Ferromagnetic Double Perovskite Semiconductors with Tunable Properties

Toward Ultra-High Sensitivity Optical Thermometers and Bright Yellow LEDs Based on Phonon-Assisted Energy Transfer in Rare Earth-Doped La2ZnTiO6 Double Perovskite

Double perovskites, a class of ceramic oxides with unique crystal structures and diverse physical properties, show promise for various technological applications including solar cells, photodetectors, and light-emitting diodes (LEDs). Despite limited research on rare earth-doped double perovskites, leveraging their ultrahigh luminous efficiency to achieve bright yellow LED emission and addressing energy transfer challenges between Yb3+ and Nd3+ ions in double perovskite La2ZnTiO6 with moderate phonon energy are explored in this work. Through phonon-assisted energy transfer, an ultrasensitive optical thermometer covering a wide temperature range is developed by utilizing the different temperature responses of Er3+ emission in the visible light region and Nd3+ emission in the near-infrared region based on the luminescence intensity ratio (LIR). All the results demonstrate that the rare earth (Yb-Er, Yb-Nd, and Yb-Nd-Er)-doped La2ZnTiO6 phosphors can be effectively utilized for ultrabright LED illumination and ultrahigh sensitivity self-calibrated temperature sensing. This research underscores the significance of phonon-assisted energy transfer in improving material properties and provides valuable insights for the advancement of multifunctional materials.

Realizing new 2D spintronic materials from the non-magnetic 1T-PdO2 monolayer through vacancy defects and doping

In this work, vacancy- and doping-based magnetism engineering in a non-magnetic 1T-PdO2 monolayer is explored in order to realize new two-dimensional (2D) spintronic materials. The pristine monolayer is an indirect gap semiconductor with a band gap of 1.45 (3.20) eV obtained using the PBE (HSE06) functional. Half-metallicity with a total magnetic moment of 3.95 μB is induced by creating a single Pd vacancy, where the magnetic properties are produced mainly by O atoms around the vacancy site. In contrast, the non-magnetic nature is preserved under the effects of a single O vacancy, however a band gap reduction in the order of 37.93% is achieved. Further doping with transition metals (TMs = V, Cr, Mn, and Fe) in the Pd sublattice and with non-metals (B, C, N, and F) in the O sublattice is investigated. TM impurities lead to the emergence of a diluted magnetic semiconductor nature, where total magnetic moments of 1.00, 2.00, and 3.00 μB are obtained in the V-, Cr(Fe)-, and Mn-doped systems, respectively. In these cases, the TMs' 3d electrons mainly originate the system's magnetism. Significant magnetization of the PdO2 monolayer is also achieved by doping with B, N, and F atoms, where either half-metallic or diluted magnetic semiconductor natures are induced. Herein, electronic and magnetic properties are regulated mainly by the interactions between the 2p orbital of the dopant, 4d orbital of the first neighbor Pd atoms, and 2p orbital of the second neighbor O atoms. Meanwhile, C impurity induces no magnetism in the PdO2 monolayer because of the strong electronic hybridization with their neighbor atoms. Results presented herein may introduce efficient approaches to engineer magnetism in a non-magnetic PdO2 monolayer, such that the functionalized systems are further recommended for prospective spintronic applications.

Crystallography at non-ambient conditions and physical properties of the synthesized double perovskites, Sr2(Co1-xFex)TeO6

Polycrystalline double perovskite-type Sr₂(Co_1-xFe_x)TeO₆ with various stoichiometric compositions (x = 0, 0.25, 0.5, 0.75, and 1) were synthesized by solid-state reactions in air. The crystal structures and phase transitions of this series at different temperature intervals were determined by X-ray powder diffraction, and from the obtained data the crystal structures were refined. It has been proven that for the compositions x = 0.25, 0.50, and 0.75, the phases crystallize at room temperature in the monoclinic space group I2/m. Down to 100 K, depending on the composition, these structures experience a phase transition from I2/m to P2₁/n. At high temperatures up to 1100 K their crystal structures show two further phase transitions. The first one is a first-order phase transition, from monoclinic I2/m to tetragonal I4/m, followed by a second-order phase transition to cubic Fm3̄m. Therefore, the phase transition sequence of this series detected at temperatures ranging from 100 K to 1100 K is P2₁/n → I2/m → I4/m → Fm3̄m. The temperature-dependent vibrational features of the octahedral sites were investigated by Raman spectroscopy, which furthermore complements the XRD results. A decrease in the phase-transition temperature with increasing iron content has been observed for these compounds. This fact is explained by the progressive diminishing of the distortion of the double-perovskite structure in this series. Using room-temperature Mössbauer spectroscopy, the presence of two iron sites is confirmed. The two different transition metal cations Co and Fe at the B sites allow exploring their effect on the optical band-gap.

Electron doping of a double-perovskite flat-band system

Electronic structure calculations indicate that the Sr2FeSbO6 double perovskite has a flat-band set just above the Fermi level that includes contributions from ordinary subbands with weak kinetic electron hopping plus a flat subband that can be attributed to the lattice geometry and orbital interference. To place the Fermi energy in that flat band, electron-doped samples with formulas Sr2-xLaxFeSbO6 (0 ≤ x ≤ 0.3) were synthesized, and their magnetism and ambient temperature crystal structures were determined by high-resolution synchrotron X-ray powder diffraction. All materials appear to display an antiferromagnetic-like maximum in the magnetic susceptibility, but the dominant spin coupling evolves from antiferromagnetic to ferromagnetic on electron doping. Which of the three subbands or combinations is responsible for the behavior has not been determined.