Optical spectra of bismuth sulfochloride crystals A. Audzijonis, R. Sereika * , R. Z ˇ altauskas, and L. Z ˇ igas Department of Physics, Vilnius Pedagogical University, Studentu 39, 08106 Vilnius, Lithuania Received 17 June 2009, revised 28 August 2009, accepted 4 September 2009 Published online 7 October 2009 PACS 71.15.Mb, 71.20.Nr, 77.22.d, 78.20.Ci * Corresponding author: e-mail sereika@vpu.lt, Phone: þ370 5 273 48 57, Fax: þ370 5 273 48 57 We present the results of the ab initio theoretical study of the optical properties for paraelectric BiSCl crystal using the full potential linearized augmented plane wave (FP-LAPW) method as implanted in the Wien 2k code. For theoretical calculations of optical constants and functions we used the generalized gradient approximation (PBE-GGA), an improve- ment of the local spin-density approximation (LSDA) and recently Wu–Cohen (WC) proposed a new WC-GGA exchange-correlation energy functional. The dielectric func- tion, refractive index, extinction coefficient, absorption coef- ficient, reflectivity, and energy loss function were calculated. The optical properties are analyzed and the origins of the peaks in the spectra are discussed in terms of the calculated density of states. ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Quasi-one-dimensional crystals attract attention due to a complicated chemical bonding and anisotropy of electron and phonon spectra. A 5 (Bi, Sb) B 6 (S, Se) C 6 (Cl, I, Br) crystals consist of chain along the [001]-axis. Crystal structure and physical properties are described in Refs. [1–3]. The crystals with Bi atoms (BiSCl, BiOCl, BiSI) have been the current focus due to special properties which have their potential applications in molecular based electronic devices, such as optical memory, display and data records [3], dielectrics for microelectronics, and nonlinear optical applications. BiSCl is isostructural to the well-known quasi-one- dimensional ferroelectric SbSI, which exhibits a number of strongly coupled semiconductive and ferroelectric proper- ties [2]. The band structure of BiSCl has been investigated using the self-consisted pseudopotential method [4, 5] in the energy region of one valence band (VB). The band structure was also obtained using the density functional theory and pseudopotential theory under generalized gradient approx- imation (GGA) concerning energy region of all VBs [6]. However, there is no ab initio calculation and experimental investigation of the optical properties of BiSCl crystals in the scientific literature. In the current paper we have investigated and calculated density of states of all VBs and optical properties of BiSCl crystals using the full potential linearized augmented plane wave (FP-LAPW) with the GGA (Perdew–Burke–Ernzerhof- GGA (PBE-GGA), Wu–Cohen-GGA (WC-GGA)) and local spin-density approximation (LSDA) by Wien 2k [7] package. 2 Computational methods The BiSCl crystal con- sists of chains of atoms along c(z)-axis and in paraelectric phase belongs to D 16 2h space group. All atoms in the BiSCl crystal are on mirror planes normal to the c-axis. This crystal has four BiSCl molecules (12 atoms) in a unit cell. Each molecule of BiSCl extends in a chain-like fashion along the c-axis. The lattice constants and positions of the atoms in the unit cell were taken from the literature [4]. The positions of all 12 atoms in unit cell may be found by the symmetry operations: (x j , y j , z j ); (x j , y j , z j þ 1=2); (x j þ 1=2, y j þ 1=2, z j þ 1=2); (x j þ 1=2, y j þ 1=2, z j ). The values of x j , y j , and z j are given in Table 1. The bond between the Bi and S atoms in the same chain is covalent, while the Cl ions are in an ionic bond with a covalently bound bridge (BiS). The interchain weak bond is Van der Waals-type. Therefore, a BiSCl-type crystal exhibits an anisotropy interatomic interaction and this creates the optical properties with strong optical anisotropy. The calculations reported in this work were carried out by means of the full-potential, linearized, augmented plane wave method using Wien 2k computer package [7]. This is Phys. Status Solidi B 247, No. 1, 176–181 (2010) / DOI 10.1002/pssb.200945288 p s s basic solid state physics b status solidi www.pss-b.com physica ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim