Journal of Alloys and Compounds 530 (2012) 186–192 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Chloride substitution induced by mechano-chemical reactions between NaBH 4 and transition metal chlorides Isabel Llamas-Jansa ∗ , Nadir Aliouane, Stefano Deledda, Jon Erling Fonneløp, Christoph Frommen, Terry Humphries, Klaus Lieutenant, Sabrina Sartori, Magnus H. Sørby, Bjørn C. Hauback Institute for Energy Technology, Physics Department, P.O. Box 40, NO-2027, Kjeller, Norway article info Article history: Received 12 January 2012 Received in revised form 15 February 2012 Accepted 19 February 2012 Available online 7 March 2012 Keywords: Metal hydrides Mechano-chemical processes Neutron diffraction SEM Thermal analysis X-ray diffraction Vibrational spectroscopy abstract Cl - to BH - 4 ion substitution was induced in NaBH 4 through mechano-chemical reactions with all the first period transition metal chlorides and CdCl 2 . The products were identified by Rietveld refinement of powder X-ray and neutron diffraction data to be mainly Na(BH 4 ) 1-x Cl x . These possess cubic NaCl-type structures, with unit cell parameters between 5.7801(3) and 5.6576(2) ˚ A, and compositions ranging from x = 0.69 (with Cu) to 0.92 (with Zn). Infrared spectroscopy of selected samples confirms the substitution through a shift of the vibrational modes of the BH - 4 group towards higher wavenumbers. An observed shape change of the vibrational features from Lorentzian to Gaussian is related to the presence of the transition metal. There is no correlation between x and the thermal behavior of the samples. The lowest decomposition temperature is found for the Zn containing sample (103 ◦ C), while Cd leads to the highest value (521 ◦ C). Their behavior is related to the presence of NaZn(BH 4 ) 3 and metallic Cd in the samples, respectively. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Hydrogen stored in metal borohydrides can act as energy car- rier and substitute fossil fuels in both stationary and mobile applications. Metal borohydrides store hydrogen with gravimet- ric hydrogen densities up to 20.7 wt% for Be(BH 4 ) 2 [1]. However, alkali and alkaline earth metal borohydrides such as LiBH 4 might be thermodynamically too stable. This translates into too high release temperatures, above the practical requirements. On the other hand, 3d/4d-transition metal borohydrides such as Zn(BH 4 ) 2 are too unstable and might lead to the release of diborane, thus hindering reversibility. The stability of the metal borohydrides in terms of their decomposition temperature has been found to cor- relate inversely with the Pauling electronegativity ( P ) of the metal cation [1]. A similar relationship has been proposed for mixed metal borohydrides with an alkali metal and a transition metal, such as Li n-4 Zr(BH 4 ) n [2], using the average  P between the two metals [3]. Thus, one of the approaches to adjust the thermody- namics of metal borohydrides is to substitute the alkali or alkaline earth metal by another metal with higher electronegativity. The combination of two metals is expected to decrease the desorption ∗ Corresponding author. Tel.: +47 63806273. E-mail addresses: isabel.llamas@ife.no, isabel.llamas@gmail.com (I. Llamas-Jansa). temperatures compared to the more stable alkali or alkaline earth metal borohydride precursor [3,4]. Similar suggestions were obtained from computational screenings of mixed metal borohy- drides based on density functional theory (DFT) [5,6]. In [5], stable mixed borohydrides with promising decomposition temperatures were predicted to have averaged metal electronegativities in the 1.3–1.6 range, and favor M ′ M ′′ (BH 4 ) 2-5 combinations with M ′ = Li, Na, K, and M ′′ = Zn, Ni, Co. The most common method for synthesis of mixed metal boro- hydrides is the metathesis reaction: mM ′ (BH 4 ) + M ′′ Cl n → M ′′ M ′ m-n (BH 4 ) m + nM ′ Cl, (1) where M ′ is an alkali metal, and M ′′ is an alkali, alkaline earth or transition metal. The reaction can either be performed by wet-chemistry or mechano-chemical (ball milling) methods. How- ever, only a limited number of mixed metal borohydrides have been successfully synthesized by mechano-chemical methods. These can be divided between those containing both metals from the alkali and/or alkaline earth groups, e.g. LiK(BH 4 ) 2 [7] or LiMg(BH 4 ) 3 [8], and those containing one alkali and one transi- tion metal, e.g. (Li–Na)Zn 2 (BH 4 ) 5 [9], Li n-4 Zr(BH 4 ) n [3], LiSc(BH 4 ) 4 [10], and NaSc(BH 4 ) 4 [11]. Anion substitutions where BH - 4 groups are replaced by a halide anion (Cl - or I - ) have also been reported [12–17]. However, details about hydrogenation and dehydrogena- tion properties for these compounds are still missing. 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.02.167