X-ray Diffraction, X-ray Photoelectron Spectra, Crystal Structure, and Optical Properties of Centrosymmetric Strontium Borate Sr2B16O26

We report results of X-ray diffraction (XRD) and valence band X- ray photoelectron (VB-XPS) spectra for strontium borate Sr2B16O26. The X-ray structural analysis shows that the single crystals of Sr2B16O26 crystallize in the monoclinic space group P21/c with a = 8.408(1) Å, b = 16.672(1) Å, c = 13.901(2) Å, β = 106.33(1)°, and Z = 4. The crystal structure consists of a 3D network of the complex borate anion [B16O20O12/2]4−, formed by 12 BO3 triangles and four BO4 tetrahedra, which can be viewed as three linked [B3O3O4/2]− triborate groups bonded to one pentaborate [B5O6O4/2]− group and two BO3 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 Sr2B16O26 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.