PHASE COMPOSITION AND ELECTROLUMINESCENT PROPERTIES OF BaS-BASED PHOSPHORS

T. N. STROGANOVA

(Presented by the scientific seminar of the Department of Experimental Physics)

It is known that, in synthesizing electroluminophors, it is necessary to introduce into the charge a larger amount of activator than in synthesizing photoluminophors. A number of studies have shown that the phenomenon of electroluminescence is associated with the formation, on the surface of phosphor crystals, of semiconductor layers such as Cu₂S, Ag₂S, etc., in the form of separate phases. For example, in works (1, 2), copper sulfide was deposited on non-electroluminescent zinc sulfide, and in work (3) a non-electroluminescent phosphor was mixed with Cu₂S or ZnO powder. In both cases the powders acquired the ability to electroluminesce. In work (4), the surface layer of electroluminophors was removed by etching them; in this case the sensitivity of the phosphor to electrical excitation decreased. In electroluminophors the concentration of activator is so high that its atoms cannot dissolve completely in the lattice of the host substance. Therefore the excess amount of activator, in the form of a sulfide, is deposited on the surface of the phosphor crystallites. Piper and Williams (5) suppose that the surface of ZnS—Cu crystals has potential barriers of the Schottky-defect type. Such barriers may arise at the boundary between the host lattice and the second phase, since the second phase, in particular Cu₂S, has an electrical conductivity an order of magnitude higher than that of the ZnS host lattice. In these barriers a very high field is created when the second phase is the cathode. But the role of the second phase is not limited only to creating a high field in the crystal. The second phase, apparently, can also serve as a source of electrons penetrating into zinc sulfide and increasing its conductivity (6). The sulfides of the activators—silver, manganese, etc.—probably also play an analogous role as the second phase. But manganese sulfide dissolves well in the ZnS lattice, forming with it a solid solution up to a Mn concentration of the order of

\[ 5 \cdot 10^{-1}\frac{\Gamma_{\mathrm{Mn}}}{\Gamma_{\mathrm{Zn}}} \]

(7). Therefore the presence of a second phase in the form of MnS cannot be assumed in ZnS phosphors. We made an attempt to obtain single-activator electroluminophors based on the sulfide of the alkaline-earth element barium, i.e., an attempt to synthesize phosphors based on another class of compounds, in comparison

with ZnS. The method for synthesizing phosphors based on BaS is described in [8]. In order to determine the structure and the presence of a second phase in the samples obtained, their X-ray structural analysis was carried out by the Debye—Scherrer method on a URS-55a apparatus. The samples studied were BaS phosphors with activators: copper, manganese, and bismuth; the activator concentration was varied within the range from \(8 \cdot 10^{-5}\) to \(8 \cdot 10^{-2}\) \(\frac{\mathrm{g\ activator}}{\mathrm{g\ BaS}}\). Barium hyposulfite, \(\mathrm{BaS_2O_3}\), was taken as the starting material. Calcined under the same conditions under which the BaS phosphors were synthesized, barium hyposulfite decomposed into the components BaS and \(\mathrm{BaSO_4}\). X-ray structural analysis showed that the \(\mathrm{BaSO_4}\) phase was present in the greater amount. Introduction into the charge of only the activator salt, without flux, did not change the quantitative phase ratio between BaS and \(\mathrm{BaSO_4}\), although the product obtained in this way is a phosphor giving luminescence in an electric field characteristic of the corresponding activator.

The phase ratio in favor of BaS changes only after adding small amounts (\(\sim 1\%\)) of flux (\(\mathrm{Na_2CO_3}\)) to the charge and after calcining the charge at a temperature above \(900^\circ\mathrm{C}\). In this case the \(\mathrm{BaSO_4}\) phase is found in the form of weak traces. From the X-ray patterns the lattice parameters were calculated (\(a \simeq 3.2\ \text{Å}\)), and it was established that all the phosphors had the cubic structure characteristic of BaS.

In copper-activated BaS phosphors, X-ray structural analysis revealed, in addition to weak traces of \(\mathrm{BaSO_4}\), a second CuS phase at an activator concentration of the order of \(8 \cdot 10^{-3}\) \(\frac{\mathrm{g\ Cu}}{\mathrm{g\ BaS}}\) and higher. When excited by an alternating electric field, these phosphors gave yellow (\(\lambda_{\max}=5880\ \text{Å}\)) and green (\(\lambda_{\max}=5180\ \text{Å}\)) emission bands, the relative intensities of which depend on the frequency of the exciting field.

At copper concentrations of the order of \(8 \cdot 10^{-5}\)—\(8 \cdot 10^{-4}\) \(\frac{\mathrm{g\ Cu}}{\mathrm{g\ BaS}}\), in the samples no second CuS phase, apart from \(\mathrm{BaSO_4}\), could be detected by X-ray structural analysis, although the phosphors luminesced in an electric field and gave two emission bands with maxima at 5180 Å and 4800 Å. The electroluminescence spectra of BaS—Cu phosphors are shown in Fig. 1.

It is possible that, at copper concentrations of the order of \(8 \cdot 10^{-4}\)—\(8 \cdot 10^{-5}\) \(\frac{\mathrm{g\ Cu}}{\mathrm{g\ BaS}}\), such a small amount of the second phase is formed that it is impossible to detect it by X-ray structural analysis (since the sensitivity of this analysis is limited). Phosphors with copper concentrations lower than \(8 \cdot 10^{-5}\) \(\frac{\mathrm{g\ Cu}}{\mathrm{g\ BaS}}\) photoluminesced well, but practically did not luminesce in an electric field.

Phosphors activated with manganese could give either two emission bands upon excitation by an electric field: \(\lambda_{\max}=5750\ \text{Å}\) and \(\lambda_{\max}=5450\ \text{Å}\), or one with \(\lambda_{\max}=5450\ \text{Å}\), depending on the calcination temperature. In BaS—Mn phosphors calcined above \(1100^\circ\mathrm{C}\), only the green band was observed (Fig. 2). Green

band appeared at manganese concentrations of \(8 \cdot 10^{-4} — 8 \cdot 10^{-2}\ \frac{\mathrm{gMn}}{\mathrm{gBaS}}\); at the same time, X-ray structural analysis revealed the presence of a second phase, MnS. At lower manganese concentrations the phosphors did not glow in an electric field and did not reveal a second phase.

The difference in the manifestation of the second MnS phase in BaS-based phosphors as compared with ZnS—Mn phosphors is possibly connected with the different solubility of manganese sulfide in the host lattices BaS and ZnS, which depends not only on the sizes of atoms and ions, but also on the degree of polarity or ionicity of the bond in the lattices of the solvent and the dissolved substance.

Fig. 1. Luminescence spectra of BaS—Cu phosphors under excitation by an alternating electric field:

\(\times\times\times\) — blue electroluminescence band \((\lambda_{\max} = 4800\ \text{Å})\) at copper concentrations \(8 \cdot 10^{-4} — 8 \cdot 10^{-5}\ \frac{\mathrm{gCu}}{\mathrm{gBaS}}\); \(\Delta\Delta\Delta\) — green electroluminescence band \((\lambda_{\max} = 5180\ \text{Å})\) at copper concentrations \(8 \cdot 10^{-2} — 8 \cdot 10^{-5}\ \frac{\mathrm{gCu}}{\mathrm{gBaS}}\); \(\bigcirc\bigcirc\bigcirc\) — yellow emission band \((\lambda_{\max} = 5880\ \text{Å})\) at copper concentrations \(\sim 8 \cdot 10^{-2} — 8 \cdot 10^{-3}\ \frac{\mathrm{gCu}}{\mathrm{gBaS}}\).

An estimate of the degree of bond ionicity from the difference in electronegativities of the elements, made by us using Pauling’s method [9], shows that the ionicity of the bond in the BaS molecule is greater (\(\sim 49\%\)) than in ZnS (\(\sim 31\%\)); therefore, the degrees of polarity of these compounds also differ strongly. The degree of ionicity of MnS (\(\sim 32\%\)) is close to the degree of ionicity of ZnS; consequently, their degrees of polarity are also close. According to [10], a different degree of bond polarity determines the different solubility of substances. Close values of the degree of polarity-

... of ZnS and MnS are the reason for the good solubility of MnS in ZnS, which is also observed experimentally. The greater degree of ionicity of BaS in comparison with ZnS accounts for the poorer solubility of MnS in BaS compared with its solubility in zinc sulfide.

In phosphors activated with bismuth, a second phase, besides BaSO$_4$, was found in the form of bismuth sulfide Bi$_2$S$_3$ at concentrations

\[ 8 \cdot 10^{-4} — 8 \cdot 10^{-2}\ \frac{\mathrm{g\,Bi}}{\mathrm{g\,BaS}}, \]

although the phosphors glowed in an electric field even at a concentration of

\[ 8 \cdot 10^{-5}\ \frac{\mathrm{g\,Bi}}{\mathrm{g\,BaS}}. \]

At lower concentrations BaS—Bi phosphors photoluminesced well, but did not glow in an electric field.

Fig. 2. Emission spectra of BaS—Mn phosphors upon excitation by an alternating electric field: ××× — green emission band ($\lambda_{\max} = 5450$ Å); ○○○ — yellow emission band ($\lambda_{\max} = 5750$ Å).

Thus, phosphors based on BaS have been obtained, i.e., based on a new class of compounds. X-ray structural analysis of the samples showed that, in copper-activated phosphors, at higher activator concentrations the CuS phase is present, while in phosphors activated with manganese and bismuth the MnS and Bi$_2$S$_3$ phases, respectively, are present. All phosphors had the cubic structure characteristic of BaS, although barium hyposulfite, which has a rhombic structure, was taken as the starting substance. In addition to the phases indicated above, the BaSO$_4$ phase was present in all phosphors.

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