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首页> 外文期刊>Physical review >Internal electric fields and color shift in Cr~(3+)-based gemstones
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Internal electric fields and color shift in Cr~(3+)-based gemstones

机译：Cr〜（3+）基宝石的内部电场和色移

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Seeking to better understand the origin of the different colors of emerald and ruby, both ab initio periodic and cluster calculations have been carried out. The calculations reproduce the interatomic distances measured for pure Be_3Si_6Al_2O_(18) and Al_2O_3 as well as the Cr~(3+)-O~(2-) distances in emerald and ruby. The mean Cr~(3+)-O~(2-) distance for Be_3Si_6Al_2O_(18):Cr~(3+) and Al_2O_3:Cr~(3+) is found to be practically equal to 1.97 A, in agreement with recent experimental values. The present calculations confirm that the variations of optical properties due to Cr~(3+) impurities along the series of ionic oxides can be understood merely through the CrO_6~(9-) unit but subject to the electric field due to the rest of the lattice ions. As a salient feature it is proved that changes in electronic density and covalency due to the internal field are not the cause of the color shift. Therefore, the red color of ruby is not due to the polarization of the electronic cloud around chromium as a result of the C_3 local symmetry. The present study also demonstrates that the variation of the ligand field splitting parameter, 10Dq, induced by the internal electric field comes mainly from the contributions of first shells of ions around the CrO_6~(9-) unit. As a consequence, 10Dq in emerald is not influenced by the internal field, as the contribution from Be~(2+) first neighbors is practically compensated by that of Si~(4+) second neighbors. In contrast, in ruby the t_(2g) levels are shifted by the internal field 0.24 eV more than the e_g ones, so explaining the color shift in this gemstone in comparison with emerald. This result is shown to arise partially from the asymmetric form of the internal electrostatic potential along the C_3 axis in Al_2O_3.
机译：为了更好地了解祖母绿和红宝石的不同颜色的起源，已经进行了从头算周期和簇计算。计算结果重现了纯Be_3Si_6Al_2O_（18）和Al_2O_3的原子间距离以及祖母绿和红宝石中的Cr〜（3 +）-O〜（2-）距离。发现Be_3Si_6Al_2O_（18）：Cr〜（3+）和Al_2O_3：Cr〜（3+）的平均Cr〜（3 +）-O〜（2-）距离实际上等于1.97 A，与最近的实验值。目前的计算结果证实，仅通过CrO_6〜（9-）单元，就可以理解由于Cr〜（3+）杂质沿着一系列离子氧化物引起的光学性能变化，而其余部分则受到电场的影响。晶格离子。作为显着特征，证明了由于内部场引起的电子密度和共价的变化不是色移的原因。因此，红宝石的红色不是由于C_3局部对称性导致铬周围电子云的极化。本研究还表明，由内部电场引起的配体场分裂参数10Dq的变化主要来自CrO_6〜（9-）单元周围离子的第一壳层的贡献。结果，祖母绿中的10Dq不受内部场的影响，因为Be〜（2+）第一邻居的贡献实际上被Si〜（4+）第二邻居的贡献所补偿。相比之下，在红宝石中，t_（2g）级别比e_g级别的内部场偏移了0.24 eV，因此，与绿宝石相比，可以解释这种宝石的颜色偏移。该结果显示出部分源自Al_2O_3中沿C_3轴的内部静电势的不对称形式。

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