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