High‐pressure behavior and phase stability of Na 2 B 4 O 6 (OH) 2 ·3H 2 O (kernite)

The high‐pressure behavior of kernite [ideally Na 2 B 4 O 6 (OH) 2 ·3H 2 O, a ~ 7.02 Å, b ~ 9.16 Å, c ~ 15.68 Å, β = 108.9°, Sp Gr P 2 1 /c , at ambient conditions], an important B‐bearing raw material (with B 2 O 3 ≈ 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 ( K V0 = β ‐1 P0,T0 , where β P0,T0 is the volume compressibility coefficient) of the ambient‐pressure polymorph of kernite was found to be K V0 = 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 K V0 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.