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

Abstract The 1:1:2 potassium aluminum phosphate KAl(HPO 4 ) 2 ·H 2 O has been synthesized and characterized by single-crystal X-ray diffraction. The compound crystallizes in the monoclinic space group P 2 1 / c with Z =8. Crystal data: a =1004.0(2) pm, b =910.7(2) pm, c =1625.4(3) pm, β =100.98(2)°, R g =0.044. A neutron-scattering experiment with subsequent refinement of the powder pattern of KAl(DPO 4 ) 2 ·D 2 O 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, β =100.97(1)°, R wp =0.044. The structure consists of two different AlO 6 -octahedra and four different HPO 2− 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. 31 P solid-state MAS spectra show only three δ iso 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 HPO 2− 4 tetrahedra it can be assumed that the first δ iso 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 σ ii of the nuclear magnetic shielding tensors of the phosphorus atoms are observed only for σ 11 of P(4) being more deshielded and for σ 22 of P(1) being more shielded.