$\mathrm{CH}{\left(\mathrm{N}{\mathrm{H}}_2\right)}_2\mathrm{PbB}{\mathrm{r}}_3 \left(\mathrm{FAPbB}{\mathrm{r}}_3\right)$ is a semiconducting material with the perovskite structure which has generated intense topical interest due to its potential applications in optoelectronic and spintronic devices. Its functional properties are related in part, at least, to a phenomenologically rich sequence of structural instabilities driven by octahedral tilting and dynamic effects of the $\mathrm{F}{\mathrm{A}}^{+}$ cations. Measurements of elastic, anelastic, and dielectric properties, linear thermal expansion, and heat capacity have been used to provide an unambiguous description of the full sequence of transitions. The octahedral tilting transitions at ∼264 and ∼183 K conform to 246 and close to 26 (tricritical) Landau potentials. First-order isosymmetric transitions driven by abrupt changes in $\mathrm{F}{\mathrm{A}}^{+}$ configurations occur at ∼161, 152, 145, and 121 K during cooling and at ∼162, 152, and 122 K during heating. A succession of further loss peaks below 100 K, relating primarily to dynamics of the $\mathrm{F}{\mathrm{A}}^{+}$ cations, reveals thermally activated relaxations with a spread of relaxation times and activation energies in the range ∼0.01–0.15 eV. Acoustic loss in the temperature interval ∼180–230 K indicates that ferroelastic domain walls in the stability field of the tetragonal structure were mobile under the low-stress conditions that apply during measurements by resonant ultrasound spectroscopy, with likely pinning by Br vacancies below ∼210 K. Overall patterns of hysteresis, particularly in dielectric loss behavior in the temperature interval ∼250–350 K, point to a thermal history dependence of the configurations of the $\mathrm{F}{\mathrm{A}}^{+}$ cations. The diversity of possible interactions of $\mathrm{F}{\mathrm{A}}^{+}$ cations with domain walls, antiphase boundaries and the perovskite framework has the potential to lead to polar or optoelectronic properties localized specifically within the domain walls.

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• Domain walls
• Entropy
• Ferroelasticity
• Landau theory
• Order parameters
• Phase transitions
• Specific heat
• Structural properties
• Thermal expansion
• Ultrasound attenuation