Thermal and compressional behavior of the natural borate kurnakovite, MgB$_{3}$O$_{3}$(OH)$_{5}$·5H$_{2}$O

Construction and building materials 266, 121094 (2021). doi:10.1016/j.conbuildmat.2020.121094
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 $α_{V(298K)}$ = 5.18(1)∙10$^{−5}$ K$^{−1}$. Volume compressional trend modelled with a Birch-Murnaghan equation of state yields a bulk modulus of K$_{P0,T0}$ = 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.
Published by Elsevier Science, Amsterdam [u.a.]