Order/disorder in YbNi_1卤xGa_2鈭搙 (x聽鈮ぢ?.08): Crystal structure, thermal expansion and magnetic properties

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• 作者：L. Vasylechko^a ; ^b ; ^{crystal-lov@polynet.lviv.ua} ; U. Burkhardt^a ; W. Schnelle^a ; H. Borrmann^a ; F. Haarmann^c ; A. Senyshyn^d ; D. Trots^e ; K. Hiebl^f ; Yu. Grin^a ; ^{grin@cpfs.mpg.de}
• 关键词：Intermetallic compounds ; Crystal structure ; Thermal expansion ; Magnetic measurements ; NMR spectroscopy ; X-ray absorption spectroscopy
• 刊名：Solid State Sciences
• 出版年：2012
• 期刊代码：98_12932558
• 类别：ch
• 出版时间：June, 2012
• 卷：14
• 期：6
• 页码：746-760
• 文件大小：1476 K

摘要

The ternary phase (x聽鈮ぢ?.08) has been synthesised from the elements by high-frequency melting in argon atmosphere with subsequent annealing at 600聽掳C or 800聽掳C. From X-ray powder and single crystal diffraction investigations the phase is found to be isotypic with MgCuAl₂: space group Cmcm, Z聽=聽4, a聽=聽4.0987-4.1147聽脜, b聽=聽9.8516-9.8620聽脜, c聽=聽6.5995-6.5916聽脜 for the specimens annealed at 800聽掳C. The quantum chemical analysis of atomic interactions in YbNiGa₂ shows Ga and Ni forming a network polyanion with covalent bonds, while ytterbium cations are located within the cavities of the polyanion. From the magnetic susceptibility data and L_III XAS spectra, the 4f¹³ electronic configuration of ytterbium (Yb³⁺) was established in samples with nickel excess (comparing to x聽=聽0), whereas an additional small contribution of the 4f¹⁴ configuration is found for compositions with the gallium excess. No change in the ytterbium valence state was detected down to 5聽K. In situ low- and high-temperature powder diffraction investigations applying both X-ray synchrotron radiation and neutrons revealed no change in structure within the broad temperature range 3.4-1173聽K. Both synchrotron and neutron diffraction data show smooth, but strongly anisotropic variation of lattice parameters below 200聽K: the thermal contraction ratio in [100], [010] and [001] directions is 4.5:3.2:1. The temperature dependence of atomic displacement parameters indicates a certain amount of static disorder for all atomic species in the crystal. At least two distinct sites with varying charge distributions in the vicinity of the Ga atoms are confirmed by ^69,71Ga NMR spectroscopy.