Covalency result is retrieved by IC-model
American Huilin Institute http://www.amhuilin.com/
Villesuzanne et al. proposed the study: “New considerations on the role of covalency in ferroelectric niobates and tantalites” . “The difference in COOP 's occurs because of more covalent due to larger radial extension of Ta 5d compared to Nb 4d orbitals, leading to a greater overlap with oxygen 2p orbitals. This effect is not accounted for in Pauling electronegativity scales , which give the 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（Xp (Ta) = 1.5，Xp (Nb)= 1.6） . In their later paper, the result is retrieved in the Allred and Rochow scale  and in Zhang electronegativity scales for ions..” The result can be accounted in IC mode l: the energies of Ta 5d and Nb 4d atomic orbitals are the same in EHTB parameters due to they have similar atomic ionicity Iav of 24.89 and 27.02 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*rc-1, in I(Iav )C(n*rc-1)=n*(Iav/R)½rc-1. The Ta 5d orbitals, compared to Nb 4d orbitals, involved the greater spatial covalency, n*rc-1, (Ta5+ = 3.246, Nb5+ = 2.869), leading to a greater overlap with oxygen 2p orbitals and a greater IC (Ta5+ = 4.393, Nb5+ = 4.043) and XIC (Ta5+ = 2.197, Nb5+ = 2.053).
 A. Villesuzanne, C. Elissalde, M. Pouchard, and J.Ravez, Eur . Phy. J. B., 1998, 6, 307.
 J.Ravez, M. Pouchard, P. Hagenmuller, Eur. J. Solid State Inorg. Chem., 1991, 25, 1107
 L. Pauling, J. Am. Chem. Soc. 1932, 54, 3570.
 A. L. Allred and E. G. Rochow, J. Inorg. Nucl.Chem.1958, 5, 264.
 Y. Zhang,, Inorg Chem., 1982, 21, 3886;
Y. Zhang,, “Zhang Electronegativity” in "Introduction to Modern Inorganic Chemistry" ( Eds: K. M. Mackay, R. A. Mackay, W.Henderson, ) 6th ed., Nelson Thornes, United Kingdom,2002, pp 3-54)
 Zhang, Y. Ionocovalency and Applications 1. Ionocovalency Model and Orbital Hybrid Scales. Int. J. Mol. Sci. 2010, 11, 4381-4406