Single crystal growth by self-flux method of the mixed valence gold   halides Cs$_2$[Au$^{I}$X$_2$][Au$^{III}$X$_4$] (X=Br,I)
  High quality single crystals of Cs$_2$Au$_2$X$_6$ (X=Br,I) were grown using a ternary self-flux method. Structural refinements based on single crystal X-ray diffraction measurements show that both materials have a distorted perovskite structure belonging to the I4/mmm space group with full site occupancy. Transport measurements reveal a large bandgap of 550 $\pm$ 100 meV for Cs$_2$Au$_2$I$_6$ and 520 $\pm$ 80 meV for Cs$_2$Au$_2$Br$_6$. Initial attempts at chemical substitution are described. 
Scott C. Riggs, M. C. Shapiro, F. Corredor, T. H. Geballe, I. R.   Fisher, Gregory T. McCandless, Julia Y. Chan
cond-mat.mtrl-sci