Ab inito determination of Bi4.86La1.14O9 monoclinic structure from powder neutron diffraction data. Characterization of the related solid solution

This paper deals with the ab initio determination of epsilon -Bi4.86La1.14O 9 monoclinic structure from powder neutron data using the Rietveld method, and with physical properties characterization of a related solid solution. epsilon -Bi4.86La1.14O9, obtained from the annealing of a quenched rhombohe dral hightemperature beta (1) sample, is monoclinic, P2/c, alpha = 9.4956(3 ) Angstrom, b = 3.9742(1) Angstrom, c = 7.0425(2) Angstrom, beta = 104.700( 2)degrees; Z = 1. The structure refinement converged to R-F = 0.043 and R-B ragg = 0.047. The structure is built from cationic slabs parallel to (100) faces of the monoclinic cell. Each cell corresponds to one slab containing a mixed Bi0.86La1.14 cationic layer (Bi-(1)) sandwiched between two equival ent bismuth layers (Bi-(2)). The cohesion of the cations in the slabs resul ts from the presence of the oxygen atoms distributed over three sites. Six O-(1) and two O-(2) atoms form a slightly distorted cubical polyhedron arou nd the mixed cationic site (Bi-(1)). Bi-(2) atoms are surrounded by seven o xygen atoms in a very distorted polyhedron. The important delocalization of Bi-(2) tone pairs toward the interslab spaces leads to significant bonds w ith the adjacent slabs and to the cohesion of the structure. epsilon -Bi4.8 6La1.14O9 is the low symmetry variety of a particular sample of a wide soli d solution domain that, formulated Bi1-xLaxO1.5, has been investigated for 0.15 less than or equal to x less than or equal to 0.325. The formation of this phase from the irreversible transformation of quenched -beta (1), on h eating and the subsequent transitions epsilon --> beta (2) --> beta (1). ha ve been evidenced by thermodiffractometry, conductivity measurements versus temperature, dilatometry, and thermal analyses. Thermal expansion coeffici ents (x 10(5)) are for a, c, and/or lattice volume, for a quenched-beta (1) , epsilon, beta (2), or beta (1) variety of Bi4.86La1.14O9 (x = 0.19) respe ctively: 1.74, - 0.72, 2.78; 5.25; 1.74, 1.81, 5.37; 0.99, 1.08 3.90. The c orresponding activation energies are 0.94, 0.83, 0.84: and 0.21 eV. The eps ilon --> beta transition, irreversible for 0.15 less than or equal to 0.275 , appears partly reversible for 0.3 less than or equal to x less than or eq ual to 0.325. (C) 2001 Academic Press.