Strong Kinetic Effects in the Order–Disorder Transformation of Cu²⁺ Hydroxyfluorides and Impact on Magnetic Properties

A Cu(OH)_1.4F_0.6 hydroxyl-rich copper hydroxyfluoride polymorph, denoted as Cu-P2/m, was synthesized through coprecipitation or microwave-assisted hydrothermal synthesis. The structural features were compared to the disordered brucite-like Cu(OH)_1.4F_0.6 polymorph (Cu-P-3m1) studied previously. The synthesis conditions were modified by playing with time and temperature in order to understand the allotropic transformation between the two polymorphs. On the basis of electron diffraction investigation supported by powder XRD analysis, a supercell relationship within the slabs exists between the monoclinic lattice (S.G. P2/m, Z = 4) and the brucite-hexagonal unit cell (S.G. P-3m1, Z = 1). Hence, considering the P2/m structural hypothesis, six different atomic positions (3 Cu and 3 X anions with different multiplicities) were refined and form folded sheets. At the local scale, the three various Cu²⁺ cations, which share common edges in a triangle, are located at elongated (4 + 2), compressed (2 + 4), and elongated with additional distortion (2 + 2 + 2) Jahn–Teller (J–T) octahedral sites, respectively. Finally, the ordered polymorph (Cu-P2/m) exhibits antiferromagnetic order with T_N = 17 K, while the disordered one (Cu-P-3m1) remains paramagnetic until 2 K. The influence of F content and the substitution of larger chlorine for fluorine in botallackite in this series will be discussed.