Isotope effects in the crystal symmetry and nature of the phase transitions in NaH3(SeO3)2 and NaD3(SeO3)2

Abstract NMR data show that the triclinic ferroelectric phase which exists in NaH 3 (SeO 3 ) 2 between -79°C and -173°C does not appear in NaD 3 (SeO 3 ) 2 . The transition from 2/m →1 in NaH 3 (SeO 3 ) 2 is connected with a doubling of the unit cell dimensions both along the a and b axes, whereas in the low temperatures phases of symmetry m in NaH 3 (SeO 3 ) 2 and NaD 3 (SeO 3 ) 2 the unit cell is doubled in the b direction only. The transitions are of the order-disorder type.