Synthesis and Characterization of the High-Pressure Nickel Borate γ-NiB₄O₇

Abstract

γ-NiBO was synthesized in a high-pressure/high-temperature experiment at 5 GPa and 900 °C. The single-crystal structure analysis yielded the following results: space group P622 (No. 179), a = 425.6(2), c = 3490.5(2) pm, V = 0.5475(2) nm, Z = 6, and Flack parameter x = -0.010(5). Second harmonic generation measurements confirmed the acentric crystal structure. Furthermore, γ-NiBO was characterized via vibrational as well as single-crystal electronic absorption spectroscopy, magnetic measurements, high-temperature X-ray diffraction, differential scanning calorimetry, and thermogravimetry. Density functional theory-based calculations were performed to facilitate band assignments to vibrational modes and to evaluate the elastic properties and phase stability of γ-NiBO.

The High‐Pressure Nickel Borate Hydrate Ni3B18O28(OH)4·H2O

“…The second problem, the lack of three positive charges, could be resolved if nickel was present in the oxidation state +3 or if the compound contained more hydrogen atoms that we could not localize from the X‐ray diffraction data. The magnetic measurements revealed a magnetic moment of µ = 3.20(1) µ B , a value in line with those observed in other Ni 2+ ‐containing borates such as γ‐NiB 4 O 7 or HP‐NiB 2 O 4 . [1j] Furthermore, the BVS and CHARDI values also point towards Ni 2+ ions instead of Ni 3+ ions (Table ).…”

“…= 2.83 µ B ) in an octahedral coordination environment. This can be explained by the additional contributions from the orbital angular momentum and spin–orbit coupling, as is also seen for γ‐NiB 4 O 7 [ µ eff = 2.98–3.08(1) µ B ] and NiB 3 O 5 (OH) [ µ eff = 3.30(1) µ B ] . Therefore, one can conclude that contributions from the orbital angular momentum or spin–orbit coupling have to be present in Ni 3 B 18 O 28 (OH) 4 · H 2 O to produce the enhanced effective magnetic moment.…”

“…The nickel atoms are coordinated octahedrally with Ni–O bond lengths between 2.064(2) and 2.184(3) Å (Table ), which are common values for nickel cations in octahedral coordination geometries with oxide anions. [1j], The three [NiO 6 ] octahedra share the common oxygen atom O6 (Figure ). As O6 is not exactly located at 0, 0, 0 but slightly shifted along the c axis, it is duplicated by the mirror plane m at x, y , 0, which results in a very small interatomic distance of 0.25(2) Å between both symmetry‐equivalent atoms.…”

“…The band at ν̃ ≈ 1150 cm –1 presumably stems from the [BO 4 ] stretching vibrations. Similarly to the IR spectrum, [BO 4 ] bending and complex vibrations give rise to bands below ν̃ ≈ 900 cm –1 , and Ni–O vibrations are expected to occur below ν̃ ≈ 350 cm –1 , …”

“…As high‐pressure syntheses of transition‐metal borates already led to the discovery of numerous compounds with previously unknown compositions or new structural motifs, for example, Co 7 B 24 O 42 (OH) 2 · 2H 2 O, Cd(NH 3 ) 2 [B 3 O 5 (NH 3 )] 2 , HP‐ M B 2 O 4 (HP = high‐pressure; M = Co, Ni),[1j], and β‐FeB 2 O 4 , we were motivated to further explore the field of high‐pressure nickel borates. The first result of these investigations was the synthesis of the third high‐pressure nickel borate, γ‐NiB 4 O 7 . Our discovery of another hitherto unknown nickel borate during the optimization of the synthesis of γ‐NiB 4 O 7 underlines the potential of high‐pressure syntheses for research focused on the development of new materials.…”