Structures and thermal behaviour of alkali metal dihydrogen phosphate HF adducts, MH2PO4 center dot HF (M = K, Rb, Cs), with hydrogen bonds of the F-H center dot center dot center dot O type

Three HF adducts of alkali metal dihydrogen phosphates, MH2PO4. HF (M = K, Rb, Cs), have been isolated from fluoroacidic solutions of MH2PO4. KH2PO4. HF crystallizes monoclinic: P2(1)/c, a=6,459(2), b=7,572(2), c= 9,457(3) An gstrom, beta =101,35(3)degrees, V = 453,5(3) Angstrom (3), Z = 4. RbH2PO4. HF and CsH2PO4. HF are orthorhombic: Pna2(1), a= 9,055(3), b=4,635(2), c=11 ,908(4)Angstrom, V=499,8(3)Angstrom (3), Z = 4, and Pbca, a = 7,859(3), b = 9,519(4), c = 14,744(5) Angstrom, V = 1102,5(7)Angstrom (3), Z = 8, respec tively. The crystal structures of MH2PO4. HF contain M+ cations, H2PO4- ani ons and neutral HF molecules. The H2PO4- anions are connected to layers by O-H . . .O hydrogen bonds (2,53-2,63 Angstrom), whereas the HF molecules ar e attached to the layers via very short hydrogen bonds of the F-H . . .O ty pe (2,36-2,38 Angstrom). The thermal decomposition of the adducts proceeds in three steps. The first step corresponds to the release of mainly HF and a smaller quantity of wat er. In the second and third steps, water evolution caused by condensation o f dihydrogen phosphate is the dominating process whereas smaller amounts of HF are also released.