As shown in Fig. (a), the peaks in the range 2θ = 5-12° (≈ 7-18 Å) in the XRD patterns were unclear, generally considered the distances for ZrP layers. The broad peaks around 2θ = 18-35° (≈ 3-5 Å) were identified and should be derived from the Zr-Zr distance (= 4.5-5.5 Å, indicated in Fig. (e)) because the atomic scattering factor of Zr is much higher than other elements in ZrHEDP (H, C, O, and P). Therefore, ZrHEDPs do not have the layer-by-layer structure but have the 2D structure, even though the plane is not so continuous because of the quite broadness of the peak. The near Zr-Zr proximity on the 2D plane is sufficient to make the distance among -PO3H2s close along the plane.