Short-range order, thermal vibration and expansion, and other properties of pseudosymmetric and mixed crystals of small organic molecules - Short-range order in crystals of anthrone and in mixed crystals of anthrone-anthraquinone

Abstract

Stationary crystal photographs of anthrone taken with white + CuK radiation and with [010] normal to the incident beam show diffuse intermediate layer lines corresponding with k=1/2, 2/3, etc. A pseudo-Weissenberg technique has been developed for the study of the h1/2l etc. layers which gives good resolution of these diffuse scattering regions. Measurement of the integral breadth of these scattering maxima show that there are, in anthrone, short-range-order domains of average length 9 molecules (36 A) in the [010] direction and 3 molecules (25 A) in the [001] direction, the arrangement along [100] being random. In mixed crystals, up to 12 mole per cent of anthraquinone can crystallize with anthrone without disturbing this short-range order. At the other extreme of the phase diagram, no short-range order is found for up to 45 mole % anthrone in anthraquinone. The region 12 to 55 mole % of anthraquinone in anthrone seems to define a miscibility gap; no crystals of these compositions could be made.