Novel compounds Sn₂₀Zn₄P_22-vI₈ (v = 1.2), Sn₁₇Zn₇P₂₂I₈, and Sn ₁₇Zn₇P₂₂Br₈: Synthesis, properties, and special features of their clathrate-like crystal structures

Tin-zinc pnictidehalides Sn₂₀Zn₄P _22-vI₈ (v=1.2), Sn₁₇Zn₇P ₂₂I₈, and Sn₁₇Zn₇P ₂₂Br₈ 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 Pm3n (no. 223) with the unit cell parameters a=10.883(2) Å (Sn₂₀Zn₄P_22-vI₈), a=10.8458(3) Å (Sn₁₇Zn₇P₂₂I₈), a=10.7449(2) Å (Sn₁₇Zn₇P₂₂Br ₈), 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 (Sn₂₀Zn₄P_22-vI₈), R=0.027 (Sn ₁₇Zn₇P₂₂I₈), and R=0.020 (Sn ₁₇Zn₇P₂₂Br₈). 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. Sn₂₀Zn₄P _22-vI₈ contains two partially filled metal sites and vacancies in one of the phosphorus sites and can be formulated as Sn ₂₀Zn₄P_20.8□_1.2I₈ (or Sn₂₀Zn₄P_22-1.2I₈). Sn ₁₇Zn₇P₂₂I₈ and Sn ₁₇Zn₇P₂₂Br₈ are vacancy-free and reveal three partially filled positions of metal atoms. Thus, Sn ₁₇Zn₇P₂₂X₈ and Sn ₂₀Zn₄P_20.8□_1.2I₈, being the limiting and the intermediate compositions, respectively, of the solid solutions of a general formula Sn_24-nZn_nP _22-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 Sn ₁₇Zn₇P₂₂I₈ and Sn ₁₇Zn₇P₂₂Br₈. The compounds Sn ₁₇Zn₇P₂₂I₈ and Sn ₁₇Zn₇P₂₂Br₈ display diamagnetic and semiconducting behavior. The heat capacity has been measured for Sn ₁₇Zn₇P₂₂I₈ in a temperature range of 6 K to 400 K. No low-temperature phase transitions have been detected in the measurements.