Two days of beamtime on HRPD will be used to examine a family of double perovskites Sr2YbMO6 that are isostructural at room temperature. When M = Mo5+ the monoclinic perovskite undergoes a spontaneous phase separation at some unknown temperature between 298 K and 32 K to give a mixture of Jahn Teller distorted material and geometrically regular MoO6 octahedra. The beamtime is requested to examine the temperature dependence of this highly unusual transition. The analogous phases Sr2YbTaO6, Sr2YbNbO6 and Sr2YbSbO6 show exceptionally strong antiferromagnetic coupling between Yb3+ cations and a loss of moment on cooling that is similar to the valence bond glass Sr2YMoO6. We will collect data from these three compounds at 5 K in order to structurally characterise them for the first time.