Divalent calcium ion conducting solid electrolyte with a three dimensional NASICON-type structure, (CaxHf1−x)4/(4−2x)Nb(PO4)31,was successfully prepared by introducing Ca2+ cations into the HfNb(PO4)3 solid. The existence of three kinds of high valence cation of Hf4+, Nb5+, and P5+ successfully realized the effective reduction of electrostatic interaction toward Ca2+ in the structure. The (Ca0.05Hf0.95)4/3.9Nb(PO4)3 solid possesses considerably higher Ca2+ cation conductivity and also lower activation energy compared with those of previously reported NASICON-type Ca0.5Zr2(PO4)32 solid.
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