Concentration dependences of the unit-cell parameters of nonstoichiometric fluorite-type Na0.5 − x R 0.5 + x F2 + 2x phases (R = rare-earth elements)

Unit-cell parameter a of the cubic phases with a varying composition Na 0.5 − x R 0.5 + x F 2 + 2 x ( R = Gd-Lu and Y) and with a fluorite-type structure, is described within an accuracy of ±0.003 Å, by the formula a = 4.454 + 0.874 r 3 + x (6.7238 r 3 − 7.259) where r 3 is the Shannon “crystal ionic radius” R 3+ at c.n. = 8.