High-Pressure Synthesis and Crystal Structure of MoC-Type Tungsten Nitride by Nitridation with Ammonium Chloride

Abstract

A novel tungsten nitride, MoC-type WN, was synthesized at 6 GPa and 1200 °C via nitridation of tungsten by ammonium chloride as a nitrogen source. This compound is isostructural with γ′-MoC, which has a hexagonal structure with a space group of P63/mmc (No. 194). Micrometer-sized single crystals of MoC-type WN were grown in molten ammonium chloride flux. In addition, NaCl-type WN and WC-type WN were synthesized via nitridation by ammonium chloride at 6 GPa and 1000 °C. Ammonium chloride is appropriate as a nitrogen source for nitride synthesis under high pressure. The new WN phase crystallizes in the hexagonal structure with unit cell parameters of a = 2.89248(2) Å and c = 10.17069(7) Å. The chemical formula of MoC-type WN refined by the Rietveld analysis from powder X-ray diffraction data was WN0.60(1). The zero-pressure bulk modulus, K 0, of MoC-type WN was determined to be 338(3) GPa, which can be expected to be a hard material.

Highly Electron-Doped TaON Single-Crystal Growth by a High-Pressure Flux Method

“…19−21 Facet planes of a crystal can be sometimes controlled by changing flux, and this is also true for crystal growth under high pressure (e.g., hexagonal BN 22 ). Hinted by a recent work on WN single crystals grown at 6 GPa using NH 4 Cl, which acts as a flux as well as a nitrogen source, 23 we attempted to grow TaON single crystals using a NH 4 Cl flux (6 GPa, 1200 °C). In this case, we obtained black rod-shaped crystals, formed as a bundle of thin needles (Figure 1d), where the growth direction is [01−1], as determined from XRD.…”

Structure stabilization effect of vacancies and entropy in hexagonal WN

“…The results indicate that mechanical properties generally decrease with an increase of the vacancy concentration, except for the hardness of the WC-type WN, which increases with a vacancy concentration up to 8 at%. Experimentally, Sasaki et al (2019) and Kawamura et al (2018) have found that the WC-type WN phase crystallizes in the structures with unit-cell parameters of a = 2.898 and c = 2.831 A ˚, and a = 2.822 A ˚and c = 2.789 A ˚; the bulk moduli were determined to be 383 and 342 GPa, respectively. The calculated bulk moduli shown in Fig.…”

Crystal and Electronic Structure of U7Te12-Type Tungsten Nitride Synthesized under High Pressure

“…42 Tungsten atoms in the U 7 Te 12 -type W 7 N 12 exist in 8-and 9-fold coordination by nitrogen. Reported tungsten nitrides, which were synthesized below 10 GPa, such as WC-type WN, 23 r-W 2 N 3 , 21 and the starting sample of MoC-type WN 0.6 , 24 have a lower coordination number (CN) of 6 (Figure S7). U 7 Te 12 -type W 7 N 12 is considered to have a higher coordination number, because it was synthesized at a higher pressure of 70 GPa.…”

“…The starting material used in this experiment follows the instructions described by Sasaki and colleagues in ref . The nitridation of tungsten was carried out by using a DIA-type cubic multianvil large volume press installed at Nagoya University.…”

“…Surprisingly, the discovery of tungsten nitrides at pressure above P > 10 GPa has been only sparsely investigated. According to previous studies for the W–N system under high pressure, new tungsten nitrides have been synthesized via a metathesis reaction or using a solid nitrogen source (NH 4 Cl) and compressed using a large volume press below 10 GPa. − On the other hand, Salke et al have recently discovered a WN 6 constructed of armchairlike hexazine (N 6 ) rings with covalent N–N single bonds at pressures above P > 120 GPa after pulsed laser heating of tungsten in molecular nitrogen using the diamond anvil cell (DAC) . They also found signs of an unknown tungsten nitride phase at lower pressures of P ≈ 90–118 GPa after thermal annealing.…”