Structural chemistry and impedance spectroscopy of Mg2+ stabilized Na+/Pr3+-beta''-aluminas

The structure with the ionic distribution in the conduction planes of a Pr3+-exchanged Mg2+ stabilized Na+-beta ''-alumina crystal, composition determined by electron microprobe analysis to Na1.19Pr0.13Mg0.49Al10.49O17 (degree of exchange xi = 25%, related to Na+ content), has been investigated by single crystal X-ray diffraction methods at room temperature (R(3) over barm$, Z = 3, a = 561.8(2)pm, c = 3361.5(11)pm). Pr3+ is slightly shifted in the ab-plane from the centrosymmetric 9 d (mO) into a 18 h site. Na+ occupies 18 h as well as 6c (BR) positions. Ionic conductivity data of Mg3+ stabilized Na+/Pr(3+)beta ''-Al2O3 crystals with varying degree of exchange determined by impedance spectroscopy are given in an Arrhenius diagram. With growing Pr3+ content the conductivity sigma decreases with increasing activation energy.