Synthesis and Disordered Crystal Structure of Al₃O_3.5C_0.5

The crystal structure of Al₃O_3.5C_0.5 (<i>Z</i> = 3) has been characterized by X-ray powder diffraction (XRPD), transmission electron microscopy, and electron probe microanalysis. The title compound is trigonal with space group <i>R</i>3̅<i>m</i> (centrosymmetric) and hexagonal unit-cell dimensions <i>a</i> = 0.29588(1) nm, <i>c</i> = 2.84080(7) nm, and <i>V</i> = 0.21538(1) nm³. The initial structural model was determined by the charge-flipping method and subsequently refined by the Rietveld method. The final structural model showed the positional disordering of one of the two types of Al sites. The maximum-entropy method-based pattern fitting (MPF) method was used to confirm the validity of the split-atom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The reliability indices calculated from the MPF were <i>R</i>_wp = 4.03%, <i>S</i> (= <i>R</i>_wp/<i>R</i>_e) = 1.17, <i>R</i>_p = 3.08%, <i>R</i>_B = 0.82%, and <i>R</i>_<i>F</i> = 0.72%. The crystal was composed of antiphase domains, suggesting the occurrence of the high-low phase transition during the cooling process. The transition would be accompanied by the loss of unit-lattice translation, and hence the disordered structural model determined by XRPD might be of the average structure of the low-temperature phase