Crystal structure and hydrogen absorption-desorption properties of Ca3Mg2Ni13

The novel compound Ca3Mg2Ni13 has been found in the Ca-Mg-Ni system, and its crystal structure has been investigated by X-ray diffraction and high-resolution transmission electron microscopy. Ca3Mg2Ni13 crystallizes in space group R3¯m (No. 166); cell parameters: a = 4.9783(2) Å and c = 36.180(2) Å; Z = 3. The Ca3Mg2Ni13 structure has three blocks stacked along the c axis in one period, and each block is composed of two sub-blocks. One only contains one layer of [CaMgNi4] units, and the other consists of one layer of [CaMgNi4] units and one layer of [CaNi5] units. Further investigations show that Ca3Mg2Ni13 has good kinetic properties but bad thermodynamic properties for hydrogen absorption-desorption. However, the Ca/Mg ratio of the Ca3Mg2Ni13-type compound can be adjusted within a wide range to improve the hydrogen absorption-desorption properties. As a result, Ca1.67Mg3.33Ni13 shows favorable thermodynamics and has good reversibility for hydrogen absorption-desorption.