= Alloy sublattice order parameter
{title2=$\rho$}
For a stoichiometric $A_3B$ <alloy> with one corner sublattice and three equivalent face sublattices, let $p_c$ and $p_f$ be the respective occupation <probabilities> of species $B$. Define $\rho=(4p_c-1)/3$. Conservation of composition, $p_c+3p_f=1$, gives $p_c=(1+3\rho)/4$ and $p_f=(1-\rho)/4$. Thus the random <alloy> has $\rho=0$ and the fully ordered corner-$B$ pattern has $\rho=1$. The coherent <superlattice reflection> amplitude is $\rho(f_B-f_A)$, so its <wave intensity> scales as $\rho^2$, while the equal-parity fundamental amplitude $f_B+3f_A$ is composition-fixed. The definition assumes equivalent face sublattices; endpoint normalization alone does not uniquely specify an arbitrary order coordinate.
Back to article page
