Georgy Dosovitskiy

The high surface-to-volume ratio in nanocrystals (NCs) enables surface energy effects that stabilize phases that are otherwise unstable in a bulk state. Double perovskites (DP) containing lithium show exactly this effect. Bulk Cs₂LiInCl₆ adopts a triclinic structure; however, we show here that the cubic phase can be stabilized in a nanocrystalline form at a wide range of lithium-sodium alloyed compositions. Density functional theory (DFT) calculations support this finding. Although bulk formation energy favors the triclinic structure for Li-rich compositions, the cubic phase becomes stable when the surface energy of the {100} facet is significant due to the large surface-to-volume ratio in nanocrystals. The importance of this Li-Na alloying is seen in the change in the physical properties, apparent in the controllable blue shift of the emission with increased Li content. This advantageous effect, which is also observed for Li-cation exchange in presynthesized colloidal nanocrystals, is overshadowed by a competing phase that we identify as an orthorhombic hydrate phase, Cs₂InCl₅·H₂O. We characterize its emergence and propose strategies to mitigate its impact.

Luminescent and scintillation ceramic materials with complex shapes, which can be created by stereolithography 3D printing, are of interest for special phosphor and detector applications. Starting powders for such ceramics may possess UV absorption bands; therefore, it is important to study the possible influence of the powders’ luminescent properties on the printing process. This paper deals with complex garnet oxides, Y₃Al₅O₁₂ and Gd₃Al₂Ga₃O₁₂—well-known hosts for luminescent materials. The photopolymerization rates of slurries based on the luminescent powders produced by various chemical routes are studied, as well as available printing regimes. The slurries containing Ce-doped powders with a broad absorption band in UV have significantly lower photopolymerization rates compared to the undoped ones; a high Ce doping virtually hinders printing with layers thicker than 25–50 μm. Furthermore, the choice of powder synthesis method is shown to influence the printing process. Slurries with Tb-doped powder, with absorption lines at shorter wavelengths, have good photopolymerization activity, close to that of the undoped powder, and can be printed with layer thicknesses of 25–100 μm.

Ceramics from cerium-doped quintuple garnet-type compounds (Gd,Y,Lu)₃Al₂Ga₃O₁₂:Ce with different contents of Y and Lu have been fabricated and studied as a prospective scintillating material enabling improvement of scintillation properties by tuning of the composition of the matrix-forming crystalline compound. The coprecipitation method was applied to produce powders of complex garnet compounds, which were subsequently compacted and sintered. The influence of the matrix composition on photoluminescence and scintillation of the ceramics is studied using luminescence spectroscopy at steady state and pulsed excitation. Partial substitution of Lu by Y ions was found to be an effective tool for tailoring of scintillation properties of the material. A light yield of the Gd₂Y_0.5Lu_0.5Al₂Ga₃O₁₂ composition was measured to be 41 000 ph/MeV, whereas a scintillation kinetics was measured to have dominant fast scintillation components: 14 ns (84%) and 78 ns (16%).

In the present article, the influence of the activator concentration and impurity content of raw materials on the luminescence and scintillation properties of Li₂CaSiO₄ was studied. Polycrystalline powder material was obtained by the sol–gel method. It was shown that europium had limited solubility in the host lattice with a limiting concentration proximate to 0.014 formula units. The maximum intensity of photoluminescence was observed with a divalent europium concentration of 0.002 formula units; the light yield under alpha-particle excitation was measured to be 21,600 photons/MeV for ~200 μm of coating, and under neutron excitation, it was calculated to be 103,800 photons/n, the scintillation kinetics was characterized by an effective decay time of 157 ns. These properties and the transparency in the visible spectrum make it possible to produce scintillation screens with a coating of Li₂CaSiO₄ for detecting neutrons, alpha particles and low-energy beta radiation. The low Z_eff (~15) of this compound makes it less sensitive to gamma rays. The 480 nm blue emission peak makes this material compatible with most commercial PMT photocathodes, CCD cameras and silicon photomultipliers, which have a maximum quantum efficiency in the blue–green spectral region.

Technological factors and processes contributing to the scintillation mechanism have been considered in quaternary garnet ceramics doped with Ce(Gd,Lu)₃Al₂Ga₃O₁₂. The super-stoichiometric additive of gadolinium in the material composition or its co-doping with a low concentration of Mg were found to be effective tools to suppress phosphorescence in the quaternary garnet, confirming that it is not an intrinsic property of the material. The Monte-Carlo simulation of electronic excitation transfer demonstrates that the hopping migration along the gadolinium sublattice plays an essential role in forming the scintillation kinetic parameters. Breaking the integrity of the gadolinium sublattice by substitution with heavier lutetium ions increases the role of self-trapped states in the excitation of Ce³⁺ ions, which ensures both an increase in the fraction of short ~20 ns and very long ~600 ns components in the scintillation kinetics.

This review article considers the latest developments in the field of inorganic scintillation materials. Modern trends in the improvement of inorganic scintillation materials are based on engineering their features at the nanoscale level. The essential challenges to the fundamental steps of the technology of inorganic glass, glass ceramics, and ceramic scintillation materials are discussed. The advantage of co-precipitation over the solid-state synthesis of the raw material compositions, particularly those which include high vapor components is described. Methods to improve the scintillation parameters of the glass to the level of single crystals are considered. The move to crystalline systems with the compositional disorder to improve their scintillation properties is justified both theoretically and practically. A benefit of the implementation of the discussed matters into the technology of well-known glass and crystalline scintillation materials is demonstrated.

The present study deals with the fabrication of light-reflecting materials used in pixelated scintillator detectors. For the first time, the reflecting surfaces for pixels of different sizes (from 0.8 to 3.2 mm) were obtained via a low-cost DLP 3D printing technique. The material for the reflectors was the new composite of transparent ultraviolet light-cured resin and TiO2 as a light-scattering filler. It was observed that TiO2 showed better performance compare to other pigments such as BaSO4, hBN or cubic zirconia. The object formation rate was about 1 cm per hour with the possibility to produce several parts simultaneously that simplifies the wrapping procedure. It was found that the regular grooves pattern of the fabricated parts (staircase effect) could increase a light collection from a scintillator. The reflective properties of such surfaces were comparable to conventional reflection coating (e.g., Teflon wrapping).Presented at the 2019 IEEE NSS & MIC conference, Manchester, UK. 14 pages, 12 figures, 1 table. Journal reference: Optical Materials V. 108, October 2020, p. 110393.