= Neighbour shells of a body-centred cubic lattice
{title2=$r_1=\sqrt3a/2,\quad r_2=a$}
In a <body-centred cubic lattice> of conventional edge $a$, a lattice site has eight nearest neighbours along body half-diagonals at distance $\sqrt3a/2$, then six next-nearest neighbours along the coordinate axes at distance $a$. There are two sites per conventional cell, so identical atoms of mass $m$ and <mass density> $\rho$ obey $\rho a^3=2m$. Specifying a neighbour shell matters: the nearest and next-nearest distances differ.
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