Swartzite, CaMg[UO2(CO3)3]·12H2O: a reversible phase transition associated with hydrogen–bond ordering below 238 K

The uranyl tricarbonate mineral swartzite, CaMg[UO 2 (CO 3 ) 3 ]·12H 2 O, was investigated using single−crystal X − ray diffraction on synthetic material between 295 and 100 K. Swartzite is composed of Mg(H 2 O) 6 octahedra and {Ca(H 2 O) 6 [UO 2 (CO 3 ) 3 ]} complexes linked into a three–dimensional framework through an extensive hydrogen–bond network. At room temperature, the compound crystallizes in the monoclinic space group P 2 1 / m ( a = 11.873(3), b = 14.638(3), c = 6.439(2) Å, β = 112.92(1)°, Z = 2, T = 295 K; non−standard unit cell). In this structure, the Mg(H 2 O) 6 octahedron is centrosymmetric, whereas the {Ca(H 2 O) 6 [UO 2 (CO 3 ) 3 ]} complex exhibits mirror symmetry. Subtle disorder in two of the seven independent H 2 O groups gives rise to directionally disordered hydrogen bonds at room temperature. At approximately 238 K (− 35 °C), a reversible phase transition occurs, producing a low−temperature structure with space group P 2 1 / c ( a = 11.850(3), b = 14.553(3), c = 12.837(2) Å, β = 113.39(1)°, Z = 4, T = 100 K). In this phase, the mirror plane is replaced by a c −glide plane, accompanied by doubling of the c axis. All atoms occupy general positions, and all H 2 O groups and hydrogen bonds become fully ordered. The phase transition is associated with small displacements and distortions of both the Mg(H 2 O) 6 octahedra and the {Ca(H 2 O) 6 [UO 2 (CO 3 ) 3 ]} complexes. Atomic displacements between the 295 K and 100 K structures range from 0.02 to 0.28 Å for non−hydrogen atoms. Final refinements converged at R 1 = 0.0164 for 2987 reflections at 295 K and R 1 = 0.0151 for 6627 reflections at 100 K. Additional diffraction measurements at 250 and 220 K constrain the phase transition process to this temperature range. Room–temperature neutron diffraction data complement the X–ray study and provide accurate hydrogen–bond geometries above the phase transition temperature.