Strontium Titanate - Properties

Properties

Strontium titanate is both much denser (specific gravity 4.88 for natural, 5.13 for synthetic) and much softer (Mohs hardness 6–6.5 for natural, 5.5 for synthetic) than diamond. Its crystal system is cubic and its refractive index (2.41—as measured by sodium light, 589.3 nm) is nearly identical to that of diamond, but the dispersion (the optical property responsible for the "fire" of the cut stones) of strontium titanate is over four times higher, at 0.19 (B–G interval). This results in an excess of fire when compared to diamond.

Synthetics are usually transparent and colourless, but can be doped with certain rare earth or transition metals to give reds, yellows, browns, and blues. Natural tausonite is usually translucent to opaque, in shades of reddish brown, dark red, or grey. Both have an adamantine (diamond-like) lustre. Strontium titanate is considered extremely brittle with a conchoidal fracture; natural material is cubic or octahedral in habit and streaks brown. Through a hand-held (direct vision) spectroscope, doped synthetics will exhibit a rich absorption spectrum typical of doped stones. Synthetic material has a melting point of ca. 2080 °C (3776 °F) and is readily attacked by hydrofluoric acid.

The synthetic material has a very large dielectric constant (300) at room temperature and low electric field. It is also used in high-voltage capacitors. Strontium titanate becomes superconducting below 0.35 K and was the first insulator and oxide discovered to be superconductive.

At temperatures lower than 105 K, its cubic structure transforms to tetragonal. It is an excellent substrate for epitaxial growth of high-temperature superconductors and many oxide-based thin films. Its monocrystals can be used as optical windows and high-quality sputter deposition targets.

SrTiO₃ is a suitable material for electronics: niobium-doped strontium titanate, is electrically conductive. High-quality, epitaxial SrTiO₃ layers can also be grown on silicon without forming silicon dioxide, thereby making SrTiO₃ an alternative gate dielectric material. This also enables the integration of other thin film perovskite oxides onto silicon.