ARTICLE DOI: 10.1002/zaac.200900418 A Peculiar Site Preference of Boron in MgAl 2–x B x O 4 (x = 0.0, 0.11, and 0.13) Spinel under High-Pressure and High-Temperature Akira Yoshiasa,* [a] Tomokazu Ito, [a] Kazumasa Sugiyama, [b] Akihiko Nakatsuka, [c] Maki Okube, [d] Masanori Kurosawa, [e] and Tomoo Katsura [f] Dedicated to Professor Arndt Simon on the Occasion of His 70th Birthday Keywords: High pressure spinel structure; Magnesium; Aluminum; Boron; X-ray diffraction; Site occupancy Abstract. Single crystals of MgAl 2–x B x O 4 (x = 0.0, 0.11 and 0.13) spinel were synthesized under high pressure (5 and 11 GPa) and high tempera- ture (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 Al 3+ ion under pressure. We estimated the cation distribution of each site from Introduction Considerable efforts have been devoted to the structural stud- ies of spinel group because of their importance as functional materials and constituents of the earth’s crust and mantle. Fac- tors influencing the cation distribution in spinel structure have been investigated by many researchers [1–5] and the system- atic of the spinel structure was reviewed by Hill et al. [6]. Despite their simple structures, many spinel type compounds exhibit complex disordering phenomena involving the mixing of cation on two sites, which have important consequences for both thermodynamic and physical properties. The general for- * Prof. Dr. A. Yoshiasa E-Mail: yoshiasa@sci.kumamoto-u.ac.jp [a] Graduate School of Sciences Kumamoto University Kumamoto, Japan [b] Institute for Materials Research Tohoku University Sendai, Japan [c] Department of Advanced Materials Science and Engineering Yamaguchi University Ube, Japan [d] Materials and Structure Laboratory Tokyo Institute of Technology Yokohama, Japan [e] Graduated School of Life and Environmental Sciences University of Tsukuba Tsukuba, Japan [f] Institute for Study of the Earth’s Interior Okayama University Misasa, Japan 472 © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2010, 636, 472–475 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 1.36 ,Mg 0.53 ,B 0.11 ]O 4 (x = 0.11) and (Mg 0.50 ,Al 0.50 )- [Al 1.37 ,Mg 0.50 ,B 0.13 ]O 4 (x = 0.13), respectively. These spinel solid solu- tions 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 MgAl 2 O 4 spinel. Only the positional shifts of oxygen ions have, therefore, been relaxing the disorder in structure. mula of the spinel group can be expressed as IV (X) VI {Y 2 }O 4 , and the X and Y cations are coordinated tetrahedrally and octa- hedrally by oxygen, respectively. The disordered cation distri- bution is represented as (X 1–i Y i ){X i Y 2–i }O 4 , where the inver- sion parameter i corresponds to degree of inversion. Cation distribution in MgAl 2 O 4 spinel is well known to be partially disordered by various conditions. The existence of an order-disorder transition was first confirmed from IR spectro- scopy by Hafner and Laves [7]. Many studies showed that the inversion parameters are below 0.4 in MgAl 2 O 4 spinel, though it varies with temperature [7–9]. The cation distribution in MgAl 2 O 4 and MgGa 2 O 4 is remarkably different, though these two spinels consist of the same group 13 element. The i value of MgGa 2 O 4 obtained by a single-crystal X-ray diffraction analysis was 0.82(1). The relation of an effective ionic radii of the constituent cation with same coordination number is Al 3+ < Ga 3+ < Mg 2+ [10]. If we considered Pauling’s first principle [11], both MgAl 2 O 4 and MgGa 2 O 4 would prefer inverse spinel (i = 1.0). MgAl 2 O 4 spinel has a peculiar site preference with higher symmetry, i.e. the octahedral sites are preferred by smaller aluminum ion than larger magnesium ion. In our previous study [12], the cation distributions and the structural variation in MgAl 2–x Ga x O 4 solid solution have been clarified using 27 Al MAS NMR measurements and single-crys- tal X-ray diffraction method. The MgAl 2–x Ga x O 4 solid solution has the characteristic that the change of the order-disorder of magnesium or aluminum in the solid solution is controlled