X-ray Diffraction, X-ray Photoelectron Spectra, Crystal Structure, and Optical Properties of Centrosymmetric Strontium Borate Sr 2 B 16 O 26 Ali Hussain Reshak,* ,† S. Auluck, ‡ I. V. Kityk, § and Xuean Chen | Institute of Physical Biology, South Bohemia UniVersity, NoVe Hrady 37333, Czech Republic, Physics Department, Indian Institute of Technology Kanpur, Kanpur (UP) 208016, India, Electrical Engineering Department, Technological UniVersity of Czestochowa, Al Armii Krajowej 17/19, Czestochowa, Poland, and College of Materials Science and Engineering, Beijing UniVersity of Technology, Ping Le Yuan 100, Beijing 100124, People’s Republic of China ReceiVed: April 6, 2009; ReVised Manuscript ReceiVed: May 12, 2009 We report results of X-ray diffraction (XRD) and valence band X- ray photoelectron (VB-XPS) spectra for strontium borate Sr 2 B 16 O 26 . The X-ray structural analysis shows that the single crystals of Sr 2 B 16 O 26 crystallize in the monoclinic space group P2 1 /c with a ) 8.408(1) Å, b ) 16.672(1) Å, c ) 13.901(2) Å,  ) 106.33(1)°, and Z ) 4. The crystal structure consists of a 3D network of the complex borate anion [B 16 O 20 O 12/2 ] 4- , formed by 12 BO 3 triangles and four BO 4 tetrahedra, which can be viewed as three linked [B 3 O 3 O 4/2 ] - triborate groups bonded to one pentaborate [B 5 O 6 O 4/2 ] - group and two BO 3 triangles. Using this structure, we have performed theoretical calculations using the all-electron full potential linearized augmented plane wave (FP- LAPW) method for the band structure, density of states, electron charge density, and the frequency-dependent optical properties. Our experimental VB-XPS of Sr 2 B 16 O 26 is compared with results of our FP-LAPW calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ of the Brillouin zone (BZ) resulting in a direct energy gap of about 5.31 eV. Our measured VB-XPS show reasonable agreement with our calculated total density of states for the valence band that is attributed to the use of the full potential method. 1. Introduction Borates are among the most interesting and therefore the most extensively studied materials. Since 1962, when the binary phase diagram Bi 2 O 3 -B 2 O 3 was investigated, many methods for growth of the borate crystals were designed and described in detail in the literature. 1-6 Theoretical exploration has shown that anionic groups and chemical bonding structures and coordination of boron atoms have a principal influence on the nonlinear optical properties of these crystals. 7,8 Borate materials are of considerable interest because they show a great variety of physical properties ranging from nonlinear optical (NLO), ferroelectric to semiconducting behaviors, and in addition, a boron atom may adopt triangler or tetrahedral oxygen coordina- tion, the BO 3 and BO 4 groups may be further linked via common oxygen atoms to form isolated rings and cages or polymerize into infinte chains, sheets, and networks, leading to the rich structural chemistry. 9,10 With a view of finding new optical materials, the alkaline earth metal borates have been extensively studied because -BaB 2 O 4 is an excellent NLO crystal primarily used in laser frequency conversion 11 and SrB 4 O 7 was shown to be a potential NLO material with excellent mechanical and optical properties including a high second harmonic generation (SHG) coefficient, high optical damage threshold, and high hardness, etc. 12 In the system of SrO-B 2 O 3 , four phases with B 2 O 3 /SrO ratios of 0.33-2.0 have been structurally characterized, which are Sr 3 B 2 O 6 , 13 Sr 2 B 2 O 5 , 14 SrB 2 O 4 , 15,16 and SrB 4 O 7 . 17 Among them, Sr 3 B 2 O 6 contains slightly distorted planar BO 3 groups separated by Sr 2+ ions. Sr 2 B 2 O 5 contains B 2 O 5 groups, each of which is formed by two BO 3 triangles linked via common vertices. SrB 2 O 4 was found to be polymorphic under high pressure. The crystal structure of SrB 2 O 4 prepared under ambient pressure consists of 1D chains of corner-sharing BO 3 triangles, 15 while that of the high-pressure phase consists of a fully connected 3D framework of corner-sharing BO 4 tetrahedra. 16 SrB 4 O 7 is also characterized by a 3D network of corner-sharing BO 4 tetrahedra. However, this network is different from that observed in the high-pressure phase of SrB 2 O 4 . In an attempt to synthesize noncentrosymmetric compounds that are potentially applicable as NLO materials, Tang et al. 18 have unexpectedly obtained single crystals of Sr 2 B 16 O 26 . X-ray structural analysis established that this compound is a B 2 O 3 -richest phase (B 2 O 3 /SrO ) 4) in the SrO-B 2 O 3 system and crystallizes in a totally new structure type (Pearson symbol mP176) which is unknown for the borates. So an investigation of the band structure may play a crucial role in understanding of the physical properties and predictions of this compound. First-principle band sructure calculations have been successfully used to obtain fundamental parameters of semiconductors and dielectrics. The structural parameters and dynamical properties of crystals determine a wide range of microscopic and macroscopic features, i.e., diffraction, sound velocity, elastic constants, Raman and infrared absorption, inelastic neutron scattering, and specific heat, etc. To the best of our knowledge, no comprehensive work, neither experimental data on the fundamental optical functions or first principles band structure calculations of the electronic features of strontium borate Sr 2 B 16 O 26 , has appeared in the literature. This work is a natural continuation of the previous * To whom correspondence should be addressed. Tel.: +420 777729583. Fax: +420-386 361231. E-mail:maalidph@yahoo.co.uk. † South Bohemia University. ‡ Indian Institute of Technology Kanpur. § Technological University of Czestochowa. | Beijing University of Technology. J. Phys. Chem. B 2009, 113, 9161–9167 9161 10.1021/jp903170p CCC: $40.75  2009 American Chemical Society Published on Web 06/12/2009 Downloaded by OAK RIDGE NATIONAL LAB on July 6, 2009 Published on June 12, 2009 on http://pubs.acs.org | doi: 10.1021/jp903170p