Optical refractive properties and phonon spectra of Na2SO4 single crystal

Abstract

Na₂SO₄ crystal was grown by slow evaporation method from an aqueous solution. Using the PXRD method, the crystal structure of Na₂SO₄ was investigated at room temperature, and it was found that the crystal belongs to the orthorhombic symmetry of the space group Fddd (no. 70) with the following unit cell parameters: a = 5.8568(4) Å, b = 12.2957(7) Å, c = 9.809(1) Å, and V = 706.52(8) ų. The dispersion of the refractive indices of Na₂SO₄ crystal in the visible and ultraviolet ranges was measured at room temperature and the principle refractive indices of optical indicatrix were determined to be n_X = 1.4703, n_Y = 1.4847, and n_Z = 1.47669 for the wavelength λ = 500 nm. The first-principle calculations of the phonon properties of the crystal were performed using the lattice dynamics method and DFPT approach. The polarized normal incidence reflection spectra of Na₂SO₄ crystal were measured in the infrared range 50–1300 cm⁻¹ by using FTIR spectrometer and synchrotron radiation. The calculated reflection spectra of Na₂SO₄ crystal are in very good agreement with the experimental ones. In Na₂SO₄ crystal, the Reststrahlen band in the spectral range of 1120–1200 cm⁻¹ is confirmed experimentally and theoretically.