Crystal Structure Evolution and Notable Thermal Expansion in Hybrid Perovskites Formamidinium Tin Iodide and Formamidinium Lead Bromide

Abstract

The temperature-dependent structure evolution of the hybrid halide perovskite compounds, formamidinium tin iodide (FASnI, FA = CH[NH]) and formamidinium lead bromide (FAPbBr), has been monitored using high-resolution synchrotron X-ray powder diffraction between 300 and 100 K. The data are consistent with a transition from cubic Pm3m (No. 221) to tetragonal P4/mbm (No. 127) for both materials upon cooling; this occurs for FAPbBr between 275 and 250 K, and for FASnI between 250 and 225 K. Upon further cooling, between 150 and 125 K, both materials undergo a transition to an orthorhombic Pnma (No. 62) structure. The transitions are confirmed by calorimetry and dielectric measurements. In the tetragonal regime, the coefficients of volumetric thermal expansion of FASnI and FAPbBr are among the highest recorded for any extended inorganic crystalline solid, reaching 219 ppm K for FASnI at 225 K. Atomic displacement parameters of all atoms for both materials suggest dynamic motion is occurring in the inorganic sublattice due to the flexibility of the inorganic network and dynamic lone pair stereochemical activity on the B-site. Unusual pseudocubic behavior is displayed in the tetragonal phase of the FAPbBr, similar to that previously observed in FAPbI.

The Effects of Doping Density and Temperature on the Optoelectronic Properties of Formamidinium Tin Triiodide Thin Films

“…These trends are generally influenced by two competing contributions—one originating from the dilation of the lattice and its effect on energy levels and the other from electron‐phonon interactions—both of which may depend on temperature with varying sign and extent . In addition, phase transitions are observed in FASnI 3 as changes in the gradients of these data at approximate temperatures of 70 and 160 K, similar to previous reports for single and poly‐crystalline FASnI 3 . We note that these transitions are less clearly observable for the highly doped (0% SnF 2 ) material, most likely because the presence of ionized tin vacancies leads to energetic broadening arising from local strain and electric fields .…”

“…We note that these transitions are less clearly observable for the highly doped (0% SnF 2 ) material, most likely because the presence of ionized tin vacancies leads to energetic broadening arising from local strain and electric fields . Such broadening may also be the reason why we cannot discern the additional phase transition (to cubic) that has been reported to occur near 225–250 K …”

Robust Tin‐Based Perovskite Solar Cells with Hybrid Organic Cations to Attain Efficiency Approaching 10%

“…Kubicki et al found that black hybrid FA + /GA + lead triiodide perovskite crystals were thermodynamically unstable and became yellow in a few hours; moreover, crystal reorientation was observed in their mother structure (FAPbI 3 ) . Phase transition occurred at room temperature has been reported for FAPbI 3 but not for FASnI 3 because it happened at much lower temperatures (125–150 K) for which the black phase of FASnI 3 is stable at room temperature . Based on our single‐crystal results, the GA x FA 1− x SnI 3 perovskites adopt the same structure as FASnI 3 until adding 30% of GA + .…”

Tracking Structural Phase Transitions in Lead‐Halide Perovskites by Means of Thermal Expansion

“…The results obtained for δ‐FAPbI 3 and FAPbBr 3 are presented in Figures and with the phase transition temperatures and phases specified in Tables 1 and 2 , respectively. It should be noted that previous studies of the crystal structure as derived from variable temperature single crystal/powder XRD studies of FAPbBr 3 gave contradictory conclusions . We first focus on δ‐FAPbI 3 in Figure a.…”