Andreas Zerr

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.

Spinel‐Si3N4: Multi‐Anvil Press Synthesis and Structural Refinement

The third known polymorph of silicon nitride, which is cubic and was only recently discovered, has been prepared from two further, different precursors—Si2N2(NH) and a-Si3N4—in a high-pressure, high-temperature synthesis using multi-anvil presses. The synthesis and characterization of the products is described, which included a structural determination by Rietveld refinement of powder X-ray diffraction data. Spinel-type c-Si3N4 is significantly harder than the α and β phases and may possibly find applications as an ultrahard material.

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.

Partitioning of nickel and cobalt between silicate perovskite and metal at pressures up to 80 GPa

The high abundance of both nickel and cobalt and the chondritic Ni/Co ratio found in samples derived from the Earths mantle are at odds with results from laboratory-based partitioning experiments conducted at pressures up to 27 GPa (refs 1,2). The laboratory results predict that the mantle should have a much lower abundance of both Ni and Co and a considerably lower Ni/Co ratio owing to the preferential partitioning of these elements into the iron core. Two models have been put forward to explain these discrepancies: homogeneous accretion, (involving changes of the Ni and Co partition coefficients with oxygen and sulphur fugacities, pressure and temperature) and heterogeneous accretion, (the addition of chondritic meteorites to the mantle after core formation was almost complete). Here we report diamond-cell experiments on the partitioning of Ni and Co between the main lower-mantle mineral ((Mg,Fe)SiO3-perovskite) and an iron-rich metal alloy at pressures up to 80 GPa (corresponding to a depth of ∼1,900 km). Our results show that both elements become much less siderophilic with increasing pressure, such that the abundance of both Ni and Co and the Ni/Co ratio observed in samples derived from the Earths mantle appear to indeed be consistent with a homogeneous accretion model.

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.

Hydrostatic compression of γ-(Mg0.6, Fe0.4)2SiO4 to 50.0 GPa

The bulk modulus, K0, and its pressure derivative K′0, of γ-(Mg0.6, Fe0.4)2SiO4 have been accurately determined to 50.0 GPa under hydrostatic conditions at room temperature in a diamond cell using synchrotron radiation. Our results agree with Brillouin and ultrasonic measurements on γ-Mg2SiO4 at low pressure, indicating normal elastic behaviour in the metastable pressure range of this high pressure mineral. Our values of K0 and k′0 are 183.0 GPa and 5.4, respectively.

Elastic moduli and hardness of c-Zr2.86(N0.88O0.12)4 having Th3P4-type structure

The equation of state of the recently discovered oxygen-bearing cubic zirconium (IV) nitride, c-Zr2.86(N0.88O0.12)4, was measured at room temperature in a diamond anvil cell using x-ray powder diffraction combined with synchrotron radiation. From these studies the bulk modulus B0=219(13)GPa and its first pressure derivative B0′=4.4(1.0) [or B0=223(5)GPa, for B0′ fixed at 4] were obtained. Applying nanoindentation techniques the reduced modulus Er≈220GPa and hardness H≈18GPa were measured for porous c-Zr2.86(N0.88O0.12)4. The shear modulus of c-Zr2.86(N0.88O0.12)4 was estimated to be at least G0=96(13)GPa using the experimental data of B0 and Er, exclusively.

Equation of state and structural phase transition in FeBO3 at high pressure

The evolution of X-ray diffraction patterns in FeBO3 under high pressures up to 63 GPa has been investigated at room temperature in a diamond anvil cell. A structural phase transition at a pressure of 53±2 GPa was found for the first time. The transition is of the first-order type with a hysteresisless drop of the reduced unit cell volume of about 8.6%. Apparently, the transition is isostructural. At pressures below the transition, the equation of state for FeBO3 was fitted. In the third-order approximation of the Birch-Murnagan equation of state, the bulk modulus K and its first pressure derivative K′ were found to be 255±25 GPa and 5.0±1.2, respectively.