Formation of a giant monoclinic P 2 SiB 6 superstructure by high-energy ball milling and spark plasma sintering

Silicon hexaboride is a structurally complex boron-rich ceramic whose diffraction response is highly sensitive to processing history and long-range ordering. Nominal SiB 6 ceramics were prepared from elemental Si and B powders by high-energy ball milling for 1 or 10 h, followed by spark plasma sintering (SPS) between 1,300 and 1,700 °C. Laboratory X-ray diffraction shows that specimens processed at 1,300–1,500 °C retain the established orthorhombic SiB 6 -type framework, whereas the patterns obtained at 1,600–1,700 °C exhibit an abrupt redistribution of intensity and intense low-angle reflections that cannot be described by the orthorhombic cell. The long-period monoclinic metric was first verified by Pawley refinement, after which parent-derived rigid polyhedral motifs were assembled in direct space with PowderSolve. For 1,700-N-1H, the PowderSolve proposal was regularized by diffraction-guided geodesic projection (DGGP), introduced here as an alternating structure-solution strategy. DGGP cycles 1–3 generated a contact-safe connected framework; comparison of coordinated rigid-object alternatives with the experimental low-angle intensities then selected variant A as the definitive cycle-4 reconstruction. The final model contains 20 M 12 /M 15 motifs and seven intercluster sites in the asymmetric unit and uses 27 bounded object-wise advancement variables rather than 777 independent Cartesian coordinates for 259 atoms. The resulting sample-specific structures crystallize in P 2 (no. 3, unique axis b ). The definitive 1,700-N-1H cycle-4 cell is a = 32.536(1) Å, b = 5.19176(4) Å, c = 30.3595(1) Å, β = 110.1602(3)°, and V = 4,814.1(2) Å 3 ; the separately deposited 1,700-N-10H cell is a = 32.493(1) Å, b = 5.1784(1) Å, c = 30.2365(7) Å, β = 110.363(2)°, and V = 4,769.7(2) Å 3 . Both cells contain B 444 Si 74 = 74(SiB 6 ), but their detailed rigid-object configurations are distinct. Strong (304) preferred orientation additionally modulates selected intensities in the polished 1H specimen. Its full-range fixed-coordinate calculation gives R wp = 12.53 %, R wp without background = 6.15 %, and R p = 5.89 %; the deposited 10H constrained refinement over 10.0001–44.9951° gives 11.09 %, 4.65 %, and 6.43 %, respectively. These sample- and range-specific values support chemically coherent, strongly constrained average P 2 models rather than unconstrained atom-by-atom determinations.