Insights into the structural variations in SmNb_1-TaO₄ and HoNb_1-TaO₄ combined experimental and computational studies

The impact of Ta doping on two orthoniobates SmNbO4 and HoNbO4 has 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 SmO8 polyhedra were invariant of the Ta content across the SmNb1-xTaxO4 solid solution whereas the average M-O (M = Nb or Ta) distance and MO6 polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb1-xTaxO4 do not form a complete solid solution when the samples were prepared at 1400 °C, rather there is a miscibility gap around x = 0.95, with HoTaO4 exhibiting the M'-type P2/c structure rather than the M-type I2/a structure of HoNbO4. Increasing the synthesis temperature to 1450 °C eliminates the miscibility gap. The energy difference between the P2/c and I2/a structures of HoTaO4 is found to be nearly 30 meV per f.u. with the total energy of the P2/c phase of HoTaO4 being 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-1 due to the symmetric stretching of the Ta-O bonds is consistent with the description of that both polymorphs of HoTaO4 contain TaO6 octahedra.