George Serghiou

Synthesis of cubic silicon nitride

Silicon nitride (Si3N4) is used in a variety of important technological applications. The high fracture toughness, hardness and wear resistance of Si3N4-based ceramics are exploited in cutting tools and anti-friction bearings; in electronic applications, Si3N4 is used as an insulating, masking and passivating material. Two polymorphs of silicon nitride are known, both of hexagonal structure: α- and β-Si3N4. Here we report the synthesis of a third polymorph of silicon nitride, which has a cubic spinel structure. This new phase, c-Si3N4, is formed at pressures above 15 GPa and temperatures exceeding 2,000 K, yet persists metastably in air at ambient pressure to at least 700 K. First-principles calculations of the properties of this phase suggest that the hardness of c-Si3N4 should be comparable to that of the hardest known oxide (stishovite, a high-pressure phase of SiO2), and significantly greater than the hardness of the two hexagonal polymorphs.

Synthesis of a cubic Ge3N4 phase at high pressures and temperatures

The two known phases of germanium nitride (Ge3N4) have hexagonal and trigonal symmetries and consist of three-dimensional networks of corner-connected Ge–N tetrahedra. A new cubic spinel phase (space-group Fd3m, a0=8.3 A, Z=8, ρ=6.36 g/cm3) containing Ge–N octahedra and tetrahedra in a 2:1 ratio was synthesized from elemental germanium and molecular nitrogen starting materials in a laser-heated diamond-anvil cell above 14 GPa. This phase is isostructural to the recently discovered cubic spinel phase of Si3N4.

Melting of CaSiO3 perovskite to 430 kbar and first in‐situ measurements of lower mantle eutectic temperatures

Melting temperatures of CaSiO3 perovskite were measured between 160 and 430 kbar in a diamond anvil cell under hydrostatic, inert conditions using CO2-laser heating. The melting temperatures are higher than those obtained in previous YAG-laser heating experiments [Shen and Lazor, 1995], where chemical reactions of the sample with rhenium might have caused a lowering in the melting temperatures. The melting temperatures of CaSiO3 perovskite are slightly higher than those of (Mg,Fe)SiO3 perovskite [Zerr and Boehler, 1993], thus making the two major minerals of the lower mantle Mg-Si-perovskite and magnesiowusite the low-melting components. First in-situ measurements of the eutectic melting temperatures of these two minerals are presented. The data suggest that eutectic melting depression in the lower mantle may be insignificant. An alternate solution for the extrapolation of the measured melting temperatures to higher pressures, based on recent observations on the highly compressible alkali halides, yields melting temperatures at the bottom of the lower mantle below 6000 K.

The Coesite‐Stishovite Transition in a laser‐heated diamond cell

The coesite-stishovite phase boundary in the temperature range 2300–2630°C was examined in a CO2-laser heated diamond anvil cell using argon as a pressure medium. Phase identification was provided by Raman spectroscopy of the temperature quenched samples at pressure. Our results suggest a coesite-stishovite phase boundary curve described by P(GPa) = 8.1 + 0.001T(°C) in good agreement with Yagi and Akimotos (1976) estimate at lower temperatures. The slope of this curve is less than one half that determined recently by Zhang et al. [1993, 1994] in a multi-anvil press.

Decomposition of alkanes at high pressures and temperatures

The stability of methane (CH4), ethane (C2H6), octane (C8H18), decane (C10H22), octadecane (C18H38), and nonadecane (C19H40) were studied in a CO2-laser heated diamond anvil cell at pressures up to 25 GPa and temperatures up to about 7300 K. Methane and ethane were found to decompose to form hydrogen and diamond. Substantially greater yields of diamond were obtained from the longer chain alkanes.

Reversible coordination changes in crystalline silicates at high pressure and ambient temperature

In silicates, generally the less dense open structures of lower pressure polymorphs have four-coordinated silicon whereas the denser more close-packed structures have six-coordinated silicon. Transformation from the fourfold coordinated crystals to the denser sixfold coordinated structures upon compression usually requires an accompanying high temperature that is much greater than ambient. We report here a first-order, reversible SiO4 -SiO6 coordination change in a crystalline silicate (MgSiO3 ) orthoenstatite at ambient temperature and quasi-hydrostatic conditions using Raman spectroscopic measurements in a diamond cell to 70 GPa. Annealing experiments using a CO2 laser, which controlled defect concentration, indicate that the four- to sixfold coordination change is facilitated by defects in the structure which may serve as nucleation sites for the formation of the octahedrally coordinated silicon phase.

Synthesis of a Nitrogen-Stabilized Hexagonal Re3ZnNx Phase Using High Pressures and Temperatures

High pressure can induce profound changes in solids. A significant barrier to new alloys and ceramics, however, is that targeted starting materials may not react with each other, even with the help of pressure. We use nitrogen, in a new capacity, to incorporate two otherwise unreactive elements, Re and Zn, in the same structure when pressure alone does not suffice, without nitrogen altering the resulting backbone structure. Synthesis experiments up to 20 GPa and 1800 K show that while no Re-Zn alloy or solid solution is formed, a novel Re(3)ZnN(x) ordered solid solution is formed, at 20 GPa, with nitrogen occupying Re-coordinated cages. We put forth that unlike pure Re(3)Zn, our novel hexagonal Re(3)ZnN(x) structure is stabilized by nitrogen bond formation with rhenium. Pressure lifts the pronounced ambient Zn anisotropy, making it more compatible with Re and likely facilitating incorporation of the structure-stabilizing nitrogen anion. This methodology and result denote further options for removing impasses to material preparation, thus opening new avenues for synthesis. These can also be pursued with other ions including carbon, hydrogen, and oxygen, in addition to nitrogen.

Metal nitride and alloy synthesis using high pressures and temperatures

In a laser-heated diamond cell, we prepare high-density metal nitrides by reaction of an element with a nitrogen pressure medium, and binary alloys by reaction of one element with another in preferably an inert pressure medium such as argon or neon. One of our objectives is to establish the parameters required for the synthesis of both metal nitrides and alloys in a multianvil press using elemental starting materials. We have already synthesized transition metal nitrides in a multianvil press using elemental starting materials, including hexagonal nickel nitride and alkali rhenium nitrides. Here, starting with the elements, we report the synthesis of magnesium sodium iron nitride and magnesium iron nitride at pressures and temperatures up to 15 GPa and 1800 K, respectively, and recovery of a GeSi alloy from 15 GPa after heating the elements at 1500 K. We also briefly present focused ion beam processing of recovered reaction products tailored for electron microscopy analysis.

Possible superlattice formation in high-temperature treated carbonaceous MgB2 at elevated pressure

We report indications of a phase transition in carbonaceous MgB2 above 9 GPa at 300 K after stress relaxation by laser heating. The transition was detected using Raman spectroscopy and X-ray diffraction. The observed changes are consistent with a second-order structural transition involving a doubling of the unit cell along c and a reduction of the boron site symmetry. Moreover, the Raman spectra suggest a reduction in electron-phonon coupling in the slightly modified MgB2 structure consistent with the previously proposed topological transition in MgB2. However, further attributes including deviatoric stress, lattice defects, and compositional variation may play an important role in the observed phenomena.

Tuning between Mixing and Reactivity in the Ge−Sn System Using Pressure and Temperature

No bulk GeSn crystal existed prior to this work. Near 10 GPa the two elements resemble each other both electronically and structurally. Synthesis experiments at 10 GPa and 1500 K followed by annealing at 770 K using Ge and Sn starting materials and ex-situ analysis using transmission electron microscopy, scanning electron microscopy, and X-ray diffraction document the recovery of a Ge(0.9)Sn(0.1) solid solution (space group P4(3)2(1)2, a = 6.014 (1) A, c = 7.057 (1) A, Z = 12).