Linear compressibilities of NaNO2 and NaNO3

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Abstract
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°.