  We study the effects of bismuth doping on the crystal structure and phase
transitions in single crystals of the perovskite semiconductor methylammonium
lead tribromide, MAPbBr3. By measuring temperature-dependent specific heat
capacity (Cp) we find that, as Bi doping increases, the phase transition
assigned to the cubic to tetragonal phase boundary decreases in temperature.
Furthermore, after doping we observe one phase transition between 135 and 155
K, in contrast to two transitions observed in the undoped single crystal. These
results appear strikingly similar to previously reported effects of mechanical
pressure on perovskite crystal structure. Using X-ray diffraction, we show that
the lattice constant decreases as Bi is incorporated into the crystal, as
predicted by density functional theory (DFT). We propose that bismuth
substitutional doping on the lead site is dominant, resulting in BiPb+ centers
which induce compressive chemical strain that alters the crystalline phase
transitions.
