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Abstract: High pressure in situ differential thermal analysis (DTA) was applied to study phase transitions of Na0.5Bi0.5TiO3 (NBT) up to 5GPa. DTA analysis at room temperature with pressure as the variable indicated NBT samples have two pressure induced phase transformations at about 0.7GPa and 2.5GPa respectively. Below 0.7GPa, two temperature induced phase transitions could be observed at around 220°C and 520°C as it was reported at ambient pressure. But under a pressure higher than 0.7GPa, these two temperature induced phase transitions would not happen. In situ electrical resistance measurements showed that the resistance of NBT changes smoothly, and the tendency is similar to that of electrical insulators like pyrophyllite. We suggest that the pressure induced phase transitions at 0.7GPa and 2.5GPa are both insulator to insulator phase transitions. [Copyright &y& Elsevier]

Binary ε -Fe 3 N 1.098 and ternary ε -Fe 2.322 Co 0.678 N 0.888 were synthesized as spherical bulk materials using novel high-pressure solid-state metathesis reactions (HPSSM). The structural and magnetic properties of the two nitrides were investigated using neutron powder diffraction (NPD), a vibrating sample magnetometer (VSM), and first-principle calculations. We found that at high pressure and high temperature (HPHT), nitrogen atoms enter the interstitial 2d sites of the nitrogen-rich ε -Fe 3 N 1.098 structure, space group P 6 3 22, which were previously reported to be vacant. The stoichiometry of the nitride (N/Me ratio) and the level of disorder have a significant influence on the site occupancies and local magnetic moments in these iron-based nitrides. The substitution of Fe by Co in ε -Fe 2.322 Co 0.678 N 0.888 did not reduce the mean magnetic moment ( μ m ) per metal atom at the Wyckoff 6 g site. NPD refinements showed that the magnetic moment per Fe atom in ε -Fe 3 N 1.098 (at 1.8 (4) μ B ) was slightly higher than that of ε -Fe 2.322 Co 0.678 N 0.888 , where the moment per Fe/Co atom is 1.7(2) μ B . [ABSTRACT FROM AUTHOR]

Single-phase ω -phase Ti with a considerable size of 3 mm × 3 mm × 4 mm and a Vickers hardness of 2.5 GPa is synthesized under high pressure and combined with a designed experimental curve at room temperature. The elastic properties of the α - and ω -phase Ti are ascertained using the non-destructive ultrasonic through-transmission technique. Based on the investigated Raman spectra and DSC, the inverse transition temperature of the metastable ω -phase and Raman spectra of the α - and ω -structures are also discussed. Modulations on the elastic and spectral properties of the two phases are simulated using first-principles calculations to systematically investigate the mechanical responses under biaxial strains. • A bulk sample consisting of single-phase of ω -Ti with considerable dimensions size of 3 mm × 3 mm × 4 mm was synthesized using HP method combined special designed experimental curve. • Due to the difficulty in prepare the bulk materials in previous studies, the obtained sample is the largest pure phase block which can be obtained so far. • A systematical investigation of the mechanical and spectral properties of α - and ω -Ti, including the Vickers hardness, elastic modulus, and Raman spectra were measured. • The ultrasonic through-transmission technique without damage was employed in the crystal sound velocity and modulus measurement. • The mechanical responses of α - and ω -Ti to strains, modulations on elastic and spectral properties of two phases Ti were simulated by using the first-principles calculation. [ABSTRACT FROM AUTHOR]

The strength of tungsten triboride (WB 3 ) was determined under nonhydrostatic compression up to 86 GPa using an angle-dispersive radial X-ray diffraction technique in a diamond-anvil cell (DAC). Analyze of diffraction data using lattice strain theory indicate that the ratio of differential stress to shear modulus ( t / G ) changes from 0.004 at ambient conditions to 0.078 at 86 GPa. Together with theoretical results on the high-pressure shear modulus, our results here show that WB 3 under uniaxial compression can support a differential stress of 26 GPa when it starts to yield to the plastic deformation at 40 GPa. The yield strength of WB 3 increases with increasing pressure, reaching a maximum value of 30 GPa at 77 GPa. By comparison, we find that the high-pressure strength of WB 3 is comparable to those of c -BC 2 N, B 6 O, and γ -Si 3 N 4 . [ABSTRACT FROM AUTHOR]

● GdB 6 is extremely stable under high pressure. ● Contrary to most other materials, the smooth X-ray diffraction rings of GdB 6 become spotty under high pressure. ● High-pressure spotty diffraction rings of GdB 6 are caused by the regional texture effect, which is reversible. ● Grain rotation behavior motivated by stress difference dominates the regional texturing in GdB 6. [Display omitted] As hard and refractory materials with high chemical resistance and mechanical strength, lanthanide hexaborides (LnB 6) have attracted much attention. Among the family of LnB 6 , gadolinium hexaboride (GdB 6) occupies a special position due to the half-filled 4 f shell of Gd. Here, using in situ synchrotron radiation angle-dispersive X-ray diffraction in a diamond anvil cell at room temperature, the structural stability and compression behavior of GdB 6 are investigated extensively, GdB 6 is observed to be structurally stable up to 73 GPa, and the bulk modulus of 177 GPa is obtained under hydrostatic compression. In this paper, an interesting observation of pressure-induced spotty diffraction rings of GdB 6 is reported, its formation mechanism can be well described by the reversible regional texture effect, which highlights the rearrangement of crystal grains under high pressure. The rearrangement mechanism has been well explained by investigating the pressure dependence of full width at half maximum, macro-differential stress and grain size. Collective grain rotation behavior motivated by stress difference is critical for the rearrangement process, the strong isotropy and strong stability of GdB 6 structure also provide necessary conditions for high-pressure grain rotation behavior. These results will help to promote the understanding of high-pressure structural properties of GdB 6 , and provide novel insights on the high-pressure grain behavior in hard materials with strong isotropy and three-dimensional skeleton constituted of strong covalent bonds. [ABSTRACT FROM AUTHOR]

Abstract: Spinel aluminium oxynitride (γ-AlON), as a kind of transparent ceramic material expectable, is studied using the ab initio density functional method, in terms of electronic, elastic, thermodynamic properties and structure disorder. The results show that γ-AlON exhibits strong ionicity, as quantitatively expressed by from our reassessment of the ionic character. We summarize and speculate that the considered oxynitride single-crystals exhibit highly elastic anisotropy. The interpretation of the thermodynamic properties of γ-AlON according to quasi-harmonic Debye model confirm the available experiments and are extended to a wider temperature/pressure range. This material holds high elastic strength under extreme environments, where dB/dT absolute value is less than 0.03GPa/K, independent of the pressure. Finally, we study the O/N structure disorder character of γ-AlON solid solution by investigating nine possible crystal structures. It is found that γ-AlON should be partially disordered, and in fact, the O/N ordering has a significant effect on the properties. [Copyright &y& Elsevier]

In this work, we report a study on the rapid densification of bulk nanograin gadolinium zirconate (Gd 2 Zr 2 O 7 , GZO) ceramics with an average grain size below 10 nm at a relatively low temperature (∼0.2 T melt). The starting nanocrystalline powders with average crystallite sizes of 6.2 nm and 12.7 nm were synthesized by a solvothermal method. Nanograin ceramics were sintered at various conditions: temperatures (280–760 °C), pressures (2.2–5.0 GPa) and sintering time (3–90 min). The average grain size of the sintered ceramic decreased with the increase of the sintering pressure but increased with the elevation of the sintering temperature, while the nearly full density was only achieved at the optimal condition (5 GPa and 520 °C) with sintering time as low as 3 min. Reducing the average crystallite size of the starting powders can promote the densification of GZO nanograin ceramic. Translucent ceramic with a relative density >99% and an average grain size of about 7 nm was successfully obtained. We consider the fast low-temperature densification mechanism under high pressure is governed by thermally-activated grain rotation and interface sliding assisted by plastic deformation. Image 1 • High-pressure sintering of Gd2Zr2O7 ceramics using lab-made nanocrystalline powders. • Translucent GZO ceramic with a relative density > 99% and an average grain size of about 7 nm was successfully obtained. • The densification mechanisms are related to nanoparticle rearrangement and interface sliding assisted by plastic deformation. [ABSTRACT FROM AUTHOR]

In-situ high pressure synchrotron angle-dispersive x-ray diffraction (AXRD) studies have been performed for HfB 2 , an ultra high temperature refractory alloy with AlB 2 structure, by using a diamond anvil cell (DAC) at room temperature. The mechanical constants (bulk modulus K , shear modulus G , Young's modulus E , Poisson's ratio ν , density ρ , and Debye temperatures Θ D) of bulk HfB 2 are measured by ultrasound experiments at room temperature. Furthermore, the compression behavior and elastic properties of HfB 2 under pressure are investigated using density functional theory. The experimental results are in good agreement with the theoretical calculations. The pressure dependence of mechanical constants and elastic anisotropy of HfB 2 are discussed. The experimental data and theoretical calculations results show that HfB 2 has a distinct anisotropic compression behavior along the a - and c -axes, and the anisotropy increases as the pressure is increasing. These markedly elastic properties are attributes to the strongly directional bonding between Hf and B atoms. • The state equation and phase stability of HfB 2 ceramics was determined. • The mechanical properties were measured accurately. • The pressure dependence of elastic constants of HfB 2 were investigated. • The change of elastic anisotropy of HfB 2 with pressure were analyzed. [ABSTRACT FROM AUTHOR]