Structures, Phase Transitions and Tricritical Behavior of the Hybrid Perovskite Methyl Ammonium Lead Iodide

Abstract

We have examined the crystal structures and structural phase transitions of the deuterated, partially deuterated and hydrogenous organic-inorganic hybrid perovskite methyl ammonium lead iodide (MAPbI3) using time-of-flight neutron and synchrotron X-ray powder diffraction. Near 330 K the high temperature cubic phases transformed to a body-centered tetragonal phase. The variation of the order parameter Q for this transition scaled with temperature T as Q ∼ (Tc−T)β, where Tc is the critical temperature and the exponent β was close to ¼, as predicted for a tricritical phase transition. However, we also observed coexistence of the cubic and tetragonal phases over a range of temperature in all cases, demonstrating that the phase transition was in fact first-order, although still very close to tricritical. Upon cooling further, all the tetragonal phases transformed into a low temperature orthorhombic phase around 160 K, again via a first-order phase transition. Based upon these results, we discuss the impact of the structural phase transitions upon photovoltaic performance of MAPbI3 based solar cells.

Molecular orientational melting within a lead-halide octahedron framework: The order-disorder transition in CH3NH3PbBr3

“…We also observe a precipitous increase in the number of active molecules at ∼ 150 K indicative of an increase in local molecular disorder as temperature is increased. This is corroborated by diffraction studies [9][10][11] , NMR studies 51 and computational investigations 17,52 .…”

“…It is tempting to draw parallels between this and liquid crystals, specifically the melting of the smectic phase to the nematic phase, though in reality the two systems are not very similar 26 . In the nematic phase of liquid crystals there is an orientational order, but no positional order 27,28 whereas in the smectic phase both positional and orientational order is obtained [8][9][10][11] . The situation in MAPbBr 3 is somewhat analogous to the molecular ionic crystals calcite and sodium nitrate where molecular positions are fixed by the crystalline lattice, however transitions from orientational order to disorder occurs with increasing arXiv:1711.07002v1 [cond-mat.mtrl-sci] 19 Nov 2017 temperatures 29,30 .…”

“…A plot of how this quasielastic full-width (2γ) averaged across the entire measured |Q| range changes with temperature is shown in Fig. 3(a), showing an initial increase, indicative of shortened lifetimes, with increasing temperature followed by a distinct drop at the 150 K phase transition, which only begins to increase again after ∼160 K. Two separate experiments are plotted in different colors as the exact temperature of the phase transition is expected to be dependent on the thermal history 11 ; here we find the results from our separate experiments agree within error.…”

“…This radius affects the size of the inorganic cages in which the MA cation sits, and thus the exact radius of rotation accessible for the molecule. In the case of Cl, the decreased size of the halide ion has been linked to the presence of two potential orientational states of the cation in the low temperature phase, as opposed to the single orientation found when I or Br is present instead [7][8][9][10][11] . The choice of halide anion also has an impact on the exact optical properties of the system, and a mixture of anions can be used to tune the band gap of the system 2-5 .…”

Rietveld Study of Cu Doped Gadolinium Orthoferrites

“…The refinement of the structural parameters is continued till convergence is optimized to a goodness of fit (GOF) between 1.0 and 1. Fig.4 it is clear that the percentage of the tetragonal phase seems to increase at the expense of orthorhombic and cubic phases with an increase in Cu concentration [20]. Based on the results of Rietveld analysis, tetragonal phase formation can be explained as follows In GdFeO 3 (ABO 3 ) structure, B ions were octahedrally coordinated by six oxygen ions (BO 6 as shown in Fig.…”