The nontoxic GLS Glass, the first chalcogenide glass( prepared in1973) has a typical structure consisting of an extended tetrahedral GaS4 units formed from Ga2S3 which is quite different from all other rare-earth glasses as follows:

[I]HERE AVERAGE COORDINATION NUMBER(CN) OF SULPHUR IS GREATER THAN TWO WHILE OTHER SULPHIDE GLASSES USUALLY HAVE C.N. LESS THAN TWO.

[II] OF ALL THE REAR –EARTH SULPHIDES ,La(III) GIVES THE LARGEST RANGE OF VITREOUS COMPOSITIONS.

[III] Ga-S(226pm) and La-S(293pm) IN CRYSTALLINE AND GLASSY STATE REMAIN TO BE SAME.

In the Ga2S3 crystal two out of three S are each bound to three Ga atoms. These sulfur atoms have two normal covalent bonds to two of the Ga atoms. The third Ga-S bond is dative which is bound to just two Ga and is thought to be a bridging atom so that the average C. N. of S is greater than two.

This structure changes to stable GaS4 tetrahedral unit by adding an ionic sulfide(La2S3) which breaks one of the Ga-S dative bonds and replace it with a S2- so that a tricoordinated S atom now becomes a dicoordinated bridging atom. This process creates a negative void which can then be filled by a La(III) . This effect of adding La2S3 gives the glass an essentially ionic character and modifies the crystalline Ga2S3 into a vitreous structure without changing Ga-S (226pm) and La-S (293pm) bond lengths.

Thanks Manohar, this is a great start! But now my question is, which of those effects leads to the dramatic increase in Tg of the glass compared to other sulfides? Is it the overcoordination of the the S atoms? Is it the mixed covalent/ionic bonding? Is it steric hinderance of pinned Ga-S tetrahedral chains? What do you think?

Sir, though I have never been an original researcher in this field, yet having some basic background while being associated with the teaching of the topic involving RE oxides/ sulphides glasses, I submit that GLS Glass differs from other RE Sulphide glasses in the following FIVE points:

AN EXTENDED STABLE TETRA HEDRAL Ga S4 UNIT NETWORK STUCTURE WITH INTERSTIES BEING OCCUPIED BY THE LARGESREST SIZED La(III) , AVERAGE CN Of SULPHUR BEING MORE THAN TWO, GaS AND LaS BOND DISTANCES IN ITS GLASSY AND CRYSTALLINE STATES BEING VIRTUALLY THE SAME , COMPARATIVE MUCH HIGHER IONIC CHARACTER AND LARGEST VITROUS RANGE.

The cumulative effect of these properties is sufficient to make its Tg much higher the other glasses of the same class