大专题——荧光粉中电荷转移和价态变化

基本参数和辐照源

还原势(redox potential)

注：
(1) 上表来自Lanthanide and Actinide Chemistry, Wiley, Chichester, United Kingdom (2006), pp. 20-21
(2) 这里的THF应该是四氢呋喃Tetrahydrofuran。
(3) Eu这里的第二行的数值是-0.34，有的认为是0.35。
(4) 上表的单位是 eV。
(5) 最下面的表说明了各种稀土常见的价态。

Rare earth elements have similar configurations in the two outermost shells. They exhibit typical metallic properties in chemical reactions. They tend to lose three electrons and exhibit a $3+$ valence state. From the Periodic Table of the elements, rare earth elements are classed as less reactive than alkali metals and alkaline earth metals but more reactive than other metals. They should be stored in an inert liquid otherwise they will be oxidized and lose their metal luster. The metal reactivity increases gradually from scandium to lanthanum and decreases gradually from lanthanum to lutetium. That is to say, lanthanum is the most reactive metal of the 17 rare earth elements. Rare earth metals can react with water and release hydrogen. They react more vigorously with acids but do not react with bases.

According to Hund's rule, electron shells are stable when empty, full or half-full. For example, the configurations $4 \mathrm{f}^0\left(\mathrm{La}^{3+}\right), 4 \mathrm{f}^7\left(\mathrm{Gd}^{3+}\right)$, and $4 \mathrm{f}^{14}\left(\mathrm{Lu}^{3+}\right)$ are stable. $\mathrm{Ce}^{3+}, \operatorname{Pr}^{3+}$, and $\mathrm{Tb}^{3+}$ have one or two more electrons than required for stable electronic configurations so they can be further oxidized to a $4+$ state. In contrast, $\mathrm{Sm}^{3+}, \mathrm{Eu}^{3+}$, and $\mathrm{Yb}^{3+}$ have one or two less electrons than required for a stable electronic configuration and they, therefore, tend to receive one or two electrons and undergo a reduction to a $2+$ state. These are the reasons for these elements having abnormal valence states.

Standard reduction potentials, $E_{\mathrm{Ln}^{4+}}^{\circ} / \mathrm{Ln}^{3+}$ and $E_{\mathrm{Ln}^{3+}}^{\circ} / \mathrm{Ln}^{2+}$, represent the driving force stability of the reduction state. The more positive the value of $E_{\mathrm{red}}^{\circ}$ the greater the driving force for reduction. The standard reduction potentials of rare earths are shown in Table $1.3$.

The data shown in the table indicate that when comparing $E_{\mathrm{Ce}^{4+}}^{\circ} / \mathrm{Ce}^{3+}$ with $E_{\mathrm{Tb}^{4+}}^{\circ} / \mathrm{Tb}^{3+}$ electronic pairs, to act as an oxidizing agent, $\mathrm{Tb}^{4+}$ is stronger than $\mathrm{Ce}^{4+}$; to act as a reducing agent $\mathrm{Ce}^{3+}$ is stronger than $\mathrm{Tb}^{3+}$. When comparing $E_{\mathrm{Eu}^{3+} / \mathrm{Eu}^{2+}}^{\circ}$ with $E_{\mathrm{Yb}^{3+} / \mathrm{Yb}^{2+}}^{\circ}$, to act an oxidizing agent $\mathrm{Yb}^{2+}$ is stronger than $\mathrm{Eu}^{2+}$ under the standard conditions. Figure $1.11$ visualizes this trend. The transverse axis is the atomic number and the length of the short lines along the vertical axis represents the trend of valence state variation. [Rare earth coordination chemistry: fundamentals and applications]

电离能(Ionization Energies)

注：
(1) 上图也是来自Lanthanide and Actinide Chemistry
(2) 电离势(ionization potential)和电离能是同一个意思，但是前者是an older and obsolete term。

电离辐射(Ionizing radiation)

按照辐射作用于物质时所产生的效应不同，人们将辐射分为电离辐射与非电离辐射两类。电离辐射包括宇宙射线、X射线和来自放射性物质的辐射。非电离辐射包括紫外线、热辐射、无线电波和微波。电离辐射是指携带足以使物质原子或分子中的电子成为自由态，从而使这些原子或分子发生电离现象的能量的辐射。

待续。。。。

价态调控及其应用

上图来自[Tuning the oxidation states of dopants: a strategy for the modulation of material photoluminescence properties-Chemistry–A European Journal, 2021]

多种价态共存-概述

For multivalent dopants (e.g. Ce, Eu or Mn) a reduction is also often required. For cerium this step clearly aims at stabilizing the luminescent Ce³⁺ instead of the commonly non-luminescent Ce⁴⁺. Even though, the possible coexistence of the two oxidations states of the dopant is not much considered, and a single oxidation state of the activator only is often expected to be stabilized. However, even under severe reducing conditions, a mixed valence may exist.

YAG:Ce即使在强还原气氛下处理，XPS测试也显示出Ce⁴⁺的存在；
CaSc₂O₄:Ce，slight Ce³⁺ concentration modifications导致drastic luminescence efficiency changes.
Strong reductions might not always be ideal to reach the best efficiencies.
Since reduction leads to a homogeneous distribution of the dopants with different oxidation states in the materials, these results are of great importance for the control of a high diversity of tunable properties such as luminescence, absorption, conductivity or magnetism. The better understanding of the dopants reduction mechanism and its control will help improving a large variety of applications, among them: LEDs, sensors, transparent semi-conductors, bio-imaging.
以上来自[Applied Materials Today-2020]

自还原理论

我的描述：In general, to obtain Eu²⁺-activated luminescent materials, a reduction-atmosphere of H₂, H₂/N₂, or CO is always required because of the necessary reduction process of Eu³⁺ → Eu²⁺. However, in some unique compounds, like phosphate Ba₃(PO₄)₂, borate SrB₄O₇, silicate BaMgSiO₄ and aluminate Sr₄Al₁₄O₂₅, it was found that the self-reduction of Eu³⁺ to Eu²⁺ could occur in an air or O₂ atmosphere at high temperatures. This anomalous self-reduction phenomenon can be partially ascribed to the rigid three-dimensional (3D) structures, with tetrahedral anion groups such as BO₄, SiO₄, AlO₄, or PO₄.

Four conditions that seem to be necessary for the reduction Eu³⁺→ Eu²⁺ in solid state compounds when prepared in air at high temperature.

no oxidizing ions are present in the host compounds;
the dopant trivalent Eu³⁺ ion replaces a divalent cation in the host;
the substituted cation has a similar radius to the divalent Eu²⁺ ion;
the host compound has an appropriate structure, based upon tetrahedral anion groups (BO₄, SO₄, PO₄ or AlO₄).

最近段昌奎发表了First-principles rationalization of self-reduction and prediction of defect levels for the lanthanide series in SrB4O7

以上是来自传统的自还原理论，下面详细记录一下Markus Suta的文章。

Inorganic hosts, such as SrB₄O₇ or certain nitrides, intrinsically stabilize Eu²⁺ even when the dopant is an Eu³⁺-based precursor and reducing conditions are not employed in the synthesis. 作者展示了trapped charge carriers, such as color centers (localized at defects), can also act as redox partners to stabilize certain oxidation states of activators.
Chemically, the stabilization of divalent lanthanides in inorganic host compounds is still a challenging task for most of these species and limits potential application areas. Eu³⁺的还原最容易，也最常见(还原化学势可以看出来)。对于Eu²⁺-based或者Eu²⁺-activated solid compounds来说，想要从Eu³⁺-based precursors得到Eu²⁺，需要
- - 将原料还原：with H₂ gas at elevated temperatures (> 400 ^oC), but also with LiH or liquefied NH₃.
  - 特殊host (oxidizable soft anions)：containing polarizable anions with appreciable covalent bonding character, such as iodides, hydrides, heavy chalcogenides, and nitrides, readily stabilize Eu²⁺, sometimes even upon use of Eu³⁺-based starting materials.
    原因：The stability of Eu²⁺ in the previously mentioned classes of compounds is chemically intuitively evident by means of Pearson's hard and soft acid and base (HSAB) principle. Moreover, these anions are generally oxidizable and it is thus also mechanistically clear how Eu³⁺ ions can be readily reduced in those compounds even without explicit reducing conditions during the synthesis at high temperatures as elemental by-products stemming from the anions can also be formed then.
  - SrB₄O₇：没有oxidizable soft anions，the mechanism of the inherent reduction is still under debate. It was speculated that the intrinsic reducing action of SrB₄O₇ stems from potential charge-compensating defects present in SrB₄O₇ that allow reduction of the trivalent activators. Alternatively, a potential impact of the rigid network built up by the composing [BO₄]^5- units was also anticipated, and it could be shown that this is clearly related to a long-range ordered crystalline structure of the host material.
Defects的多面性：
- - 猝灭发光：Mostly, defects are considered undesirable in phosphors as they can act as trapping sites for charge carriers and thereby diminish the luminescence efficiency or completely quench it.
  - 长余辉：In fact, this particular, apparently undesirable feature of defects has found a useful application in the field of persistent phosphors.
  - 稳定价态：However, a role as redox-active centers to help stabilize luminescent dopants in a desirable oxidation state could prove to be a beneficial alternative concept for a simple chemical access to new phosphors.
STEs: are commonly encountered in both halidoperovskites and other halides, especially if the band gap is successively lowered and they obtain more semiconducting character.
CsMgX₃基质的特点：
- - 特殊的晶体结构feature strongly favors polaron formation and self-trapping of excitons within these chains.
  - Eu²⁺和Mg²⁺的半径不匹配 favors exciton localization at the Eu²⁺ impurities based on the very strong induced distortion as was also demonstrated by us earlier.

标号上面两个分别为(a)和(b)，下面两个分别为(c)和(d)；
图(b)中的红色宽带发光只有在低温下才能观察到，室温下完全猝灭，作者认为这个发光峰对应于 F centers，更确切地说是activator localized F_A center，因为：
- - 峰位置、宽度都和文献报道的F centers的发光类似；
  - UV激发，观察到Eu³⁺的发光，图(c)。
图(d)，检测Eu³⁺的发光，得到的激发光谱是和Eu²⁺的激发谱类似。
Thus, the observed luminescence of Eu³⁺ is intrinsically related to the presence of Eu²⁺ ions, which is already a strong indication for the anticipated charge carrier trapping mechanism explained above.
测试了F_A center的寿命为0.1 ms, which is expected for defects。
采用EuCl₃作为原料，no reductive conditions during synthesis yet leads to an observation of strong Eu²⁺.
为了排除a potential thermally induced reduction of EuCl₃ during synthesis，采用了Eu₂O₃，同样得到了Eu²⁺的发光。
Eu³⁺作为 potential electron scavengers，之前Eu²⁺作为原料观察到的 broad red emission band在这里vanishes，而部分没有被还原的Eu³⁺的发光现在可以观察到了。
the Eu³⁺-IL-STE entity now more resembles a “Eu²⁺ + h⁺” Frenkel-type exciton. (紧束缚激子或者叫弗仑克尔(Frenkel)激子，它的束缚半径比较小，大约在一个原子的范围内。激子是一种localized，想要变为delocalized需要吸收足够的激子结合能。)
对于之前EuCl₂作原料的情况，作者说Eu²⁺-IL-STE entity can be chemically envisioned as a “Eu³⁺ + e^-” Frenkel-type exciton.
文章最后也提到了，Strong evidence for the presence of trapped electrons was given by combined thermoluminescence and electron paramagnetic resonance studies in the borate Zn₄B₆O₁₃. 在A Meijerink的这篇文章中，作者通过测试UV辐照后的样品的EPR表面electrons trapped in two types of oxygen vacancies, electrons trapped on or near Mn²⁺ impurities and holes trapped by borate groups.

参考文献：
(1) Z. W. Pei, Q. Su and J. Y. Zhang, J. Alloys Compd., 1993, 198, 51.
(2) M. Y. Peng, Z. W. Pei, G. Y. Hong and Q. Su, J. Mater. Chem., 2003, 13, 1202
(3) Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials-Angew-2020

原料(或者加还原剂)和气氛(热处理工艺)调控价态

(1) Eu原料
stabilization of Eu²⁺ could be improved when the doping was executed using EuF₂ or EuS in the reacting mixture of oxides. 这句话来自[JAC-Eugeniusz Zych-2020] 比如[M.Nikl-JPCC-2016]用的就是EuS、EuF₂。[Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials-Angew-2020] 也用了EuCl₂。

(2) 湿化学合成中的还原剂
amine作为reducing agent制备了EuSe nanocrystals，观察到了Eu²⁺的发光[A Simple Reducing Approach Using Amine To Give Dual Functional EuSe Nanocrystals and Morphological Tuning-Angew-2011]。

(3) 气氛-CO
By adding fluxes and sintering in a CO reducing atmosphere, the IQE is greatly enhanced to 92.3%. 之前别人报道的相同基质掺杂Cr³⁺效率不高是因为Cr³⁺ luminescence suffered from impurities and oxidation of Cr⁴⁺ when the material was sintered in air.
[Strategies to approach high performance in Cr³⁺-doped phosphors for high-power NIR-LED light sources] 之前研究的固相法烧制的Cr³⁺多集中在含Ga/Ge的基质，这些元素在还原性气氛下容易挥发，不能形成纯相。
我：注意气氛既可以是烧制过程中用还原性气氛，也可以是air中烧制好后，在还原气氛下post-treatment。

(4) 各种气氛-概述
One valence state of dopant can be preferentially stabilized at the expense of the other according to the partial pressure of oxygen. Thus a highly oxidative atmosphere such as O₂ or air, will favor the stabilization of the oxidized form of dopant. On the contrary, with the use of a reductive atmosphere such as CO or H₂, the reduced form of dopant will be predominantly stabilized. This has been demonstrated for example through the study of cerium-doped materials.
Thus, under oxidative atmosphere (in air), oxidation of Ce³⁺ ions into Ce⁴⁺ is favored, which leads to the decrease of the luminescence intensity. In reductive atmosphere (CO), the ratio Ce³⁺/Ce⁴⁺ increases, improving the luminescence intensity. In N₂ atmosphere,the material is isolated from any oxidative or reducing sources. Thus, an intermediate emission intensity corresponding to a limited stabilization of Ce³⁺ ions is observed.The charge balance is preserved owing to the formation (respectively filling) of oxygen vacancies in reducing (respectively oxidative) atmosphere. The study of Ce⁴⁺/Ce³⁺ doped materialsillustra tes the possibility of tuning the ratio of the dopant valence state according to synthesis conditions, which can be generalized to most of thedopants.
[Tuning the oxidation states of dopants: a strategy for the modulation of material photoluminescence properties-Chemistry–A European Journal, 2021]

(5) oxygen getter(hydrides)
Controlled reduction of dopant can also be performed using oxygen getter such as hydrides (e.g. CaH₂, NaH, LiH) as reducing agent. This method can be implemented after material synthesis if the dopants are the only reducible elements in the material. It consists in the thermally activated disinsertion of oxygen from the crystal structure of the phosphor. Disinserted oxygen can then react with the getter to form thermodynamically stable oxides. Consequently, the dopants are reduced to keep the charge balance. Since this process is temperature and time dependent, it is possible to accurately control the valence states of dopants as it has been demonstrated by Behrh etal. through the investigation of SrAl₂O4:Eu phosphor. The reduction of Eu³⁺ ions into Eu²⁺ is performed with CaH₂ as oxygen getter. With the increase of reduction temperature, the Eu²⁺ emission intensity increases at the expense of Eu³⁺ emission intensity.
[A Chemical Route Towards Single-Phase Materials with Controllable Photoluminescence-Angew-2015] [Tuning the oxidation states of dopants: a strategy for the modulation of material photoluminescence properties-Chemistry–A European Journal, 2021]

Electron beam(处理)和laser beam(合成/处理)

electron beam参考菲利普的ome，

laser beam: Comparison among the spectra revealed the presence of the single trivalent state of Tb in the YAG laser-sintered ceramics. These results agree with those previously reported that suggested laser sintering in air can create a reducing environment at high temperature and this causes the release of oxygen, creating oxygen vacancies inside the sample. This oxygen deficiency associated with the high cooling rate promote the stabilization of the lower chemical state of rare earthss, including Eu²⁺ and Ce³⁺, and transition metals, Ti³⁺, even when the samples were sintered in air. Therefore, during the sample cooling down, the re-oxidation is frustrated and just the Tb³⁺ became stable at room temperature. [Laser sintering and photoluminescence study of Tb-doped yttrium aluminum garnet ceramics]

共掺杂调控价态(电荷补偿)

概述：In order to prevent defect formation, charge compensators can also be inserted inside the material. This strategy is widely investigated for preferential stabilization of one oxidationstate of dopant, leading to the enhancement of luminescence properties. Li⁺, N^a+, and K⁺ are among the most widely used monovalent charge compensators but other multivalent charge compensators such as Mg²⁺, Si⁴⁺ or Ge⁴⁺ ions have alsobeen investigated.
[Tuning the oxidation states of dopants: a strategy for the modulation of material photoluminescence properties-Chemistry–A European Journal, 2021]

(1) 共掺杂Mg²⁺/Ca²⁺

T. Murata等在2005年在CaAl₁₂O₁₉:Mn⁴⁺中通过添加MgF₂或者CaF₂来促进Mn⁴⁺的发光[JL-2005]，减少样品中Mn2+的出现。如图是CaAl₁₂O₁₉:Mn⁴⁺和加入了 CaF₂/MgF₂的CaAl₁₂O₁₉:Mn⁴⁺的EPR谱，虽然作者没有发现Mn²⁺的发光，但是在 CaAl₁₂O₁₉:Mn⁴⁺样品中仍显示出明显的Mn²⁺的信号，而加入CaF₂/MgF₂之后， Mn²⁺的信号消失，以Mg²⁺为例，作者把原因归结为Mn⁴⁺-O-Mn²⁺这种电荷补偿形式被Mn⁴⁺-O-Mg²⁺所取代。MgF₂不仅可以作为助熔剂，促进高温固相反应的进行，也作为电荷补偿剂，抑制了Mn²⁺，因而最终Mn⁴⁺的发光得到了显著增强。

Lu₃Al₅O₁₂:Ce,Mg Optical Ceramics Scintillators [AOM-2016]通过共掺杂Mg²⁺，促进了Ce³⁺部分地转变为Ce⁴⁺。The Ce⁴⁺ center, however, does not impair the scintillation performance due to its capability to positively influence the scintillation process. The importance of simultaneous application of such co-doping and annealing treatment is also demonstrated. With 0.3 at% Mg, our ceramics display a light yield of ≈25000 photons/MeV with short 1 μs shaping time, a relative fast component intensity as high as 60%, and very low afterglow.

In Ce³⁺-doped garnet scintillators, Ce⁴⁺ is intentionally added to accelerate the decay and, hence, improve the time response of the radiation detector. [Charge transfer from Eu²⁺ to trivalent lanthanide co-dopants: Systematic behavior across the series-JCP-2021]

闪烁体缺陷工程，补充文献1，文献2，文献3

Role of Ce4+ in the Scintillation Mechanism of Codoped Gd3Ga3Al2O12∶Ce

The Carrier Capture and Recombination Processes in Ln3+-Activated Scintillators

(2) 共掺杂Ln³⁺/Bi³⁺

潘跃晓等人研究了在 CaAl₁₂O₁₉中[RSC-2013]，通过共掺杂三价离子(Bi³⁺， La³⁺等) ,在空气气氛下实现 Mn⁴⁺→Mn²⁺空气气氛下的自还原。

(3) 共掺杂Hf⁴⁺

Stable Eu²⁺ ions in LuPO₄ and YPO₄ single crystals were obtained by co-doping with Hf⁴⁺. [JAC-2020]

Electron transfer between Eu and Tb in complex fluorides

(4) 共掺杂Li⁺

[Li substituent tuning of LED phosphors with enhanced efficiency, tunable photoluminescence, and improved thermal stability-夏志国-Science Advances-2019]

补充

费米能级和稀土价态-Dorenbos

Dorenbos finally indicated that the redox stability of a given oxidation state of a lanthanide ion in an inorganic host should depend on the relative position between the ground state of the incorporated activator within the band gap and the Fermi level of the inorganic host material. [Underestimated Color Centers: Defects as Useful Reducing Agents in Lanthanide-Activated Luminescent Materials-Angew-2020]

参考[Dorenbos_CM_2005]，这篇文章的主要目的不是深入解释镧系稀土离子价态的稳定性(在特定基质中)。目的是提供一个包含不同类别化合物的overview。采用phenomenological approach的方法来indetify价态的稳定性，研究的参数包括阴离子的种类，格位的大小，以及材料的band gap。

费米能级处于禁带中央，基本不随温度变化(简化处理)，近似认为费米能级的位置在低掺杂浓度下不变，在低温热平衡下，E_F以下的能级被占据，而E_F之上的能级没有被占据。Energy difference between the electron in the Eu²⁺ ground state and the Fermi energy. $$E_{\mathrm{Ff}} \equiv E^{\mathrm{CT}}-0.54 E^{\mathrm{ex}}$$这里的0.54来源于1.08的一半，$E^{\mathrm{ex}}$表示的是the energy needed to excite the host lattice (i.e. the energy to create a bound electron-hole pair (exciton))。

按照Dorenbos的说法，如果Eu²⁺基态的能级高于费米能级($E_{\mathrm{Ff}}>0$)，那么Eu³⁺更稳定，如果Eu²⁺的能级低于费米能级，那么Eu²⁺更稳定。按照Dorenbos的说法，还原性气氛使得费米能级上移，氧化氢气氛使得费米能级下移，他还讨论了电荷补偿剂对Eu能级的影响(局域电荷补偿，不影响费米能级的位置，但是会影响Ln²⁺和Ln³⁺基态的位置。)。可以参考下图

规律总结：

对氧化物来说，$E_{\mathrm{Ff}}$都是大于零，也就是说倾向于形成Ln³⁺；
Ln半径越大，$E_{\mathrm{Ff}}$越小；
bandgap越大，$E_{\mathrm{Ff}}$越小；
Indeed there are very few reports about Eu²⁺ on trivalent rare earth sites in oxide compounds even when prepared under reducing conditions. (这是因为Eu²⁺的激发态往往很靠近导带或者直接在导带里面，导致发光不强或者猝灭)
对于硼酸盐来说，the rigid tetrahedron structure of anions prevents the attack of oxygen from air on Eu during synthesis. This is also the case for tetrahedral phosphate, silicate and aluminate group；
SrB₄O₇这个物质值得特别注意，在这种基质里面，除了可以稳定Eu²⁺，也可以稳定Sm²⁺。它的禁带宽度在氧化物里面来说很大(10 eV)。
$E_{\mathrm{Ff}}$比较大时，比如CaO (1.2 eV)、SrO (1.5 eV)，即使还原气氛但是还是不能被完全还原。

Jonas上课讲的是Eu³⁺/Eu²⁺能级和Eu²⁺/Eu³⁺能级，这种表示方法应该更科学合理，需要拓展这一部分。

应用：

Eu²⁺掺杂的样品发光很弱。The energy difference between the first 5d state of Eu²⁺ and the bottom of the CB is 0.1 eV, which is consistent with the weak luminescence of Eu²⁺ in CaMgSiO₄ and an incomplete reduction in Eu³⁺ in it. The energy difference (E_Ff) between the 4f ground level of Eu²⁺ and the Fermi level for CaMgSiO₄ is calculated to be 0.95 eV, and this high E_Ff value may account for the incomplete reduction in Eu²⁺ in this compound. [Photoluminescence Properties and Valence Stability of Eu in CaMgSiO4-JES-2010]

Degradation和价态变化

(1) [Thermal degradation of the green-emitting SrSi₂O₂N₂:Eu²⁺ phosphor for solid state lighting-JMCC-2014] the thermal degradation of SrSi₂O₂N₂:Eu²⁺ can be ascribed to (i) oxidation of activators (Eu²⁺ → Eu³⁺) and (ii) oxidation of the host crystal (SrSi₂O₂N₂ → SrSiO₃).

压力导致的价态变化

[Pressure-Induced Valence and Structural Changes in YbMn₂Ge₂—Inelastic X-ray Spectroscopy and Theoretical Investigations-IC-2013]

YAG中Yb²⁺和Yb³⁺的动态变化

参考[Dynamics of Yb²⁺ to Yb³⁺ ion valence transformations in Yb:YAG ceramics used for high-power lasers—Optical Materials—2020]

Cr3+/Cr4+的调控

[Controlling Cr3+/Cr4+ concentration in single-phase host towards tailored super-broad NIR luminescence for multifunctional applications]

能带工程调控Charge Transfer

参考[Control of electron transfer between Ce³⁺ and Cr³⁺ in the Y₃Al_5−xGa_xO₁₂ host via conduction band engineering-JMCC-2015]

不同价态离子含量的Quantification

It is important to note that it is not possible to precisely quantify the ratio of two oxidation states of one dopant using the photoluminescence properties only. Thus, the efficiencies of excitation and emission of activators depend strongly on many parameters including the excitation wavelength (see associated section), or absorption cross-section. Energy and charge transfers can also occur between ions, as well as non-radiative de-excitations (e.g. multiphonon relaxations via lattice vibration), leading to the variation of luminescence properties of emitting centers. Thus, in order to identify and quantify different oxidation states of dopant, several methods can be considered among which Electron Paramagnetic Resonance (EPR), Xray Photoelectron Spectroscopy (XPS), or X-ray Absorption Near Edge Spectroscopy (XANES).
[Tuning the oxidation states of dopants: a strategy for the modulation of material photoluminescence properties-Chemistry–A European Journal, 2021]

阅读[Method to Measure the Degree of Reduction of Eu³⁺ to Eu²⁺: How Anion and Cation Vacancies Influence the Degree of Reduction-JPCC-2021]

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