  We report a comparative study of temperature-dependent photoluminescence and
structural dynamics of two perovskite semiconductors, the chalcogenide
BaZrS$_3$ (BZS) and the halide CsPbBr$_3$ (CPB). These materials have similar
crystal structures and direct band gaps, but we find that they have quite
distinct optoelectronic and vibrational properties. Both materials exhibit
thermally-activated non-radiative recombination, but the non-radiative
recombination rate in BZS is between two and four orders of magnitude faster
than in CPB. Raman spectroscopy reveals that the effects of phonon
anharmonicity are far more pronounced in CPB than in BZS. Further, although
both materials feature a large dielectric response due to low-energy polar
optical phonons, the phonons in CPB are substantially lower in energy than in
BZS. Our results suggest that electron-phonon coupling in BZS is more effective
at non-radiative recombination than in CPB, and that BZS may also have a
substantially higher concentration of non-radiative recombination centers than
CPB. The low defect concentration in CPB may be related to the ease of lattice
reconfiguration, typified by anharmonic bonding. It remains to be seen to what
extent these differences are inherent to the chalcogenide and halide
perovskites and to what extent they can be affected by materials processing;
comparing BZS single-crystals and thin films provides reason for optimism.
