在以第一原理密度泛函理論計算出來Ru2VGa1-xAlx (x=0, 0.25, 0.5, 0.75, 1)在費米能階上的態密度主要貢獻為Ru與V的d電子，推測此系列樣品屬於金屬，但態密度比起一般金屬還要小。
利用電弧熔煉法並且將樣品置於800°C 下退火三天，使晶格中的原子排列整齊，從XRD粉末繞射圖來看，樣品成功製作且隨著參雜量的增加晶格常數變小，表示有成功參雜。
利用NMR測量樣品之共振譜線與自旋-晶格鬆弛時間，比較奈特位移與自旋-晶格鬆弛率，可以發現兩者趨勢大致上相反，可以猜測兩者主要貢獻皆來自d電子的核心極化作用，而以Korringa realation計算出來V的d電子在費米能階上態密度，可以看出系列樣品態密度皆不大，與理論計算結果相符合。

SUMMARY

We report the results of a 51V Nuclear Magnetic Resonance (NMR) study on the Heusler-type compounds Ru2VGa1-xAlx (x=0, 0.25, 0.5, 0.75, 1). Ru2VGa crystallizes in the L21 structure, and Ru2VAl crystallizes in the B2 structure. All materials of the series sample possess high electrical resistivity as compared to ordinary metals. The temperature-dependent NMR spin-lattice relaxation rates 1/T1 for all materials follow a Korringa relation, indicative of metallic behavior. From the analysis of experimental 1/T1 data, we extract the V 3d partial Fermi-level density of states (DOS) for each individual compound. The obtained results indicate a small DOS at around the corresponding Fermi level for all samples of Ru2VGa1-xAlx. We also compare the experimental results with the theoretical electronic structures revealed from recent band structure calculations.

Key words: Nuclear Magnetic Resonance, Heusler-type, relaxation, Korringa realation

INTRODUCTION

In 1903 Heusler-type intermetallic compounds, also called Heusler alloys, began to attract attention when Germany chemist Friedrich Heusler discovered a ferromagnetic compound Ru2MnSn which is synthesized by three non-magnetic elements, Ru, Mn and Sn.

The generic formulas of Heusler-type alloys are X2YZ or XYZ. X2YZ is called full Heusler alloy and crystallizes in the L21 structure, while XYZ is called semi-Heusler alloy and crystallizes in the C1b structure. Compared to X2YZ, XYZ is formed by removing one of the X sites. X and Y atoms are transition metals, while Z atoms are the elements of groups IIIA~VA. The properties of these compounds are very wide, including metals, semiconductors and semimetals. Many Heusler alloys have potential to be thermoelectric materials, so these alloys attract much attention recently.

The theoretical electronic properties of Ru2VGa1-xAlx were calculated by first principle density functional theory in a paper. And this paper indicated that the Fermi-level density of state (DOS) is dominated by Ru-d electrons and V-d electrons and the DOS are small for every samples. We used NMR technique to measure the properties of Ru2VGa1-xAlx.

MATERIALS AND METHODS

All samples were prepared by an arc-melting method and then annealed under 800 ºC for three days. Room temperature X-ray diffractions (XRD) on the powder Ru2VGa1-xAlx samples are displayed in Fig. 1. Ru2VGa crystallizes in the L21 structure while Ru2VAl in the B2 structure. Each XRD shows a single phase and all reflection peaks in the diffraction spectrum are in agreement with the corresponding structure. It is clear in Fig. 2. that the lattice parameter decreases with the Al concentration, indicative of a successful replacement between Ga and Al atoms in this series of samples.

RESULT AND DISCUSSION

Figure 4 shows the 51V NMR central transition line of each component. We determined the Knight shift from the peak position of each spectrum. The evolution of the 51V NMR Knight shift exhibits an opposite trend of the spin-lattice relaxation rate 1/T1T, as demonstrated in Fig. 5. This result suggests that both variations essentially arise from the change of the V-3d electronic states since the hyperfine field association with the core polarization via 3d electrons is negative.

CONCLUSION

From the analysis of 1/T1T, we can deduce the V-3d partial Fermi-level density of states with the results shown in Fig. 5. It is noted that our NMR results for the V-3d electronic states of Ru2VGa1-xAlx are quite consistent with those obtained from the theoretical band structure calculations.

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