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 Bi2-xLaxSiO5 (x∼0.1) was determined from powder X-ray and neutron diffraction data (space group I4/mmm, a=b=3.8307(3)A, c=15.227(1)A, V=224.18A3, Z=2; reliability factors: RBragg=5.65%, Rp=14.6%, Rwp=16.8%, Rexp=8.3%, χ2=8.3 (X-ray) and RBragg=2.40%, Rp=8.1%, Rwp=7.5%, Rexp=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