Neutron diffraction study crystal structures of KAlO₂ and Cs₃PO₄ in wide range of temperatures

Voronin V.I.^a, Proskurnina N.V.^a, Shekhtman G.Sh.^b, Byrmakin E.I.^b, Stroev S.S.^b

^a Institute of Metal Physics, Ural Division of Russian Academy of Sciences,

S. Kovalevskoi Str.18, Ekaterinburg GSP-170, Russia

^b Institute of High Temperature Electrochemistry, Ekaterinburg, Russia

Solid orthophosphate ceria and potassium aluminate electrolytes are desirable materials for industrial applications. It is well known that the main factor determining an electrical property of the solid electrolytes is their crystal structure. Therefore, cation conductivity and crystal structure of KAlO₂ and Cs₃PO₄ has been studied in present work. Conductivity dependences on temperature in Arrhenius plots – lg(sT)-vs 1/T are shown in Figure 1 and 2.

It is seen that behavior of conductivity is very similar in these compositions. In initial KAlO₂ and Cs₃PO₄ samples conductivity jump are observed in high temperature region and the values of activation energy are decrease. The conductivities are significantly increased as the doped element content increases, end bending points are on Arrhenius plots at some temperatures. To understand the reason of such behavior of conductivity neutron-diffraction experiments in a wide range of temperatures were carried out. Because of samples chemical activity and their hygroscopicity and to prevent their contacts with air all samples were placed in quartz-closed ampoules. Therefore, there is a wide diffused halo of amorphous quartz on neutron diffraction patterns. The neutron diffraction patterns obtained for KAlO₂ and Cs₃PO₄ (fig. 3,4) show that the samples were monophasic. The Rietveld analysis of the diffraction pattern revealed that the KAlO₂ and Cs₃PO₄ crystallize in the orthorhombic space groups, Pbca for KAlO₂ (a = 5.4446(9) Å, b = 10.931(1) Å, c = 15.458(2) Å) and Pmmm for Cs₃PO₄ (a = 14.590(5) Å, b = 10.219(3) Å, c = 7.780(2) Å). Analyze also showed that KAlO₂ structure has the tetrahedra AlO₄ forming three-dimensional framework with full connectivity. On the contrary in Cs₃PO₄ the tetrahedra PO₄ are separated spatially. We found that Cs₃PO₄ and KAlO₂ exhibits structural phase transition at T ~ 790 K and ~ 813 K, correspondingly (fig. 5,6), followed by the thermal conductivity jump (fig. 1,2).

Using high temperature structure b-cristobalite study results [111] it has been showed that at Т>540^oC the aluminum, potassium and oxygen atoms occupy, respectively, the special equivalent positions 8a, 8b and 16c in the space group Fd-3m. It is an “ideal” model that gives a bad experiment description. Disordered models give good accordance with experiments. In the first model the oxygen atoms do not lie on well-defined positions, but the Al-O bond precesses about its average orientation so that the oxygen atoms lie on an annulus of fixed radius (~0.5 Å). In the second model the oxygen atoms are placed in the 96h sites, with partial occupancy of 1/6. We cannot prefer any of these models from our experimental data. More precise example is given in Fig. 5.

In high temperature phase orientationally disordered phosphate ions occupy an FCC lattice, and the sodium cations occupy all tetrahedral and octahedral interstices (neutron diffraction patterns show at fig.6).

Crystal structures of KAlO₂ and Cs₃PO₄ are cubic at room temperature because of doping them by titanium and barium, respectively. However these cubic structures are disordered as for according high temperature phases, but this disordering is static and dynamic at high temperature. There is an increase of disordering degree of the K_1-xAl_2-xTi_xO₂ (x=0.2) sample as the temperature increases and Debay-Waller factor significant increases.

Because of crystal structure features it was suggested the conductivity mechanism at high temperatures in so called superionic state. As it was mentioned earlier there are rigid tetrahedra PO₄ groups in orthophosfat cesium which are saved their form in whole temperature region. However, large Debye -Waller factors in superionic state suggest dynamic disordering. These facts mean group motion as a whole. Such model was suggested for isostructural compound Na₃PO₄ based on complex study of structural state at high temperatures using different methods [666]. Inelastic neutron scattering experiments showed rotor motion of these complexes. It’s the model suggests strong coupling between the reorientation of anions and the mobility of cations – the “paddle-wheel mechanism”. The cubic structure appears at room temperature at doping of compounds that is why such correlated tetrahedra PO₄ rotation and cesium ions diffusion appears at lower temperature. This process is illustrated by bending point on Arrhenius plots; bending point temperature decreases with barium concentration (Fig.2).

Additional conducting channels appear at phase transition into cubic phase in KAlO₂ lattice. At the same time such “paddle-wheel mechanism” occurs in KAlO₂. Because of tetrahedrals link by oxygen atoms there are correlating AlO₄ vibrations and motion of K cations with increasing temperature. This correlation is confirmed by sharp activation energy decreasing at doping with titanium (Fig.1). And the bending point temperature decreases as the Ti concentration increase.

Work supported by State Scientific Research Program “Neutron Investigations of Condensed Matter” (State control No. 40.012.1.1.11.50) and the basic research program of the Department of Physical Sciences of the Russian Academy of Sciences "Neutron studies of the substance structure and fundamental properties of matter”.

Swainston I.P. and Dove M.T., J.Phys.: Condens. Matter., 7 (1995) 1771.

Wilmera D., Feldmanna H., Combetb J., Lechner R.E., Physica B 301 (2001) 99