The new high-pressure gallium borate Ga2B3O7(OH) was synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 10.5 GPa and 700 °C. For the system Ga-B-O-H, it is only the second known compound next to Ga9B18O33(OH)15·H3B3O6·H3BO3. The crystal structure of Ga2B3O7(OH) was determined by single-crystal X-ray diffraction data collected at room temperature. Ga2B3O7(OH) crystallizes in the orthorhombic space group Cmce (Z = 8) with the lattice parameters a = 1050.7(2) pm, b = 743.6(2) pm, c = 1077.3(2) pm, and V = 0.8417(3) nm(3). Vibrational spectroscopic methods (Raman and IR) were performed to confirm the presence of the hydroxyl group. Furthermore, the band gap of Ga2B3O7(OH) was estimated via quantum-mechanical density functional theory calculations. These results led to the assumption that our gallium borate could be a suitable substance to split water photocatalytically, which was tested experimentally.

The trigonal indium borate InBO(OH) was synthesized in a Walker-type multianvil apparatus under high-pressure/high-temperature conditions of 13 GPa and 1150 °C. The crystal structure could be determined by single-crystal X-ray diffraction data collected at room temperature. InBO(OH) crystallizes in the trigonal space group R3̅ (Z = 3) with the lattice parameters a = 1802.49(6) pm, c = 1340.46(5) pm, and V = 3.7716(3) nm. The structure of InBO(OH) contains alternating B-O T2 supertetrahedra units. The presence of hydroxyl groups was confirmed with vibrational spectroscopic methods such as Raman and IR. Besides HInBO, InBO(OH) is now the second known compound in the system In-B-O-H.

The high-pressure ammonium triborate HP-(NH 4 )B 3 O 5 was synthesized in a high-pressure/high-temperature experiment at 3 GPa and 600°C. It crystalizes in the monoclinic space group C2/c (No. 15) with the parameters a = 981.0(2), b = 880.2(2), c = 922.2(2) pm, β = 103.8(1)°, V = 0.7735(3) nm 3 , Z = 8, R 1 = 0.0436, and wR 2 = 0.1059 (all data). The compound is built up from ribbons of interconnected corner-and edge-sharing BO 4 tetrahedra that are linked by planar 2753 groups to form a three-dimensional structure. The ammonium ions are situated in channels, forming four hydrogen bonds to oxygen of the boron oxygen network. The structure is a substitutional variant of the compounds HP-MB 3 O 5 , (M = K, Rb) and HP-Cs 1-x (H 3 O) x B 3 O 5 (x = 0.5-0.7). The collected IR and Raman spectra of HP-(NH 4 )B 3 O 5 show great similarities to the spectra of the analogous phases.through single-crystal X-ray diffraction, IR and Raman spectroscopy. Results and Discussion Crystal StructureThe structure of HP-(NH 4 )B 3 O 5 is depicted in Figure 1 exhibiting great similarities to HP-MB 3 O 5 (M = K, Rb) [3,4] and HP-Cs 1-x (H 3 O) x B 3 O 5 (x = 0.5-0.7) [6] due to a similar boron

Explorative solid‐state chemistry led to the discovery of the two new compounds Ga5B12O25(OH) and In5B12O25(OH). Extreme synthetic conditions within the range of 12 GPa and a temperature of 1450 °C realized in a Walker‐type multianvil apparatus resulted in the formation of an unprecedented tetragonal structure with the exclusive presence of condensed BO4 tetrahedra, forming cuboctahedral cavities. Doping of these cavities with Eu3+ in In5B12O25(OH) yielded in an orange–red luminescence. Photocatalytic tests of In5B12O25(OH) revealed a hydrogen production rate comparable to TiO2 but completely co‐catalyst free.

The new acentric indium borate InBO(OH) was synthesized in a Walker-type multianvil apparatus at extreme pressure and temperature conditions of 12.3 GPa and 1500 °C. Single-crystal X-ray diffraction provided the data for the crystal structure solution and refinement. InBO(OH) crystallizes in the orthorhombic space group Fdd2 ( Z = 8) with the lattice parameters a = 39.011(8), b = 4.4820(9), c = 7.740(2) Å, and V = 1353.3(5) Å. The structure of InBO(OH) is basically built of corner-sharing BO tetrahedra and isolated InO octahedra. The presence of hydroxyl groups was confirmed with vibrational spectroscopic methods (IR and Raman). Furthermore, the second harmonic signal of an InBO(OH) powder sample yielded more than twice the intensity of quartz.