Structure Determination and Analysis of the Ceramic Material La_0.987Ti_1.627Nb_3.307O₁₃ by Synchrotron and Neutron Powder Diffraction and DFT Calculations

In this paper, the ternary system La₂O₃-TiO₂-Nb₂O₅ is studied to find new ternary phases with useful electrical properties. The solid solution La_3−xTi_5−3xNb_10−2xO_39.5−12.5x was recently identified, and this study focuses on the structural characterization of this solid solution with x = 0.04. The crystal structure, representing a new structural type, was determined from synchrotron and neutron powder diffraction data. The unit cell parameters are a = 7.332 Å, b = 7.421 Å, c = 10.673 Å, α = 84.15°, ß = 80.16°, γ = 60.37°, and space group $P\overline{1}$. The titanium and niobium atoms are disordered in five different crystallographic sites coordinated octahedrally by oxygen atoms. The eight-coordinated La atoms are embedded in the octahedral framework. Ti and Nb preferentially occupy different sites, and this feature was studied using periodic density functional theory methods. Energies of possible Ti/Nb distributions were calculated and the results agree well with the site occupancies obtained by combined Rietveld refinement of synchrotron and neutron powder diffraction patterns. The geometries optimized by DFT also agree well with the structural parameters determined by diffraction. The general agreement between the theoretical calculations and the experimental data justifies the quantum chemical methods as reliable complementary tools for the structural investigation of ceramic materials.