Abstract. In this work we have grown single crystals K x (NH 4 ) 2-x SO 4 (x  1.75), which belong to a potassium-ammonium sulfate family, and studied their roomtemperature structure. Spectral dependences of the principal refractive indices and principal optical birefringences are measured in the visible region. Anomalous birefringence dispersion is observed along the light propagation direction parallel to the principal x axis. According to our birefringence extrapolation based on the Sellmeier fit for the refractive indices, the symmetry of optical indicatrix at the room temperature should increase at the light wavelength λ IP  1350±60 nm. This corresponds to a specific 'isotropic point' defined by the condition n x (λ IP ) = n y (λ IP ), which has not been detected in either K 2 SO 4 or (NH 4 ) 2 SO 4 crystals.

The structural, electronic properties and ionicity of the lithium ammonium sulfate dielectric crystals are examined using a complex approach including experimental studies of X-ray spectroscopy and the first principles band structure techniques within a framework of local electron density functional theory (DFT). Band energy dispersion, density of electronic states and dielectric function dispersion in the wide spectral range corresponding to electronic excitations were calculated using the plane wave basis and Vanderbilt ultra-soft pseudopotentials. The origin of the energy bands are estimated using density of states diagrams and the band gap magnitudes for different exchange correlation functions. To verify the data of the performed band structure calculations, the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES) are used. The XPS core-level and valence-band spectra as well as the XES bands representing the energy distribution of the O 2p and N 2p states are studied. Theoretical refractive indices dispersion for the main crystallographic directions (n a , n b and n c ) as well as birefringence spectral dependences (Dn a , Dn b and Dn c ) in the visible spectral range are obtained. All the calculated properties are compared with the available experimental results and good agreement between both sets of data is demonstrated. a Institute of Physics, J. Dlugosz Academy, Armii Krajowej 13/15, PL-42-201,

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