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Ferroelectric materials are used in applications ranging from energy harvesting to high-power electronic transducers. However, industry-standard ferroelectric materials contain lead, which is toxic and environmentally unfriendly. The preferred alternative, BaTiO₃, is non-toxic and has excellent ferroelectric properties, but its Curie temperature of ∼130 °C is too low to be practical. Strain has been used to enhance the Curie temperature of BaTiO₃ (ref. ) and SrTiO₃ (ref. ) films, but only for thicknesses of tens of nanometres, which is not thick enough for many device applications. Here, we increase the Curie temperature of micrometre-thick films of BaTiO₃ to at least 330 °C, and the tetragonal-to-cubic structural transition temperature to beyond 800 °C, by interspersing stiff, self-assembled vertical columns of Sm₂O₃ throughout the film thickness. The columns, which are 10 nm in diameter, strain the BaTiO₃ matrix by 2.35 …