Linear compressibilities of NaNO₂ and NaNO₃

The linear compressibilities of sodium nitrite and sodium nitrate have been determined from x‐ray diffraction of synthetic single crystals mounted in a miniature diamond‐anvil high‐pressure cell at pressures to 27 kbars. Compressibilities of both compounds are anisotropic; compression perpendicular to rigid N‐O bonds is approximately 3.5 times greater than compression within the plane of N‐O bonding. In NaNO₂ axial compression ratios are a : b : c=1.0 : 0.46 : 0.28. For a second‐order Birch‐Murnaghan equation of state, K₀=219±2 kbars and K′=4.3±0.8. The a : c axial compression ratio of NaNO₃ is 0.35 : 1.0. Linear compressibilities are in agreement with data of Bridgman, who determined bulk modulus, K₀=258±6 kbars, and K′=6.6±1.5. The product of bulk modulus and molar volume (K₀V₀) is a constant for alkali halides with the NaCl structure, and the same value is observed for NaNO₂, which is only slightly distorted from an NaCl arrangement of Na⁺ and NO⁻₂. Sodium nitrate, which does not posssess the NaCl topology of Na⁺ and NO⁻₃, deviates from this constant. Broadening of x‐ray diffraction maxima of NaNO₂ above 10 kbars may be due to twinning in a high‐pressure phase of NaNO₂. Such twinning could indicate reduced symmetry; however, the high‐pressure diffraction data indicate a deviation from orthogonality of less than 1°.