Phase diagrams and crystal-fluid surface tensions in additive and nonadditive two-dimensional hard disk mixtures

Using density functionals from fundamental measure theory, phase diagrams and crystal-fluid surface tensions in additive and nonadditive (Asakura-Oosawa model) two-dimensional hard disk mixtures are determined for the whole range of size ratios $q$ between disks, assuming random disorder in the crystal phase. The fluid-crystal transitions are first-order due to the assumption of a periodic unit cell in the density functional calculations. Qualitatively, the shape of the phase diagrams is similar to the case of three-dimensional hard sphere mixtures. For the nonadditive case, a broadening of the fluid-crystal coexistence region is found for small $q$ whereas for higher $q$ a vapor--fluid transition intervenes. In the additive case, we find a sequence of spindle type, azeotropic and eutectic phase diagrams upon lowering $q$ from 1 to 0.6. The transition from azeotropic to eutectic is different from the three-dimensional case. Surface tensions in general become smaller (up to a factor 2) upon addition of a second species and they are rather small. The minimization of the functionals proceeds without restrictions and optimized graphics card routines are used.