Structures of the paraelectric and ferroelectric phases of NaD₃(SeO₃)₂by neutron diffraction

Abstract

Abstract The crystal and molecular structure of the paraelectric and the ferroelectric phase of NaD3(SeO3)3 has been determined and refined using neutron diffraction data. Above Tc = 270°K the paraelectric phase is monoclinic, belonging to space group P21/n with Z=2, a=10.365(2), b=4.850(1), c=5.792(1) and β=91.16(3) at 298°K. The paraelectric phase is characterized by deuterium atoms in positions of two-fold disorder. The ferroelectric phase below Tc belongs to space group Pn with Z=4, a=10.314(3), b=9.663(2), c=5.768(2) and β=91.23(3) at 173°K. To collect proper neutron diffraction data on the ferroelectric phase, it was necessary to prevent small domain formation by poling the crystal in an external field of 15 Kv/cm during cooling-warming cycles through Tc. The deuterium atoms are ordered in the ferroelectric phase and the present determination of the particular way in which they are ordered resolves a disagreement between two previous resonance1,2 studies. Our results are in agreement with the model proposed by Soda and Chiba1. Final agreement factors for the paraelectric structure are R(F2)=0.038 and RW(F2)=0.049; for the ferroelectric structure, R(F2)=0.035 and RW(F2)=0.044.