COMPACTNESS IN RELATION TO THE MEAN COORDINATION NUMBER IN GLASSY GexBi6S94-x

Chalcogenide glasses have recently attracted much interest due to mainly two reasons. The first stems from their interesting physical and optical properties and consequently of their potential technological applications. A recent review of all these potential applications appeared in the proceedings of the international workshop on amorphous and nanostructured chalcogenides [1]. These applications include the well-known xerography process with Se or As-Se , lithography with Ge25Se75 [2], CD compatible erasable disk with Ge-Te or Ge-Te-Se [3], chemical sensors with AsS(-Se) based alloys [4], long lengths of low loss IR transmitting devices with As40S60-xSex [5], the use of memory and switching effects as observed in Bi29Tl35Se36 [6] or photodarkening properties as in As2S3(-Se3), Ge-S2(Se2) [7,8], and their use for the preparation of electrical memories as in Al23Te77 [9]. The second one is linked to the relative ease of their preparation from the melt. The best example is demonstrated by pure Se which is known to be the only element able to give monoatomic glass [10]. As a consequence, chalcogenide glasses can be used as reference materials in order to develop a better understanding of the glassy state and its specific properties. Moreover, because these materials show a continuous change of their various properties with change in their chemical composition, it is possible to investigate the correlation of the features observed in the property-composition dependence with the structural arrangement in the glass. Such studies on thermal [11], mechanical [12], electrical [13], optical [14] and physicochemical [15] have recently been reported. The Ge-S-Bi is a prototypical chalcogenide system. The first paper that dealt with the addition of bismuth in a chalcogenide matrix was published by Bowman and Schottmiller [16] in 1968. The goal was the preparation of new infrared photoconductors. At the same time, it was discovered that a small amount of Bi atoms (≈ 7 at% Bi) incorporated in a Se matrix leads to the change of the conductivity type from p to n [17]. More recently, the possibility of making an all chalcogenide glass p-n junction based on this ability of Bi was demonstrated [18,19]. Since the results of [17], many studies were performed to propose an explanation of this specific property that the Bi atom seems to possess. Different atomic configurations were proposed. Most of these works were recently discussed in a review article of Vautier [20]. With regard to this review article, one conclusion was that after three decades of researches and debates on this matter, the origins of this