Kendrick E, Sansom J E H, Tolchard J R, Islam M S, Slater P R
Chemistry, UniS Materials Institute University of Surrey, Guildford, Surrey, UK.
Faraday Discuss. 2007;134:181-94; discussion 215-33, 415-9. doi: 10.1039/b602258h.
In this paper, detailed studies of the effect of Mg doping in the apatite-type oxide ion conductor La9.33Si6O26 are reported. Mg is confirmed as an ambisite dopant, capable of substituting for both La and Si, depending on the starting composition. A large enhancement in the conductivity is observed for Si site substitution, with a reduction for substitution on the La site. Neutron powder diffraction studies show that in agreement with cation size expectations, an enlargement of the unit cell is observed on Mg substitution for Si, with a corresponding increase in the size of the tetrahedral sites. For Mg substitution on the La site, a contraction of the unit cell is observed, and the neutron diffraction results indicate that there is preferential occupancy of Mg on the La2 (1/3, 2/3, approximately 0.5) site. Atomistic simulation studies show significant local structural changes affecting the oxide ion channels in both cases. Mg doping on the Si site leads to a local expansion of the channels, while doping on the La site results in a large displacement of the silicate O4 site, such that it encroaches the oxide ion channels. The observed differences in conductivities are discussed with respect to these observations.
本文报道了对磷灰石型氧化物离子导体La9.33Si6O26中Mg掺杂效应的详细研究。Mg被确认为一种双位掺杂剂，根据起始组成，它能够替代La和Si。对于Si位替代，观察到电导率有大幅提高，而对于La位替代，电导率则降低。中子粉末衍射研究表明，与阳离子尺寸预期一致，Mg替代Si时观察到晶胞增大，四面体位置尺寸相应增加。对于Mg替代La位，观察到晶胞收缩，中子衍射结果表明Mg优先占据La2(1/3, 2/3, 约0.5)位置。原子模拟研究表明，在这两种情况下，都会发生显著的局部结构变化，影响氧化物离子通道。Si位的Mg掺杂导致通道局部扩张，而La位的掺杂导致硅酸盐O4位置发生大的位移，从而侵入氧化物离子通道。针对这些观察结果，讨论了观察到的电导率差异。
