where x is the ratio of the structure with P/Zr = 4. Then, the value of "(P/Zr -2)/2 × 100 (%)" represents the minimum ratio of -PO3H2 per whole phosphonate site, while why the ratio is the "minimum" is because there is a possibility of remaining ZrO2. Thus, P/Zr = 3.0 indicates that more than 50% of entire phosphonate sites are -PO3H2 (Fig. )), i.e., one of every two phosphonate sites in the Zr-O-network is -PO3H2, and the other is immobilized -PO3-. Therefore, we expected that the higher P/Zr means a closer distance between -PO3H2.