The ionic conductivity s of KYF4 and K2
RF5 single crystals (R = Gd, Ho, Er) and KNdF4 and K2
RF5 ceramic samples (R = Dy, Er) has been studied in the temperature range of 340–500°C. A comparative analysis of the σ values for these objects has been performed. Binary fluorides of potassium and rare earth elements were synthesized by the hydrothermal method (temperature 480°C, pressure 100–150 MPa) in the R
2O3–KF–H2O systems. The σ values of tetraf luorides are 3 × 10–5 S/cm (KYF4 single crystal) and 3 × 10–6 S/cm (KNdF4 ceramics) at 435°C. A K2ErF5 single crystal with σ = 1.2 × 10–4 S/cm at 435°C has the maximum value of ionic conductivity among pentafluorides. The anisotropy of ionic transport was found in K2HoF5 single crystals, σ∥c
= 2.5, where σ∥c
are, respectively, the conductivities along the crystallographic c axis and in the perpendicular direction.