“…The perovskite structure, [1] with its almost infinitely adaptable array of derivatives, must count as one of the most important in materials science with the essential ABX 3 (A = a large cation; B = a smaller cation; X = an anion) structural archetype contributing to ferroelectric, [2] piezoelectric, [3] superconducting, [4] photochemical, [5] and many other technologically important properties. Interest in perovskites has recently been further accelerated by rapid developments in the fabrication of hybrid [3,[6][7][8] or all-inorganic halide perovskite ABX [9,10] structures where A is an organic or alkali metal counterion, B typically lead or tin, and X a halogen, allowing materials with optical and photovoltaic characteristics [11,12] exploitable in solar cells, [13,14] ion-conducting materials, [15] supercapacitors, [16] and other energy storage devices [17] to be developed. Bulk halide perovskites are however reactive, suffering from surface hydration, [18] phase transformations, [19,20] and high defect densities, [21] reducing their performance and longevity.…”