Structure of Cu-As-Se glasses investigated by neutron diffraction with copper isotope substitution

Neutron diffraction with copper isotope substitution was used to study the structure of glassy Cu(5)As(46.25)Se(48.75), which lies at x=0.0513 on the (Cu(2)Se)(x)(AsSe)(1-x) pseudobinary tie line. The results are consistent with fourfold coordination of copper to matrix (As or Se) atoms at a mean distance of 2.40(2) angstrom and with threefold coordination of arsenic to one As and two Se atoms at a mean distance of 2.43(2) angstrom. A comparison is made with the structure of glassy AsSe and with the structure of a high-Cu content glass with x=0.25. It is found that the short-range order of glassy AsSe is not changed when 5.13 mol % of Cu(2)Se is added. However, the addition of 25 mol % leads to a disruption of the intermediate-range order and to an alteration of the short-range order, which is consistent [within a scenario where arsenic remains threefold coordinated by matrix atoms at a mean distance of 2.42(2) angstrom] with a substitution of As-As bonds by As-Se bonds. In the x=0.25 material, the Cu to matrix atom coordination number is 3 at 2.42(2) A and the Cu-Cu nearest-neighbor coordination number is 1.0(3) at 2.70(4) angstrom. A comparison between (i) the measured structure of the glassy network formers AsSe and As(2)Se(3) and (ii) the results obtained from first-principles molecular-dynamics methods points to a need for more accurate simulations in order to help elucidate the structure and properties of these materials.