Tolerance Factor and Cooperative Tilting Effects in Vacancy-Ordered Double Perovskite Halides

Lattice
dynamics and structural instabilities are strongly implicated
in dictating the electronic properties of perovskite halide semiconductors.
We present a study of the vacancy-ordered double perovskite Rb2SnI6 and correlate dynamic and cooperative octahedral
tilting with changes in electronic behavior compared to those of Cs2SnI6. Though both compounds exhibit native n-type semiconductivity, Rb2SnI6 exhibits
carrier mobilities that are reduced by a factor of ∼50 relative
to Cs2SnI6. From synchrotron powder X-ray diffraction,
we find that Rb2SnI6 adopts the tetragonal vacancy-ordered
double perovskite structure at room temperature and undergoes a phase
transition to a lower-symmetry monoclinic structure upon cooling,
characterized by cooperative octahedral tilting of the [SnI6] octahedra. X-ray and neutron pair distribution function analyses
reveal that the local coordination environment of Rb2SnI6 is consistent with the monoclinic structure at all temperatures;
we attribute this observation to dynamic octahedral rotations that
become frozen in to yield the low-temperature monoclinic structure.
In contrast, Cs2SnI6 adopts the cubic vacancy-ordered
double perovskite structure at all temperatures. Density functional
calculations show that static octahedral tilting in Rb2SnI6 results in marginally increased carrier effective
masses, which alone are insufficient to account for the experimental
electronic behavior. Rather, the larger number of low-frequency phonons
introduced by the lower symmetry of the Rb2SnI6 structure yield stronger electron–phonon coupling interactions
that produce larger electron effective masses and reduced carrier
mobilities relative to Cs2SnI6. Further, we
discuss the results for Rb2SnI6 in the context
of other vacancy-ordered double perovskite semiconductors, in order
to demonstrate that the electron–phonon coupling characteristics
can be predicted using the geometric perovskite tolerance factor.
This study represents an important step in designing perovskite halide
semiconductors with desired charge transport properties for optoelectronic
applications.