Anion Substitution in Ca(BH4)2-CaI2: Synthesis, Structure and Stability of Three New Compounds

The substitution of the complex borohydride anion BH4- in calcium borohydride by the larger iodide anion I- is explored in order to tailor the hydrogen storage properties. Three new compounds are identified in the Ca(BH4)(2)-CaI2 system and are structurally characterized using the Rietveld method and synchrotron radiation powder X-ray diffraction (SR-PXD) data. Calcium borohydride readily dissolves in the trigonal calcium iodide structure during ball milling, forming a solid solution Ca((BH4)(1-x)I-x)(2) with a CaI2-type structure and an anisotropically contracted trigonal unit cell, a = 4.311(1) and c = 6.867(2) angstrom for x similar to 0.3 (T = 28 degrees C), space group P (3) over bar m1. The trigonal tri-Ca((BH4)(0.70)I-0.30)(2) transforms at similar to 180 degrees C to an orthorhombic phase of similar composition, ort-Ca((BH4)(0.64)I-0.36)(2), with a CaCl2-type structure (a distorted beta-Ca(BH4)(2) type structure) and cell parameters a = 7.271(2), b = 7.042(1), and c = 4.4601(7) angstrom (T = 322 degrees C), space group Pnnm. Further heating of the CaCl2-type compound to similar to 330 degrees C leads to a transition to a tetragonal phase with cell parameters, a = 4.1062(2) and c = 24.822(2) angstrom (T = 340 degrees C, x similar to 0.62), space group I4mm. This iodide-rich compound tet-Ca((BH4)(0.38)I-0.62)(2), has a new structure type. The tetragonal phase finally decomposes to CaHI and CaB6 at T > 345 degrees C. All three novel compounds found in the Ca(BH4)(2)-CaI2 system are stable at room temperature. The anion substitution ultimately changes the decomposition reaction pathway in which hydrogen is released from the tetragonal Ca((BH4)(1-x)I-x)(2) via CaHI, but unfortunately the temperature of hydrogen release is still fairly high and similar to that for Ca(BH4)(2).