Synthesis and luminescence properties of color-tunable Ba1−ySryLa4−xTbx(WO4)7 (x = 0.02-1.2, y = 0-0.4) phosphors

Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ (x = 0.02-1.2, y = 0-0.4) phosphors were prepared via a solid-state reaction and their photoluminescence properties were investigated. An analysis of the decay behavior indicates that the energy migration between Tb³⁺ ions is conspicuous in the ⁵D₃ → ⁷F₄ transition due to the cross-relaxation in BaLa₄(WO₄)₇. A partial substitution of Ba²⁺ by Sr²⁺ can not only enhance the emission intensity but also increase the solid solubility of Tb³⁺ in Ba_1−ySr_yLa_4−xTb_x(WO₄)₇. The emission intensity of the ⁵D₄ → ⁷F_J (J = 4, 5, 6) transitions can be enhanced by increasing Sr²⁺ and Tb³⁺ concentrations, with the optimal conditions being x = 1.2, y = 0.4 (Ba_0.6Sr_0.4La_2.8Tb_1.2(WO₄)₇). Under near-UV excitation at 379 nm, the CIE color coordinates of Ba_1−ySr_yLa_4−xTb_x(WO₄)₇ vary from blue (0.212, 0.181) at x = 0.04, y = 0, to green (0.245, 0.607) at x = 1.2, y = 0.4.     

Synthesis and electrical properties of ordered perovskite oxide Cd3−x−yCuxAyTeO6 (A = Li, Na; 0.00 ≤ x, y ≤ 0.15)

Polycrystalline Cd_3−x−yCu_xA_yTeO₆ (A = Li, Na) samples were prepared by solid-state reaction, and their crystal structure and electrical properties were investigated. In Cd_3−xCu_xTeO₆ and Cd_3−yA_yTeO₆ (A = Li, Na), the maxim solubility of x and y was 0.15 and 0.15 for A = Li, 0.05 for A = Na, respectively. For co-substituted samples Cd_2.9−yCu_0.1Li_yTeO₆ and Cd_2.9−yCu_0.1Na_yTeO₆, the maxim solubility of x was the same as single substitution above-mentioned. The alkali-metal substituted samples Cd_3−yA_yTeO₆ (A = Li, Na) showed a negative Seebeck coefficient, which indicates that the major conduction carriers are electron. On the other hand, the co-substituted samples Cd_2.9−yCu_0.1A_yTeO₆ (A = Li, Na) represented a positive Seebeck coefficient, and major conduction carriers were hole through substitution by copper ions.     

Luminescence characterization of (Ca1−xSrx)(S1−ySey):Eu,M (M = Sc and Y) for high color rendering white LED

The highly efficient red phosphors (Ca_1−xSr_x)(S_1−ySe_y):Eu²⁺,M³⁺ (M = Sc and Y) were prepared, starting from CaCO₃, SrCO₃, Eu₂O₃, Sc₂O₃, Y₂O₃, S, and SeO₂ with a flux, by a conventional solid-state reaction. The optimized red phosphors converted 11.8% (Sc³⁺) and 11.7% (Y³⁺) of the absorbed blue light into luminescence. These quantum values are much higher than Q = 3.0% of CaS:Eu²⁺. For the fabrication of light-emitting diodes (LEDs), the prepared phosphors were coated with MgO from non-aqueous solution to overcome their weakness against moisture. White LEDs were fabricated by pasting the prepared red phosphors and the yellow YAG:Ce³⁺ phosphor on an InGaN blue chip (λ_ems = 446.5 nm). The incorporation of the red phosphor to the YAG:Ce³⁺ phosphor resulted in an improved color rendering index (Ra) from 70 to 80.     

Synthesis, low-temperature sintering and the dielectric properties of the ZnO-(1 − x)TiO2-xSnO2 (x = 0.04-0.2)

ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. Fine particle powders were prepared by chemical processing to activate the formation of compound and to improve the sinterability. One wt.% of V₂O₅ and B₂O₃ with the mole ratios of 3:1 were used to lower the sintering temperature of ceramics. The effect of Sn content on phase structure and dielectric properties were investigated. The results show that the substituting Sn for Ti accelerates the hexagonal phase transition to cubic phase, and an inverse spinel structure Zn₂(Ti_1−xSn_x)O₄ solid solution forms. The best dielectric properties obtained at x = 0.12. The ZnO-0.88TiO₂-0.12SnO₂ ceramics sintered at 900 °C exhibit a good dielectric property: ?_r = 29 and tan δ = 9.86 × 10⁻⁵. Due to their good dielectric properties, low firing characteristics, ZnO-(1 − x)TiO₂-xSnO₂ (x = 0.04-0.2) can serve as the promising microwave dielectric capacitor.     

Effect of B-site doping on Sm0.5Sr0.5MxCo1−xO3−δ properties for IT-SOFC cathode material (M = Fe, Mn)

The crystal structure, thermal expansion rate, electrical conductivity and electrochemical performance of Sm_0.5Sr_0.5M_xCo_1−xO_3−δ (M = Fe, Mn) have been investigated. Two crystal structures have been observed in the specimens of Sm_0.5Sr_0.5Fe_xCo_1−xO_3−δ (SSFC) at room temperature, the perovskite structure of SSFC has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5 ≤ x ≤ 0.9. The specimens of Sm_0.5Sr_0.5Mn_xCo_1−xO_3−δ (SSMC) crystallize in an orthorhombic structure. The adjustment of thermal expansion rate to electrolyte, which is one of the main problems of SSC, can be achieved to lower TEC values with more Fe and Mn substitution. Especially, Sm_0.5Sr_0.5Mn_0.8Co_0.2O_3−δ exhibits good thermal compatibility with La_0.8Sr_0.2Ga_0.8Mg_0.2O₃. High electrical conductivities are obtained for all the specimens and they demonstrate above 100 S/cm at 800 °C in SSFC system. The polarization resistance increases with increasing Mn content, Nevertheless, the polarization resistance of SSFC increases with increasing Fe content, but when the amount of Fe reaches to 0.4, the maximum is obtained while the resistance will decrease when the amount of Fe reaches above 0.4. Sm_0.5Sr_0.5Fe_0.8Co_0.2O_3−δ electrode exhibits high catalytic activity for oxygen reduction operating at temperature from 700 to 800 °C.     

Structure, dielectric and ferroelectric anisotropy of Sr2−xCaxBi4Ti5O18 ceramics

Sr_2−xCa_xBi₄Ti₅O₁₈(x = 0, 0.05) powders synthesized by solid state route were uniaxially pressed and sintered at 1225 °C for 2 h. The obtained dense ceramics exhibited crystallographic anisotropy with a dominant c axis parallel to the uniaxial pressing direction which was quantified in terms of the Lotgering factor. Microstructure anisotropy of both compositions (x = 0, 0.05) consisted of plate like grains exhibiting their larger surfaces mostly perpendicular to the uniaxial pressing direction. Dielectric and ferroelectric properties of Sr_2−xCa_xBi₄Ti₅O₁₈ ceramics were measured under an electric field (E) applied parallel and perpendicularly to uniaxial pressing direction. The obtained dielectric ?_R(T) and polarization (P-E) curves depended strongly on E direction thus denoting a significant effect from microstructure and crystallographic texture. Sr_2−xCa_xBi₄Ti₅O₁₈ properties were also significantly affected by Ca content (x): Curie temperature increased from 280 °C (x = 0) to 310 °C (x = 0.05) while ?_R and remnant polarization decreased for x = 0.05. The present results are discussed within the framework of the processing and crystal structure-properties relationships of Aurivillius oxides ceramics.     

Eu-Mössbauer spectroscopic and X-ray diffraction study on EuyM1−yO2−x (0 ≤ y ≤ 1.0) (M = Th, U)

¹⁵¹Eu-Mössbauer spectroscopic and powder X-ray diffraction (XRD) study has been performed for the Eu_yM_1−yO_2−x (M = Th and U) systems over the entire composition range of 0 ≤ y ≤ 1.0. The XRD results of the Eu-Th system showed that a very wide defect-fluorite (DF) type phase in which oxygen vacancies (V_O) are disordered (x = y/2) is formed for 0 ≤ y < 0.5 and that two-phase regions sandwitching a narrow C-type (C) single phase around y ≈ 0.8 appear for 0.5 < y < 0.8 (DF + C) and 0.82 < y < 1.0 (C + B-type (monoclinic) Eu₂O₃). The Mössbauer results show that the isomer shifts (ISs) of Eu³⁺ in this system smoothly increase with Eu composition, y. The decrease of average coordination number (CN) of O²⁻ around Eu³⁺ with increasing y (CN = 8 − 2y) (x = y/2) results in the decrease of the average EuO bond length, which is due to the decrease of repulsion force between O²⁻ anions. This result confirms that the IS of Eu³⁺ correlates well with the average EuO bond length in oxide systems. For the Eu-U system, the lattice parameter, a₀, of the system decreases almost linearly with y, in accordance with the calculated a₀ versus y curve for the oxygen-stoichiometric (i.e. x = 0) fluorite-type dioxide (CN = 8). The ISs of Eu³⁺ in this composition range remain almost constant around 0.5 mm/s, which is comparable to those of pyrochlore oxides (Eu₂Zr₂O₇ and Eu₂Hf₂O₇ (y = 0.5)) with O²⁻-eight-fold coordinated Eu³⁺(CN = 8).     

Oxygen vacancy ordering in strontium doped rare earth cobaltate perovskites Ln1−xSrxCoO3−δ (Ln = La, Pr and Nd; x > 0.60)

A family of Sr-doped perovskite compounds Ln_1−xSr_xCoO_3−δ (Ln = La³⁺, Pr³⁺ and Nd³⁺; x > 0.60), were prepared by sol-gel chemistry and reaction at 1100 °C under 1 atm of oxygen. This structural family has been shown to be present only for rare earth ions larger than Sm³⁺ and an upper limit of Sr²⁺ solubility in these phases was found to exist between x = 0.90 and 0.95. X-ray diffraction shows oxygen-deficient, simple cubic (Pm-3m) perovskite crystal structures. The combination of electron and powder neutron diffraction reveals that oxygen vacancy ordering occurs, leading to a tetragonal (P4/mmm) superstructure and a doubling of the basic perovskite unit along the crystallographic c-axis. No additional Ln³⁺/Sr²⁺ cation ordering was observed.     

Structural, magnetic, and electronic characterization of double perovskites BixLa2−xMnMO6 (M = Ni, Co; x = 0.25, 0.50)

A newly substituted series of perovskites, Bi_xLa_2−xMnMO₆ with M = Ni, Co and x = 0.25, 0.50, were synthesized using a citrate sol-gel technique. The crystal structure, established from neutron diffraction, is a distorted double perovskite with partial transition metal B-site ordering. These perovskites crystallize in the centrosymmetric space group, P2₁/n, with structures that are similar to the x = 0 end members. All samples are prone to non-stoichiometry involving substitution of Mn onto the Ni/Co sites, in addition to varying degrees of antisite disorder. The neutron powder diffraction and magnetization measurements reveal ferromagnetism in all samples with ordering temperatures between 220 K and 280 K. The M = Co samples have lower Curie temperatures, but higher coercivities. There is a clear link between the degree of transition metal site disorder and the saturation magnetization values. Electrical measurements demonstrate the presence of mixed ionic and electronic conductivity.     

Structural and microwave dielectric behavior of (Li1/4Nb3/4) substituted ZrxSnyTizO4 (x + y + z = 2) system

The phase structure, microwave dielectric properties, and their stability with different annealing conditions have been investigated in (Li_1/4Nb_3/4) substituted Zr_xSn_yTi_zO₄ system. The sintering temperature of Zr_xSn_yTi_zO₄ ceramic was lowered from 1500 to 1140 °C by (Li_1/4Nb_3/4) substitution. Both X-ray diffraction (XRD) analysis and electron diffraction (ED) analysis revealed that the (Li_1/4Nb_3/4) substituted Zr_xSn_yTi_zO₄ ceramic crystallized as the high-temperature disordered ZrTiO₄ phase. As the content of Sn increased from 0.10 to 0.30, the permittivity of the (Zr_1−xSn_x)(Li_1/4Nb_3/4)_0.4Ti_0.6O₄ ceramic decreased gradually from 35.5 to 31.5, the Q_f value increased from 37,800 to 58,300 GHz, and TCF value shifted slightly from −4.5 to −33.0 ppm °C⁻¹. Both the phase structure and microwave dielectric properties of (Zr_1−xSn_x)(Li_1/4Nb_3/4)_0.4Ti_0.6O₄ ceramics were stable with annealing conditions.     

Structure and magnetism in the oxygen-deficient perovskites Ce1−xSrxCoO3−δ (x ≥ 0.90)

We have examined the structure and phase behaviour of strontium-doped Ce_1−xSr_xCoO_3−δ and found that the perovskite form is stabilised over a relatively narrow solid solution range (x > 0.85). A combination of electron, powder X-ray and neutron diffraction has revealed tetragonal superstructures of the basic perovskite unit; (I4/mmm) 2a_p × 2a_p × 4a_p (x = 0.90) and (P4/mmm) a_p × a_p × 2a_p (x = 0.95). Magnetisation measurements show ferromagnetic behaviour under applied magnetic fields. Low temperature neutron diffraction of Ce_0.10Sr_0.90CoO_2.80 in zero field reveals a magnetic cell of dimension 2a_p × 2a_p × 4a_p with an ordered cobalt moment of 1.7 B.M. at 25 K.     

Synthesis of perovskite-type (La1−xCax)FeO3 (0 ≤ x ≤ 0.2) at low temperature

Gel formation was realized by adding citric acid to a solution of La(NO₃)₃·5H₂O, Ca(NO₃)₂·4H₂O, and Fe(NO₃)₂·9H₂O. Perovskite-type (La_1−xCa_x)FeO₃ (0 ≤ x ≤ 0.2) was synthesized by firing the gel at 500 °C in air for 1 h. The crystallite size (D_1 2 1) decreased with increasing x, while the specific surface area was 6.8-9.4 m²/g and independent of x. The XPS measurement of the (La_1−xCa_x)FeO₃ surface indicated that the Ca²⁺ ion content increased with increasing x, while the Fe ion content was independent of x. Catalytic activity for CO oxidation increased with increasing x.     

Phase formation, microstructure, and dielectric properties of (1 − x)PZT-(x)PCN ceramics

Ceramics in a PZT-PCN system with the formula (1 − x)Pb(Zr_1/2Ti_1/2)O₃-(x)Pb(Co_1/3Nb_2/3)O₃, where x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, and 1.0, were prepared using a solid-state mixed-oxide technique with columbite−CoNb₂O₆ and wolframite−ZrTiO₄ precursors. The crystal structure of the specimens studied with X-ray diffraction (XRD) analysis showed a coexistence between tetragonal and pseudo cubic phases at composition x = 0.2. The SEM micrograph showed that the average grain size significantly decreased with increasing PCN content. A maximum dielectric constant was observed at composition x = 0.2, while the transition temperature strongly decreased with increasing PCN content. All ceramics also showed diffused phase transition behaviors with a minimum diffusivity at x = 0.2. The morphotropic phase boundary (MPB) lay at the 0.8PZT-0.2PCN composition.     

Order-disorder phase transitions and high-temperature oxide ion conductivity of Er2+xTi2−xO7−δ (x = 0, 0.096)

The Er_2+xTi_2−xO_7−δ (x = 0.096; 35.5 mol% Er₂O₃) solid solution and the stoichiometric pyrochlore-structured compound Er₂Ti₂O₇ (x = 0; 33.3 mol% Er₂O₃) are characterized by X-ray diffraction (phase analysis and Rietveld method), thermal analysis and optical spectroscopy. Both oxides were synthesized by thermal sintering of co-precipitated powders. The synthesis study was performed in the temperature range 650-1690 °C. The amorphous phase exists below 700 °C. The crystallization of the ordered pyrochlore phase (P) in the range 800-1000 °C is accompanied by oxygen release. The ordered pyrochlore phase (P) exists in the range 1000−1200 °C. Heat-treatment at T ≥ 1600 °C leads to the formation of an oxide ion-conducting phase with a distorted pyrochlore structure (P2) and an ionic conductivity of about 10⁻³ S/cm at 740 °C. Complex impedance spectra are used to separately assess the bulk and grain-boundary conductivity of the samples. At 700 °C and oxygen pressures above 10⁻¹⁰ Pa, the Er_2+xTi_2−xO_7−δ (x = 0, 0.096) samples are purely ionic conductors.     

Structural and electrical properties of La2−xBaxMo2O9−x/2 (x = 0-0.18)

La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) have been prepared by solid state reaction method. The lattice parameter of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) determined by XRD data refinement shows a linear dependence on the dopant Ba content x. For the specimen with a La/Ba molar ratio of 0.18-0.2, additional reflection of secondary phase exists in the XRD pattern, so the value of solubility limit for Ba in La₂Mo₂O₉ is defined in range of 0.18 < x < 0.2. As the replacement degree of La³⁺ by Ba²⁺ increases, the bulk conductivity of La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) decreases initially and then increases, a minimum value at La_1.9Ba_0.1Mo₂O_8.95 exists. Hebb-Wagner studies in argon atmosphere, which use an oxide-ion blocking electrode, show that La_2−xBa_xMo₂O_9−x/2 (x ≤ 0.18) are predominantly oxide-ion conducting in the temperature ranging from 773 to 1173 K. The average thermal expansion coefficient of La_1.84Ba_0.16Mo₂O_8.92 determined by high-temperature XRD was deduced as great as 17.5 × 10⁻⁶ K⁻¹ between 298 and 1173 K.     

Effect of the Mn ion on electrical and magnetic properties of orthorhombic perovskite-type Ca(Mn1−xTix)O3−δ

Orthorhombic perovskite-type Ca(Mn_1−xTi_x)O_3−δ (0 ≤ x ≤ 0.7) was synthesized at 1173 K for 12 h in a flow of oxygen from a precursor gel prepared using citric acid and ethylene glycol. The Mn³⁺ ion was generated by substituting a Ti⁴⁺ ion in CaMnO₃. The average particle size was 100-300 nm and did not depend on x. The lattice constants and the (Mn, Ti)-O distance increased linearly with increasing x. The variation in global instability index (GII) indicated that the instability of the structure increases monotonically with increasing x. Ca(Mn_1−xTi_x)O_3−δ was an n-type semiconductor that had its minimum values of electrical resistivity (ρ) and activation energy (E_a) at x = 0.1. Ca(Mn_1−xTi_x)O_3−δ (x = 0 and 0.1) exhibited a weak ferromagnetic behavior. The variation in μ_eff indicated that the spin state of the Mn³⁺ ion changes from low to high at x = 0.1, then reverts to low in the range of 0.2 ≤ x ≤ 0.7. The variations in ρ and E_a are explained by the number of electrons according to the change in the spin state of the Mn³⁺ ion.     

High-frequency magneto-electrical properties of Zn1 − x − yAlxCoyO thin films

The Al doping effects on high-frequency magneto-electric properties of Zn_{1 − x − y}Al_xCo_yO (x = 0-10.65 at.%) thin films were systematically studied. In the current work, the Zn_{1 − x − y}Al_xCo_yO thin films were deposited by magnetron co-sputtering onto quartz substrates. The magneto-impedance spectra of the thin films were measured by an impedance analyzer. Among all the doped films studied, the thin film with 6.03 at.% Al-doping showed the highest ac conductivity and relaxation frequency. To characterize the relaxation mechanism underlying the magneto-electric properties, a Cole-Cole impedance model was applied to analyze the impedance spectra. The analyzed result showed that the magneto-impedance of the Zn_{1 − x − y}Al_xCo_yO is contributed by multiple processes of magnetization dynamics and dielectric relaxation. The results imply that Zn_{1 − x − y}Al_xCo_yO may be applicable for high-frequency magneto-electric devices.     

Negative thermal expansion and electrical properties of Mn3(Cu0.6NbxGe0.4 − x)N (x = 0.05-0.25) compounds

Bulk materials with the general formula of Mn₃(Cu_0.6Nb_xGe_0.4 − x)N (x = 0.05, 0.1, 0.15, 0.2, 0.25), Mn₃(Cu_0.6Ge_0.4)N and Mn₃(Cu_0.7Ge_0.3)N were fabricated by mechanical ball milling and solid state sintering. Their thermal expansion coefficients and electrical conductivities were investigated in the temperature range of 80-300 K. It is found that the temperature interval of negative temperature expansion behavior is about 95 K in the samples of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N and Mn₃(Cu_0.6 Nb_0.2Ge_0.2)N, which is twice as large as that of Mn₃(Cu_0.7Ge_0.3)N. The negative thermal expansion of Mn₃(Cu_0.6Nb_0.15Ge_0.25)N can reach to − 19.5 × 10⁻⁶ K^− 1 in the temperature range of 165 to 210 K. The electrical conductivity of this series materials is in a level of about 2.5 × 10⁶ (Ω m)^− 1.     

Synthesis, structure and magnetic properties of Sr2Fe1−xGaxMoO6 (0 ≤ x ≤ 0.6) double perovskites

Polycrystalline Sr₂Fe_1−xGa_xMoO₆ (0 ≤ x ≤ 0.6) materials have been synthesized by solid state reaction method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in the tetragonal symmetry in the space group I4/m. The anti-site (AS) defects concentration increases with Ga doping, giving rise to highly B-site disordered materials. Ga doping at the Fe-site decreases the cell volume. The evolution of bond lengths and the cation oxidation states was determined from the Rietveld refinement data. The saturation magnetization and Curie temperature decreased with the increasing Ga content in the samples. Low temperature neutron diffraction data analysis and magnetization measurements confirm the magnetic interaction as ferrimagnetic in the sample.     

Itinerant ferromagnetism to insulating spin glass in SrRu1−xCuxO3 (0 ≤ x ≤ 0.3)

The ferromagnetic metallic oxide, SrRuO₃ (T_C ∼ 165 K) undergoes structural, magnetic and metal-insulator transitions upon substitution of Cu at the Ru-site. For x = 0.2 in SrRu_1−xCu_xO₃, the structure becomes a tetragonal with the space group I4/mcm and there is a signature of both ferromagnetic (T_C = 65 K) and antiferromagnetic (T_N = 32 K) ordering due to possible magnetic phase separation. The antiferromagnetism arises due to short range ordering of Cu- and Ru-moments. Jahn-Teller distortion of (Ru,Cu)-O₆ octahedra indicates that the copper ions are in 2+ oxidation state with ⁶t2g³eg electronic configuration. For x ≥ 0.1, narrowing of Ru-4d bandwidth by the substitution of Cu ions results in semiconducting behavior. For x = 0.3, the ac and dc susceptibility measurements indicate a spin glass behavior. The origin of spin glass behavior has been attributed to competing ferromagnetic and antiferromagnetic interactions.