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

Abstract

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−xIx)2 with a CaI2-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 P3̅m1. The trigonal tri-Ca((BH4)0.70I0.30)2 transforms at ∼180 °C to an orthorhombic phase of similar composition, ort-Ca((BH4)0.64I0.36)2, with a CaCl2-type structure (a distorted β-Ca(BH4)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 CaCl2-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 I4mm. This iodide-rich compound tet-Ca((BH4)0.38I0.62)2, has a new structure type. The tetragonal phase finally decomposes to CaHI and CaB6 at T > 345 °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−xIx)2 via CaHI, but unfortunately the temperature of hydrogen release is still fairly high and similar to that for Ca(BH4)2.