STRUCTURE DETERMINATION OF THE FERROELASTIC TRIPLE-TWINNED PHASE OF K3NA(SEO4)2 AT 291-K AND ITS PARENT PHASE AT 390-K

K3Na(SeO4)2, M(r) = 426.19, lambda(Mo Kalpha) = 0.71073 angstrom, two phases. At 291 K, monoclinic, C2(1)c, a = 10. 162 (2), b = 5.867 (1), c = 15.021 (2) angstrom, beta = 90.00 (1)-degrees, V = 895.56 angstrom 3, mu = 96.29 cm-1, Z = 4, F(000) = 800, D(x) = 3.160 g CM-3, R = 0.02 02 for 1779 observed reflections. Lattice parameters of the pseudohexa gonal cell: a = 5.867 (1), b = 5.867 (1), c = 15.021 (2) angstrom, alp ha = 90.0, beta = 90.00 (1), gamma = 120.0-degrees. At 390 K, trigonal , P3m1BAR, a = 5.906 (3), c = 7.552 (1) angstrom, V = 228.13 angstrom3 , mu = 94.50 cm-1, Z = 1, F(000) = 200, D(x) = 3.101 g cm-3, R = 0.012 6 for 222 observed reflections. The high-temperature phase is isomorph ous with the room-temperature structures of K3Na(CrO4)2 and K3Na(SO4)2 and at 346 K undergoes a reversible ferroelastic phase transition fro m point group 3m1BAR to 2/m. Below this transition point the c axis, o riginally coincident with the trigonal axis, doubles its length. As a consequence, below the transition point the crystals are triple-twinne d with a 120-degrees domain structure, and the mirror plane is replace d by the glide plane c. Both structures consist of [SeO4]2- tetrahedra l anions, Na cations coordinated by six 0 atoms in a deformed octahedr on, and two crystallographically independent K(1) and K(2) cations wit h 10 and 12 0-atom coordination, respectively. The phase transition is accompanied by the shift of K(1) and K(2) cations as well as by the s hift and tilting of [SeO4]2- by 7.6 (1)-degrees. The phase transition mainly affects the environment of K(2). The cation-O-atom distances an d temperature factor as well as the bond-valence sums indicate that th is K atom is the most loosely bound atom in the high- and low-temperat ure phases. The thermal stability of isomorphous K3Na(CrO4)2 and K3Na( SO4)2 is related to the bonding of this cation.