Molten Salt Synthesis of an Open-Frame Aluminum Phosphate Ba3Al2P4O16 with a Rare-Pyramidal AlO5 Group

A new open-frame aluminophosphate, Ba₃Al₂P₄O₁₆, was synthesized by a high-temperature flux method. Ba₃Al₂P₄O₁₆ features discrete [Al₄P₈O₃₂]_∞^12- chains with rare square-pyramidal AlO₅ groups adopted. Especially, this pyramidal AlO₅ group acts as an ideal "plane builder" or "vacancy inducer" to construct open-frame structures compared with an ordinary trigonal-bipyramidal AlO₅ group.

Joint crystallization of KCuAl[PO4]2 and K(Al,Zn)2[(P,Si)O4]2: crystal chemistry and mechanism of formation of phosphate-silicate epitaxial heterostructure

Two novel phases, potassium copper aluminium bis(phosphate), KCuAl[PO4]2 (I), and potassium zinc aluminium bis(phosphate-silicate), K(Al,Zn)2[(P,Si)O4]2 (II), were obtained in one hydrothermal synthesis experiment at 553 K. Their crystal structures have been studied using single-crystal X-ray diffraction. (I) is a new member of the A + M 2+ M 3+[PO4]2 family. Its open 3D framework built by AlO5 and PO4 polyhedra includes small channels populated by columns of CuO6 octahedra sharing edges, and large channels where K+ ions are deposited. It is assumed that the stability of this structure type is due to the pair substitution of Cu/Al with Ni/Fe, Co/Fe or Mg/Fe in different representatives of the series. From the KCuAl[PO4]2 structural features, one may suppose it is a potentially electrochemically active material and/or possible low-temperature antiferromagnet. In accordance with results obtained from X-ray diffraction data, using scanning electron microscopy, microprobe analysis and detailed crystal chemical observation, (II) is considered as a product of epitaxial intergrowth of phosphate KAlZn[PO4]2 and silicate KAlSi[SiO4]2 components having closely similar crystal structures. The assembly of `coherent intergrowth' is described in the framework of a single diffraction pattern.

Rb2CaCu6(PO4)4O2, a novel oxophosphate with a shchurovskyite-type topology: synthesis, structure, magnetic properties and crystal chemistry of rubidium copper phosphates

Single crystals of Rb₂CaCu₆(PO₄)₄O₂ were synthesized by a hydrothermal method in the multicomponent system CuCl₂-Ca(OH)₂-RbCl-B₂O₃-Rb₃PO₄. The synthesis was carried out in the temperature range from 690 to 700 K and at the general pressure of 480-500 atm [1 atm = 101.325 kPa] from the mixture in the molar ratio 2CuO:CaO:Rb₂O:B₂O₃:P₂O₅. The crystals studied by single-crystal X-ray analysis were found to be monoclinic, space group C2, a = 16.8913 (4), b = 5.6406 (1), c = 8.3591 (3) Å, β = 93.919 (3)°, V = 794.57 (4) ų. The crystal structure of Rb₂CaCu₆(PO₄)₄O₂ is similar to that of shchurovskyite and dmisokolovite and is based upon a heteropolyhedral open framework formed by polar layers of copper polyhedra linked via isolated PO₄ tetrahedra. The presence of well-isolated 2D heteropolyhedral layers in the title compound suggests low-dimensional magnetic behavior which is masked, however, by the fierce competition between multiple ferromagnetic and antiferromagnetic exchange interactions. At T_C = 25 K, Rb₂CaCu₆(PO₄)₄O₂ reaches a magnetically ordered state with large residual magnetization.

The rich crystal chemistry of the AMM'(PO4)2 morphotropic series: NaZnAl(PO4)2, the first Na representative with a new structure type

A novel phosphate, sodium zinc aluminium bis(phosphate), NaZnAl(PO₄)₂, was obtained under mild-temperature hydrothermal conditions at 553 K. The crystal structure has been studied using single-crystal X-ray experimental data. The pseudo-hexagonal phase NaZnAl(PO₄)₂ crystallizes in the monoclinic space group P2₁/c. Its unique crystal structure is based on a three-dimensional (3D) framework built by Zn-, Al- and P-centred tetrahedra sharing vertices. Channels parallel to the [101] and [-101] directions are limited by six- and eight-membered windows, and incorporate Na atoms. The new compound is discussed as a member of the morphotropic series AMM'PO₄, where A = Na, K, Rb or NH₄, M = Cu, Ni, Co, Fe, Zn or Mg and M' = Fe, Al or Ga. The title compound is the first Na representative within the series and is characterized by a 3D architecture of tetrahedra populated in an ordered manner by Zn²⁺, Al³⁺ and P⁵⁺ ions.