Synthesis, crystal structure and thermal behavior of Ba3B6Si2O16

Abstract

Single crystals of Ba 3 B 6 Si 2 O 16 were obtained by cooling a melt of stoichiometric composition. The crystal structure was determined by direct methods from X-ray single crystal diffractometry data and refined to R1 = 0.066. It is triclinic, P̄1̄, a = 5.038(1), b = 7.657(1), c = 8.526(1) Å , α = 77.68(1), β = 77.88(1), γ = 86.32(1) °. The structure consists of a layered silicoborate anion formed by four crystallographically independent polyhedra with boron and silicon distributed in different proportion over the polyhedra. Ideally, there are one BO 3 triangle, two BO 4 tetrahedra and one SiO 4 tetrahedron forming the independent anionic unit. Silicoborate layers are located parallel to the (010) plane. The Ba(1) and Ba(2) atoms are located in two non-equivalent sites with 8-fold and 12-fold coordination, respectively. The Ba(2) atoms form Ba-O layers parallel to (010) interchanged with anionic layers. Ba-O layers are connected via Ba-O columns build up by Ba(1)O 8 polyhedra along [100]. Formation of the compound is studied by heat treatments in the range 800-970 °C. According to high-temperature X-ray diffraction data the thermal expansion of the compound is anisotropic and relatively low in comparison with other borates.