Pressure induced semiconductor—metal transition in TmSe0.17Te0.83

SoLid State Communications, Vol. 38, pp. 75-77. Pergamon Press Ltd. 1981. Printed in Great Britain.

0038-1098/81/010075-03502.00/0

PRESSURE INDUCED SEMICONDUCTOR-METAL TRANSITION IN TmSeo.tvTeo.s3 H. Boppart and P. Wachter Laboratorium fiJr Festkorperphysik, ETH Ztirich, 8093 Ztirich, Switzerland and B. Batlogg and R. G. Maines Bell Laboratories, Murray Hill, N.J. 07974, U.S.A.

(Received 16 October 1980 by A.R. Miedema) In the TmSe t_xTex pseudobinary compounds a transition from semiconductor to metal (SMT) with intermediate Tm valence is observed in function of composition. Here, we investigate the pressure induced SMT for TmSeo.tTTe0.~3 in a continuous measurement of the electrical resistivity and the specific volume on the same single crystal up to 3.5 GPa (= 35 kbar). The SMT around 2 GPa is continuous and above this pressure the Tm ions are in an intermediate valent metallic state. 1. INTRODUCTION

investigated because of the coe~stence of spontaneous magnetization and strong valence mixing [7]. The chemical aspects of the TmSe t _xTex mixed crystals are discussed in [3]. In the semiconducting compositions (x > 0.5) a SMT can also be induced by external pressure. This SMT is driven by an increase of the ligand field splitting of the 5d-conduction band as the anion-cation distance decreases. Consequently, the 4fl3-5d ener~" gap is reduced and finally 4felectrons flow into the conduction band that means, the valence of the Tm ions changes. In this paper we study in detail the pressure induced SMT for one particular composition, TmSe0jTTe0.83. The two crucial quantities, characterizing such a SMT due to a valence change of the rare earth ion, have been measured :

THE TmSe-TmTe SYSTEM is unique among the rare earth monochalcogenides as it shows a compositionally induced semiconductor to metal transition (SMT) [1-3]. The electronic behavior of the mixed crystals TmSe,_xTex can be seen at one glance in Fig. 1. To start from semiconducting Tm2*Te with a gap of ~ 0.3 eV, one observes that the increasing incorporation of Se reduces the energy gap (symbols). This gap is determined by the separation in energ~ of the 4/"t3 and the 4f ~2 (5d6s) * confi~rations. The substitution of the Te ions by smaller Se ions leads to a reduction of the lattice zonstant and exerts a chemically induced pressure on the Tm ions. Under the increasing strength of the ligand field, the 5dt2~ and 5deu parts of the conduction band move apart in energy, keeping the 4f~3-5d center of gravity essentially the same. The broken line in Fig. 1 represents the closing of the gap as predicted from a comparison with the analogous divalent Eu and Sm compounds and we find agreement forx > 0 . 5 [4]. The reduction of the 4f-5dt2u separation leads to an increasing mixture of the 4fand 5d states. This confi~ration mixing manifests itself, e.g. in a reduction of the 4fmultiplet splitting due to 4f-5d exchange contributions as has been studied in detail for the 7£'0~ 7F~ excitations in semiconducting SInS [5, 6]. Also the radius of the rare earth ion is affected by such a configuration mixing. However, the Se rich crystals are metallic despite the expected f'mite value of ~0.1 eV for the gap in TmSe (Fig. 1). The compositionally induced accellerated closing of the gap has been discussed in [2]. The metallic compounds (x ~< 0.2) have already been

(1) the electrical resistivity determined by the number of itinerant electrons and (2) the specific volume which is sensitive to the 4fconfiguration.

2. EXPERIMENTAL Hydrostatic pressure up to 3.5 GPa has been generated in a piston-cylinder device using a teflon cell [8] with isoamyl alcohol as pressure transmitting medium. Resistivity measurements were carried out using a conventional four probe method. The volume-pressure dependence was determined by means of a strain gauge technique. Two strain gauges were glued onto opposite faces of both the crystals to be measured and a reference of known compressibility (e.g. Cu). The four strain gauges are then connected to a resistance bridge and 75

76

SEMICONDUCTOR-METAL TRANSITION IN TmSeojTTeo.s3

Vol. 38, No. 1

• d.

.3

> z; . 2 2-

:5 :i .I ~e:a~g

Se-::onSuc%'n

I

I

2)

.2

i

.4

TmSe

9

I

.6 TmSel-x Tex

.3

1 TrnTe

Fig. I. The energy gap for the 4]" ~s -~ 4[~25dexcitation in TmSe, _~Te~. The square indicates the result from optical measurements of [ 13] for TmTe. The dashed line shows the variation of the bottom of the 5dt~ conduction band as taken from measurements of the related Eu- and Sm compounds [4].

c

-

~mSe.:TTe.

83

3~K

tD

c

\

Tm

>.95 .9g .85

. . . . . . . . . . . . . . . . . . . .............

3im

.8~

>p.

72 m

....

8

t,nt 'o7 'u

"~-

5 U

r

4 --

~2 '

2 PRESSURE

3 [GPa]

4

5

Fig. 2. Pressure dependence of (a) the resistivity; (b) the specific volume;and (c) the compressibility of TmSeo.17Teo.83 at 300 K.

uncontrolled pressure effects on the strain gauge material (constantan) are thus eliminated to a lai# degree. Then the difference in the change of len~h of both crystals yields the volume-pressure dependence of the sample because the materials are cubic. From the temperature dependence of the resistivity (Arrhenius plot) of TmSe0.1~Teo.s3, the slope of the high temperature data yields an activation energy of 0 . 2 0.25 eV [2]. On the other hand, the activation energy can also be obtained from the pressure dependence of the resistivity p, assuming simple statistics for the number of carriers in the conduction band. Then p is given by p(AE, 7) = Po exp (AE/kT), where Oo "" 400/l~ cm denotes the experimental value o f p if the gap AE = 0 [Fig. 2(a)]. From the exponential drop of the experimental curve and with a resistivity at ambient pressure of 1.5 92 cm, one obtains 0.21 eV for AE, in agreement with the value determined from the Arrhenius plot. The exponential resistivity-pressure relation implies that the gap is closing linearly under pressure, thus, -~E(p) = A E o + (dAE/dp)p. The numerical results for dAE/dp are given in Table 1. Figure 2(a) also reveals around 0.9 GPa a kink in the resistivity-pressure curve with the resistivity still dropping exponentially for larger pressures, but only with a rate of 2/3 the one at low pressure. Above approximately 2 GPa, the resistivity remains at about 400/i~ cm. The results of the volume-pressure measurement and the derived compressibility-pressure curve are

Vol. 38, N o . I

SEMICONDUCTOR-METAL TRANSITION IN TmSeo.wTeo.83

Table 1. Some physical data o f semiconducting TmSeo.l~Teo.~ Lattice constant at ambient conditions (A) Resistivity at ambient pressure (f2 cm) high pressure (p > 2.5 GPa) 4[ 13 -* 5d gap for Tm from pressure data (meV) from temperature data (d&E/dp) for Tm 4Y13 ~ 5d 0 - 0 . 9 GPa (meV GPa-t) Compressibility K at ambient conditions (10 -t i pa-t )

6.28 1.5 4 x 10-4 210 200-250 140 2.4

shown in Fig. 2(b, c). The initial value of the compressibility K is 2.4 x I0 -tt Pa -~. Above 0.9 GPa the crystal gets markedly weaker and at about 1.9 GPa K reaches the high value of 9.5 x 10-It Pa-t. Above this pressure, the crystal stiffens and at 3.3 GPa the compressibility is only 1.8 x 10-11Pa -1. The value for K at ambient pressure fits well into the Andersen-Nafe plot for other semiconducting and isostructural RE monochalcogenides. AccordinNy, the compressibility and the specific volume Vo for the various compounds are connected by a power law K ~ Vo" [9].

the larger both effects are. Finally, we observe the h i ~ est compressibility just when the resistivity indicates that the gap is closed (~ 2 GPa). Now the question arises, whether the Tm ions are trivalent or intermediate valent for still larger pressures. The answer becomes obvious in Fig. 2(b). In addition to the measured curve, the expected behavior of TmSeo.,7Teo.s3 with the Tm ions being either purely di. or trivalent is also indicated. To calculate the dashed curves, we used the first order Birch-Murnaghan equation [11 ] and parameters (ao, K) deduced from comparison with other RE chalcogenides [9]. The main conclusions can be drawn if p(p) and V(p) are compared. This is now possible since both quantities have been measured continuously on the same crystal. (1) The valence of Tm is appreciably mixed already on the semiconducting side. (2) The valence varies continuously through the SMT and (3) stays intermediate in the metallic phase. Pressures of more than say 6 GPa can be estimated to be necessary to drive Tm completely trivalent. This experiment shows again that the compressibility of metallic intermediate valent RE compounds is anomalously high but decreases under pressure [9, 12].

Acknowledgement - The authors are very ~ateful to Dr E. Kaldis for growing and chemically characterizing the single crystal and to Dr A. Jayaraman for permitting the use of his high pressure equipment. REFERENCES

3. DISCUSSION A characteristic feature of the T m S e - T m T e alloy system is the substitution of the anions, keeping the cations constant with their potential to become intermediate valent. However, it must be considered that each Tm ion has a statistically different anion surrounding which will influence the position of the bottom of the conduction band due to the different ligand field splitting. Thus, in spatial coordinates, the bottom of the conduction band will appear somewhat warped. The definition of an energy gap can thus mean only an average value. If this gap is large compared with the "ripple" Gf the conduction band, these local effects are not very important. But they become recognizable with increasing pressure as the gap closes. This might well be the reason for the change in the slope of the log p -- p relation in Fig. 2(a) around 0.9 GPa. A similar behavior in the resistivity pressure dependence was observed in the SmS-SmSe system, in which the anion S is substituted by Se [ 10]. However, while the gap is closing under pressure, we get a larger and larger admixture of 5d wavefunctions to the 4/"13 state. This manifests itself in a strong reduction of the specific volume [see Fig. 2(b)] and a softening of the crystal [see Fig. 2(c)]. The smaller the gap already is,

77

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

B. Batlogg, E. Kaldis & P. Wachter, J. de Phys. 40-C5,370 (1979). B. Batlogg & P. Wachter, Proc. Int. Conf. on Magnetic Semiconductors, Montpellier (I 979); J. de Phys. 41-C5, 59 (1980). E. Kaldis, B. Fritzler, E. Jilek & A. Wisard, J. de Phys. 40-C5,366 (1979). B. Batlogg, E. Kaldis, A. Sch.legel & P. Wachter, Phys. Rev. B14, 5503 (1976). M.I. Nathan, F. Holtzberg, .I.E. Smith Jr., J.B. Torrance & J.C. Tsang, Phys. Rev. Lett. 34,467 (1975). S.M. Shapiro, R.J. Birgeneau & E. Bucher, Phys. Rev. Lett. 34,470 (1975). B. Batlogg, H.R. Ott & P. Wachter,Phys. Rev. Lett. 42,278 (1979). A. Jayaraman, A.R. Hutson, J.H. McFee, A.S. Coriell & R.G. Maines, Rev. Scient. Instrum. 38, 44 (1967). A. Jayaraman, A.K. Singh, A. Chatterjee & S. Usha Devi,Phys. Rev. B9, 2513 (1974). E. Bucher & R.G. Maines, Solid State Commun. 11, 1441 (1972). A.L. Ruoff & L.C. Ch.habildas, J. Appl. Phys. 47, 4182 (1976). B. Batlogg, H.R. Ott, E. Kaldis, W. Thoni & P. Wachter,Phys. Rev. B19,247 (1979). R. Suryanarayanan, G. Giintherodt, J.L. Freeouf & F. Holtzberg, Phys. Rev. B12,4215 (1975).