PHOTO- AND ELECTROLUMINESCENCE OF BARIUM SULFIDE SINGLE CRYSTALS

T. N. STROGANOVA, V. A. SOKOLOV

Until now, no data have appeared in the literature on the luminescence of barium sulfide single crystals, apparently because their preparation by ordinary methods is associated with great difficulties. First, the compound BaS itself is very unstable. The presence of moisture in the atmosphere leads to its decomposition with the formation of an alkaline-earth metal oxide, hydrogen sulfide, sulfur dioxide, and free sulfur. Second, barium sulfide belongs to the class of refractory compounds. Its melting point is 2205°C. Third, in comparison, for example, with zinc sulfide, BaS is much heavier and, consequently, its sublimation temperature is higher than the sublimation temperature of ZnS. For these reasons, methods of growing barium sulfide crystals from the melt or from the gas phase are practically difficult to carry out.

We obtained BaS crystals from barium hyposulfite \((\mathrm{BaS_2O_3})\), which was preliminarily heated at a temperature of \(\sim 500^\circ\mathrm{C}\). On heating, the hyposulfite decomposes into BaS, \(\mathrm{BaSO_4}\), and \(\mathrm{SO_2}\). Simultaneously with heating, air and gaseous decomposition products were pumped out of the ampoule. When the vacuum reached the order of \(10^{-2}\) mm Hg, iodine was introduced into the ampoule at a rate of 5 mg per \(1\ \mathrm{cm^3}\) of ampoule volume (in the absence of iodine, crystals do not grow). After this the ampoule was sealed and kept for a long time at room temperature. Under these conditions, a time of the order of 2–3 months is necessary for the formation of crystals. The crystals formed in the charge itself and had sizes from \(0.2\ \mathrm{mm} \times 0.5\ \mathrm{mm}\) to \(7\ \mathrm{mm} \times 8\ \mathrm{mm}\). The crystals had a yellowish tint because of the incorporation of iodine into the lattice. In open air the crystals gradually became decolorized. At places where iodine accumulated on the surface of the decolorized crystal, macroscopic pores were visible, which made the crystal surface rough. The crystals were resistant to heat treatment. The results on the growth of BaS crystals were reproducible.

X-ray diffraction analysis showed that the crystals obtained are indeed single-crystal formations with the cubic structure characteristic of barium sulfide. No traces of other compounds were detected.

Under the action of ultraviolet light \((\lambda_{\max} = 365\ \mathrm{nm})\), the crystals photoluminesced with a yellow-orange color. For comparison,

photoluminescence spectra were taken of single-crystalline and powder barium sulfide. The BaS powder was obtained from barium carbonate (BaCO\(_3\)), which was calcined in a porcelain boat at a temperature of 900°C in a stream of hydrogen and hydrogen sulfide for two hours and then slowly cooled in a stream of hydrogen. X-ray analysis showed that the powders obtained in this way possess the structural properties characteristic of BaS. The photoluminescence spectra of the crystals and powders are shown in Fig. 1. As can be seen from the figure, their spectra are identical. The maxima of the emission bands lie at \(\lambda \approx 610\) nm.

Fig. 1. Photoluminescence spectra of single-crystalline (1) and powder (2) barium sulfide.

Thus, X-ray structural analysis, as well as the identity of the luminescence spectra of powder and crystalline barium sulfide, prove that the crystals described above are indeed BaS single crystals.

Barium sulfide crystals with fused indium electrodes, placed in silicone oil, emitted light in alternating and direct electric fields. In a direct field the glow was many times weaker than in an alternating one. The emission spectrum of the crystals in the electric field coincided with the photoluminescence spectrum. The brightest glow was observed at the electrodes. Figure 2 gives the dependences of the average brightness \(B\) of the electroluminescence of a crystal on the voltage \(U\) for excitation by an alternating (1) and a direct (2) field. In the case of an alternating field this dependence can be satisfactorily described by the law:

\[ B \sim \exp\left(-\frac{\delta}{\sqrt{u}}\right). \]

The luminescence of barium sulfide under the action of various types of excitation is apparently associated with activation by its superstoichiometric barium. This capacity for self-activation of powder barium sulfide was indicated in work [1], in which the concentration of superstoichiometric barium was determined as a function of the conditions of thermal treat-

of powder grinding. In our case, for BaS single crystals, the incorporation of iodine atoms into the crystal lattice may apparently promote the formation of superstoichiometric barium, as well as the formation of defects responsible for the luminescence.

Fig. 2. Dependence of the average brightness of electroluminescence of a BaS single crystal on voltage under excitation by alternating (1) and constant (2) fields

REFERENCES

1. P. V. Kovtunenko, B. V. Kondakov, B. P. Nikonov. Transactions of the Moscow D. I. Mendeleev Institute of Chemical Technology. Issue XXXI. Moscow, 1960.