Double Kondo-lattice-like system in the ytterbium deuterides

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Abstract

Heat capacity measurements on the β-ytterbium deuteride samples with the composition D/Yb=2.71 and 2.46 have been carried out over a temperature range from 1.9 to 260 K. The low temperature specific heat anomalies at around 4 K in YbD2.71 and at 6 K in YbD2.46 are analysed in terms of two different Kondo-lattice systems which, depending on the hydrogen concentration, can be considered in the β-ytterbium deuterides. The specific heat anomaly in YbD2.71 has been analysed as a non-magnetically doped Kondo semiconductor whereas anomaly at about 6 K in YbD2.46 was interpreted as a result of the free carriers vanished owing to the bound electron–hole pairs.

Introduction

The ytterbium hydride phases belong to the interesting rare earth interstitial compounds whose structural and electronic properties depend on the hydrogen concentration [1], [2], [3], [4], [5]. Ytterbium metal reacts with hydrogen gas to form an orthorhombic dihydride, α-YbH2 phase which with a hydrogen concentration increasing to over H/Yb≈2.2 undergoes a phase transition to the β-YbH2+δ, with a cubic crystallographic structure. In the ytterbium dihydride a divalent, non-magnetic 4f14 electronic state of the ytterbium ions have been established, contrary to the other rare earth elements, which are trivalent. Only when an orthorhombic into cubic phase transition takes place the Yb2+→Yb3+ valence transition is observed [1]. However, the investigations of an electronic structure of cubic ytterbium hydrides [4], [5], [6] have shown that the ytterbium atom changes its valence from 2 in the YbH2 to non-integer values 2.66–2.72 only, instead of 3 as one can expect. It means, that the hydride ground state consists of the Yb with two different occupations of the 4f shell (valences), which have comparable energies, or becomes in an intermediate valence state.

In this paper, we present the low temperature heat capacity measurements of the β-ytterbium deuteride samples with the D/Yb ratios equal to 2.46 and 2.71. These two compositions mark off the β-cubic hydride phase existence range in the Yb-H phase diagram. It would be shown that even such small difference in hydrogen concentration (Δc=0.25) has strong consequences in electronic properties of both samples. For instance, the presence of high temperature anomaly in Cp(T) at about 230 K for YbD2.71 deuteride and the absence of such anomaly in the YbD2.46 composition [6]. The heat capacity measurements presented in this paper were performed in order to determine the thermodynamical differences of both compositions at the lowest temperatures.

Section snippets

Experimental

The ytterbium deuteride samples were prepared by direct reaction of gaseous hydrogen with ytterbium metal as it was described in [7]. The hydrogen concentration in the sample was determined volumetrically from the pressure changes in the glass apparatus of known volume. To get the β-ytterbium deuteride sample with the lowest composition we had to use some tricky method. From the PTC diagram analysis it appears that the best method to achieve this goal is to synthesize two phases sample with a

Results and discussion

X-ray diffraction analysis at room temperature showed both the samples to be of fcc phase with the lattice parameters a=5.1459 Å for YbD2.46 and a=5.1697 Å. for YbD2.71, respectively. The net specific heat for the YbD2.46 deuteride has been obtained from experimental Cp(T) data of the YbD2.26 deuteride by subtracting appropriate quantity (33%) of the specific heat of YbD1.85 phase [5]. The molar specific heats versus temperature for YbD2.46 and YbD2.71 samples are presented in Fig. 1. As it is

Conclusion

The low temperature anomaly in the specific heat of the YbD2.71 deuteride has been explained as a characteristic feature of the Kondo semiconductor with a small hybridization gap suppressed by the ytterbium Yb2+ ions, which disrupt the Yb3+ lattice periodicity. The idea is based on the Anderson lattice Hamiltonian model of non-magnetic doping effect on Kondo magnetic lattice. This model suggests that the non-magnetic impurities form Kondo holes breaking the translational invariance of the

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