Ferroelectricity and High Tunability in Novel Strontium and Tantalum Based Layered Perovskite Materials

The present study concerns the achievement of new layered-type perovskite ferroelectrics materials with a Curie temperature tailored to be at ambient temperature. This was achieved by mixing two ferroelectrics (Sr2Ta2O7 (STO),... [ view full abstract ]

The present study concerns the achievement of new layered-type perovskite ferroelectrics materials with a Curie temperature tailored to be at ambient temperature. This was achieved by mixing two ferroelectrics (Sr₂Ta₂O₇ (STO), T_C = -107°C, and La₂Ti₂O₇ (LTO), T_C = 1461°C) in a (Sr₂Ta₂O₇)_100-x(La₂Ti₂O₇)_x solid solution. Composition (x) was varied between 0 and 5 and led to an ideal solid solution with an evolution of the lattice parameters and cell volume following the Vegard's law. The dielectric characterization has shown that both relative permittivity (k) and dielectric loss (tand) are stable in frequency from 1 to 100 kHz. At RT and fixed frequency, the relative permittivity increases then decreases with the composition (x); a maximum (k = 365) is reached for x = 1.65 (@10 kHz). Permittivity also peaks for a temperature (T_M) shifting towards higher values with the amount of La₂Ti₂O₇ (x) in the solid solution; compositions with x ≥ 1.85 have a T_M higher than room temperature. Polarization-electric field (P-E) measurements have highlighted non-open loops for the compositions x = 0 and 1.65, while hysteresis were obtained for x = 1.85, 2.0, 2.5 and 3.0, along with switching current peaks in I-E, thus confirming that these compositions are ferroelectrics at RT. The variation of the relative permittivity as a function of a DC electric field (i.e. tunability) was demonstrated for x ≥ 1.5. A very high tunability (17.6 % @0.38 kV/mm; 10 kHz) was obtained for the composition x = 1.65, associated with very low dielectric losses (tand < 2.10^-3). These new and high level performances make (Sr₂Ta₂O₇)_100-x(La₂Ti₂O₇)_x promising lead-free ferroelectric materials which can compete with standard ferroelectrics.