STUDY OF PARTIAL ISOMORPHIC SUBSTITUTION ON REFRACTIVE PARAMETERS OF K₂SO₄ SINGLE CRYSTALS

It is well known that ammonium sulphate and potassium sulphate form a continuous series of mixed systems of the formula K_2-x[NH₄]_xSO₄ (0 < x < 1). The above system has been studied for the ferroelectric phase transition, structural changes and electrical and thermal behavior [1, 2]. All these studies were aimed at looking at the paraelectric to ferroelectric transition in ammonium sulphate at 223 K which fades out with rise in x value. But to our knowledge optical properties only for extreme compounds of this system, namely ammonium sulphate and potassium sulphate crystals were considered.

Optical parameters of such promising isomorphic crystals as potassium sulphate, K₂SO₄ and ammonium sulphate, NH₄SO₄ are rather well studied [3-6].

On the basis of this data by study of intermediate compounds with the K_2-x[NH₄]_xSO₄ general formula, the influence of partial isomorphic substitution on optical parameters of single crystals can be revealed.

In present work K_1,75[NH₄]_0,25SO₄ single crystals were obtained from aqueous solution using slow evaporation method. Obtained samples were transparent and of good optical quality. Structure quality has been approved using XRPD.

Investigation of the dispersion of refractive indices was performed using Obreimov immerse method.

It was found that the dispersion of refractive indices n_i of K_1,75[NH₄]_0,25SO₄ single crystal at room temperature is normal dn_i/dλ < 0 for all crystal physics directions and significantly increases while moving towards the absorption edge (fig.1). Behavior of refractive indices of K_1,75[NH₄]_0,25SO₄ crystals confirms classical crystal optics rule: refractive index in the direction perpendicular to optical axes plane takes middling values while in the direction of acute bisectrix between the optical axes its values are maximal. Correlation between obtained results for K_1,75[NH₄]_0,25SO₄ and similar for K₂SO₄ from literature is discussed.

Further optical birefringence study of K_1,75[NH₄]_0,25SO₄ single crystals made it possible to reveal peculiarities due to partial substitution of potassium with NH₄⁺ ions. Optical birefringence changing in a terms of uniaxial stresses is also discussed here.

References:

1. Sawada, S. Ohya, Y. Ishibashi and Y. Takagi, J. Phys. Soc. Japan 38, 1408 (1975)

2. K. Ohi, J. Osaka and H. Uno, J. Phys. Soc. Japan 44, 529 (1978)

3. Zheludev I.S., Gaba V.M., Romanyuk N.A., Ursul Z.M., Izv. AN USSR, 7 (2) (1986), 386 (in Russian)

4. Stadnyk V.Yo., Romanyuk M.O., Chyzh O.Z., Vachulovych V.F., Condens. Matter Phys., 10 (49) (2007), 45

5. V.Yo. Stadnyk, M.O. Romanyuk, Ferroelectrics 317, 255 (2005)

6. V.Yo. Stadnyk, O.S. Kushnir, R.S. Brezvin, V.M. Gaba, Opt. Spectrosc. 106, 614 (2009)