Two new magnesium-niobium hydrides, Mg6.5NbH∼14 with a face centred cubic unit cell a = 9.548(1) Å (space group F m over(3, ̄) m (No. 225), Z = 4, V = 870.49 Å3) and MgNb2H∼4 with a C-centred monoclinic unit cell a = 5.685 (4) Å, b = 3.2914(6) Å, c = 7.924(2) Å and β = 103.82(3)° (space group C2/m (No. 12), Z = 2, V = 143.98 Å3), were synthesized by reacting MgH2 and Nb in a high-pressure anvil cell at 8 GPa and 600 °C. The structures of the new hydrides were refined from synchrotron radiation data. Mg6.5NbH∼14 has a structure related to Mg7TiH∼16 (a = 9.532(2) Å space group F m over(3, ̄) m (No. 225)) (D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 372 (2004) 213). Where the metal atom structure is of the Ca7Ge type. Mg6.5NbH∼14, however, has some vacancies on one of the magnesium sites similar to the recently discovered Mg6VH∼14 (D. Kyoi, T. Sato, E. Rönnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Compd. 375 (2004) 253). The metal atoms in MgNb2H∼4 have the AuTe2 type structure.
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