  Controlling grain orientations within polycrystalline all-inorganic halide
perovskite solar cells can help increase conversion efficiencies toward their
thermodynamic limits, however the forces governing texture formation are
ambiguous. Using synchrotron X-ray diffraction, we report meso-structure
formation within polycrystalline CsPbI2.85Br0.15 powders as they cool from a
high-temperature cubic perovskite ({\alpha}-phase). Tetragonal distortions
(\b{eta}-phase) trigger preferential crystallographic alignment within
polycrystalline ensembles, a feature we suggest is coordinated across multiple
neighboring grains via interfacial forces that select for certain lattice
distortions over others. External anisotropy is then imposed on polycrystalline
thin films of orthorhombic ({\gamma}-phase) CsPbI3-xBrx perovskite via
substrate clamping, revealing two fundamental uniaxial texture formations; (i)
I-rich films possess orthorhombic-like texture (<100> out-of-plane; <010> and
<001> in-plane), while (ii) Br-rich films form tetragonal-like texture (<110>
out-of-plane; <1-10> and <001> in-plane). In contrast to relatively
uninfluential factors like the choice of substrate, film thickness and
annealing temperature, Br incorporation modifies the {\gamma}-CsPbI3-xBrx
crystal structure by reducing the orthorhombic lattice distortion (making it
more tetragonal-like) and governs the formation of the different, energetically
favored textures within polycrystalline thin films.
