Lkhamsuren Bayarjargal

Synthesis of Binary Transition Metal Nitrides, Carbides and Borides from the Elements in the Laser-Heated Diamond Anvil Cell and Their Structure-Property Relations

Transition metal nitrides, carbides and borides have a high potential for industrial applications as they not only have a high melting point but are generally harder and less compressible than the pure metals. Here we summarize recent advances in the synthesis of binary transition metal nitrides, carbides and borides focusing on the reaction of the elements at extreme conditions generated within the laser-heated diamond anvil cell. The current knowledge of their structures and high-pressure properties like high-(p,T) stability, compressibility and hardness is described as obtained from experiments.

Diamond formation from CaCO3 at high pressure and temperature

We studied the decomposition of CaCO3 by laser heated diamond anvil cell experiments at pressures between 9 and 21 GPa up to 4000 K. The quenched samples were characterized by micro-Raman spectroscopy. From the results we conclude that calcite decomposes into CaO + O2 + C across the whole pressure range investigated at temperatures around 3500 K, initially forming graphite nanoparticles with dimensions around 3–11 nm. The graphite particles may aggregate and transform into diamond with dimensions around 20 nm if the sample is annealed in the diamond stability field. We therefore conclude that diamond can be crystallized directly from carbonatitic melts by decomposition of CaCO3 at high pressures and temperatures, and that phase diagrams showing a decomposition into CaO + CO2 in this P , T -range need to be reevaluated.

Temperature dependence of Bi2O3 structural parameters close to the α–δ phase transition

This article presents the results of in situ X-ray powder diffraction, Raman spectroscopy, and electrical impedance spectroscopy of the α-phase of Bi2O3, at 0.1 MPa in the temperature range below and above the α–δ-phase transition. This work demonstrated subtle nonlinear temperature variations of the cell parameters, of the hard-mode Raman shift, and of the activation energy of electrical conductivity in the temperature range about 100–120°C below the α–δ phase transition temperature T Tr ≈ 725°C in Bi2O3. At T < 600°C, the linear variation of the inverse dielectric susceptibility (χ −1) correlates well with the hard mode frequency shift Δ(ω 2) of Raman A1g mode as Δ(χ −1)/Δ(ω 2) ≈ 5.5 × 10−7 cm2. A structural model describing the mechanism of O2− anion distribution and electric dipole disordering in the vicinity of T Tr is discussed.

Influence of deviatoric stress on the pressure-induced structural phase transition of ZnO studied by optical second harmonic generation measurements

The pressure-induced B4⇆B1 structural phase transition of ZnO has been studied with the second harmonic generation (SHG) technique. Measurements in nonhydrostatic and hydrostatic pressure transmitting media show slightly different transition pressures (9–11 GPa) and a different pressure dependence of the SHG intensities. These observations are consistent with the presence of a tetragonal and hexagonal intermediate phase as a result of hydrostatic and axial compression, respectively. In contrast to earlier work, it is shown that it is not necessary to use nanocrystalline starting material to be able to recover the B1 phase at ambient conditions.

The Local Atomic Structures of Liquid CO at 3.6 GPa and Polymerized CO at 0 to 30 GPa from High‐Pressure Pair Distribution Function Analysis

The local atomic structures of liquid and polymerized CO and its decomposition products were analyzed at pressures up to 30 GPa in diamond anvil cells by X-ray diffraction, pair distribution function (PDF) analysis, single-crystal diffraction, and Raman spectroscopy. The structural models were obtained by density functional calculations. Analysis of the PDF of a liquid CO-rich phase revealed that the local structure has a pronounced short-range order. The PDFs of polymerized amorphous CO at several pressures revealed the compression of the molecular structure; covalent bond lengths did not change significantly with pressure. Experimental PDFs could be reproduced with simulations from DFT-optimized structural models. Likely structural features of polymerized CO are thus 4- to 6-membered rings (lactones, cyclic ethers, and rings decorated with carbonyl groups) and long bent chains with carbonyl groups and bridging atoms. Laser heating polymerized CO at pressures of 7 to 9 GPa and 20 GPa resulted in the formation of CO(2).

Influence of grain size, surface energy, and deviatoric stress on the pressure-induced phase transition of ZnO and AlN

The high pressure behavior of aluminum nitride (AlN) and zinc oxide (ZnO) nanocrystals was studied up to 30 GPa using second harmonic generation. ZnO (10 nm) crystals transform to the high pressure B1 phase at 16.6 GPa, ≈5 GPa higher than the corresponding value for a bulk sample. The transition of AlN (20 nm) and AlN (100 nm) occurred at 14 and 21.5 GPa at lower values than for bulk samples. Under non-hydrostatic pressure conditions, the transition pressures of ZnO (10 nm) and AlN (100 nm) decrease to 12.5 and 18 GPa, respectively. We determined the surface energy ( and ) of the B1 polymorphs. We show that the main reason for the size-dependent decrease of the transition pressure of AlN nanocrystals is due to the higher surface energy of the B4 phase relative to the surface energy of the B1 phase. We predict that it is possible to quench or synthesize pure B1 AlN to ambient conditions if the grain size is less than 8.5 nm.

High (pressure, temperature) phase diagrams of ZnO and AlN from second harmonic generation measurements

The pressure-induced B4 → B1 structural phase boundaries of ZnO and AlN have been determined with the second harmonic generation (SHG) technique at high temperature. The SHG measurements of AlN show that between 15.6 and 18 GPa, the phase boundary has a negative slope of nearly −627 K/GPa, and that below 15.6 GPa, the slope is significantly smaller (−77 K/GPa). ZnO has a phase boundary with a negative slope of nearly −1427 K/GPa around 5.3–6 GPa and −228 K/GPa below 5 GPa. The phase transition pressure of AlN is sensitive to deviatoric stress and varies from 18 to 24.5 GPa.

Synthesis and Characterization of the High-Pressure Nickel Borate γ-NiB4O7

γ-NiB4O7 was synthesized in a high-pressure/high-temperature experiment at 5 GPa and 900 °C. The single-crystal structure analysis yielded the following results: space group P6522 (No. 179), a = 425.6(2), c = 3490.5(2) pm, V = 0.5475(2) nm3, Z = 6, and Flack parameter x = -0.010(5). Second harmonic generation measurements confirmed the acentric crystal structure. Furthermore, γ-NiB4O7 was characterized via vibrational as well as single-crystal electronic absorption spectroscopy, magnetic measurements, high-temperature X-ray diffraction, differential scanning calorimetry, and thermogravimetry. Density functional theory-based calculations were performed to facilitate band assignments to vibrational modes and to evaluate the elastic properties and phase stability of γ-NiB4O7.

Pressure-induced magnetic phase transition in Cr2O3 determined by second harmonic generation measurements

We demonstrate that pressure-induced magnetic phase transitions can be detected by optical second harmonic generation (SHG) measurements in diamond anvil cells and show that the transition from an acentric to a centrosymmetric magnetic structure occurs in Cr2O3 at 10(1) GPa. The pressure dependence of the Neel temperature, dTN/dP = −1.0(5) K/GPa from our SHG measurements, which differs significantly from earlier results where dTN/dP ranged from −16 K/GPa to +15 K/GPa.

Synthesis and Crystal Structure of the Acentric Indium Borate InB6O9(OH)3

The new acentric indium borate InB6O9(OH)3 was synthesized in a Walker-type multianvil apparatus at extreme pressure and temperature conditions of 12.3 GPa and 1500 °C. Single-crystal X-ray diffraction provided the data for the crystal structure solution and refinement. InB6O9(OH)3 crystallizes in the orthorhombic space group Fdd2 ( Z = 8) with the lattice parameters a = 39.011(8), b = 4.4820(9), c = 7.740(2) Å, and V = 1353.3(5) Å3. The structure of InB6O9(OH)3 is basically built of corner-sharing BO4 tetrahedra and isolated InO6 octahedra. The presence of hydroxyl groups was confirmed with vibrational spectroscopic methods (IR and Raman). Furthermore, the second harmonic signal of an InB6O9(OH)3 powder sample yielded more than twice the intensity of quartz.