Materials Science and Engineering A 521–522 (2009) 169–171 Contents lists available at ScienceDirect Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea An investigation of the structural phase transition of ammonia borane Annalisa Paolone a,b,∗ , Oriele Palumbo a,c , Pasquale Rispoli a,d , Rosario Cantelli a , Tom Autrey e a Università di Roma “La Sapienza”, Dipartimento di Fisica, Piazzale A. Moro 2, I-00185 Roma, Italy b Laboratorio Regionale SuperMAT, CNR-INFM, Salerno, Italy c CNISM – Dipartimento di Fisica, Università di Roma “La Sapienza”, Piazzale A. Moro 2, I-00185 Roma, Italy d CNR – Department of Physics, University “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy e Pacific Northwest National Laboratory, 908 Battelle Blvd., Richland, WA 99352, USA article info Article history: Received 26 August 2008 Accepted 23 September 2008 Keywords: Elastic properties Ammonia borane Hydrogen storage Structural phase transition abstract A detailed anelastic spectroscopy study of the structural phase transition of ammonia borane was con- ducted for the first time. The transformation from the tetragonal high temperature phase into the orthorhombic low temperature one is detected on cooling around 220K by a huge drop of the elastic modulus and a spike of the elastic energy dissipation. We find clear indications of a hysteresis, which led us to conclude that the transition is of first-order. The kinetics of the transition was investigated in detail. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Ammonia borane (NH 3 BH 3 ) is attracting noticeable interest as a potential hydrogen storage material given the high density of hydrogen (about 19 wt%) [1]. Hydrogen release from solid ammonia borane can be obtained by heating and takes place around 373K if the heating rate is very low, and even at 363K by isothermal treatments [2]. Recently, a reduction in the dehydrogenation tem- perature (<363 K) and a significant improvement in the kinetics were reported in nanocomposites of ammonia borane and meso- porous silica [3] or coherent carbon cryogel [4]. Ammonia borane is a prototype of a donor–acceptor complex, with the B–N bond resulting from the donation of the ammonia lone-pair electrons to the empty p-orbital of the borane [5]. The NH 3 BH 3 lattice symmetry is tetragonal at room temperature, with the B–N bond oriented parallel to the c-axis [6], and undergoes a structural transition to the orthorhombic phase at about 225 K [6,7], with the B–N bonds oppositely tilted with respect to the c-axis [8,9]. A rotationally order–disorder [6,10] character has been observed for the transition, with a displacive component due to a distortion in the NH 3 unit [7]. It has been suggested that the transition is trig- gered by the slowing down of the NH 3 motion [7], but at present the driving mechanism of the tetragonal to orthorhombic transfor- mation is still not known, and little is known also on its kinetics and hysteresis features. ∗ Corresponding author at: Università di Roma “La Sapienza”, Dipartimento di Fisica, Piazzale A. Moro 2, I-00185 Roma, Italy. Tel.: +39 06 49914400; fax: +39 06 4957697. E-mail address: Annalisa.Paolone@roma1.infn.it (A. Paolone). The present work reports a detailed study of the phase transformation in ammonia borane, by anelastic spectroscopy (elastic modulus and energy dissipation), which is a powerful tool to study transformations, since the modulus is very sen- sitive to the formation of new phases or of atomic complexes in materials. The structural phase transformation is detected, on cooling, at nearly 220 K and its first-order character is dis- cussed. 2. Experimental Ammonia borane (NH 3 BH 3 ) powder (99%) obtained from Avia- bor was purified by vacuum sublimation. The consolidated samples necessary to perform anelastic spectroscopy measurements were obtained by pressing the powder in a die at 3 kbar, following a new procedure recently introduced to study sodium alanates [11,12]. A recent high pressure Raman spectroscopy study [13] reported two solid phase transitions, occurring in NH 3 BH 3 under pressure, at 5 and 14kbar. These pressure-induced transformations were reversible, although some hysteresis was observed upon unload- ing [14]. The pressure applied to the samples investigated in the present work, 3 kbar, has been selected in the pressure range where no structural change is induced, and indeed it is well below the for- mer values. The obtained samples were rectangular bars 40 mm long, 5 mm wide and 0.7–1.5 mm thick. The bars are composed of pure NH 3 BH 3 , as the use of a compactant, such as KBr, was not necessary. Anelastic spectroscopy measurements are conducted suspend- ing the bars on thin wires located at the nodal lines of flexural vibration modes and electrostatically exciting the corresponding 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.09.145