Hydrogen bonding in the crystalline state. The crystal structure of NaH2PO4.2H2O by X-ray and neutron diffraction

Sodium dihydrogen phosphate dihydrate (NaH2PO4.2H20) is orthorhombic, space group P212121, with a=7.275 (9), b=11-384 (6) and c=6-606 (4) A; Z=4. The structure was solved from X-ray measurements by direct methods and refined anisotropically with X-ray (R = 0.032 over 898 reflexions) and neutron data (R=0.059 over 751 reflexions); both sets of intensities were collected on singlecrystal diffractometers (Mo Ks radiation and 2= 1-035 A respectively). A correction for secondary extinction was applied to the neutron data. While the two acidic hydrogens form strong hydrogen bonds (2.577 and 2.536 ,&), one of those of the water molecules is involved in a very weak O H . . . O contact of 3-106 ,&. The P tetrahedra are linked by hydrogen bonds in a distorted diamond-type framework. Six systems of pseudo-hexagonal channels are present; the largest of them, along [001], accommodates a chain of Na octahedra. The configuration of the hydrogen-bonded groups is described. Bond-strength/bond-length correlations are discussed and their weakness for the case of strongly hydrogen-bonded atoms is pointed out. A comparison of X-ray and neutron results by Z 2 tests and half-normal probability plots shows significant differences for the thermal parameters only.