Hydrogen bonding in the crystalline state. Structure of NaH₂PO₄.H₂O (orthorhombic phase), and crystal chemistry of the NaH₂PO₄.nH₂O series

Abstract

NaH2PO4. H20, orthorhombic phase, crystallizes in space group Pna21, with a = 7"616 (5), b = 7-899 (3), c=7.382 (2) /~, Z=4. The structure was solved (direct methods) and refined anisotropically (least squares, R= 0.034) with 678 reflexions measured on a diffractometer with Mo K~ radiation. H atoms were located on difference maps, but were not refined. Pnam pseudo-symmetry explains the formation of (001) twins by merohedry.[001] chains of Na coordination octahedra sharing edges develop in pseudotetragonal channels present in the framework of P tetrahedra; these are tied by medium-strength acidic hydrogen bonds. Water hydrogen bonds are weak. Bond-strength/bond-length correlations are discussed. Chains of Na polyhedra are present in all structures of the series NaH2PO4. nH20; their linking accounts for the decreasing packing density with increasing hydration. The close similarity of structures of the bihydrated and of the two monohydrated forms is analysed, and a mechanism is proposed for the solidstate reaction NaH2PO4.2H20 .~-NaH2PO4. H20 + H20.

Thermo-Raman Studies on NaH2PO4·2H2O for Dehydration, Condensation, and Phase Transformation

“…Sodium dihydrogen phosphate is mainly found in the hydrated form as NaH 2 PO 4 ·2H 2 O at room temperature. However, NaH 2 PO 4 ·4H 2 O was reported to be present at −20 °C . The solubility studies suggest the existence of two hydrated forms, NaH 2 PO 4 ·2H 2 O and NaH 2 PO 4 ·H 2 O, and an anhydrous form, NaH 2 PO 4 , above room temperature…”

“…The crystal structure of NaH 2 PO 4 ·2H 2 O was well-resolved by X-ray diffraction and by neutron scattering by Bartl et al The four oxygen atoms are not equivalent, but the structural stability is achieved due to the strong hydrogen bonding. The two acidic hydrogens of the H 2 PO 4 - group form strong hydrogen bonds with the oxygen of the adjacent H 2 PO 4 - , and the water molecules are weakly bounded in the structure. , This should be the reason for the complicated spectrum of NaH 2 PO 4 ·2H 2 O at room temperature and for the first dehydration to occur at such a low temperature.…”

New insights into oxygen environments generated during phosphate glass alteration: a combined 17O MAS and MQMAS NMR and first principles calculations study

“…The two structures, NaH 2 PO 4 ÁH 2 O and NaH 2 PO 4 Á2H 2 O (Fig. 1c and d, respectively), have been described by Catti and Ferraris 60 and Bartl et al, 61 respectively. The structure of NaH 2 PO 4 ÁH 2 O, orthorhombic phase, was determined using single crystal X-ray measurements and direct methods of refinement.…”

Mechanism of the reaction Ca5H2(AsO4)4 · 9 H2O (ferrarisite) → Ca5H2(AsO4)4 · 5 H2O) (dimorph of vladimirite), and structure of the latter phase

“…the analysis of the reaction NaH2P04'2H20~NaH2P04 . H 2 0 (Catti and Ferraris, 1976), and the study of the topotactic dehydration CaiAs0 4 )2' 11 H 2 0 (new mineral) -> Ca 3 (As0 4 h . 10 H 2 0 (rauenthalite), which has been recently completed in our laboratory (Catti and Ivaldi, in preparation).…”