I have a problem to find out bond length of Ba-O.
I have find out that Ba-O value is 0.276nm and other paper show 0.194nm.
The Ba-O bond length will depend on the material composition because it depends on the local structure around Ba. In particular the Ba-O coordination number. The empirical Bond Valence (BV) method provides a means to relate bond length to coordination number.
Making the simplifying assumption that all Ba-O bonds to a given Ba are the same length, then Bond Valence gives the following lengths in Angstrom:
2.696 2.754 2.803 2.847 2.885 2.921 2.953
for coordination numbers of 6 to 12 respectively (i.e. [BaO6], [BaO7] ... etc.)
Hence your number 0.194nm seems too short. But 0.276nm is about what would be expected for 7-coordinated Ba, [BaO7].
Remember that in reality, the bond lengths are rarely all the same, so some can be shorter, and some longer than the simplified BV predictions above.
Already simple addition of ionic radii gives a decent estimate of acceptable values for the bond length: e.g. from my old periodic table r(Ba[6]2+) = 0.135 nm, r(O[4]2-) = 0.138 nm, Ba-O = 0.273 nm. Ergo a value of 0.194 nm is way too short. I have the suspicion of a typing error: The distance would be good for Be-O.
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