High-pressure synthesis, structure and properties of cubic zirconium(IV)- and hafnium(IV) nitrides

This thesis is concerned with recently discovered high-pressure (HP) zirconium- and hafnium nitrides having cubic Th3P4-type structure, c-M3N4 (M=Zr or Hf). These compounds belong to a rapidly growing group of new hard HP nitrides exhibiting advanced properties. The research was focused on (i) synthesis of macroscopic amounts of c-M3N4, (ii) investigation of their solid state structure, composition and morphology and (iii) measurement of properties related to potential industrial applications of these compounds as hard wear-resistant materials. Nitrogen-rich starting materials for high-pressure high-temperature (HP/HT) synthesis of c-M3N4, namely nanocrystalline powders of M3N4+x with distorted NaCl-type structure, had to be prepared in this work because they were not commercially available. They were obtained via HT ammonolysis of the corresponding metal dialkylamides, M[N(C2H5)2]4, at moderate temperatures up to 873 K. Both, c-Zr3N4 and c-Hf3N4, were synthesized from the nanocrystalline M3N4+x powders applying a pressure of 12 GPa and a temperature of 1873 K using a multi-anvil HP-apparatus. The products were characterized using various techniques including powder XRD (Rietveld refinement), TEM, EPMA, SEM/EDX and Raman spectroscopy. In the case of zirconium nitride, formation of a single phase crystalline material was verified by both XRD and TEM. The presence of a small amount of oxygen in the sample, revealed by EPMA, suggested the formation of oxygen-bearing zirconium(IV) nitride (or oxynitride) having Th3P4-type structure, c-Zr2.86(N0.88O0.12)4. The measured composition was found to correspond to the general formula Zr3-u(N1-uOu)4 which fulfils the electrical neutrality condition. A high quality of the Rietveld structure refinement of the powder XRD data for c-Zr2.86(N0.88O0.12)4 supported the above findings. Further, the assignment of the structure of the obtained oxygen-bearing zirconium nitride to the Th3P4-type was confirmed by Raman spectroscopic measurements. In contrast to zirconium nitride, a minor oxidation of the hafnium nitride sample led to formation of a mixture of oxygen-poor c-Hf3N4 and an oxidic material. The latter was evident from the XRD, EPMA, and SEM/EDX measurements. A detailed analysis of the powder XRD patterns and Rietveld refinement suggested that the oxidic material is comprised of a mixture of the known γ-Hf2N2O and oI-HfO2. The last part of the thesis concerned the investigation of the properties of the synthesized materials. The bulk moduli of c-Zr2.86(N0.88O0.12)4 and c-Hf3N4 were determined via their quasi-hydrostatic compression in a diamond anvil cell up to 45 GPa. The obtained values are B0=219 GPa (B'0=4.4) and B0=227 GPa (B'0=5.3) for c-Zr2.86(N0.88O0.12)4 and c-Hf3N4, respectively. The reduced elastic modulus, Er=224 GPa, of porous c-Zr2.86(N0.88O0.12)4 (volume fraction porosity of 0.3) was measured using nanoindentation techniques. Combining B0 and Er values, the lower limits of the shear modulus G0=96 GPa, and of the Young’s modulus E0=252 GPa were determined for the oxygen-bearing c-Zr3N4. The nanoindentation hardness and Vickers microhardness, HV(1), of the porous zirconium nitride sample were measured to be 18 GPa and 12 GPa, respectively. Using a relation between hardness and volume fraction porosity, suggested in the literature, the HV(1) of the fully dense c-Zr3N4 was estimated to exceed 25 GPa. The indentation fracture toughness of 3.2 MPa m^1/2 for porous c-Zr2.86(N0.88O0.12)4 was evaluated from the Vickers indentation cracks. The linear thermal expansion coefficient of the oxygen-bearing c-Zr3N4 was found to increase from 6.6×10^6 1/K at room temperature to about 14×10^6 1/K at 873 K. Onset of the material oxidation in air was observed at 773 K. Finally, the obtained results were compared with existing experimental and theoretical data for c-M3N4 and for other related technological materials and discussed with respect to potential industrial applications of c-M3N4

Synthesis of cubic zirconium(IV) nitride, c-Zr3N4, in the 6-8 GPa pressure region

We report on the lowermost pressure and temperature conditions where cubic zirconium(IV) nitride having Th3P4-type structure, c-Zr3N4, forms in a belt-type apparatus. This novel hard material having exceptional wear resistance by machining of low-carbon steels appears at pressures between 6.5 and 7.7 GPa when heated to 1400-1600 degrees C. Single-phase samples were obtained at P = 7.7 GPa thus indicating that the industrial scale synthesis of this innovative material is feasible. Chemical reaction of the sample material with the Pt-capsule led to formation of a Pt Zr alloy, as a minor admixture, which can potentially be used as a binder for cementing of polycrystalline c-Zr3N4 tools

High-pressure high-temperature behavior of polymer derived amorphous B-C-N

Dense diamond-like BCN compounds are of interest due to their extreme hardness and predicted excellent thermal and chemical stability, which are superior to those of diamond and c-BN. Here, we report on the high-pressure high-temperature (HP-HT) behavior of amorphous BC2N and BC4N -as potential precursors for HP-HT synthesis of diamond-like BCN. Prepared via hydroboration reaction of piperazine borane and pyridine borane, respectively, amorphous BC2N and BC4N are characterized by well-mixed B-N, C-C and C-N bonds, confirmed by XPS analysis. These BCN compositions were subjected to pressures between 5-12 GPa and temperatures up to 1700 °C using multi-anvil apparatus and toroid-type press. In- and ex-situ X-ray diffraction reveals the decomposition of BC4N to graphite and h-BN between 5 and 12 GPa above 500 °C, in contrast to BC2N which remains amorphous up to 1600 °C

Novel binary and ternary phases in the Si-C-N system

The present article reviews recent advances in synthesis of novel phases in the ternary Si-C-N system. A dense carbon nitride phase, C(2)N(2)(NH), was synthesized for the first time at high pressures and high temperatures in a laser heated diamond anvil cell (LH-DAC). Based on results of electron diffraction, EELS- and SIMS-measurements combined with theoretical calculations the structure of this new C-N-H compound was analysed to be a defect wurtzite structure of the sinoite (Si(2)N(2)O)-type. Farther, a variety of amorphous SiCN phases and the first ternary crystalline phases, namely Si(NCN)(2) and Si(2)N(2)(NCN), were synthesized at ambient pressure. In general, the high-pressure polymorphs of Si-C-N materials are predicted to exhibit a unique combination of high hardness, thermal stability and oxidation resistance with interesting optoelectronic properties

High Pressure Synthesis of Marcasite-Type Rhodium Pernitride

Marcasite-type rhodium nitride was successfully synthesized in a direct chemical reaction between a rhodium metal and molecular nitrogen at 43.2 GPa using a laser-heated diamond-anvil cell. This material shows a low zero-pressure bulk modulus of <i>K</i>₀ = 235(13) GPa, which is much lower than those of other platinum group nitrides. This finding is due to the weaker bonding interaction between metal atoms and quasi-molecular dinitrogen units in the marcasite-type structure, as proposed by theoretical studies

Possible Superhardness of CrB₄

Chromium tetraboride [orthorhombic, space group <i>Pnnm</i> (No. 58), <i>a</i> = 474.65(9) pm, <i>b</i> = 548.0(1) pm, <i>c</i> = 286.81(5) pm, and <i>R</i> value (all data) = 0.041], formerly described in space group <i>Immm</i>, was found not to be superhard, despite several theory-based prognoses. CrB₄ shows an almost temperature-independent paramagnetism, consistent with low-spin Cr^I in a metallic compound. Conductivity measurements confirm the metallic character

Possible Superhardness of CrB4

Chromium tetraboride [orthorhombic, space group Pnnm (No. 58), a = 474.65(9) pm, b = 548.0(1) pm, c = 286.81(5) pm, and R value (all data) = 0.041], formerly described in space group Immm, was found not to be superhard, despite several theory-based prognoses. CrB4 shows an almost temperature-independent paramagnetism, consistent with low-spin CrI in a metallic compound. Conductivity measurements confirm the metallic character