Noninvasive Quality Assessment of Melt-Grown Cesium Lead Bromide Perovskite by Nuclear Quadrupole Resonance Spectroscopy

Melt-grown, highly crystalline CsPbBr₃ has been intensely investigated as a semiconductor for direct hard radiation detection. While the phase purity and crystallinity of the CsPbBr₃ ingots are assessed by X-ray diffraction and optical microscopy, the overall quality of the material is ultimately judged by the performance of the final device. The iterative evaluation of crystal quality would greatly benefit from broadening readily accessible structural methods. In this work, we establish nuclear quadrupole resonance (NQR) spectroscopy as a versatile, noninvasive technique for evaluating the quality of melt-grown CsPbBr₃ ingots. We show that in addition to its inherent utility for probing the local environment around a quadrupolar nucleus, NQR spectroscopy is highly sensitive to crystal orientation and crystallinity, as further supported by ab initio calculations. The key spectroscopic descriptors (linewidth and integrals) can thus be correlated with both macroscopic and microscopic structural features, thereby establishing a robust and rapid method for evaluating crystal quality. Customized resonators can accommodate large ingots and enable measurements directly in the quartz ampule used for melt growth, as well as semiautomated spatial mapping of spectroscopic features across the ingots. For instance, we show that removing the impurities collected near the top of the ingot and subsequent recrystallization improve the homogeneity and overall crystallinity of the samples, highlighting the need for multiple purification steps. We also observe that different crystallographic orientations of crystal domains along the ingot are obtained and preserved in cut crystal disks. These findings pave the way for integrating NQR spectroscopy as a practical, noninvasive tool for in-line or in-situ crystal quality control and guided sample selection.