Understanding the crystallographic phase relations in alkali-trihalogeno-germanate perovskites in terms of ferroelectric or antiferroelectric arrangements of the tetrahedral GeX$_3$ units

The alkali-trihalogeno-germanates AGeX$_3$ with A a large single positive ion such as Rb, Cs, or organic radicals such as methyl ammonium (MA), and X a halogen (I, Br, Cl, F) along with the corresponding stannates (ASnX$_3$) and plumbates (APbX$_3$) exhibit a large variety of crystal structures, some of which are of the perovskite type. These materials, better known as "halide perovskites'' have recently gained worldwide attention as promising photovoltaic and more broadly opto-electronic materials. But their stability problems relative to the non-perovskite phases is a major issue. Here we show that the phase relations in these materials can be understood in terms of the relative orientation of the GeX$_3$ tetrahedral units, which is ferroelectric in the perovskite phase but antiferroelectric in the competing phases. This suggests that an applied electric field could be used to stabilize the desired phases and trigger a phase transition between two phases of the material with widely different optical and electronic properties.

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