Continuously Tunable Negative Pressure for Engineering Ideal Octahedral Rotations in CsPbI₃

Despite the fascination it has held for theorists for decades, the phenomenon of triaxial tensile strain, or equivalently ‘negative pressure,’ has had few experimental demonstrations and those have been limited to a material-specific, chemically induced phase transition. In this work, the well-known phenomenon of strain induced by thermal-expansion-coefficient-mismatch between a thin film and its substrate is reimagined, replacing the canonical planar support with a three-dimensional, nano-confining scaffold in which we embed the material of interest. In this manner we demonstrate a general approach to exert a continuously tunable, triaxial, tensile strain, defying the Poisson ratio and achieving the exotic condition of ‘negative pressure.’ This approach is generalizable to any material with a low modulus and high thermal expansion coefficient, and we use it here to achieve negative pressure in perovskite-phase CsPbI₃ embedded within the cylindrical pores of anodic aluminum oxide (AAO) membranes. Thus, it is possible to continuously transform the perovskite structure towards higher symmetry, in contrast with the symmetry reducing action of any other mechanical perturbation. We use this effect to control the octahedral rotation angle that is critical to the remarkable photovoltaic attributes of halide perovskites. Up to hundreds of megapascals of apparent negative pressure, the bandgap tunability is observed to follow the same quantitative trend observed for hydrostatic positive pressure, exploring the negative pressure region for the first time and demonstrating the dominance of bond stretching effects in comparison to the influence of average octahedral rotation angle on electronic structure. The possibility for novel dynamics under the reduced interatomic forces could have application to a variety of ferroic phenonomena, wherein at a fixed temperature, dipolar and external forces would have a greater relative influence on the equilibrium ordering, as desirable, for example, in photovoltaics and ferroelectric switching.