CHEN CHUANG-TIAN. AN IONIC GROUPING THEORY OF THE ELECTRO-OPTICAL AND NON-LINEAR OPTICAL EFFECTS OF CRYSTALS (Ⅰ)——A THEORETICAL CALCULATION OF ELECTRO-OPTICAL AND SECOND OPTICAL HARMONIC COEFFICIENTS OF BARIUM TITANATE CRYSTALS BASED ON A DEFORMED OXYGEN-OCTAHEDRAJ. Acta Physica Sinica, 1976, 25(2): 146-161. DOI: 10.7498/aps.25.146