Refractive indices of (NH4)2SO4 crystals under uniaxial pressure

The effect of uniaxial pressure σm ≤ 200 bar on the spectral (300-800 nm) and temperature (300-77 K) dependences of the refractive indices of(NH4)2SO4 crystals has been investigated. The baric dependences of the electronic polarizability, specific refraction, and parameters of the Sellmeier formula have been calculated. 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 anomalies 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.