Galina V. Kiriukhina

Crystal structure of a new polymorphic modification of niahite, NH4MnPO4 · H2O

The crystal structure of a new modification of the niahite mineral NH4MnPO4 · H2O, obtained in the form of single crystals under hydrothermal conditions in the Mn2O3-(NH4)3PO4-H2O system, is determined by X-ray diffraction (Xcalibur-S-CCD diffractometer, R = 0.0259): a = 17.582, b = 4.909, and c = 5.731 Å; space group D2h16 = Pnam, Z = 4, Dx = 2.497 g/cm3. It is shown that the new compound is a polymorphic modification of niahite; it has a layered structure, which differs from the structure of the mineral by the presence of the center of inversion and the doubled a unit cell parameter. It is proposed to consider the minerals and synthetic analogues of the niahite and triphylite groups as members of the common polysomatic series.

Canted anti­ferromagnet superimposed on a buckled kagomé network in RbMn4(PO4)3

Rubidium tetramanganese tris(phosphate), RbMn4(PO4)3, has been synthesized as single crystals under hydrothermal conditions. The crystal structure was refined in the space group Pnnm (D2h12). It is argued that the size factor RM/RA, i.e. the ratio of the A+ ionic radius to the M2+ ionic radius, within the morphotropic series AM4(TO4)3 corresponds to a specific type of crystal structure. At low temperatures, the antiferromagnet superimposed on a buckled kagomé network in RbMn4(PO4)3 experiences a transition into a long-range ordered state with finite spontaneous magnetization. First principles calculations provide the dominant magnetic exchange interactions both within and between the kagomé layers. The analysis of these interactions allows us to suggest a model of alternating ferromagnetic and antiferromagnetic arrangements within chains of Mn3 atoms.

Crystal Structure of Rb2Mn3(H2O)2[P2O7]2, a New Representative of the Family of Hydrated Diphosphates

The crystal structure of Rb2Mn3(H2O)2[P2O7]2, a new phase obtained in the form of single crystals under hydrothermal conditions in the MnCl2–Rb3PO4–H2O system, is determined by X-ray diffraction (Xcalibur-S-CCD diffractometer, R = 0.0270): a = 9.374(2), b = 8.367(2), c = 9.437(2) Å, ß = 99.12(2)°, space group P21/c, Z = 2, Dx = 3.27 g/cm3. A correlation between the unit-cell parameters and the size of cations forming the crystal structures of isostructural A2M3(H2O)2[P2O7]2 diphosphates (A = K, NH4, Rb, or Na; M = Mn, Fe, Co, or Ni) is revealed. It is shown that, due to the topological similarity, the structures of diphosphates and orthophosphates of the farringtonite structural type can undergo mutual transformations.

A microporous potassium vanadyl phosphate analogue of mahnertite: hydrothermal synthesis and crystal structure

Abstract The novel phase K2.5Cu5Cl(PO4)4(OH)0.5(VO2)•H2O was prepared by hydrothermal synthesis at 553 K. Its crystal structure was determined using low-temperature (100 K) single-crystal synchrotron diffraction data and refined against F2 to R = 0.035. The compound crystallizes in the tetragonal space group I4/mmm, with unit-cell parameters a = 9.8120(8), c = 19.954(1) Å, V = 1921.1(2) Å3, and Z = 4. Both symmetrically independent Cu2+ sites show elongated square-pyramidal coordination. The V5+ ions reside in strongly distorted five-vertex VO5 polyhedra with 50% occupancy. The structure is based on a 3D anionic framework built from Cu- and V-centered five-vertex polyhedra and PO4 tetrahedra. Channels in the [100] and [010] directions accommodate large K atoms and H2O molecules. The compound is a new structural representative of the topology shown by the lavendulan group of copper arsenate and phosphate minerals. Their tetragonal or pseudotetragonal crystal structures are characterized by two types of 2D slabs alternating along one axis of their unit cells. One slab, described by the formula [Cu4X(TO4)4]∞ (where X = Cl, O and T = As, P), is common to all phases, whereas the slab content of the other set differs among the group members. We suggest interpreting this family of compounds in terms of the modular concept and also consider the synthetic phase Ba(VO)Cu4(PO4)4 as a simplest member of this polysomatic series.

Synthetic shibkovite K(K1.67H2O0.33)(Ca1.3Na0.7)[Zn3Si12O30]: the crystal structure and comparative crystal chemistry

The structure of a single crystal of a synthetic analog of mineral shibkovite K(K1.67H2O0.33)(Ca1.3Na0.7)[Zn3Si12O30] (milarite structure type) obtained by hydrothermal synthesis in the AlPO4-K3PO4-CaCO3-Na2CO3-ZnCO3-SiO2-H2O system has been solved (R = 0.0406) by X-ray diffraction analysis: a = 10.5327(2) Å, c = 14.2019(3) Å, sp. gr. P6/mcc, Z = 2, and ρcalcd = 2.90 g/cm3. The crystal-chemical features of the new phase are studied in comparison with the other terms of the milarite group. It is shown that the crystallization conditions for minerals and synthetic analogs of this group determine the presence or absence of crystallization water in the structures of compounds.