Mar 29 2004
Russian researchers produce crystals of various colours and shades based on yttrium, aluminium and oxygen. Outwardly, they practically do not differ from well-known semiprecious garnet stones. However, artificially produced crystals possess higher solidity, and the colour variety is much wider than that of their natural “relatives”.
Sometimes a minor thing is sufficient to change the situation beyond recognition. That is particularly important in chemistry, especially in chemistry of crystals. A crystal is like a huge building constructed from atom “bricks”: in case of one redundant atom or vice versa – and the building changes the shape, the quality of such structure decreasing. To colour the crystal building, small amounts (hundredth parts) of certain metals (colour promoters) are required. Such admixtures of chromium and iron make the first-class gems – rubies and sapphires – from aluminium oxide.
Nature spends several years to achieve the result, however the laboratories need one or two days to produce the same. Laboratories also grow natural garnets, but the quality has to be sacrificed to the speed. Crystals of a large size (in this case, they are convenient for the jeweller's art) can be grown up from the melt containing silicon oxide, aluminium oxide, ferric oxide. Yttrium-aluminium garnets without admixtures are colourless. By adding different rare-earth metals in the course of preparing these crystals, not only the desired colour can be ensured to crystals, but also the required shade. The advantage of these crystals is also that the colour promoters are better distributed in them, therefore, producing the crystals of uniform colour and high degree of purity.
To receive the yttrium-aluminium crystal with the structure of garnet, aluminium oxide and yttrium oxide were melted down at the 3:5 proportion. The obtained mixture was ground through to formation of the 5 millimeter granules. Then the calcined powders of rare-earth metals and zirconium, hafnium or titanium were mixed with granules of the yttrium-aluminium crystal, and the mixture was placed evenly in the boat container along its length. Then the boat container was placed in the gas-vacuum chamber. The mixture melted under the heater installed in the chamber, as the boat container moved away from the heater to a cooler area, crystallization was taking place. The so-called inoculum - nucleus of crystal - is preliminarily allocated for programming the direction of crystal growth, the inoculum possessing the miniparameters of the future macrocrystal.
Thus the researchers produced analogues to natural garnets of red and green colours, as well as pale blue garnets which do not exist in the natural environment. The red colouring of yttrium-aluminium garnets is caused by inclusion of zirconium ions in the crystal structure. Increasing the amount of zirconium may intensify the colour. However, the researchers failed to get the desired variety of red shades. Introduction of scandium oxide in the melt solves the problem –the deepness and evenness of the colour is increased. This is how garnets of bright red through crimson colour were produced. The researchers can ensure various shades of violet colour, to this end, part of yttrium ions should be additionally replaced by the europium ions – and garnets of multiple violet shades will be available to jewellers.
To produce the crystals imitating all natural garnets of green shades, yttrium-aluminium garnets are used with adding of ytterbium, zirconium and cerium. Increasing the amount of cerium ions in the crystal, the colour may be changed from dark-green via bright green through yellow-green. Thus, can be achieved grass green, swamp green and emerald shades. If the researchers introduce in the crystal content the metal ions, the size of which are larger than that of the yttrium ions, then the blue-green crystal will be produced to imitate emerald stones.
The colour of blue garnets which do not exist in the natural environment are caused by presence of the europium, zirconium and terbium ions in the structure of yttrium-aluminium garnets. It is possible now to produce sky-blue garnet (which is outwardly very similar to aquamarine) or garnets of violet-blue colour and even of bright blue colour with green shade.
Transparency, high solidity, large size – all these properties make the artificially produced garnets serious rivals to natural stones in the jeweller's art. Natural stones yield in this connection to artificial ones and cost much more than artificial stones. The produced crystals turned out to be excellent cutting material and fit for making elegant jewellery. The main consumers of artificial crystals are major associations and jewellery enterprises in Russia and foreign countries.
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