were densified enough to generate acid-acid interactions. DSC results also showed the same tendency because ZrHEDPs with a higher P/Zr exhibited a lower freezing temperature (Fig. ). Generally, the freezing point is reduced when the water is firmly bound to other groups. Concentrated acids can bind water molecules intensely because multiple acids can form H-bonds to a single water molecule. Therefore, the decrease of freezing point in ZrHEDP should be originated from the densified -PO3H2.