Crystal structure, polymorphism, and thermal properties of yttrium borohydride Y(BH4)3

Y(BH4)3 was synthesized by cryo-milling mixtures of LiBH4 and YCl3 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) A from 11B and D (2H) substituted samples using PND. It was found to contain Y3+ cations in a highly distorted octahedral environment formed by six [BD4]- 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-3c (no. 226) and lattice constant a = 11.0086(1) A. This high-temperature phase shows an ideal and undistorted octahedral coordination around the central Y3+ 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(BH4)3 with presumably orthorhombic symmetry, and lattice constants a = 12.170(14) A, b = 7.670(5) A, and c = 7.478 (6) A, in a narrow temperature region between 473 K and 520 K.