New Data on the Isomorphism in Eudialyte-Group Minerals. V: Crystal Structure of an Intermediate Member of the Manganoeudialyte‒Ilyukhinite Isomorphous Series

Crystallography Reports - The crystal structure of a high-symmetry representative of the eudialyte group from the Norra Kärr massif (Sweden) has been investigated by X-ray diffraction analysis...     

Crystal structure refinement of Sr<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript>

An accurate X-ray diffraction study of Sr₃TaGa₃Si₂O₁₄ (STGS) crystal (a = 8.3023(10) Å, c = 5.0853(2) Å, sp. gr. P321, Z = 1, R/wR = 0.59/0.52%, 4004 independent reflections) is performed. The use of two independent data sets obtained on diffractometers with point and 2D detectors made it possible to determine the model structure characterized by the best reproducibility of parameters. The ordered distribution of atoms over crystallographic positions and the anharmonic character of displacements of all cations and one oxygen atom are established.     

Accurate Refinement of the Crystal Structure of Ba<Subscript>3</Subscript>TaGa<Subscript>3</Subscript>Si<Subscript>2</Subscript>O<Subscript>14</Subscript> from Langasite Family and Analysis of Structural Transformations Due to Isomorphous Cation Substitutions

An accurate structure analysis of a Ba₃TaGa₃Si₂O₁₄ single crystal from langasite family was performed using four X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector (sp. gr. P321, Z = 1, sinθ/λ ≤ 1.35 Å^–1; at 295 K a = 8.516(1) Å, c = 5.1910(6) Å, R/wR = 0.58/0.56%, Δρ_min/Δρ_max =–0.73/0.42 e/ų, 4414 independent reflections; at 106 K a = 8.5109(9) Å, c = 5.1861(9) Å, R/wR = 0.75/0.86%, Δρ_min/Δρ_max =–0.81/1.06 e/ų, 4382 reflections). The distinguishing feature of the Ba₃TaGa₃Si₂O₁₄ structure is a strong disorder of the Ga atom at the 3f site. Structural transformations in the series of Сa₃TaGa₃Si₂O₁₄–Sr₃TaGa₃Si₂O₁₄–Ba₃TaGa₃Si₂O₁₄–Ba₃TaFe₃Si₂O₁₄ crystals were analyzed.     

Anisotropy of Microhardness and Fracture of Langasite Crystal and Its Structural Basis

Crystallography Reports - The anisotropy of microhardness and fracture of the (0001) basal plane of langasite crystal has been studied using Berkovich indentation under a load of 100 g at different...     

Structural study of K<Subscript>0.93</Subscript>Ti<Subscript>0.93</Subscript>Nb<Subscript>0.07</Subscript>OPO<Subscript>4</Subscript> single crystals at 30 K

The structure of a K_0.93Ti_0.93Nb_0.07OPO₄ single crystal is studied at the temperature 30 K. The measurements are performed on a four-circle HUBER-5042 diffractometer with a DISPLEX DE-202 cryostat. Processing of the diffraction data and the preliminary refinement of the model are performed using the ASTRA program package. The final refinement of the structure model is made using the JANA2000 program complex. The refinement shows that the structure of a K_0.93Ti_0.93Nb_0.07OPO₄ crystal at T = 30 K is similar to its structure at room temperature. No phase transitions are revealed. Slight temperature-induced displacements of the potassium positions in the large cavities of the mixed framework are established.     

X-ray diffraction study of a Nd3Ga5SiO14 crystal at 295 and 90 k and the structural basis for chirality

An accurate X-ray diffraction study of a Nd₃Ga₅SiO₁₄ crystal (sp. gr. P321, Z = 1) has been performed using repeated X-ray diffraction data sets collected on a diffractometer equipped with a CCD area detector at 295 and 90 K. The asymmetry of the disorder in the atomic positions in Nd₃Ga₅SiO₁₄ is described in two alternative ways: in terms of anharmonic atomic displacements (at 295 K R/wR = 0.80/0.92%; at 90 K R/wR = 0.82/0.76%) and using a split model (at 295 K R/wR = 0.85/0.97%; at 90 K R/wR = 0.80/0.74%). An analysis of the probability density function of finding an atom at a particular point in space shows that most of the atoms in the unit cell are asymmetrically or highly anisotropically disordered in the vicinity of their sites. Electron density regions for cations and anions that form pseudohelices were found. Such regions mimic a screw axis of symmetry and are responsible for the chirality of the structure.     

On automation of the procedure for crystal structure model refinement

The methods of automation of the procedure for crystal structure model refinement from experimental diffraction data, implemented in the ASTRA program package, are described. Such tools as statistical tests, parameter scanning, and data scanning reduce the time necessary for structural investigation. At strong correlations between parameters, especially when the data set is limited, parameter scanning has an advantage over the full-matrix refinement.