Optical studies of [N(CH3)4]2CoCl4, [N(CH3)4]2MnCl4 single crystals in the normal paraelectric phase

The absorption edge in the main crystallographic directions of [N(CH3)(4)](2)CoCl4 and [N(CH3)(4)](2)MnCl4 single crystals were studied in the normal paraelectric phase (at 303 K). The optical transmittance T and absorbance A were measured and applied to deduce the absorption coefficient alpha and optical band gap E-g. The analysis of the data revealed the existence of two optical transition mechanisms. The calculated values of the direct band gap E-gd for both crystals were estimated to be between 4.553 and 4.667 eV for the Co compound; and between 5.351 and 5.383 eV for the Mn compound. The allowed indirect transition data were analyzed and interpreted in terms of the two valence bands which were suggested to be originated from the spin-orbit interaction and crystal-field splitting. The momenta E-p were attributed to the energy difference between the heavy and the split valence bands. The anomalous behavior of the polar c-axis for [N(CH3)(4)](2)CoCl4 crystals was attributed to the presence of domain walls in the paraelectric phase. The refractive index n, and both the real epsilon(r) and imaginary parts epsilon(i) of the dielectric permittivity were calculated and the results are discussed. (C) 2003 Elsevier Science B.V. All rights reserved.