# Polycrystalline Formamidinium Lead Bromide X-ray Detectors
## 3 Results and Discussion
### Microstructure and Surface Topology  
The top and cleaved surfaces of the pellets were imaged with SEM to examine the microstructural changes as a function of varying pressures and annealing conditions. Figures 4a-c shows the evolution of the FAPbBr3 pellet morphology as a function of different pressures. At the lowest pressure of 24 MPa the material shows a high degree of porosity, and also a large variation in grain sizes due to the manual process used to grind the as-grown crystals. As the pressure increases, the amount of porosity appears to decrease, which correlates with the measured increase in density. This reduction of porosity with increased pressure may be due to the annihilation of large particle agglomerates, additional compaction forcing grains closer together, and/or pressure induced sintering resulting in additional neck growth between individual particles. Figure 4d,e are SEM micrographs of the pellet fracture surface comparing the microstructure of the pellet before and after annealing, with the pellets formed using a pressure of 100 MPa. The annealing process was carried out in ambient air, at a temperature of 145 \({}^{\circ}\)C for 15 min. Annealing  
Figure 3: Room temperature PL emission from FAPbBr3 pellets as a function of a pressure used during formation (**a**) PL spectroscopy and peak positions, inset: PL intensity of 12 MPa FAPbBr3 pellet before and after annealing. (**b**) PL peak intensity. (**c**) Peak FWHM line width.  
at these conditions successfully induced additional sintering and grain growth, with the grain size appearing to increase and the overall porosity decreasing.  
XRD measurements, shown in Figure 4f, were conducted on an annealed and a non-annealed pellet pressed at the same 100 MPa pressure and compared to a simulated XRD pattern. No significant differences in peak position or overall phase were observed comparing the two samples, suggesting that the phase and stress state of the pellet did not change due to annealing. Compared to the simulated diffraction pattern [20], the pellets pressed at 100 MPa before and after annealing have preferred orientation along the (1 0 0) family of planes. This preferred orientation may be associated with the anisotropic pressing process used to consolidate the powder into pellets. Also, the orientation of the  
Figure 4: SEM images and XRD of FAPbBr\({}_{3}\) pellets fabricated with various conditions; (**a–c**) Top-surface SEM images showing a comparison of structure at different pressures of 12, 74, and 124 MPa, without annealing. (**d,e**) SEM images of cleaved surfaces of pellets formed at 100 MPa pressure, before and after annealing at 145 \({}^{\circ}\)C for 15 min. (**f**) XRD patterns of 100 MPa FAPbBr\({}_{3}\) pellet before and after annealing.  
FA molecule within the cubic FAPbBr\({}_{3}\) crystal structure may not be uniform throughout the sintered pellet. As a result, the simulation may not demonstrate a representative distribution of FA molecule orientations, which may impact the overall structure factor of the crystal and subsequent observed diffraction intensities. The FWHM of the diffraction peaks narrowed after annealing, suggesting an increase in crystallite size. This is consistent with the observed changes in microstructure seen in the fracture surface SEM.