SEMICONDUCTOR-METAL TRANSITIONS IN TmSe-TmTe AND TmSe-EuSe

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SEMICONDUCTOR-METAL TRANSITIONS IN TmSe-TmTe AND TmSe-EuSe

B. Batlogg, P. Wachter

To cite this version:

B. Batlogg, P. Wachter. SEMICONDUCTOR-METAL TRANSITIONS IN TmSe-TmTe AND TmSe- EuSe. Journal de Physique Colloques, 1980, 41 (C5), pp.C5-59-C5-63. �10.1051/jphyscol:1980511�.

�jpa-00219946�

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SEMICONDUCTOR-METAL TRANSITIONS IN TmSe-TmTe AND TmSe-EuSe B. ~atlogg* and P. Wachter

Laboratorium fiir ~ e s t k d @ e ~ h ~ s i k ETH 8093 Ziirich, SUISSE.

Resume

.-

Une transition de semiconducteurPm~ta1 (SMT) a lieu soit en fonction de la composition ou de la pression externe dans les compos6s pseudobinaires TmSe-TnlTe et TmSe-EuSe. La transition est caus6e par la dhlocalisation des electrons de la couche 4f l3 du Tm divalent dans la bande de conduction 5d. La con£ iguration divalente

(4f7) des ions Eu est plus stable, et donc n'intervient pas dans la SMT. On observe une similarit6 frappante entre la SMT dans TmSel-yTex en fonction de x et la SMT in- duite par pression dans SmS, et il est sugg6r6 qu une combinaison particulisre des paramstres Glectroniques et Blastiques est le facteur d6terminant l'ordre de transition.

Abstract.- A semiconductor to metal transition (SMT) occurs either as a function of composition or external pressure in the TmSe-TmTe and TmSe-EuSe pseudobinary com ounds. The transition is caused by the delocalization of electrons from the 4flq shell of divalent Tm into the 5d conduction band. The divalent configuration

(4f7) of the Eu ions is more stable and therefore is not involved in the SMT. A striking similarity between the SMT in TmSel-xTex as a function of x and the pressure induced SMT in SmS is observed and it is suggested that a particular combination of the electronic and elastic parameters is the factor determining the order of transition.

1. Introduction.- The TmSe-TmSe pseudobinary system is unique among the rare-earth monochalcogenides in that one end member of the series (the selenide) is metallic and the other semiconducting. A detailed study of this semiconductor-to-metal transition

(SMT) may be expected to throw light on the question why the pressure or chemical subs- titution in Sm, Eu, Tm and Yb monochalcogenides result in discontinuous in some cases, but mostly continuous, SMT transitions.

1 ' '

Both Tm and Eu can be divalent or trivalent. The chemical environment dictates the valence. Obviously this must be the energetically most favorable valence state.

From the SMT in the EuSe-TmSe alloy system we expect to learn quantitatively why the Tm monochalcogenides are such excep-

tional cases in the above context.

2. TmSe-TmTe.- 2.1. Experiments.- Electrical resistivity, 'optical, magnetic and compres- sibility measurements have been performed on either single-or polycrystalline mate- rial of the chemical composition TmSel-xTex with x = 0; .09, .17, .5, -77, .83,

.91 and 1. Detailed information on the sam- ple preparation and the related solid state chemical problems has been published in ref.

/3 /

. .

f Present address : Bell Laboratories, Murray Hill, NJ 07974 USA.

The semiconducting behavior of TmTe has been established in earlier investiga- tions r4/ and according to these an energy gap of 0.2 or 0.35 eV separates the lo- calized 4f13 levels of the divalent Tm ions from the 5d-6s conduction band. On substitu- tion of Te by Se we find an exponential drop in the electrical resistivity (at ambient conditions) from % 50C2 cm in TmTe to % .25 51 cm in TmSe - 5 Te - 5 . As an example, the Arrhenl.uspl0t of the resistivity of TmSe

.17 Te.83 is shown in figure 1 over a wide temperature range. From the slope at the high temperature end, an activation energy of

0.2 to 0.25 eV is deduced. Aternately, the activation energy can be obtained if one assumes simple statistics for the number of carriers in the conduction band. The resistivity is then given by.

P (AEtT) = p o exp (AE/kT)

where p o denotes the experimental value of

p when the gap AE = 0 and equals to % 2 0 0 p a cm, typical for metallic TmSe. The lat-

ter method leads to very similar results for AE and they are shown in figure 2 for the different compositions. Evidently, TmSel-x Tex is semiconducting for x 1 0.5 and metallic for x 5 0.2. Included in figure 2 is also the value of the gap as determined from optical spectroscopy in ref 14/ and we

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980511

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JOURNAL DE PHYSIQUE

noti-ce the good agreement.

A strong indication for the divalent state of Tm in semiconducting TmSelqxTex is the magnetic susceptibility X. Between 1.5 and 300 K no ordering could be detected and above c 100 K

x

obeys the Curie law

( 8 +.5K). The splitting of the J = 7/2 P = . -

free lon (Hund's rule) ground state by the crystal-electric field accounts for the deviation of X-l from the Curie straight line below % 100 K. The effective magnetic moment amounts to 4.7

%

/ mole and there-

fore bears clear evidence for a 4f13 ( ~ m ~ + ) configuration, which in the case of a free ion carries a peff of 4.58

%.

For compari- son peff equals 7.56 ps f a r a free triva-, lent Tm ion.

When these samples are subject to external hydrostatic pressure their electrical resistance decreases exponentially with p and the reduction of the energy gap dAE/dp can be calcul3ted to be 10-13 meV/kbar at pressures u~ to 10 kbar. The pressure induced SMT should therefore be completed at p % 2P kbar for x = .5 and

% 35 kbar for x = 1. The latter value agrees reasonably well with an earlier p(p)

result /I/.

Fig. 1 Arrhenius plot of the electrical resistivity of TmSe 17 Tea8?. From the slope at the highest tem- peraturGs (see lnset) the activation energy is de- termined to 200-250 meV.

Within the pressure range of this study (up to 20 kbar) the resistance decreases conti-

nuously for all x, but dRogp/dP is pressure dependent and is smaller by a factor of 2 at the highest p. A detailed discussion of the latter effectr which appears to be in- trinsic in originr will be presented else- where.

Furthermore, the optical reflectivity spectum is typical for semiconductors with a small energy gap: interband excitation peaks above c 0.5 eV and a plasma resonance minimum at some tenth of an eV. The plasma is formed by the electrons thermally excited across the gap and its collective mode (plas- mon) frequency is consistent with a carrier density of a few percent of the cations.

TmSel-xTex is metallic for x < - 2 with the Tm ions in a mixed valent state. This means that the 4f13 and the 4f 5d configu- 12

rations at each cation site are equivalent in energy. As a consequence the f-d interac- tion at the Fermi energy strongly modifies the metallic behavior, which becomes most obvious in the dc conductivity and the infrared dielectric response. Peculiarities in the magnetic properties can be ascribed to the instability of the 4f shell and a 4f- double exchange model has been proposed to explain the gradual transition from metamag- netism in TmSe to an order carrying sponta- neous magnetization in the samples with 9 and 17% Se replaced by Te.

METALLIC SEMICONDUCTING

I I I

0 5 I

TmSe TmSe,_,Te, TmTe

?ig. 2 The energy gap for the 4f13 + 4f125d excita- tion in TmSe Te

.

The squaFe indicates the result from Ref. 4

&o$

^&l'e. The dashed line shows the variation of the gap as predicted from the measure- ments on the related Sm, E u and ~b compounds.

These compounds are of outstanding interest in connection with valence mixing and magne-

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in references 5 and 6.

2.2 Discussion.- The delocalization of 4f electrons into the 5d-6s conduction band and the concomitant valence change of Tm is the reason for the SMT in the TmSe-TmTe alloy system. The energy gap to be overcome in this process is given by ~ ( 4 f l ~ 5 d ) - ~ ( 4 f 13 )

.

The crucial point is the volume dependence of the 5d conduction band contribution to this difference. It is the strength of the ligand field (acting on the 5d states) which increases as the ligand distance is reduced and in turn lowers the bottom of the 5d conduction band. This is observed in all rare- earth monochalcogenides.

The starting point for our discussion is the dashed line in figure 2. It represents the reduction of the energy gap on going from the tellurides to the selenides and amounts to 0.2 eV. Evidently the Te rich samples behave similarly as the related Sm, Eu and Yb compounds. The most striking ano- maly, however, is the metallic character of the Se rich compositions. From the universal behavior, even TmSe itself would be predic-

ted to be semiconducting. The energy gap in TmSel-xTex apparently drops very fast to zero in the range 0.2 < x < 0.5.

Two important quantities of this SMT are worth comparing with the ones in the pressure induced first order SMT of SmS :

-

(1) In both cases, the energy gap has shrunk to 0.15-0.20 eV when the SMT starts. (Here the zero pressure gap in SmS is taken to be 0.2-0.25 eV according to the high temperature activation energy. A similar value results from the energy of the first optical absorption peak (fad) and as- cribing to it the same width as in SmSe and SmTe 1 7 / ) .

-

(2) The lattice constants are also

0

similar : % 5.92 A for SmS at 6.5 kbar and

0

a 6.0 A for a fictitious TmSe .65Te.35(to ta- ke a composition closest to the SMT).

The similarities could be accidental, but the correlation is striking and perhaps there is a deeper reason behind it : it is probable that a particular combination of

directly related / I / ) energies favor a first order SMT. Numerous theories exist for the SMT in the Sm chalcogenides and in many cases an appropriate choice of the parameters allows even to reproduce both the observed discontinuous and the continuous transitions.

From the present study, we are lead to the speculation that an energy gap of a .15 eV and a bulk modulus of 450 kbar (or perhaps larger) are the (experimentally accessible) parameters necessary for a discontinuous SMT

in the rare-earth monochalcogenides.

Figure 3 represents a schematic did- gram of f13 + f125d excitation energies along the TmS-Se-Te series and the correspon- ding level structure around the Fermi energy.

F i g . 3 Upper a r t : The d i f f e r e n c e i n energy o f the d i v a l e n t (4f

P)

and t r i v a l e n t (4£l25d) c o n f i g u r a t i o n of t h e Tm i o n s i n TmS-TrnSe-TmTe. Pinning of t h e 4f l e v e l a t t h e Fermi energy i s t y p i c a l f o r t h e mixed v a l e n t s t a t e and o c c u r s o v e r a wide range o f compo- s i t i o n .

Lower p a r t : Schematic energy diagram around t h e Fermi energy. The maximum a t EF i n TmSe should demons- t r a t e t h e £-d h y b r i d i z a t i o n (mlxedvalence s i t u a t i o n )

Only the concurrence of an effect typical for mixed valent rare-earth compounds is shown : the pinning of the 4f levels at the Fermi energy over a finite range of the valence in the intermediate valence phase. The Tm ions in TmS are known to be trivalent but separated from the divalent state by a couple of meV 18'. We therefore predict valence mixing in nearly the entire TmSe-TmS alloy system. This offers the very appealing possi- bility to vary the valence of Tm from close to 3+ at the sulphide rich end to 2.7 in TmSe

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JOURNAL DE PHYSIQUE

and to 2.5 in TmSe

.83Te.17'

3. TmSe-EuSe.- 3.1. Experiments.- We have applied the same variety of experimental techniques to the Tm Eu Se system as

I-x x

mentioned in section 1.1. and found a gradual s&~T for .4 < x < .5. On the Eu rich half of the system (x 20.5) the magnetic susceptibility and the lattice constant cle- arly indicate both Tm and Eu to be divalent.

The measured effective moments (from the Curie-Weiss behavior) correspond within percents to a mixture of 4f13 (Tm 2+ ) and 4f (Eu 2+ ) electron configurations.

The crucial information about the whole alloy system has been obtained from the resistivity and optical reflectivity measurements on Tm.5Eu.5Se at ambient and high pressures. Firstly a continuous SMT is induced by pressure and appears to be completed above 15 kbar. Secondly, the optical spectra show that only the Tm ions undergo a valence transition.(delocalization of 4f electrons into the conduction band), whereas the Eu ions remain unaffected. This is cle- arly demonstrated in figure 4 .

I I I -

NORMAL PRESSURE

I

PHOTON ENERW (eV)

Fig. 4 Optical reflectivity spectrum of Tm

!jEuO sSe

The vertical bars represent the expected rnu?6lple€s after excitation of an electron from the 4f13

(l'm2+) and the 4f 7 (Eu 2+) configuration, respecti- vely.

The optical reflectivity spectrum of the semiconducting phase of Tmo.5Euo.5Se consists

of excitations from both the Eu and the Tm 4f shells into the ligand field split conduction band. They are easy to identify because of their fingerprint-like multiplet structure, indicated by the vertical bars.

In the high pressure phase,which is obtained by mechanical polishing, the transitions from the 4f7 (Eu 2+ ) states are still present, but the ones involving the divalent Tm ions are missing. In addition, the reflectivity edge caused by the conduction band plasma has shifted to higher energies therefore being another evidence for the valence change of the Tm ions.

Thirdly, the identification of the 4f13 +- 4f125d and 4f7+ 4 f 6 5d optical excitations reveal for the Eu ions a higher stability against valence transitions by 148 eV compared to the Tm ions. Accordingly th$

Eu ions would become trivalent only at pressures higher than % 150 kbar. We note that, a similar phenomenon has been published in the meantime for Sml,xYbxS. / 9 / -

In the Tm rich samples the Tm ions are in a mixed valent state. But in contradt to pure TmSe, there are indications (effec*

tive magnetic moment, resistance) that a fraction of the Tm ions are either di- or trivalent. The presence of the much larger Eu ions can account for such inhomogeneities through local strain effects. Also, if the Tm ions are surrounded by Eu ions the divalent state would be preferred.

3.2 Discussion.- Here we will not comment on all experimental details (the magnetic properties /6/ have even not been mentioned in section 3.1)

,

but we will concentrate only on the difference between the Tm and Eu ions. The high stability of divalent Eu, which prevents it to participate in the SMT in these crystals and under the given conditions, is based in the high stability of the just halffilled 4f shell. This mani- fests itself already in the third ioniza- tion energy which is smaller by Q, 1.3 eV for Tm compared to Eu /lo/. The same arguments hold also for the comp?irison between the Sm and Eu chalcogenides /7/. It would therefore be possible to construct a unified energy level scheme for all semiconducting rare-earth monochalcogenides with the posi-

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ble parameter.

4. Conclusions.- Our main conclusions are the following points :

(1) The SMT in the compounds studied are caused by delocalization of 4f electrons into the 5d conduction band.

(2) In TmSel-xTeX the SMT occurs in a narrowrange of composition (.2 < x < .5).

This SMT is strikingly similar to the first order SMT in SmS and the coincidence of the electronic (gap s .15 eV) and the elas-

0

tic (lattice parameter 5.9-6.0 A) energy parameters are concluded to be most favorable for a discontinuous SMT in the rare- earth monochalcogenides.

(3) The observed pinning of the 4f levels at the Fermi energy is characteris- tic for mixed valent compounds. We predict Tm to remain mixed valent up to 60-80% of Se in TmSel-xSx.

(4) The divalent Eu ions in Tml,x EuxSe are by 1.8 eV more stable than the divalent Tm ions. This explains our obser- vations that only Tm participates in the SMT and Eu retains its highly stable halffilled 4f shell ( ~ u ~ + : 4 f ~ ) .

Acknowledgments.- We are very grateful to Dr. E. Kaldis and B. Fritzler for preparation and characterization of the materials and to Dr. A. Jayaraman for helpful discus- sions. We thank also the Swiss National Science Foundation for financial support.

Jayaraman, A., Singh, A.K., Chatterjee, A., and Usha Devi, S., Phys. Rev. B

2

(1974) 2513.

(~orth-~ollknd, i979) p. 575. -

Robinson, J.M., Phys; Reports

51

⁽¹⁹⁷⁹⁾

1 and many theoretical studies cited therein.

Kaldis, E., Fritzler, B., Jilek, E., and Wisard, A,, J. Physique Colloq.

40 (1979) C5

-

^366.

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Suryanarayanan, R., Giintherodt,G., Freeouf, J.L., and Holtzberg, F., Phys. Rev. B

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(1975) 4215.

Batlogg, B., 0tt.H.R. and Wachter, P., Phys. Rev. Lett.

42

(1979) 278.

Batlogg, B., Proc. Int. Conf. Magn.

(Munich, 1979) in Magn. J., Magn, Met.;

Ph. D. thesis Nr. 6335, ETH Ziirich (1979 unpublished.

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