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

Abstract

Neutron diffraction with copper isotope substitution was used to study the structure of glassy ${\text{Cu}}_{5}{\text{As}}_{46.25}{\text{Se}}_{48.75}$, which lies at $x=0.0513$ on the ${({\text{Cu}}_{2}\text{Se})}_{x}{(\text{AsSe})}_{1\ensuremath{-}x}$ pseudobinary tie line. The results are consistent with fourfold coordination of copper to matrix (As or Se) atoms at a mean distance of $2.40(2)\text{ }\text{\AA{}}$ and with threefold coordination of arsenic to one As and two Se atoms at a mean distance of $2.43(2)\text{ }\text{\AA{}}$. 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 $5.13\text{ }\text{mol}\text{ }%$ of ${\text{Cu}}_{2}\text{Se}$ is added. However, the addition of $25\text{ }\text{mol}\text{ }%$ 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 $2.42(2)\text{ }\text{\AA{}}$] 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 $\ensuremath{\simeq}3$ at $2.42(2)\text{ }\text{\AA{}}$ and the Cu-Cu nearest-neighbor coordination number is 1.0(3) at $2.70(4)\text{ }\text{\AA{}}$. A comparison between (i) the measured structure of the glassy network formers AsSe and ${\text{As}}_{2}{\text{Se}}_{3}$ 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.