Formation, stability and crystal structure of mullite-type Al 6  x B x O 9 K. Hoffmann a,b,n , T.J.N. Hooper c , M.M. Murshed b,d , O. Dolotko e,1 , Z. Révay e , A. Senyshyn e , H. Schneider a , J.V. Hanna c , Th.M. Gesing b,d , R.X. Fischer a,d a Kristallographie, FB05, Klagenfurter Straße/GEO, Universität Bremen, D-28359 Bremen, Germany b Institut für Anorganische Chemie und Kristallographie, FB02, Leobener Straße/NW2, Universität Bremen, D-28359 Bremen, Germany c Department of Physics, University of Warwick, Coventry CV4 7AL, UK d MAPEX Center for Materials and Processes, Universität Bremen, Bibliothekstraße 1, D-28359 Bremen, Germany e Heinz Maier-Leibnitz Zentrum (MLZ), Technische Universität München, Lichtenbergstr.1, D-85748 Garching, Germany article info Article history: Received 4 May 2016 Received in revised form 12 August 2016 Accepted 15 August 2016 Available online 16 August 2016 Keywords: Aluminum borate, chemical synthesis Mullite-type structure X-ray diffraction, NMR spectroscopy, ther- mal expansion abstract Mullite-type Al 6x B x O 9 compounds were studied by means of powder diffraction and spectroscopic meth- ods. The backbones of this structure are chains of edge-connected AlO 6 octahedra crosslinked by AlO- and BO-polyhedra. Rietveld refinements show that the a and b lattice parameters can be well resolved, thus representing an orthorhombic metric. A continuous decrease of the lattice parameters most pronounced in c- direction indicates a solid solution for Al 6x B x O 9 with 1.09rx r2. A preference of boron in 3-fold co- ordination is confirmed by 11 B MAS NMR spectroscopy and Fourier calculations based on neutron diffraction data collected at 4 K. Distance Least Squares modeling was performed to simulate a local geometry avoiding long B-O distances linking two octahedral chains by planar BO 3 groups yielding split positions for the oxygen atoms and a strong distortion in the octahedral chains. The lattice thermal expansion was calculated using the Grüneisen first-order equation of state Debye-Einstein-Anharmonicity model. & 2016 Elsevier Inc. All rights reserved. 1. Introduction Many of the described ternary aluminum borates belong to the family of mullite-type materials [1,2]. Exceptions are, e.g., zeolite PKU-5, Al 4 B 6 O 15 reported by Ju et al. [3] to be synthesized under hydrothermal conditions at 623 K, and the high-pressure phase AlBO 3 described by Vegas et al. [4] to be isotypic to CaCO 3 . Mullite- type aluminum borates are characterized by chains of edge-shar- ing AlO 6 octahedra. Depending on the individual phases these AlO 4 chains are linked by different arrangements of AlO 4 - AlO 5 -, BO 3 - and eventually BO 4 -polyhedra. However, little is known about the exact conditions of formation, stability ranges and de- tails of the crystal structures. According to the phase diagram gi- ven by Gielisse and Foster [5] two thermodynamically stable phases Al 18 B 4 O 33 (A 9 B 2 ) and Al 4 B 2 O 9 (A 2 B) with mullite-type structures (Fig. 1) are described to exist in the binary system Al 2 O 3 -B 2 O 3 . The exact chemical composition of A 9 B 2 is an ongoing discussion (see, e.g., Fisch et al. [6]). Based on an ordered dis- tribution of Al and B, Al 20 B 4 O 36 (A 5 B) is the idealized model, re- presenting Al 18 B 4 O 33 (A 9 B 2 ) composition [7] if about 2% of the Al is replaced by B (Al 20 B 4 O 36 vs. Al 19.6 B 4.4 O 36 ). In this work we use the designation A 9 B 2 as commonly employed in the material science literature. Accordingly, the exact stoichiometric composition of Al 18 B 4 O 33 was used for the synthesis, slightly shifted to a lower Al content compared to Al 20 B 4 O 36 and Al 6 x B x O 9 with x ¼ 1. Mazza et al. [8] synthesized a series of probably metastable com- pounds with chemical composition according to Al 6x B x O 9 (1rx r3) and disordered crystal structures different from A 9 B 2 and A 2 B. Further heat treatment yields the stable equilibrium phases. The compounds with x ¼ 1 (Al 5 BO 9 ) and x ¼ 2 (Al 4 B 2 O 9 ) are described by Mazza et al. [8] to crystallize in the orthorhombic space group Pbam with lattice parameters a ¼ b, representing a pseudo-tetragonal metric, and an average structure close to mullite (Fig. 1). In order to distinguish between different phases with identical chemical compositions we refer to the metastable compounds as Al 6x B x O 9 with x ¼ 1 and x ¼ 2, respectively, in contrast to the stable compounds A 9 B 2 and A 2 B. According to Mazza et al. [8], the Al-rich compound Al 6 x B x O 9 (x ¼ 1) consists of octahedrally and tetrahedrally coordinated alu- minum and trigonal borate groups, whereas in the boron-rich compound Al 6 x B x O 9 (x ¼ 2) one B atom substitutes for one Al atom in the tetrahedral site, leading to the occurrence of an ad- ditional BO 4 tetrahedron and a slight distortion of the octahedral chains. In this fourfold coordination an additional longer bond to a non-octahedral oxygen atom occurs. As boron cannot be detected accurately from X-ray powder diffraction data due to its low Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry http://dx.doi.org/10.1016/j.jssc.2016.08.018 0022-4596/& 2016 Elsevier Inc. All rights reserved. n Corresponding author at: Kristallographie, FB05, Klagenfurter Straße/GEO, Universität Bremen, D-28359 Bremen, Germany. E-mail address: Kristin.Hoffmann@uni-bremen.de (K. Hoffmann). 1 239 Spedding, Ames Laboratory, Iowa State University, Ames, IA 50011-3020, USA. Journal of Solid State Chemistry 243 (2016) 124–135