= Smectic phase
A smectic phase is a layered <liquid crystal> with periodic density or composition variation along one direction and fluid-like positional organization within a layer. A simple order-parameter profile is $\phi(\mathbf r)=A\cos(\mathbf q_0\cdot\mathbf r)$, with layer spacing $2\pi/|\mathbf q_0|$.
Layer displacement and orientational fluctuations are additional degrees of freedom, so a fixed cosine is only the simplest mean profile. A <smectic A phase> has average molecular alignment along the layer normal; a <smectic C phase> has a tilted director. Scattering can reveal the preferred layer spacing and orientation through peaks near $\mathbf q_0$. The layer structure does not imply a full three-dimensional crystal lattice of fixed molecular positions.
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