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‐1P0,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. High‐pressure phase transitions in kernite.