Generally, 6s electron and 6p electron of Bi3+ ions are located in its outermost layer; thus, luminescence properties of Bi3+ are strongly associated with the coordination environment around Bi3+. Bi3+ ions occupy different cationic positions in hosts, which may cause the movement of the emission spectrum. In order to investigate the luminescent property of Bi3+, a series of Bi3+ doped Ca5(BO3)3F species are synthesized. There are three types of Ca2+ sites in the host, which could be substituted by Bi3+. Upon the 322 nm excitation of Bi3+, a broad emission band can be observed, which is ascribed to the 1s0 → 3p1 transition of Bi3+. Meanwhile, there is the emission shift of Ca5(BO3)3­F:xBi3+, and its emission color can be altered from blue to cyan. It may result from Bi3+ occupying different positions of Ca2+ in the host, which can give rise to different degrees of a nephelauxetic effect and crystal field splitting. In order to explore the relationship between the luminescence properties of Bi3+ and the nepelauxetic effect, the value of the centroid shift (∈c) is calculated. Centroid shift (∈c) is related to the covalence and average bond length of an octahedron in which the influence of covalence is primary. The relationship between the luminescence properties of Bi3+ and the crystal field splitting is discussed. The crystal field splitting is related to the interaction between the Bi3+ species, the crystal field splitting energy (Δ), and the distortion of the crystal. Emission spectra are asymmetric; meanwhile, the emission spectra have remarkable changes at various excitation wavelengths. This proves that the broadband emission band consists of at least two emission centers. In order to assess this hypothesis, the decay curves are measured. This confirms that there are three luminescence centers in a host. On one hand, considering the effect of the centroid shift (∈c) and crystal field splitting (∈cfs), the sources of three luminescence centers are confirmed by calculating the total shift (D(A)) of the 6s6p level of Bi3+ in a host. On the other hand, the source of three luminescence centers is determined by the changing trend of the average bond length of the octahedron. In addition, the luminescence properties of Ca5(BO3)3F:Bi3+, Eu3+, are investigated as well. There is efficient energy transfer from the Bi3+ to the Eu3+ ion, and the color-tunable phosphor can be achieved by the combination of the appropriate proportion of Bi3+ and Eu3+ ions. The emission color can gradually change from cyan to red.

High mass loading and high areal capacity are key metrics for commercial batteries, which are usually limited by the large charge‐transfer impedance in thick electrodes. This can be kinetically deteriorated under low temperatures, and the realization of high‐areal‐capacity batteries in cold climates remains challenging. Herein, a low‐temperature high‐areal‐capacity rechargeable potassium–tellurium (K–Te) battery is successfully fabricated by knocking down the kinetic barriers in the cathode and pairing it with stable anode. Specifically, the in situ electrochemical self‐reconstruction of amorphous Cu1.4Te in a thick electrode is realized simply by coating micro‐sized Te on the Cu collector, significantly improving its ionic conductivity. Meanwhile, the optimized electrolyte enables fast ion transportation and a stable K‐metal anode at a large current density and areal capacity. Consequently, this K–Te battery achieves a high areal capacity of 1.25 mAh cm−2 at −40 °C, which greatly exceeds those of most reported works. This work highlights the significance of electrode design and electrolyte engineering for high areal capacity at low temperatures, and represents a critical step toward practical applications of low‐temperature batteries.

Phosphors with the emission spectra located at the biological window I (650–950 nm) are significant for biological imaging. In this work, a series of deep red and near-infrared phosphors InMgGaO4: xCr3+ and In0.9‑y MgGaO4: 0.1Cr3+, yAl3+ are designed and successfully synthesized by a high temperature solid state method. InMgGaO4 is selected as the host considering its special crystal structure that one of the Mg/Ga–O bonds is affected by the surrounding environment. Therefore, when Cr3+ is substituted into the lattice, the longer Mg/Ga–O bonds are easily broken, which provides a tunable crystal field. The emission spectra of InMgGaO4: xCr3+ cover from 650 to 1200 nm, including one sharp line emission peak (peak 1) and two broad emission bands (peak 2 and peak 3). The Racha parameters D q/B and the decay curves are analyzed to distinguish the origins of these three peaks. Meanwhile, these three emission peaks show different degrees of red shift, which is related to the covalency, crystal field splitting (D q), bond breaking of Mg/Ga–O, and decrease of band gap. However, comparing with the luminescent property of Cr3+ single doped samples, In0.9‑y MgGaO4: 0.1Cr3+, yAl3+ shows a contrasting luminescence property and the reason is analyzed. In summary, the emission spectra of these samples can be tuned between a narrow peak and broad band continuously by controlling the concentration of Cr3+ ions or Al3+ ions, which shows a potential application in biological imaging.

A series of color-tunable NaBa1-zSrzB9O15:Ce3+,Mn2+ phosphors were synthesized by a high temperature solid state method. Luminescence property, energy transfer, thermal stability and cation substitution were investigated in detail. Due to energy transfer, NaBaB9O15:Ce3+,Mn2+ presents violet to green luminescence and manifest a broad excitation range from 200 to 350 nm. The energy transfer mechanism of Ce3+-Mn2+ is identified as a dipole-dipole interaction. NaBa1-zSrzB9O15:Ce3+,Mn2+ displays both Ce3+ violet and Mn2+ green and orange emissions under ultraviolet excitation. It is observed that Sr2+ partial substitution for Ba2+ could adjust the ratio of Mn2+ emission intensity in different cation sites, which results from preferred sites' occupation with modification of the crystal structure. Furthermore, increase in temperature can enhance the energy transfer from Ce3+ to Mn2+, which enhances the Mn2+ emission intensity sharply. The highly thermal-sensitive property of NaBa1-zSrzB9O15:Ce3+,Mn2+ makes it feasible for its potential application in luminescent ratiometric thermometers with wide temperature range.