Synthesis and Study of the Effect of Ba2+ Cations Substitution with Sr2+ Cations on Structural and Optical Properties of Ba2-xSrxZnWO₆ Double Perovskite Oxides (x = 0.00, 0.25, 0.50, 0.75, 1.00)

Phosphors Ba2 LaTaO6 :Mn4+ and its alkali metal charge compensation for plant growth illumination

A series of Mn4+ -doped and Mn4+ ,K+ -co-doped Ba2 LaTaO6 (BLT) double-perovskite phosphors was synthesized using a high-temperature solid-state reaction. The phase purity and luminescence properties were also studied. The optimum doping concentration of Mn4+ and K+ was obtained by investigating the photoluminescence excitation spectra and photoluminescence emission spectra. The comparison of BLT:Mn4+ phosphors with and without K+ ions shows that the photoluminescence intensity of K+ -doped phosphors was greatly enhanced. This is because there was a charge difference when Mn4+ ions were doped with Ta5+ ions in BLT. Mn4+ -K+ ion pairs were formed after doping K+ ions, which hinders the nonradiative energy transfer between Mn4+ ions. Therefore, the luminescence intensity, quantum yield, and thermal stability of phosphors were enhanced. The electroluminescence spectra of BLT:Mn4+ and BLT:Mn4+ ,K+ were measured. The spectra showed that the light emitted from the phosphors corresponded well with chlorophyll a and phytochrome PR . The results show that the BLT:Mn4+ ,K+ phosphors had good luminescence properties and application prospects and are ideal materials for plant-illuminated red phosphors.

Dielectric, AC Conductivity, and DC Conductivity Behaviours of Sr2CaTeO6 Double Perovskite

Relatively new double perovskite material, Sr₂CaTeO₆, has been prepared through conventional solid-state procedures. Structural, dielectric, and optical characteristics of this exquisite solid-state material were analysed in this study. The single-phase monoclinic P2₁/n structure of this prepared compound was well correlated with the literature review. Good distribution of grain sizes and shapes was observed in the morphological study of this compound. The discussions on its optical and dielectric properties are included in this manuscript. High dielectric real permittivity, low dielectric loss, and good capacitance over a range of temperatures possessed by this compound, as shown in dielectric and electrical modulus studies, indicated good potential values for capacitor applications. The R_o(R_gQ_g)(R_gbQ_gb) circuit fitted well with the impedance and electrical modulus plot of the compound. Its relatively high electrical DC conductivity in grain at high frequencies and its increasing value with the temperature are typical of a semiconductor behaviour. This behaviour might be attributed to the presence of minor oxygen vacancies within its lattice structure and provides a long-range conduction mechanism. A small difference between activation energy and E_a of DC conductivity indicates that the same charge carriers were involved in both grains and the grain boundaries’ long-range conduction. The electrical AC conductivity of this compound was found to contribute to the dielectric loss in grain structure and can be related to Jonscher’s power law. The presence of polarons in this compound was exhibited by non-overlapping small polaron tunnelling (NSPT) and overlapping large polaron tunnelling (OLPT) conduction mechanisms over a range of temperatures. Wide optical band gap and E_opt in the range of 2.6 eV to 3.6 eV were determined by using an indirect and direct allowed mechanism of electrons transitions. These values supported the efficient semiconducting behaviour of the grain in this material and are suitable for applications in the semiconductor industry.

Site Preference-Driven Mn4+ Stabilization in Double Perovskite Phosphor Regulating Quantum Efficiency from Zero to Champion

The tetravalent-state stability of manganese is of primary importance for Mn⁴⁺ luminescence. Double perovskite-structured A₂B'B″O₆:Mn⁴⁺ has been recently prevalent, and the manganese ions are assumed to substitute for the B″(IV-VI)O₆ site to stabilize at the tetravalent charge state to generate far-red emissions. However, some Mn-doped A₂B'B″O₆-type materials show no or weak luminescence such as typical Ca₂MgWO₆:Mn. In this work, a cation-pair co-substitution strategy is proposed to replace 2Ca²⁺ by Na⁺-La³⁺ to form Ca_2-2xNa_xLa_xMgWO₆:Mn. The significant structural distortion appears in the solid solution lattices with the contraction of [MgO₆] but enlargement of [WO₆] octahedron. We hypothesize that the site occupancy preference of Mn migrates from Mg²⁺ to W⁶⁺ sites. As a result, the effective Mn⁴⁺/Mn²⁺ concentration enhances remarkably to regulate nonluminescence to highly efficient Mn⁴⁺-related far-red emission. The optimal CaNa_0.5La_0.5MgWO₆:0.9%Mn⁴⁺ shows an internal quantum efficiency of 94% and external quantum efficiency of 82%, reaching up to the top values in Mn⁴⁺-doped oxide phosphors. This work may provide a new perspective for the rational design of Mn⁴⁺-activated red phosphors, primarily considering the site occupancy modification and tetravalent-state stability of Mn.

Effects of iron substitution and anti-site disorder on crystal structures, vibrational, optical and magnetic properties of double perovskites Sr2(Fe1−xNix)TeO6

We report a new series of DP Sr2(Fe1−xNix)TeO6, which have different transition metal Fe and Ni on B sites, providing an opportunity to investigate their effect on crystal structure, vibrational, optical and magnetic properties.

Structural, optical and dielectric properties of tellurium-based double perovskite Sr2Ni1-x Zn x TeO6

In this paper, Sr₂Ni_1−xZn_xTeO₆ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) double perovskite compounds were synthesised by the conventional solid-state method, and the structural, optical and dielectric properties were investigated. The Rietveld refinement of X-ray diffraction data shows that all compounds were crystallised in monoclinic symmetry with the I2/m space group. Morphological scanning electron microscopy reported that the grain sizes decreased as the dopant increased. The UV-vis diffuse reflectance spectroscopy conducted for all samples found that the optical band gap energy, E_g, increased from 3.71 eV to 4.14 eV. The dielectric permittivity ε′ values increased for the highest Zn-doped composition, Sr₂Ni_0.2Zn_0.8TeO₆, being ∼1000 and ∼60 in the low- and high-frequency range, respectively. All samples exhibited low dielectric loss (tan δ ≤ 0.20) in the range of 10⁴–10⁵ Hz frequency. Impedance measurement revealed that grain resistance decreased with enhancement in Zn content in the Sr₂NiTeO₆ crystal lattice.

In this paper, Sr₂Ni_1−xZn_xTeO₆ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) double perovskite compounds were synthesised by the conventional solid-state method, and the structural, optical and dielectric properties were investigated.

Aluminum doping effects on photoresponse characteristics of hydrothermal tin disulfide nanosheets

Photoresponse characteristics of Al-doped SnS₂ nanosheets have been improved significantly by aluminum doping, compared to pristine SnS₂. The response time was reduced by two orders of magnitude and the responsivity was increased one hundredfold.

Hydrothermal synthesis and characterization of novel yellow pigments based on V5+ doped BiPO4 with high near-infrared reflectance

A series of new yellow pigments was synthesized (BiP_1−xV_xO₄), coatings colored with synthesized pigments have high NIR solar reflectance.