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Perovskite photovoltaic ABX$_3$ systems are being studied due to their highenergy-conversion efficiencies with current emphasis placed on pure inorganicsystems. In this work, synchrotron single-crystal diffraction measurementscombined with second harmonic generation measurements reveal the absence ofinversion symmetry below room temperature in CsPbBr$_3$. Local structuralanalysis by pair distribution function and X-ray absorption fine structuremethods are performed to ascertain the local ordering, atomic paircorrelations, and phase evolution in a broad range of temperatures. Thecurrently accepted space group assignments for CsPbBr$_3$ are found to beincorrect in a manner that profoundly impacts physical properties. Newassignments are obtained for the bulk structure: $Im$$\bar{3}$ (above $\sim$410 K), $P$2$_1$/$m$ (between $\sim$ 300 K and $\sim$ 410 K), and the polargroup $Pm$ (below $\sim$ 300 K), respectively. The newly observed structuraldistortions exist in the bulk structure consistent with the expectation ofprevious photoluminescence and Raman measurements. High-pressure measurementsreveal multiple low-pressure phases, one of which exists as a metastable phaseat ambient pressure. This work should help guide research in the perovskitephotovoltaic community to better control the structure under operationalconditions and further improve transport and optical properties.

High-Resolution In-situ Synchrotron X-ray Studies of Inorganic Perovskite CsPbBr$_3$: New Symmetry Assignments and Structural Phase Transitions