A density-wave-like transition in the polycrystalline V3Sb2 sample with bilayer kagome lattice

Recently, transition-metal-based kagome metals have aroused much research interest as a novel platform to explore exotic topological quantum phenomena. Here we report on the synthesis, structure, and physical properties of a bilayer kagome lattice compound V3Sb2. The polycrystalline V3Sb2 samples were synthesized by conventional solid-state-reaction method in a sealed quartz tube at temperatures below 850 ℃. Measurements of magnetic susceptibility and resistivity revealed consistently a density-wave-like transition at T dw ≈ 160 K with a large thermal hysteresis, even though some sample-dependent behaviors are observed presumably due to the different preparation conditions. Upon cooling through T dw, no strong anomaly in lattice parameters and no indication of symmetry lowering were detected in powder x-ray diffraction measurements. This transition can be suppressed completely by applying hydrostatic pressures of about 1.8 GPa, around which no sign of superconductivity is observed down to 1.5 K. Specific-heat measurements reveal a relatively large Sommerfeld coefficient γ = 18.5 mJ/mol-K2, confirming the metallic ground state with moderate electronic correlations. Density functional theory calculations indicate that V3Sb2 shows a non-trivial topological crystalline property. Thus, our study makes V3Sb2 a new candidate of metallic kagome compound to study the interplay between density-wave-order, nontrivial band topology, and possible superconductivity.

Microwave Dielectric Properties of β-CaSiO3 Glass–Ceramics Prepared Using Two-Step Heat Treatment

The microwave dielectric properties of β-CaSiO3 glass–ceramics are compared with those of α-CaSiO3 ceramics. β-CaSiO3 is prepared using glass–ceramics method with two-step heat treatment at 730 °C for 1–7 h and at 900 °C for 3 h, and α-CaSiO3 is prepared using conventional solid-state reaction and sintered at 1460–1500 °C for 3 h. With increasing holding time at 730 °C, the degree of crystallisation and Qf of the β-CaSiO3 glass–ceramics increased. The β-CaSiO3 specimens heat-treated at 730 °C for 3 h and 900 °C for 3 h exhibit the following dielectric properties: K = 6.57, TCF = −36.22 ppm/°C, and Qf = 52,400 GHz (highest) for the entire range of heat treatment conditions. The Qf difference between β-CaSiO3 and α-CaSiO3 could be explained by the bond characteristics using Rietveld refinement. FT-IR analysis shows that the Ca–O bond is the dominant factor for the Qf of CaSiO3 ceramics compared to the Si–O bond. The higher Qf of β-CaSiO3 than that of α-CaSiO3 can be attributed to the higher bond strength of Ca–O for β-CaSiO3 than that for α-CaSiO3.

Effects of B2O3 doping on the dielectric and piezoelectric properties of (K,Na,Li)NbO3-BaZrO3-(Bi,Na)TiO3 ceramics

(K,Na,Li)NbO3-BaZrO3-(Bi,Na)TiO3-x B2O3 (abbreviated as KNLN-BZ-BNT-x B2O3) ceramics were prepared by conventional solid state reaction method. The effects of B2O3 content on dielectric and piezoelectric properties of the KNLN-BZ-BNT ceramics have been studied. Based on the identification by X-ray diffraction (XRD), all the samples show a two-phase coexistence zone consisting of rhombohedral (R) and tetragonal (T) phases. For the ceramics with x = 0.5 wt%, the sintering temperature was observed to reduce from 1200 oC to 940 oC, while the samples maintained a high relative density around 97%. Because of the high densification and large grain size, the dielectric and piezoelectric properties were improved. As a result, the ceramics with x = 0.5 wt% sintered at 940 oC exhibited good properties of d 33 = 197 pC/N, k p= 0.29, tan δ = 0.029 and ε r = 1889, together with a Curie temperature (T C) of 222 oC.

Origins of Giant Dielectric Properties with Low Loss Tangent in Rutile (Mg1/3Ta2/3)0.01Ti0.99O2 Ceramic

The Mg2+/Ta5+ codoped rutile TiO2 ceramic with a nominal composition (Mg1/3Ta2/3)0.01Ti0.99O2 was synthesized using a conventional solid-state reaction method and sintered at 1400 °C for 2 h. The pure phase of the rutile TiO2 structure with a highly dense microstructure was obtained. A high dielectric permittivity (2.9 × 104 at 103 Hz) with a low loss tangent (<0.025) was achieved in the as-sintered ceramic. After removing the outer surface, the dielectric permittivity of the polished ceramic increased from 2.9 × 104 to 6.0 × 104, while the loss tangent also increased (~0.11). The dielectric permittivity and loss tangent could be recovered to the initial value of the as-sintered ceramic by annealing the polished ceramic in air. Notably, in the temperature range of −60–200 °C, the dielectric permittivity (103 Hz) of the annealed ceramic was slightly dependent (<±4.4%), while the loss tangent was very low (0.015–0.036). The giant dielectric properties were likely contributed by the insulating grain boundaries and insulative surface layer effects.

Micro-structural, magnetic and dielectric performance of rare earth ion (Sm3+) doped MgCd ferrites

The combined effects of Sm3+ substitution together with the addition of 3 wt% Bi2O3 endowed the MgCd ferrites with excellent magnetic permeability and dielectric permittivity. Various concentrations of Sm3+ (x=0.00, 0.03, 0.06, 0.09, 0.12, and 0.15) were employed to modify the permeability (μ') and permittivity (ε') of the MgCd ferrites. XRD, SEM, VSM and vector network analysis techniques were used to characterize the samples. The measurement results reveal that the ferrites processed a saturation magnetization of up to 36.8 emu/g and coercivity of up to 29.2 Oe via the conventional solid-state reaction method. The surface morphology SEM confirms that with increasing Sm3+ concentration, the grain shape changes from a polygon to a circle. Moreover, the outstanding dielectric performance of dielectric permittivity can achieve 23. The excellent properties obtained in Sm3+-substituted Mg ferrites suggest that they could be promising candidates for modern high-frequency antenna substrates or multilayer devices.

Existence of Partially Degenerate Electrical Transport in Intermetallic Cu2SnSe3 Thermoelectric System Sintered at Different Temperatures

A detailed investigation on the temperature dependent electrical properties of Cu2SnSe3 system, synthesized via conventional solid-state reaction at different sintering temperatures are presented in this communication. All the samples exhibit degenerate semiconducting nature at low temperatures. The existence of small polarons and hence electron–phonon interactions are confirmed at temperatures below 400 K. A transition was observed from degenerate to non-degenerate semiconducting behaviour at high temperatures (T > 400 K). The study confirms the unusual transition in electrical resistivity as well as thermopower at high temperatures in all the compounds, demonstrating the existence of minority carrier excitation along with temperature-triggered ionisation of the defects. The transport behaviour is further supported by an upward movement of Fermi level away from the valence band. Highest weighted mobility of 8.2 cm2 V−1 s−1 at 673 K was obtained for the sample sintered at 1073 K. A considerable decrease in electrical resistivity with increase in temperature (T > 400 K) has driven the power factor to increase exponentially, thereby achieving highest value of 188 µV/mK2 (at 673 K) for the sample sintered at 673 K. Graphic abstract

Effects of Oxide Additives on the Phase Structures and Electrical Properties of SrBi4Ti4O15 High-Temperature Piezoelectric Ceramics

In this work, SrBi4Ti4O15 (SBT) high-temperature piezoelectric ceramics with the addition of different oxides (Gd2O3, CeO2, MnO2 and Cr2O3) were fabricated by a conventional solid-state reaction route. The effects of oxide additives on the phase structures and electrical properties of the SBT ceramics were investigated. Firstly, X-ray diffraction analysis revealed that all these oxides-modified SBT ceramics prepared presented a single SrBi4Ti4O15 phase with orthorhombic symmetry and space group of Bb21m, the change in cell parameters indicated that these oxide additives had diffused into the crystalline lattice of SBT and formed solid solutions with it. The SBT ceramics with the addition of MnO2 achieved a high relative density of up to 97%. The temperature dependence of dielectric constant showed that the addition of Gd2O3 could increase the TC of SBT. At a low frequency of 100 Hz, those dielectric loss peaks appearing around 500 °C were attributed to the space-charge relaxation as an extrinsic dielectric response. The synergetic doping of CeO2 and Cr2O3 could reduce the space-charge-induced dielectric relaxation of SBT. The piezoelectricity measurement and electro-mechanical resonance analysis found that Cr2O3 can significantly enhance both d33 and kp of SBT, and produce a higher phase-angle maximum at resonance. Such an enhanced piezoelectricity was attributed to the further increased orthorhombic distortion after Ti4+ at B-site was substituted by Cr3+. Among these compositions, Sr0.92Gd0.053Bi4Ti4O15 + 0.2 wt% Cr2O3 (SGBT-Cr) presented the best electrical properties including TC = 555 °C, tan δ = 0.4%, kp = 6.35% and d33 = 28 pC/N, as well as a good thermally-stable piezoelectricity that the value of d33 was decreased by only 3.6% after being annealed at 500 °C for 4 h. Such advantages provided this material with potential applications in the high-stability piezoelectric sensors operated below 500 °C.

Ce and Y Co-Doping Effects for (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 Lead-Free Ceramics

CeO2 and Y2O3 were co-doped to (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics and sintered by conventional solid-state reaction process to form x wt.% CeO2-y wt.% Y2O3 doped (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (CexYy-BCZT) ceramics. The effects of different contents of CeO2-Y2O3 dopants to the (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 composition were analyzed by studying the phase, surface microstructure, piezoelectric and ferroelectric properties of BCZT ceramics. In this study, we have shown that co-doping a small amount of CeO2 and Y2O3 will not change the phase structure of (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramics. However, the proper introduction of CeO2 and Y2O3 can improve the piezoelectric constant and electromechanical coupling coefficient of BCZT ceramic samples. Moreover, these dopants can promote the grain growth process in (Ba0.85Ca0.15) (Zr0.1Ti0.9)O3 ceramics. C0.04Y0.02 doped (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramic has the best piezoelectric properties compared with other composition, the results are as follows: Relative density = 96.9%, Kp = 0.583, and d33 = 678 pC/N, V = 8.9 V. It means that this Ce0.04Y0.02 doped (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramic is a desired material in the application of lead-free ceramics.

Microstructures and Electrical Conduction Behaviors of Gd/Cr Codoped Bi3TiNbO9 Aurivillius Phase Ceramic

In this work, a kind of Gd/Cr codoped Bi3TiNbO9 Aurivillius phase ceramic with the formula of Bi2.8Gd0.2TiNbO9 + 0.2 wt% Cr2O3 (abbreviated as BGTN−0.2Cr) was prepared by a conventional solid-state reaction route. Microstructures and electrical conduction behaviors of the ceramic were investigated. XRD and SEM detection found that the BGTN−0.2Cr ceramic was crystallized in a pure Bi3TiNbO9 phase and composed of plate-like grains. A uniform element distribution involving Bi, Gd, Ti, Nb, Cr, and O was identified in the ceramic by EDS. Because of the frequency dependence of the conductivity between 300 and 650 °C, the electrical conduction mechanisms of the BGTN−0.2Cr ceramic were attributed to the jump of the charge carriers. Based on the correlated barrier hopping (CBH) model, the maximum barrier height WM, dc conduction activation energy Ec, and hopping conduction activation energy Ep were calculated with values of 0.63 eV, 1.09 eV, and 0.73 eV, respectively. Impedance spectrum analysis revealed that the contribution of grains to the conductance increased with rise in temperature; at high temperatures, the conductance behavior of grains deviated from the Debye relaxation model more than that of grain boundaries. Calculation of electrical modulus further suggested that the degree of interaction between charge carriers β tended to grow larger with rising temperature. In view of the approximate relaxation activation energy (~1 eV) calculated from Z’’ and M’’ peaks, the dielectric relaxation process of the BGTN−0.2Cr ceramic was suggested to be dominated by the thermally activated motion of oxygen vacancies as defect charge carriers. Finally, a high piezoelectricity of d33 = 18 pC/N as well as a high resistivity of ρdc = 1.52 × 105 Ω cm at 600 °C provided the BGTN−0.2Cr ceramic with promising applications in the piezoelectric sensors with operating temperature above 600 °C.

Enhanced Photoluminescence Properties of BaAl2O4: Ce3+/Li+ Yellow Phosphors

Ce3+/ Li+ activated barium aluminate phosphor (BAO) was synthesized by conventional solid-state reaction method. The crystal structure of the synthesized phosphor was analyzed by X-ray diffraction (XRD) and Raman spectroscopy analysis. FT-IR spectrum results revealed the characteristic vibration bands present in the synthesized phosphor. Surface composition analysis of the prepared samples was examined using X-ray photoelectron spectroscopy (XPS). PL emission band observed at 589 nm was assigned to 5D excited level corresponding to 2F5/2 transition in yellow region under the excitation wavelength of 320 nm. Yellow light emission was confirmed by the Commission Internationale de L’Eclairage (CIE) chromatic coordinate graph. The color purity of BAO: 0.5Ce3+, BAO: 0.5Ce3+, 0.1Li+ was found to be 78.4 %, 81.3 % whereas the measured lifetime was 4.333, 4.738 ns respectively.