EFFECT OF DEUTERON SUBSTITUTION ON STRUCTURAL PHASE-TRANSITIONS IN NATURAL AND DEUTERATED (NH4)(2)SECL6

(NH4)(2)SeCl6 undergoes a structural phase transition at T-cl = 24 K. By substitution of hydrogen by deuterium an additional phase transitio n can be induced at T-c2 = 37 K, whence the transition temperature of the first transition changes to T-cl = 48 K. Furthermore the transitio n temperatures have been analysed in partially deuterated (NH4(1-x)D-4 x)(2)SeCl6-crystals with deuteron concentrations x = 0.5, 0.7. Both co mpounds exhibit a similar behaviour like (ND4)(2)SeCl6, only the trans ition temperatures are shifted. These strong isotope effects have been investigated by Raman scattering, X-ray powder diffraction and Cl-35- nuclear quadrupole resonance.