Cation ordered structure and F-ionic conductivity of Er0.7Ca0.3F2.7 tysonite-type crystals

• Results of electron microscope studies of Er1 − yCayF3 − y samples are reported.
• Structure model of the Er0.7Ca0.3F2.7 cation ordered tysonite-type phase is proposed.
• Link between cation ordering and suppression of ionic conductivity is suggested.

Cation ordering in the Er0.7Ca0.3F2.7 monoclinic phase with the tysonite-type (LaF3) structure, which was formed in Er1 − yCayF3 − y samples of the 65–72.5 mol% ErF3 composition range at solid state synthesis (905 °C, 230 h), has been studied in a transmission electron microscope. It is found that the three space groups C2, Cm and C2/m obey the reflection conditions, which were recognized through observation of supercell spots in electron diffraction patterns obtained by studying seven samples. Separate occupation of 20 available cation positions in the Er14Ca6F54 supercell by erbium and calcium ions can be only realized in the C2 space group. A structure model of the supercell has been proposed by fitting simulated electron diffraction patterns to experimental ones. A link between cation ordering and suppression of F-ionic conductivity of the Er0.7Ca0.3F2.7 phase, which belongs to the least conducting congruent melting phase R1 − yCayF3 − y with end elements of the rare-earth element family, has been suggested.