Structural investigations of two new ternary magnesium-niobium hydrides, Mg_6.5NbH_∼14 and MgNb₂H_∼4

Two new magnesium-niobium hydrides, Mg_6.5NbH_∼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 ų) 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 ų), 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. Mg_6.5NbH_∼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. Mg_6.5NbH_∼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.