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The structure of tetragonal NH4H2PO 4 has been determined in a single-crystal neutron-diffraction study. The N-If • • • O and O-H • • • O bond systems suggested in earlier X-ray investigations were confirmed. The H2PO [ network is very similar to that found in KH2PO 4 ; the only significant difference observed was in the angle between the P-O and O-i • • • O bonds. The ammonium ion was found to be only slightly distorted from a regular tetrahedral arrangement. Considerable distortion would be necessary for linear N-i • • • O bonds. The observed angle between the Eli bond and the line joining nitrogen and oxygencenters was about 14 °. In order to achieve good agreement between calculated and observed data it was necessary to introduce anisotropic thermal vibration parameters.The final value of the discrepancy factor was 8.9 %.

The CaWO4 structure has been refined, starting with the parameters of Sillén and Nylander, using two zones of neutron diffraction data. The oxygen coordinates with the origin of the cell at the W atom (4̄) are x=0.2413 (σ=0.0005), y=0.1511 (σ=0.0006), z=0.0861 (σ=0.0001). The slightly distorted WO4 tetrahedron contains a W–O distance of 1.788 Å (σ=0.003) and angles of 113° 27′ (σ=14′) and 107° 56′ (σ=0.09′). Close nonbonded approaches are Ca–O=2.48 Å and O–O=2.77 Å. Anisotropic thermal vibration parameters were also determined.