The powder mixture was loaded into a small fused quartz tube (9 mm outer diameter and 7 mm inner diameter), with one end always open. Next, the small tube was loaded into a larger fused quartz tube (12 mm outer diameter and 10 mm inner diameter). The large tube was then sealed under vacuum and the smaller tube was situated so that the reagents lie near the middle of the larger tube. The double-tube assembly was heated in a hori-zontal two-zone furnace with high and low temperature sides at 480 • C and 280 • C, with the starting ingredients at around 420 • C. The furnace was programmed so that both sides reached their temperature set points simultaneously after 10 hours, and the assembly was held for 80 hours before cooling naturally. Figure S1 in Supporting Information 39 shows the reaction tube before and after the reaction.