Navigation through high-dimensional chemical space: discovery of Ba5Y13[SiO4]8O8.5 and Ba3Y2[Si2O7]2

Abstract

Two compounds were discovered in the well-studied BaO–Y₂O₃–SiO₂ phase field. Two different experimental routines were used for the exploration of this system due to the differences of synthetic conditions and competition with a glass field. The first phase Ba₅Y₁₃[SiO₄]₈O_8.5 was isolated through a combination of energy dispersive X-ray spectroscopy analysis and diffraction techniques which guided the exploration. The second phase Ba₃Y₂[Si₂O₇]₂ was located using iterative algorithmic identification of target compositions. The structure solution of the new compounds was aided by continuous rotation electron diffraction, and the structures were refined against combined synchrotron and neutron time-of-flight powder diffraction. Ba₅Y₁₃[SiO₄]₈O_8.5 crystallizes in I2m, a = 18.92732(1), c = 5.357307(6) Å and represents its own structure type which combines elements of structures of known silicates embedded in columns of interconnected yttrium-centred polyhedra characteristic of high-pressure phases. Ba₃Y₂[Si₂O₇]₂ has P2₁ symmetry with a pseudo-tetragonal cell (a = 16.47640(4), b = 9.04150(5), c = 9.04114(7) Å, β = 90.0122(9)°) and is a direct superstructure of the Ca₃BaBi[P₂O₇]₂ structure. Despite the lower symmetry, the structure of Ba₃Y₂[Si₂O₇]₂ retains disorder in both Ba/Y sites and disilicate network, thus presenting a superposition of possible locally-ordered fragments. Ba₅Y₁₃[SiO₄]₈O_8.5 has low thermal conductivity of 1.04(5) W m⁻¹ K⁻¹ at room temperature. The two discovered phases provide a rich structural platform for further functional material design. The interplay of automated unknown phase composition identification with multiple diffraction methods offers acceleration of the time-consuming exploration of high-dimensional chemical spaces for new structures.