The crystal chemistry and crystallography of the compounds SrR 2 CuO 5 (Sr-121, R=lanthanides) were investigated using the powder X-ray Rietveld refinement technique. Among the 11 compositions studied, only R=Dy and Ho formed the stable SrR 2 CuO 5 phase. SrR 2 CuO 5 was found to be isostructural with the “green phase”, BaR 2 CuO 5 . The basic structure is orthorhombic with space group Pnma. The lattice parameters for SrDyCuO 5 are a=12.08080(6)Å, b=5.60421(2)Å, c=7.12971(3)Å, V=482.705(4)Å 3 , and Z=8; and for the Ho analog are a=12.03727(12)Å, b=5.58947(7)Å, c=7.10169(7)Å, V=477.816(9)Å 3 , and Z=8. In the SrR 2 CuO 5 structure, each R is surrounded by seven oxygen atoms, forming a monocapped trigonal prism (RO 7 ). The isolated CuO 5 group forms a distorted square pyramid. Consecutive layers of prisms are stacked in the b-direction. Bond valence calculations imply that residual strain is largely responsible for the narrow stability of the SrR 2 CuO 5 phases with R=Dy and Ho only. X-ray powder reference diffraction patterns for SrDy 2 CuO 5 and SrHo 2 CuO 5 were determined.