GROWTH AND CATION TRANSPORT PROPERTIES OF SEVERAL MIXED OXIDE SINGLE CRYSTALS
V.B.Nalbandyan, V.A.Volotchayev
Rostov
State University, 7, Zorge str., Rostov-on-Don, 344090, Russia
E-mail: vnalband@uic.rnd.runnet.ru
Keywords: superionic conductors, crystal growth,
ion exchange, ion conduction
Preliminary results on growth, X-ray characterization, ion exchange and ion conduction of Na2O-Li2O-TiO2 and Na2O-MgO-TiO2 crystals are reported. The crystal structures are described in a separate report.
The former system has been studied below the solidus, and several mixed oxides were identified by powder methods [1]. Two of them are layered with general formula NaxLix/3Ti1-x/3O2. At x close to unity, O3 phase (alpha-NaFeO2 type) exists, and at x = 0,65-0,69, P2 phase is formed (here O and P stand for octahedral and trigonal prismatic Na+ coordination, and number of layers in a hexagonal cell is also denoted). P2 ceramics shows high sodium ion conductivity.
The crystals have been grown from the melts with slight excess of sodium oxide (Na/Li = 4), but the chemical analysis of crystals confirms the proposed Na/Li ratio of 3, at least for P2 phase. Single crystal X-ray data are in fair agreement with powder data [1]. P2 phase is best obtained at 64-66 mole % TiO2 in the form of mica-like hexagonal plates 0.1-1 mm thick and up to 10 mm in diameter. At higher titania content, orthorhombic Na1+yLi1-yTi3O7 appears which does not exhibit ion transport properties. Decreasing TiO2 content gives rise to rhombohedral O3 crystals of the same mica-like habit. However, at x close to unity (i.e. at 50 % TiO2) some additional reflections are observed indicating a 31/2 superlattice, obviously due to Li/Ti ordering. From the structural point of view, P3112 space group is proposed for the ordered phase, despite apparent 6-fold symmetry of the superlattice spots due to twinning.
Impedance measurements on P2 single crystals confirm high ion conductivity in the (001) plane: about 7 S/m at 300 C. It is only 2-2,5 times higher than that of ceramics [1] with the same activation energy 0,31 eV.
Ion exchange in P2 crystals have been studied in aqueous solutions of Li+, K+, Ag+, NH4+, Tl+, Ca2+, Ba2+, Cd2+, Pb2+ salts. At room temperature, only lithium and silver exchange is observed. The diffusion in crystals proceeds via two-phase mechanism rather than via solid solution formation. One of the phases is unchanged sodium precursor in the middle of the crystal, while the other is half-exchanged product (Na1/2Ag1/2)0.66(Li0.22Ti0.78)O2 with very strong superlattice reflections (00l with l odd) indicating alternation of Na and Ag layers. These reflections disappear again after the complete removal of Na+.
Similar superlattice is observed after the treatment in molten KNO3. However, no additional reflections are found upon Li+ exchange. We suppose that the first large cation (Ag+ or K+) in Na+ site serves as a pillar: it promotes further exchange in this layer and prohibites exchange in the two adjacent layers (which are compressed locally) leading to alternation of layers.
The exchange is accompanied by serious degradation of the crystals. The complete Ag+ substitution for Na+ is difficult to achieve without lithium loss, whereas complete Li+ substitution for Na+ leads to amorphous products.
Growth of Na2O-MgO-TiO2 crystals was reported previously [2,3], however one additional phase was discovered in this work. It is monoclinic Na5.4Mg0.7Ti7.3O18 of the Na2+xTi4O9 type [4]. The structure contains one-dimensional non-intersecting triple channels where Na+ resides. Counter-diffusion in such channels seems quite improbable. However, we have found fast ion exchange in aqueous or molten AgNO3. E.g., less than an hour at 320oC is necessary for complete exchange in a 3 mm crystal elongated in the channel direction. Another crystal with non-intersecting channels, Na0.9Mg0.45Ti1.55O4 [2], shows similar behaviour. Exchange reaction in these crystals is much more fast than in P2 crystals of similar size, although the latter have higher ionic conductivity and provide a bidimensional system of intersecting diffusion paths.
These facts are an indication of possible concerted directional move of all the ions in a channel, instead of a random walk, as a mechanism of ion exchange and ion conduction in the structures with non-intersecting channels.
The work was supported by the Russian Foundation for Basic Research under the grant No. 97-03-33807a.