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）
报告内容：Langasite-family Ca3Nb(Ga1-xAlx)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 Eg 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.