کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
1626852 | 1516452 | 2006 | 11 صفحه PDF | دانلود رایگان |
عنوان انگلیسی مقاله ISI
Polar binary Zn/Cd-rich intermetallics: Synthesis, crystal and electronic structure of A(Zn/Cd)13 (A = alkali/alkaline earth) and Cs1.34Zn16
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کلمات کلیدی
موضوعات مرتبط
مهندسی و علوم پایه
مهندسی مواد
فلزات و آلیاژها
پیش نمایش صفحه اول مقاله
چکیده انگلیسی
The binary alkali/alkaline earth (A) zinc and cadmium (M) compounds that form the NaZn13 structure type (cubic, space group Fm3¯c), which were known only from indexed X-ray powder diagrams, have been synthesized from stoichiometric mixtures of the elements and have been structurally characterized using a combination of single crystal data and the Rietveld method. The alkali zinc phases AIZn13 are only formed with the lighter alkali elements sodium to rubidium (AI=Na: a â=â1227.3(2)âpm, R1=0.0205;AI=K: a â=â1237.9(3)âpm, R1=0.0154;AI=Rb: a â=â1245.19(1)âpm, Rp=0.0250), whereas the cadmium compounds AI Cd13 are stable only for the heavier alkali metals potassium to cesium (AI=K: a â=â1379.2(4)âpm, R1=0.0118;AI=Rb: a â=â1384.5(2)âpm, R1=0.0139;AI=Cs: a â=â1392.0(3)âpm, R1=0.0381). In the crystal structures M-centered M(2)M(1)12 icosahedra are connected via strong exobonds (forming tetrahedral stars, stella quadrangula) to form a 3D net, in the interstices of which the alkali metals are located in a snub cube 24 coordination. In contrast, the alkaline earth (AII) zinc compounds with calcium, strontium and barium (AII=Ca: a â=â1215.4(1)âpm, R1=0.0274;AII=Sr: a â=â1222.2(2)âpm, R1=0.0154;AII=Ba: a â=â1235.8(2)âpm, R1=0.0227) show statistical defects at the Zn(2) position centering the Zn(1) icosahedra. Both the geometric and the electronic stability of the NaZn13 structure type, which can be assessed from the pseudo-bandgap calculated using FP-LAPW-DFT methods, are discussed. In contrast to the aforementioned systems, the NaZn13 type ceases to exist in the binary system Cs-Zn. In this case a phase Cs1.36Zn16 with a new structure type but very similar stoichiometry emerges. This compound is the first and only known phase in the system Cs-Zn and crystallizes with a new incommensurably modulated structure: its average structure (orthorhombic, space group Imma, a=264.2(5) âpm,b=720.8(14) âpm,c=1760(5) âpm, R1=0.0198) already indicates the representative structural features: in a 3D net of Zn atoms tubular channels filled by the Cs atoms are formed. This Cs filling is, due to geometric requirements, not commensurate with the Zn host structure resulting in a modulated structure with satellite reflections that can be indexed with a qâ vector of (0,â0,â0.2755). The final refinement in the superspace group (Pcn2(00g)sso,a=1760(5) âpm,b=720.8(14) âpm,c=264.2(5) âpm, wRmain=0.0550,wRsatellites=0.140) shows a strong occupational and a weak positional modulation of Cs in the channels formed by the Zn partial structure.
ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: Journal of Alloys and Compounds - Volume 421, Issues 1â2, 14 September 2006, Pages 24-34
Journal: Journal of Alloys and Compounds - Volume 421, Issues 1â2, 14 September 2006, Pages 24-34
نویسندگان
Marco Wendorff, Caroline Röhr,