Pressure effect on the zero-phonon line emission of Mn4+ in K2SiF6

Luminescence and Formation of Cubic and Hexagonal (K,Rb)2SiF6:Mn4

The efficient red-emitting phosphor K2SiF6:Mn4+ (KSF) is widely used for low-power LED applications. The saturated red color and sharp line emission are ideal for application in backlight LEDs for displays. However, the long excited state lifetime lowers the external quantum yield (EQY) at high photon flux, limiting the application in (higher power density) lighting. Here, we report the synthesis of a new crystalline phase: hexagonal (K,Rb)SiF6:Mn4+ (h-KRSF). Due to the lower local symmetry, the Mn4+ emission in this new host material shows a pronounced zero phonon line, which is different from Mn4+ in the cubic KSF. The lower symmetry reduces the excited state lifetime, and thus, the loss of EQY under high photon fluxes, and the spectral change also increases the lumen/W output. Temperature-dependent emission and lifetime measurements reveal a high luminescence quenching temperature of ∼500 K, similar to that of KSF. The formation mechanism of h-KRSF was studied in situ by measuring the emission spectra of the precipitate in solution over time. Initially, nanocrystalline cubic KRSF (c-KRSF) is formed, which transforms into a microcrystalline hexagonal precipitate with a surprising exponential increase in the transformation rate with time. The stability of the new phase was studied by temperature-dependent XRD, and an irreversible transition back to the cubic phase was seen upon heating to temperatures above 200 °C.

Evolution of the full energy structure of Mn4+ in fluoride phosphors under high pressure conditions

This paper analyzes the photoluminescence excitation and emission spectra of fluoride phosphors doped with Mn⁴⁺: KNaSiF₆:Mn⁴⁺, Rb₂GeF₆:Mn⁴⁺, and Na₃HTiF₈:Mn⁴⁺ under high pressure conditions. From the optical spectra, the pressure-dependent energies of optically active ⁴T₂, ⁴T₁, and ²E crystal field subterms of Mn⁴⁺ have been determined in the 0-30 GPa pressure range. A strong blueshift of the ⁴T₂ and ⁴T₁ subterms was found, as expected from the Tanabe-Sugano diagram for Mn⁴⁺ (d³). At the same time, the ²E emitting state exhibited a redshift under pressure - an effect opposite to the prediction of the Tanabe-Sugano diagram. This is a manifestation of the pressure-driven nephelauxetic effect, governed by pressure induced changes of Racah parameters, which demonstrates the necessity of taking into account the Racah parameters for a correct description of Mn⁴⁺ emission under pressure. The high pressure experimental data allowed to determine the pressure dependence of crystal field strength parameter Dq and Racah parameters B and C. Finally, obtaining the pressure dependence of Dq and Racah parameters allowed to calculate the full energy structure of the d³ configuration of Mn⁴⁺ in KNaSiF₆, Rb₂GeF₆, and Na₃HTiF₈ in the pressure range of 0-30 GPa. The calculations reproduced the redshift of the ²E emitting state under pressure, as well as gave the pressure shift direction and magnitude for all crystal field subterms of Mn⁴⁺ up to 50 000 cm^-1 (i.e. the equivalent of the Tanabe-Sugano diagram for high-pressure experiments). The approach presented in this paper can be easily extended for calculating the energy structure of materials doped with isoelectronic Cr³⁺ as well as other transition metal ions.

Inverse‐Tunable Red Luminescence and Electronic Properties of Nitridoberylloaluminates Sr 2− x Ba x [BeAl 3 N 5 ]:Eu 2+ ( x =0–2)

The nitridoberylloaluminate Ba₂[BeAl₃N₅]:Eu²⁺ and solid solutions Sr_2−xBa_x[BeAl₃N₅]:Eu²⁺ (x=0.5, 1.0, 1.5) were synthesized in a hot isostatic press (HIP) under 50 MPa N₂ atmosphere at 1200 °C. Ba₂[BeAl₃N₅]:Eu²⁺ crystallizes in triclinic space group P1‾ (no. 2) (Z=2, a=6.1869(10), b=7.1736(13), c=8.0391(14) Å, α=102.754(8), β=112.032(6), γ=104.765(7)°), which was determined from single‐crystal X‐ray diffraction data. The lattice parameters of the solid solution series have been obtained from Rietveld refinements and show a nearly linear dependence on the atomic ratio Sr : Ba. The electronic properties and the band gaps of M₂[BeAl₃N₅] (M=Sr, Ba) have been investigated by a combination of soft X‐ray spectroscopy and density functional theory (DFT) calculations. Upon irradiation with blue light (440–450 nm), the nitridoberylloaluminates exhibit intense orange to red luminescence, which can be tuned between 610 and 656 nm (fwhm=1922–2025 cm⁻¹ (72–87 nm)). In contrast to the usual trend, the substitution of the smaller Sr²⁺ by larger Ba²⁺ leads to an inverse‐tunable luminescence to higher wavelengths. Low‐temperature luminescence measurements have been performed to exclude anomalous emission.

Influence of Isostatic Pressure on the Elastic and Electronic Properties of K2SiF6:Mn4

Isostatic pressure effects on the elastic and electronic properties of non-doped and Mn4+-doped K2SiF6 (KSF) have been investigated by first-principles calculations within density functional theory (DFT). Bulk modulus was obtained by the Murnaghan's equation of states (EOS) using the relationship between volume and pressures at pressures between 0 and 40 GPa, and elastic constants were calculated by the stress-strain relationship giving small distortions at each pressure point. The other elastic parameters such as shear modulus, sound velocity and Debye temperature, which can be obtained from the elastic constants, were also estimated. The influence of external isostatic pressure on the electronic properties, such as crystal field strength 10Dq and emission energy of 2E → 4A2 transition (Eem), of KSF:Mn4+ was also studied. The results suggest that 10Dq and Eem linearly increase and decrease, respectively, with increasing pressure.

Thermally Stable and Deep Red Luminescence of Sr1-xBax[Mg2Al2N4]:Eu2+ (x = 0-1) Phosphors for Solid State and Agricultural Lighting Applications

The systematic substitution of Ba in the Sr site of Sr[Mg₂Al₂N₄]:Eu²⁺ generates a deep-red-emitting phosphor with enhanced thermal luminescence properties. Gas pressure sintering (GPS) of all-nitride starting materials in Molybdenum (Mo) crucibles yields pure-phase red-orange-colored phosphors. Peaks in the synchrotron X-ray diffraction (SXRD) data show a systematic shift toward smaller angles due to the introduction of the larger Ba cation in the same crystal structure. The photoluminescence property reveals that Ba substitution shifts the original emission wavelength of Sr[Mg₂Al₂N₄]:Eu²⁺ (625 nm) toward ∼690 nm for Ba[Mg₂Al₂N₄]:Eu²⁺. Thermal stability measurement of Sr_1-xBa_x[Mg₂Al₂N₄] indicates a systematic increase in stability from x = 0 to x = 1. X-ray absorption near-edge spectroscopy (XANES) results demonstrate the coexistence of Eu²⁺ and Eu³⁺. The red-shift and the enhanced thermal stability reveals that the distance of the emitting 5d level to the conduction band of Ba[Mg₂Al₂N₄]:Eu²⁺ is large. The ionic size mismatch of Eu occupying a Ba site reduces the symmetry, thereby further splitting the degenerate emitting 5d level and lowering the energy of the emitting center. The development of deep-red phosphors emitting at 670-690 nm (x = 0.8-1.0) offers possible candidates for plant lighting applications.

A strong zero-phonon line red phosphor BaNbF7:Mn4+ for white LEDs

A novel red fluoride phosphor BaNbF₇:Mn⁴⁺ was synthesized by co-precipitation method. Under UV and blue light excitation, this phosphor shows strong ZPL emission intensity at 630 nm and which makes it an attractive red phosphor candidate for WLEDs.

A novel Cs2NbOF5:Mn4+ oxyfluoride red phosphor for light-emitting diode devices

The addition of a red-emitting phosphor to YAG:Ce3+-based white light-emitting diodes (WLEDs) greatly facilitates their applications in the field of high-color-rendering-index warm solid-state lighting. It is highly desirable to develop a red phosphor with satisfactory spectral features and low synthesis cost. In this study, a novel non-rare-earth and nonequivalent doping type of Cs2NbOF5:Mn4+ oxyfluoride red-emitting phosphor with high luminous efficiency was obtained via a facile room-temperature co-precipitation method, and its morphology and luminescent properties were investigated in detail. The Cs2NbOF5:Mn4+ phosphor with micro-rod-like morphology exhibited broad band absorption at blue light region (∼474 nm) and narrow bandwidth emissions at red region (∼633 nm). The color purity of the Cs2NbOF5:Mn4+ phosphor was calculated to be about 99%, and the internal quantum yield (QY) under 474 nm excitation was 63.4%. The concentration quenching of Mn4+ in Cs2NbOF5 matrix was mainly due to dipole-dipole interactions, and the activation energy of temperature quenching was calculated to be ∼0.2610 eV. The demonstration of a blue InGaN LED chip in combination with a blend of newly developed Cs2NbOF5:Mn4+ red phosphor and YAG:Ce3+ yellow phosphor greatly decreased the correlated color temperature (CCT) from 6255 to 3517 K while significantly improving the color rendering index (CRI) from 72.5 to 87.5. It deserves to be mentioned that the brand-new matrix to phosphor in the present study can be extended to various niobium/tantalum oxyfluoride series, which is very helpful for developing and designing new red phosphors.

A novel red phosphor of seven-coordinated Mn4+ ion-doped tridecafluorodizirconate Na5Zr2F13 for warm WLEDs

A novel red phosphor, Na₅Zr₂F₁₃:Mn⁴⁺, exhibits a strong zero phonon line at 616 nm, which is attributed to the low symmetry of Mn⁴⁺ ions occupied in a seven-coordinated environment. A warm white light has been produced by a WLED fabricated with the red phosphor Na₅Zr₂F₁₃:Mn⁴⁺.

Synthesis and photoluminescence of Mn4+ activated ternary-alkaline fluoride K2NaGaF6 red phosphor for warm-white LED application

A novel ternary-alkaline red emitting fluoride phosphor K₂NaGaF₆:Mn⁴⁺ was successfully synthesized. The crystal structure, morphology, electronic band structure and luminescence properties of K₂NaGaF₆:Mn⁴⁺ phosphors were investigated in detail.

Temperature effect on the emission spectra of narrow band Mn4+phosphors for application in LEDs

In narrow-band Mn⁴⁺phosphors, as the temperature increases, the intensity of both Stokes and anti-Stokes phonon sidebands increases with respect to zero-phonon line.

Abnormal site occupancy and high performance in warm WLEDs of a novel red phosphor, NaHF2:Mn4+, synthesized at room temperature

A novel red phosphor, NaHF₂:Mn⁴⁺, was obtainedviasubstituting Na⁺located at the center of the octahedron with Mn⁴⁺. This work provides a novel strategy to develop novel Mn⁴⁺doped red phosphors with controlled luminescence properties by substituting for alkali metal ions.

Mn4+doped (NH4)2TiF6and (NH4)2SiF6micro-crystal phosphors: synthesis through ion exchange at room temperature and their photoluminescence properties

Two red micro-crystal phosphors: (NH₄)₂TiF₆:Mn⁴⁺and (NH₄)₂SiF₆:Mn⁴⁺have been obtained through ion exchange at room temperature for a few hours. The reaction mechanism and luminescence properties of them have been comprehensively invested.