A solid solution of magnesium and manganese borohydrides was studied by in situ synchrotron radiation X-ray powder diffraction and infrared spectroscopy. A combination of thermogravimetry, mass and infrared spectroscopy, and atomic emission spectroscopy were applied to clarify the thermal gas desorption of pure Mn(BH_4)_2 and a solid solution of composition Mg_(0.5)Mn_(0.5)(BH_4)_2. Mg_xMn_(1-x))(BH_4)_2 (x = 0-0.8) conserves the trigonal structure of Mn(BH_4)_2 at room temperature. Manganese is dissolved in the hexagonal structure of a-Mg(BH_4)_2, with the upper solubility limit not exceeding 10 mol.% at room temperature. There exists a two-phase region of trigonal and hexagonal borohydrides within the compositional range x = 0.8-0.9 at room temperature. Infrared spectra show splitting of various vibrational modes, indicating the presence of two cations in the trigonal Mg_xMn_(1-x)(BH_4)_2 solid solutions, as well as the appearance of a second phase, hexagonal α-Mg(BH_4)_2, at higher magnesium contents. All vibrational frequencies are shifted to higher values with increasing magnesium content. The decomposition temperature of the trigonal Mg_xMn_(1-x))(BH_4)_2 (x = 0-0.8) does not vary significantly as a function of the magnesium content (433-453 K). The desorbed gas contains mostly hydrogen and 3-7.5 mol.% diborane B_2H_6, as determined from analyses of the Mn(BH_4)_2 and Mg_(0.5)Mn_(0.5)(BH_4)_2 samples. An eutectic relation between α-Mg(BH_4)_2 and LiBH_4 is observed. The solid solution Mg_xMn_(1-x))(BH_4)_2 is a promising material for hydrogen storage as it decomposes at a similar temperature to Mn(BH_4)_2, i.e. at a much lower temperature than pure Mg(BH_4)_2 without significantly losing hydrogen weight capacity thanks to substitution of Mn by Mg up to 80 mol.%. The questions of diborane release and reversibility remain to be addressed.
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