Insights into the structural variations in SmNb < inf > 1−x < /inf > Ta < inf > x < /inf > O < inf > 4 < /inf > and HoNb < inf > 1−x < /inf > Ta < inf > x < /inf > O < inf > 4 < /inf > combined experimental and computational studies

Abstract

The impact of Ta doping on two orthoniobates SmNbO4and HoNbO4has been studied using a combination of high-resolution powder diffraction and Density-Functional Theory calculations. In both ANb1−xTaxO4(A = Sm, Ho) series the unit cell volume decreases as the Ta content increased demonstrating that the effective ionic radii of Ta is smaller than that of Nb in this structure. The average Sm-O distance and volume of the SmO8polyhedra were invariant of the Ta content across the SmNb1−xTaxO4solid solution whereas the average M-O (M = Nb or Ta) distance and MO6polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb1−xTaxO4do not form a complete solid solution when the samples were prepared at 1400 °C, rather there is a miscibility gap aroundx= 0.95, with HoTaO4exhibiting the M′-typeP2/cstructure rather than the M-typeI2/astructure of HoNbO4. Increasing the synthesis temperature to 1450 °C eliminates the miscibility gap. The energy difference between theP2/candI2/astructures of HoTaO4is found to be nearly 30 meV per f.u. with the total energy of theP2/cphase of HoTaO4being more negative. First-principles calculations, carried out using Density-Functional Theory, reveal significant covalent character in the Nb-O bonds, which is reduced in the corresponding tantalates. Anisotropy in the Born Effective Charge tensors demonstrates the impact of the long M-O bond identified in the structural studies showing that the Nb and Ta cations are effectively six-coordinate. The similarity in the frequency of the intense Raman peak near 800 cm−1due to the symmetric stretching of the Ta-O bonds is consistent with the description of that both polymorphs of HoTaO4contain TaO6octahedra.