We propose to use two days of beamtime on POLARIS to determine the structure and magnetic ordering that occurs in a series of cation-ordered perovskites. These cubic compounds are intermediate between the semi-metallic phase Ba2FeMoO6 and the localised compounds Ba2YMoO6 and Ba2YbMoO¬6 and show a complex series of magnetic transitions that indicate a switch from ferrimagnetism to antiferromagnetism around the percolation limit for Fe. The beamtime will determine the magnetic structure at low temperature in these compounds and look at the thermal evolution of structure, in order to identify magnetostrictive effects around the magnetic transitions. This will help in the development of new magnetoresistive phases and provide insights into the anomalous Jahn Teller effect exhibited by Mo5+.