S.F. Wu

Superconductivity and the metal-semiconductor transition in the septenary oxide system, (Tl0.5Pb0.5)(Ca1−yYy)Sr2Cu2O7−δ

Abstract The septenary compound (Tl0.5Pb0.5)(Ca1−yYy)Sr2Cu2O7−δ exhibits the highest superconducting transition temperature (110 K) yet observed in the TlCaBa2Cu2O7−δ (1122) structure type. This complex, six-blend combination of metallic elements which make up the material is, however, compensated by a relatively simply crystal structure, which bears many similarities to that of 90 K superconductor YBa2 Cu3O7−δ. In this note we report some important features of the cation-substitution chemistry of the title compound, drawing attention to the fundamental changes in the electronic properties of the (Tl0.5Pb0.5) (Ca1−yYy)Sr2Cu2O7−δ system as Ca2+ is replaced by Y3+. Superconductivity is observed over the homogeneity range y = 0–0.5, with the superconducting transition temperature showing a maximum (108 K) at y = 0.2. Across the homogeneity range y = 0.6–1.0, the material undergoes a metal-semiconductor transition.

Facile Atmospheric Pressure Synthesis of High Thermal Stability and Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor for High Color Rendering Index White Light-Emitting Diodes

Red phosphors (e.g., SrLiAl3N4:Eu(2+)) with high thermal stability and narrow-band properties are urgently explored to meet the next-generation high-power white light-emitting diodes (LEDs). However, to date, synthesis of such phosphors remains an arduous task. Herein, we report, for the first time, a facile method to synthesize SrLiAl3N4:Eu(2+) through Sr3N2, Li3N, Al, and EuN under atmospheric pressure. The as-synthesized narrow-band red-emitting phosphor exhibits excellent thermal stability, including small chromaticity shift and low thermal quenching. Intriguingly, the title phosphor shows an anomalous increase in theoretical lumen equivalent with the increase of temperature as a result of blue shift and band broadening of the emission band, which is crucial for high-power white LEDs. Utilizing the title phosphor, commercial YAG:Ce(3+), and InGaN-based blue LED chip, a proof-of-concept warm white LEDs with a color rendering index (CRI) of 91.1 and R9 = 68 is achieved. Therefore, our results highlight that this method, which is based on atmospheric pressure synthesis, may open a new means to explore narrow-band-emitting nitride phosphor. In addition, the underlying requirements to design Eu(2+)-doped narrow-band-emitting phosphors were also summarized.

Superconductivity with Tc(zero) above 105 K in Tl-containing septenary oxides with Y1Ba2Cu3Oy-like structure

Abstract A family of high- T c superconductors with T c(zero) above 105 K is described. The compounds are seven-element blends with the general formula Tl 0.5 Pb 0.5 Ca 0.8 A 0.2 Sr 2 Cu 2 O x ( A = Y ), and rare-earth elements La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb). The results represent one of the first cases where chemical substitution has significantly raised T c in a known compound. The septenary compounds are tetragonal in structure with almost identical lattice parameters to those of now-famous Y 1 Ba 2 Cu 3 O y structure. The samples were highly reproducible, stable and remarkably homogeneous both in composition and structure. The preparative conditions were found to be much less stringent than those of other copper-based high- T c superconductors.

Formation mechanism study of REBa2Cu4O8+δ (RE=Y and Dy) accelerated by nitric acid

An accelerated formation of REBa2Cu4O8+δ (RE=Y and Dy) was observed as a suitable amount of nitric acid was added. This provides an excellent route for preparing high‐ purity REBa2Cu4O8+δ in ∼ 30 h. X‐ray diffraction and thermal analyses (TGA/DTA) were conducted to reveal the formation mechanism. The addition of nitric acid did not change the reaction routes but formed more Ba2Cu3O5 which might play an important role for the formation of REBa2Cu4O8+δ. Magnetization data confirmed the Tc’s of 80 and 75 K for as‐prepared Y‐124 and Dy‐124, respectively.

First example of indium as a practical alternative to thallium in high-Tc superconductors

Abstract The substitution of indium for thallium in the material Tl x Pb 1− x Ca 1− n Y n Sr 2 Cu 2 O y has been carried out. A stable and reproducible T c(mid-point) of circa 60 K for the new material, In 0.3 Pb 0.7 Ca 0.8 Y 0.2 Sr 2 Cu 2 O y was observed in both electrical resistivity and magnetization measurements. The X-ray diffraction pattern shows that the superconducting phase can be indexed on the basis of the tetragonal thallium-based 1122-type compound.

Improvement of phase purity and accelerated formation of the Tl-1223 phase from the stoichiometric compositions (Tl0.6Pb0.2Bi0.2) (Sr2-xBax)Ca2Cu3O9 (x=0.2-0.3)

Abstract We report an efficient and highly reproducible method for the preparation of the almost monophasic Tl-1223 phase by partial substitution of Ba 2+ into Sr 2+ sites in the stoichiometric compositions of (Tl 0.6 Pb 0.2 Bi 0.2 )(Sr 2- x Ba x )Ca 2 Cu 3 O 9 ( x =0.2-0.3). The as-synthesized samples have a superconducting transition temperature around 115 K which can be further increased up to 122 K after post-annealing at 820°C for 20 h in oxygen.

The fabrication and characterization of superconducting Tl-Pb-Ca-Pr-Sr-Cu-O compounds with Y1Ba2Cu3Oy-like structure and Tc (zero) up to 106 K

Bulk superconductivity with T c (zero) up to 106 K in a Tl 0.5 Pb 0.5 Ca 0.8 Pr 0.2 Sr 2 Cu 2 oxide with Y 1 Ba 2 Cu 3 O y -like structure was observed. A single phase, tetragonal structure with a = 0.382 ± 0.001 and c = 1.200 ± 0.001 nm and space group P4/mmm, was found in Tl 0.5 Pb 0.5 Ca 0.8 Pr 0.2 Sr 2 Cu 2 oxide samples by X-ray diffraction. T c (zero)s were found to be 78, 96, 106, 98, 45 and 25 K for Tl 0.5 Pb 0.5 Ca 1− x Pr x Sr 2 Cu 2 oxide samples with x = 0, 0.1, 0.2, 0.3, 0.5 and 0.7, respectively. For x = 1, the samples exhibited semiconducting behavior. All sintered samples were found to be remarkably homogeneous both in composition and structure. They were also exceptionally regular in microstructure and highly reproducible and stable. Although the six-metal blend makes the system appear to be rather complicated composition-wise, it is compensated by the relatively simple crystal structure. Furthermore, the preparation conditions are much less stringent than those for other copper-based high- T c superconductors.

Accelerated formation of the three Cu‐O layered Tl‐(Pb,Bi)‐Sr‐Ca‐Cu‐O compound

Bulk samples containing nearly single phase of the three Cu‐O layered compound in the Tl‐Pb(Bi)‐Sr‐Ca‐Cu‐O system have been readily synthesized. By partially substituting Ca for Sr, an accelerated formation of the desired three Cu‐O layered phase was observed when the Ca/Sr ratio reached up to 2.4/1.6. The phase purity was clearly evidenced by x‐ray diffraction and magnetization measurement. The synthesis is valuable for further application interests.

A new high-Tc superconducting Tl-Pb-Ca-Sr-Cu-O system

Abstract A new family of high-Tc superconducting material in the Tl-Pb-Ca-Sr-Cu-O system has been synthesized. Multiple phases with two-step superconducting transitions around 110 K and 80 K were observed resistively and magnetically. The 80 K phase has been tentatively identified to be (Tl, Pb)2Sr2CuOy by X-ray diffraction analysis. This material appears to have a relatively high flux pinning.

Synthesis and characterization for a new family of Tl-containing septenary oxides with Tc,zero above 105 K

Abstract Bulk superconductivity with Tc,zero above 105 K in Tl 0.5 Pb 0.5 (Ca 1−x A x )Sr 2 Cu 2 O y oxides (x=0.2, A=Y and rare-earth elements La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) with YBa 2 Cu 3 O y -like structure was observed. For A=Ce and mischmetal, Tc,zero was found to be 95 K and 101K with x=0.1. The results represent one of the few cases where chemical substitution has significantly raised Tc in a known compound. Single phase samples were prepared and were found to be highly reproducible and stable. Furthermore, the preparation conditions are much less stringent than those of other copper-based high-Tc superconductors.