Depressed Phase Transitions and Thermally Persistent Local Distortions in CsPbBr3 Quantum Dots

The optoelectronic properties of CsPbX 3 quantum dots (where X = Cl, Br, or I) are influenced by both their local and average structures. Variable-temperature synchrotron X-ray diffraction measurements of CsPbBr 3 quantum dots show that the temperatures for both the orthorhombic-to-tetragonal (50 °C < T γ-β < 59 °C) and tetragonal-to-cubic (108 °C < T β-α < 117 °C) phase transitions of the average structure are depressed relative to their temperatures in bulk CsPbBr 3 . Simultaneously, pair distribution function analysis of synchrotron X-ray total scattering measurements indicates that the local crystal structure of the quantum dots is best described as orthorhombically distorted over the temperature range of 22 °C < T < 160 °C, with only small changes in the magnitude of the distortion occurring during the observed changes in the average structure. Taken together, these results suggest that phase transitions in CsPbBr 3 quantum dots are order–disorder, involving the gradual ordering of individually coherent domains that cannot be attributed to changes in the surface area or to ferroelectric phenomena.