An X-ray- and neutron-diffraction study of synthetic ferricopiapite, Fe_(14/3)(SO_4)_6(OD,OH)_2(D_2O,H_2O)_(20), and ab initio calculations on the structure of magnesiocopiapite, MgFe_4(SO_4)_6(OH)_2(H_2O)_(20)

摘要

We determined the positions of all atoms, including the light ones (D, H), in the crystal structure of synthetic fenicopiapite Fe_(14/3)(SO_4)_6(OD,OH)_2(D_2O,H_2O)_(20), using X-ray synchrotron [lambda 0.64905( 1) A] and neutron (lambda2.0787 A) powder diffraction. The H/(H + D) ratio in the sample is 0.09, as determined by infrared spectroscopy. The results of the diffraction study are supported by ab initio calculations of the structure of magnesiocopiapite, MgFe_4(SO_4)_6(OH)_2(H_2O)_(20). Neutron-diffraction data were collected at both room and low (approx16 K) temperature. The X-ray-diffraction pattern was collected at room temperature only. The room-temperature diffraction data (X-ray and neutron) were refined simultaneously. The structural model was refined in space group P1-bar, with unit-cell parameters a 7.3926(1), b 18.3806(1), c 7.3361(1) A, alpha 93.933(1), beta 102.212(1) and gamma 98.900(1) deg (XRD data). Positions of the heavy atoms determined in this study are virtually identical to those determined in the earlier studies of the structure of copiapite. The structure consists of infinite heteropolyhedral chains parallel to [101] and isolated [A(D_2O)_6] sites. The occupancy of the isolated A site in our ferricopiapite sample is 0.63(1). According to an analysis of the hydrogen-bonding network, the infinite chains in the structure have the composition [Fe_4(SO_4)_6(D_2O)_8(OD)_2](D_2O)_2. The isolated sites form slabs of composition {[A(D_2O)_6](D_2O)_4} with isolated D?O molecules attached to the infinite chains via a complicated set of hydrogen bonds. Ab initio calculations provide insight into the bonding around the isolated A site, where occupied by a cation (Mg~(2+)) and where vacant. The octahedral coordination of the A site is preserved whether the site is occupied or vacant. However, vacating the site results into a 17 percent increase of the volume of the octahedron and significant re-arrangement of the ligands. Ab initio calculations also suggest that the high-spin state of Fe~(3+) in copiapite is the stable electronic configuration.