Phase transition at high pressure in Cu2CO3(OH)2 related to the reduction of the Jahn–Teller effect

Hydroxycarbonates with the general formula Me2(CO3)(OH)2 are widely used materials in industrial processes and are widespread in nature. The Cu term, malachite, Cu2CO3(OH)2, is monoclinic, P21/a. Substitution of Cu2+ with other bivalent cations such as Mg, Zn, Fe, Cu or Ni is possible and leads to a different structure type, rosasite, P21/a or P21/b11 in the same cell setting as malachite. Rosasite structure is topologically similar to malachite, but the symmetry elements are oriented differently with respect to structural units. The stability of the malachite-like structure (MS) compared with the rosasite-like structure (RS) has been suggested to be related to the Jahn–Teller effect in CuO6 coordination polyhedra. For this reason the hypothesis of the phase transition of malachite, Cu2CO3(OH)2, to a rosasite structure at high pressure, as a result of the reduced Jahn–Teller effect, has been tested and confirmed by powder and single-crystal diffraction structural studies: above 6 GPa the malachite structure is no longer stable and transforms to a RS structure. RS Cu2CO3(OH)2 is 3% more dense than malachite and the bulk modulus is remarkably higher, 80 (2) GPa compared with 48 (4) GPa. The longer apical Cu—O bonds in the distorted Me1 octahedral site are progressively shortened with increasing pressure, revealing a decrease in the Jahn–Teller effect at high pressure. The transition has a first-order character, is reversible with a significant hysteresis, and there is no evidence of any intermediate phase between the two structures. We then have further evidence that in the Me2(CO3)(OH)2 compounds, the two main structural types, MS and RS, are closely related. The former structure is stabilized only when Cu is the prevalent cation in the octahedral sites, and it can transform directly to the RS as a function of thermodynamic changes.