Structure and Deuterium Desorption from Ca3Mg2Ni13 Deuteride: A Neutron Diffraction Study

The Ca3Mg2Ni13 unit cell can be viewed as the stacking of three blocks along the c axis. Each block is composed of two sub-blocks; one sub-block contains one layer of a [CaMgNi4] unit, and the other sub-block consists of one layer of a [CaMgNi4] unit and one layer of a [CaNi5] unit. To understand the deuterium release from the Ca3Mg2Ni13 deuteride, crystal structures of Ca3Mg2Ni13D15.6, Ca3Mg2Ni13D5.9, and Ca3Mg2Ni13D0.3 corresponding to before, during, and after deuterium desorption were determined by neutron diffraction. In Ca3Mg2Ni13D15.6, D atoms occupy interstitial sites within [CaNi5] and [CaMgNi4] units as well as sites at two unit borders. Upon deuterium desorption, the D atoms located at unit borders are released first. Then D atoms located within [CaNi5] and [CaMgNi4] units are simultaneously released which leads to the coexistence of Ca3Mg2Ni13D5.9 and a deuterium-poor solid solution phase. With further desorption, Ca3Mg2Ni13D5.9 transforms into Ca3Mg2Ni13D0.3 where D atoms reside in [CaNi5] units only.