Monoclinic and triclinic high-pressure phases of KHCO3 were identified using in situ high-pressure single-crystal X-ray analysis. These monoclinic and triclinic phases, designated as phases IV and V, respectively, differ from three previously identified phases: I–III. The lattices of the two phases are superimposed along aIV* = 2 × aV* and hk0 and h0l nets of both phases lie on the same plane, i.e., the (100) plane is common in both lattices in real space. The space group of phase IV is P21/b11 with lattice constants of a = 10.024(3) Å, b = 6.912(5) Å, c = 4.1868(11) Å, α = 115.92(4)°, and V = 260.9(2) Å3. The crystal structure of phase IV, excluding the hydrogen atoms, was successfully determined by direct methods and is isostructural with a cesium hydrogen carbonate CsHCO3.

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