Study of the Effect of Gallium Replacement by Thallium in CuGa&amp;lt;sub&amp;gt;(1-x)&amp;lt;/sub&amp;gt;Tl&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;S&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Structure by First-Principles Calculations on CASTEP