The recent discovery of superconductivity in a series of iron pnictide materials has provided impetus to the field of new materials discovery. Like in the cuprate superconductors there are many iron arsenides and phosphides that display superconductivity. The latest materials which have emerged in 2009 are compounds in which the FeAs layers are widely separated by transition metal oxide layers. This experiment on POLARIS is to investigate the detailed structures of the series Sr2CrFe1-xCoxAsO3 around the region in which superconductivity occurs and the superconductor Sr2VFeAsO3. Complementary magnetic measurements are proposed on WISH. Detailed comparison will be made between the Cr and V-containing compounds, and these results will be important for interpretation of the properties.