LiNbO_3∶Mg 晶体的点阵常数和密度

Lattice Parameter and Density Investigation of LiNbO_3∶Mg

用西门子 D-500型衍射仪,CuK_α辐射测量了一组不同掺镁浓度 LiNbO_3的 X 射线粉末衍射谱。样品的掺镁浓度分别为0、1.0、2.7、5.0、6.0、9.0(mol%,x=0.945)。X 射线衍射结果表明各样品均未发生相变或相分离。用最小二乘法(2θ=90°～150°)得到了各组分的精密点阵常数 a、c。结果表明掺镁浓度低于阈值样品的点阵常数无明显变化,而高于阈值样品的点阵常数则有明显增大。用阿基米德法测量了上述样品的密度,按我们提出的缺陷结构模型计算所得的 X 射线密度与实测密度比较,发现两者变化趋势和绝对值都较吻合。上述实验结果证明:在掺镁浓度低于阈值时,Mg^(2+)进入 Li 位;而当掺镁浓度超过阈值时,Mg^(2+)则进入 Nb 位。这与我们早先二篇文章的结果是一致的。

The X-ray powder diffraction patterns of LiNbO_3 doped with different MgOlevels were obtained by using a D-500 diffractometer,Cu K_α radiation.The MgOdoping levels in samples were 0,1.0,2.7,5.0,6.0,9.0 mol% respectively.According to these diffraction patterns no phase transition was found.The latticeparameters of the LiNbO_3 were accurately calculated by means of the least squaremethod (2θ=85-150°;13-16 diffraction lines).The densities were also measuredby using the Archimedes method.These results show that lattice parameters a and cincrease proportionally to the doping levels in the heavy Mg-doped samples (abovethreshold level),and keep unchanging in light doped or undoped samples (belowthreshold level).The changing tendency and the absolute values of the measureddensities show to be congruent with that of the densities calculated on the basis ofour earlier suggested defect structure model of LiNbO_3∶Mg.For light Mg-doped Li-NbO_3 crystals,Mg^(2+)ions should occupy Li sites;but for heavy Mg-doped LiNbO_3where doping level is higher than its threshold level,Mg^(2+) ions should get intoNb sites.This conclusion agrees with the argument suggested by us in a series ofprevious papers.