Maria G. Krzhizhanovskaya

XRD and DSC study of the formation and the melting of a new zeolite like borosilicate CsBSi5O12 and (Cs,Rb)BSi5O12 solid solutions

Polycrystalline CsBSi5O12 was prepared from a stoichiometric mixture by solid-state reaction above 1000 °C. The solid solutions Cs1–xRbxBSi5O12 were obtained at 1000 °C during a long heat treatment of polycrystalline Cs1–xRbxBSi2O6 boropollucites (xRb = 0, 0.05, 0.2, 0.4). A new borosilicate compound and its solid solutions were studied using X-ray powder diffraction (XRD), annealing, differential scanning calorimetry (DSC), and thermogravimetry (TG). For Cs,Rb-boropollucites the new phase formation is accompanied by significant mass losses detected by DSC and TG. The following mechanism of phase transformations is assumed: (Cs,Rb)BSi2O6 → (Cs,Rb)BSi5O12 + (Cs,Rb)BO2↑. The zeolite phase forms as a result of the boropollucite decomposition over 1000 °C. Zeolite decomposes also on further heating and the SiO2 reflections are observed in the XRD pattern only. Thus above 1000 °C both boropollucite and zeolite phases are unstable presumably due to the ability of the alkali cations to leave the structure. Using XRD the unit cell parameters of CsBSi5O12 have been determined in the orthorhombic crystal system: a = 16.242(4) Å, b = 13.360(4) Å, c = 4.874(1) Å. The compound is isostructural with the zeolite compound CsAlSi5O12. In the crystal structure of Cs1–xRbxBSi5O12 solid solutions the changes of cell parameters are insignificant under the substitution of Cs by Rb atoms that indicates a very limited substitution range.

Crystal structure and thermal expansion of β-RbB5O8 from powder diffraction data

Using Rietveld refinement the crystal structure of the β-rubidium pentaborate has been found to be isotypic with β-potassium pentaborate: (61) Pbca - (c)14, oP112, a = 7.550(1) Å, b = 11.842(1) Å, c = 14.805(1) Å, V = 1323.7(2) Å3, Z = 8, Dcalc = 2.68×103 kg/m3. With the aid of high temperature X-ray diffractometry a strongly anisotropic thermal expansion has been observed: αa= 61·10-6 K-1, αb= 23·10-6 K-1 and αc= 4.7·10-6 K-1. This anisotropy may be caused by anisotropic thermal vibrations of heavy atoms as rubidium.

Crystal structure of the low-temperature modification of α-RbB3O5

The crystal structure of α-RbB3O5 was refined by the Rietveld method with due regard for anisotropic vibrations of rubidium atoms to Rp = 2.93, Rwp = 3.80, RB = 2.53, RF = 2.84, and s = 1.54. The compound is isostructural to CsB3O5: it is orthorhombic, sp. gr. P212121, a = 8.209(1), b = 10.092(1), c = 5.382(1) Å, and V = 445.9 Å 3. The framework structure is formed by the boron-oxygen [B2IIIBIVO5] − rings consisting of two [BO3]-triangles and a [BO4]-tetrahedron. The rings are linked to form systems of helical chains running along the twofold screw axes parallel 21 to the a-and b-axes and infinite channels parallel to the a-and c-axes, which accommodate Rb atoms. The data were collected on an ADP-2 diffractometer [CuKα radiation, Ni-filter, 12.00° < 2θ < 110.00°, a step in 2θ equal to 0.02°, count time 8 s per step, and 711 reflections α1 + α2)]. All the calculations were performed using version 3.3 of the WYRIET program. The comparison of the structures of α-and β-RbB3O5 and CsB3O5 revealed that the type of deformations in the framework structures of alkali-metal borates due to the changes of the temperature or the substitution of cations is determined by the role played by metal atoms, and especially, by large and heavy ions.

Thermal expansion and polymorphism in a series of rubidium cesium boroleucites

The crystal structure of the boroleucite solid solution Rb0.40Cs0.54B0.94Si2.06O6 is refined in space group I-43d by the Rietveld method with the use of the X-ray powder diffraction data. The refinement data complement the available crystal chemical characteristics of Rb1−xCsxBSi2O6 solid solutions. The thermal expansion and phase transformations of Rb1−xCsxBSi2O6 borosilicates are investigated in parallel by high-temperature X-ray diffraction with conventional powdered samples and by the dilatometric method with samples in the form of pressed pellets. It is demonstrated that the thermal expansion coefficients, as well as the temperatures and sequence of polymorphic transitions, which are determined from the data obtained by two methods are in close agreement. The temperature curve of the I-43d ⇄ Ia3d phase transition for the Rb1−xCsxBSi2O6 solid solution system is constructed from the data obtained by both methods. It is shown with the use of the structural data obtained by the Rietveld method that, at temperatures above 800°C, rubidium-cesium boroleucites undergo decomposition due to the release of alkali cations.

Synthesis and Characterization of the High-Pressure Nickel Borate γ-NiB4O7

γ-NiB4O7 was synthesized in a high-pressure/high-temperature experiment at 5 GPa and 900 °C. The single-crystal structure analysis yielded the following results: space group P6522 (No. 179), a = 425.6(2), c = 3490.5(2) pm, V = 0.5475(2) nm3, Z = 6, and Flack parameter x = -0.010(5). Second harmonic generation measurements confirmed the acentric crystal structure. Furthermore, γ-NiB4O7 was characterized via vibrational as well as single-crystal electronic absorption spectroscopy, magnetic measurements, high-temperature X-ray diffraction, differential scanning calorimetry, and thermogravimetry. Density functional theory-based calculations were performed to facilitate band assignments to vibrational modes and to evaluate the elastic properties and phase stability of γ-NiB4O7.

Crystal structure of new polymorphic modification β-Ca3B2SiO8, β-α phase transition and thermal expansion of α- and β-modifications

Single crystals of the β-Ca3B2SiO8 new monoclinic modification have been obtained by cooling the melt of a stoichiometric composition. The crystal structure has been determined from the single crystal X-ray diffraction data and refined with R = 0.059 (wR = 0.069) in the monoclinic space group P21/m. The thermal behavior of the synthetic borosilicate has been studied. At 472 ± 5°С, a reversible phase transition of the first order occurs, leading to the formation of the orthorhombic α-Ca3B2SiO8 modification. The thermal expansion of α- and β-modifications of Ca3B2SiO8 is anisotropic: (α11 = 15, α22 = 16, α33 =–1, αV = 30 × 10–6°С–1) and α11 = 9, α22 = 28, α33 = 1, αV = 38 × 10–6°C–1, respectively.

Perovskites with the Framework‐Forming Xenon

The Group 18 elements (noble gases) were the last ones in the periodic system to have not been encountered in perovskite structures. We herein report the synthesis of a new group of double perovskites KM(XeNaO6) (M = Ca, Sr, Ba) containing framework-forming xenon. The structures of the new compounds, like other double perovskites, are built up of the alternating sequence of corner-sharing (XeO6) and (NaO6) octahedra arranged in a three-dimensional rocksalt order. The fact that xenon can be incorporated into the perovskite structure provides new insights into the problem of Xe depletion in the atmosphere. Since octahedrally coordinated Xe(VIII) and Si(IV) exhibit close values of ionic radii (0.48 and 0.40 Å, respectively), one could assume that Xe(VIII) can be incorporated into hyperbaric frameworks such as MgSiO3 perovskite. The ability of Xe to form stable inorganic frameworks can further extend the rich and still enigmatic chemistry of this noble gas.

Incommensurate modulation and thermal expansion of Sr3B2 + xSi1 − xO8 − x/2 solid solutions

Crystal structures of Sr3B(2 + x)Si(1 - x)O(8 - x/2) solid solutions with nominal compositions x = 0.28, 0.53, 0.78 in the Sr3B2SiO8-Sr2B2O5 section of the SrO-B2O3-SiO2 system are refined using single-crystal X-ray diffraction data. Incommensurate structure modulations are mainly associated with various orientations of corner-sharing (B,Si)-polyhedra. Preference is given to the (3 + 2)-dimensional symmetry group Pnma(0βγ)000(0βγ)000 for a single crystal compared with an alternate model of a twin formed by monoclinic components, each of them corresponding to the (3 + 1)-dimensional symmetry group P2(1)/n(0βγ). Single-phase polycrystalline samples of solid solutions are investigated by high-temperature X-ray powder diffraction in air. Orientation preferences of the BO3 units lead to a strong anisotropy of thermal expansion. Negative expansion is observed along the a axis over the temperature range 303-753 K. Anisotropy decreases both on heating and decreasing of the boron content.

Temperature-dependent evolution of RbBSi2O6glass into crystallineRb-boroleucite according to X-ray diffraction data

Abstract The temperature-dependent evolution of the glass into a crystalline phase is studied for a rubidium borosilicate glass of composition 16.7 Rb2O · 16.7 B2O3 · 66.6 SiO2 employing X-ray diffraction (XRD) data. A glass sample was prepared by melt quenching from 1500°С within 0.5 hour. The glass sample was step-wise annealed at 13 distinct temperatures from 300 °C up to 900 °C for 1 h at every annealing step. To investigate changes in the glass structure, angle-dispersive XRD was applied by using an energy-resolving semiconductor detector. The radial distribution functions (RDFs) were calculated at every stage. For polycrystalline states the crystal structure of the samples with different thermal history was refined using the Rietveld method. Comparing correlation distances estimated from RDFs of glass and polycrystalline samples and mean interatomic distances calculated for polycrystalline samples by using atomic coordinates after Rietveld refinement, it is concluded that the borosilicate glass under study is converted into the crystalline state in the temperature range of 625–750 °C (i.e. in the temperature range close to the glass transition range 620–695 °C as determined by differential scanning calorimetry by using of heating rate of 20 K/min) at an average heating rate of about 0.35 K/min. When the heating rate is increased up to 10 or 20 K/min, the crystallisation temperature shifts sharply up to 831–900 °C and 878–951 °C, respectively. XRD data give evidence that distinctive traces of cubic RbBSi2O6 appear from glass at about 625 °C and a two-phase range exists up to 750 °C. After annealing at higher temperatures (800–900 °C) the crystal structure practically does not change any more.

Temperature hysteresis of AgI phase transition in AgI–chalcogenide glass nanolayered films

Using a method of laser ablation, a number of AgI–chalcogenide glass nanolayered films has been obtained with different thicknesses of the layers (10, 25, 50, and 100 nm). In order to study α ⇆ β phase transition in AgI, X-ray phase analysis has been carried out in the temperature range from 30 to 200°C. A correlation between the layer thickness and the temperature of the α → β phase transition during the lowering of the temperature is found. An explanation of the correlation is proposed.