STRUCTURE OF RB3D(SEO4)2 AT 25, 110 AND 297 K

Trirubidium deuteriumbiselenate, Rb3D(SeO4)2, M(r) = 544.3, A2/a, Z = 4, lambda(Mo K-alpha) = 0.71073 angstrom, mu = 22.4 mm-1, F(000) = 976. At T = 297 K, a = 10.4775 (6), b = 6.086 (1), c = 15.421 (1) angstrom, beta = 102.919 (9)-degrees, V = 958.1 (3) angstrom 3, D(x) = 3.773 Mg m-3; at T = 110 K, a = 10.4019 (7), b = 6.049 (1), c = 15.321 (1) angstrom, beta = 102.827 (8)-degrees, V = 939.9 (2) angstrom 3, D(x) = 3.846 Mg m-3; at T = 25 K, a = 10.3827 (7), b = 6.039 (1), c = 15.294 (1) angstrom, beta = 102.816 (8)-degrees, V = 935.0 (2) angstrom 3, D(x) = 3.866 Mg m-3. R(F2) = 0.0808 for 2514 unique reflections at 297 K, R(F2) = 0.0550 for 2487 unique reflections at 110 K, and R(F2) = 0.0488 for 2444 unique reflections at 25 K. The isotope effect in the bond distances and angles is not significant. The temperature effects on the structure are mainly noticeable in the Rb-O distances in the oxygen coordination polyhedra of Rb (0.015 angstrom/100 K on average). No space-group change was detected at 25 K; the possible nature of the low-temperature phase below the 92 K transition is discussed.