Theoretical prediction of Jahn-Teller distortions and orbital ordering in Cs2CuCl2Br2

With the use of the density function calculations we show that the actual crystal structure of Cs$_2$CuCl$_2$Br$_2$ should contain elongated in the $ab-$plane CuCl$_4$Br$_2$ octahedra, in contrast to the experimentally observed compression in $c-$direction. We also predict that the spins on Cu$^{2+}$ ions should be ferromagnetically ordered in $ab-$plane, while the exchange interaction along $c-$direction is small and its sign is uncertain.