Crystal structure and electrochemical hydrogenation of LaZn5-хMnx.solid solution

Abstract

Systematic investigation of the interaction between the components in La–Mn–Zn ternary system revealed that binary intermetallic compound LaZn 5 (structure type CaCu 5 , space group P 6 /mmm ) dissolve some amount of Mn forming a solid solution of substitution LaZn 5– x Mn x with the homogeneity range of about 6 аt. %. Alloys for investigation have been synthesized by arc melting of pure metals, further annealing at 500 °С over a two-month period and quenching in cold water. Phase analysis of the samples has been performed on data obtained by powder X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The crystal structure of solid solution has been investigated using single crystal X-ray intensity data collected on Xcalibur Oxford Diffraction diffractometer with graphite-monocromated Mo K α–radiation using w scans. The space group P 6 /mmm was chosen and the initial atomic parameters were found by direct methods, confirming the expected CaCu 5 -type structure. The refinement of coordinates and displacement parameters of atoms has been carried out by least squares method in anisotropic approximation to R F = 0.0563 and wR = 0.1391. The atoms in structure occupy the same positions as in CaCu 5 structure, yet in 3 с position there is a statistic mixture M (12 % Mn + 88 % Zn). That is why the calculated composition of single crystal corresponds to the LaZn 4.64 Mn 0.36 formula. The effectiveness of electrochemical hydrogenation of the solid solution has been studied on LaZn 5– x Mn x alloy, x = 0.12, which served as anode material, in a prototype of nickel-metal hydride batteries. Ni(OH) 2 and graphite mixture served as cathode material and a 6 М solution of KOH served as electrolyte. Investigation of the alloy over 30 cycles of charge-discharge showed that it is electrochemically active and can reversibly absorb up to 0.044 Н/f.u. The crystal structure of obtained LaZn 4.88 Mn 0.12 H 0.044 hydride remained unchanged, though the unit cell volume naturally increased as compared to the initial sample. Keywords: solid solution, crystal structure, X-ray structure analysis, electrochemical hydrogenation, nickel-metalhydride battery.