Structural transition of the Mg2Ni hydride

Abstract

It is well established that the ternary hydride Mg2NiH4 undergoes a structural transition at 210 to 245°C [1-7]. Although the high-temperature (HT) hydride phase is confirmed to be the antifluoride-type structure (space group: Fm3m) with a = 0.649nm [1 7], there is still some confusion about the crystal structure and the unit-cell dimensions of the low-temperature (LT) hydride phase. It seems that these contradictory structural determinations have been caused by the possible existence of more than one LT modification of Mg2NiH4 . In fact, Darnaudery et al. [8] have proposed two allotropic varieties of the LT hydride phase Mg2NiH4; one of them has a monoclinic structure (a = 1.299nm, b = 0.6390nm, c = 0.6598nm and fl = 93.22°), which transforms irreversibly into the HT hydride phase at 245°C under a hydrogen pressure of I bar, whereas the other LT hydride phase has an orthorhombic structure (a = 0.6499nm, b = 0.6415nm and c = 0.6589nm) and transforms reversibly into the HT hydride phase at 235 ° C. Nor6us and Werner have also reported that the X-ray diffraction pattern obtained by cooling the HT hydride phase down to room temperature is a mixed pattern of at least two phases with a monoclinic structure (a = 0.6497nm, b = 0.6414nm, c = 0.6601 nm and fl = 93.23 °) and an unidentified structure [9]. Gavra et al. [10] have reported that under stress-free hydriding conditions, a two-phase hydride mixture is formed below 210 ° C, and that one of the two phases has a hydrogen-deficient monoclinic structure (a = 1.3137nm, b = 0.6419nm, c = 0.6461nm and/? = 93.60 °) whereas the other coexisting phase has a semiconducting stoichiometric orthorhombic structure (a = 1.1434nm, b = 1 . 1 2 5 7 n m a n d c 0 . 4 5 1 9 n m ) . Furthermore, it is interesting that there are two simultaneous relaxation rates in the temperature range 192 to 207 ° C and below 37 ° C, according to the measurements of the nuclear spin-lattice relaxation time of the proton in Mg2NiH4, suggesting the coexistence of two phases [11, 12]. Quite recently, Zolliker et al. [13] have demonstrated that the X-ray diffraction pattern of the monoclinic LT hydride phase of Mg2NiH 4 can be explained in terms of a structure model which allows for microtwinning, and that the intensity profiles of broad reflections and of reflections due to a second orthorhombic phase are reproduced by different propabilities of microtwinning. The purpose of this letter is to present brief results and comment on the structural transition of the Mg2Ni hydride. The intermetallic compound Mg2Ni was prepared by melting 99.99wt% Mg and 99.99wt % Ni in a graphite crucible under an atmosphere of sulphur