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

The substitution of the complex borohydride anion BH 4 − 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(BH 4 ) 2 −CaI 2 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((BH 4 ) 1− x I x ) 2 with a CaI 2 -type structure and an anisotropically contracted trigonal unit cell, a = 4.311(1) and c = 6.867(2) Å for x ∼ 0.3 ( T = 28 °C), space group P 3̅ m 1. The trigonal tri -Ca((BH 4 ) 0.70 I 0.30 ) 2 transforms at ∼180 °C to an orthorhombic phase of similar composition, ort -Ca((BH 4 ) 0.64 I 0.36 ) 2, with a CaCl 2 -type structure (a distorted β-Ca(BH 4 ) 2 type structure) and cell parameters a = 7.271(2), b = 7.042(1), and c = 4.4601(7) Å ( T = 322 °C), space group Pnnm . Further heating of the CaCl 2 -type compound to ∼330 °C leads to a transition to a tetragonal phase with cell parameters a = 4.1062(2) and c = 24.822(2) Å ( T = 340 °C, x ∼ 0.62), space group I 4 mm . This iodide-rich compound tet -Ca((BH 4 ) 0.38 I 0.62 ) 2, has a new structure type. The tetragonal phase finally decomposes to CaHI and CaB 6 at T > 345 °C. All three novel compounds found in the Ca(BH 4 ) 2 −CaI 2 system are stable at room temperature. The anion substitution ultimately changes the decomposition reaction pathway in which hydrogen is released from the tetragonal Ca((BH 4 ) 1− x I x ) 2 via CaHI, but unfortunately the temperature of hydrogen release is still fairly high and similar to that for Ca(BH 4 ) 2 .