Nonstoichiometric La_2-xGeO_5-δ Monoclinic Oxide as a New Fast Oxide Ion Conductor

Abstract

Oxide ion conductivity in La(2)GeO(5)-based oxide was investigated and it was found that La-deficient La(2)GeO(5) exhibits oxide ion conductivity over a wide range of oxygen partial pressure. The crystal structure of La(2)GeO(5) was estimated to be monoclinic with P2(1)/c space group. Conductivity increased with increasing the amount of La deficiency and the maximum value was attained at x = 0.39 in La(2 - x)GeO(5 - delta). The oxide ion transport number in La(2)GeO(5)-based oxide was estimated to be unity by the electromotive force measurement in H(2)-O(2) and N(2)-O(2) gas concentration cells. At a temperature higher than 1000 K, the oxide ion conductivity of La(1.61)GeO(5 - delta) was almost the same as that of La(0.9)Sr(0.1)Ga(0.8)Mg(0.2)O(3 - delta) or Ce(0.85)Gd(0.15)O(2 - delta), which are well-known fast oxide ion conductors. On the other hand, a change in the activation energy for oxide ion conductivity was observed at 973 K, and at intermediate temperature, the oxide ion conductivity of La(1.61)GeO(5 - delta) became much smaller than that of these well-known fast oxide ion conductors. The change in the activation energy of the oxide ion conductivity seems to be caused by a change in the local oxygen vacancy structure. However, doping a small amount of Sr for La in La(2)GeO(5) was effective to stabilize the high-temperature crystal structure to low temperature. Consequently, doping a small amount of Sr increases the oxide ion conductivity of La(2)GeO(5)-based oxide at low temperature.

A Structural Investigation of La2(GeO4)O and Alkaline-Earth-Doped La9.33(GeO4)6O2

“…High values of the conductivity have been also reported for La 9.66 Ge 6 O 30Àd with s ¼ 0:199 S cm À1 at 9501C. However, a monoclinic structure was reported for this compound (14).…”

“…However, such a comparison is only possible with Gd 2 GeO 5 as Dy 2 GeO 5 crystallizes in a different space group (I2=a), the a-axis parameter reported for Eu 2 GeO 5 is too large and not in agreement with the powder data reported in the JCPDS (21) and the calculated positions for the oxygen ions in La 2 GeO 5 and Nd 2 GeO 5 result in very distorted GeO 4 tetrahedra. In the work by Ishihara et al (14), the authors claimed to have prepared La-deficient samples with composition La 2Àx GeO 5Àd and with a monoclinic structure for values of xr0:45: However, their structural data reveal that the change in the unit cell parameters is not significant although a large decrease would be expected by such an La and O ion deficiency. Furthermore, their structural models reveal very distorted GeO 4 tetrahedra.…”

Synthesis and conductivities of the apatite-type systems, La9.33+xSi6−yMyO26+z (M=Co, Fe, Mn) and La8Mn2Si6O26

“…Recently high oxide ion conductivity has been reported in the oxide apatitetype system Ln10.ySi6026_+ z (Ln = rare earths) as well as AI doped variants, with conductivities higher than YSZ at intermediate temperatures (4.3 • 10 .3 S-cm -~ vs. 1 • 10 .3 S-cm -~ at 500 ~ [1][2][3][4][5]. Arikawa et al have subsequently reported that the Ge analogue La~oGe60 z and in particular the Sr doped version LagSrtGe60 ~ exhibit even higher conductivity [6], although the exact nature of the conducting phase in this latter system is still the subject of discussion, with the same authors subsequently claiming that it was cation deficient La2GeO5 type [7]. The reason for this confusion appears to be due to the fact that loss of Ge occurs at high temperatures leading to a trans-"Corresp.…”

Relation between microstructure and synthetic procedure of Ln2TO5(Ln = La and Nd, T = Ge and Si)

“…However, powder diffraction data contains information about disorder and defects in the form of diffuse scattering which is often ignored. The use of real-space pair distribution function (PDF) techniques is becoming increasingly popular in the study of disordered and nanoscale materials [1][2][3]. The PDF has peaks corresponding to inter atomic distances.…”

“…[ Rare earth oxy-apatites are nowadays interesting due to their high oxide ion conductivities and low activation energies, properties that are making these materials promising electrolytes for solid oxide fuel cells, SOFCs [1]. An attractive system related with the apatite-type phases is the monoclinic La 2-x GeO 5-3x/2 series, synthesized and studied by Ishihara et al [2]. The structural formula of this series is better expressed as La 2-x (GeO 4 )O 1-3x/2 to highlight its oxy-germanate character.…”