Phase transition and high pressure behavior of Na2B4O6(OH)2·3H2O (kernite)

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, is an important B-bearing material (B2O3 51 wt%)[1]. Despite 75% of world borates consumption is due to the production of ceramics, detergents, fertilizers, and glass. Moreover borates, due to the high cross section of the 10B(n,α)7Li reaction (~3840 barns), could be added to concretes or epoxy resins in order to enhance the shielding efficiency towards neutron radiations [2,3]. In order to assess if kernite could be a valuable candidate in this kind of application, a comprehensive knowledge of its thermodynamic parameters is advisable. However, despite few several studies were conducted at non-ambient conditions [4,5] highlighting 3 possible phase transitions at about 2.5, 7.0 and 11.0 GPa, the structure of the high pressure polymorphs and the compressional parameters of kernite (and those of its high-pressure polymorphs) are still unknown. For these reasons, we investigated the high-pressure behavior of kernite by in-situ single-crystal X-ray diffraction up to 14.5 GPa. Data collected at high-pressure show that: 1. Kernite is stable, in its ambient-conditions polymorph, up to (only) 1.6 GPa. Between 1.6 and 2 GPa kernite undergoes a first-order isometric phase transition, which is reconstructive in character (Fig1). 2. The changes in the compressional anisotropic scheme resulted from the phase transitions are described. 3. The structure of the kernite-II polymorph has been resolved. The kernite-to-kernite-II phase transition leads to an increase in the average coordination number of the two crystallographically-independent Na sites (to VII), along with a change of the topological configuration: ribbons of Na-polyhedra connected to form elliptically distorted 10-mRs of polyhedra. 4. Between 6.5 and 7.5 GPa, a second P-induced first-order phase transition occurs (fig.1 ). Unfortunately, a sharp decrease in the number of the observed Bragg reflections prevents the structure solution of the kernite-III polymorph.