Uncommon conductivity of R–Mn–Al (R = Gd, Tb) ternary compounds

Crystal structure investigations, electrical resistivity and magnetoresistance measurements were carried out for GdMn_xAl_12−x (2.6 ≤ x ≤ 6.1), TbMn_xAl_12−x (2.3 ≤ x ≤ 7.2) (ThMn₁₂ structure-type, space group I4/mmm) and Gd₂Mn_xAl_17−x (6.6 ≤ x ≤ 12.2), Tb₂Mn_xAl_17−x (8.5 ≤ x ≤ 10.2) (Th₂Zn₁₇ structure-type, space group R-3m) ternary compounds. These aluminides are characterized by either disordered or partially ordered distributions of Mn and Al atoms in the crystal structure. However, the RMn_xAl_12−x phases exhibit a fully ordered structure (CeMn₄Al₈ structure-type) at the 1:4:8 stoichiometry. A relation between the crystal structure and conductivity of the R–Mn–Al ternaries was evaluated. Hopping conductivity in terms of variable range-hopping was used to describe electrical transport below 150–200 K. The width of the hopping conductivity regime appears to be related to the atomic disorder of the crystal structure. The maximum range was detected for Gd₂Mn_9.5Al_7.5 (50–200 K). The magnetic state of rare earth atoms does not substantially influence this type of conductivity since the magnetoresistance is below 1% in a large temperature range (T > 0.5 K) and magnetic fields up to 12 T.