Europium doped crystalline Ba₇F₁₂Cl₂ phosphors have been prepared at temperatures between 650 and 900 °C using alkali chloride fluxes, yielding both disordered (with the incorporation of small amounts of Na) and ordered crystal modifications. The white emission spectrum excited in the near UV consists roughly of two broad emission bands at ca 450 and 590 nm, as well as weak sharp Eu²⁺ 4f-4f emission bands around 360 nm. The incorporation of Eu²⁺ is further studied using EPR spectroscopy on single crystals, and reveals a significant zero field splitting. The emission spectrum can be significantly tuned by varying the excitation wavelength between 300 and 390 nm. Fine tuning may also be achieved by chemical substitutions to form Ba_7-xM_yF₁₂Cl_2-zBr_z (M = Na, Ca,Eu). Quantitative measurements of the light produced using commercial near UV LEDs show that the color temperature ranges between 4000 and 9700 K with CIE chromaticity coordinates close to the ideal values of x=y=0.333. The best color rendering index (CRI) found was 0.83, and the highest light to light conversion yield was 171 lumen/W. These results show that the title compound is a very promising candidate for white light generation using near UV LED excitation.

Mixed matlokite hosts of composition BaFBr_xI_1−x(0≤x≤1) (pure and doped with Sm²⁺, Eu²⁺) were studied with x-ray crystallography, luminescence, Raman, and electron paramagnetic resonance (EPR) spectroscopy. Results are presented for BaFBr_0.5I_0.5 which demonstrate that a ferrielectric domain structure is formed due to the fact that the heavy halogen ions form separate sublattices with randomly distributed domain walls. The space group of a domain is P4  mm (No. 99). The EPR data from Eu²⁺ allowed to determine the volume fraction of domains.

An optical investigation of the properties of europium (II) ions introduced in Ba₂Mg₃F₁₀ single crystals is presented. The spectra and time-dependence of both the inter-configurational (f–d) and the intra-configurational (f–f) transitions are described. The emission spectrum consists of two emitting Eu²⁺ centres, each showing one broad f–d band and one ⁶P_7/2 quartet. The thermal equilibrium between the f–f and f–d emitting states is investigated and found to take place on a microsecond timescale for one of the two Eu²⁺ centres. The spectroscopic results combined with numerical modelling of the influence of the host crystal on the ⁶P_7/2 energy scheme allow the assignment of each f–f and f–d emission to its corresponding Eu²⁺ centre.

The absorption and photostimulated spectra of single crystals of the new substance Ba₁₂F₁₉Cl₅ doped with Europium ions were studied. Creation of color-center-type absorption bands was observed under C band UV irradiation of the doped crystals. These samples show photostimulated luminescence when subsequently excited with the 20,492 cm^−1 line of an Ar ion laser. Our experiments support the assignment that the PSL signal is from the Eu²⁺ ions. This system may be of interest as an UV storage phosphor.

Single crystals of the new host Ba₁₂F₁₉Cl₅ doped with Eu²⁺ were grown and studied by electron paramagnetic resonance (EPR) and luminescence emission spectroscopy. Three different Eu²⁺ sites were observed. Two of them had orthorhombic point symmetry while the last one was monoclinic. Physico-chemical and symmetry arguments allowed us to establish correspondence between the different Eu²⁺centres and the host cation lattice sites. All three centres presented in their ground state important crystal field splitting. The 80 K luminescence emission spectrum consisted of one broad unsymmetrical f-d band peaking at 22 700 cm^-1. No 4f-4f transitions of the Eu²⁺ ion were observed between room temperature and 80 K.

A recent investigation of the (BaF₂–MgF₂) phase diagram produced several new compounds which are suitable hosts for Rare Earth impurities. We present results on single crystals of Ba₂Mg₃F₁₀ doped with Eu²⁺. The local structure and optical properties of this system were investigated by luminescence emission and by EPR. We observed two different Eu²⁺ sites. Both show C_s point symmetry and an important ground state splitting. Correlating our EPR and optical results with the new Ba₂Mg₃F₁₀ structure data allowed the assignment of each of them to a specific barium lattice site. The luminescence emission of both the 4f⁷–4f⁶5d and the 4f⁷–4f⁷ transitions is observed. The relative importance of the two emissions is strongly temperature dependent. The emission intensities of the intra f-shell ⁶P_7/2→⁸S_7/2transitions increase strongly on going from 295 K to 77 K. Thus, the lowest levels of the 4f⁶5d configuration are approximately degenerate with the ⁶P_7/2 manifold.

Eu²⁺ was introduced into pure and oxygen codoped BaMgF₄ single crystals. A detailed EPR study of this ion (S=7/2) was realized on both types of systems. The result is that only one spectrum was observed involving a strong crystal field. The associated site symmetry of the impurity is C_s. It occupies very closely a Barium lattice site as was established by correlating the EPR results with those of a refined X-ray structure analysis on a Ba_0.8Eu_0.2MgF₄ single crystal realized in our laboratory. The oxygen codoped crystals exhibited this same Eu²⁺EPR spectrum (the only one). Optical emission and excitation experiments were performed between 13 000 and 53 000 cm^−1. The results due to the Eu²⁺ impurity are given and discussed qualitatively within the 4f⁷ ↔ 4f⁶5d¹Â scheme.