The relation between the melt topology and glass-forming ability for liquid Ge-Se alloys

The topology of liquid Ge_ySe_1-y (0<or=y<or=1) alloys, at the compositions y=0.33, 0.4, 0.5 and 1, is studied using neutron diffraction to measure the Bhatia-Thornton number-number partial structure factor S_NN(kr₁), where k and r₁ are, respectively, the scattering vector and nearest-neighbour distance in real space. S_NN(kr₁) has, for the range 0<or=kr ₁<or=10, a prominent three-peak character in the case of the glass-forming melt GeSe₂ (y=0.33). This character changes significantly with increasing y: the so-called first sharp diffraction peak (FSDP) is virtually eliminated at y=0.5, a composition for which the amorphous state is not formed by usual bulk-quenching methods, and the third peak becomes a shoulder to the second at y=1. The concomitant changes on both the short and intermediate real-space length scales are discussed in terms of a break-up of the GeSe₂ network structure. It is found that liquid GeSe is not characterized by a regular octahedral coordination environment but that it is topologically similar to liquid As.