Spectral Reflectivity as a Function of Temperature ofβ-Brass Type Alloys

Abstract

A number of $\ensuremath{\beta}$-brass type alloys undergo reversible color changes as a function of temperature. Normally, yellow $\ensuremath{\beta}$-brass becomes copper-red at about 250\ifmmode^\circ\else\textdegree\fi{}C, the color change taking place gradually. $\ensuremath{\beta}$-AgZn is pink at room temperature; it is silver gray at 300\ifmmode^\circ\else\textdegree\fi{}C. Similar changes have been observed in $\ensuremath{\beta}$-AgCd, $\ensuremath{\beta}$-AuZn, and $\ensuremath{\beta}$-AuCd. Color changes consistent with the above observations have been noted on cooling to liquid nitrogen temperatures. Although disordering takes place in some of these alloys, it is not thought that these color change phenomena are primarily due to disordering or to surface films. Quantitative measurements of reflectivity as a function of wavelength at several temperatures have been made on $\ensuremath{\beta}$-CuZn, $\ensuremath{\beta}$-AgZn, $\ensuremath{\beta}$-AuZn, $\ensuremath{\beta}$-AgCd, $\ensuremath{\beta}$-AuCd, and $\ensuremath{\alpha}$-brass. The reflectivity edge for $\ensuremath{\beta}$-brass shifts from 5000 \AA{} at 27\ifmmode^\circ\else\textdegree\fi{}C to 5300 \AA{} at 275\ifmmode^\circ\else\textdegree\fi{}C equivalent to a shift of about -5.6\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ eV/\ifmmode^\circ\else\textdegree\fi{}C. Edge shifts of this magnitude were not observed with $\ensuremath{\alpha}$-brass, copper, gold, ${\mathrm{Au}}_{3}$Zn, CuAu, or ${\mathrm{Cu}}_{3}$Au. A number of possible explanations of the optical results are discussed; it is believed that these results are primarily due to plasma effects strongly influenced by interband transitions.