Thermal and compressional behavior of the natural borate kurnakovite, MgB3O3(OH)5·5H2O

The thermal and compressional behaviors of kurnakovite, a potential B-rich aggregate in neutron radiation-shielding concretes, were studied by in situ single-crystal synchrotron X-ray diffraction (100– 393 K, 0–13.36(5) GPa). Above 393 K, the crystal structure collapses in response to dehydration. The bulk thermal expansion coefficient is aV(298K) = 5.18(1)∙105 K1. Volume compressional trend modelled with a Birch-Murnaghan equation of state yields a bulk modulus of KP0,T0 = 35(3) GPa. A phase transition occurs between 9.23(5) and 11.11(5) GPa, leading to a triclinic high-P polymorph (with triple unit-cell volume) and to an increase in coordination of one third of the trigonal-planar boron sites to a tetrahedral geometry.