| Summary: | In this paper, the review of the new class of ionic conductors was made. For the last several years, the layered perovskites with Ruddlesden-Popper structure A<sup>II</sup>LnInO<sub>4</sub> attracted attention from the point of view of possibility of the realization of ionic transport. The materials based on Ba(Sr)La(Nd)InO<sub>4</sub> and the various doped compositions were investigated as oxygen-ion and proton conductors. It was found that doped and undoped layered perovskites BaNdInO<sub>4</sub>, SrLaInO<sub>4</sub>, and BaLaInO<sub>4</sub> demonstrate mixed hole-ionic nature of conductivity in dry air. Acceptor and donor doping leads to a significant increase (up to ~1.5–2 orders of magnitude) of conductivity. One of the most conductive compositions BaNd<sub>0.9</sub>Ca<sub>0.1</sub>InO<sub>3.95</sub> demonstrates the conductivity value of 5 × 10<sup>−4</sup> S/cm at 500 °C under dry air. The proton conductivity is realized under humid air at low (<500 °C) temperatures. The highest values of proton conductivity are attributed to the compositions BaNd<sub>0.9</sub>Ca<sub>0.1</sub>InO<sub>3.95</sub> and Ba<sub>1.1</sub>La0<sub>.9</sub>InO<sub>3.95</sub> (7.6 × 10<sup>−6</sup> and 3.2 × 10<sup>−6</sup> S/cm correspondingly at the 350 °C under wet air). The proton concentration is not correlated with the concentration of oxygen defects in the structure and it increases with an increase in the unit cell volume. The highest proton conductivity (with 95−98% of proton transport below 400 °C) for the materials based on BaLaInO<sub>4</sub> was demonstrated by the compositions with dopant content no more that 0.1 mol. The layered perovskites A<sup>II</sup>LnInO<sub>4</sub> are novel and prospective class of functional materials which can be used in the different electrochemical devices in the near future.
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