Bismuth/Silver-Based Two-Dimensional Iodide Double and One-Dimensional Bi Perovskites: Interplay between Structural and Electronic Dimensions

New structures with favorable band
structures and optical properties
are of broad interest to the halide perovskite community. Recently,
lead-free two-dimensional (2D) double perovskites have emerged as
dimensionally reduced counterparts of their three-dimensional analogues. In addition to the
structural diversity provided by the organic cation, the achievement
of 2D lead-free iodide double perovskites has prompted researchers
to explore more structures in this new material family. Here, we report
the synthesis and structures of a series of 2D iodide double perovskites
based on cyclic diammonium cations (aminomethyl)­piperidinium (AMP)
and (aminomethyl)­pyridinium (AMPY), (4AMP)2AgBiI8 and (3AMPY)2AgBiI8, and compare them with
one-dimensional (1D) structures with Bi only (x-AMP)­BiI5 and (x-AMPY)­BiI5 (x = 3 and 4). The crystallographic structures of the double perovskite
phases are highly distorted, because of the inability of Ag to form
regular octahedral coordination with iodine. The experimental bandgaps
of the double perovskite phases are surprisingly similar to ((4AMP)2AgBiI8) or even larger than ((3AMPY)2AgBiI8) those in the 1D structures with the same cations
((4AMP)­BiI5 and (3AMPY)­BiI5). Density functional
theory calculations suggest that the effective electronic dimensionality
of the double perovskites is on a par with or lower than that of the
1D structures. The reduced electronic dimension of the 2D compounds
originates from the weak electronic coupling between the corner-sharing
Ag and Bi octahedra. The band structures of the 1D compounds are dispersive
in the chain direction, suggesting that their electronic and structural
dimensions are similar. Low-frequency Raman spectra exhibit distinct
peaks at room temperature for all compounds reported here, suggesting
rigid lattices.