Cs3As7, a compound with cage-like As73- anions, has been prepared from the elements in sealed quartz ampoules at 820 K. The ruby-red compound undergoes a first order phase transition at 640 K and is obtained as microcrystalline samples. The structure of the low-temperature α-modification is not known, but β-Cs3As7 belongs to the plastically crystalline β-Rb3P7 type of structure (F m 3̄ m , a (673 K) = 1169,2(1) pm). Cs3As7 melts under its own vapour pressure at 1061 K, and is easily soluble in liquid ammonia. From those solutions ruby red crystals of Cs3(NH3)As7 precipitate. In dynamical vacuum NH3 is evaporated in the region of 300 to 520 K, resulting in Cs3As7. Cs3(NH3)As7 crystallizes in the space group P21/a (a = 2759.0(12) pm, b = 744.1(3) pm, c = 751.5(3) pm, β = 90.16(5)°; Z = 4). The structure is closely related to the plastically crystalline β-Rb3P7 type structure (Li3Bi variant). The heptaarsanortricyclene anion is coordinated by cesium atoms, connecting the anions to a three dimensional arrangement. The anion exhibits a differentiation of As - As bond lengths typical for ionic nortricyclenes (d(basis) = 251.2 pm; d(basis-bridge) = 235.5 pm; d(bridge-bridgehead) = 240.1 pm). The distances Cs to As are in the range 372 to 401 pm. The cesium atoms are surrounded by three resp. four As atoms and the NH3 molecules. The FIR and Raman spectra are given for different temperatures as well as of an en-solution of Cs3(NH3)As7. The valence vibrations of NH3 are found at 3175 (A1) and 3320 cm-1 (E ). The fundamental vibrations of the three-membered ring of different heteronortricyclenes with P and As are compared and yield a correlation factor χ = 0.57(1) for the observed frequencies between P3 and As3. The band gaps EG are determined from the diffuse reflexion at 2.14 eV for Cs3(NH3)As7 and 2.00 eV for Cs3As7.
THERMODYNAMICAL PHASES OF A REGULAR SAdS BH
