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

Crystal growth, structure elucidation and CHARDI/BVS investigations of β-KCoFe(PO4)2

Single crystals of β-KCoFe(PO4)2, potassium cobalt(II) iron(III) bis-(ortho-phosphate), were grown from the melt under atmospheric conditions. This phosphate crystallizes isotypically with KZnFe(PO4)2 in space group C2/c, adopting a zeolite-ABW type of structure. The structure of the present phosphate is distinguished by an occupational disorder of the two transition-metal sites with ratios Fe:Co of 0.5725:0.4275 for the first and 0.4275:0.5725 for the second site. In the crystal structure, PO4 and (Co,Fe)O4 tetra-hedra are linked through vertices to form elliptical rings with the sequence DDDDUUUU of up (U) and down (D) pointing vertices. Each eight-membered ring is surrounded by four other rings of the same type, delimiting inter-stices with rectangular shape. This arrangement leads to the formation of [(Co/Fe)(PO4)]- ∞ sheets parallel to (001). Stacking of the sheets into a three-dimensional framework results in the formation of two types of channels. The first one is occupied by potassium cations, whereas the second one remains vacant. Calculations of bond-valence sums and charge distribution were used to confirm the structure model.

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.

Novel K/Mn phosphate hydrates, K2Mn3(H2O)2[P2O7]2 and KMn(H2O)2[Al2(PO4)3]: hydrothermal synthesis and crystal chemistry

Two novel K/Mn phosphate hydrates, namely, dipotassium trimanganese dipyrophosphate dihydrate, K₂Mn₃(H₂O)₂[P₂O₇]₂, (I), and potassium manganese dialuminium triphosphate dihydrate, KMn(H₂O)₂[Al₂(PO₄)₃], (II), were obtained in the form of single crystals during a single hydrothermal synthesis experiment. Their crystal structures were studied by X-ray diffraction. Both new compounds are members of the morphotropic series of phosphates with the following formulae: A₂M₃(H₂O)₂[P₂O₇]₂, where A = K, NH₄, Rb or Na and M = Mn, Fe, Co or Ni, and AM²⁺(H₂O)₂[M³⁺₂(PO₄)₃], where A = Cs, Rb, K, NH₄ or (H₃O); M²⁺ = Mn, Fe, Co or Ni; and M³⁺ = Al, Ga or Fe. A detailed crystal chemical analysis revealed correlations between the unit-cell parameters of the members of the series, their structural features and the sizes of the cations. It has been shown that a mixed type anionic framework is formed in (II) by aluminophosphate [(AlO₂)₂(PO₄)₂]_∞ layers, with a cationic topology similar to the Si/Al-topology of the crystal structures of feldspars. A study of the magnetic susceptibility of (II) demonstrates a paramagnetic behaviour of the compound.

Synthesis and crystal structure of NaCuIn(PO4)2

Single crystals of sodium copper(II) indium bis-[phosphate(V)], NaCuIn(PO4)2, were grown from the melt under atmospheric conditions. The title phosphate crystallizes in the space group P21/n and is isotypic with KCuFe(PO4)2. In the crystal, two [CuO5] trigonal bipyramids share an edge to form a dimer [Cu2O8] that is connected to two PO4 tetra-hedra. The obtained [Cu2P2O12] units are inter-connected through vertices to form sheets that are sandwiched between undulating layers resulting from the junction of PO4 tetra-hedra and [InO6] octa-hedra. The two types of layers are alternately stacked along [101] and are joined into a three-dimensional framework through vertex- and edge-sharing, leaving channels parallel to the stacking direction. The channels host the sodium cations that are surrounded by four oxygen atoms in form of a distorted disphenoid.