La(NiMMn)_(5.61)储氢合金的结构特性

Structural Properties of La(NiMMn)_(5.61) Hydrogen Storage Alloy

利用粉末中子衍射技术对过化学计量储氢合金La(NiMMn)5.61(M=Ni,Cu,Al,Fe,Sn)的晶体结构进行研究。研究结果显示:不同替代元素所形成的储氢合金晶体结构均为CaCu5型结构,P6/mmm空间群。与正化学计量AB5型储氢合金A原子占1a位、B原子占2c和3g有所不同,过化学计量的AB5+x储氢合金中部分A原子被一对沿c轴定向排列的B-B"哑铃"对替代形成2e位,致使2c位的B原子向2e收缩,进而部分占据6l位,随过化学计量程度的增加,B原子在2e、6l晶位的占位数逐渐增加,而在2c位的占位数则逐渐减少,自始至终,3g位的B原子均为满占位。由于"哑铃"对的形成,使得过化学计量合金出现a轴减小c、轴增大的奇异现象,但与正化学计量相比,晶胞体积仍减小。

All of the over-stoichiometric La （NiMMn） 5. 61 （M = Ni, Cu, Al, Fe, Sn） hydrogen storage alloys were studied by neutron powder diffraction. The result shows that the crystal structures of all hydrogen storage alloys are CaCus type with space group of P6/mmm, but the occupancy site is different from AB5 compounds with the CaCu5 structure. In AB5-type alloys, A atoms occupy la site in P6/mmm space group, B atoms occupy 2c and 3g sites. In the over-stoichiometric AB5＋x alloys, B atoms occupy site 2e in P6/mmm space group, orient along the c-axis and replace A atoms. Moreover, the B atoms forming a hexagon in the z=0 plane around the dumbbell are no longer on site 2c but shrink into position 61. The occupation number of B atoms on site 2e and 61 goes up while that of B atoms on site 2c goes down with increasing the degree of overstoichiometric, and 3g site is fully occupied by B atoms from beginning to end. As compare with ABs-type stoichiometric alloys, the induction of the dumbbells decreases α-axis while increases c-axis, ultimately decreases unit cell volume.