Martin C. Wilding

Pressure-induced amorphization and an amorphous–amorphous transition in densified porous silicon

Crystalline and amorphous forms of silicon are the principal materials used for solid-state electronics and photovoltaics technologies. Silicon is therefore a well-studied material, although new structures and properties are still being discovered. Compression of bulk silicon, which is tetrahedrally coordinated at atmospheric pressure, results in a transition to octahedrally coordinated metallic phases. In compressed nanocrystalline Si particles, the initial diamond structure persists to higher pressure than for bulk material, before transforming to high-density crystals. Here we report compression experiments on films of porous Si, which contains nanometre-sized domains of diamond-structured material. At pressures larger than 10 GPa we observed pressure-induced amorphization. Furthermore, we find from Raman spectroscopy measurements that the high-density amorphous form obtained by this process transforms to low-density amorphous silicon upon decompression. This amorphous–amorphous transition is remarkably similar to that reported previously for water, which suggests an underlying transition between a high-density and a low-density liquid phase in supercooled Si (refs 10, 14, 15). The Si melting temperature decreases with increasing pressure, and the crystalline semiconductor melts to a metallic liquid with average coordination ∼5 (ref. 16).

Detection of First-Order Liquid/Liquid Phase Transitions in Yttrium Oxide-Aluminum Oxide Melts

We combine small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS) with aerodynamic levitation techniques to study in situ phase transitions in the liquid state under contactless conditions. At very high temperatures, yttria-alumina melts show a first-order transition, previously inferred from phase separation in quenched glasses. We show how the transition coincides with a narrow and reversible maximum in SAXS indicative of liquid unmixing on the nanoscale, combined with an abrupt realignment in WAXS features related to reversible shifts in polyhedral packing on the atomic scale. We also observed a rotary action in the suspended supercooled drop driven by repetitive transitions (a polyamorphic rotor) from which the reversible changes in molar volume (1.2 ± 0.2 cubic centimeters) and entropy (19 ± 4 joules mole–1 kelvin–1) can be estimated.

Polyamorphic transitions in yttria–alumina liquids

Abstract There has been much recent discussion given to the phenomenon of polyamorphism where distinct, different states of amorphous liquids and solids are observed as a function of density. Underlying this phenomenon is the possibility of a first order liquid–liquid phase transition driven by the density and entropy differences between the two amorphous phases. Differential scanning calorimetry (DSC) of aluminate glasses containing 32–20 mol% Y2O3 has enabled characterisation of the thermodynamic differences between the high and low density amorphous phases. The DSC measurements show a glass transition temperature with onset at 1135 K. The width and the magnitude of jump in heat capacity at this glass transition indicates an extremely fragile liquid, that is there is a pronounced non-Arrhennian viscosity-temperature relation. There are two additional features of interest. A second glass transition at 1300 K is attributed to the low-density liquid (LDA) more stable at low temperature. This low-density liquid has a more Arrhenian viscosity temperature curve. A strong exothermic signal is seen between the two glass transitions and represents the transition between the supercooled, high-density liquid (HDA) and a low-density glass. Neutron scattering data suggest that the polyamorphic transition is accompanied by minor changes in the aluminate framework and more prominent changes in the local environment surrounding the yttrium ion, seen as an increase in the first neighbor yttrium-oxygen distance.

A neutron diffraction study of yttrium- and lanthanum-aluminate glasses

Abstract Neutron diffraction data for three aluminate glasses have been collected for a range of scattering vector from 0.3 to 25 A −1 . A single phase glass, representing the high-density glass polymorph of yttrium-aluminate (AY25) composition has a structure resembling that of levitated YAG liquid. This glass has a first-neighbor Al–O distance of 1.82 A and a first-shell coordination number of 4.16±0.21. The Y–O distance is represented by a single Gaussian peak centered at 2.28 A which yields a first-neighbor coordination number of 6.64±0.33. These values indicate an increase in volume on melting of YAG solid solution. An equivalent lanthanum-aluminate glass (La25), also produced from the stable high-temperature aluminate liquid, has an Al–O distance of 1.79 A and a coordination number of 4.55±0.23. The La–O distance is increased due to an increase in ionic radius for La(III). The magnitude of this La–O peak is lower than expected and there is evidence for a second La–O distance at 2.79 A. The T ( r ) for the lanthanum-aluminate glass shows stronger features than the yttrium-aluminate glass at radial distances greater than 3 A; these suggest a higher degree of order in the lanthanum-aluminate liquid. A two-phase yttrium-aluminate (AY20) glass, which represents a partly complete transition between a high-density liquid and a lower density polymorph, has an Al–O distance of 1.81 A and a mean Al–O coordination number of 4.38±0.22. There is an increase in the Y–O radial distance on transition and also additional mid-range ordering. This mid-structure is similar to that seen in the La25 glass. The data show that rare earth aluminate liquids are dominated by a tetrahedral aluminate framework and that the liquid–liquid (transition) in the yttrium-aluminate liquids results from differences in packing among AlO 4 and Y–O polyhedra and not from a change in the mean Al–O coordination number.

High temperature calorimetric studies of the heat of solution of La2O3 in silicate liquids

Abstract High temperature calorimetry at 1760 K has been used to measure the heat of solution of La2O3 in a series of simple alkali and alkaline earth silicate liquids. The heat of solution in these solvents is strongly exothermic and varies as a function of liquid composition. However, the variation of the heat of solution does not follow simple trends related to cation size or charge and varies little with La2O3 concentration. The variation of heat of solution with composition of the liquid reflects the ability of La(III) to perturb the transient silicate framework and compete with other cations for oxygen. This complex pattern of melt energetics is consistent with recent spectroscopic measurements which suggest extreme perturbation of the silicate framework by La(III), sufficient to isolate oxygen from silicon. This interpretation suggests the presence of phase-ordered regions rich in La(III) consistent with incipient liquid–liquid immiscibility suggested by previous calorimetric studies. Within error the heat capacity of La-bearing silicate liquids is the same over the super-cooled liquid range as in the stable liquid, with no evidence for the large heat capacities associated with melt restructuring. Thus the energetics of the liquid are dominated by the exothermic reactions which form La-clusters and these phase-ordered regions do not dissociate as temperature increases up to 1760 K.

Cooling rates of hyaloclastites: applications of relaxation geospeedometry to undersea volcanic deposits

Abstract Glass fragments from three different hyaloclastites have been used to evaluate the range of cooling rates experienced by undersea volcanic deposits. We found that the glass fragments retain structures with a range of apparent quench rates from 25 to 0.15 K min–1. The most rapid cooling rates are interpreted to be those resulting from cooling of the lava near the water interface. Simple conductive cooling models produce a range of quench rates comparable to those of the more rapidly cooled samples. The very slow apparent quench rates are unlikely to result from simple linear cooling through the glass transition, because of the onset of crystallization; instead, they are indicators of a more complex thermal history that involves the annealing of glasses at temperatures within the glass transition interval for a dwell time sufficient to allow the relaxation of the glass to lower temperature structures. The thermal history recorded in these samples illustrates the complexity of eruptive processes and demonstrates that quench rates for natural glasses retain information relevant to more complex cooling models.

Polyamorphism in aluminate liquids

McMillan, P. F., Wilson, M., Wilding, M. C. (2003). Polyamorphism in aluminate liquids. Journal of Physics: Condensed Matter, 15 (36), 6105-6121 RAE2008

Thermodynamic and structural aspects of the polyamorphic transition in yttrium and other rare-earth aluminate liquids

A first-order transition between two liquids has been reported in yttrium–aluminate liquids close to Y3Al5O12 (YAG) composition. This transition is seen as the nucleation and growth of a low-density phase in a matrix of a higher density liquid when Y2O3–Al2O3 liquids are cooled below the liquidus. Both liquids are quenched to glass before the transition is complete. Analysis of the resulting composite samples shows that the two glasses are identical in composition although the two glasses differ in density by 4%. There are also mechanical differences between the two glasses; the low-density glass is more resistant to polishing, while the high-density glass becomes highly scratched.

Density-driven structural transformations in network forming glasses: a high-pressure neutron diffraction study of GeO2 glass up to 17.5 GPa

The structure of GeO(2) glass was investigated at pressures up to 17.5(5) GPa using in situ time-of-flight neutron diffraction with a Paris-Edinburgh press employing sintered diamond anvils. A new methodology and data correction procedure were developed, enabling a reliable measurement of structure factors that are largely free from diamond Bragg peaks. Calibration curves, which are important for neutron diffraction work on disordered materials, were constructed for pressure as a function of applied load for both single and double toroid anvil geometries. The diffraction data are compared to new molecular-dynamics simulations made using transferrable interaction potentials that include dipole-polarization effects. The results, when taken together with those from other experimental methods, are consistent with four densification mechanisms. The first, at pressures up to approximately equal 5 GPa, is associated with a reorganization of GeO(4) units. The second, extending over the range from approximately equal 5 to 10 GPa, corresponds to a regime where GeO(4) units are replaced predominantly by GeO(5) units. In the third, as the pressure increases beyond ~10 GPa, appreciable concentrations of GeO(6) units begin to form and there is a decrease in the rate of change of the intermediate-range order as measured by the pressure dependence of the position of the first sharp diffraction peak. In the fourth, at about 30 GPa, the transformation to a predominantly octahedral glass is achieved and further densification proceeds via compression of the Ge-O bonds. The observed changes in the measured diffraction patterns for GeO(2) occur at similar dimensionless number densities to those found for SiO(2), indicating similar densification mechanisms for both glasses. This implies a regime from about 15 to 24 GPa where SiO(4) units are replaced predominantly by SiO(5) units, and a regime beyond ~24 GPa where appreciable concentrations of SiO(6) units begin to form.

Development of structural order during supercooling of a fragile oxide melt.

The authors have studied the structural evolution of the fragile glass-forming liquid CaAl2O4 during supercooling from the stable liquid phase to the cold glass below Tg. The evolution is characterized by a sharpening of the first diffraction peak and a shortening of the average nearest-neighbor bond length around 1.25Tg, indicating an increase in the degree of both intermediate-range and short-range orders occurring close to the dynamical crossover temperature. The cooling curve developed a kink at this temperature, indicating a simultaneous change in thermodynamic properties.