interpolyhedral OO distance of 2.5045(5) A in this structure is the shortest known in any solid and is shorter than the intrapolyhedral OO distances, which are of 2.6208(5) A. These short distances are the origin of the very low compressibility of this oxide phase, which approaches that of diamond. The Raman spectrum of Pa3-type RuO2 is consistent with group-theoretical calculations.

The high-pressure behaviour of the “moganite” polymorph of SiO2

The high-pressure behaviour of the “moganite” polymorph of SiO 2 has been investigated by angle-dispersive, X-ray powder diffraction in a diamond anvil cell at room temperature. The bulk modulus was calculated from the P-V data: B 0 = 32.2(3) GPa with B’ 0 fixed to 5.2 as for quartz. The molar volume of moganite becomes lower than the volume of quartz above 5 GPa. Pressure-induced amorphisation occurred progressively; it was complete above 25 GPa. Upon decompression, amorphisation was partially reversible if the maximum pressure reached was below 25–30 GPa and irreversible when the pressure reached was above 45 GPa. The pressure-induced amorphisation of the quartz and moganite polymorphs of silica and phosphorus oxynitride PON, a silica analogue, is related to the distortion of the tetrahedra and, in the case of silica, to the presence of a much denser phase, in which the cation has a higher coordination number.