The crystal structure of Gd(CO3)OH was solved using synchrotron powder X-ray diffraction. Gd(CO3)OH was known to exist in a form Gd2O(CO3)2·H2O and its powder pattern has been listed in JCPDF (#430604) for decades, but the crystal structure has not yet been elucidated. The crystal structure is solved with simulated annealing and the DASH program. The final Rietveld refinement converged to Rwp= 6.28%, Rp = 4.47%and χ2= 1.348, using the GSAS program. Gd(CO3)OH crystallizes in orthorhombic system with lattice parameters a = 7.08109(9), b = 4.88436(7), c = 8.45010(13)Å and space group P nma. Gd(CO3)OH forms a three-dimensional framework with an eight-membered ring, a one-dimensional channel and OH- in the cavity. XANES of Gd LIII-edge indicates that the oxidation state of Gd is 3+. Two phase transitions of Gd(CO 3)OH were found at 500 and 650 °C to yield Gd2O 2CO3 and Gd2O3 respectively.
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