Raman phonon study of Jahn-Teller distortion in Ba$_3$CuSb$_2$O$_9$*

The frustrated magnet Ba$_3$CuSb$_2$O$_9$ does not exhibit either structural or magnetic ordering down to the lowest measured temperatures and is of great current interest as a spin-liquid candidate. It has been proposed recently that the lack of ordering is due to a static or dynamic Jahn-Teller distortion that leads to orbital disorder [1]. We use phonon Raman scattering at temperatures between 20 and 380 K to investigate Jahn-Teller distortion in crystals with different Sb:Cu stoichiometry. We focus on phonons in the range of 500-800 cm$^{-1}$ attributable to oxygen vibrations. In addition to signatures of the strong disorder due to Cu-Sb site mixing present in these materials, we observe mode-splitting due to a static Jahn-Teller distortion below 200 K in samples that undergo a transition to an orthorhombic phase. In contrast, samples that remain hexagonal to the lowest temperatures do not show such mode splitting. References: [1] S. Nakatsuji et al. Science 336, 559 (2012)