Sodium hydrogencarbonate (NaHCO3): coincidence-site lattice twinning and structure refinement

摘要

Sodium hydrogencarbonate (NaHCO3) has been recrystallised hydrothermally using a cooling ramp from 250 degrees C to 190 degrees C at 0.069 GPa, and investigated by single crystal X-ray diffraction at room temperature and pressure. Representative crystal data are: monoclinic, space group P2(1), Z = 4, a = 7.4688(4), b = 9.6842(6), c = 3.4791(2) angstrom, = 93.321(2), R = 0.026, and D-x = 2.221 g/cm(3). The crystals investigated are complexly twinned by reticular pseudomerohedry, but give two coincidence-site lattices with pseudocell parameters: a = 3.4791(2), b = 9.6842(6), c = 29.875(2) angstrom, beta = 93.321(2)degrees, twin axis [001], twin index (Sigma) = 4; and, a = 8.4360(6), b = 9.6811(7), c = 24.941(2) angstrom, beta = 99.109(5)degrees, twin axis [10 (1) over bar], Sigma = 8. The Sigma = 4 and Sigma = 8 twins occur in separate twinned crystals but in similar twin proportions (5.5:1), and are most likely secondary in origin. The two coincidence-site lattices are permitted in NaHCO3 because beta corresponds closely to the theoretical value given by the positive root of [sin(2) (beta - pi/2) = 3/912].