Enthalpies of formation of selected Co2YZ Heusler compounds

Highlights: •Enthalpies of formation of selected Co2YZ were measured by drop calorimeters. •Enthalpy decreases as the Z element approaches the top right corner of the periodic table. •For the Y element, enthalpy increases on increasing the number of d electrons. •Result of L21 structured compounds agrees with first principles data. •Lattice parameters and related phase relationships were consistent with literature data. -- Abstract: Standard enthalpies of formation at 298 K of selected ternary Co2-based Heusler compounds Co2YZ (Y = Fe, Hf, Mn, Ti, V, Zr; Z = Al, Ga, In, Si, Ge, Sn) were measured by high temperature direct synthesis calorimetry. The measured enthalpies of formation (in kJ/mole of atoms) of the L21 compounds are: Co2FeGa (−25.8 ± 2.6); Co2FeSi (−38.4 ± 2.2); Co2FeGe (−11.6 ± 2.1); Co2MnGa (−30.1 ± 2.3); Co2MnSi (−42.4 ± 1.2); Co2MnGe (−31.6 ± 3.0); Co2MnSn (−15.6 ± 2.8); Co2TiAl (−55.0 ± 3.7); Co2TiGa (−54.2 ± 2.6); Co2TiSi (−61.4 ± 1.7); Co2TiGe (−59.3 ± 3.8); Co2TiSn (−38.4 ± 2.0); Co2VGa (−28.4 ± 1.1) and for the B2 compounds: Co2FeAl (−22.5 ± 2.5), Co2MnAl (−27.6 ± 2.7). Values are compared with those from first principles calculation when available and the extended semi-empirical model of Miedema. Trends in enthalpy of formation with element atomic number are discussed. Lattice parameters of the compounds with L21 structure are determined by X-ray diffraction analysis