The relationships governing variation of the photoluminescence of graphitic carbon nitride synthesized by heat treatment of melamine in a closed air medium containing oxygen in the temperature range of 10-300 K were investigated. It was shown that the concentration of oxygen in the obtained material 4-5 at.% increases with increase of temperature and decreases with increase in the duration of the synthesis process. By measurements at reduced temperatures right down to 10 K it was possible to resolve bands due to radiative recombination processes in the photoluminescence spectra of the graphitic carbon nitride. It was found that increase of the synthesis temperature from 500 to 600 o C and also increase of the duration at the given temperature from 30 to 240 min shift the maximum in the photoluminescence spectrum from 2.74 eV into the region of lower energies to 2.71-2.67 eV. This is due to the bigger role of the molecular system formed by the π bonds of carbon and nitrogen atoms with sp 2 hybridization and characterized by a smaller forbidden band width in the emission of light. Transitions due to recombination through oxygen-induced levels in the forbidden band of the semiconductor lead to the appearance of a "tail" in the photoluminescence spectra in the region of low energies (2.40-2.33 eV). Increase of the carbon nitride synthesis temperature to 600 o C leads to a change in the structure of the energy bands and to increase of the energy of the radiative transitions as a result of increase in the degree of doping with oxygen atoms and thermal stratifi cation.

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The photoluminescence (PL) in the visible spectral range of Ca 0.5 Ba 0.5 Ga 2 S 4 solid solutions activated with ions of rare-earth elements Eu 2+ and Er 3+ has been studied in the temperature range of 10-300 K. The PL spectrum contains a strong luminescence band peaking at 530 nm, which is due to the 4 f 6 5d → 4 f 7 transitions in Еu 2+ ions and weak lines in the range of 650-675 nm, which are due to the 4 F 9/2 → 4 I 15/2 transitions in Er 3+ ions. The position of the PL spectrum and the total PL intensity are characterized by high temperature stability in the range of 10-300 K; the total PL intensity decreases by only 13% with respect to the maximum value at 10 K. The PL decay time and the activation energy of nonradiative recombination channels are found to be τ e = 291 ns and E a = 41 meV, respectively. INTRODUCTIONPhotoluminescence (PL) of ions of rare-earth elements (REEs) in wide-gap semiconductors has been intensely investigated, because these compounds are characterized by high luminescence efficiency due to the excitation of REE-ion PL through the semiconductor matrix [1][2][3]. A study of the luminescence properties of REEs in semiconductors and insulators yields information about the nearest REE environment and the symmetry of PL centers and crystal field. Ternary alkali-earth chalcogenide semiconductors of the II-III 2 -VI 4 type (II: Ba, Sr, or Ca; III: Ga or Al; and VI: S, Se, or O), activated by REE (Eu, Ce, Er, Yb, Pr, or Sm) ions, are promising phosphors.Calcium thiogallates doped with Еr 3+ ions exhibit strong Stokes and anti-Stokes luminescence in the visible spectral range at wavelengths of 530 nm ( 2 H 11/2 → 4 I 15/2 ), 550 nm ( 4 S 3/2 → 4 I 15/2 ), and 660 nm ( 4 F 9/2 → 4 I 15/2 ) and in the near-IR region at 870-920 nm ( 4 S 3/2 → 4 I 13/2 ), 976 nm ( 4 I 11/2 → 4 I 15/2 ), and 1530 nm ( 4 I 13/2 → 4 I 15/2 ) [4,5]. Barium, strontium, and calcium thiogallates activated by Eu 2+ ions are efficient phosphors emitting in the blue-green spectral region due to the 4 f 6 5d → 4 f 7 electronic transitions [6][7][8][9][10].Luminescence efficiency can be increased using more than one type of REE ions as dopants due to the excitation-energy transfer from one REE ion to another. In particular, in barium thiogallates activated

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