Mn4+-activated K2SiF6 phosphors for use in light emitting diode (LED) applications have recently attracted a great deal of attention since they exhibit an advantage over conventional wide band-type red-light-emitting phosphors.

The mechano-luminescence (ML) of phosphors has stirred a great deal of interest for its potential application in inexpensive, non-destructive load sensors. However, the most serious drawback of ML phosphors has been responses that differ according to the loading conditions. This has led to a lack of standardization in realizing smart ML sensor applications. We improved the applicability of ML phosphors to that of a smart, standardized load sensor by detecting ML based on the UV excitation above the threshold power density during the entire loading process. The ML behavior under these conditions was completely different from that of conventional ML behavior with UV excitation turned off. The ML output was clearly represented as a simple linear function of the applied load under conditions that could be either static or dynamic. In addition, neither a ML loss angle nor hysteresis behavior was observed under these ML measurement conditions.

Phosphors that exhibit a narrow red emission are particularly interesting due to the advantage of providing a more extensive color gamut and better rendering in LED applications such as displays and solid‐state lighting. Although some Eu2+‐activated nitridosilicates have been discovered in this regard, K2SiF6:Mn4+ phosphors are the only option in actual LED applications thus far. We discovered a novel phosphor, K3SiF7:Mn4+, with P4/mbm symmetry. The luminescent properties of K3SiF7:Mn4+ are almost identical to those of the K2SiF6:Mn4+ phosphor, but its materials identity is distinct due to a completely different crystallographic structure, which leads to reduced decay time. The fast decay is one of the most serious disadvantages of existing K2SiF6:Mn4+ phosphors. The K3SiF7:Mn4+ phosphor was examined in comparison to the K2SiF6:Mn4+ via density functional theory calculation, Rietveld refinement, X‐ray photoelectron spectroscopy, X‐ray absorption near‐edge structure spectroscopy, and time‐resolved photoluminescence.

A solid-state combinatorial chemistry approach, which used the A-Ge-O (A = Li, K, Rb) system doped with a small amount of Mn as an activator, was adopted in a search for novel red-emitting phosphors. The A site may have been composed of either a single alkali metal ion or of a combination of them. This approach led to the discovery of a novel phosphor in the above system with the chemical formula LiRbGeO:Mn. The crystal structure of this novel phosphor was solved via direct methods, and subsequent Rietveld refinement revealed a trigonal structure in the P3̅1m space group. The discovered phosphor is believed to be novel in the sense that neither the crystal structure nor the chemical formula matches any of the prototype structures available in the crystallographic information database (ICDD or ICSD). The measured photoluminescence intensity that peaked at a wavelength of 667 nm was found to be much higher than the best intensity obtained among all the existing AGeO (A = Li, K, Rb) compounds in the alkali-germanate system. An ab initio calculation based on density function theory (DFT) was conducted to verify the crystal structure model and compare the calculated value of the optical band gap with the experimental results. The optical band gap obtained from diffuse reflectance measurement (5.26 eV) and DFT calculation (4.64 eV) results were in very good agreement. The emission wavelength of this phosphor that exists in the deep red region of the electromagnetic spectrum may be very useful for increasing the color gamut of LED-based display devices such as ultrahigh-definition television (UHDTV) as per the ITU-R BT.2020-2 recommendations and also for down-converter phosphors that are used in solar-cell applications.