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

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 = β-1
P0,T0, where βP0,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 increases 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.