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The electrical conductivity of Ln 2 + x Zr 2x O 7x /2 (Ln = Sm-Gd) solid solutions prepared from mechanically activated Ln 2 O 3 and ZrO 2 is shown to correlate with their structural properties. In the three systems, the x -T regions are determined in which electrical transport is dominated by oxygen-ion conduction. In the Sm 2 O 3 -ZrO 2 system, ionic conductivities from 5 × 10 -4 to 6 × 10 -3 S/cm at 740 ° C are found in Sm 2 + x Zr 2 − x O 7x /2 with 26.6, 33.3, 35.5, 37, and 40 mol % Sm 2 O 3 prepared at 1450, 1530, and 1600 ° C. Eu 2 + x Zr 2x O 7x /2 and Gd 2 + x Zr 2x O 7x/2 containing 33.3 to 37 mol % Ln 2 O 3 have 740°C ionic conductivities of 10 -3 to ~7.5 × 10 -3 and 10 -3 to 7 × 10 -3 S/cm, respectively. The activation energy of conduction in Ln 2 + x Zr 2 − x O 7 − x/2 (Ln = Sm-Gd), E a = 0.84-1.04 eV, increases with the atomic number of Ln and x. The highest ionic conductivity is offered by the stoichiometric Ln 2 Zr 2 O 7 (Ln = Sm-Gd) pyrochlores prepared at 1600°C, owing to the optimal concentration of Ln Zr + Zr Ln antistructure pairs (~5-22%). The grains in the ceramic samples studied range in size from 0.5 to 2 µm.

The high-temperature (>1600 ° C) order-disorder phase transition of Tm 2 Ti 2 O 7 is shown to be irreversible. The 740 ° C ionic conductivity of nanocrystalline Tm 2 Ti 2 O 7 ceramics synthesized at 1670 ° C is 2 × 10 − 3 S/cm and remains unchanged upon heat treatment in air at 860 ° C for 240 h. The conductivity of the hightemperature (disordered pyrochlore) phase of Tm 2 Ti 2 O 7 is independent of grain size in the range 20-30 nm.

Saturated vapor pressure p° and enthalpy of sublimation (ΔsH°) of cerium tetrafluoride CeF4 were determined by means of Knudsen effusion mass spectrometry in the range of 750-920 K. It was discovered that sublimation of cerium tetrafluoride from a platinum effusion cell competes with thermal decomposition to CeF3 in the solid phase, but no accompanying release of fluorine to the gas phase occurs. Thus, fluorine atoms migrate within the surface layer of CeF4(s) to the regions of their irreversible drain. We used scanning electron microscopy to study the distribution of the residual CeF3(s) across the inner surface of the effusion cell after complete evaporation of CeF4(s). It was observed that CeF3 accumulates near the edge of the effusion orifice and near the junction of the lid and the body of the cell, that is, in those regions where the fluorine atoms can migrate to a free platinum surface and thus be depleted from the system. Distribution of CeF3(s) solid particles indicates the ways of fluorine atoms migration providing CeF3(s) formation inside the CeF4(s) surface layer.

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