The layer charges of the three layers of YBa2Cu3Ox, namely the CuO chain layers, the BaO layers, and the cuprate-planes (combined with the rare-earth charge) have been extracted from the data (using the bond-valence-sum method [10]), and have the following features: (1) they are all virtually linear in x and almost the same as the layer charges for PrBa2Cu3Ox; (2) the CuO chain layers have decreasing charge as oxygen content x increases; (3) the BaO layers and the combined CuO2/Rare-earth/CuO2 layers have increasing charge with oxygen content x; and (4) only the BaO layers have charges that appear to change sign as the oxygen content x increases beyond x=6.4, and the superconductivity sets in [11]. These facts cause us to assign the superconductivity of YBa2Cu3O7 to the BaO layers. The same facts also caused us to believe that PrBa2Cu3O7 would superconduct [12], which was later show [13,14].
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