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We report high-pressure photoluminescence, Raman scattering, and x-raydiffraction measurements on a lead-free halide perovskite $Cs_3Sb_2Br_9$. Atabout 3 GPa, an electronic transition manifests itself through a broad minimumin linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, andthe unusual change in the $c/a$ ratio of the trigonal lattice. The largecompressibility and observed Raman anomalies indicate to a soft material withstrong electron-phonon coupling. The observed below bandgap broadband emissionin the photoluminescence measurement indicates the recombination ofself-trapped excitons. The initial blueshift of the photoluminescence peakreinforces itself to the redshift at around 3 GPa due to the change in theelectronic landscape. A first order trigonal to a monoclinic structuraltransition is also seen at 8 GPa. The first-principles density functionaltheory (DFT) calculations reveal that the electronic transition is associatedwith direct-to-indirect bandgap transition due to changes in the hybridizationof $Sb-5s$ and $Br-4p$ orbitals near the Fermi level in the valence band. Theexperimentally observed Raman modes are assigned to their symmetry using thedensity functional perturbation theory. In addition, the DFT calculationspredict a 27.5\% reduction of the bandgap in the pressure range 0-8 GPa.

physical review. b./physical review. b

Pressure induced emission enhancement and bandgap narrowing:  experimental investigations and first principles theoretical simulations on a  model halide perovskite