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

## TL;DR

This paper introduces nuclear quadrupole resonance spectroscopy as a noninvasive method to assess the quality of cesium lead bromide perovskite crystals used in radiation detection.

## Contribution

The novel use of NQR spectroscopy for evaluating crystal quality and orientation in melt-grown CsPbBr3 is established.

## Key findings

- NQR spectroscopy is sensitive to crystal orientation and crystallinity in CsPbBr3.
- Key spectroscopic features correlate with structural properties of the material.
- Multiple purification steps improve homogeneity and crystallinity of the samples.

## Abstract

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.