ORIENTATIONAL DISORDER OF SULFATE IONS IN THE CRYSTAL STRUCTURE OF NH <sub>4</sub>Al <sub>0.43</sub>Fe <sub>0.57</sub>(SO <sub>4</sub>) <sub>2</sub>×12H <sub>2</sub>O
The crystal structure of the solid solution of alum NH 4Al 0.43Fe 0.57(SO 4) 2×12H 2O (I) is studied in the space group Ра 3(-) and its maximal subgroups R 3(-), P 1(-), and P 1. In centrosymmetric space groups sulfate ions occupy one of two opposite orientations, and in the triclinic group P 1 four of eight tetrahedra have the same orientation. The second harmonic generation is observed when a train of chirped optical pulses of ytterbium diode pumped solid state laser passes through the crystal of I.