Crystal growth and anisotropy of ionic conductivity of trifluoride DyF$_3$

Crystallographically oriented single crystals of DyF$_3$, a representative of the third structural group of rare-earth trifluorides (structure $\beta$-YF$_3$, sp. gr. Pnma, unit cell parameters $a$ = 6.4603(2), $b$ = 6.9104(1), $c$ = 4.3808(2)$\mathring{\mathrm{A}}$, were grown for the first time by the directional crystallization method. Temperature (386–783K) measurements of the ionic conductivity of this crystal were carried out along and perpendicular to the crystallographic $b$ axis. It was found that DyF$_3$ crystals have weak anisotropy of electrical conductivity, the coefficient $\sigma_{\parallel b}/\sigma_{\perp b}$ = 2.2 $\pm$ 0.1 and $\sigma_{\parallel b}$ = 2.5 $\cdot$ 10$^{-6}$ S/cm (at 500K). The relationship between the characteristics of ion transport and the crystal structure is discussed for the general family of rare earth trifluorides with the $\beta$-YF$_3$ structure, including $R$F$_3$ compounds ($R$ = Dy, Tb, Ho), low-temperature modifications of $\beta$-$R$F$_3$ ($R$ = Er, Y) and solid solutions Gd$_{0.3}$Er$_{0.7}$F$_3$, Gd$_{0.5}$Y$_{0.5}$F$_3$. It is shown that for this family of rhombic trifluorides, with an increase in the radius of rare earth cations, the activation enthalpy of ion transfer (vacancy mechanism) decreases, which leads to an increase in ionic conductivity.