用X射线衍射(XRD)、中子衍射(NRD)和Rietveld全谱拟合方法分析和研究PuNi_3型La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5)合金及其充氘后的氘化物晶体结构.结果表明,退火合金La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5)由主相(La, Mg)Ni_3相(PuNi_3-type)和少量LaMgNi_4相(MgCu_4Sn-type) 及La_2Ni_7相(Ce_2Ni_7-type)组成;Co元素在(La, Mg)Ni_3相中主要分布在AB_5单元中6c和AB_5 /AB_2单元共格处的18 h位置上;合金充满氘后形成了La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5)D_(4.0),D原子在(La, Mg)Ni_3相中主要占据RM_5单元中的36i_2、18h_5、6c_4及RM_2单元内的6c_1、18h_3和18h_1位置上,其中RM_5单元中的氘含量为6.5(1)D/ f.u.,而RM_2单元中吸纳的氘量为3.2(2)D/ f.u..充氘后La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5)D_(4.0)晶胞整体基本呈各向同性膨胀(δ_a/a＝7.1%,δ_c/c=9.1%),但在RM_2单元中其各向异性膨胀较大(δ_c/c =15.8%,δ_V/V_(RM2)=32.8%),而在RM_5单元中各向异性膨胀较小(δ_c/c=2 .4%,δ_V/V_(RM5)=17.5%).
Crystal structure of annealed alloy La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5) and (La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5))D_(4.0) deuterides were studied by X-ray diffraction, neutron diffraction and Rietveld refinement. It is found that the alloys are mainly composed of (La,Mg)Ni_3 phase with rhombohedra PuNi_3 type (space group: R3m) structure, and the second phases are LaMgNi_4 phase (MgCu_4Sn type) and La_2Ni_7 phase (Ce_2Ni_7 type). In the (La,Mg)Ni_3 phase, the Mg atoms only occupy at 6c sites in the RM_2 unit, while the Co atoms mainly occupy at 18 h in the RM_5 unit and 6c positions in the boundary between the RM_5 and RM_2 blocks. The full deuteride of the main phase (La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5))D_(4.0) has a rhombohedral structure. For the (La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5))D_(4.0) deuteride, the significant occupancy of deuterium atoms are mainly six sites: 6c_1, 18h_3 and 18h_1 within the RM_2 units and 36i_2, 18h_5, 6c_4 within the RM_5 units. At the same time, it is found that the 6.5(1)D/f.u. is located within the RM_5 units whereas the 3.2(2) )D/f.u. belongs to the RM_2 units. For the PuNi_3 type structure of (La_(0.67)Mg_(0.33)Ni_(2.5)Co_(0.5))D_(4.0), the total isotropic lattice expansions of the a-axis and c-axis are δ_a/a＝7.1% and δ_c/c=9.1%, respectively. A significant anisotropic volume expansion (δ_c/c=15.8%, δ_V/V_(RM2)=32.8%) is found inside the RM_2 unit, but inside the RM_5 unit, the volumetric expansions are 2.4% (δ_c/c) and 17.5% (δ_V/V_(RM5)).
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