Elastic properties of single crystal Bi12SiO20 as a function of pressure and temperature and acoustic attenuation effects in Bi12 MO20 (M = Si, Ge and Ti)

A comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd

Nowotny phase Mo3+2xSi3C0.6 dispersed in a porous SiC/C matrix: A novel catalyst for hydrogen evolution reaction

The ternary Nowotny phase (NP), with a composition Mo3+2xSi3C0.6 (x = 0.9‐0.764), is found to be catalytically active in the field of electrochemical water splitting. The NP embedded in a porous SiC/C nanocomposite matrix is synthesized via a single‐source‐precursor approach which involves the reaction of allylhydridopolycarbosilane with MoO2(acac)2. Thermal treatment of the single‐source‐precursor up to 1400°C in a protective atmosphere results in the in situ formation of nanocrystalline Mo3+2xSi3C0.6 immobilized in a thermally and corrosion‐stable SiC/C matrix. The weight fractions of the observed crystalline phases Mo3+2xSi3C0.6 and SiC amount to ca. 28 (26) and 72 (74) wt%, respectively, when prepared at 1400°C (1350°C). The porosity of the formed nanocomposite is adjusted by the addition of polystyrene (PS) as a pore former to the single‐source‐precursor resulting in a specific surface area up to 206 m2/g. The electrocatalytic activity of the Mo3+2xSi3C0.6/C/SiC nanocomposite with respect to the hydrogen evolution reaction (HER) is characterized by low over potentials of 22 and 138 mV vs reversible hydrogen electrode (RHE) for applying 1 and 10 mA cm−2 of current density, respectively. The analyzed electrocatalytic performance exceeds that of most Mo‐based electrocatalysts and shows high stability (over 90%) during 35 hours

Conversion of a polysilazane‐modified cellulose‐based paper into a C/SiFe(N,C)O ceramic paper via thermal ammonolysis

Cellulose-based paper samples were surface-modified by a polymeric single-source precursor prepared from perhydropolysilazane (PHPS) and iron(III)acetylacetonate (Fe(acac)(3)) and ammonolyzed at 500 degrees C, 700 degrees C, 900 degrees C, and 1000 degrees C, leading to C/SiFe(N,C)O-based ceramic papers with in situ-generated hierarchical micro/nano-morphology. As reference, cellulose-free samples were prepared under the same conditions. Upon thermal treatment, the microstructure evolutions of the resulting ceramic paper and the reference sample were comparatively investigated. Scanning electron microscopy (SEM) showed that for all temperatures, the ceramic papers exhibit the same morphology as the template, however, with noticeable shrinkage and curling, particularly evident at higher temperatures. X-ray diffraction (XRD) measurements of the reference samples and the ceramic papers showed a similar crystallization behavior and phase evolution in both materials. In the ceramic paper, the crystallization process seems to occur at a later time. The results provide a comprehensive understanding of the investigated C/SiFe(N,C)O-based ceramic system. It was shown that use of the cellulose-based paper template has the benefit of retaining the microstructure and furthermore, apart from transforming the cellulose fibers into turbostratic carbon, does not change the phase evolution during the polymer-to-ceramic transformation, allowing at the same time the manufacturing of novel morphologically complex parts by a convenient one-pot synthesis approach

Discovery of Ternary Silicon Titanium Nitride with Spinel-Type Structure

Here we report on the discovery of a ternary silicon titanium nitride with the general composition (Si1−x,Tix)3N4 with x = 0 < x < 1 and spinel-type crystal structure. The novel nitride is formed from an amorphous silicon titanium nitride (SiTiN) precursor under high-pressure/high-temperature conditions in a large volume high-pressure device. Under the conditions of 15–20 GPa and 1800–2000 °C, spinel-type γ-Si3N4 and rock salt-type c-TiN are formed. In addition, crystals of the discovered nano-sized ternary phase (Si1−x,Tix)3N4 embedded in γ-Si3N4 are identified. The ternary compound is formed due to kinetically-controlled synthesis conditions and is analyzed to exhibit the spinel-type structure with ca. 8 atom% of Ti. The Ti atoms occur in both Ti3+ and Ti4+ oxidation states and are located on the Si sites. The ternary nano-crystals have to be described as (Si,Ti)3N4 with N-vacancies resulting in the general composition (Si4+1−x Ti4+x-δTi3+δ)3N4-δ

Phase evolution of SiOC‐based ceramic nanocomposites derived from a polymethylsiloxane modified by Hf‐ and Ti‐alkoxides

SiOC/HfO2‐based ceramic nanocomposites with in situ formed HfO2 nanoparticles were prepared via a single‐source precursor (SSP) approach starting from a polymethylsilsesquioxane (PMS) modified by Hf‐ and Ti‐alkoxides. By varying the alkyl‐group of the employed Hf‐alkoxides, SiOC/HfO2‐based ceramic nanocomposites with different HfO2 polymorphs formed via thermal decomposition of the SSP under the same heat‐treatment conditions. Using PMS chemically modified by Hf(OnBu)4, tetragonal HfO2 phase was formed after the synthesis at 1100°C in Ar, whereas both, tetragonal and monoclinic HfO2 nanocrystals, were analyzed when replacing Hf(OnBu)4 by Hf(OiPr)4. After oxidation of the synthesized nanocomposites in air at 1500°C, a facile formation of oxidation‐resistant HfSiO4 (hafnon) phase occurred by the reaction of HfO2 nanocrystals with silica present in the SiOC nanocomposite matrix derived from Hf(OiPr)4‐modified SSPs. Moreover the amount of hafnon is dramatically increased by the additional modification of the polysiloxane with Ti‐alkoxides. In contrast, ceramic nanocomposites derived from Hf(OnBu)4‐modified SSPs, almost no HfSiO4 is detected after oxidation at 1500°C even though in the case of Ti‐alkoxide‐modified single‐source precursor

A Novel High-Pressure Tin Oxynitride Sn2N2O

We report the first oxynitride of tin, Sn2N2O (SNO), exhibiting a Rh2S3-type crystal structure with space group Pbcn. All Sn atoms are in six-fold coordination, in contrast to Si in silicon oxynitride (Si2N2O) and Ge in the isostructural germanium oxynitride (Ge2N2O), which appear in four-fold coordination. SNO was synthesized at 20 GPa and 1200-1500 degrees C in a large volume press. The recovered samples were characterized by synchrotron powder X-ray diffraction and single-crystal electron diffraction in the TEM using the automated diffraction tomography (ADT) technique. The isothermal bulk modulus was determined as B-o=193(5) GPa by using in-situ synchrotron X-ray diffraction in a diamond anvil cell. The structure model is supported by DFT calculations. The enthalpy of formation, the bulk modulus, and the band structure have been calculated

Frontispiece: A Novel High‐Pressure Tin Oxynitride Sn2_{2}N2_{2}O

We report the first oxynitride of tin, Sn$_{2}$N$_{2}$O (SNO), exhibiting a Rh2S3‐type crystal structure with space group Pbcn. All Sn atoms are in six‐fold coordination, in contrast to Si in silicon oxynitride (Sn$_{2}$N$_{2}$O) and Ge in the isostructural germanium oxynitride (Ge$_2$N$_2$O), which appear in four‐fold coordination. SNO was synthesized at 20 GPa and 1200–1500 °C in a large volume press. The recovered samples were characterized by synchrotron powder X‐ray diffraction and single‐crystal electron diffraction in the TEM using the automated diffraction tomography (ADT) technique. The isothermal bulk modulus was determined as B$_o$=193(5) GPa by using in‐situ synchrotron X‐ray diffraction in a diamond anvil cell. The structure model is supported by DFT calculations. The enthalpy of formation, the bulk modulus, and the band structure have been calculated

A Novel High‐Pressure Tin Oxynitride Sn2N2OSn_{2}N_{2}O

We report the first oxynitride of tin, Sn2N2O (SNO), exhibiting a Rh2S3‐type crystal structure with space group Pbcn. All Sn atoms are in six‐fold coordination, in contrast to Si in silicon oxynitride (Si2N2O) and Ge in the isostructural germanium oxynitride (Ge2N2O), which appear in four‐fold coordination. SNO was synthesized at 20 GPa and 1200‐1500 °C in a large volume press. The recovered samples were characterized by synchrotron powder X‐ray diffraction and single crystal electron diffraction in the TEM using the automated diffraction tomography (ADT) technique. The isothermal bulk modulus was determined as Bo=193(5) GPa using in‐situ synchrotron X‐ray diffraction in a diamond anvil cell. The structure model is supported by DFT calculations. The enthalpy of formation, the bulk modulus, and the band structure have been calculated