Growth of Apatite-Type Lanthanide Silicates Single Crystals by the Floating Zone Method and Mechanism of Their Oxide Ionic Conduction

具有磷灰石结构的稀土硅酸盐是一种新型的氧化物离子导体，在600℃的低温下，其电导率优于稳定化的氧化锆。然而，到目前为止，这些材料还没有生长出大尺寸的单晶，因此它们的基本性质还没有弄清楚。本研究采用浮区法生长了磷灰石型稀土硅酸盐单晶，并对其电学性能进行了测试，从光吸收的角度，生长了Pr、Nd、Sm硅酸盐晶体。所有的晶体都很容易用浮区法生长，因为它们是一致熔化的。对于ND系统，偶尔会含有气泡包裹体，但使用贫钕的送料棒能够生长出无包裹体的晶体。同分组成可能与化学计量组成不同，并位于贫钕一侧。对于Sm-系统，由于微裂纹的形成，还没有得到宏观上无缺陷的晶体。然而，由于裂纹是平行于c平面形成的，因此它们不会显著阻碍电导率的测量。对于所有类型的晶体，平行于c轴的电导率比垂直于c轴的电导率大约一个数量级。这些结果证明，2a位的氧离子优先通过平行于磷灰石结构c轴的通道迁移。单晶的电导率与稀土离子的种类无关，但烧结体的电导率明显降低。这是因为2a位的氧离子与6h位的3个稀土离子在同一平面水平上配位。

Lanthanide silicates with an apatite structure are a new type of oxide ionic conductors; at low temperatures blow 600 ℃ the conductivities of these materials are superior to that of stabilized zirconia. However, no large-size single crystals of these materials have so far been grown, and accordingly their essential properties have not yet clarified. In this study, single crystals of apatite-type lanthanide silicates were grown by the floating zone method and their electrical properties were evaluated.From the viewpoint of light-absorption, praseodymium, neodymium and samarium silicates crystals were grown. All the crystals were easily grown by the floating zone method since they melt congruently. For the Nd-system, bubble inclusions were occasionally contained but the use of a Nd-poor feed rod enabled to grow an inclusion-free crystal. The congruent composition may be different form the stoichiometric one and is located at Nd-poor side. For the Sm-system, macroscopic defect-free crystals have not yet been obtained due to the formation of microcracks. Since the cracks were formed parallel to the c-plane, however, they do not hinder significantly the measurements of conductivities. For all the kinds of crystals, the conductivities parallel to the c-axis were larger by about one order of magnitude than those perpendicular to the c-axis. These results proved that the oxide ions at 2a site migrate preferentially through the channel parallel to the c-axis of the apatite structure. The conductivities of the single crystals were independent on the kinds of rare-earth ions although remarkable decrease in the conductivities were observed for the sintered compacts. This is because the oxide ions at 2a site is coordinated by three rare-earth ions at 6h site on the same plane level.