Polymorphic composition of alane after cryomilling with fluorides Jon Erling Fonneløp ⇑ , Sabrina Sartori, Magnus H. Sørby, Bjørn C. Hauback Institute for Energy Technology, Department of Physics, P.O. Box 40, NO-2027 Kjeller, Norway article info Article history: Received 14 April 2012 Received in revised form 18 June 2012 Accepted 19 June 2012 Available online 28 June 2012 Keywords: Hydrogen storage Alane Cryomilling Polymorphs Additives Catalytic effect abstract The effect of additives and mechanochemical processing at low temperature on b-AlD 3 has been studied. b-AlD 3 synthesized by wet chemistry was mixed with selected fluorides and cryomilled. Investigations of the fractions of a-, a 0 - and b-AlD 3 in the samples show that the additive and cryomilling affect the ratio of the polymorphs. Addition of AlF 3 leads to the highest ratio of a-AlD 3 , while addition of TiF 3 leads to full decomposition to Al. Formation of a 0 -AlD 3 during milling with NaF or MgF 2 is observed. The presence of fluorides in the synthesis of alane by cryomilling showed no notable impact on the ratios of the alane polymorphs compared to cryomilling without additives. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Alane (aluminium hydride, AlH 3 ) contains 10.1 wt.% of hydro- gen, has a H density of 148 g/L and releases hydrogen at tempera- tures below 100 °C [1,2]. Despite its metastable and irreversible nature, the high hydrogen content and low decomposition temper- ature make it promising for hydrogen storage applications, and have led to extensive studies of AlH 3 during the last years [3–9]. AlH 3 has been found to take at least six different crystal structures (a, a 0 , b, c, d and e) depending on the synthesis route [10]. Among them a-AlH 3 has been found to be the least unstable modification. Its dehydrogenation enthalpy of only 7.6 kJ/molH 2 (determined by high-precision bomb calorimeter study) [11], gives an equilibrium pressure in the kilo bar range even at ambient temperature. How- ever, a-AlH 3 is kinetically stabilized by an oxide layer of Al 2 O 3 and can be stored for years [1,12–14]. The other polymorphs are more unstable; c-AlH 3 is found to decompose to aluminium and hydro- gen at room temperature [10], and both the b- and c-modification transform to a-AlH 3 upon storage [6]. a 0 -AlH 3 is reported to release hydrogen at 40 °C [3], and transforms to a-AlH 3 prior to decompo- sition [15]. Sandrock et al. [1,16] demonstrated the potential to im- prove the hydrogen desorption kinetics of AlH 3 by grain-size control and use of additives like LiH. AlH 3 is typically synthesized from LiAlH 4 and AlCl 3 in diethyl ether and, depending on the conditions, usually in a, b or c struc- ture [10]. AlD 3 has been synthesised directly by cryomilling LiAlD 4 and AlCl 3 , giving a mixture of a-AlD 3 , a 0 -AlD 3 and LiCl [3]. In this mixture a 0 -AlD 3 starts to decompose at 80–85 °C, while a decom- poses from 95 °C when heated with 1 °C/min [15]. Thus above 95 °C both the transformation from a 0 to a and the direct decom- position of both polymorphs to Al and D 2 occur in parallel. a 0 - AlD 3 is fully decomposed at 140 °C, and at 165 °C only traces of a can be seen. TDS and in situ SR-PXD studies of a-AlD 3 synthesised by wet chemistry show a gradual decomposition starting already at 80 °C, but the main event occurs at 130–150 °C, depending on the heating rate [7]. In situ SR-PXD of b-AlD 3 from wet chemistry show transformation from b- to a-AlD 3 starting at 80 °C, and the decomposition to Al and D 2 starts at 130 °C [6]. The formation of the different alane polymorphs is highly sen- sitive to the synthesis conditions. It is therefore desirable to find methods to better control which polymorphs are being formed. Altering the kinetics by selective use of additives is possible in var- ious metal hydride and complex hydride systems [17–20]. Fluo- rides have shown to be very efficient as additives in several compounds [21,22], and it is well known that many hydrides are isostructural with the corresponding fluorides [23,24]. For alane this is valid since a-AlH 3 takes the same structure as a-AlF 3 ,a rhombohedral structure with space group R  3c [3]. a 0 -AlH 3 is iso- structural to b-AlF 3 , taking an orthorhombic structure with space group Cmcm [4]. Similarities in the crystal structure could play a role in the way additives interact with the alane polymorphs. Thus, a selection of fluorides was chosen as additives in the present study. The main goal is to determine the influence of selected addi- tives and cryomilling on the ratio between the alane polymorphs in the samples. 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.098 ⇑ Corresponding author. Tel.: +47 97534211; fax: +47 63810920. E-mail address: j.e.fonnelop@fys.uio.no (J.E. Fonneløp). Journal of Alloys and Compounds 540 (2012) 241–247 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom