Thermal and compressional behavior of the natural borate kurnakovite, MgB_3O_3(OH)_5·5H_2O

摘要

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 alpha(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,K-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. (C) 2020 Elsevier Ltd. All rights reserved.