“…Subsequently, De Hair et al reported the compositions A2MIO6 (A = Ba, M = Ag, K, Na, Li; and A = Sr, M = Na), using similar methods, and reported the Raman and infra-red spectra 21 . Kubel et al reported the first crystal structure determinations for the periodate double perovskites, A2NaIO6 (A = Ba, Sr, Ca, Pb), in 2013, synthesised by: solid state reaction between AF2 and excess of NaI (A= Ba, Sr, Ca); reaction of A(OH)2•8H2O and NaIO4 (A = Ba, Sr), at 650 °C in air; or precipitation from a solution of nitrates with NaOH and NaIO4 (A = A = Ba, Sr, Ca, Pb) 22 . From Rietveld analysis of powder X-ray diffraction data, Ba2NaIO6 was reported to adopt the undistorted Fm-3m aristotype structure, whereas, Sr2NaIO6, Ca2NaIO6 and Pb2NaIO6 were reported to adopt the P21/c hettotype structure, with cooperative tilting of the rock-salt ordered B-site octahedra (Glazer notation, a + b − b − ) [22][23][24] .…”