Stability at high pressure, elastic behavior and pressure-induced structural evolution of "Al5BO9", a mullite-type ceramic material

Elastic behavior and pressure-induced structural evolution of synthetic boron-mullite "Al5BO9" (a = 5.678(2) , b = 15.015(4) and c = 7.700(3) , space group Cmc2(1), Z = 4) were investigated up to 7.4 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase transition or anomalous compressional behavior occurred within the investigated P range. Fitting the P-V data with a truncated second-order (in energy) Birch-Murnaghan Equation-of-State (BM-EoS), using the data weighted by the uncertainties in P and V, we obtained: V (0) = 656.4(3) (3) and K (T0) = 165(7) GPa (beta (V0) = 0.0061(3) GPa(-1)). The evolution of the Eulerian finite strain versus normalized stress (f (E)-F (E) plot) leads to an almost horizontal trend, showing that a truncated second-order BM-EoS is appropriate to describe the elastic behavior of "Al5BO9" within the investigated P range. The weighted linear regression through the data points gives: F (E)(0) = 159(11) GPa. Axial compressibility coefficients yielded: beta (a) = 1.4(2) x 10(-3) GPa(-1), beta (b) = 3.4(4) x 10(-3) GPa(-1), and beta (c) = 1.7(3) x 10(-3) GPa(-1) (beta (a) :beta (b) :beta (c) = 1:2.43:1.21). The highest compressibilities observed in this study within (100) can be ascribed to the presence of voids represented by five-membered rings of polyhedra: Al1-Al3-Al4-Al1-Al3, which allow accommodating the effect of pressure by polyhedral tilting. Polyhedral tilting around the voids also explains the higher compressibility along [010] than along [001]. The stiffer crystallographic direction observed here might be controlled by the infinite chains of edge-sharing octahedra running along [100], which act as "pillars", making the structure less compressible along the a-axis than along the b- and c-axis. Along [100], compression can only be accommodated by deformation of the edge-sharing octahedra (and/or by compression of the Al-O bond lengths), as no polyhedral tilting can occur. In addition, a comparative elastic analysis among the mullite-type materials is carried out.