LAUSR

Epitaxial strain can unlock enhanced properties in oxide materials, but restricts substrate choice and maximum film thickness, above which lattice relaxation and property degradation occur. Here we employ a chemical alternative to epitaxial strain by providing targeted chemical pressure, distinct from random doping, to induce a ferroelectric instability with the strategic introduction of barium into today’s best millimetre-wave tuneable dielectric, the epitaxially strained 50-nm-thick n  = 6 (SrTiO 3 ) n SrO Ruddlesden–Popper dielectric grown on (110) DyScO 3 . The defect mitigating nature of (SrTiO 3 ) n SrO results in unprecedented low loss at frequencies up to 125 GHz. No barium-containing Ruddlesden–Popper titanates are known, but the resulting atomically engineered superlattice material, (SrTiO 3 ) n − m (BaTiO 3 ) m SrO, enables low-loss, tuneable dielectric properties to be achieved with lower epitaxial strain and a 200% improvement in the figure of merit at commercially relevant millimetre-wave frequencies. As tuneable dielectrics are key constituents of emerging millimetre-wave high-frequency devices in telecommunications, our findings could lead to higher performance adaptive and reconfigurable electronics at these frequencies. Strain can modify properties, but to prevent cracking is limited to films below a critical thickness. Here, by inserting atomic layers into a ferroelectric superlattice, chemical pressure is generated in thicker films, with enhanced figure of merit for tuneable millimetre-wave dielectrics.