Ionicity and birefringence of α-LiNH₄SO₄crystals: ab initio DFT study, X-ray spectroscopy measurements

Abstract

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,

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

“…This is a reason why mixed crystals with varying content or solid solutions with the IP have been studied so extensively (see, e.g., Refs. [9,12,15,[17][18][19][20]). On the other hand, efficient control of the IP characteristics anticipates also ascertaining the underlying interrelations between the structural and optical anisotropy characteristics.…”

Electronic Structure, Optical, and Elastic Properties of AgGaS2 Crystal: Theoretical Study

“…To compare the optimized crystal structure with the experimental (Table 2), the parameter of relative deviation of the unit cell volume d r and relative root mean squared deviation D r of the distance matrix d ij were used as was done, for example, in refs. [44,45] The relative deviation of the unit cell volume d r can be found as…”

Structure and refractive properties of LiNaSO4 single crystals

“…Notice also that the refractivity parameters R for LiRbSO 4 and, especially, α-LiNH 4 SO 4 are essentially (~ 50%) larger than that of LSS (see Refs. [18,19]). Nonetheless, the cationic substitution K → Na leads to nearly twice as large increase in the refractive indices, if compared with the substitutions K → Rb and K → NH 4 (see discussions in Refs.…”

“…Nonetheless, the cationic substitution K → Na leads to nearly twice as large increase in the refractive indices, if compared with the substitutions K → Rb and K → NH 4 (see discussions in Refs. [17][18][19]). Fig.…”

“…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].…”