Ab Initio Determination of Bi4.86La1.14O9 Monoclinic Structure from Powder Neutron Diffraction Data. Characterization of the Related Solid Solution

Abstract

This paper deals with the ab initio determination of ϵ-Bi4.86La1.14O9 monoclinic structure from powder neutron data using the Rietveld method, and with physical properties characterization of a related solid solution. ϵ-Bi4.86La1.14O9, obtained from the annealing of a quenched rhombohedral high- temperature β1 sample, is monoclinic, P2/c, a=9.4956(3) Å, b=3.9742(1) Å, c=7.0425(2) Å, β=104.700(2)°; Z=1. The structure refinement converged to RF=0.043 and RBragg=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 equivalent bismuth layers (Bi(2)). The cohesion of the cations in the slabs results 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 around the mixed cationic site (Bi(1)). Bi(2) atoms are surrounded by seven oxygen atoms in a very distorted polyhedron. The important delocalization of Bi(2) lone pairs toward the interslab spaces leads to significant bonds with the adjacent slabs and to the cohesion of the structure. ϵ-Bi4.86La1.14O9 is the low symmetry variety of a particular sample of a wide solid solution domain that, formulated Bi1−xLaxO1.5, has been investigated for 0.15≤x≤0.325. The formation of this phase from the irreversible transformation of quenched-β1 on heating and the subsequent transitions ϵ→β2→β1 have been evidenced by thermodiffractometry, conductivity measurements versus temperature, dilatometry, and thermal analyses. Thermal expansion coefficients (×105) are for a, c, and/or lattice volume for a quenched-β1, ϵ, β2 or β1 variety of Bi4.86La1.14O9 (x=0.19) respectively: 1.74, −0.72, 2.78; 5.25; 1.74, 1.81, 5.37; 0.99, 1.08, 3.90. The corresponding activation energies are 0.94, 0.83, 0.84, and 0.21 eV. The ϵ→β2 transition, irreversible for 0.15≤x≤0.275, appears partly reversible for 0.3≤x≤0.325.