Rimma S. Bubnova

Software for determining the thermal expansion tensor and the graphic representation of its characteristic surface (theta to tensor-TTT)

A software is developed for determining the parameters of the thermal expansion tensor of the crystals of any system by a set of experimental diffraction data received using X-ray, synchrotron and other radiations at various temperatures. An algorithm is realized which allows carrying out all calculations from the experimental determination of the Bragg reflection angles to the calculation of the parameters of the thermal expansion tensor of the crystals, including the orientation of the tensor axes with respect to the crystallographic axes. The drawing of the 3D characteristic surface of the tensor and its 2D sections is also provided. The software is aimed at studying the anisotropy of the thermal expansion of the crystal materials and phase transitions, as well as revealing the mechanism and nature of the thermal behavior of substances.

High-temperature borate crystal chemistry

Abstract The paper presents a brief review of the present state of high-temperature borate crystal chemistry. This review summarizes the results of high- and low-temperature single crystal X-ray diffraction studies for more than 10 borate structures and high-temperature powder Xray diffraction data for about 65 borates. Thermal behavior of their crystal structures, thermal expansion, polymorphic transitions and their relationship to borate glasses are presented. These studies allow to formulate the basic principles of high-temperature borate crystal chemistry and to reveal the regularities of thermal behavior of borates. On heating, the BO3 and BO4 polyhedra and rigid groups consisting of these polyhedra, practically maintain their configuration and size, but they are able to rotate like hinges exhibiting highly anisotropic thermal expansion, including linear negative expansion. Based on these results, we generalize the term “rigid group” and render thermal vibrations as the key ingredient for the self-assembly of borate rigid groups.

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 of K1-xCsxBSi2O6 (x = 0.12, 0.50) boroleucite solid solutions and thermal behaviour of KBSi2O6 and K0.5Cs0.5BSi2O6

Abstract The crystal structures of two K1-xCsxBSi2O6 solid solutions have been refined at room temperature by the Rietveld method: x = 0.12, a = 12.6858(4) Å, Rwp = 7.66%, RF = 5.56% and x = 0.50, a = 12.8480(2) Å, Rwp = 7.64%, RF = 3.10%. They are isostructural to cubic KBSi2O6 with the space group I4̅3d. The structure is built up from (Si,B)O4 tetrahedra linked in four-, six- and eightfold rings which are forming a three-dimensional borosilicate framework. The framework contains large cavities that are placed in continuous channels along the [111] directions. The Cs and K atoms occupy the positions in the channels statistically. Thermal behaviour of KBSi2O6 and K0.5Cs0.5BSi2O6 has been studied by high-temperature powder X-ray diffraction within the temperature range of 293-1073 K. A new tetragonal polymorph of KBSi2O6 has been found in situ under heating. The new polymorphic I4̅3d (cubic) – Ia3̅d (cubic) transition and the new Ia3̅d cubic polymorphic phase has been proposed for K1-xCsxBSi2O6 from our experimental and literature data on crystal structures and thermal expansion of leucites. The structural relaxation under cationic (K, Cs) substitutions and under heating has been investigated.

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.

Synthesis, crystal structure and thermal expansion of a novel borate, Ba3Bi2(BO3)4

Abstract Single crystals of novel metastable borate Ba3Bi2(BO3)4, were obtained from melt with stoichiometric composition. The compound is structurally related to the A3Ln2(BO3)4 family of luminescent borates (A = Ca, Sr, Ba; Ln = Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb). The crystal structure was solved by direct methods and refined to R = 0.041 (wR = 0.045), orthorhombic, Pnma, a = 7.9508(5), b = 17.399(1), c = 8.9791(5) A˚ , V = 1242.1(1) A˚ 3, Z = 4. The structure contains isolated BO3 triangles linked through Ba2+and Bi3+ cations distributed among three cation sites, M1, M2 and M3. The mixture of the Ba3Bi2(BO3)4 and BaBiBO4 phases in the ~1 : 1 ratio was investigated by means of high-temperature X-ray powder diffraction in air. Ba3Bi2(BO3)4 demonstrates an anisotropic thermal expansion with the following coefficients: aa = 16, ab = 11, ac = 11, aV = 38 x 10-6 K-1 at 25 ºC. The anisotropy increases on heating: aa = 32, ab=-2, ac = 7, aV = 37 x 10-6 K-1 at 700 ºC. The reason for the strong anisotropy of thermal expansion is the preferable orientation of the BO3 triangles.

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.

Crystal formation from glass, crystal structure refinement and thermal behavior of K1−xRbxBSi2O6 boroleucite solid solutions from X-ray powder diffraction data

Abstract The crystal chemistry of K1−xRbxBSi2O6 mixed boroleucites has been studied by means of X-ray powder diffraction at room temperature. Boroleucites under study were prepared by crystallization from glass at 700–1000 °C for a few to 1000 hours time. First glass crystallization process starts from formation of two solid solutions: one of them is related to space group I-43d, another to Ia-3d. After heat-treatment at 950 °C/51 h, K1−xRbxBSi2O6 solid solutions crystallize in the cubic space group I-43d, in a wide range of compositions as Rietveld refinement of the structures of solid solutions (nominal composition x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) demonstrated. Within a narrow range of x near 0.2 area of immiscibility was found. Under substitution of K by Rb structural parameters (cubic lattice parameter a, (K,Rb)—O bond lengths in (K,Rb)O15 polyhedron, (K,Rb)—(K,Rb) bond lengths, (K,Rb)—O—(K,Rb) äangles between (K,Rb)O15 polyhedra, and T—O—T angles between tetrahedra) of the phase increased nonlinearly due to cationic size rising. Non-linear behavior of the composition dependence of structural parameters was observed near x = 0.2 ÷ 0.4. A comparative analysis has been made for the K—Rb—Cs substitution in the two sequential boroleucite series KBSi2O6—RbBSi2O6 and RbBSi2O6—CsBSi2O6.

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.