A new and typical valence fluctuating system, YbB12

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Journal of Magnetism and Magnetic Materials 31-34 (1983) 437-438 A N E W A N D T Y P I C A L V A L E N C E F L U C T U A T I N G S Y S T E M , YbBi2 M. K A S A Y A , F. I G A , K. N E G I S H I , S. N A K A I * a n d T. K A S U Y A

Department of Physics, Tohoku University, Sendai, Japan

Magnetic susceptibility, electrical resistivity, Hall effect and L m absorption measurements indicate that Y'bBl2 exhibits unique behavior uncharacteristic of hitherto studied ytterbium compounds. We suggest that YbBt2 is a valence fluctuating system accompanied with the formation of an energy gap.

Valence fluctuating compounds have been extensively studied in the past ten years, both experimentally and theoretically. However, some fundamental properties remain still unsolved. For example, the archetypal valence fluctuation occurs in SmB6, but the origin of the formation of an energy gap as well as the disappearance of Sm3+ 's contribution to the susceptibility have not yet been explained definitely. The fact that examples exhibiting an energy gap in valence fluctuating compounds are few makes the problem more complex. Therefore, it is worthwhile to look for new valence fluctuating materials whose ground state behavior is semiconducting. In this paper, we report experimental results of magnetic and electrical measurements on YbB12. The samples were prepared by borothermal reduction under vacuum at 2200°C. It is difficult to prepare a single phase of YbB~2; the product usually contains a slight amount of YbB6 [1]. However, it is found that YbB6 can be dissolved away using diluted nitric acid. For the measurements of transport properties, we used polycrystals prepared from the powders by melting in an arc furnace and slowly cooling. The lattice constant a = 7.469 ,~ is equal to that reported in the literature. As a reference sample of YbB12, we have also prepared TmBx2 by the same procedure. The lattice constant is 7.475 ,~. The crystal structure of YbBt2 is of UBx2 type. The magnetic susceptibility, X, vs. temperature curve is shown in fig. 1. It is to be noted that X of YbB~2 above 170 K shows a Curie-Weiss behavior with Op= 95 K and Pal = 3.93#a. As the temperature decreases X goes through a pronounced peak and then decreases rapidly below 75 K. The sharp rise in the low temperature region may arise from the presence of Yb203 impurity with a concentration of the order of 2 wt%. To investigate the correlation between the anomalous magnetic property and the transport properties, we have measured the electrical resistivity and Hall effect from 1.7 up to 300 K. Electrical resistivity at 300 K is 5 × 10-412 cm. As temperature decreases 0 increases exponentially between 100 and 20 K with an activation

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atom is nearly trivalent in YbBI2 at room temperature. Existing data on M6ssbauer effect of YbBi2 suggest that the Yb atom is nearly trivalent at 4.2 K, too [3]. For comparison we have also measured some physical properties of TmBi2. The effective moment of 7.32 is almost equal to that of theoretical value, 7.56, for Tm 3÷ (4f 12) (fig. 2). TmB12 is metallic with the resistivity of the order of 5/d~ cm at low temperature (fig. 3). These results suggest that the anomalous properties observed in YbBi2 arise from the valence fluctuation accompanied with the formation of an energy gap. In valence fluctuating ytterbium compounds, maxima in × vs. T curve is a common property. However, YbBt2 is the first example which shows semiconducting property. These

results along with the experimental results on SmB6 would shed light on the problem of valence fluctuation•

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References

[1] Z. Fisk, B.T. Matthias and E. Corenzwit, Proc. Natl. Acad. Sci. US 64 (1969) 1151. [2] M. Kasaya, H. Kimura, Y. Isikawa, T. Fujita and T. Kasuya, Valence Fluctuations in Solids (North-Holland, Amsterdam, 1981) p. 251. [3] P. Bonville, P. Imbert, G. Jehanno and F. Gonz/dez-Jimenez, J. Phys. Chem. Solids 39 (1978) 1273.