Clathrate formation in the systems Ba-Ir-Ge and Ba-{Rh, Ir}-Si: Crystal chemistry and phase relations

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• 作者：M. Falmbigl^a ; A. Grytsiv^a ; P. Rogl^a ; ^{peter.franz.rogl@univie.ac.at} ; G. Giester^b
• 关键词：A. ternary alloy systems ; B. crystallography ; phase diagrams ; thermal stability ; F. diffraction (X-ray) ; electron microscopy ; scanning
• 刊名：Intermetallics
• 出版年：2013
• 出版时间：May, 2013
• 年：2013
• 卷：36
• 期：Complete
• 页码：61-72
• 全文大小：1847 K

文摘

Phase relations in the {Si,Ge}-rich part of the three systems Ba-Ir-Ge and Ba-{Rh,Ir}-Si were investigated by means of X-ray single crystal (SC) and powder diffraction (XPD), EPMA and DTA measurements. For Ba-Ir-Ge, partial isothermal sections were constructed at 800?¡ãC and 700?¡ãC. The clathrate solid solution extends from binary Ba₈Ge₄₃¡õ₃ to Ba₈Ir_0.4Ge₄₃¡õ_2.6 with a slightly enlarged lattice parameter a?=?2.13402(2)?nm (, Ba₈Ge₄₃¡õ₃-type). Two new ternary stoichiometric compounds were found and their crystal structures were determined by X-ray diffraction: ¦Ó₁-BaIr₂Ge₇ (Pmna, own type, a?=?3.7782(1), b?=?0.64099(2), c?=?0.64401(2)?nm; R_F?=?0.0331, SC-data) and ¦Ó₂-Ba₃Ir₄Ge₁₆ (P4/mmm, Ba₃Rh₄Ge₁₆-type, a?=?0.653948(6), c?=?2.22849(3)?nm; R_F?=?0.0388, XPD-Rietveld analysis).

The isothermal sections of the Ba-{Rh,Ir}-Si systems were investigated at 950?¡ãC. Whereas in the Ba-Rh-Si system a clathrate-type I solid solution ¦Ê_I-Ba₈Rh_xSi_46?x extends within 2.6?¡Ü?x?¡Ü?2.9, the corresponding clathrate-I phase in the Ba-Ir-Si system, ¦Ê_I-Ba₈Ir_1.8Si_44.2 (, Na₄Si₂₃-type, a?=?1.0346(1) nm), is not stable at this temperature, but was clearly observed in the as cast state.

The analysis of temperature dependent atomic displacement parameters (100-300?K) of guest and framework atoms for ¦Ê_I-Ba₈Ir_xGe_46?x?y¡õ_y revealed Debye (¦È_D) and Einstein (¦È_E) temperatures of ¦È_D?=?244?K, ¦È_E,11?=?85?K, and ¦È_E,22,33?=?64?K proving the rattling nature of the Ba-atoms in the large Ge-based tetrakaidecahedral cages. A similar analysis for ¦Ê_I-Ba₈Rh_xSi_46?x yields significantly higher Debye?and Einstein temperatures, ¦È_D?=?508?K, ¦È_E,11?=?115?K, ¦È_E,22,33?=?99?K, i.e. a tight bonding of Ba-atoms to the smaller Si-based cages. This behavior is further reflected by the pronounced difference in the thermal expansion coefficients: ¦Á_{100-300 K}?=?13.7?¡Á?10^?6?K^?1 (¦Ê_I-Ba₈Ir_xGe_46?x?y¡õ_y) and ¦Á_{100-300 K}?=?7.3?¡Á?10^?6?K^?1 (¦Ê_I-Ba₈Rh_xSi_46?x).