K.Bekmyrza, M.K.Myrzakhmet, M.Nikl*, V.Jery*, K.S.Baktybekov

L.N.Gumilyov Eurasian National university, Astana,

*Institute of Physics of Academy of Sciences of the Czech Republic, Prague

ABOUT LUMINESCENT PROPERTIES OF MONOCRYSTALS OF THE DIFFICULT SULPHATES ACTIVATED BY THALLIUM

Introduction

Crystals of sulphate of ammonium and the mixed crystals of sulphate of potassium and lithium are interesting with some phases caused by various behavior of cations and annions at various temperatures.

Ammonium sulphate at 223 K to turns from the high-temperature paraelectric form I in ferroelectric phase II [1]. Both forms are ortorombical, group forms I Ðïàò, and forms II — Ðïà2₁ is spatial. Under a condition for trimetric elementary cells b > à > ñ with, the axis c with in a phase II is a direction spontaneous polarization. The phase I possesses three mirror planes àb, bc and àñ and the inversion center, and the phase II doesn't have center of inversion and a reflection plane àb.

In both forms, paraelectric and ferroelectric, for ammonium ions in a lattice there is environment of two types. Types of ions corresponding to them are designated as NH₄ (I) and NH₄ (II [1]. Ion NH₄ (I) has five sulphates-ions in the nearest environment, and ion NH₄ (II) — six (in both phases). Ions NH₄ (I) are less mobile, than ions of the second type. With increasing concentrations of potassium ions, the transition temperature decreases and activation energy of the ions NH₄ (I) decreases. Moreover, last type of ions is mainly replaced, and in enough enriched ions potassium the mixed crystals there are only ions NH₄ (II). From measurements of time of a relaxation on a phase I of salt (NH₄)₂S0₄ follows that above 300 K ammonium ions begin to diffuse in a lattice with energy of activation 75 kj/mol [2].

Transition II→I partially smooth as thermal capacity measurements have shown. It is accompanied by the gradual reduction of volume reaching as a result approximately 1,5%, thus compression is concentrated only along an axis and [3]. However, though transition is also stretched on an interval approximately in 50 K, it comes to the end nevertheless isothermally. Thus, in the Curie point sharp change of spontaneous polarization [3] and factors of an elastic pliability [4] is observed. Nevertheless, there is no a sharp change in frequencies of librational (torsional) oscillations, that has been established by long-wavelength infrared spectra [5], infrared spectra and Raman scattering [6], nor in the effective cross section of neutron scattering.

For ammonium sulphates, and also the mixed sulphates of potassium and the lithium activated by thallium, remain unsolved questions:

• distinction of optical properties of ions Tl⁺ for two various positions of potassium with which they replace,

• optical properties of pair centers Tl⁺₂ in these crystals.

Experiment

Crystals of sulphate of ammonium and the mixed sulphates of lithium and potassium pure and in the presence of a thallium impurity are grown up from a water solution by a method of slow evaporation at room temperature at the Eurasian national university of L.N.Gumilyov. Measurements of optical properties of these crystals are carried by the equipment of department of optical materials of Institute of Physics of Academy of Sciences of the Czech republic in Prague.

Absorption of crystals was measured at room temperature by means of installation Shimadzu 3101PC, a luminescence in a wide range of temperatures from temperature of liquid nitrogen to 500°Ñ – with help spectrofluorimeter HJY 5000M.

In fig. 1 - 2 are shown the comparison with standard scintillator BGO spectra X-ray all measured by us it is exemplary. The samples activated by thallium always show more intensive strip of radiation with various intensity (depending on the form of the sample, concentration of thallium, quality of a material).

Fig. 1. Radio luminescence of crystals LiKSO₄

Fig.2. Radio luminescence of crystals NH₄SO₄ in comparison with the data for ammonium chloride.

Fig. 3. Spectrum of absorption of samples at room temperature.

In fig. 3 spectra of absorption of samples with thallium impurity – A-strip Tl⁺ with a maximum about 220-230 nanometers are shown.

Fig. 4. Samples LiKSO_4.

Fig. 5 Spectrum of photoluminescence of the investigated samples

The excitation spectrum in this strip approximately repeats an absorption A-strip (Fig. 6), there is a high intensity of this strip, most likely, because of high value spectral absorptive abilities and arising geometrical effects.

Fig. 6. Spectrum of excitation of the investigated samples.

The photoluminescence and excitation spectra in all crystals are measured by us corresponded to X-ray and to absorption, accordingly. Time of attenuation of luminescence is homogeneous everywhere for a strip of radiation and 410 nanoseconds, the disintegration form single-exponent give for crystals LiKSO₄-Tl approximately, that is the radiation center is unique and well defined. In radiation spectrum (NH₄) 2SO₄ there are two strips approximately 295 nanometers and weaker of 380 nanometers. The spectrum of excitation dominating corresponds to absorption spectrum. Time of attenuation of a photoluminescence of the basic strip of 300 nanometers approximately 360 nanoseconds, and for weaker strip of 380 nanometers is longer - approximately 600-800 nanoseconds.

Photoluminescence NH₄Cl:Tl is well studied earlier [8]. The photoluminescence spectrum reaches a maximum approximately 370-380 nanometers and there correspond to one strip X-rayed and its time of attenuation approximately 360 nanoseconds.

Conclusions

We investigated the luminescence centers in the crystals of sulphate of ammonium activated by thallium and in the mixed crystals. It is shown that these centers are monovalent ions of thallium. Two various positions of replacement poorly affect luminescent characteristics of these crystals.

References:

1. Parsonige N., Staveli L. Disorder in crystals. V.1. Ì.: Mir, 1982. (ýòî íå ñòàòüÿ, à êíèãà, ññûëêà íà êíèãó)

2. Knispel R.R., Petch H.E., Pintar M.M. // J.Chem.Phys., 63, 390 (1975).

3. Hoshino S., Vedam K., Okaya Y., Pepinsky R. // Phys. Rev., 112, 405 (1958).

4. Ikeda T., Fujibayashi K., Nahai T., Kobayashi J. // Phys. St. Solidi, A16, 279 (1973).

5. Trefler M. // Can. J. Phys., 49, 1694 (1971). (íîìåð æóðíàëà 49, ñòð. 1694, ãîä 1971)

6. Torrie B.H., Lin C.C., Binbrek O.S., Anderson A. (J.) Phys. Chem. Solids, 33, 697 (1972).

7. Leung P.S., Taylor T.I., Havens W.W. // J. Chem. Phys., 48, 4912 (1968).

8. Murzakhmetov M.K. Complex luminescence centers in crystals NH₄Cl, activated by ions Tl. // In the book. "Phototransformation of energy in atomic and molecular systems ". - Karaganda, 1984.- pp.13-19.