Preparation, properties, and crystal structure of the solid electrolytes Cu 2P 3I 2 and Ag 2P 3I 2

The new compound Cu ₂P ₃I ₂ is a diamagnetic semiconductor with a susceptibility of χ = -193 × 10 ^-6 cm ³/mole and a band gap of E_a = 0.72 eV. Its crystal structure is monoclinic, space group {P2 ₁}/{c}, with the lattice constants a = 15.343(2) Å, b = 12.925(2) Å, c = 15.260(2) Å, β = 116.38(1)°, and Z = 16 formula units per cell. Twinning wrongly suggests orthorhombic and tetragonal symmetry. The crystal structure was determined from single-crystal X-ray data and refined to a residual of R = 0.068 and 4330 independent structure factors and 197 variable parameters. The P atoms form tubes similar to those in the Hittorf modification of phosphorus. They are surrounded by Cu atoms on 15 equipoint positions with occupancies varying between 0.15(1) and 0.97(1). These in turn are surrounded by iodine atoms which are well localized and form one-dimensional infinite channels. The structural characteristics suggest high mobility of the Cu ⁺ ions. This was proven by an ion exchange reaction of the Cu ₂P ₃I ₂ crystals in aqueous AgNO ₃ solution, which forms the new isotypic compound Ag ₂P ₃I ₂. Transference measurements in a cell Ag/Ag ₂P ₃I ₂/Ag show that the electrical conductivity is essentially due to the transport of Ag ⁺ ions. The Ag ⁺ conductivity of Ag ₂P ₃I ₂ at 140°C was determined to σ = 3.5 × 10 ^-3 Ω ^-1 cm ^-1. Ag ₂P ₃I ₂ is diamagnetic with a susceptibility of χ = -227 × 10 ^-6 cm ³/mole.