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

Two novel phases, potassium copper aluminium bis(phosphate), KCuAl[PO] (I), and potassium zinc aluminium bis(phosphate-silicate), K(Al,Zn)[(P,Si)O] (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 AMM[PO] family. Its open 3D framework built by AlO and PO polyhedra includes small channels populated by columns of CuO 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[PO] 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[PO] and silicate KAlSi[SiO] components having closely similar crystal structures. The assembly of `coherent intergrowth' is described in the framework of a single diffraction pattern.