Low-temperature phase transitions and twin-domain structure of (NH ₄)₃H(SeO₄)₂ studied by the electron paramagnetic resonance of Cu²⁺, VO²⁺and SeO ₃^-

EPR of Cu²⁺, VO²⁺, SeO₃^-, and other paramagnetic impurities and species were used to study the low-temperature phase transitions and twin-domain structure of (NH₄) ₃H(SeO₄)₂ crystals. The major findings are; (i) twinning in the monoclinic phases is of the merohedral type (there are three kinds of small domain related by rotations of +or-120 degrees abut the trigonal c axis), (ii) EPR of Cu²⁺ or SeO₃^- reveals the coexistence of the high-temperature and the low- temperature phases below the trigonal R3m to monoclinic A2/a phase transition and on further transitions, however, into the two other low-temperature monoclinic phases a single phase was again observed and (iii) impurities and lattice point defects created by ionizing radiations were observed to influence the twinning and crystal structure of the host compound.