High‐pressure behavior and phase stability of Na₂B₄O₆(OH)₂·3H₂O (kernite)

Abstract

The high-pressure behavior of kernite [ideally Na2B4O6(OH)2•3H2O, a ~7.02 Å, b ~9.16 Å, c ~15.68 Å, β = 108.9°, Sp. Gr. P21/c, at ambient conditions], an important B-bearing raw material (with B2O3  51 wt%) and a potential B-rich aggregate in radiation shielding materials, has been studied by single-crystal synchrotron X-ray diffraction up to 14.6 GPa. Kernite undergoes an iso-symmetric phase transition at 1.6-2.0 GPa (to kernite-II). Between 6.6-7.5 GPa, kernite undergoes a second phase transition, possibly iso-symmetric in character (to kernite-III). The crystal structure of kernite-II was solved and refined. The isothermal bulk modulus (KV0= -1 P0,T0, where P0,T0 is the volume compressibility coefficient) of the ambient-pressure polymorph of kernite was found to be KV0 = 29(1) GPa and a marked anisotropic compressional pattern, with K(a)0 : K(b)0 : K(c)0 ~ 1:3:1.5., was observed. In kernite-II, the KV0 increase to 43.3(9) GPa and the anisotropic compressional pattern increases pronouncedly. The mechanisms, at the atomic scale, which govern the structure deformation, have been described.

Crystal structure of the high-P polymorph of Ca2B6O6(OH)10·2(H2O) (meyerhofferite)

“…The deformation mechanisms above described for meyerhofferite have already been observed in other hydrated borates: e.g. in the kernite-to-kernite-II phase transition, where the coordination number of the cation site Na1 increases from VI to VII with similar mechanisms (Comboni et al, 2020b). In detail, one oxygen site leaves the coordination sphere of Na1 and is replaced by another one, which is pushed towards Na1 by the tilting of the BO x units.…”

“…In the previous study by Comboni et al (2020a), a sort of transient behaviour, detected also in other borates experiencing P-induced phase transitions with a relevant change in volume (e.g. kernite; Comboni et al, 2020b), was observed. In order to avoid this phenomenon, which could potentially be the sign of an incomplete re-equilibration of the crystal structure, for all the measurements preceding and following the phase transition, an average equilibration time of 1.5 h was applied.…”

“…Such a dramatic and abrupt decrease in the unit-cell volume is the highest among the hydrated borates investigated so far (e.g. ÁV $6% in ulexite, $7.5% and $7% in kernite, $4.7% in kurnakovite and $9.2% in colemanite) (Lotti et al, 2017;Comboni et al, 2020bComboni et al, , 2021Pagliaro et al, 2021).…”

“…In this study, based on new experimental data, we provide the crystal structure model of meyerhofferite-II. The phase-transitionand pressure-induced deformation mechanisms of meyerhofferite-II up to 7.4 GPa are described and discussed, among the other high-pressure phase transitions occurring in other hydrated borates (Comboni et al, 2020b(Comboni et al, , 2021Pagliaro et al, 2021;Lotti et al, 2017).…”

Atomic-scale deformation mechanisms at high-pressure in inderborite, CaMg[B3O3(OH)5]2(H2O)4⋅2H2O

“…In both the experiments, crystals did not recover after the phase transition. This was the most destructive phase transition observed in hydrated borates to date (e.g., Comboni et al, 2020bComboni et al, , 2022b, since the number of observed reflections was barely enough to properly index the diffraction pattern of the high-pressure polymorph, inderborite-II, which was found to be metrically monoclinic. The space group has not unambiguously determined.…”