Corner-connected Octahedra Research Articles

Redefinition of the formula for aldermanite, [Mg(H2O)6][Na(H2O)2Al3(PO4)2(OH,F)6]⋅H2O, and its crystal structure

AbstractAldermanite from Tom's quarry in the Kapunda–Angaston area of the Mount Lofty Ranges, South Australia has been characterised by electron microprobe analyses and single-crystal structure analysis. The empirical formula is Na0.72K0.13Ca0.06Mg1.15Al2.92(PO4)2.05[(OH)2.92F2.96]Σ5.88⋅8.91H2O, based on 23 anions. Analysis of a specimen from the type locality, the nearby Klemm's quarry, Moculta, gave a similar formula, Na0.59K0.06Ca0.36Mg0.92Al3.16(PO4)1.97[(OH)4.08F2.70]Σ6.78⋅8.36H2O. Na and F were not analysed in the original description of the mineral. The ideal end-member formula is [Mg(H2O)6][Na(H2O)2Al3(PO4)2(OH)6]⋅H2O, compared to the original formula of Mg5Al12(PO4)8(OH)22⋅nH2O withn≈ 32. Aldermanite is monoclinic,P21/cwitha= 13.524(3),b= 9.958(2),c= 7.013(1) Å and β = 97.40(3)°. The crystal structure of aldermanite is built from sawtooth layers ofcis- andtrans-corner-connected, Al-centred octahedra, decorated with corner-connected PO4tetrahedra to give (100) layers of composition Al3(PO4)2(OH,F)6. Interlayer Mg(H2O)6octahedra and H2O molecules hold the layers together through H bonding. The corner-connected octahedra form 6-membered rings that are centred by 8-coordinated Na and have a topology identical to a 3-octahedra-wide {110} slice of the pyrochlore structure. This pyrochlore element contains intersecting kagomé nets of Al atoms, parallel to (111) and (11$\bar{1}$) of cubic pyrochlore. Minerals of the walentaite group, as well as zirconolite-3Opolytypes have the same type of intersecting kagomé nets of small cations.

First-Principles Prediction of a Stable Hexagonal Phase of CH3NH3PbI3

Methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) perovskite is a promising new photovoltaic material with high power conversion efficiency. However, its perovskite phase with corner-connected PbI6 octahedra shows poor environmental stability. More recently, MAPbI3 has been shown to be thermodynamically unstable with a positive formation enthalpy. Here, using first-principles density functional theory calculations, we predict a layered hexagonal phase of MAPbI3 consisting of infinite chains of face-shared PbI6 octahedra with P63mc space-group symmetry to be thermodynamically the most stable phase for a wide range of volume and temperature compared to any of the experimentally observed perovskite phases with a different tilt pattern of the corner-connected octahedra. The predicted hexagonal phase is also dynamically stable without any soft phonon modes. The change from corner to face-shared connectivity in the hexagonal phase leads to a predicted band gap of 2.6 eV and a band structure that favors highly a...

Jarosite–butlerite intergrowths in non-stoichiometric jarosites: crystal chemistry of monoclinic natrojarosite–hydroniumjarosite phases

AbstractMonoclinic, non-stoichiometric natrojarosite—hydroniumjarosite solid solution phases have been synthesized hydrothermally over a range of temperatures, starting compositions and reaction times, and have been characterized using Rietveld refinement of synchrotron X-ray diffraction data, and chemical and thermal analyses. The H atom locations have been obtained from refinement of neutron diffraction data on a deuterated sample. The results confirm a direct relationship between the monoclinic distortion and the ordering of iron site vacancies in one of two independent iron sites. Ordering of iron vacancies gives rise to domains containing butlerite-like 7 Å chains of corner-connected octahedra and tetrahedra. The formation of these chains within (100) planes results in an expansion of the monoclinic a lattice parameter and a contraction of the c parameter relative to stoichiometric jarosites. The results support a recent model for iron deficiency, whereby an iron vacancy is compensated by the replacement of four coordinated OH– ions by H2O molecules, with one of the H2O molecules coming from deprotonation of H3O+. The general formula, based on intergrowth of stoichiometric jarosite and non-stoichiometric, butlerite-like regions, is [(Na,H3O)Fe3(SO4)2(OH)6]1–y[(H2O)Fe2(SO4)2(OH)2(H2O)4]y.

A NEW CRYSTAL-CHEMICAL VARIATION OF THE ALUNITE-TYPE STRUCTURE IN MONOCLINIC PbZn0.5Fe3(AsO4)2(OH)6

The crystal structure of a segnitite-related mineral with composition Pb[Zn0.50.5]Fe3(AsO4)2(OH)6,  = vacancy, from Broken Hill, Australia, has been determined using single-crystal X-ray data. The mineral has monoclinic symmetry, space group C2/c, with a 25.8898(6), b 14.8753(2), c 12.1700(2) A, b 110.681(1)°, Z = 16. All crystals examined were found to be multiply twinned by 120° rotations about c*, which corresponds with ch of the hexagonal alunite-type subcell. The structure was determined on a twinned crystal with one dominant individual, and refined to R = 0.055 for 4089 reflections with F > 4sF. The general features of the monoclinic structure match those of the prototype rhombohedral alunite-type structure. This involves a rhombohedral stacking of hexagonal tungsten-bronze-type sheets of corner-connected octahedra that share their apical anions with AsO4 tetrahedra on either side of the layer of octahedra, with Pb atoms located between the layers. However, the structure differs from those of other alunite-type minerals in having zinc atoms ordered in trigonal bipyramidal sites in half of the available six-membered rings of the layers of octahedra. These sites are unoccupied in previously reported alunite-type structures. Ordered displacements of the Pb atoms occur in response to the Zn– ordering.

(C6H5CH2CH2NH3)2[CdCl4

The title compound, bis(β-phenylethylammonium) tetrachlorocadmate, (C8H12N)2[CdCl4] crystallizes as a layer structure; the heavy atoms form a two-dimensional network of corner-connected octahedra, the organic cation being connected to the network through hydrogen bonding.

AB3 nets built from corner-connected octahedra: geometries, electrostatic lattice energies, and stereochemical discussion

Abstract Ideal AB6 octahedra are known to form, by sharing all corners (but no edges), three topologically different kinds of three-dimensional periodic nets: perovskite type, hexagonal tungsten bronze type, and pyrochlore type. The geometries of the arrangements with the highest possible symmetry as well as some of the distortions are discussed and electrostatic lattice energies are computed. The electrostatic lattice energies are in part compatible with experience in stereochemistry. Notable exceptions are several trifluorides that do not adopt the perovskite arrangement with the highest possible symmetry but rather rhombohedrically distorted structures. This behaviour is tentatively interpreted to be a consequence of the polarizability of the F−and/or the admixture of covalent bonds.

EXPERIMENTAL PROGRESS IN UNDERSTANDING HIGH-TEMPERATURE SUPERCONDUCTIVITY

In this paper first the structure of the newest layered compounds is presented, which do not contain rare-earth ions, instead either Bi- or Tl-oxide layers are sandwiched between the copper-oxide layers. The latter consist of either corner-connected octahedra, pyramids or planar squares: the transition temperature T/sub c/ increases with the density of copper-oxide layers per unit cell and is accounted for by single-electron interlayer tunneling. The Bi- or Tl-oxide layers can either be double or single, with the former having the higher T/sub c/. Of considerable scientific interest is the discovery of a cubic perovskite with a high T/sub c/ of 29.6 {Kappa} which contains no copper. Ba/sub 1-x/K/sub x/BiO/sub 3/. Owing to its doping with K/sup +/ for Ba/sup 2+/, holes must again be present. If they are on the oxygens, this would be a common feature with the cuprates. This paper is aimed at determining the superconducting gap {Delta} and its anisotropy. Some measurements of infrared reflectivity on single crystals and nuclear magnetic resonance in powders indicate that the ratio is 2{Delta}/{kappa}T/sub c/ {ge} 7. Infrared data supporting this ratio in BaY/sub 2/Cu/sub 3/O/sub 7/ have now been extended. Others however yield values near the BCS weak couplingmore » limit of 3.5. Determination of this ratio is important, because it indicates whether the coupling is weak or strong. A Kramer-Kronig analysis of IR data in the normal-conducting regime supports intermediately strong coupling, but weaker than in many fermion systems. It is also pointed out that the tunneling experiments yield gaps which are somewhat too low because {Delta} is depressed at the surface owing to the intrinsic short coherence lengths.« less