Phase Transformation of a K2GeF6 Polymorph for Phosphors Driven by a Simple Precipitation-Dissolution Equilibrium and Ion Exchange

Formation and Polymorphism of Semiconducting K2SiH6 and Strategy for Metallization

K2SiH6, crystallizing in the cubic K2PtCl6 structure type (Fm3̅m), features unusual hypervalent SiH62- complexes. Here, the formation of K2SiH6 at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH3 as a precursor. At the investigated pressures, 8 and 13 GPa, K2SiH6 adopts the trigonal (NH4)2SiF6 structure type (P3̅m1) upon formation. The trigonal polymorph is stable up to 725 °C at 13 GPa. At room temperature, the transition into an ambient pressure recoverable cubic form occurs below 6.7 GPa. Theory suggests the existence of an additional, hexagonal, variant in the pressure interval 3-5 GPa. According to density functional theory band structure calculations, K2SiH6 is a semiconductor with a band gap around 2 eV. Nonbonding H-dominated states are situated below and Si-H anti-bonding states are located above the Fermi level. Enthalpically feasible and dynamically stable metallic variants of K2SiH6 may be obtained when substituting Si partially by Al or P, thus inducing p- and n-type metallicity, respectively. Yet, electron-phonon coupling appears weak, and calculated superconducting transition temperatures are <1 K.

Ultraintense Zero-Phonon Line from a Mn4+ Red-Emitting Phosphor for High-Quality Backlight Display Applications

The zero-phonon line (ZPL) of Mn⁴⁺, which highly depends on its local environment, is usually much weaker than the vibrational phonon sidebands. In this work, an ultraintense ZPL emission, coming from a brand new red-emitting Rb₂LiGaF₆:Mn⁴⁺ (RLGFM) phosphor upon blue light excitation, is presented. The interesting spectral characteristic originates from the nonequivalent substitution of Mn⁴⁺ for Ga³⁺ in a rigid octahedral structure with a low symmetry, which induces neighboring cation vacancies that distort the local symmetry of the [MnF₆] octahedra. Benefiting from the ultraintense ZPL emission, a wide color gamut is achieved using RLGFM and β-SiAlON:Eu²⁺ as color converters. Moreover, a comprehensive investigation on the thermal quenching behavior is also conducted to provide detailed insights to explore novel Mn⁴⁺ red phosphors for high-quality backlight display applications.

Enhancing Structural Rigidity via a Strategy Involving Protons for Creating Water-Resistant Mn4+-Doped Fluoride Phosphors

The poor water resistance property of a commercial Mn⁴⁺-activated narrow-band red-emitting fluoride phosphor restricts its promising applications in high-performance white LEDs and wide-gamut displays. Herein, we develop a structural rigidity-enhancing strategy using a novel KHF₂:Mn⁴⁺ precursor as a Mn source to construct a proton-containing water-resistant phosphor K₂(H)TiF₆:Mn⁴⁺ (KHTFM). The parasitic [HMnF₆]^- complexes in the interstitial site from the fall off the KHF₂:Mn⁴⁺ are also transferred to the K₂TiF₆ host by ion exchange to form KHTFM with rigid bonding networks, improving the water resistance and thermostability of the sample. The KHTFM sample retains at least 92% of the original emission value after 180 min of water immersion, while the non-water-resistant K₂TiF₆:Mn⁴⁺(KTFM) phosphor maintains only 23%. Therefore, these findings not only illustrate the effect of protons on fluoride but also provide a novel insight into commercial water-resistant fluoride phosphors.