Lanthanum 이 첨가된 투명세라믹 strontium barium niobate $〔(Sr_{0.75}Ba_{0.25})_{5-7.5x}La_{5x}〕Nb_{10}O_{30}$ 과 물리적 성질Lanthanum -modified transparent strontium barium niobate $〔(Sr_{0.75}Ba_{0.25})_{5-7.5x}La_{5x}〕Nb_{10}O_{30}$ ceramics and their physical properties