Valence changes and semiconductor-to-metal transitions in Tm1-xEuxSe and TmSe1-xTex

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Valence changes and semiconductor-to-metal transitions in Tm1-xEuxSe and TmSe1-xTex

B. Batlogg, E. Kaldis, P. Wachter

To cite this version:

B. Batlogg, E. Kaldis, P. Wachter. Valence changes and semiconductor-to-metal transitions in Tm1-xEuxSe and TmSe1-xTex. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-370-C5-371.

�10.1051/jphyscol:19795131�. �jpa-00218917�

JOURNAL DE PHYSIQUE Collogue C5, supplement au n° 5, Tome 40, Mai 1979, page C5-370

Valence changes and semiconductor-to-metal transitions in Tm^^Eu^Se and

TmSei-^Te*

B. Batlogg, E. Kaldis and P. Wachter

Laboratorium fur Festkorperphysik ETH, CH-8093 Zurich, Switzerland

Résumé. — En al liant TmSe à EuSe ou TmTe il est possible de faire varier la valence de Tm entre environ 3 + et 2 + . Les propriétés magnétiques, optiques et de transport mettent en évidence la transition métal-semiconducteur asso- ciée au changement de valence. Une pression hydrostatique permet d'inverser la situation en faisant passer Tm d'un état divalent à un état trivalent.

Abstract. — Alloying TmSe with either EuSe or TmTe results in a Tm valence varying from nearly 3 + to 2 + . Optical, transport and magnetic properties show the concomittant metal-to-semiconductor transition. Hydro- static pressure then converts Tm from divalent back to trivalent.

From investigations of TmSe with a systematically varying Tm to Se ratio an average Tm valence of

~ 2.75 has been deduced for the stoichiometric compound [1, 2]. The purpose of alloying TmSe with either TmTe or EuSe is twofold : first, to shift the Tm valence towards 2 and second, to prepare systems which contain only divalent rare earth ions but are so close to a valence crossover that even moderate changes of external parameters (temperature, pressure) are sufficient to induce drastic changes in the electronic structure.

It is the energy difference A£ between the two configurations 4f 1 3 (Tm 2 + ) and 4f 12 5d(Tm 3 + ) which determines the valence state in any particular case.

All the following considerations are based on the fact that this AE can be influenced externally. The 5d band states, derived from the RE 5d atomic states, are split by the crystal electric field of the octahedral surroundings and the energetically lower « 5d? ^2g » part shifts to even lower energies as the interionic distances are reduced. Therefore AE depends sensi- tively upon variations of the lattice parameter.

TmSe-TmTe. — The partial replacement of Se ions by the larger Te ions offers the possibility of applying negative pressure through the anion lattice.

TmTe is a semiconductor [3, 4] in which Tm is purely divalent. In the alloy system TmSe^^Te^ with 0.5 < x < 1 the electrical resistivity at 300 K drops exponentially on going from TmTe ( ~ 10 ² Qcm) to TmSe ^{0 5} Te ^{0 5} ( ~ 0.3 Qcm) and the temperature dependence shows typical semiconducting behaviour (see figure 1). As pointed out above, hydrostatic pressure is expected to close the energy gap and indeed we find an exponential decrease of the resistivity p by 0.17 decades per kbar in all semiconducting samples. Assuming p ~ exp(AE/kT), the value of AE

Fig. 1. — Temperature dependence of the electrical resistivity of TmSe 0 5 Te 0 5 showing semiconducting behaviour at normal pres- sure.

can then be estimated using the measured change of p under pressure and the measured p for AE < 0, corresponding for instance to the metallic

TmSe 0 8 3 Te 0 1T (~ 120 uficm) .

Accordingly AE decreases linearly within the experi- mental accuracy from 320 + 30 meV for TmTe (in excellent agreement with ref. [4]) to 170 ± 30 meV for T m S e ^{0 5} T e ^{0 5} . The divalency of Tm for x > 0.5 at normal pressure is also confirmed by the magnetic susceptibility and a representative example is shown in figure 2. The effective magnetic moment

p e(f = 4.73 p B /Tm

is very close to the calculated free ion value of 4.58 for Tm 2 + (4f 13 ). Below ~ 90 K x' 1 starts to deviate from the Curie-Weiss straight line as expected from the crystal electric field effects on the 4f electrons.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19795131

VALENCE CHANGES AND SEMICONDUCTOR-TO-METAL TRANSITIONS IN Tm, - ,Eu,Se A N D TmSe, _ ,Te, C5-371

The paramagnetic Curie temperature (+ 0.5 K) and the possible indication of magnetic ordering well below 1.5 K (see insert in figure 2) are very similar to the data of TmTe [5].

0 100 ZOO 300 I

Temperature (K )

Fig. 2. - Reciprocal magnetic susceptibility of TmSe, ,Tea ,.

For x < 0.25, Tm is intermediate valent. This

follows from the resistivity at 300 K (100-200 pQcm), which is typical for RE^+ monochalcogenides, and from the high concentration of delocalized elec- trons giving rise to the high metallic reflectivity in the long wavelength part of the visible spectrum.

Below 300 K the resistivity steadely increases on cooling and either stays constant below

3 K (x = 0.09) or passes through a maximum at - ^{5.5 K}

(x = 0.17). The over-all behaviour (figure 3) strongly resembles that of Tm,.,,Se. From the shape of the magnetoresistance curves the magnetic ordering tem- peraturescan be estimated as 3 f 0.5 K and 5.5 + ^{0.5 K}

for x = 0.09 and 0.17, resp. Additional magnetization measurements on TmSe,,,,Te,,,, show for the first time a spontaneous net magnetic moment (T 5 5 K) in a mixed valent RE compound.

TmSe-EuSe. - Here, we will concentrate only on Trn,~,Eu,~,Se. Just the lattice constant (6.042 A)

and the effective magnetic moment (6.47 p,/RE) suffice to show the divalency of both the Tm and Eu ions. This is also confirmed by the reflection spectrum,

Tm Sem3Tea,,,

100o ^{0 3 10 30} Temperature 100 (K) 300

Fig. 3. - Temperature dependence of the electrical resistivity of TmSeo ,,Te, ,, at zero applied magnetic field. Magnetic ordering occurs at 5.5 + 0.5 K.

where the fingerprint-like final-state multiplets of the excitations from the 4f shells of Tm and Eu can be resolved and identified unambiguously. Correspon- dingly the Tm 4f13 level lies very close to the 4f 125d state ( N 0.1 eV) whereas the Eu 4f7 level is 1.8 eV lower in energy. At room temperature hydrostatic pressure induces a semiconductor-to-metal transition which appears to be completed by - 14 kbar. Reflec- tivity measurements of the high pressure phase show the electronic phase transition to be caused by the continuous valence change of only the Tm from 2 towards 3. The temperature dependence of the resis- tance at 1 bar is very interesting in itself. Below

- ²⁰⁰ K the conduction is bandlike (dp/dT > 0) and pronounced magnetic scattering occurs around the magnetic ordering point (T, = 20 K, 8, = 12 K) similar to what happens in EuSe with a few percent of free charge carriers [6]. But above - ^{200 K,}

dp/dT becomes negative and the conduction changes to a temperature-activated type with an activation energy of - 50 meV. Experiments are planned to study the influence of the carrier concentration (which can be regulated by pressure) on the magnetic properties.

References

[I] BATLOGG, B., KALDIS, E., OTT, H. R., Phys. Lett. 62A (1977) [4] SURYANARAYANAN, R., GUNTHERODT, G., FREEOUF, J. L.,

270. HOLTZBERG, F., Phys. Rev. B 12 (1975) 4215.

[Z] OTT, H. R., BATLOGG, B., KALDIS, E., WACHTER, P., J. Appl. [5] BUCHER, E., ANDRES, K., ^DI SALVO, F. J., MAITA, J. P., GOS-

Phys. 49 (1978) 21 18. SARD, A. C., COOPER, A. S., HULL, G. W. Jr, Phys. Rev.

[3] BUCHER, E., NARAYAMAMURTI, V., JAYARAMAN, A,, J. Appl. B 11 (1975) 500.

Phys. 42 (1971) 1741. [6] VON MOLNAR, S., ZBM J. Res. Develop. 14 (1970) 269.