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Abstract
(NH4)2SeCl6 undergoes a structural phase transition at Tc1 = 24 K. By substitution of hydrogen by deuterium an additional phase transition can be induced at Tc2 = 37 K, whence the transition temperature of the first transition changes to Tc1 = 48 K. Furthermore the transition temperatures have been analysed in partially deuterated (NH4(1–X)D4′)2SeCl6-crystals with deuteron concentrations ′ = 0.5, 0.7. Both compounds exhibit a similar behaviour like (ND4)2SeCl6, only the transition temperatures are shifted. These strong isotope effects have been investigated by Raman scattering, X-ray powder diffraction and 35Cl-nuclear quadrupole resonance.