The Potassium Aluminum Phosphate KAl(HPO4)2⋅H2O: X-Ray Diffraction, Neutron-Scattering, and Solid-State NMR Characterization

Author： Dick S. Gosner U. Weiss A. Robl C. Grossmann G. Ohms G. Muller M.

Publisher： Academic Press

ISSN： 0022-4596

Source： Journal of Solid State Chemistry, Vol.132, Iss.1, 1997-08, pp. : 47-55

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Abstract

The 1:1:2 potassium aluminum phosphate KAl(HPO4)2⋅H2O has been synthesized and characterized by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic space group P21/c with Z=8. Crystal data: a=1004.0(2) pm, b=910.7(2) pm, c=1625.4(3) pm, beta=100.98(2)#°, Rg=0.044. A neutron-scattering experiment with subsequent refinement of the powder pattern of KAl(DPO4)2⋅D2O elucidated the D atom positions in the deuterated compound. Crystal data: a=1003.7(1) pm, b=910.5(1) pm, c=1622.3(2) pm, beta=100.97(1)#°, Rwp=0.044. The structure consists of two different AlO6-octahedra and four different HPO2-4 tetrahedra. Corner sharing between these polyhedra creates a complicated three-dimensional framework with K+ in cavities. Building units of that framework are Al phosphate chains that form various interconnections by corner sharing of polyhedra and hydrogen bonds. The hydrogen-bond donor and acceptor properties of the phosphates and water molecules are discussed along with their function in the structure. 31P solid-state MAS spectra show only three deltaiso values at -6.5, -10.7 and -12.6 ppm instead of the expected four different isotropic chemical shifts. The sideband system at -10.7 ppm has twice the intensity of each of the outer systems. Considering the surroundings of the four HPO2-4 tetrahedra it can be assumed that the first deltaiso value results from P(4), the second from P(2) and P(3), and the third from P(1). Significant differences between a given value of the principal axes sigmaii of the nuclear magnetic shielding tensors of the phosphorus atoms are observed only for sigma11 of P(4) being more deshielded and for sigma22 of P(1) being more shielded.