γ-BaB₂O₄: High-Pressure High-Temperature Polymorph of Barium Borate with Edge-Sharing BO₄ Tetrahedra

Abstract

The α- and β-modifications of barium metaborate are important functional materials used in optoelectronic devices. A new theoretically predicted modification of BaB2O4 has been synthesized under conditions of 3 GPa and 900 °C, using the DIA-type apparatus. The new high-pressure modification, γ-BaB2O4, crystallizes in a centrosymmetrical group of monoclinic syngony (P21/n (#14), a = 4.6392(4) Å, b = 10.2532(14) Å, c = 7.066(1) Å, β = 91.363(10)°, Z = 4). A distinctive feature of the γ-BaB2O4 structure is the presence of edge-sharing tetrahedra [B2O6] which form infinite double chains ∞[B4O4O8/2] stretching along the a axis. The number of known structural types with the [B2O6] group is limited. Phase γ-BaB2O4 has the shortest distance between boron atoms of shared tetrahedra among all currently known compounds. The [B2O6] group angles are 95.5° and 105.5°. Thermodynamic stability and electronic properties of the γ-BaB2O4 modification were studied. The width of the band gap, calculated using the HSE06 functional, is 7.045 eV which implies transparency in the deep-UV region. Experimental and numerical methods which demonstrate a good match were used to the study the Raman spectra of γ-BaB2O4 and β-BaB2O4 modifications. In the Raman spectra of γ-BaB2O4, the most intense band at a frequency of 853 cm–1 was found to correspond to the symmetric bending mode of the B–O–B–O ring in edge-sharing tetrahedra.

High-Pressure Synthesis, Electronic Properties, and Raman Spectroscopy of Barium Tetraborate BaB4O7 Polymorphs

“…36,38 The spectrum registered in this work is in good agreement with the data of ref 36 (Figure S4) and does not agree with the data for α-BaB 4 O 7 doped with samarium. 38 We believe that the Raman spectra given in the latter work contain bands typical of β-BaB 2 O 4 , 15 namely, the strongest band at about 637 cm −1 , the bands at about 788, 1524, and 1540 cm −1 . A symmetry-representation analysis of β-BaB 4 O 7 gives the following modes as Γ = 19A g + 17A u + 17B 1g + 19B 1u + 19B 2g + 17B 2u + 17B 3g + 19B 3u , where 1B 1u , B 2u , and B 3u are the acoustic modes and 72 modes (19A g + 17B 1g + 19B 2g + 17B 3g ) are Raman-active modes.…”

“…The general view of the synthesized sample is shown in Figure a. It consists of relatively large BaB 4 O 7 crystals up to 150 μm in size, confined mainly to the low-temperature zone of the sample, and a melt solidified into a mixture of β-BaB 4 O 7 and newly discovered γ-BaB 2 O 4 phases (Figure b,c). The γ-BaB 2 O 4 phase was identified by both energy-dispersive X-ray spectroscopy (EDX) analysis and Raman spectroscopy .…”

“…For the synthesis of β-BaB 4 O 7 , a "Discoverer-1500" DIA-type apparatus installed at the Sobolev Institute of Geology and Mineralogy SB RAS (IGM SB RAS) was used. Detailed information on the high-pressure cell used can be found in ref 15. The cell has four graphite cassettes with holes 0.9 mm in diameter and 1.1 mm deep.…”

“…The γ-BaB 2 O 4 phase was identified by both energy-dispersive X-ray spectroscopy (EDX) analysis and Raman spectroscopy. 15 The compositions of the phases formed are given in Table 1. We attribute the relatively large standard deviation both to the small size of the crystals synthesized and the difficulties in determining the concentration of boron.…”

“…For the second modification, named β-BaB 4 O 7 in ref 36, an unexpected conclusion was drawn. It was considered to be identical to that described by Knyrim et al 13 To clarify this issue, the Gibbs 15 The results of the total and partial DOS calculation of BaB 4 O 7 polymorphs are given in Figure S3. They are typical of barium borates, in which the valence bands are formed by O 2p orbitals slightly hybridized with the B2p, while conduction bands are formed by unoccupied Ba 5d and O 2p orbitals.…”