Solution
= Solution
The even indices give an allowed reflection. Its spacing is $d_{200}=0.280\,\mathrm{nm}$, and
$$
\theta_B=\sin^{-1}\frac{0.154}{2(0.280)}=15.96^\circ,\qquad \boxed{2\theta_B=31.92^\circ.}
$$
The $(200)$ planes are parallel to the cut $(100)$ face, so the ordinary coupled reflection scan reaches them directly. \b[This is an easy, strong peak], with the two ionic amplitudes adding rather than subtracting.