Insights into the structural variations in SmNb1−xTaxO4 and HoNb1−xTaxO4 combined experimental and computational studies

Abstract

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 ANb_1−xTa_xO₄ (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 SmNb_1−xTa_xO₄ solid solution whereas the average M–O (M = Nb or Ta) distance and MO₆ polyhedral volume decrease with Ta doping. The analogous Ho oxides HoNb_1−xTa_xO₄ 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.