Novel compounds Sn20Zn4P22−vI8 (v=1.2), Sn17Zn7P22I8, and Sn17Zn7P22Br8: Synthesis, properties, and special features of their clathrate-like crystal structures

Tin-zinc pnictidehalides Sn20Zn4P22−vI8 (v=1.2), Sn17Zn7P22I8, and Sn17Zn7P22Br8 have been prepared as single-phase samples by a standard ampoule procedure. The compounds are isotypic and possess the structure of type-I gas hydrate (clathrate-I type). They all crystallize in the cubic space group Pm3¯n (no. 223) with the unit cell parameters a=10.883(2)Å (Sn20Zn4P22−vI8), a=10.8458(3)Å (Sn17Zn7P22I8), a=10.7449(2)Å (Sn17Zn7P22Br8), and Z=1. The crystal structures of all compounds have been solved from single-crystal X-ray diffraction data sets and refined to R=0.032 (Sn20Zn4P22−vI8), R=0.027 (Sn17Zn7P22I8), and R=0.020 (Sn17Zn7P22Br8). In all structures tin, zinc, and phosphorus atoms form the clathrate-I framework while the guest halogen atoms are encapsulated in the cavities of the framework. The compositions of the synthesized compounds are in agreement with those predicted by using the Zintl-Klemm concept. Sn20Zn4P22−vI8 contains two partially filled metal sites and vacancies in one of the phosphorus sites and can be formulated as Sn20Zn4P20.8□1.2I8 (or Sn20Zn4P22−1.2I8). Sn17Zn7P22I8 and Sn17Zn7P22Br8 are vacancy-free and reveal three partially filled positions of metal atoms. Thus, Sn17Zn7P22X8 and Sn20Zn4P20.8□1.2I8, being the limiting and the intermediate compositions, respectively, of the solid solutions of a general formula Sn24−nZnnP22−vX8 (v=(14−2n)/5, X=Br, I), show different mechanisms of the structural stabilization. The 119Sn Mössbauer together with the 31P and 119Sn NMR spectra confirm the local environment of tin and phosphorus atoms in the structures of Sn17Zn7P22I8 and Sn17Zn7P22Br8. The compounds Sn17Zn7P22I8 and Sn17Zn7P22Br8 display diamagnetic and semiconducting behavior. The heat capacity has been measured for Sn17Zn7P22I8 in a temperature range of 6 K to 400 K. No low-temperature phase transitions have been detected in the measurements.