Zhang Ionocovalency correlates with structure of oxides

Y. Zhang Iono-covalent theory applications (25)

Yonghe Zhang

American Huilin Institute   http://www.amhuilin.com/

On the basis of Zhang ionic-covalent theory that the chemical bond strength (complex stability) is directly proportional to its ionic force (the effective nuclear charge) and inversely proportional to its covalent force (the covalent radius)[16,17], Mi-Hyae Park and Yu-Ju Shin [58] proposed the work “Influences of Structural Feature on the Ionic Conduction in Layered Oxides Na0.67[Co0.33-yMgyTi0.67]O2 (0£y£0.28)” and they interestingly found: “the lattice parameter increases with Mg-content (Table 1) despite the small size of Mg2+(r=0.72A) over Co2+ (HS: 0.745A). It strongly implies that the overall covalency in MO2 sheet is diminished, because of the small electronegativity of Mg2+ (1.22) with respect to that of Co2+ (1.57) [15, 48]. As a result, the covalency of Na-O should be enhanced in the competitive bond Na-O-M. It is apparent that the evolution of bond character in Na-O-M also affects the value  c /a, which actually decreases with Mg-content indicating the lattice expansion is rather two-dimensional. The structure change from P2 to O3 was observed near y=0.15. the drastic change of  a implies that the bond character in Na-O-M is greatly influenced by structure change (or vice versa): Comparing with O3, P2 induces highly enhanced covalency in M-O and thus gives rise to stronger interaction within MO2 sheet than across the sheets. It means that the bond Na-O is essentially ionic in P2, whereas it takes some covalency in O3.”.

 

Table 1. Lattice constants and b values of Na0.67Co0.33-yMgyTi0.67O2

Na0.67Co0.33-yMgyTi0.67O2

a(Å)

c(Å)*

c/a

b=xfi(Na-O)fi(M-O)

y=0.00(P2)

2.965(3)

16.75(2)

5.649

0.324

0.10(P2)

2.975(1)

16.74(3)

5.627

0.337

0.15(P2)

2.974(6)

16.77(1)

5.639

0.323;

0.15(O3)

2.993(2)

16.45(3)

5.496

0.343

0.20(O3)

2.994(3)

16.44(1)

5.491

0.35

0.28(O3)

2.996(1)

16.42(2)

5.481

0.358

*For P2 phases, each c was multiplied by 1.5 for comparison

 

[16] Y. Zhang, Inorg. Chem. 1982a, 21, 3886).

[17] Y. Zhang, Inorg. Chem. 1982b, 21, 3889).

[48] J.Portier, G. Campet, J. Etoumeau and B.Tanguy, Alloys Comp.,1994, 209, 285.

[58] Park Mi-Hyae and ShinYu-Ju Journal of the Korean Chemical Society, 2004, Vol.48, No.1, pp 94-98.

 

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