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Abstract The tetragonal tungsten bronze, Sr2NaNb5O15, shows promise for application in high-temperature high-efficiency capacitors vital for the sustainable energy revolution. Previously, the structural complexity of this and related materials has obscured the mechanisms underpinning two large anomalies in relative permittivity (ε r) which give rise to their exceptionally broad dielectric response. Here, we comprehensively investigate the structural evolution from −173 to 627 °C, combining electron, X-ray and neutron diffraction, electron microscopy, and first principles electronic structure calculations to unambiguously identify the structural origins of both anomalies. The peak in ε r at 305 °C is associated with a polar-nonpolar phase transition, wherein cations displace along the c axis. Guided by DFT, we identify a further transition upon cooling, associated with the second peak at −14 °C, linked to the softening of an in-plane polar distortion with a correlation length limited by ferroelastic nano-domains arising from rigid-unit-like tilting of NbO6 octahedra at high temperature, imparting relaxor-like behaviour. Thus, the two dielectric anomalies in Sr2NaNb5O15 are associated with two distinct crystallographic phase transitions and their interplay with a microstructure that arises from a third, non-polar structural distortion. Chemical control of these will enable development of tuneable materials with dielectric properties suitable for high-temperature energy storage applications.

A conduction-cooled cryogenic permanent magnet undulator, CU15, with a period length of 15 mm and 133 periods has been successfully developed. This undulator has been operated at the Taiwan Photon Source (TPS) storage ring at a beam current of 500 mA for more than 3 years. In CU15, two cryocoolers are used to cool the PrFeB magnets to 80 K. The effective magnetic field measures 1.33 Tesla for a magnet gap of 4.2 mm, maintaining an rms phase error below 2.6°. The bunch length of the electron beam in TPS is as short as 16 psec, leading to a serious issue with high beam-induced heating in CU15. This report addresses the above issues and provides detailed adaptations of CU15 to operate at a beam current of 500 mA.

Abstract Definitive understanding of superconductivity and its interplay with structural symmetry in the hole-doped lanthanum cuprates remains elusive. The suppression of superconductivity around 1/8th doping maintains particular focus, often attributed to charge-density waves (CDWs) ordering in the low-temperature tetragonal (LTT) phase. Central to many investigations into this interplay is the thesis that La1.875Ba0.125CuO4 and particularly La1.675Eu0.2Sr0.125CuO4 present model systems of purely LTT structure at low temperature. However, combining single-crystal and high-resolution powder X-ray diffraction, we find these to exhibit significant, intrinsic coexistence of LTT and low-temperature orthorhombic domains, typically associated with superconductivity, even at 10 K. Our two-phase models reveal substantially greater tilting of CuO6 octahedra in the LTT phase, markedly buckling the CuO2 planes. This would couple significantly to band narrowing, potentially indicating a picture of electronically driven phase segregation, reminiscent of optimally doped manganites. These results call for reassessment of many experiments seeking to elucidate structural and electronic interplay at 1/8 doping.

Abstract The lithiation/delithiation properties of α‐Si3N4 and β‐Si3N4 are compared and the carbon coating effects are examined. Then, β‐Si3N4 at various fractions is used as the secondary phase in a Si anode to modify the electrode properties. The incorporated β‐Si3N4 decreases the crystal size of Si and introduces a new NSiO species at the β‐Si3N4/Si interface. The nitrogen from the milled β‐Si3N4 diffuses into the surface carbon coating during the carbonization heat treatment, forming pyrrolic nitrogen and CNO species. The synergistic effects of combining β‐Si3N4 and Si phases on the specific capacity are confirmed. The operando X‐ray diffraction and X‐ray photoelectron spectroscopy data indicate that β‐Si3N4 is partially consumed during lithiation to form a favorable Li3N species at the electrode. However, the crystalline structure of the hexagonal β‐Si3N4 is preserved after prolonged cycling, which prevents electrode agglomeration and performance deterioration. The carbon‐coated β‐Si3N4/Si composite anode shows specific capacities of 1068 and 480 mAh g−1 at 0.2 and 5 A g−1, respectively. A full cell consisting of the carbon‐coated β‐Si3N4/Si anode and a LiNi0.8Co0.1Mn0.1O2 cathode is constructed and its properties are evaluated. The potential of the proposed composite anodes for Li‐ion battery applications is demonstrated.