Stability of Ce(III) activator and codopant effect in MHfO3 (M = Ba, Sr) scintillators by XANES
Stability of Ce(III) activator and codopant effect in MHfO3 (M = Ba, Sr) scintillators by XANES
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DOI：
10.1111/j.1551-2916.2004.00004.x
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发表时间：
2005-01-01
影响因子：
3.9
通讯作者：
Venkataramani, V
中科院分区：
文献类型：
--
作者：
Loureiro, SM;Gao, Y;Venkataramani, V
我们在多种还原条件下合成了具有不同电荷补偿机制（Lu、Al和Ta）的Ce激活BaHfO3和SrHfO3钙钛矿，并利用X射线吸收近边结构（XANES）研究了样品中Ce3+/Ce4+相对百分比的变化。我们发现铝和钽共掺杂比非共掺杂或镥共掺杂样品产生更高的相对 Ce3+。然而，似乎只有非常高的温度才能有效稳定 BaHfO3 主晶格中的 Ce3+。对于铪酸锶主晶格，我们发现在更温和的条件下可以获得更高的相对百分比的 Ce3+。这一结果表明，SrHfO3 是更好的基质，可以最大限度地提高铪酸盐钙钛矿中 Ce3+ 的发光性能。最近，发现了一类具有ABO3钙钛矿晶体结构的新型闪烁材料MHfO3：Ce（M5Ba，Sr，Ca），用于高能核医学应用。 1 我们发现，低含量的 Ce3+ 激活剂 (% 0.5–1%) 可在材料中产生理想的发光特性。 z 无补偿地用Ce3+取代结构A位中的碱土离子，会产生引起辐射损伤和余辉的缺陷。因此，需要合成具有作为共掺杂剂的附加元素的电荷补偿相，并彻底评估它们的性能。将铪酸盐粉末转化为透明陶瓷的过程需要材料完全致密化，同时仍保持活化剂的三价氧化态。烧结步骤期间的氧分压可能导致Ce3+氧化成Ce4+。激发和发射测量等光学技术可以很好地表明特定晶格位置是否存在 Ce3+。 Ce4+ 不是发光发射体，因此无法使用光学表征来确定。然而，这种物质的存在可能会导致闪烁体有效光输出的降低。 Ce XANES 实验在布鲁克海文国家实验室国家同步加速器光源光束线 X18B 上进行。对于 SrHfO3，测量是在 Ce L3 边缘进行的，该边缘通常用于根据标准 Ce3+ 和 Ce4+ 化合物的独特光谱特征确定氧化态。对于 BaHfO3，我们使用 Ce K 边缘，因为 Ce L3 边缘与 Ba L 边缘重叠。由于 Ce 浓度较低，因此使用 13 元素 Ge 探测器测量 X 射线荧光。入射 X 射线能量通过通道切割 Si (111) 单色仪进行调谐，L 边缘的扫描步长为 0.5 eV，K 边缘的扫描步长为 3 eV。 y 对于 L3 边缘测量，单色仪失谐 30%，以消除高次谐波。将一薄层样品粉末铺在透明胶带上并相对于入射光束和Ge探测器安装451。入射光束高度设置为 1 毫米，并根据入射强度调整其宽度，以将探测器死区时间减少到 25% 以下。通过与样品同时测量，整个测量过程中使用 CeO2 粉末进行能量校准。 CeF3 和CeO2 分别用作Ce3+ 和Ce4+ 的标准品。 z 在L3边缘，根据白线高度确定氧化态。在 K 边缘，氧化是通过相对边缘位移来确定的。使用WinXASJ程序对数据进行分析，并基于CeO2参考光谱进行校准，边缘位置设置为40.447 keV。对于Ce K边缘测量，吸收边缘定义在半高能量处。这 …
WE have synthesized Ce-activated BaHfO3 and SrHfO3 perovskites with different charge compensation mechanisms (Lu, Al, and Ta) under several reducing conditions, and studied the variation of the relative percentage of Ce3+/Ce4+ present in the samples using X-ray absorption near-edge structure (XANES). We have found that aluminum and tantalum codoping yield higher relative Ce3+ than non-codoped or lutetium codoped samples. However, only very high temperatures seem to effectively stabilize Ce3+ in the BaHfO3 host lattice. For the strontium hafnate host lattice we found that higher relative percentages of Ce3+ can be obtained at much milder conditions. This result indicates that SrHfO3 is a better host to maximize the luminescent properties of Ce3+ in the hafnate perovskites. Recently, a new class of scintillating materials MHfO3: Ce (M5Ba, Sr, Ca) with ABO3 perovskite crystal structure for high-energy nuclear medical applications has been discovered. 1 We have found that low levels of Ce3+ activator (% 0.5–1%) yield desirable luminescent properties in the material. z Uncompensated substitution of Ce3+ for the alkali-earth ion in the A-site of the structure, will result in defects causing radiation damage and afterglow. Therefore, the synthesis of charge compensated phases with additional elements acting as codopants needs to be carried out, and their properties thoroughly evaluated. The process used to convert hafnate powders to transparent ceramics requires full densification of material while still maintaining the trivalent oxidation state of the activator. The oxygen partial pressure during the sintering step may cause Ce3+ to oxidize to Ce4+. Optical techniques such as excitation, and emission measurements give good indication for the presence of Ce3+ at a particular lattice site. Ce4+ is not a luminescent emitter, and thus cannot be determined using optical characterization. However, the presence of this species may contribute to the decrease of the effective light output of the scintillator. The Ce XANES experiments were carried out at beamline X18B, National Synchrotron Light Source, Brookhaven National Laboratory. For SrHfO3, the measurements were performed at the Ce L3-edge, which is commonly used for oxidation state determination based on the distinct spectroscopic features from standard Ce3+ and Ce4+ compounds. For BaHfO3, we used Ce K-edge since Ce L3-edge overlaps with Ba L-edges. Due to the low Ce concentration, X-ray fluorescence was measured with a 13-element Ge-detector. Incident X-ray energy was tuned by a channel-cut Si (111) monochromator, and the scan step size for the L-edge was 0.5 eV and for the K-edge was 3 eV. y For L3-edge measurements, the monochromator was detuned by% 30% to eliminate higher harmonics. A thin layer of sample powders were spread over Scotch tape and mounted 451 with respect to the incident beam and the Ge detector. The incident beam was set at 1-mm high and its width was adjusted according to the incident intensity to decrease detector dead time below 25%. CeO2 powders were used for energy calibration throughout the measurement by simultaneous measurement with the sample. CeF3 and CeO2 were used as standards for Ce3+ and Ce4+, respectively. z At the L3-edge, the oxidation state was determined based on the white-line height. At the K-edge, the oxidation was determined by relative edge shift. The data were analyzed using WinXASJ program, and calibrated based on CeO2 reference spectrum with its edge position set to be 40.447 keV.For Ce K-edge measurements, the absorption edge was defined at the energy at half-height. This …