Electronic structure of hydrothermally synthesized single crystal U_0.22Th_0.78O₂

Abstract

Single crystals of ThO2, UO2, and their solid solutions, UxTh1–xO2, have been obtained through various hydrothermal growth conditions. This technique offers the better of two other growth processes: (i) single crystal purity as by photochemical growth of nanocrystals; and (ii) large/bulk sizes as obtained by the arc melt method. The band gap of the UxTh1–xO2 single crystal solid solution, along with the luminescence transition, have been characterized. The occupied and unoccupied structures are determined using ultraviolet and inverse photoemission spectroscopy and the electronic band gap was measured to be 3–4 eV. The strain of incorporating U into the ThO2 is analyzed through Vegard's law. In this crystal there are defect and impurity sites, likely arising from the kinetic growth process, giving rise to a similar yet slightly different optical gap evident with cathodoluminescence spectroscopy. There is a major luminescence feature spanning the range from 3.18 to 4.96 eV (250–390 nm) with a maximum at 4.09 eV (303 nm), corresponding with the measured electronic band gap. In this paper, the electronic properties of a solid solution U0.22Th0.78O2 are measured and interpreted compared to the pure actinide oxides, ThO2 and UO2.

Interband Transitions and Critical Points of Single‐Crystal Thoria Compared with Urania

“…Surface effects, not discussed here, are expected to be a persistent problem and caution is needed in interpreting experimental results with a strong surface contribution. For example, the photoelectric work function of the (111) hydrothermally grown UO 2 was measured at 3–4 eV, [ 39 ] and 3.19 ± 0.03 eV. [ 27 ] More recently, the photoelectric determined work function of nearly stoichiometric hydrothermally grown UO 2 (111) and (100) was measured to be 6.28 ± 0.36 and 5.80 ± 0.36 eV, respectively.…”

Interband Transitions And Critical Points Of Single Crystal Thoria Compared To Urania

“…Surface effects, not discussed here, are expected to be a persistent problem and caution is needed in interpreting experimental results with a strong surface contribution. For example, the photoelectric work function of the (111) hydrothermally grown UO2 was measured at 3-4 eV [20], and 3.19 ± 0.03 eV [41]. More recently, the photoelectric determined work function of nearly stoichiometric hydrothermally grown UO2(111) and (100) was measured to be 6.28 ± 0.36 eV and 5.80 ± 0.36 eV, respectively [21].…”

“…Single crystals of UO2 were grown by hydrothermal synthesis [12,13,[16][17][18][19][20][21][22], as described in [12,13,22]. Further hydrothermal growth information is detailed in [16].…”

“…Absorption of UO2 thin films, by optical transmission has also been measured [14], while the optical properties for both ThO2 and UO2 have been previously calculated [15] using the Heyd-Scuseria-Ernzerhof (HSE) functional. The challenges of growing uranium oxide singlecrystals have been addressed with the fabrication of high-quality, single crystal actinide oxide samples using a hydrothermal synthesis growth technique [12,13,[16][17][18][19][20][21][22]. This growth process has produced bulk single crystals of near-stoichiometric UO2 [12,13,17,[20][21][22] and ThO2 [13,18,19,22].…”

“…The challenges of growing uranium oxide singlecrystals have been addressed with the fabrication of high-quality, single crystal actinide oxide samples using a hydrothermal synthesis growth technique [12,13,[16][17][18][19][20][21][22]. This growth process has produced bulk single crystals of near-stoichiometric UO2 [12,13,17,[20][21][22] and ThO2 [13,18,19,22]. These improved single crystals make higher quality assessments of the UO2 and ThO2 optical and semiconductor properties possible, as defect contributions to the complex dielectric function are much suppressed [13].…”