Strontium titanate

strontium titanate

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Strontium titanate

Synthetic strontium titanate was one of several titanates patented during the late 1940s and early 1950s; other titanate included barium titanate and calcium titanate. Research was conducted primarily at the National Lead Company (later renamed NL Industries) in the United States, by Leon Merker and Langtry E. Lynd. Merker and Lynd first patented the growth process on February 10, 1953; a number of refinements were subsequently patented over the next four years, such as modifications to the feed powder and additions of coloring dopants.

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 micrometers 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 crystallizes to form a single pedunculated pear or boule crystal. This boule is usually no larger than 2.5 centimeters in diameter and 10 centimeters long; it is an opaque black to begin with, requiring further annealing in an oxidizing atmosphere in order to make the crystal colorless and to relieve strain. This is done at over 1000 °C for 12 hours.

Thin films of SrTiO3 can be grown epitaxially by various methods, including pulsed laser deposition, molecular beam epitaxy, RF sputtering and atomic layer deposition. As in most thin films, different growth methods can result in significantly different defect and impurity densities and crystalline quality, resulting in a large variation of the electronic and optical properties.

A diamond simulant

Its cubic structure and high dispersion once made synthetic strontium titanate a prime candidate for simulating diamond. Beginning ca. 1955, large quantities of strontium titanate were manufactured for this sole purpose. Strontium titanate was in competition with synthetic rutile (“titania”) at the time, and had the advantage of lacking the unfortunate yellow tinge and strong birefringence inherent to the latter material. While it was softer, it was significantly closer to diamond in likeness. Eventually, however, both would fall into disuse, being eclipsed by the creation of “better” simulants: first by yttrium aluminium garnet (YAG) and followed shortly after by gadolinium gallium garnet (GGG); and finally by the (to date) ultimate imitation in terms of diamond-likeness and cost-effectiveness, cubic zirconia.

Despite being outmoded, strontium titanate is still manufactured and periodically encountered in jewellery. It is one of the costliest 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.

Strontium titanate

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