Anion-deficient perovskite-like Ba3SmFe2O7.5 was prepared using a glycerol–nitrate synthesis. Using high-temperature X-ray diffraction in situ a crystal structure transition temperature range 800 and 840 °C was established. These results were further confirmed by high-temperature dilatometric analysis. The average thermal expansion coefficient (TEC) of Ba3SmFe2O7.5 is about 12.8 × 10−6 K−1 between 25 °C and 800 °C. Magnetic experiments proved an excellent phase purity of the oxide and reveal that Fe3+ ions stay in high and intermediate spin states in a ratio of 75% and 25% respectively.