Preparation, crystal structure, vibrational and electronic properties of Cd4Si2O7F2

Abstract

Herein we report preparation and crystal of Cd4Si2O7F2 and details of its crystal structure, vibrational and electronic properties from powder XRD, Raman scattering, FTIR studies and ab initio density functional theory (DFT) calculations. Cd4Si2O7F2 crystallizes in a monoclinic lattice (space group: P21/c) with unit cell parameters: a = 7.3211(1), b = 10.2708(2), c = 10.9123(2) Ǻ and β = 107.172(2)°, V = 775.02(3) Ǻ3. Highly distorted (CdO,F)6 and Si2O7 units form the building units of the crystal structure. The structural and vibrational properties were investigated by using Raman and IR spectroscopic studies and supported by DFT calculations. Electronic structure calculations revealed an indirect band gap (∼1.8 eV) in Cd4Si2O7F2 which reasonably agrees with that observed in DRUV measurements. For comparison DFT calculations on structure and electronic properties of Ca4Si2O7F2, were also carried out. Ca4Si2O7F2 is also found to be an indirect band gap semiconductor, but the band gap is appreciably higher (∼4.6 eV). More diffused 5s orbital of Cd atoms in Cd4Si2O7F2 lower the conduction band minima and hence show lower band gap.