Three-Dimensional Methylhydrazinium Lead Halide Perovskites: Structural Changes and Effects on Dielectric, Linear, and Nonlinear Optical Properties Entailed by the Halide Tuning

Three-dimensional
lead halide perovskites are promising materials
for optoelectronic applications. The most famous representative comprise
methylammonium (MA+) and formamidinium (FA+)
cations, but recently, this group was enlarged by methylhydrazinium
(MHy+) analogues that crystallize in polar structures at
room-temperature. Properties of three-dimensional (3D) perovskites
can be tuned by mixing of molecular cations or halide anions. Here,
we report synthesis and physicochemical characterization of mixed-halide
MHyPbBrxCl3–x (x = 0.40, 0.58, 0.85, 1.33, 1.95, 2.25,
and 2.55) and MHyPbBr2.8I0.2 perovskites. X-ray
diffraction data show that all materials feature a polar monoclinic P21 symmetry at room temperature. With the temperature
increase, all MHyPbBrxCl3–x perovskites undergo a displacive phase transition
to another polar orthorhombic Pb21m phase at T2 ≥ 318 K.
The bromine rich crystals (x ≥ 1.33) exhibit
an additional order–disorder phase transition to the archetypal
cubic Pm3̅m phase at T1 ≥ 409 K. In contrast to MHyPbBrxCl3–x perovskites,
MHyPbBr2.8I0.2 undergoes a direct P21 to Pm3̅m phase
transition. The temperature at which the cubic phase is stabilized,
stability range of the Pb21m phase, and distortion of the lead-halide octahedra decrease with
the increase of Br– content. The structural changes
affect dielectric, conductivity, and optical properties. In particular,
the Br-rich samples show switchable dielectric behavior near 410–420
K. Furthermore, the activation energy of Cl– ionic
conductivity increases with the increase of Br– content
in phases Pb21m and P21, whereas in phase Pm3̅m, the conductivity of Br– ions increases
with the increase of Cl– content. The energy band
gap narrows and the photoluminescence (PL) bands exhibit red shift
when going from Cl to Br and then to I. Interestingly, whereas PL
of the Br-rich and Cl-rich samples is dominated by bound exciton and
self-trapped exciton bands, respectively, these bands are suppressed
for 2.25 ≥ x ≥ 0.85. The PL color is
strongly tuned by doping and changes from greenish-blue for the Cl-rich
samples to yellowish-green for MHyPbBr2.8I0.2. SHG studies demonstrate that doping of MHyPbCl3 with
Br– ions reduces the difference between SHG signal
intensities of the monoclinic and orthorhombic phases, to the extent
that beyond x = 1.95, the SHG response of these phases
becomes essentially the same. The relative SHG efficiencies of Br–Cl
mixed materials at room temperature increase with the increase in
Br content.