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 KMCO 3 F (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-KCaCO 3 F) was solved using neutron powder diffraction. A new Mn-based phase KMnCO 3 F 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 KMnCO 3 F and HT-KCaCO 3 F structures are not fixed to two distinct orientations corresponding to mono- and bidentate coordinations of the M cation. In KMnCO 3 F, 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 MCO 3 layer and enables diminishing the mismatch between the MCO 3 and KF layers. This conclusion is corroborated by the HT-KCaCO 3 F 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. KMnCO 3 orders antiferromagnetically below T N = 40 K.