Ferroelectric phase transition and crystal structure of the mixed crystals (NH4)2SO4-(NH4)2SeO4

Abstract

Solid solutions of general formula (NH4)2(SeO4)x(SO4)1-x (0.14 ≥ × ≥ 0) have been prepared. The orthorhombic symmetry of the crystals at room temperature was confirmed. The substitution of sulfate by selenate causes an increase in the Curie temperature for the paraelectric-ferroelectric, Pnam (D162h) to Pna21 (C92v), phase transition. The crystal structure of (NH4)2(SeO4)0.10(SO4)0.90 has been resolved: Fw = 143.86, orthorhombic, Pnam, a = 7.797(1) A, b = 10.664(2) A, c = 5.995(1) A, V = 49889.5(2) A3, Dx = 1.916 g cm-3, Z = 4, F(000) = 298.0, μ(Mo Kα) = 0.71069 A, μ(Mo Kα) = 24.20 cm-1 at 20°C, and Pna21, a = 7.890(2) A, b = 10.589(3) A, c = 5.957(2) A, V = 491.7(4) A3, Dx = 1.919 g cm-3, Z = 4, F(000) = 298.0, μ(Mo Kα) = 24.24 cm-1 at -50°C. The final R factor was 0.041 (wR = 0.045) for 20°C and 0.062 (wR = 0.070) for -50°C with all observed reflections. The mechanism that better justifies the phase transition is a strengthening of hydrogen bond which produces a rotation in sulfate or selenate ions, together with a modification in the intramolecular bond lengths and angles in these anions. In view of this we propose a displacive mechanism for the phase transition.