ANIONIC CONDUCTIVITY OF MONNOCLINIC BaR2F8 (R-RARE EARTHS) SIGLE CRYSTALS
N.I. Sorokin1 and M.W. Breiterb2
1Institute of
Crystallography, Russian Academy of Sciences, 59 Leninsky
prospekt, 117333 Moscaw, Russia
2Technical University of Vienna, 9
Getreidemarkt, A-1060 Vienna, Austria
Fluoride compounds form an important class of materials with diverse technological applications as solid electrolytes. Monoclinic fluorides with BaTm2F8-type structure {space group C2/m) are fluoride-ion conductors having a pronounced conductivity [1]. The aim of present communication is the electrochemical study of oriented single crystals of BaR2F8 (R-rare earth) at high temperatures up to 1100 K. Single crystals of monoclinic BaR2F8 (R=Y0.9Er0.1 and Er0.945Tm0.05Ho0.005) were grown by Bridgman-Stockbarger technique under fluorinating atmosphere in Institute of Crystallography in Moscow. The electrical properties of RaR2F8 were studied by impedance spectroscopy in a wide frequency range from 0.1 to 107 Hz. Investigations were carried out in a constant flow of N2 gas. Impedance spectra reflected the presence of the polarization/reaction resistance and the film resistance as well as the bulk crystal resistance. There existed a good reproducibility of bulk conductivity data for a given sample in different heating/cooling runs. The conductivity measurements ot sample with R=Y0.9Er0.1 were made along the b axis (the 2 symmetry axis of s. gr. C2/m) and normal to the b axis (along the arbitrary direction on the m symmetry plane of s. gr. C2/m). The conductivity plot along the b axis has the fifm resistance region with the activation enthalpy of H=0.14-0.17 eV at T<420-460 K and the bulk resistance region with H=0.50 eV at 420-460 K<T<970 K. Up to 970 K the conductivity plot normal to the b axis has two bulk resistance regions with H1=0.59 eV at T<Tknee and H2=0.44 eV at T>Tknee (Tknee=520 K). At T>970 K the bulk conductivities along these two directions were the same and display a considerable deviation from low-temperature Arrhenius-type behaviour, caused by probably the transition from extrinsic conductivity to intrinsic one. Also the conductivity measurements were made for BaR2F8 with R= Y0.5Yb0.5 along the <211> direction and with R=Er0.945Tm0.05Ho0.005 along the c axis. Up to 1073 K the conductivity plots have two bulk resistance regions with H1=0.72 or 0.66 eV at T<Tknee and H2=0.63 or 0.55 eV at T>Tknee (Tknee =620 K) for samples with R= Y0.5Yb0.5 or R= R=Er0.945Tm0.05Ho0.005, respectively. The fluoride vacancies are the most likely mobile species in the monoclinic fluorides. The anionic transport was explained taking into account the fluoride vacancies migration by different F1, F2 and F3 crystallographic positions. The fluoride-ion conductivity at 500 K of the BaR2F8 with R=Y0.9Er0.1 along and normal to the b axis, with R=Y0.5Yb0.5 along the <211> direction and with R=Er0.945Tm0.05Ho0.005 along the c axis were 1.7x10-6, 3.0x10-7, 7.3x10-7 and 2.5x10-6 Scm-1, respectively.