Анизотропия анионной проводимости в монокристаллах суперионного проводника CeF-=SUB=-3-=/SUB=-

The anisotropy of anionic conductivity in crystals of a superionic conductor CeF3 with the tysonite structure (sp. gr. P-3с1) has been studied for the first time. The conductivity measurements at temperatures from 300 K to 600 K were carried out along the principal a- and c-axes of trigonal unit cell of the crystal. The maximum value of electrical conductivity is observed along the c-axis. The superionic CeF3 crystals have the weak anisotropy of electrical conductivity equal to σ||c/σ||a = 2.4 and σ||c = 5.6 10–4 S/cm at 500 K. The anisotropy effect of anionic conductivity in individual fluorides with the tysonite structure is discussed in connection with the peculiarities of their atomic structure.

Fluorides containing lanthanides and yttrium at extreme conditions

We are interested in crystal structures and stabilities of fluoride materials containing lanthanides and yttrium that are related to the CaF2 structure. These compounds are laser hosts and luminescent materials, oxygen sensors as well as components of solar cells. They exhibit various schemes of (dis)ordering of cations and anions in fluorite superstructures and anion-excess fluorites. In the last few years, we have performed a series of studies on the bulk AMF4 and MF3 materials (A = Li, Na, K; M = Y, lanthanide) at different pressure-temperature conditions. Among them, ordered LiYF4 is a commercial host for solid state lasers, while partially ordered NaYF4 doped with lanthanides is the most efficient material for green and blue up-conversion known to date. In the system KF–YF3, we have studied not only KYF4 [1] but also KY3F10, which is an anion-excess 2×2×2 superstructure of fluorite at atmospheric conditions. At high temperatures and high pressures, it converts to another fluorite superstructure with disordered fluorine atoms. The pressure-induced LaF3 post-tysonite structure is another example of the anion-excess fluorite [2]. Our work on the fluorite-related materials at extreme conditions provides information on their structural instabilities that could further be used to better understand and control their materials properties. For instance, we demonstrated that the NaMF4 up-converters are unstable and that the ordering of the cations and vacancies in their structure is a slow process [3]. Consequently, the order–disorder transformations have a profound influence over the luminescent properties of these materials when doped.