Is Bi2+ Responsible for the Red-Orange Emission of Bismuth-Doped SrB4O7?

Red-orange luminescence
from bismuth-doped SrB4O7 has previously been
reported and assigned to 6p-6p transitions of divalent Bi. To provide
support for this assignment and for the stability of this unusual
valence state of Bi, we report here results from low-temperature luminescence
spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy,
electron paramagnetic resonance (EPR) spectroscopy, and wave function
based ab initio calculations. Low-temperature luminescence spectra
reveal zero-phonon lines (ZPLs) in excitation and emission spectra,
allowing an accurate determination of the energies for the electronic
transitions. The influence of the Bi concentration on the emission
intensity is shown to be small, and only a small increase of the red-orange
emission is observed upon raising the nominal Bi concentration from
0.02% (200 ppm) to 2%. This result indicates that only a very low
concentration of Bi2+ can be incorporated in SrB4O7. This observation is supported by EPR experiments,
which do not show a signal that can be assigned to Bi2+, and by XANES experiments showing that most Bi is in the trivalent
state. An upper limit of the Bi2+ concentration is estimated
to be 20 ppm. Ab initio calculations on the (BiO9)16– cluster embedded in SrB4O7 give energies for excited states that are close to the experimentally
observed energies. Also, the luminescence lifetime for the red-orange
emission (∼12 μs) is consistent with the lifetime for
6p-6p emission calculated for the Bi2+ emission (3.5 μs).
Equivalent ab initio calculations for Bi2+ luminescence
are very far from the experimental results, providing independent
evidence and additional support for the interpretation of stable Bi2+ species being responsible for the red-orange luminescence.
The calculations provide a new interpretation of the third excitation
band, which is not due to a 2S1/2 state of the
6s27s configuration of Bi2+, as previously assumed,
but is due to a state with important characters of 6s6p2–4P (63%) and doublets of the 6s6p2,
6s26d, and 6s26p configurations; its higher
intensity is due to its character of parity-allowed 6s → 6p
and 6p → 6d excitations.

红橙色的荧光现象先前已在铋掺杂的SrB4O7中报道，并被归因于二价铋的6p-6p跃迁。为支持这一归属并证实这种不寻常价态的稳定性，本文报道了低温荧光光谱学、X射线吸收近边缘结构（XANES）光谱学、电子顺磁共振（EPR）光谱学和基于波函数的从头计算的结果。低温荧光光谱揭示了激发和发射光谱中的零声子线（ZPLs），从而能够精确测定电子跃迁的能量。研究表明，铋浓度的变化对发射强度的影响微乎其微，仅在将铋浓度从0.02%（200 ppm）提高到2%时观察到红橙色发射的轻微增强。这一结果表明，在SrB4O7中只能掺入极低浓度的Bi2+。这一观察结果得到了EPR实验的支持，实验未显示出可归因于Bi2+的信号，以及XANES实验表明大多数铋处于三价态。估计Bi2+浓度的上限为20 ppm。在SrB4O7中嵌入的（BiO9）16–簇的从头计算给出了与实验观察到的能量接近的激发态能量。此外，红橙色发射的荧光寿命（∼12 μs）与计算得到的Bi2+发射的6p-6p发射寿命（3.5 μs）一致。对于Bi2+荧光的等效从头计算与实验结果相差甚远，这为解释稳定的Bi2+物种是红橙色荧光的成因提供了独立证据和额外的支持。计算为第三激发带提供了新的解释，该激发带并非如先前所假设的那样是由Bi2+的6s27s配置的2S1/2态引起的，而是由具有6s6p2–4P（63%）和6s6p2、6s26d、6s26p配置的双重态的重要特征的状态引起的；其高亮度归因于其偶宇称允许的6s→6p和6p→6d激发的特征。