Wide-Bandgap Rare-Earth Iodate Single Crystals for Superior X-Ray Detection and Imaging

Semiconductor-based X-ray detectors with low detectable thresholds become critical in medical radiography applications. However, their performance is generally limited by intrinsic defects or unresolved issues of materials, and developing a novel scintillation semiconductor for low-dose X-ray detection is a highly urgent objective. Herein, a high-quality rare-earth iodate Tm(IO₃ )₃ single crystal grown through low-cost solution processing is reported with a wide bandgap of 4.1 eV and a large atomic number of 53.2. The roles of IO and TmO groups for charge transport in the Tm(IO₃ )₃ are revealed with the structural difference between the [101] and [ 1 ¯ 01 ] $[{\bar{1}}01]$ crystal orientations. Based on anisotropic responses of material properties and detection performances, it is found that the [ 1 ¯ 01 ${\bar{1}}01$ ] orientation, the path with fewer IO groups, achieves a high resistivity of 1.02 × 10¹¹ Ω cm. Consequently, a single-crystal detector exhibits a low dark current and small baseline drifting due to the wide bandgap, high resistivity and less ion migration of Tm(IO₃ )₃ , resulting in a low detection limit of 85.2 nGy_air s^-1 . An excellent X-ray imaging performance with a high sensitivity of 4406.6 µC Gy_air ^-1 cm^-2 is also shown in the Tm(IO₃ )₃ device. These findings provide a new material design perspective for high-performance X-ray imaging applications.

Understanding the role of flux, pressure and temperature on polymorphism in ThB2O5

A novel polymorph of ThB₂O₅, denoted as β-ThB₂O₅, was synthesised under high-temperature high-pressure (HT/HP) conditions. Via single crystal X-ray diffraction measurements, β-ThB₂O₅ was found to form a three-dimensional (3D) framework structure where thorium atoms are ten-fold oxygen coordinated forming tetra-capped trigonal prisms. The only other known polymorph of ThB₂O₅, denoted α, synthesised herein using a known borax, B₂O₃-Na₂B₄O₇, high temperature solid method, was found to transform to the β polymorph when exposed to conditions of 4 GPa and ∼900 °C. Compared to the α polymorph, β-ThB₂O₅ has smaller molar volume by approximately 12%. Exposing a mixture of the α and β polymorphs to HT/HP conditions ex situ further demonstrated the preferred higher-pressure phase being β, with no α phase material being observed via Rietveld refinements against laboratory X-ray powder diffraction (PXRD) measurements. In situ heating PXRD measurements on α-ThB₂O₅ from RT to 1030 °C indicated that α-ThB₂O₅ transforms to the β variant at approximately 900 °C via a 1st order mechanism. β-ThB₂O₅ was found to exist only over a narrow temperature range, decomposing above 1050 °C. Ab initio calculations using density functional theory (DFT) with the Hubbard U parameter indicated, consistent with experimental observations, that β is both the preferred phase at higher temperatures and high pressures. Interestingly, it was found by switching from B₂O₃-Na₂B₄O₇ to H₃BO₃-Li₂CO₃ flux using consistent high temperature solid state conditions for the synthesis of the α variant, β-ThB₂O₅ could be generated. Comparison of their single crystal measurements showed this was identical to that obtained from HT/HP conditions.

Enhancing photosensitivity via the assembly of a uranyl coordination polymer

Synergistic assembly of uranyl centres and luminescent 2,6-bis(pyrazol-1-yl)pyridine-4-carboxylates (bppCOOH) gives rise to a uranyl coordination polymer, namely U-bppCOO, which exhibits a luminescence quenching response toward UV or X-ray irradiation doses. Notably, the photosensitivity of U-bppCOO has been significantly enhanced via metal-ligand assembly compared with that of the naked bppCOOH ligand.

Transuranium Sulfide via the Boron Chalcogen Mixture Method and Reversible Water Uptake in the NaCuTS3 Family

The behavior of 5f electrons in soft ligand environments makes actinides, and especially transuranium chalcogenides, an intriguing class of materials for fundamental studies. Due to the affinity of actinides for oxygen, however, it is a challenge to synthesize actinide chalcogenides using non-metallic reagents. Using the boron chalcogen mixture method, we achieved the synthesis of the transuranium sulfide NaCuNpS₃ starting from the oxide reagent, NpO₂. Via the same synthetic route, the isostructural composition of NaCuUS₃ was synthesized and the material contrasted with NaCuNpS₃. Single crystals of the U-analogue, NaCuUS₃, were found to undergo an unexpected reversible hydration process to form NaCuUS₃·xH₂O (x ≈ 1.5). A large combination of techniques was used to fully characterize the structure, hydration process, and electronic structures, specifically a combination of single crystal, powder, high temperature powder X-ray diffraction, extended X-ray absorption fine structure, infrared, and inductively coupled plasma spectroscopies, thermogravimetric analysis, and density functional theory calculations. The outcome of these analyses enabled us to determine the composition of NaCuUS₃·xH₂O and obtain a structural model that demonstrated the retention of the local structure within the [CuUS₃]^- layers throughout the hydration-dehydration process. Band structure, density of states, and Bader charge calculations for NaCuUS₃, NaCuUS₃·xH₂O, and NaCuNpS₃ along with X-ray absorption near edge structure, UV-vis-NIR, and work function measurements on ACuUS₃ (A = Na, K, and Rb) and NaCuUS₃·xH₂O samples were carried out to demonstrate that electronic properties arise from the [CuTS₃]^- layers and show surprisingly little dependence on the interlayer distance.

Recent advances in the applications of thorium-based metal-organic frameworks and molecular clusters

This perspective highlights the recent advances in the structural and practical aspects of thorium-based metal-organic frameworks (Th-MOFs) and molecular clusters. Thorium, as an underexplored actinide, features surprisingly rich coordination geometries and accessibility of the 5f orbital. These features lead to a myriad of topologies and electronic structures, many of which are undocumented for other tetravalent metal-containing MOFs or clusters. Moreover, Th-MOFs inherit the modularity, structural tunability, porosity, and versatile functionality of the state-of-the-art MOFs. Recognizing the radioactive nature of these thorium-bearing materials that may limit their practical uses, Th-MOFs and Th-clusters still have great potential for various applications, including radionuclide sequestration, hydrocarbon storage/separation, radiation detection, photoswitch, CO₂ conversion, photocatalysis, and electrocatalysis. The objective of this updated perspective is to propose pathways for the renaissance of interest in thorium-based materials.

Advances and perspectives of actinide chemistry from ex situ high pressure and high temperature chemical studies

High pressure high temperature (HP/HT) studies of actinide compounds allow the chemistry and bonding of among the most exotic elements in the periodic table to be examined under the conditions often only found in the severest environments of nature. Peering into this realm of physical extremity, chemists have extracted detailed knowledge of the fundamental chemistry of actinide elements and how they contribute to bonding, structure formation and intricate properties in compounds under such conditions. The last decade has resulted in some of the most significant contributions to actinide chemical science and this holds true for ex situ chemical studies of actinides resulting from HP/HT conditions of over 1 GPa and elevated temperature. Often conducted in tandem with ab initio calculations, HP/HT studies of actinides have further helped guide and develop theoretical modelling approaches and uncovered associated difficulties. Accordingly, this perspective article is devoted to reviewing the latest advancements made in actinide HP/HT ex situ chemical studies over the last decade, the state-of-the-art, challenges and discussing potential future directions of the science. The discussion is given with emphasis on thorium and uranium compounds due to the prevalence of their investigation but also highlights some of the latest advancements in high pressure chemical studies of transuranium compounds. The perspective also describes technical aspects involved in HP/HT investigation of actinide compounds.

Unveiling the new function of uranyl molecular clusters as fluorometric sensors for UV and X-ray dosimetry

Simple synthetic modulation based on uranyl acetate and phenanthroline has resulted in two uranyl clusters (1 and 2) with different topologies and nuclearities. Notably, the dimeric complex exhibits distinct luminescence quenching upon UV and X-ray irradiation with detection limits of 4.30 × 10^-6 J and 0.32 Gy, respectively. To advance the practical application, 1 was further fabricated with polyvinylidene fluoride into a flexible strip as a UV and X-ray indicator.

Luminometric dosimetry of X-ray radiation by a zwitterionic uranium coordination polymer

A novel X-ray dosimeter based on a uranium coordination polymer has been developed by the judicious synergy between the luminescent uranyl centres and zwitterionic tritopic ligands.

Emergence of Thorium-Based Polyoxo Clusters as a Platform for Selective X-ray Dosimetry

A brand-new application of thorium-bearing clusters in the field of ionizing radiation detection is exemplified by two novel hexanuclear thorium clusters, Th-bppCOO-1 and Th-bppCOO-2, which incorporate carboxylate-functionalized 2,6-di(pyrazol-1-yl)pyridine ligands. Notably, Th-bppCOO-1 is composed of an unprecedented [Th₆(OH)₄O₄(H₂O)₅]¹²⁺ secondary building unit, the Th₆ core of which is decorated by five H₂O molecules. Furthermore, selective photoluminescence quenching responses of Th-bppCOO-1 and Th-bppCOO-2 toward X-ray over UV radiation have been demonstrated for the first time.

The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

Herein, the novel uranyl selenate and selenite compounds Rb2[(UO2)2(SeO4)3], Rb2[(UO2)3(SeO3)2O2], Rb2[UO2(SeO4)2(H2O)]·2H2O, and (UO2)2(HSeO3)2(H2SeO3)2Se2O5 have been synthesized using either slow evaporation or hydrothermal methods under acidic conditions and their structures were refined using single crystal X-ray diffraction. Rb2[(UO2)2(SeO4)3] synthesized hydrothermally adopts a layered 2D tetragonal structure in space group P42/ncm with a = 9.8312(4) Å, c = 15.4924(9) Å, and V = 1497.38(15) Å, where it consists of UO7 polyhedra coordinated via SeO4 units to create units UO2(SeO4)58− moieties which interlink to create layers in which Rb+ cations reside in the interspace. Rb2[(UO2)3(SeO3)2O2] synthesized hydrothermally adopts a layered 2D triclinic structure in space group P1¯ with a = 7.0116(6) Å, b = 7.0646(6) Å, c = 8.1793(7) Å, α = 103.318(7)°, β = 105.968(7)°, γ = 100.642(7)° and V = 365.48(6) Å3, where it consists of edge sharing UO7, UO8 and SeO3 polyhedra that form [(UO2)3(SeO3)2O2] layers in which Rb+ cations are found in the interlayer space. Rb2[UO2(SeO4)2(H2O)]·2H2O synthesized hydrothermally adopts a chain 1D orthorhombic structure in space group Pmn21 with a = 13.041(3) Å, b = 8.579(2) Å, c = 11.583(2) Å, and V = 1295.9(5) Å3, consisting of UO7 polyhedra that corner share with one H2O and four SeO42− ligands, creating infinite chains. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 synthesized under slow evaporation conditions adopts a 0D orthorhombic structure in space group Cmc21 with a = 28.4752(12) Å, b = 6.3410(3) Å, c = 10.8575(6) Å, and V = 1960.45(16) Å3, consisting of discrete rings of [(UO2)2(HSeO3)2(H2SeO3)2Se2O5]2. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 is apparently only the second example of a uranyl diselenite compound to be reported. A combination of single crystal X-ray diffraction and bond valance sums calculations are used to characterise all samples obtained in this investigation. The structures uncovered in this investigation are discussed together with the broader family of uranyl selenates and selenites, particularly in the context of the role acidity plays during synthesis in coercing specific structure, functional group, and topology formations.

High-Performance Second-Harmonic-Generation (SHG) Materials: New Developments and New Strategies

ConspectusSecond-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ² = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO₃^3--based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO₃^3- and NO₃^-) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO₄-based compounds, highly polarizable cations such as Hg²⁺ and Bi³⁺ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I⁵⁺ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO₆ octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.

Beyond structural motifs: the frontier of actinide-containing metal-organic frameworks

In this perspective, we feature recent advances in the field of actinide-containing metal-organic frameworks (An-MOFs) with a main focus on their electronic, catalytic, photophysical, and sorption properties. This discussion deviates from a strictly crystallographic analysis of An-MOFs, reported in several reviews, or synthesis of novel structural motifs, and instead delves into the remarkable potential of An-MOFs for evolving the nuclear waste administration sector. Currently, the An-MOF field is dominated by thorium- and uranium-containing structures, with only a few reports on transuranic frameworks. However, some of the reported properties in the field of An-MOFs foreshadow potential implementation of these materials and are the main focus of this report. Thus, this perspective intends to provide a glimpse into the challenges, triumphs, and future directions of An-MOFs in sectors ranging from the traditional realm of gas sorption and separation to recently emerging areas such as electronics and photophysics.