Wei Yi

We report a large magnetocaloric effect (MCE) under a relatively low magnetic field in an n = 1 Ruddlesden–Popper-type layered perovskite oxide Eu ₂ TiO ₄ . A maximum magnetic entropy change −Δ S _M = 15 J·kg ^–1 ·K ^–1 is achieved under a magnetic field change of μ ₀ Δ H = 0−1 T, which is significantly larger than that of perovskite-type EuTiO ₃ (−Δ S _M = 9.9 J·kg ^–1 ·K ^–1 ). This enhancement originates from the higher mass fraction of Eu ²⁺ cations (73.1 wt% for Eu ₂ TiO ₄ vs 61.2 wt% for EuTiO ₃ ), together with the ferromagnetic ground state of Eu ₂ TiO ₄ , in contrast to the antiferromagnetic ordering in EuTiO ₃ . This pronounced low-field MCE highlights Eu ₂ TiO ₄ as a promising candidate for cryogenic magnetic refrigeration and suggests that Eu ²⁺ -based layered perovskites are useful platforms for designing efficient low-temperature magnetocaloric materials.

We demonstrate a strategy for tuning the Curie temperature (TC) of hybrid improper ferroelectric La2SrSc2O7, an n = 2 Ruddlesden–Popper-type perovskite oxide. By kinetically controlling the distribution of A-site cations (La/Sr) through different post-sintering cooling rates, TC can be varied widely from ∼400 to ∼700 K. Structural mode analysis reveals that increasing A-site cation disorder weakens ScO6 octahedral deformation and enhances octahedral rotation, thereby stabilizing the polar phase and leading to a higher TC. A linear correlation between TC and A-site cation disorder is established, highlighting A-site cation disorder as a key descriptor for tailoring ferroelectricity in La2SrSc2O7. Consequently, controlling the cooling rate provides an effective, composition-independent pathway for designing hybrid improper ferroelectrics with tunable functionalities, offering a distinct alternative to conventional compositional design.

Band engineering in Cu(Nb,Ta)O₃via Ta substitution and symmetry transition enables efficient visible-light-driven H₂ evolution.

Rational design of ferroelectrics in layered perovskites, like n = 2 Ruddlesden-Popper (RP) phase A₃B₂O₇, has been achieved by the hybrid-improper ferroelectric (HIF) mechanism, in which an electric polarization is induced via a trilinear coupling to nonpolar BO₆ octahedral rotation and tilt distortions around crystallographic axes. In the present work, hybrid improper ferroelectricity in n = 2 RP-type La₂SrSc₂O₇ induced by the disordering of Sr²⁺/La³⁺ cations on the A-sites in rocksalt ([Sr/La]_Rs = 25/75) and perovskite ([Sr/La]_Pv = 50/50) layers is demonstrated through experimental and theoretical investigations. The ferroelectric A2₁am structure (a^-a^-c⁺ in Glazer notation) at room temperature and the second-order phase transition to paraelectric Amam structure (a^-a^-c⁰) at T_C ∼ 600 K are determined by a combination of X-ray and neutron diffraction and optical second harmonic generation. The ferroelectric hysteresis loop measurements prove the switchable electric polarization indicative of ferroelectricity. These results represent an unprecedented example of ferroelectricity in the n = 2 RP family of Ln₂AB₂O₇ with inequivalent Ln³⁺ and A²⁺ cations. Combining the abovementioned experimental results with the first-principles calculations, we verify the role of Sr/La distributions in regulating the interlayer rumpling, which, in addition to the structural tolerance factor, is key to controlling the structural distortions of RP phases. The stabilization of the ferroelectric, a^-a^-c⁺ distorted structure is a consequence of the disordered Sr/La distribution on the A-sites, which suppresses the rumpling-induced octahedral deformations in competition with the octahedral rotations and thus enables the concurrence of a⁰a⁰c⁺ rotations and a^-a^-c⁰ tilts required for the HIF mechanism. This work demonstrates the possibility of altering the crystal symmetry of RP phases through the A-site cation disorder and provides a complementary approach to the rational design of new HIF materials.

We have investigated the impact of high-temperature Mg-implantation in GaN layers on distribution of Mg-enriched defects using scanning transmission electron microscopy and atom probe tomography. For this, 1 × 1019 cm−3 Mg ions have been implanted in GaN layers at room temperature (RT) and 1000 °C, followed by annealing at 1300 °C. A smaller number of Mg-enriched defects were observed in the sample implanted at 1000 °C in comparison to the sample implanted at RT. The implantation of Mg ions at 1000 °C resulted in a higher amount of randomly distributed Mg in the GaN matrix, which, in turn, leads to more uniform and enhanced donor–acceptor pair emission, leading to higher Mg activation.

An area selective doping via ion implantation is a key technology to realize gallium nitride (GaN) based energy-efficient power devices; however, conventional annealing leads to the formation of numerous Mg-enriched defects, which result in inefficient p-type activation. The recent invention of ultra-high-pressure annealing (UHPA) has enabled a significant improvement in p-type activation efficiency. In this study, we investigated the formation of Mg-enriched defects in Mg implanted GaN followed by annealing under either conventional atmospheric pressure or ultra-high-pressure. Unlike the conventional annealing, UHPA leads to a much lower number density of Mg-enriched defects. Correlative scanning transmission electron microscopy, atom probe tomography, cathodoluminescence, and secondary ion mass spectrometry analyses have shown that the number density of Mg-enriched defects is substantially suppressed by the UHPA. The dissolved Mg concentrations in the GaN matrix for both the conventional and the UHPA samples are almost of the same value, approximately 2 × 10¹⁸ cm⁻³; however, the UHPA sample shows over one order of magnitude stronger intensity of donor–acceptor-pair emission than the conventional one. Thus, the implanted Mg is effectively activated as acceptors through the UHPA technique.

Electron-beam-induced current (EBIC) and cathodoluminescence (CL) have been applied to investigate the electrical and optical behaviors of dislocations in SrTiO₃. The electrical recombination activity and defect energy levels of dislocations have been deduced from the temperature-dependent EBIC measurement. Dislocations contributed to resistive switching were clarified by bias-dependent EBIC. The distribution of oxygen vacancies around dislocations has been obtained by CL mapping. The correlation between switching, dislocation and oxygen vacancies was discussed.

Efficient acceptor activation in gallium nitride (GaN) achieved through Mg ion-implantation depends mainly on the concentration of implanted Mg ions and the post-implantation annealing process. In this study, we conducted correlative scanning transmission electron microscopy, atom probe tomography, and cathodoluminescence (CL) measurements on Mg-implanted GaN layers with the implanted concentration ranging from 1 × 10¹⁷ cm⁻³ to 1 × 10¹⁹ cm⁻³. It was found that at the implanted concentration of ∼1 × 10¹⁸ cm⁻³, Mg atoms were randomly distributed with defects likely to be vacancy clusters whereas at the implanted concentration of ∼1 × 10¹⁹ cm⁻³, Mg-enriched clusters and dislocation loops were formed. From the CL measurements, the donor–acceptor pair (DAP) emissions from the implanted and un-implanted regions are obtained and then compared to analyze Mg activation in these regions. In the sample with Mg ∼1 × 10¹⁹ cm⁻³, the existence of Mg-enriched clusters and dislocations in the implanted region leads to a weaker DAP emission, whereas the absence of Mg-enriched clusters and dislocations in the sample with Mg ∼1 × 10¹⁸cm⁻³ resulted in a relatively stronger DAP emission.