Structure and optical anisotropy of K1.75(NH4)0.25SO4 solid solution

Abstract

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.

Structure and refractive properties of LiNaSO4 single crystals

“…The difference of the bandgaps calculated for the x(y) and z axes, which gives the bandgap anisotropy, is equal to 0.50 eV. It is comparable to the corresponding parameter obtained for K 1.75 (NH 4 ) 0.25 SO 4 (~ 0.53 eV [20]) and less than the result for LiKSO 4 (1.21 eV -see Ref. [28]).…”

“…They showed excellent optical quality. The studies of crystal structure were performed on powders using a standard X-ray technique, of which peculiarities are described in the work [20]. Spectral dependences of the refractive indices ) ( To study the effect of uniaxial mechanical stresses on the birefringence, we employed a special cryostat attachment that allowed for compressing crystal samples at fixed temperatures.…”

“…Li and Na atoms occupy respectively tetrahedral and octahedral cavities within the environment. [20], the structural units in LSS are tightly arranged, which is a feature of ionic type of bonding. The other feature of domination of ionic bonds is a high-symmetry character of the second-coordination environments of anions in the shape of hexagonal analogue of cuboctahedron (see Fig.…”

“…Single crystals of LSS were grown from aqueous solution, using a slow-evaporation method (see, e.g., the work [20] and references therein). The starting compounds were commercially available Li 2 SO 4 and Na 2 SO 4 salts with the purity of 99.9%.…”

“…As far as we know, there is no data concerned with these characteristics, except for the refractive indices for a single wavelength (~ 589.3 nm) and a mention that LSS is optically uniaxial [4]. Among different aspects, an interest to the optical anisotropy is due to the fact that the ABSO 4 -group crystals often reveal inversion of sign of the optical birefringence, or simply 'isotropic point' [17][18][19][20]. The latter is interesting from the fundamental point of view [21,22] and, moreover, finds its applications in a number of practical fields [23][24][25].…”