Crystal structure of lanthanum bismuth silicate Bi2−xLaxSiO5 (x∼0.1)

A melting and glass recrystallization route was carried out to stabilize a new tetragonal form of Bi₂SiO₅ with bismuth partially substituted by lanthanum. The crystal structure of Bi_2−xLa_xSiO₅ (x∼0.1) was determined from powder X-ray and neutron diffraction data (space group I4/mmm, , c=15.227(1) Å, V=224.18 Å³, Z=2; reliability factors: R_Bragg=5.65%, R_p=14.6%, R_wp=16.8%, R_exp=8.3%, χ²=8.3 (X-ray) and R_Bragg=2.40%, R_p=8.1%, R_wp=7.5%, R_exp=4.2%, χ²=3.3 (neutrons); 11 structural parameters refined).The main effect of lanthanum substitution is to introduce, by removing randomly some bismuth 6s² lone pairs, a structural disorder in the surroundings of (Bi₂O₂)²⁺ layers, that is in the (SiO₃)²⁻ pyroxene files arrangement. It results in a symmetry increase relatively to the parent compound Bi₂SiO₅, which is orthorhombic. The two structures are compared.