On the High-Temperature Ca ²⁺ Conduction in NASICON-Type Ca _{(1+ <i>x</i> )/2} In _<i>x</i> Zr _{2– <i>x</i>} (PO ₄ ) ₃

Lithium-based batteries are currently the leading battery technology. To develop more diverse and sustainable energy storage solutions, alternative battery chemistries and materials must be explored. In this work, we investigated the aliovalent substitution series of NASICON-type Ca(1+x)/2InxZr2–x(PO4)3 (x = 0, 0.125, 0.25, 0.375, 0.5, 0.75, and 1). To establish the structure–transport relations in the materials, a combination of impedance spectroscopy, direct current (DC) polarization measurements, and temperature-dependent powder X-ray diffraction (XRD) measurements up to 800 °C was used. At 800 °C, where the materials are most conductive, there is an increase of the ionic conductivity from x = 0 to 0.375, reaching a maximum of 2.4·10–4 S·cm–1. For compositions with x ≥ 0.5, however, the conductivity decreases significantly. The maximum conductivity and its subsequent decrease are linked to an interplay of crystal–chemical factors, namely, the bottleneck size and the number of available vacancies. This study shows that it is possible to increase the Ca2+ ion conductivity through aliovalent substitution; however, due to distinct, crystallographic differences, design principles from lithium- or sodium-based NASICONs may not be directly applicable to their calcium analogues.