Denticity and Mobility of the Carbonate Groups in AMCO₃F Fluorocarbonates: A Study on KMnCO₃F and High Temperature KCaCO₃F Polymorph

We report on a thorough structural study on two members of layered fluorocarbonates KMCO₃F (M = Ca, Mn). The Ca-based member demonstrates a phase transition at ∼320 °C, evidenced for the first time. The crystal structure of the high temperature phase (HT-KCaCO₃F) was solved using neutron powder diffraction. A new Mn-based phase KMnCO₃F was synthesized, and its crystal structure was solved from electron diffraction tomography data and refined from a combination of X-ray synchrotron and neutron powder diffraction. In contrast to other members of the fluorocarbonate family, the carbonate groups in the KMnCO₃F and HT-KCaCO₃F structures are not fixed to two distinct orientations corresponding to mono- and bidentate coordinations of the M cation. In KMnCO₃F, the carbonate group can be considered as nearly "monodentate", forming one short (2.14 Å) and one long (3.01 Å) Mn-O contact. This topology provides more flexibility to the MCO₃ layer and enables diminishing the mismatch between the MCO₃ and KF layers. This conclusion is corroborated by the HT-KCaCO₃F structure, in which the carbonate groups can additionally be tilted away from the layer plane thus relieving the strain arising from geometrical mismatch between the layers. The correlation between denticity of the carbonate groups, their mobility, and cation size variance is discussed. KMnCO₃ orders antiferromagnetically below T_N = 40 K.