New alkaline earth aluminium fluorides with the formula (M, M ')AlF5 (M, M' = Ca, Sr, Ba)

Two new modifications, alpha ' -SrAlF5 and delta -BaAlF5, as well as two so lid solutions, Ca0.08Sr0.92AlF5 and Ca0.33Sr0.67AlF5, have been synthesized in the system (M, M ' )F-2/AlF3 with M, M ' = Ca, Sr, Ba. The preparation of single crystals has been performed under hydrothermal conditions and sol id state reactions, respectively. All crystal structures have been refined from single crystal diffractometer data. A common feature of the structures are the corner linked AlF6 octahedra forming chains [AlF4/1F2/2]. alpha ' -SrAlF5 is the second well characterized modification known up until now ha ving this composition. Its crystal structure (P2(1)/n (No. 14), a = 8.9574( 5), b = 7.4614(4), c = 10.7212(6) Angstrom, beta = 94.158(1)degrees, Z = 8, R[F-2 > 2 sigma (F-2)] = 0.020, omegaR(2)(F-2 all) = 0.045) is isotypic wi th alpha ' -BaFeF5. Incorporation of small amounts of calcium into the stru cture leads to the solid solution Ca0.08Sr0.92AlF5 (P21/n (No. 14), a = 8.9 351(7), b = 7.4387(6), c = 10.6845(8) Angstrom, beta = 94.194(2)degrees, Z = 8, R[F-2 > 2 sigma (F-2)] = 0.025, omegaR(2)(F-2 all) = 0.053) with sligh tly smaller lattice parameters. Increase of the calcium amount in the SrAlF 5 host lattice stabilizes the second modification of SrAlF5 and the tetrago nal superlattice structure of Ca0.33Sr0.67AlF5 (I4(1)/a (No. 88), a = 19.73 45(3), c = 14.1982(2) Angstrom, Z = 64, R[F-2 > 2 sigma (F-2)] = 0.039, ome gaR(2)(F-2 all) = 0.099) is obtained. delta -BaAlF5 is the fourth modificat ion of BaAlF5 so far; it crystallizes in the triclinic system, P (1) over b ar (No. 2), a = 5.2146(7), b = 7.3933(10), c = 14.946(2) Angstrom, alpha = 79.365(2)degrees, beta = 88.525(2)degrees, gamma = 88.183(2)degrees, Z = 6, R[F-2 > 2 sigma (F-2)] = 0.033, omegaR(2)(F-2 all) = 0.089. The delta -BaA lF5 crystals show ferroelastic behaviour; no first order phase transition c ould be observed up to 580 degreesC. The bridging fluorine atoms are disord ered around one Al atom. The relations between different BaAlF5 polymorphs are briefly discussed. (C) 2001 Editions scientifiques et medicales Elsevie r SAS. All rights reserved.