Strontium titanate is an oxide of strontium and titanium with the chemical formula SrTiO3. It is a centrosymmetric paraelectric material with a perovskite structure. At low temperatures it approaches a ferroelectric phase transition with a very large dielectric constant ~104 but remains paraelectric down to the lowest temperatures measured. It was long thought to be a wholly artificial material, until 1982 when its natural counterpart—discovered in Siberia and named tausonite—was recognised by the IMA. Tausonite remains an extremely rare mineral in nature, occurring as very tiny crystals. Its most important application has been in its synthesized form wherein it is occasionally encountered as a diamond simulant, in precision optics, in varistors, and in advanced ceramics.
The name tausonite was given in honour of Lev Vladimirovich Tauson (1917-1989), a Russian geochemist. Disused trade names for the synthetic product include strontium mesotitanate, Fabulite, Diagem, and Marvelite.
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.
Synthetic material has a very large dielectric constant (300), and is used in high-voltage capacitors. At very low temperatures, it exhibits piezoelectric and superconductive properties. (It superconducts at 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.
Nb:SrTiO3, niobium doped strontium titanate, is electrically conductive.
A modification to the basic Verneuil process (also known as flame-fusion) is the favoured method of growth. An inverted oxy-hydrogen blowpipe is used, with feed powder mixed with oxygen carefully fed through the blowpipe in the typical fashion, but with the addition of a third pipe to deliver oxygen—creating a tricone burner. The extra oxygen is required for successful formation of strontium titanate, which would otherwise fail to oxidize completely due to the titanium component. The ratio is ca. 1.5 volumes of hydrogen for each volume of oxygen. The highly purified feed powder is derived by first producing titanyl double oxalate salt (SrTiO(C2O4)2·2H2O) by reacting strontium chloride (SrCl2) and oxalic acid ((COOH)2.2H2O) with titanium tetrachloride (TiCl4). The salt is washed to completely eliminate chloride, heated to 1000°C in order to produce a free-flowing granular powder of the required composition, and is then ground and sieved to ensure all particles are between 0.2–0.5 micrometres in size.
The feed powder falls through the oxyhydrogen flame, melts, and lands on a rotating and slowly descending pedestal below. The height of the pedestal is constantly adjusted to keep its top at the optimal position below the flame, and over a number of hours the molten powder cools and crystallises to form a single pedunculated pear or boule crystal. This boule is usually no larger than 2.5 centimetres in diameter and 10 centimetres long; it is an opaque black to begin with, requiring further annealing in an oxidizing atmosphere in order to make the crystal colourless and to relieve strain. This is done at over 1000°C for 12 hours.
Despite being outmoded, strontium titanate is still manufactured and periodically encountered in jewellery. It is one of the most costly of diamond simulants, and due to its rarity collectors may pay a premium for large i.e. >2 carat (400 mg) specimens. As a diamond simulant, strontium titanate is most deceptive when mingled with melée i.e. <0.20 carat (40 mg) stones and when it is used as the base material for a composite or doublet stone (with, e.g., synthetic corundum as the crown or top of the stone). Under the microscope, gemmologists distinguish strontium titanate from diamond by the former's softness—manifested by surface abrasions—and excess dispersion (to the trained eye), and occasional gas bubbles which are remnants of synthesis. Doublets can be detected by a join line at the girdle ("waist" of the stone) and flattened air bubbles or glue visible within the stone at the point of bonding.