The presence of strong excitonic interactions in this material family is unsurprising, given the low structural and electronic dimensionality (Figs. and), weak band dispersion and large carrier effective masses (Table ) discussed above. We find the exciton binding strength to be governed by the conduction band character in these compounds, giving the expectation for similar strongly-bound Frenkel excitons in A 2 BX 6 compounds with isoelectronic (d 0 ) B 4+ cations, such as Zr and Hf. Indeed, strong excitonic interactions have been recently been reported in Cs 2 ZrX 6 -promising white-light emitters, and a distinct excitonic feature is seen at the absorption onset in Cs 2 HfCl 6 67,68 -which has emission and radiation detection applications. Moreover, the bound excitonic behavior in this material class is very similar to that witnessed in the double perovskites, which despite a greater structural connectivity, exhibit a low effective electronic dimensionality due to orbital mismatch between the B-site cations. Likewise, extension of theoretical models to include explicit electron-hole interactions was required in order to reproduce the experimental spectrum, explaining the excitonic origin of the direct absorption onset.