Cover illustration: The packing of catena-poly[bis(tert-butylammonium) [plumbate(II)-tri--iodo] iodide dihydrate], viewed along the b axis. See Billing & Lemmerer [Acta Cryst. (2006), C62, m264-m266].
The relaxor ferroelectric Pb(Zn1/3Nb2/3)O3, known as PZN, was found to be rhombohedral in R3m at both 4.2 and 295 K from very high resolution neutron powder diffraction data. Atomic coordinates at both temperatures show strong electric polarization.
The title structures belong to the β-K2SO4 family. Cs2PO3F is twinned owing to the proximity of its lattice parameters to those of the orthohexagonal cell. Phase transitions were observed in slightly humid samples at 175 and 177 K for Rb2PO3F and Cs2PO3F, respectively, but were not observed in well dried samples.