Crystal structures of the series of double perovskites Ca_(2−x)Ba_xLaNbO₆:Eu³⁺ phosphors have been examined by powder X-ray diffraction and Rietveld refinements. Ca₂LaNbO₆ has a monoclinic (P2₁/n) and Ba₂LaNbO₆ has a monoclinic (C2/m) structure. The structural phases of Ca_(2−x)Ba_xLaNbO₆:Eu³⁺ samples are divided into three sections depending on different Ca/Ba ratios: (1) monoclinic phase (P2₁/n) as Ca₂LaNbO₆ in the range of x = 0–0.1, (2) mixed phases containing Ca₂LaNbO₆ and Ba₂LaNbO₆ between 0.15 and 1.2, and (3) monoclinic phase (C2/m) as Ba₂LaNbO₆ for x = 1.4–2. Eu³⁺ ions act as the structural probes to study the structural phase transitions, and the evolution of the photoluminescence properties and thermal stability behaviours has been also comparatively investigated depending on different structural symmetries from Ca₂LaNbO₆ to Ba₂LaNbO₆ phase. The strong red emission from ⁵D₀–⁷F₂ peaking at 618 nm can be found in Ca₂LaNbO₆:Eu³⁺ phosphors, which is attributed to the low crystal field effect of the activator ions located in the highly distorted [LaO₈] polyhedra sites. The composition-optimized phosphors can find applications in white light emitting diodes (LEDs).