Single Crystal Growth of Double Honeycomb Lattice Superconductor Related Materials

利用x射线粉末衍射数据对Na_<0.9>Zr_2N_2S_<1.5>、Na_<0.85>Hf_2N_2S_<1.5>、Zr_2N_2S、Hf_2N_2S等双蜂窝晶格相关材料进行了结构研究。Zr_2N_2S和Hf_2N_2S的反射峰-101和10-3明显变宽，表明α-M_2N_2S与β-M_2N_2S相共生。这是由于层错导致的二维化合物的典型特征。结果表明，α-M_2N_2S与β-M_2N_2S相的叠加能差很小。这些化合物的结构分析没有这些宽峰。根据结果，我们估计了这些化合物的能带宽度。这些估计的能带宽度比Na_<0.28>HfNCl的能带宽度窄，Na_<0.28>HfNCl是迄今为止所有双蜂窝晶格化合物中最窄的能带宽度，表明态的电子密度很大。然而，这些频带宽度不符合我们先前的期望。即两个阴离子相对于正离子的价差越小，则能带宽度越窄。碱金属嵌入的HfNCl和ZrNCl超导体表现出比原始化合物更宽的频带宽度。因此，电子载流子掺杂后的双蜂窝晶格化合物也可以表现出更宽的能带。载流子掺杂的YOCl化合物比HfNCl和ZrNCl超导体具有更窄的带宽。这表明，即使在大量载流子密度下，窄带宽度造成的强载流子局部化对绝缘性能也有重要作用。在这些绝缘子中，研究具有强配对相互作用的局域自旋行为是很有意义的。这是目前双蜂窝晶格化合物研究工作中最重要的课题。

Structural study of double honeycomb lattice related materials such as Na_<0.9>Zr_2N_2S_<1.5>, Na_<0.85>Hf_2N_2S_<1.5>, Zr_2N_2S, and Hf_2N_2S has been performed by using X-ray powder diffraction data. As for Zr_2N_2S, and Hf_2N_2S, some reflection peaks such as -101 and 10-3 were broadened significantly, suggesting the intergrowth of α-M_2N_2S and β-M_2N_2S phases. This is a typical characteristic for two-dimensional compounds due to stacking faults. It indicates that the energy difference in stacking between α-M_2N_2S and β-M_2N_2S phases is very small. The structural analysis for these compounds has been carried out without these broad peaks. Based on the results, we estimated the band widths for these compounds. These estimated band widths were narrower than the band width of Na_<0.28>HfNCl, which had been the narrowest band width in any double honeycomb lattice compounds so far, suggesting large electron density of states. These band widths, however, do not correspond to our previous expectation. That is, the smaller the valence difference of two anions relative to the cation valence is, the narrower the band width is. The alkali metal intercalated HfNCl and ZrNCl superconductors exhibit wider band widths than the pristine compounds. So, the present double honeycomb lattice compounds may also show wider band widths after electron carrier doping. Carrier doped YOCl compounds have narrower band widths than the HfNCl and ZrNCl superconductors. It suggests that strong carrier localization due to the narrow band width plays an important role for the insulating property even with large number of carrier density. In these insulators, it is interesting to study the localized spin behavior with strong pairing interaction. This is the most important subject in the remaining research works about the present double honeycomb lattice compounds.