Isomorphism and mixed crystals in 2-R-naphthalenes: evidence of structural subfamilies and prediction of metastable forms

Solid–liquid binary phase diagrams and isothermal unit-cell parameters as a function of composition are given for the high-temperature form (P2₁/a, Z = 2) mixed crystals generated by 2-fluoronaphthalene, naphthalene and 2-naphthol with four other β derivatives of naphthalene. The study leads to the distinction between two high-temperature forms (or types of packing). The first one (type 1) is taken by five 2-R-naphthalenes (R = F, Cl, Br, SH, CH₃; the first subfamily), the second one (type 2) by naphthalene itself and 2-naphthol (R = H, OH; the second subfamily). The crystallographic data also allow an estimation of unit-cell parameters for the metastable forms of naphthalene (type 1, a = 8.0, b = 5.95, c = 8.6 Å, β = 116°) and 2-fluoronaphthalene (type 2, a = 8.336, b = 5.915, c = 9.000 Å, β = 122.23°), supporting an interpretation, in terms of crossed isodimorphism, of the observed two-phase regions in systems in which the components do not belong to the same subfamily.