The cubic unit cell structure of a compound containing cation M and...

${M}_{2 \times \dfrac{1}{2}}{X}_{4 \times \dfrac{1}{4}} = MX$

${d}^{2} = {\left(\dfrac{a}{\sqrt{2}}\right)}^{2} + {\left(\dfrac{a}{2}\right)}^{2}$

${d}^{2} = \dfrac{{a}^{2}}{2} + \dfrac{{a}^{2}}{4} = \dfrac{3{a}^{2}}{4}$

$\dfrac{{d}^{2}}{{a}^{2}} = \dfrac{3}{4}$

$\dfrac{d}{a} = \dfrac{\sqrt{3}}{2} = \dfrac{1 . 732}{2} = 0 . 866$

$\dfrac{{r}_{+}}{{r}_{-}} = 0 . 732$