Examination of the solid phases in the system AgI–NaI–H₂O

By means of several low temperature devices the solid hydrous phases of the AgI–NaI–H₂O system have been extensively studied by X-ray diffraction, transmission Laue photography, and polarizing microscopy. The density, composition and unit-cell parameters have been determined for three distinct AgI–NaI hydrates for which the stability regions correspond to phases C, E, and F previously reported by Davis [J. Atmos. Sci. (1969), 26, 1042]. The cell assignments are based on determinations of axial ratios from the Laue photographs and interplanar spacing analyses utilizing the Hesse–Lipson technique, Cohen cell-parameter refinement and the de Wolff cell-reliability criteria. All phases are of the composition AgI–NaI–nH₂O with phase C at 25 °C having n = 4, ρ = 3.0 g.cm⁻³ (measured and calculated) and indexing on the basis of a monoclinic cell with parameters a₀ = 7.573, b₀ = 9.454, c₀ = 12.403 Å, and β = 92.43° with Z = 4. Phase F at −10 °C is also monoclinic with a₀ = 5.757, b₀ = 13.563 and c₀ = 12.541 Å and β = 103.05° with Z = 4, and n = 4. Phase E appears to consist of two structures, very similar but differing in cell volume by 2.8%. We believe this to be due to varying water content, the n = 3 phase being metastable. The cell reliability factor is less than 10 for both cells and indexing is considered to be tentative.