Oxidation Behavior and Microstructural Evolution of ZrB2–35MoSi2–10Al Composite Coating

The problem of creating and implementing high-temperature coatings for the protection of carbon–carbon (C/C) composites remains relevant due to the extremely low or insufficient heat resistance of C/C composites in an oxygen-containing environment. In the present work, detonation spraying was used for preparing new ZrB2–35MoSi2–10Al coatings on the surface of C/C composites without a sublayer. As a stabilizer of high-temperature modification of zirconia, and to increase the wettability of the surface of C/C composites, 5 wt.% Y2O3 and 10 wt.% Al were added to the initial powder mixture, respectively. The structure of the as-sprayed coating presents many lamellae piled up one upon another, and is composed of hexagonal ZrB2 (h- ZrB2), tetragonal MoSi2 (t-MoSi2), monoclinic ZrO2 (m-ZrO2), tetragonal ZrO2 (t-ZrO2), monoclinic SiO2 (m-SiO2), and cubic Al phases. The oxidation behavior and microstructural evolution of the ZrB2–35MoSi2–10Al composite coating were characterized from RT to 1400 °C in open air. During oxidation at 1400 °C, a continuous layer of silicate glass was formed on the coating surface. This layer contained cubic ZrO2 (c-ZrO2), m-ZrO2, and small amounts of mullite and zircon. The results indicated that a new ZrB2–35MoSi2–10Al composite coating could be used on the surface of C/C composites as a protective layer from oxidation at elevated temperatures.

The polymorphs of the Na+ ion conductor Na3PS4 viewed from the perspective of a group-subgroup scheme

The Na+ solid state electrolyte Na3PS4 is currently being investigated due to its high ionic conductivity and its synthesis-dependent crystal structure. Na3PS4 adopts a tetragonal low-temperature modification with space group P 4̅21c that transforms to a cubic high-temperature modification with space group I 4̅3m (Tl3VS4 type). These two modifications are related by a group-subgroup scheme. The symmetry reduction proceeds via a translationengleiche transition from I 4̅3m to I 4̅2m and subsequently via a klassengleiche transition to P 4̅21c. The tetragonal phase with space group I 4̅2m corresponds to the K2HgSnSe4 type. The group-subgroup scheme of this tetragonal branch of the Bärnighausen tree is discussed along with the crystal chemical consequences and results of diffraction experiments. The structure of K3SbSe4 (space group R 3c) belongs to the rhombohedral branch of the aristotype Tl3VS4.

Alkali sulfates with aphthitalite-like structures from fumaroles of the Tolbachik Volcano, Kamchatka, Russia. I. MetathÉnardite, a natural high-temperature modification of Na2SO4

A new mineral, metathénardite, ideally Na2SO4, the high-temperature hexagonal dimorph of thénardite, a natural analogue of the synthetic phase Na2SO4(I), was found in the sublimates of active fumaroles at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure eruption, Tolbachik volcano, Kamchatka, Russia. The holotype originates from the Glavnaya Tenoritovaya fumarole in which metathénardite is associated with hematite, tenorite, fluorophlogopite, sanidine, anhydrite, krasheninnikovite, vanthoffite, glauberite, johillerite, and lammerite. The cotypes 1 and 2 are from the Arsenarnaya (with hematite, tenorite, fluorophlogopite, sanidine, euchlorine, wulffite, anhydrite, fluoborite, johillerite, nickenichite, calciojohillerite, badalovite, tilasite, cassiterite, and pseudobrookite) and the Yadovitaya (with tenorite, euchlorine, fedotovite, dolerophanite, langbeinite, krasheninnikovite, anhydrite, and hematite) fumaroles, respectively. All specimens with metathénardite were collected from areas with temperatures of 350–400 °C. Metathénardite forms hexagonal tabular, lamellar, or dipyramidal crystals (forms: {001}, {100}, {102}, and {201}) up to 3 mm combined in crusts up to several hundred cm2 in area. The mineral is transparent to semitransparent, colorless, white, light-blue, greenish, yellowish, grayish or brownish, with vitreous luster. Dmeas. = 2.72(1), Dcalc. = 2.717 g/cm3. Metathénardite is optically uniaxial (–), ω = 1.489(2), ε = 1.486(2). The empirical formulae are (Na1.92K0.05Ca0.02Zn0.01)[S0.99O4] (holotype), (Na1.54K0.22Ca0.09Cu0.01Mg0.01)[S1.00O4] (cotype 1), and Na1.65K0.11Ca0.05Cu0.04Mg0.01)[S1.01O4] (cotype 2). Admixed K and bivalent cations probably stabilize the hexagonal aphthitalite-like structure of metathénardite at room temperature. The crystal structure was solved using single crystals of all three samples, R1 = 0.0852, 0.0452, and 0.0449 for holotype and cotypes 1 and 2, respectively. The space group is P63/mmc, and the unit-cell parameters of the holotype are a = 5.3467(9), c = 7.0876(16) Å, V = 157.47(6) Å3, and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I)(hkl)] are: 4.667(27)(100), 3.904(89)(101), 3.565(33)(002), 2.824(94)(102), 2.686(100)(110), and 1.939(35)(202). Metathénardite and thénardite clearly differ from one another in X-ray diffraction data and infrared and Raman spectra.

Synthesis and Raman spectra of K-Ca double carbonates: K2Ca(CO3)2 bütschliite, fairchildite, and K2Ca2(CO3)3 at 1 ATM

Here we present results on synthesis of double K-Ca carbonates at atmospheric pressure in closed graphite capsules. The mixtures of K2CO3 and CaCO3 corresponding to stoichiometry of K2Ca(CO3)2 and K2Ca2(CO3)3 were used as starting materials. The low-temperature modification of K2Ca(CO3)2 was synthesized by a solid-state reaction at 500°C during 96 h. The high-temperature modification of K2Ca(CO3)2 as well as the K2Ca2(CO3)3 compound were synthesized both by a solid-state reaction at 600°C during 72 h and during cooling of the melt from 830 to 650°C for 30 min. The obtained carbonates were studied by Raman spectroscopy. The Raman spectrum of bütschliite is characterized by the presence of an intense band at 1093 cm-1 and several bands at 1402, 883, 826, 640, 694, 225, 167 and 68 сm-1. The Raman spectrum of fairchildite has characteristic intense bands at 1077 and 1063 cm-1, and several bands at 1760, 1739, 719, 704, 167, 100 сm-1. In the Raman spectrum of K2Ca2(СO3)3 intense bands at 1078 and 1076 cm-1 and several bands at 1765, 1763, 1487, 1470, 1455, 1435, 1402, 711, 705, 234, 221, 167, 125 and 101 сm-1 were found. The collected Raman spectra can be used to identify carbonate phases entrapped as microinclusions in phenocrysts and xenoliths from kimberlites and other alkaline rocks.

Influence of partial substitution of Cu atoms by Zn and Cd atoms on polymorphic transformation in the Cu1.75Te crystal

The [Formula: see text], [Formula: see text] and [Formula: see text] compounds have been synthesized and low-temperature modification single crystals obtained from the high temperature modification by polymorphic transformation. The method of high temperature X-ray diffractometer has been used to study [Formula: see text] (x = 0.05 Zn and Cd) layered single crystal. It was established that such a substitution has a significant impact on the number and temperature of polymorphic transformations. Crystallgraphic parameters were determined for each phase. Temperature dependence of lattice parameters were obtained and determined change mechanism of lattice parameters by influence of temperature.

Structural Evolution in the Systems TAl3-xGex (T = Zr, Hf)

The crystal structures of the binary compounds ZrAl3 and HfAl3 at 600°C belong to the structure type ZrAl3 (Pearson symbol tI16, space group I4/mmm, a = 4.00930(11), c = 17.2718(7) Å for ZrAl3 and a = 3.9849(3), c = 17.1443(15) Å for HfAl3). Substitution of Ge atoms for Al atoms in ZrAl3 and HfAl3 led to the formation of the ternary compounds ZrAl2.52(1)Ge0.48(1) and HfAl2.40(1)Ge0.60(1), respectively, where the latter is probably part of a solid solution extending from the high-temperature modification of HfAl3. The crystal structures belong to the tetragonal structure type ht-TiAl3 (tI8, I4/mmm, a = 3.92395(11), c = 9.0476(4) Å for ZrAl2.52Ge0.48 and a = 3.9021(2), c = 8.9549(8) Å for HfAl2.40Ge0.60). The structure types ZrAl3 and ht-TiAl3 are both members of the family of close-packed structures.

RhSn3 and the Modifications of RhSn4 – Structure and 119Sn Mössbauer spectroscopic characterization

Single crystals of the high-temperature modification of RhSn4 were obtained from a tin flux (1:20 molar ratio; final annealing at 920 K; dissolution of the tin matrix in 2N HCl). The structure was refined from single-crystal X-ray diffractometer data: I41/acd, a=629.73(5), c=2288.36(18) pm, wR2=0.0382, 447 F2 values and 14 variables. β-RhSn4 is isotypic with β-IrSn4. The rhodium atoms have slightly distorted square-antiprismatic tin coordination with Rh–Sn distances of 4×273.4 and 4×274.1 pm. The RhSn8 units are condensed via common edges to layers that are staggered with respect to each other and stacked in ABCD sequence. A 119Sn Mössbauer spectroscopic characterization of ß-RhSn4 and the stannides RhSn3 and α-RhSn4 shows the typical isomer shifts for transition metal stannides. Only for α-RhSn4 the three crystallographically independent tin sites could be resolved, a consequence of the different s-electron density. Treatment of α-RhSn4 under high-pressure (up to 10 GPa)/high-temperature (up to T=1370 K) conditions leads to decomposition into Rh1.5Sn, RhSn2 and β-Sn.

Barium titanate via thermal decomposition of Ba,Ti-precursor complexes: The nature of the intermediate phases

The thermal decomposition of Ba,Ti-precursor complexes, containing organic ligands and suitable for the single-source preparation of nanocrystalline BaTiO3, leads firstly to the segregation of specific Ba-rich and Ti-rich phases. Quantitative electron energy loss spectroscopy and powder X-ray diffraction data indicated that the (i) Ba-rich phase is a BaO-stabilised variant of the calcite-type high-temperature modification of BaCO3 and (ii) Ti-rich phases are represented by low crystalline barium titanates with the general Ba:Ti ratio close to 1:4. The subsequent solid state reaction between these phases results then in the formation of BaTiO3.