Borides Mn_3-x{Rh,Ir}₅B₂ with Ti₃Co₅B₂-type: X-ray single crystal and TEM data, physical properties

A cursory investigation of the phase relations in the Mn-{Rh,Ir}-B systems prompted for each system a ternary compound, Mn_3-x{Rh,Ir}₅B₂, the crystal structure of them was determined from X-ray single crystal data to be isotypic with the Ti₃Co₅B₂-type (space group P4/mbm, No. 127). In both cases the Mn-site in 2a at the origin of the unit cell exhibits a significant defect (or Mn/B substitution). Transmission electron microscopy studies confirm the absence of a superstructure related to these defects/disorder. The two phases, Mn_3-xRh₅B₂ (x∼0.34) and Mn_3-xIr₅B₂ (x∼0.85) at 950 °C show rather limited homogeneity regions pointing towards higher Mn-contents. Whereas temperature dependent magnetization and specific heat measurements of Mn_2.15Ir₅B₂ do not reveal any indication for a magnetic phase transition in the temperature range from 3 to 300 K, a broad specific heat anomaly at around 200 K and a tilde shape of the temperature dependent magnetization of Mn_2.66Rh₅B₂ appears indicative of an antiferromagnetic phase transition. The latter is also reflected by an anomaly of the electrical resistivity (ρ; 4–300 K) of Mn_2.66Rh₅B₂ which displays a non-monotonous temperature dependence, whereas ρ(T) of Mn_2.15Ir₅B₂ displays a simple metallic-like behavior dominated by scattering from defects. Elastic moduli and Poisson's ratio were determined at room temperature from Resonant Ultrasonic Spectroscopy (RUS) data yielding Young's moduli of E ∼ 170 GPa for Mn_2.66Rh₅B₂ and E ∼ 210 GPa for Mn_2.15Ir₅B₂. Vickers hardness HV is lower for Mn_2.66Rh₅B₂ (HV = 590 ≅ 5.79 GPa) than for Mn_2.15Ir₅B₂ (HV = 655 ≅ 6.42 GPa). The indentation fracture toughness for Mn_2.15Ir₅B₂ was IK_C = 0.71 ± 0.5 MPa m^1/2.