Crystal structure, polymorphism, and thermal properties of yttrium borohydride Y(BH 4) 3

Abstract

Y(BH 4) 3 was synthesized by cryo-milling mixtures of LiBH 4 and YCl 3 and characterized by powder X-ray and neutron diffraction (PXD, PND), differential scanning calorimetry (DSC), and temperature programmed desorption (TPD). The crystal structure was refined in the space group Pa−3 (no. 205) with lattice constant a = 10.8522(7) Å from 11B and D ( 2H) substituted samples using PND. It was found to contain Y 3+ cations in a highly distorted octahedral environment formed by six [BD 4] − complex anions. Heat treatment under 10 MPa of deuterium at 475 K led to a phase transformation from the primitive cubic room-temperature phase to a face-centered cubic high-temperature phase with space group Fm−3 c (no. 226) and lattice constant a = 11.0086(1) Å. This high-temperature phase shows an ideal and undistorted octahedral coordination around the central Y 3+ cation. In situ synchrotron radiation powder X-ray diffraction experiments (SR-PXD) show the presence of an intermediate phase during the thermal decomposition of Y(BH 4) 3 with presumably orthorhombic symmetry, and lattice constants a = 12.170(14) Å, b = 7.670(5) Å, and c = 7.478 (6) Å, in a narrow temperature region between 473 K and 520 K.