Magnesium bis(tetrahydridoborate) Mg(BH4)2 has been applied as single-source precursor for CVD of Mg- and B–contaning thin films, f.e MgB2 [[i]]
Magnesium bis (tetrahydroborate) (magnesium borhydride) Mg(BH4)2 has been synthesized by several wet chemical and mechanochemical method, resulting in solvated and desolvated Mg(BH4)2. Mg(BH4)2 quantitatively and in pure form has been prepared by a direct synthesis, i.e. reaction of MgH2 with aminoboranes. The products were characterized by elemental analysis, FTIR spectroscopy, in situ XRD, and thermal analysis (high-pressure calorimetry under a H2 atmosphere (HP-DSC) as well as TGA-DSC). [[ii]]
Two phases of unsolvated Mg(BH4)2 were obtained depending on the synthesis conditions, their structures have been determined [[iii]]. The first, a hexagonal phase with space group P61, is stable below 180°C (453 K). At higher temperature it transforms to an orthorhombic phase with space group Fddd, which is stable up to 340°C (613 K), above which it decomposes with release of hydrogen. Both phases consist of complex networks of corner-sharing tetrahedra consisting of a central Mg atom and four BH4 units. The high-temperature orthorhombic phase has a strong antisite disorder in the a lattice direction, which can be understood on the basis of atomic structure [[iv]]
Mg(BH4)2 for MgB2 films by CVD
Mg(BH4)2 has been applied as single source precursor for preparation of MgBx thin films on Si(100) by CVD. The films were characterized by XRD and XPS. Ar ion sputtering in combination with cyclic XPS measurements has been used to study the contributions of various chemical species (boride and oxides) in the depth profiles (which were distinguished by the peak-fitting analysis of photoelectron B1s and Auger MgKLL spectra. It was determined that the layers were composed of MgBx film having an overlayer of Mg and boron oxides.[[v]] , [[vi] ]
The surface of the films was enriched with Mg, while the volume was characterized by the B : Mg ratio exceeding 2. According to XPS data, the bulk decomposition of Mg(BH4)2 precursor was successful; therefore, some other phenomenon, such as Mg segregation, was responsible for the formation of MgBx films with x > 2. Film composition dependence on the deposition time / layer thickness was studied; a prolonged deposition time resulted in a thicker surface overlayer of Mg and B oxides, but it did not change the stoichiometry of the boride film. [[vii]]
[i] L. Crociani, G. Rossetto, S. Kaciulis, A. Mezzi, N. El-Habra, V. Palmieri
Chemical Vapor Deposition, Volume 13, Issue 8, pages 414–419, August, 2007, “Study of Magnesium Boride Films Obtained From Mg(BH4)2 by CVD”
[ii]K. Chłopek, C. Frommen, A. Léon, O. Zabara, M. Fichtner, J. Mater. Chem., 2007, 17, 3496-3503, DOI: 10.1039/B702723K, “ Synthesis and properties of magnesium tetrahydroborate, Mg(BH4)2”
[iii] J.-H. Her, P. W. Stephens, Y. Gao, G. L. Soloveichik, J. Rijssenbeek, M. Andrus, J.-C. Zhao , Acta Cryst. (2007). B63, 561-568 [ doi:10.1107/S0108768107022665 ], “Structure of unsolvated magnesium borohydride Mg(BH4)2”
[iv] J.-H. Her, P. W. Stephens, Y. Gao, G. L. Soloveichik, J. Rijssenbeek, M. Andrus, J.-C. Zhao , Acta Cryst. (2007). B63, 561-568 [ doi:10.1107/S0108768107022665 ] , “Structure of unsolvated magnesium borohydride Mg(BH4)2”
[v]L. Crociani, G. Rossetto, S. Kaciulis, A. Mezzi, N. El-Habra, V. Palmieri, Chemical Vapor Deposition, Volume 13, Issue 8, pages 414–419, August, 2007. “Study of Magnesium Boride Films Obtained From Mg(BH4)2 by CVD”
[vi] http://master.lnl.infn.it/slideshow/tuesday/CROCIANI_ATTEMPTS%20TO%20DEPOSIT%20MgB2%20BY%20LOW%20PRESSURE%20CVD.pdf
[vii]L. Crociani, G. Carta, S. Kaciulis, A. Mezzi, G. Rossetto, P. Zanella, Surface and Interface Analysis, Special Issue: Papers Presented at ECASIA'07: The 12th European Conference on Applications of Surface and Interface Analysis, Brussels, Belgium, 9–14 September 2007, Volume 40, Issue 3-4, pages 741–745, March - April 2008, “ Chemical composition of magnesium boride films obtained by CVD
Bis(octahydrotriborate)magnesium Mg(B3H8)2
Bis(octahydrotriborate)magnesium Mg(B3H8)2 and its adducts with ethers Mg(B3H8)2(Et2O)2 and Mg(B3H8)2(Me2O)2 are potential precursors for the chemical vapor deposition of MgB2 thin films. Preliminary attempts to employ them as CVD sources produced boron-rich MgBx films instead, with x ≈ 7.[114]
Mg(B3H8)2 was synthesized by the solid-state reaction of MgBr2 and NaB3H8 at 20°C, with subsequent sublimation at 80°C/0.05 Torr, producing a white solid sublimate. Similar reactions with MgBr2(Et2O) and MgBr2(Me2O)1.5 result in the crystalline ether adducts Mg(B3H8)2(Et2O)2 and Mg(B3H8)2(Me2O)2. In contrast, reactions of MgBr2 with NaB3H8, in the solvent result in the formation of nonvolatile [MgLx][B3H8]2.
The crystal structure of Mg(B3H8)2(Et2O)2 and Mg(B3H8)2(Me2O)2 reveal magnesium center adopting a distorted cis-octahedral geometry with two bidentate B3H8 groups and two Et2O ligands. [114]
Magnesium bis(N,N-dimethylaminodiboranate) Mg(H3B-NMe2-BH3)2
The homoleptic complex magnesium N,N-dimethylaminodiboranate, Mg(H3BNMe2BH3)2 and its monoadducts with THF and 1,2-dimethoxyethane, as well as mixed ligand complex (C5Me5)Mg(H3BNMe2BH3)(thf) have been prepared.
Mg(H3BNMe2BH3)2 has a vapor pressure of 800 mTorr at 25 °C, making it the most volatile reported magnesium complex. Crystal structures and NMR data for all complexes were determined.
Magnesium N,N-dimethylaminodiboranate, Mg(H3BNMe2BH3)2 and its THF and DME adducts, as well as C5Me5)Mg(H3BNMe2BH3)(thf) were proposed as potentially useful mas precursors for the growth of MgB2 and MgO,films by CVD. [[i]]
[i]D.Y. Kim, G.S. Girolami, Inorg. Chem., 2010, 49 (11), pp 4942–4948, Highly Volatile Magnesium Complexes with the Aminodiboranate Anion, a New Chelating Borohydride. Synthesis and Characterization of Mg(H3BNMe2BH3)2 and Related Compounds”