A peculiar site preference of boron in MgAl_2-xB _xO₄ (x = 0.0, 0.11, and 0.13) spinel under high-pressure and high-temperature

Single crystals of MgAl_2-xB_xO₄ (x = 0.0, 0.11 and 0.13) spinel were synthesized under high pressure (5 and 11 GPa) and high temperature (1873 and 1273 K) using a 1000 ton "6-8" type uniaxial split-sphere apparatus. Single-crystal X-ray diffraction measurements were carried out using a laboratory diffractometer with a Weissenberg-type imaging plate detector and using a four-circle diffractometer with synchrotron radiation at Photon Factory, Japan. The maximum content of boron was about x = 0.13 at 1273K and 11GPa. The smallest boron ion occupies the octahedral site in top priority in the spinel solid solution of the Mg/Al/B systems. The B ³⁺ ions can replace considerably bigger Al³⁺ ion under pressure. We estimated the cation distribution of each site from the reproduction of the observed, average distance using the observed local bond lengths for Mg-O and Al-O and the normal B-O distance. The chemical formula of boron-bearing spinels are estimated as (Mg_0.47,Al_0.53) [Al_1.36Mg_0.53,B_0.11]O₄ (x = 0.11) and (Mg_0.50Al_0.50)-[Al_1.37,Mg _0.50B_0.13]O₄ (x = 0.13), respectively. These spinel solid solutions are largely disordered crystals and the inversion parameter grow up to 0.5 in the tetrahedral site. The temperature factor B _eq for oxygen site in boron-bearing spinels were significantly larger than that in pure MgAl₂O₄ spinel. Only the positional shifts of oxygen ions have, therefore, been relaxing the disorder in structure.