Phase transition and high-pressure behavior of ulexite, a potential aggregate in radiation-shielding concretes

The high-pressure behavior of ulexite [ideally NaCaB5O6(OH)6·5(H2O)], a B-bearing raw material (with B2O3 ≈ 43 wt%) and potential B-rich aggregate, has been studied by in-situ single-crystal synchrotron X-ray diffraction up to 9.5 GPa, in order explore its stability field at high pressure, the anisotropy compressional deformation mechanisms atomic scale. Ulexite undergoes first-order iso-symmetric phase transition, reconstructive character, between 6.44 7.13 GPa (to ulexite-II polymorph); crystal structure was successfully solved refined. For low-P polymorph (stable all working conditions as aggregate concretes), isothermal bulk modulus here obtained is KV0 = 36.9(5) with strongly anisotropic pattern (finite-Eulerian principal components unit-strain ellipsoid: ε1:ε2:ε3 ∼7:3:1). Ulexite-II also shows marked resulting softening (KV0 26(4) GPa).