The high-pressure crystal structure of potassium hydrogen carbonate (KHCO3)

The crystal structure of the high-pressure phase of potassium hydrogen carbonate, here termed KHCO3 form III, has been solved using single-crystal X-ray diffraction techniques. It adopts triclinic P (1) over bar symmetry and is formed on direct compression of the monoclinic ambient-pressure phase (kalicinite, form 1) via a first-order phase transition at 3.2 GPa (Delta VIV similar to 4%). We have also used time-of-flight high-pressure neutron powder-diffraction to determine the structural changes in KDCO3 of the form I and III polymorphs to 8.3 GPa. This study indicates that the transition appears to be in response to pronounced displacements of the K+ cations and the cooperative tilting of the (DCO3-)(2), dimers. The O center dot center dot center dot O hydrogen bond distance shows a monotonic compression over the entire pressure range studied with no obvious discontinuity at the phase transition. However, the O-D center dot center dot center dot O bond angle appears to exhibit an abrupt similar to 4 degrees decrease across the I-III phase transition with a concomitant change in its pressure dependence. Birch Murnaghan fits to the equation of state data above and below the transition indicate that form III [B-0 = 26.5(2.8) GPa, B' = 5.2(6)] is marginally less compressible than form 1 [B-0 = 22.7(8) GPa, B'= 4.1(5)].