In-gap states with nearly free electron characteristics in layered structure trivalent iridates

Iridium oxides (iridates) provide good platform to study the complex interplay of spin-orbit coupling (SOC) interactions, correlation effects, Hund coupling and lattice degree of freedom. However, previous studies primarily focus on tetravalent (Ir4+, 5d5) and pentavalent (Ir5+, 5d4) iridates. Here, we turn our attention to a recently reported unprecedented trivalent (Ir3+, 5d6) iridates, K0.75Na0.25IrO2, crystalizes in a triangular lattice with edge-sharing IrO6 octahedra and alkali ions intercalated [IrO2]- layers. We theoretically determine the preferred occupied positions of the alkali ions from energetic viewpoints, and reproduce the experimentally observed semiconducting behavior and nonmagnetic (NM) properties. The SOC interactions play a critical role in the band dispersion, resulting in NM Jeff = 0 states. More intriguingly, our electronic structure not only confirms the experimental speculation of the presence of in-gap states and explains the abnormal low activation energy in K0.75Na0.25IrO2, but also puts forward the in-gap states featured with nearly free electron characteristics. Our theoretical results provide new insights into the unconventional electronic structures of the trivalent iridates and imply its promising applications in nanoelectronic devices such as ideal electron transport channels.Comment: 14+15pages,6+6figure

Entropy regulation in LaNbO4-based fergusonite to implement high-temperature phase transition and promising dielectric properties

High-entropy effect is a novel design strategy to optimize properties and explore novel materials. In this work, (La1/5Nd1/5Sm1/5Ho1/5Y1/5)NbO4 (5RNO) high-entropy microwave dielectric ceramics were successfully prepared in the sintering temperature (S.T.) range of 1210–1290 ℃ via a solid-phase reaction route, and medium-entropy (La1/3Nd1/3Sm1/3)NbO4 and (La1/4Nd1/4Sm1/4Ho1/4)NbO4 (3RNO and 4RNO) ceramics were compared. The effects of the entropy (S) on crystal structure, phase transition, and dielectric performance were evaluated. The entropy increase yields a significant increase in a phase transition temperature (from monoclinic fergusonite to tetragonal scheelite structure). Optimal microwave dielectric properties were achieved in the high-entropy ceramics (5RNO) at the sintering temperature of 1270 ℃ for 4 h with a relative density of 98.2% and microwave dielectric properties of dielectric permittirity (εr) = 19.48, quality factor (Q×f) = 47,770 GHz, and resonant frequency temperature coefficient (τf) = –13.50 ppm/℃. This work opens an avenue for the exploration of novel microwave dielectric material and property optimization via entropy engineering

UltrahighQ Sr1+xY2O4+x (x = 0–0.04) microwave dielectric ceramics for temperature-stable millimeter-wave dielectric resonator antennas

Microwave dielectric ceramics should be improved to advance mobile communication technologies further. In this study, we prepared Sr1+xY2O4+x (x = 0–0.04) ceramics with nonstoichiometric Sr2+ ratios based on our previously reported SrY2O4 microwave dielectric ceramic, which has a low dielectric constant and an ultrahigh quality factor (Q value). The ceramic exhibited a 33.6% higher Q-by-frequency (Q×f) value (Q ≈ 12,500) at x = 0.02 than SrY2O4. All Sr1+xY2O4+x (x = 0–0.04) ceramics exhibited pure phase structures, although variations in crystal-plane spacings were observed. The ceramics are mainly composed of Sr–O, Y1–O, and Y2–O octahedra, with the temperature coefficient of the resonant frequency (τf) of the ceramic increasing with Y2–O octahedral distortion. The ceramic comprises uniform grains with a homogeneous elemental distribution, clear grain boundaries, and no obvious cavities at x = 0.02. The Sr1+xY2O4+x (x = 0–0.04) ceramics exhibited good microwave dielectric properties, with optimal performance observed at x = 0.02 (dielectric constant (εr) = 15.41, Q×f = 112,375 GHz, and τf = −17.44 ppm/℃). The τf value was reduced to meet the temperature-stability requirements of 5G/6G communication systems by adding CaTiO3, with Sr1.02Y2O4.02+2wt%CaTiO3 exhibiting εr = 16.14, Q×f = 51,004 GHz, and τf = 0 ppm/℃. A dielectric resonator antenna prepared using Sr1.02Y2O4.02+2wt%CaTiO3 exhibited a central frequency of 26.6 GHz, with a corresponding gain and efficiency of 3.66 dBi and 83.14%, respectively. Consequently, Sr1.02Y2O4.02-based dielectric resonator antennas are suitable for use in 5G millimeter-wave band (24.5–27.5 GHz) applications

A first-principle study on the electronic properties of substitutionally Cu (I, II)-doped LiNbO3

The electronic properties of Cu-doped lithium niobate (LiNbO3) systems are investigated by first-principles calculations. In this work, we focus on substitutionally Cu→Li-doped LiNbO3 system with cuprous and cupric doping, which corresponds to the Li5∕6Cu1∕6NbO3 and Li4∕6Cu1∕6NbO3 [abbreviated as (Li, Cu I)NbO3 and (Li, Cu II)NbO3]. The density functional theory (DFT) calculations show that the electronic property of LiNbO3 is completely different from (Li, Cu I)NbO3 and (Li, Cu II)NbO3. The calculated band structure and density of state (DOS) of (Li, Cu I)NbO3 show a small band gap of 1.34eV and the top of valance band (VB) is completely composed of a doping energy level originating from Cu 3d filled orbital. However, the calculated band structure and DOS of (Li, Cu II)NbO3 show a relatively large band gap of 2.22eV and the top of VB is mainly composed of Cu 3d unfilled orbital and O 2p orbital

Giant permittivity and good thermal stability of LiCuNb3O9-Bi(Mg0.5Zr0.5)O3 solid solutions

(1−x)LiCuNb3O9-xBi(Mg0.5Zr0.5)O3 ceramics ((1−x)LCN-xBMZ) with 0≤x≤0.08 were synthesized by a solid-state reaction method. The phase structure of (1−x)LCN-xBMZ ceramics was characterized by X-ray diffraction (XRD), which revealed that the ceramics were distorted cubic perovskite structures. Apparent giant permittivity of 1.98×104–1.05×105 at 100kHz over the measured temperature range (25∘C–250∘C) was observed in the sintered (1−x)LCN-xBMZ (0≤x≤0.08) ceramics. Especially for the sample of x=0.04, the temperature stability of permittivity was markedly increased (Δε/ε100∘C≤±15%), and high relative permittivity (>8.3×104) were obtained over a wide temperature range from 100∘C to 250∘C at 100kHz, which indicates that this ceramic is a promising dielectric material for elevated temperature dielectrics. The giant dielectric property of (1−x)LCN-xBMZ ceramics are profoundly concerned with the Maxwell–Wagner effect

Dielectric temperature stability and energy storage performance of NBT-based lead-free ceramics for Y9P capacitors

In this work, novel (1 [Formula: see text])(0.75Na[Formula: see text]Bi[Formula: see text]TiO3)-0.25Sr(Zr[Formula: see text]Sn[Formula: see text]Hf[Formula: see text]Ti[Formula: see text]Nb[Formula: see text])O3-[Formula: see text]NaNbO3 (NBT-SZSHTN-[Formula: see text]NN, [Formula: see text] = 0.1, 0.15, 0.2, 0.25) ceramics were fabricated. The influence of co-doping of NN and high entropy perovskite oxide (SZSHTN) on the phase structure, microstructure and dielectric properties of NBT-based lead-free ceramics was investigated. Dense microstructure with a grain size of [Formula: see text]5 [Formula: see text]m is observed. When [Formula: see text] = 0.25, a wide dielectric temperature stable range of −35.4–224.3[Formula: see text]C with a low temperature coefficient of capacitance of [Formula: see text] 10% is achieved, fulfilling the industry standard of Y9P specification. Furthermore, excellent energy storage performance with recoverable energy density of 2.4 J/cm3, discharge efficiency of 71%, power density of 25.495 MW/cm3 and discharge rate [Formula: see text] 200 ns are simultaneously obtained, which shows great potential for high temperature capacitor applications

Sintering characteristic, structure, microwave dielectric properties, and compatibility with Ag of novel 3MgO-B2O3-xwt% BaCu(B2O5)-ywt% H3BO3 ceramics

In this study, 3MgO-B2O3-xwt%BaCu(B2O5) (BCB)-ywt%H3BO3 (2 ≤ x ≤ 8, 0 ≤ y ≤ 20) ceramics were sintered at the optimum temperature to form Mg3B2O6 and MgO phases. The effects of H3BO3 and BCB on the product characteristics, phase transition, microstructure, and microwave dielectric properties of 3MgO-B2O3 ceramics were investigated. The intensities of diffraction peaks of two phases varied with changing the x and y values. After sintering at 950°C, the ceramics with x = 6 and y = 15 achieved the excellent microwave properties with a εr of 6.72, Q × f of 83,205 GHz and τf of – 65.05 ppm/°C. Besides, the ceramics with x = 8 and y = 5 sintered at 925°C also achieved good microwave dielectric properties with a εr of 6.64, Q × f of 78,173 GHz and τf of – 57.27 ppm/°C. The sintering temperatures of above both ceramics are well lower than the melting point of Ag, showing promising applications in low temperature cofired ceramic devices. In particular, these two ceramics can be used as the potential candidate materials for microwave ceramics for 5 G technology, provided that τf can be further optimized