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From superconductors to photovoltaics, perovskites have been intensely studied for their range of applications in solid-state physics. Double perovskites with formula A₂BB’X₆ are of particular interest for the chemical, electrical, and magnetic properties made possible by the versatility of their structure. As such, predicting the crystalline symmetry of double perovskite compositions can allow for optimization of these properties. Here, double perovskites with formula Ba₂BNbO₆ with B = Lu, Yb, Y, Gd, Eu, Nd, Pr, and La were surveyed to identify structural transitions. Additionally, Ba₂(Eu,Nd)MoO₆ perovskites were surveyed to probe a structural transition observed by a previous investigation. Samples were synthesized with solid-state techniques and analyzed with X-ray diffractometry. CeO₂ was used as an internal standard to accurately determine lattice parameters. The data were analyzed by Rietveld refinement to determine the dependence of lattice parameter and space group symmetry on B-site ionic radius and Goldschmidt tolerance factor. These findings provide a reference for optimizing a compound’s electronic, magnetic, and chemical properties as they are intertwined with the material’s crystalline structure.