Double perovskite halides (aka elpasolites), A2MIMIIIX6, are the environmentally benign alternatives of the popular Pb halide perovskite. The number of structurally characterized double perovskite halides decreases linearly and steeply as the halide size increases from F to I, and there is a void at iodide. Iodide double perovskites are highly desirable since their band gaps are expected to be similar to those found in the parent lead iodides, but the synthesis of iodides appears to have been a challenge. One may wonder, why there is such a trend?
One of the design strategies based on the octahedral ratio of the trivalent metal size to the halide size for a hybrid double perovskite with fluoride (radius 1.33 Å), chloride (radius 1.81 Å), bromide (radius 1.96 Å), or iodide (radius 2.2 Å) is that the radii of their trivalent metals (MIII) need to be greater than 0.55, 0.74, 0.80, or 0.90 Å, respectively, to ensure that the octahedral factor exceeds 0.41. Because the radii of most of the trivalent metals tend to be quite small, this condition is much easier to meet with fluorides than with chlorides and bromides, and it is particularly challenging in the cases of iodides. In addition, there are other challenges of synthesizing iodides which hamper their formation.
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