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} 5 B 2 , the crystal structure of them was determined from X-ray single crystal data to be isotypic with the Ti 3 Co 5 B 2 -type (space group P 4/ mbm , No. 127). In both cases the Mn -site in 2 a 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-x Rh 5 B 2 (x-0.34) and Mn 3-x Ir 5 B 2 (x-0.85) at 950 degrees C show rather limited homogeneity regions pointing towards higher Mncontents. Whereas temperature dependent magnetization and specific heat measurements of Mn 2.15 Ir 5 B 2 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 2.66 Rh 5 B 2 appears indicative of an antiferromagnetic phase transition. The latter is also reflected by an anomaly of the electrical resistivity (rho; 4-300 K) of Mn 2.66 Rh 5 B 2 which displays a non-monotonous temperature dependence, whereas rho(T) of Mn 2.15 Ir 5 B 2 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 2.66 Rh 5 B 2 and E - 210 GPa for Mn 2.15 Ir 5 B 2 . Vickers hardness HV is lower for Mn 2.66 Rh 5 B 2 (HV = 590 - 5.79 GPa) than for Mn 2.15 Ir 5 B 2 (HV = 655 - 6.42 GPa). The indentation fracture toughness for Mn 2.15 Ir 5 B 2 was IK C = 0.71 +/- 0.5 MPa m 1/2 .