题目：High-temperature electrical properties of langasite Ca₃Nb(Ga,Al)₃Si₂O₁₄ single crystals 

　　报告人：Prof. Tomoaki Karaki （陈智明、唐木智明） 

　　时间：2018年11月13日（周二）, 下午14:30 

　　地点：中科院上海硅酸盐研究所（嘉定），10号楼（青年公寓）2楼会议室 

　　联系人：罗豪甦（021-69987760）  

　　报告人简介：陈智明教授，中科院上海硅酸盐研究所获得硕士学位，在日本京都大学大学院工学研究科博士，现为日本富山县立大学工学系智能设计工程专业教授，中国科学院上海硅酸盐研究所兼职教授。主要研究方向为压电陶瓷的制备和应用（无铅压电陶瓷、厚膜）；铁电压电晶体的制备和应用（KLN,KN,CGNS铁电压电,电光晶体）；声表面波器件的模拟计算（基板以及多层结构器件）；压电单晶声波相关物理参数及温度系数测试；以及水热合成纳米功能晶体（片状模板粉体），以及低中强度超声波对水产养殖和生物等的影响。 

　　报告内容：Langasite-family Ca₃Nb(Ga_1-xAl_x)₃Si₂O₁₄ (CNGAS) (x=0-0.6) single crystals were grown by a Czochralski technique along the x-axis. When the Al substituent was 0.5, electrical resistivity was greatly improved to 1010 Ωcm at 500 oC, which was more than two orders of magnitude higher than that of Ca₃NbGa₃Si₂O₁₄ (CNGS), as shown in Fig. 1. This value is the highest resistivity among members of the langasite family at present. However, the dielectric features associated with loss tangent jump at about 400 ^oC were not improved by the Al substituent. The bandgap Eg was measured by spectroscopic ellipsometry, and E_g was changed by both temperature and amount of Al substituent. Real and imaginary dielectric constants at a frequency from 0.1 to 20,000 Hz were measured from 150 to 600 ^oC. Cole-Cole plots showed one dielectric relaxation existing in the crystals. There was no marked difference between specimens with x=0.1 and 0.5. Oxygen vacancies associated with Nb ions were discussed for DC conductivity and AC loss tangent. Both the DC conductivity and AC loss tangent in CNGAS were caused by Vo¨ related to B-site Nb. Schematic draw of band structure of CNGAS single crystals. The DC conductivity corresponded to the carriers excited from the NbNb’-Vo¨ pair level E’ and density of states. The density of E’ states could be reduced by thermal treatment in oxygen atmosphere or Al substituent. The AC loss tangent corresponded to the short-range motion of Vo¨ that was hardly removed completely. 

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