It is well known that ammonium sulphate and potassium sulphate form a continuous series of mixed systems of the formula K2-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 K2-x[NH4]xSO4 general formula, the influence of partial isomorphic substitution on optical parameters of single crystals can be revealed.
In present work K1,75[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 ni of K1,75[NH4]0,25SO4 single crystal at room temperature is normal dni/dλ < 0 for all crystal physics directions and significantly increases while moving towards the absorption edge (fig.1). Behavior of refractive indices of K1,75[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 K1,75[NH4]0,25SO4 and similar for K2SO4 from literature is discussed.
Further optical birefringence study of K1,75[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)
