Crystal structures of the compounds RAl0.5-xSi0.5+x (R = La, Ce, Pr, Nd, Sm, Gd), R3Al4Si6 (R = La, Pr), and RAlSi2 (R = Pr, Nd)

Nine ternary compounds were found in R-Al-Si systems (R=rare-earth element) and their crystal structures were determined by X-ray powder diffraction. Compounds RAl0.5-xSi0.5+x with CrB structure type (oS8, Cmcm) were found for R=La, Ce, Pr, Nd, Sm; compounds R3Al4Si6 with Ce3Al4Si6 structure type (hP13, P-3m1) for R=La, Pr; and compounds RAlSi2 with CeAlSi2 structure type (hP8, P-3m1) for R=Pr, Nd. The structure of the compound PrAl0.5Si0.5 was refined from X-ray powder diffraction data (RB=0.038): Pr in 4(c), y=0.3566(1), and a statistical mixture Al0.5Si0.5 in 4(c), y=0.0731(5). Within the rows of isostructural compounds, the volumes of the unit cells regularly decrease on increasing the atomic number of the rare-earth element