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Gallium lanthanum sulfide glass
The glass forming ability of gallium(III) sulfide and lanthanum sulfide was discovered in 1976 by Loireau-Lozac’h, Guittard, and Flahut.〔A.M. Loireau Lozac'h, M. Guittard and J. Flahaut. “Glasses formed by rare earth sulphides La2S3 with gallium sulphide Ga2S3” Material Research Bulletin, 11:1489-1496, 1976〕 This family of chalcogenide glasses, referred to as gallium lanthanum sulfide (Ga-La-S) glasses, have a wide region of glass formation centred about the 70Ga2S3:30La2S3 composition and can readily accept other modifiers into their structure.〔J. Flahaut, M. Guittard and A.M Loireau-Lozac'h, “Rare earth sulphide and oxysulphide glasses”, Glass Technology, 24:149-156, 1983.〕 This means that Ga-La-S can be compositionally adjusted to give a wide variety of optical and physical properties. Optically, Ga-La-S has a high refractive index, a transmission window covering most of the visible wavelengths and extending to about 10 µm and a low maximum phonon energy, approx. 450 cm−1. Thermally, the refractive index of Ga-La-S glasses has a strong temperature dependence and low thermal conductivity, which results in strong thermal lensing. However, the high glass transition temperature of Ga-La-S makes it resistant to thermal damage, it has good chemical durability and unlike many chalcogenides which are based on arsenic, its glass components are non-toxic. A clear advantage over other chalcogenides is its high lanthanum content which allows excellent rare-earth solubility and dispersion of the ions in the glass matrix for active devices.〔T. Schweizer, D.W. Hewak, B.N. Samson, and D.N. Payne, “Spectroscopy of potential mid-infrared laser transitions in gallium lanthanum sulphide glass”, J. Lumin. 419:72-74, 1997.〕 Ga-La-S can exist in both glassy and crystalline phases,〔 in a glassy phase, it is a semiconductor with a bandgap of 2.6 eV corresponding to a wavelength of 475 nm; consequently Ga-La-S glass takes a deep orange colour. As with all chalcogenides the phase of the bulk is determined by two key factors; the material composition and the rate at which the molten material is cooled. These variables can be controlled to manipulate the final phase of the material.
==Chemistry==
The structure of Ga-La-S glass consists of Ga-S bonds, with a length of 2.26 Å, and La-S bonds of length 2.93 Å. It has been reported that the Ga-S bond lengths in the glassy state are identical to those in the crystalline state.〔A. Loireau-Lozac'h, H. Dexpert, P. Lagarde, J.Flahaut, S. Benazeth, M Tuilier, “An EXAFS structural approach of the gallium-lanthanum-sulphur glasses”, Journal of Non-Crystalline Solids, 110:89-100, 1989〕 Therefore it is only necessary to change the bond angles and, thus, it is hypothesised that Ga-La-S has the potential to be a fast switching phase change memory material. In the Ga2S3 crystal shown in (figure 2 below) it should be noticed that two out of three sulfur atoms (S1 and S2) are each bound to three gallium atoms. These sulfur atoms have two normal covalent bonds to two of the gallium atoms. The third Ga-S bond is dative or coordinate covalent (one of the atoms provides both electrons). The third sulfur atom, S3, is bound to just two gallium atoms and is thought to be a bridging atom. The average sulfur coordination number is greater than two; sulfide glasses usually have coordination numbers less than two. Experimentally, Ga2S3 has not been observed in a glassy state. There exists however a GaS4 unit within the Ga-S crystal which has been noted as the glass former. The La-S bond is ionic and likely to be a network modifier. By adding an ionic sulfide to the crystal, like La2S3, it is possible to modify the crystalline Ga2S3 into a vitreous structure. Of all the rare-earth sulfides, lanthanum gives the largest range of vitreous compositions. The effect of adding an ionic sulfide modifier such as a La2S3 molecule to the crystal is to break one of the Ga-S dative bonds and replace it with a S2− anion. This anion links the gallium atom such that its tetrahedral environment is not altered, but what was a tricoordinated S atom now becomes a dicoordinated bridging atom. This process creates a negative void which can then be filled by a La3+ cation. Electrically, the effect of adding La2S3 is to give the glass an essentially ionic character.〔
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