New compounds in the potassium-aluminium-hydrogen system observed during release and uptake of hydrogen

Three new compounds are observed in the potassium-aluminium-hydrogen system, which are characterised using in-situ synchrotron radiation powder X-ray diffraction (SR-PXD), thermal analysis (TG and DSC) and Siverts measurements (PCT). All three new compounds (denoted 1, 2 and 3) are observed during release and uptake of hydrogen in the potassium-aluminium-hydrogen system and may be new intermediates. All three compounds were indexed and the following unit cells were found, 1: cubic, a = 17.0248(9) Å, 2: cubic, a = 14.2746(4) Å and 3: orthorhombic, a = 10.46(1), b = 6.661(6) and c = 6.173(5) Å. Formation and observation of 1, 2 and 3 depends on the mechano-chemical sample preparation (ball milling), temperature, (heating rate), and hydrogen pressure (and temperature for pressure change). Compound 1 is often observed in the temperature range 55-150 °C for a medium ball-milled sample heated under a constant hydrogen pressure of 50 bar 1 is clearly an intermediate for formation of KAlH₄ and may have the composition K_yAlH_x with 1 ≤ y ≤ 3 and 4 ≤ x ≤ 6. Applying hydrogen pressure abruptly at elevated temperatures leads to faster hydrogenation, which can then be performed at lower hydrogen pressures. In the latter case, 1 is only observed shortly in a few PXD patterns. Compounds 2 and 3 are mainly observed during dehydrogenation of KAlH₄ in the temperature ranges of ca. 200 to 350 °C and 200 to 390 °C respectively, as relatively weak Bragg diffraction peaks.