= Solution
The direct <body-centered cubic lattice> has a face-centred cubic <reciprocal lattice>, whereas the direct <face-centered cubic lattice> has a body-centred cubic <reciprocal lattice>. Thus the <reciprocal lattice change in a structural phase transition> reverses the two cubic types: the reciprocal lattice changes from FCC to BCC when the real material changes from BCC to FCC. If the direct conventional side is $a$, the reciprocal conventional side is $4\pi/a$ in either case, with the appropriate phase-specific $a$.
Within one phase, ordinary <thermal expansion> increases direct spacings and decreases reciprocal spacings. At the structural <phase transition>, reciprocal points acquire the new symmetry and allowed positions. A first-order transformation may show both phases simultaneously and may create differently oriented domains; it need not continuously preserve one perfect reciprocal array throughout.
Using the fixed-radius estimates from part (b), $\Omega_F/\Omega_B\simeq0.919$, so the reciprocal primitive cell volume would increase by about $8.9\%$. That comparison belongs to the ideal contact model. The question supplies neither the actual <thermal expansion> nor a crystallographic orientation relationship, so it does not determine a unique trajectory of individual reciprocal points during the transition.
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