STRUCTURAL PHASE-TRANSITION IN KHCO3

The structural phase transition of KHCO3 has been studied using three-dimensional data collected both below and above the transition point. Above 318 K, the crystal structure changes from the room temperature phase (monoclinic space group P21 a) to a high temperature phase (monoclinic space group C2 m), both containing four formula units. The cell dimensions are a = 15.195(1) Å, b = 5.640(1) Å, c = 3.734(1) Å, and β = 104.86(1)°, and v = 309.28 A3 (at 353 K). The structure is refined by the full-matrix least-squares method and final R-factor is 0.023. From the hydrogen atoms revealed in the difference Fourier map, it is concluded that the phase transition is ascribed to the order-disorder of the (HCO3)2 dimer. © 1990.