= Reciprocal lattice change in a structural phase transition
The <reciprocal lattice> of a <body-centered cubic lattice> is face-centred cubic, and that of a <face-centered cubic lattice> is body-centred cubic. In both cases its conventional cubic side is $4\pi/a$ if the direct conventional side is $a$. Thus a direct BCC-to-FCC <phase transition> reverses the reciprocal lattice types. The reciprocal primitive cell volume is $(2\pi)^3/\Omega$, so <thermal expansion> contracts reciprocal spacings, while a discontinuous change in direct <primitive unit cell> volume gives an inverse change in reciprocal cell volume.
A first-order structural transition need not continuously deform one perfect reciprocal array into another. Phase coexistence, new orientation variants, domains and loss of single-crystal alignment can produce two sets of diffraction peaks or many differently oriented sets. The two endpoint lattice types alone do not specify the transformation path or orientation relationship.
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