Crystal structure of LiH2PO4, structural topology and hydrogen bonding in the alkaline dihydrogen orthophosphates

Abstract

Abstract LiH2PO4 crystallizes in space group Pna21; a = 6.253(4), b = 7.656(4), c = 6.881(4) Å, Z = 4. With 1268 observed reflections measured on a single-crystal diffractometer (MoKα radiation), the structure was solved (Patterson methods) and refined anisotropically (least-squares) to R = 0.022. The hydrogen atoms had been located from difference maps. LiO4 coordination polyhedra are linked by vertices and form [100] isolated chains. The two independent hydrogen bridges, 2.684 and 2.564 Å long, connect the PO4 polyhedra building up a three-dimensional framework; [100] channels containing H atoms are observed in the structure. Similarities with the structure of NaH2PO4 · H2O are discussed. The effect of increasing the alkali cation size in the series MH2PO4 causes a progressive condensation of the coordination polyhedra by sharing vertices and edges, from single chains (LiH2PO4) to very compact frameworks (CsH2PO4) of polyhedra. Shorter hydrogen bonds are observed when larger alkali cations are present; this effect is discussed in the light of bond-strength balance concepts.