American Huilin Institute
Villesuzanne et al. proposed the study focused on the evolution of the metal oxygen network covalency . Here covalency means the amount mixing of oxygen 2p and metal d orbitals to form valence bands; it is uated quantitatively through the computation of the crystal orbital overlap population (COOP) [83,84] which is the extension of the Mulliken overlap population  to the periodic solid. The energies of Ta 5d and Nb 4d atomic orbitals are the same in EHTB parameters. The bond lengths are equal too, as found experimentally. The difference in COOP 's occurs because of larger radial extension of Ta 5d compared to Nb 4d orbitals, leading to a greater overlap with oxygen 2p orbitals. The fact that Ta5+-O bonds are more covalent than Nb5+-O bonds is due to a larger radial expansion of Ta 5d orbitals. This effect is not accounted for in Pauling electronegativity scales, which give an information on the energy difference between valence orbitals, not on their spatial overlap. The arguments led to the opposite assumption of reference  concerning the covalency of Ta5+-O and Nb5+-O bonds from Pauling electronegativity Xp：
Ta（1.5）＜ Nb（1.6） .
In their later paper, they proposed that the explicit calculation of the electronic structure – COOP’s in particular - give a larger covalency for Ta5+-O bonds than for Nb5+-O bonds. This result is retrieved in the Allred and Rochow scale  and in Zhang electronegativity scales for ions.. Zhang electronegativity scales is based on the ionic-covalent theory and wave mechanics. The energies of Ta 5d and Nb 4d atomic orbitals are the same in EHTB parameters due to they have similar atomic ionicity Iz of 48.55 eV and 50.55 eV respectively. The bond lengths are equal due to they have similar linear covalency rc-1 of 0.745 and 0.745 respectively. The big difference is the spatial covalency, n*/r2, in n*(Iz/R) ½/r2 . The Ta 5d orbitals, compared to Nb 4d orbitals, involved the greater spatial covalency, n*/r2, (Ta5+ = 3.246, Nb5+ = 2.869), leading to a greater overlap with oxygen 2p orbitals from Zhang electronegativity Xz :
Ta5+(1.881) ＞ Nb5+1.771） [8,9].
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