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

The optoelectronic properties of CsPbX3 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 CsPbBr3 quantum dots show that the temperatures for both the orthorhombic-to-tetragonal (50 degrees C < T gamma-beta] < 59 degrees C) and tetragonal-to-cubic (108 degrees C < T beta-alpha, < 117 degrees C) phase transitions of the average structure are depressed relative to their temperatures in bulk CsPbBr3. 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 degrees C < T < 160 degrees 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 CsPbBr3 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.