Polymorphs of the Gadolinite-Type Borates ZrB2O5 and HfB2O5 Under Extreme Pressure

Based on the results from previous high-pressure experiments on the gadolinite-type mineral datolite, CaBSiO₄(OH), the behavior of the isostructural borates β-HfB₂O₅ and β-ZrB₂O₅ have been studied by synchrotron-based in situ high-pressure single-crystal X-ray diffraction experiments. On compression to 120 GPa, both borate layer-structures are preserved. Additionally, at ≈114 GPa, the formation of a second phase can be observed in both compounds. The new high-pressure modification γ-ZrB₂O₅ features a rearrangement of the corner-sharing BO₄ tetrahedra, while still maintaining the four- and eight-membered rings. The new phase γ-HfB₂O₅ contains ten-membered rings including the rare structural motif of edge-sharing BO₄ tetrahedra with exceptionally short B−O and B⋅⋅⋅B distances. For both structures, unusually high coordination numbers are found for the transition metal cations, with ninefold coordinated Hf⁴⁺, and tenfold coordinated Zr⁴⁺, respectively. These findings remarkably show the potential of cold compression as a low-energy pathway to discover metastable structures that exhibit new coordinations and structural motifs.     

Role of nickel and manganese in recovery of resistivity in iron-based alloys after low-temperature proton irradiation

This study investigates the recovery of electric resistivity in pure iron, Fe–0.6Ni and Fe–1.5Mn as related to isochronal annealing following 1 MeV proton irradiation at lower temperature than 70 K, focusing on the relationship between solute atoms and irradiation defects. Both nickel and manganese prevent stage I_D recovery, which corresponds to correlated recombination. Stage II recovery is also changed by the addition of a solute, which corresponds to the migration of small interstitial clusters. In both pure iron and Fe–0.6Ni, no evident difference was observed in the stage III region, which corresponds to the migration of vacancies. In contrast, two substages appeared in the Fe–1.5Mn at a higher temperature than stage III_B appeared in pure iron. These substages are considered to represent the release of irradiation-induced defects, which was trapped by manganese.     

Interband and intraband radiation from the n-InGaAs/GaAs heterostructures with quantum wells under the conditions of injection in high lateral electric fields

The interband and intraband radiation from the n-InGaAs/GaAs heterostructures with the double and triple tunnel coupled and selectively doped quantum wells (QWs), which is appeared under the lateral electric field and in the presence of hole injection from the anode contact, has been investigated. A steep increase of the interband radiation intensity was found at the fields of E≥1.7 kV/cm. This effect should be related to the big lifetime of the injected charge carriers (~10⁻⁶ s) which exceeds by three orders of magnitude the lifetime in the similar bulk direct-band semiconductor. Its reason lies in spatial separation of the injected holes and electrons between coupled wells, firstly, by the built-in transverse electric field between wells and, secondly, due to the real-space transfer of carriers heated by the lateral electric field from the wide well to the narrow δ-doped one. Furthermore, an increase of the carrier concentration due to injection leads to an increase of that transition intensity and, consequently, to an intensity increase of the radiative intersubband transitions of carriers in QWs which results in a steep intensity increase of the far (50–120 µm) infrared radiation.     

Observation of radially polarized terahertz radiation generated by a sub-picosecond electron beam

The fundamental property of radially polarized terahertz radiation, which is axial symmetry of the polarization state in the radial direction, was measured with a Schottky diode detector and with a terahertz camera and wire-grid linear polarizer. Radially polarized terahertz radiation was generated from coherent transition radiation using a 30-MeV sub-picosecond electron beam. Bow-tie intensity distributions, aligned along the polarization direction, were clearly observed with the terahertz camera and could be rotated by changing the direction of the linear polarizer. The measured intensity distribution agreed with the calculated value. A video of the data can be found online.     

Structures,g-tensors,and hyperfine coupling constants of L-α-alanine radicals in radiation dosimetry: An ab initio molecular dynamics simulation study

Alanine is used as a transfer standard dosimeter for gamma ray and electron beam calibration. An important factor affecting its dosimetric response is humidity which can lead to errors in absorbed dose calculations. Ab initio molecular dynamics calculations were performed to determine the environmental effects on the electron paramagnetic resonance (EPR) parameters of L-α-alanine radicals in acidic and alkaline solutions. A new result, not dissimilar to the closed-shell amino acid molecule alanine, is that the non-zwitterionic form of the alanine radical is the stable form in the gas phase while the zwitterionic neutral alanine radical is not a stable structure in the gas phase. Geometric and EPR parameters of radicals in both gas and solution phases are found to be dependent on hydrogen bonding of water molecules with the polar groups and on dynamic solvation. Calculations on the optimized free radicals in the gas phase revealed that for the neutral radical, hydrogen bonding to water molecules drives a decrease in the magnitudes of g-tensor components g _xx and g _yy without affecting neither g _zz component nor the hyperfine coupling constants (HFCCs). The transfer from the gas to solution phase of the alanine radical anion is accompanied with an increase in the spin density on the carboxylic group's oxygen atoms. However, for the neutral radical, this transfer from gas to solution phase is accompanied with the decrease in the spin density on oxygen atoms. Calculated isotropic HFCCs and g-tensor of all radicals are in good agreement with experiment in both acidic and alkaline solutions.     

Nitride Spinel: An Ultraincompressible High‐Pressure Form of BeP2N4

Owing to its outstanding elastic properties, the nitride spinel γ‐Si₃N₄ is of considered interest for materials scientists and chemists. DFT calculations suggest that Si₃N₄‐analog beryllium phosphorus nitride BeP₂N₄ adopts the spinel structure at elevated pressures as well and shows outstanding elastic properties. Herein, we investigate phenakite‐type BeP₂N₄ by single‐crystal synchrotron X‐ray diffraction and report the phase transition into the spinel‐type phase at 47 GPa and 1800 K in a laser‐heated diamond anvil cell. The structure of spinel‐type BeP₂N₄ was refined from pressure‐dependent in situ synchrotron powder X‐ray diffraction measurements down to ambient pressure, which proves spinel‐type BeP₂N₄ a quenchable and metastable phase at ambient conditions. Its isothermal bulk modulus was determined to 325(8) GPa from equation of state, which indicates that spinel‐type BeP₂N₄ is an ultraincompressible material.     

顽火辉石的高温高压同步辐射研究

 采用同步辐射能量色散X射线衍射技术、激光加热技术和金刚石对顶砧（DAC）高压装置,在温度为2 000 K和压力为23 GPa的范围内,对采自地幔二辉橄榄岩中的顽火斜方辉石,进行了原位的高温高压能量色散X射线衍射（EDXRD）测量。实验结果表明：当压力为15.3 GPa、温度为1 600 K时（相当于地球内部410 km处的地震波不连续界面的温压环境）,顽火斜方辉石转变为橄榄石的β相——瓦兹利石（Wadsleyite）相;继续加温加压至2 000 K、23 GPa时（相当于地球内部670 km处的地震波不连续界面的温压环境）,顽火斜方辉石相变为钛铁矿（Ilmenite）结构和钙钛矿（Perovskite）结构的混和相。实验结果进一步证明,在地幔中存在的两个地震波不连续界面是由橄榄石、顽火斜方辉石等矿物的相变引起的。     

抛物面式太阳能聚能系统聚光特性模拟

结合集热器的光学特性,本文应用蒙特卡罗法对理想的槽型和碟型两种抛物面集热器在不同焦距、不同边缘角时焦面上的热流密度进行了计算,获得了焦距、边缘角和聚光比之间的关系。在考虑太阳不平行度、跟踪指向误差、镜面的反射误差条件下,对碟型抛物面太阳能聚能系统的辐射特性进行了数值模拟分析,获得了吸收器壁面热流分布以及边缘角对焦面热流分布的影响,为太阳能聚能系统的设计和安装提供参考依据。