Department of Earth Sciences, University of Ferrara, Via Saragat 1, 44100 Ferrara, Italy.
J Phys Chem A. 2009 Dec 10;113(49):13772-8. doi: 10.1021/jp9043072.
The structural relaxation around Cr(3+) in YAl(1-x)Cr(x)O(3) perovskites was investigated and compared with analogous Cr-Al joins (corundum, spinel, garnet). Eight compositions (x(Cr)((3+)) from 0 to 1) were prepared by sol-gel combustion and were analyzed by a combined X-ray diffraction (XRD) and electron absorption spectroscopy (EAS) approach. The unit cell parameters and the XRD averaged octahedral (Cr,Al)-O and ([VIII])Y-O bond distances scale linearly with the chromium fraction. The optical parameters show an expected decrease of crystal field strength (10Dq) and an increase of covalency (B(35)) and polarizability (B(55)) toward YCrO(3), but a nonlinear trend outlines some excess 10Dq below x(Cr)((3+)) approximately 0.4. The local Cr-O bond lengths, as calculated from EAS, indicate a compression from 1.98 A (x(Cr)((3+)) = 1.0) down to 1.95 A (x(Cr)((3+)) = 0.035) so that the relaxation coefficient of perovskite (epsilon = 0.54) is the lowest in comparison with garnet (epsilon = 0.74), spinel (epsilon = 0.68), and corundum (epsilon = 0.58) in contrast with its structural features. The enhanced covalent character of the Cr(3+)-O-Cr(3+) bond in the one-dimensional arrangement of corner-sharing octahedra can be invoked as a factor limiting the perovskite polyhedral network flexibility. The increased probability of Cr-O-Cr clusters for x(Cr)((3+)) greater than approximately 0.4 is associated to diverging trends of nonequivalent interoctahedral angles. The relatively low relaxation degree of Y(Al,Cr)O(3) can be also understood by considering an additional contribution to 10Dq because of the electrostatic potential of the rest of the lattice ions upon the localized electrons of the CrO(6) octahedron. Such an "excess" of 10Dq increases when the point symmetry of the Cr site is low, as in perovskite, and would be affected by the change of yttrium effective coordination number observed by XRD for x(Cr)((3+)) greater than approximately 0.4. This would justify the systematic underestimation of local Cr-O bond distances, as inferred from EAS, compared to what is derived from X-ray absorption (XAS) studies implying a stronger degree of relaxation around Cr(3+) of all the structures considered and supporting the hypothesis that 10Dq from EAS contains more information than previously retained particularly an additional contribution from the next nearest neighboring ions.
研究了 YAl(1-x)Cr(x)O(3)钙钛矿中 Cr(3+)周围的结构弛豫，并将其与类似的 Cr-Al 连接（刚玉、尖晶石、石榴石）进行了比较。通过溶胶-凝胶燃烧法制备了八种组成（x(Cr)((3+))从 0 到 1），并通过 X 射线衍射（XRD）和电子吸收光谱（EAS）相结合的方法进行了分析。晶胞参数和 XRD 平均八面体（Cr、Al）-O 和 ([VIII])Y-O 键长与铬分数呈线性关系。光学参数显示晶体场强度（10Dq）预期降低，共价性（B(35))和极化率（B(55))增加到 YCrO(3)，但一些多余的 10Dq 呈非线性趋势，低于 x(Cr)((3+))约 0.4。根据 EAS 计算的局部 Cr-O 键长表明，从 1.98 A（x(Cr)((3+)) = 1.0）压缩至 1.95 A（x(Cr)((3+)) = 0.035），因此钙钛矿的弛豫系数（epsilon = 0.54）在石榴石（epsilon = 0.74）、尖晶石（epsilon = 0.68）和刚玉（epsilon = 0.58）中是最低的，与结构特征相反。一维顶角共享八面体排列中 Cr(3+)-O-Cr(3+)键的增强共价性质可以作为限制钙钛矿多面体网络灵活性的因素。对于 x(Cr)((3+))大于约 0.4，Cr-O-Cr 簇的概率增加与不等价的八面体间角的发散趋势有关。由于 CrO(6)八面体的局域电子对晶格离子的静电势的影响，Y(Al,Cr)O(3)的相对低弛豫度也可以通过考虑 10Dq 的额外贡献来理解。当 Cr 位的点对称度较低时，如钙钛矿中，这种 10Dq“过剩”会增加，并且会受到 XRD 观察到的 yttrium 有效配位数变化的影响对于 x(Cr)((3+))大于约 0.4。这将解释为什么从 EAS 推断的局部 Cr-O 键距离会系统地低估，与从 X 射线吸收（XAS）研究中推断出的相比，这意味着所有考虑的结构中 Cr(3+)周围的弛豫程度更强，并支持 EAS 中的 10Dq 包含比以前保留的更多信息，特别是来自最近邻离子的额外贡献的假设。
