Volume 217, Issue 6

Crystallisation and annealing behaviour of two caesium borosilicate glass samples with approximate boroleucite composition were characterised by means of differential scanning calorimetry measurements as well as X-ray powder diffraction investigations with subsequent profile fitting of the observed patterns according to the Rietveld and/or Pawley method. While one sample crystallised primarily in an orthorhombic phase ( a = 6.592(3) Å, b = 11.825(5) Å, c = 12.620(6) Å; possible space groups: Pmc 2 1 ((26), setting: P 2 1 am ), Pma 2 (28) and Pmma ((51), setting: Pmam )) which has been unknown until now and transformed irreversibly into the well-known cubic boroleucite phase at T ≈ 850 °C, cubic caesium boroleucite (space group: Ia 3̅ d (230)) was crystallised directly from the other sample. In this case, the orthorhombic phase was metastable. At the onset temperature of glass crystallisation T x , on = 770 °C, it existed only for about 75 min. The change of the lattice parameter a of the cubic unit cell of caesium boroleucite in the course of the thermal treatments was explained by the alteration of the size of the TO 4 tetrahedra (T = Si, B).

The layer silicate H-RUB-18 (chemical compo-sition: Si 4 O 7 (OH) 2 ) was prepared by ion exchange from the parent material Na-RUB-18. The high degree of structural disorder typical for this material precluded a classical structure analysis based on diffraction data only. Nevertheless, a detailed picture of the structure was obtained by a step by step process using a combination of complementary methods: Crystal chemical considerations and solid state NMR spectroscopy allowed to establish a rough model of the structure which was optimized by forcefield molecular dynamics (MD) simulations. A subsequent Rietveld refinement in space group I 4 1 / amd confirmed the model consisting of a sequence of pseudo tetragonal silicate layers with an intra-layer repeat unit a = 7.38 Å and inter-layer distances of c = 7.44 Å. The main type and degree of disorder was analyzed by the simulation of various stacking sequences of rigid silicate layers which served as the building blocks. The random displacement of consecutive layers is accompanied by a slight distortion of the layers as indicated by an additional density functional theory (DFT) energy minimization calculation in triclinic symmetry. The given displacement and distortion allow for the formation of hydrogen bonds between the terminal OH groups necessary for the stabilization of the structure. The concept of a structure analysis presented in this paper may be suitable as a guide line to be applied to many other disordered materials.

The crystal structures of the four synthetic double salt compounds K 2 Mn(SO 4 ) 2 ·2H 2 O (space group P 1̅, no. 2, Z = 2, a = 6.574(1), b = 7.332(1), c = 10.700(1) Å, α = 72.89(1), β = 73.91(1), γ = 69.77(1)°, V = 453.6(1) Å 3 ) and K 2 Cd(SeO 4 ) 2 ·2H 2 O(space group P 1̅, no. 2, Z = 2, a = 6.625(1), b = 7.542(1), c = 11.349(1) Å, α = 72.17(1), β = 74.55(1), γ = 70.13(1)°, V = 499.3(1) Å 3 ) as well as (NH 4 ) 2 Cu(SeO 4 ) 2 ·2H 2 O (space group P 1̅, no. 2, Z = 1, a = 5.784(1) Å, b = 7.107(1) Å, c = 7.371(1) Å, α = 104.21(2)°, β = 93.04(2)°, γ = 112.27(2)°, V = 268.19(7) Å 3 ), and KFeH(SO 4 ) 2 ·2H 2 O(space group P 1̅, no. 2, Z = 1, a = 4.626(1), b = 5.820(1), c = 8.265(1) Å, α = 103.86(1), β = 99.77(1), γ = 95.66(1)°, V = 210.64(2) Å 3 ) have been determined from single-crystal X-ray diffraction data collected with a CCD area detector at 293 K, and were refined to R ( F ) = 0.0231, 0.0251, 0.0230 and 0.0194, respectively. The first two compounds are isostructural. The atomic arrangements are described, with a focus on the hydrogen bonding. All compounds contain kröhnkite [Na 2 Cu(SO 4 ) 2 ·2H 2 O]-type tetrahedral-octahedral chains but show different topological arrangements of these chains. A classification is proposed in an accompanying paper. A particular feature of KFeH(SO 4 ) 2 ·2H 2 O is a disordered H atom involved in a short hydrogen bond, which shows no ordering down to 110 K.

The crystal structures of the compounds Ba M GaO 4 ( M = Al, Fe) have been investigated using conventional single-crystal X-ray diffraction data for M = Fe and with synchrotron radiation data for M = Al. BaAlGaO 4 (space group P 6 3 22, a = 5.2788(4) Å and c = 8.7835(7) Å, V = 211.97(4) Å 3 , Z = 2, D calc . = 4.67 g cm –3 , wR 2 = 0.089 for 242 independent reflections with I > 2σ( I )) is isostructural with BaAl 2 O 4 -I. Main building units are layers of six-membered rings of (Al,Ga)O 4 -tetrahedra perpendicular to [001]. The sequence of directedness of the tetrahedral apices in all single six membered rings (S6R) is UDUDUD (U: up, D: down). Stacking of layers parallel to c results in a three-dimensional framework containing tunnels, where the Ba atoms are located. They are coordinated by nine oxygen ligands in form of distorted tricapped trigonal antiprisms. BaFeGaO 4 also belongs to the hexagonal crystal system (space group P 6 3 , a = 10.8377(6) Å and c = 8.6865(6) Å, V = 883.59(9) Å 3 , Z = 8, D calc . = 4.92 g cm –3 , wR 2 = 0.099 for 1042 independent observed reflections). The crystal showed twinning by merohedry which was accounted for in the refinement calculations. Within a single tetrahedral layer of the framework structure two different types of ditrigonal-shaped rings are distinguishable: one fourth of the rings have an UDUDUD topology, whereas the sequence of the remaining rings is UUUDDD. The Fe 3+ - and Ga 3+ -cations show an ordered distribution among the four symmetrically independent T-sites: two positions contain either Fe or Ga, whereas on the remaining two sites there is a definite preference for one of the two cation species. The Ba atoms in the cavities of the framework are coordinated by eight to nine oxygen neighbors. The observed twinning in BaFeGaO 4 can be attributed to the existence of pseudo twofold axis within the tetrahedral layers, running parallel [100] and the corresponding symmetrically equivalent directions.

The crystal structure of Bi 4 O 5 I 2 has been solved from a 3:1 twinned crystal; accurate lattice parameters have been determined by a Rietveld refinement. Bi 4 O 5 I 2 crystallizes with space group P 2 1 (no. 4), Z =4, a =14.944(1), b =5.6983(3), c =11.263(1) Å, β = 99.81(1)°. Its structure is isotypic to the structure of the homologous Bi 4 O 5 Br 2 for which more reliable lattice parameters have been determined by a Rietveld refinement. The effects of replacement of Br by I on the structure are discussed and compared with corresponding effects in the BiOX family (X = Cl, Br, I). A structural reason for the deviation of the twin supercells of the two compounds from orthogonality is discussed.