LiNbO_3: Cr^(3+),Mg^(2+)晶体中Cr^(3+)离子的光谱精细结构,零场分裂和占位研究

STUDY OF OPTICAL FINE STRUCTURE AND ZERO FIELD SPLITTING OF SUBSTITUTIONAL SITES IN LiNbO 3:Cr 3+ ,Mg 2+

采用完全对角化方法，首先计算了ＬｉＮｂＯ３：Ｃｒ３＋，Ｍｇ２＋晶体中Ｃｒ３＋（Ⅰ）离子的光谱精细结构，并重点分析了Ｃｒ３＋（Ⅰ）离子的零场分裂和Ｒ线（２Ｅ→４Ａ２跃迁）分裂．然后，根据ＬｉＮｂＯ３：Ｃｒ３＋，Ｍｇ２＋晶体的ＥＰＲ谱和吸收光谱，进一步研究了Ｃｒ３＋（Ⅰ）和Ｃｒ３＋（Ⅱ）离子的占位情况．计算结果表明：Ｃｒ３＋（Ⅱ）离子应占Ｎｂ５＋位，同时沿着Ｃ３轴向八面体中心移动０．０１３±０．００１ｎｍ，对于该占位，Ｃｒ３＋（Ⅱ）离子的零场分裂以及２Ｅ、４Ｔ２态的能量均与实验值符合；对于Ｃｒ３＋（Ⅰ）离子，存在着两种可能占位，即占Ｎｂ５＋位和Ｌｉ＋位，并且都沿着Ｃ３轴向八面体中心移动约０．０４９ｎｍ，它们的零场分裂值和２Ｅ态分裂都能与实验值相符合．

By means of the complete diagonalization of Hamiltonian matrix, we calculate the optical fine structure of Cr 3+ (Ⅰ) ion in LiNbO 3:Cr 3+ ,Mg 2+ , and then carefully analyze the zero field splittings and the R lines ( 2E→ 4A 2transition). According to the EPR and absorption spectra, we further study the substitutional sites for Cr 3+ (Ⅰ) and Cr 3+ (Ⅱ) ions in LiNbO 3:Cr 3+ ,Mg 2+ . The results indicate that Cr 3+ (Ⅱ) ions should be at the Nb 5+ sites and move 0.013±0.001nm along the C 3 axis towards the centre of the oxygen octahedron, and the calculated values of the zero field splitting parameter D and energies of the  2E and  4T 2states are both in good agreement with the experimental data. For Cr 3+ (Ⅰ), there exist two possibilities: at the Nb 5+ and Li +sites, both with moving along the C 3 axis about 0.049nm towards the center of the octahedron. The calculated results for the  2E state energy and the zero field splitting agree well with experimental ones.