Transition metal oxynitride perovskites are an emerging class of functional materials. However, their oxide/nitride ordering principles were unclear until a recent study of the representative compounds SrMO2N (M = Nb, Ta) by neutron and electron diffraction revealed a robust partial anion order where disordered zig-zag MN chains lie within two-dimensional (2D) planes. However BaTaO2N contains the same cis-chains propagating in three dimensions (3D). The discovery of 2D anion chains in SrTaO2N but 3D chains for BaTaO2N raises the question of how the ordering dimensionality changes in intermediate Ba1-xSrxTaO2N perovskites. We propose to collect HRPD data from 4 Ba1-xSrxTaO2N samples to explore this. We will also explore how the tilting transition to I4/mcm (3D anion order) or Fmmm (2D) changes as it is suppressed from 450 K for x = 1 to zero temperature for small x.