Cr-substituted polycrystalline Ba₂(In_2-xCr_x)O₅·(H₂O)_δ powders (0.04 ≤ x ≤ 0.60) were synthesized by solid state reaction to investigate the relation of crystal structure, thermochemical, magnetic, and optical properties. The Cr-substitution results in an unit cell expansion and formation of the higher-symmetric tetragonal phase together with increased oxygen and hydrogen contents. Magnetic property measurements reveal that the diamagnetic pristine Ba₂In₂O₅·(H₂O)_δ becomes magnetically ordered upon Cr-substitution. By UV–vis spectroscopy a gradual shift of the absorption-edge energy to lower values was observed. Numerical calculations showed that the observed bandgap narrowing was ascribed to the Cr induced states near the Fermi level. The correlation between the changes of crystal chemistry, magnetic, and optical properties of Cr-substituted Ba₂(In_2-xCr_x)O₅·(H₂O)_δ can be explained by the replacement of In by Cr. Consequently, an enhanced photocatalytic CO₂ reduction activity was observed with increasing Cr substitution, compatible with the state-of-the-art high surface area TiO₂ photocatalyst (P-25).

Cr-substituted and pristine Ba₂In₂O₅·(H₂O)_x powders were synthesized by solid state reaction. The influence of Cr-substitution on the crystal structure, chemical composition, magnetic and optical properties were investigated. Powder X-ray diffraction (XRD), elemental analysis and TGA-MS reveal that with substitution of In for Cr, the unit cell volume and the unit cell parameter b increase together with the oxygen and hydrogen content. Magnetic property measurements indicate that Ba₂In₂O₅·(H₂O)_x is diamagnetic in the temperature range of 2 K < T < 300 K becoming ferromagnetic upon Cr-substitution. In the UV–vis spectra of the Cr-substituted sample a distinctive shift of the absorption-edge energy from 430 to 690 nm was observed corresponding to a bandgap narrowing from 2.88 to 1.80 eV. The replacement of tetrahedral InO₄ units by octahedral CrO₆ units was found to be the main factor for the drastic change of the magnetic and optical properties.

Persistent luminescence of SrAl₂O₄:Eu²⁺ has attracted considerable attention due to their high initial brightness, long-lasting time and excellent thermal stability. Here the influence of boric acid on the persistent luminescence and thermal oxidation resistance of SrAl₂O₄:Eu²⁺ was investigated in detail. Crystal structural analysis and scanning electron microscopy revealed that with the addition of boron, the unit cell volume decreased and the morphology of the particles became more irregular with sharp edges. Thermogravimetric analysis showed better thermal oxidation resistance accompanied by a change in oxygen vacancy concentration when boron acid is used. Photoluminescence spectra and afterglow decay curves confirm an improved afterglow performance for boron-added SrAl₂O₄:Eu²⁺. Thermoluminesence allowed monitoring the changes in the trap states due to the presence of B. Our results imply that the substantial improvement of afterglow performance and the thermal stability in SrAl₂O₄:Eu²⁺ can be attributed to the incorporation of boron into the aluminate network.

Fluorine-substituted CaTiO₃:Pr phosphors were prepared by a solid-state reaction. Rietveld refinements of powder X-ray diffraction patterns revealed that increasing fluorine-substitution leads to the gradual shrinkage of the unit-cell. Enhanced afterglow intensities were observed with fluorine-substitution. Furthermore, the effect of annealing atmosphere was investigated by thermochemical treatment in different atmospheres (Ar, air and NH₃). UV-Vis diffuse reflectance spectra and photoluminescence excitation spectra revealed that Pr⁴⁺ in the pristine CaTi(O,F)₃:Pr phosphor was partially reduced to Pr³⁺ under NH₃ flow leading to an intensity improvement of ca. 450% compared to CaTiO₃:Pr. The substantial improvement of afterglow intensity by fluorine substitution and annealing in NH₃ is considered to be connected with the generation of oxygen vacancies and the partial reduction of Pr⁴⁺ to Pr³⁺.

The phosphor CaTiO₃:Pr³⁺ was synthesized via a solid-state reaction in combination with a subsequent annealing under flowing NH₃. Comparatively large off-center displacements of Ti in the TiO₆ octahedra were confirmed for as-synthesized CaTiO₃:Pr³Â by XANES. Raman spectroscopy showed that the local crystal structure becomes highly symmetric when the powders are ammonolyzed at 400 °C. Rietveld refinement of powder X-ray diffraction data revealed that the samples ammonolyzed at 400 °C have the smallest lattice strain and at the same time the largest average Ti-O-Ti angles were obtained. The samples ammonolyzed at 400 °C also showed the smallest mass loss during the thermal re-oxidation in thermogravimetric analysis (TGA). Enhanced photolumincescence brightness and an improved decay curve as well as the highest reflectance were obtained for the samples ammonolyzed at 400 °C. The improved photoluminescence and afterglow by NH₃ treatment are explained as a result of the reduced concentration of oxygen excesses with simultaneous relaxation of the lattice strain.

Red emitting CaTiO₃:Pr phosphors with a nominal composition of Ca_0.998+xPr_0.002TiO_3+δ (0.02≤x≤0.04) were prepared by solid state reactions with different thermal post treatments and characterized by X-ray diffraction, transmission electron microscopy and photoluminescence. The Ca excess exhibited complete solubility up to 4% in the samples treated at 1400 °C but segregation in the form of Ruddlesden-Popper phases (Ca₃Ti₂O₇ - Ca₄Ti₃O₁₀) was observed in samples prepared at 1500 °C. The increase in temperature for stoichiometric samples showed a monotonic increase of decay time due to the reduction of non-radiative recombination defects. It was found that the Ca excess favored the formation of oxygen vacancies which are known to act as trap. In the samples treated at 1400 °C, 3% of Ca excess showed to be the best concentration to increase the decay time of persistent luminescence. For the samples treated at 1500 °C, the segregation of Ruddlesden-Popper phases left a constant amount of Ca soluble in all the CaTiO₃ samples. This constant concentration of Ca caused the same density of defects and, consequently, the same decay time in all samples.