Single Crystal Growth of Double Honeycomb Lattice Superconductor Related Materials

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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.