NONHYDROGEN ISOTOPE EFFECTS ON STRUCTURAL PHASE-TRANSITIONS IN DIELECTRIC CRYSTALS

The effect of isotope substitutions on the phase-transition temperatures in LiTaO3 (Li), PbZrO3 (Zr), KH2PO4 (K and O), NH4H2PO4 (N), K2SeO4 (O), Rb2ZnCl4 (Rb and Zn), NaNO2 (N), and KNO3 (N) are given. Parentheses denote atoms for which isotopes have been substituted. Except for NaNO2, all of the materials studied show higher phase-transition temperatures when lighter isotopes are substituted, and vice versa for heavier isotopes. This is opposite to the effect of deuteron substitution in potassium dihydrogen phosphate (KDP)-type crystals. The results suggest that the role of the ionic masses for atoms heavier than hydrogen differs completely from the role of hydrogen. NaNO2 scarcely showed any isotope effect despite heavy N15 substitution; this suggests that NaNO2 undergoes a pure Weiss-type order-disorder phase transition due to rigid permanent dipoles on NO2-, whereas the other materials cannot be described by the simplest Weiss model. © 1990 The American Physical Society.