What are the dielectric constant and crystal structure of strontium titanate?

Strontium titanate (SrTiO3) is a perovskite-type dielectric material characterized by a high dielectric constant (especially at low temperatures), frequency- and temperature-tunable properties, and the absence of ferroelectric hysteresis; it is primarily used in high-value capacitors. At room temperature, SrTiO3 exhibits a cubic crystal structure, with titanium atoms at the body centers, oxygen atoms at the face centers (forming oxygen octahedral cages), and strontium atoms at the corner sites. As the temperature decreases, the crystal structure undergoes a transition to a tetragonal system at 110 K and subsequently to an orthorhombic system at 65 K. The pure crystal acts as an insulator at room temperature, with a bandgap of 3.15 eV. Upon reduction by heating in a hydrogen atmosphere, this transparent, colorless insulating crystal transforms into an n-type semiconductor, and its color shifts to varying shades of blue due to free-carrier absorption.

Its dielectric constant is closely related to crystal orientation and temperature. Dielectric constant measurements taken from thin slices cut along the [100], [110], and [111] directions yield the following values: at room temperature, the relative dielectric constant is approximately 330 ± 10 in the [001] direction, ~458 in the [011] direction, and ~448 in the [111] direction. At 77 K, the values are approximately 1880 in the [001] direction, ~2640 in the [011] direction, and ~2580 in the [111] direction [1].

References

[1] Neville, R. C., Hoeneisen, B., Mead, C. (1972). Permittivity of Strontium Titanate. Journal of Applied Physics, 43 1, 2124–2131. https://doi.org/10.1063/1.1661463