ISSN 1063-7745, Crystallography Reports, 2009, Vol. 54, No. 2, pp. 313–319. © Pleiades Publishing, Inc., 2009. Original Russian Text © V.Yo. Stadnyk, M.O. Romanyuk, B.V. Andrievskii, N.R. Tuzyak, 2009, published in Kristallografiya, 2009, Vol. 54, No. 2, pp. 338–344. 313 INTRODUCTION Ammonium sulfate (AS) ((NH 4 ) 2 SO 4 ) crystals at room temperature are nonpolar and belong to the sp. gr. Pnma of the orthorhombic system. Below T C = 223 K, they undergo a ferroelectric phase transition without a change in the number of formula units per unit cell to the sp. gr. Pna2 1 [1, 2]. This phase transition is accom- panied by a significant deformation step, while the spontaneous polarization P c below T C decreases in magnitude and changes sign near 85 K [3]. This fact indicates the presence of several temperature-depen- dent contributions to P c from different sublattices and compensation for these contributions in the vicinity of 85 K [4]. The properties of AS crystals can be described on the basis of the theory of pseudoproper ferroelectrics [5, 6]. The peculiar temperature behavior of P c is explained within the model of two sublattice polariza- tions P c1 and P c2 (P c = P c1 + P c2 ), caused by the antipar- allel, different in magnitude, moments of the (1) and (2) groups and their different temperature dependences [7, 8]. A unit cell of AS crystals contains four formula units, which possess two nonequivalent ammonium ions (1) and (2) in both phases. Structural studies of the para- and ferroelectric phases of AS crystals revealed ordered elements [6]: NH 4 (1), NH 4 (2), and SO 4 groups. It was concluded that SO 4 tetrahedra in the paraelectric phase do not have two equilibrium positions and move in a single-mini- mum potential [8]. NH 4 + NH 4 + NH 4 + NH 4 + At room temperature, an AS crystal is optically biaxial, positive, and has an acute angle bisector ori- ented along the Z axis; the angle between the optical axes 2V = 60° (λ = 632.8 nm) and 58° (λ = 441.1 nm). The temperature and spectral dependences of the refractive indices and birefringence of a mechanically free AS crystal were investigated in [9]. Despite certain interest in AS crystals, the effect of uniaxial pressures on their optical characteristics and behavior of the phase transition point has not been investigated. Such pressures generally do not change the crystal symmetry but make it possible to selectively affect some groups of bonds and the corresponding structural units, thus obtaining additional information for analysis of the phase transition mechanisms. Study- ing the refractive indices and birefringence, one can derive useful information about the structure and phys- ical properties of crystals. Study of the effect of uniaxial pressure on the spec- tral and temperature dependences of the birefringence ∆n i and refractive indices n i of crystals isomorphous to AS revealed a strong baric sensitivity of the electron subsystem of these crystals, which manifests itself in high frequency shifts of the effective bands of UV and IR oscillators [10]. Our purpose was to investigate the effect of uniaxial pressure on the temperature and spectral changes in the refractive indices n i and the position of the phase tran- sition point and analyze the temperature spectral dependences of the combined piezooptic constants of AS crystals. π im 0 Refractive Indices of (NH 4 ) 2 SO 4 Crystals under Uniaxial Pressure V. Yo. Stadnyk a , M. O. Romanyuk a , B. V. Andrievskii b , and N. R. Tuzyak a a Franko National University of Lviv, Lviv, 79005 Ukraine b Koszalin University of Technology, Koszalin, Poland e-mail: vasylstadnyk@ukr.net Received August 4, 2008 Abstract—The effect of uniaxial pressure σ m ≤ 200 bar on the spectral (300–800 nm) and temperature (300– 77 K) dependences of the refractive indices of (NH 4 ) 2 SO 4 crystals has been investigated. The baric dependences of the electronic polarizability, specific refraction, and parameters of the Sellmeier formula have been calcu- lated. It is established that uniaxial pressure increases the refractive indices, mainly because of the narrowing of the band gap, increase in the oscillator density, and redshift of the UV absorption band maximum. The anom- alies arising as a result of ferroelectric phase transition are related to the occurrence of spontaneous deformation and polarization (the latter is a superposition of two sublattice polarizations). The spectral and temperature dependences of the piezooptic constants are analyzed and the values of electro-optic coefficients are estimated. PACS numbers: 78.20.Fm, 78.30.Am DOI: 10.1134/S1063774509020229 PHYSICAL PROPERTIES OF CRYSTALS