Mixed Halide Ordering as a Tool for the Stabilization of Ruddlesden–Popper Structures

While
the constraints on the choice of organic cations are greatly
relaxed for layered two-dimensional perovskites compared to three-dimensional
perovskites, the shape of the spacer cation is still subject to limitations
due to the size of the inorganic pocket between four adjacent corner-sharing
octahedra. To investigate the effect of the spacer cation branching
on the formation of Ruddlesden–Popper (RP) structures, we performed
a comprehensive investigation of structures formed using tert-butyl ammonium (t-BA). We demonstrate that in contrast
to pure bromides and pure iodides, the use of mixed halides enables
the formation of the t-BA2PbBr2I2 RP perovskite structure with the specific ordering
of the bromide and iodide anions. The t-BA spacer,
despite its branched and bulky shape that prevents its deeper penetration,
is able to form significant H-bonds that lead to the stabilization
of the RP assembly if the inorganic pocket is designed in such a way
that the bromide anions occupy terminal axial positions, while the
iodides occupy equatorial positions. We obtain excellent agreement
between experimentally determined and theoretically predicted structures
using global optimization via a minima hopping algorithm
for layered perovskites, illustrating the ability to predict the structure
of RP perovskites and to manipulate the perovskite structure by the
rational design of the inorganic pocket.

相较于三维钙钛矿（three-dimensional perovskites），层状二维钙钛矿（layered two-dimensional perovskites）对有机阳离子选择的限制已大幅放宽，但由于相邻四个共角八面体所形成的无机腔尺寸限制，间隔阳离子的形状仍受到一定约束。为探究间隔阳离子的支化结构对鲁德斯登-波珀（Ruddlesden–Popper, RP）相形成的影响，我们针对叔丁基铵（tert-butyl ammonium, t-BA）构筑的相结构开展了系统性研究。研究发现，与纯溴化物或纯碘化物体系不同，采用混合卤化物可构筑出溴离子与碘离子有序排布的t-BA₂PbBr₂I₂型RP钙钛矿结构。尽管叔丁基铵支化且体积庞大，难以深入无机层间，但只要无机腔的设计使溴离子占据轴向末端位点、碘离子占据赤道位点，其即可通过形成大量氢键稳定RP组装结构。我们通过针对层状钙钛矿的最小跳变算法（minima hopping algorithm）开展全局优化，实现了实验测定结构与理论预测结果的高度吻合，这证明通过合理设计无机腔，可精准预测RP钙钛矿的结构并实现钙钛矿结构的精准调控。