Shtender V. Crystal structure and physico-chemical properties of new intermetallic compounds and their hydrides based on rare-earth metals, magnesium and d-transition metals

The phase equilibria diagrams of the {Nd, Tb}-Mg-Co systems have been established at 500 °C for the first time. The crystal structure of new compounds, namely, RMgCo4, RMgNi2Co2, NdMgCu4, R2MgCo9 (R = Y, Nd, Tb), NdMg2Co9, Nd23Mg4Co7 have been studied. The existence of known compounds in the {Y, Nd, Tb}-Mg-Co systems was confirmed. Hydrides of the selected compounds have been synthesized and their crystal structures were determined for the first time. Hydrogen sorption-desorption properties were studied from the gas phase and by electrochemically. It is shown that in the RMgT4 compounds the increasing of Co content provide to the increasing gas hydrogen capacity. The results of the interaction between hydrogen and the R3-хMgхCo9 (R = Y, Nd, Tb) alloys show that with increasing Mg content gas hydrogen capacity and their thermodynamic stability are reduced. The Nd3-хMgхCo9Hx hydrides are stable up to 80 °C in a vacuum, and after hydrogen desorption the parent structure of the IMC is retained. For already known compounds, R4MgCo (R = Y, La, Nd, Tb) and R4Mg3Co2 (R = Nd, Tb), hydrogen absorption properties were studied for the first time. The R4Mg3Co2 compounds absorb 2 wt.% hydrogen, but then they amorphous after hydrogenation. R4MgCo compounds absorb hydrogen and form R4MgCoH12 hydrides, which under the appropriate conditions of hydrogenation preserve the structure of the parent metal matrix and are stable in air. For NdMgNi4-xCox electrodes have been studied that their electrochemical discharge capacity range is 250 mAh/g. It shown that substitution Ni for Co has not improved the discharge capacity. Electrochemical studies of electrodes based on Nd3-xMgxCo9 showed increasing discharge capacity with a decreasing discharge of current. The influence of annealing on electrochemical capacity has been studied for La2MgNi9 alloys. Additional heat treatment of the alloy leads to reduce the number of phases and improve cyclic stability of the electrodes. Mechanical grinding impairs the electrochemical characteristics. Hydrogen in the alloys reduces the relative magnetization hydrides compare to the initial compounds. Magnetic features pseudobinary compounds are similar to the binary compounds.