Ionic monochalcogenides under pressure

Physica 139 & 140B (1986) 277-283 North-Holland, Amsterdam

IONIC MONOCHALCOGENIDES UNDER PRESSURE* K. SYASSEN Physikalisches Institut II1, Universitdt Diisseldorf, D-4000 Diisseldorf 1, Fed. Rep. Germany Among the high pressure phenomena in alkaline earth and divalent rare earth chalcogenides we discuss (i) systematic trends in structural transitions, (ii) the changes in optical properties associated with the structural transitions and the approach to a valence-conductionband overlap in alkaline compounds, and (iii) recent investigationsof the 4f-5d mixing in Eu and Yb compounds.

I. Introduction The heavy alkaline earth chalcogenides (AEX, A E = C a , Sr, Ba, X = O, S, Se, Te) and the divalent rare earth monochalcogenides ( R E X ) with cations from the middle (Sm, Eu) and the end (Tm, Yb) of the lanthanide row form a group of closely related ionic compounds. Under normal conditions they crystallize in the rocksalt structure. The optical gaps between the p-like valence and sd-like conduction band states in the A E X range from about 2.5 to 6 eV. The valenceconduction band structure of the divalent R E X is similar to that of the related A E X except for the correlated 4f n÷l level falling into the band gap. Three partly related aspects of the high pressure physics of ionic monochalcogenides will be discussed here: in section 2 we are concerned with systematic trends in high pressure structural transitions from octahedral to eightfold coordination [1, 2], which have become evident from a number of recent X-ray investigations [1-8]. Band structure theory predicts that a lowering of the d-like conduction band states relative to the p-like valence band states is a common phenomenon in A E X under pressure, which ultimately results in an insulator-metal (IM) transition due to a valence-conduction band overlap [9-11]. Experimental studies of these changes in elec* Work performed in collaboration with H. Winzen, H.G. Zimmer, (University of Diisseldorf), G. Schmiester (FU Berlin), K. Fischer (KFA J/ilich), and J. Evers (University of Miinchen).

tronic structure are presented in section 3 with main emphasis on the pressure dependence of the optical reflectivity of BaTe [12]. In divalent (semiconducting) R E X the lowering of the d-like conduction band states may result in a 4f-5d mixing or intermediate valence (IV) ground state [13, 14]. IV behavior in SmX and TmX is observed at moderate pressures (P < 50 kbar) or even at zero pressure (TmSe), and the physical properties of these materials at fractional 4f occupation remain a topic of continued interest [15]. On the other hand, the f - d gaps in EuX and YbX (except YbO) are larger than 1 eV and correspondingly larger pressures are needed in order to possibly induce an energetic overlap of f and d states. In section 4 we present recent experimental results [16-19] which probe the pressureinduced f - d mixing in the two R E compounds EuS and YbS.

2. Structural changes in monochalcogenides A high-pressure phase transformation from the NaCl-type (B1) to the eightfold coordinated CsCl-type (B2) structure (or a closely related structure [3]) is a common p h e n o m e n o n in the A E X [1-8]. The relative volume changes at the B 1 - B 2 transitions range from about 10.5 to 14%. The transition pressures are, however, considerably higher compared to B 1 - B 2 transitions in isoelectronic alkali halides. The pressures Pv needed to stabilize an eightfold coordinated structure in the A E X exhibit a clear dependence

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K. Syassen / Ionic monochalcogenides under pressure

278

on cation (rc) and anion (rA) radius [1,2], as shown in fig. 1 by plotting PT as a function of ionic radius ratio. Decreasing anion or cation radius requires an increasing stabilization pressure for eightfold coordination. The transition pressures in AEX approximately scale with the empirical coordinate [1, 2] R M' = (rc 2 + rA2) '/2 " Fig. 2 shows a plot of PT versus RM~. The solid line roughly extrapolates to the predicted B1-B2 transition (8 Mbar [20]) in MgO. The relative arrangement of compounds in fig. 1 bears a strong resemblance to dual-coordinate classification schemes for the crystal structures of AB compounds under normal conditions, as may be readily verified by the inspection of various structural maps [21]. The ionic radius ratio plays the role of a charge transfer (or ionicity) coordinate, whereas the inverse of In(PT) correlates with semiempirical metallization parameters. Therefore, R M is considered to be at least in part a measure of the degree of metallization present in the compound. I

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Transition pressures to eightfold coordination in A E X as a function of the empirical coordinate RM~. The line connects experimental (BaTe) and estimated or theoretical (BaO, SrO) band overlap pressures.

The empirical relations presented in figs. 1 and 2 allow for a prediction of as yet unknown phase transitions in other AEX. The B1-B2 transitions known for the divalent (or close to divalent) REX seem to follow the same systematic trends, as is evidenced by the inclusion in fig. 1 of the corresponding transition pressures for the EuX [22]. Since the cation radii of Ca and divalent Yb are very similar, we would expect a sixfold coordinated structure of YbX to be stable up to at least 300 kbar.

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As a representative example of the effect of pressure on optical properties and electronic structure of the AEX we consider BaTe. Reflection spectra of single crystalline starting material at different pressures are shown in fig. 3. Throughout this paper absolute reflectivities (denoted Rd) refer to a sample-diamond interface. The reflectivity in the B1 phase exhibits two sharp bands in the near-UV, which are separated by roughly 0.6eV. With increasing pressure both

279

K. Syassen I Ionic monochalcogenides under pressure

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reflection bands shift almost in parallel to lower energy, which clearly identifies the final state of the corresponding interband excitations as 5dlike. These transitions are interpreted as excitonic transitions at the Brillouin zone boundary (Xpoint), as indicated in the schematic band structure in fig. 4a. The 0.6 eV separation results from spin-orbit splitting of valence band states. This assignment is supported by the observation of similar pressure effects and an almost identical splitting in SrTe [17] and is also consistent with optical investigations of other A E X under normal conditions [23]. Absorption m e a s u r e m e n t s of B a T e (B1) indicate the indirect ( F - X ) gap to be about 0.5 eV below the first direct exciton, but at 300 K the gap energy is difficult to locate due to an overlapping weak defect adsorption band. At the B 1 - B 2 transition the lowest energy structure in reflection drops discontinuously from

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K. Syassen / Ionic monochalcogenides under pressure

280

about 2.6 eV (B1) to 1.6 eV (B2) and then shifts further to lower energy with increasing the pressure. This indicates a considerably smaller direct gap in the B2 phase, which is now expected at the zone center (see fig. 4b). Fig. 5 shows the energetic positions of reflectivity edges as a function of relative density, where the experimental PV relation [8] is used for pressure to density conversion. The position of the reflectivity edges is taken from maxima in the first derivative of the reflectivity with respect to photon energy. Also included in fig. 5 is the energy of the dominant absorption edge in the B2 phase. A linear extrapolation of the data for the B2 phase seems appropriate in order to estimate the relative density at which overlap of the direct gap is expected. The corresponding pressure is about 270 kbar [8]. On the other hand, electrical conductivity measurements indicate an IM transition at about 200 kbar [8]. We offer two possible explanations for this discrepancy: the IM transition may arise from a closure of the indirect ~/-M gap (see fig. 4b) which, according to calculations in ref. [11], is the lowest gap in the B2 phase of BaTe. However,

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this prediction may critically depend on the choice of the crystal potential. Unfortunately, our optical absorption data do not allow for an unambiguous characterization of the nature of the band gap just above the B1-B2 transition due to the interference of defect absorption below the direct gap. The observation of defect absorption leads to a second possible explanation in terms of a precursor effect, namely the delocalization of electrons captured at defect sites. A similar mechanism has been proposed to explain the IM transition in EuO at low temperatures [24]. We thus expect that stoichiometry and the related defect concentration plays an important role in determining the electrical transport properties of BaTe and other AEX near band overlap. At pressures above 300 kbar BaTe shows an increase of the near-IR reflectivity, as would be expected beyond the band overlap transition. In this situation, direct interband transitions extend down to very low energies, and a simple interpretation of the reflectivity edge in terms of a freeelectron-like reflection edge would not be appropriate. A more detailed discussion will be presented elsewhere [12]. According to energy band calculations all the AEX are expected to undergo a similar band overlap transition [9-11]. From high pressure optical studies of SrTe [17] we extrapolate a transition pressure of roughly 550kbar. The dashed line in fig. 2 connects approximate band overlap pressures in BaTe and SrTe with the predicted metallization pressures in BaO and SrO [9]. The sequence of band overlap pressures seems to positively correlate with the sequence of B1-B2 transitions.

4. 4f-5d mixing in rare earth compounds

Characteristic properties indicating a pressureinduced f-d mixing in SmX and TmX are a bulk modulus softening or volume anomaly [13], a change in the profile of the X-ray absorption edges (for a discussion of the method see J. R6hler in ref. [15]), and a strong enhancement of optical excitations below 2eV photon energies, resulting in a drastic increase of the near-IR

K. Syassen I Ionic monochalcogenides under pressure

reflectivity followed by a plasma reflection edge at higher energies. We use these criteria to investigate the 4f instability in EuS are YbS. The experimental PV relation of YbS (B1) is shown in fig. 6 together with the hypothetical curves for the d i v a l e n t (4f ]4) and trivalent (4f135d) electronic configurations of the Yb ion [18]. The softening of the P V relation near 100 kbar clearly indicates a change in the electronic structure. However, in contrast to the SmX, an additional pressure of 150 kbar is not sufficient to transform YbS into a nearly trivalent material. Near 250 kbar the volume remains approximately half way in between the divalent and trivalent reference curves. The course of the valence change in YbS as determined from recent investigations of the L m absorption edge profile [19], is shown in fig. 7. In agreement with the interpretation of the P V relation, these data indicate the onset of f-electron delocalization near 100 kbar and a mean valence approaching a value of 2.5 near 300 kbar. An important difference between YbS and the SmX is observed in the optical response of the IV phases. Reflection spectra of YbS and SmS are compared in fig. 8. Whereas IV SmX (10kbar) and nearly trivalent SmX (250kbar) exhibit a high Drude-like reflectivity throughout the nearIR spectral range, we find a pronounced edge in the near-IR reflectivity of IV YbS, which corres-

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ponds to a strong optical absorption band of approximately 0.3 eV width [17, 18]. The position of this band is indicated by arrows in fig. 8. This unusual feature is characteristic of the Yb ion, because it is observed in all IV YbX at high pressure. It has been interpreted [18] as an excitation of predominantly d to f character where the small width of the excitation band indicates a partial localization of the initial d states. Possible explanations for the formation of a narrow dcomponent in the IV ground state assume a strong Coulomb interaction between 4f hole and 5d electron or a relatively large f-d hybridization effect [18].

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282

K. Syassen I Ionic monochalcogenides under pressure

The behavior of EuS under pressure is an example of the competition between a structural B 1 - B 2 transition (at 200 kbar [22]) and f - d mixing. Fig. 9 shows reflection spectra of EuS at different pressures. The reflection band centered near 2.3 eV at normal pressure arises from 4f 7 to 4f6(7Fj)5d excitations, the corresponding absorption edge being at about 1.7eV. With increasing pressure this f - d excitation band shifts to lower energy at a rate of about 11 meV/kbar, which extrapolates to an energetic overlap of 4f and 5d states near 160 kbar. Accordingly, above about 160kbar EuS shows an increase of the near-IR reflectivity which is typical for the optical response of other R E X near the onset of f - d mixing [18]. Thus, the optical reflectivity provides clear evidence for f - d mixing in the B1 phase of EuS. By analogy to the results for YbS we estimate a mean valence of - 2 . 2 just before the B 1 - B 2 transition. At the structural B 1 - B 2 transition the near-IR reflectivity decreases discontinuously, and, as in

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In this report we have briefly summarized the experimental results of recent high pressure investigations of ionic monochalcogenides with emphasis on three topics of current interest, namely, systematic trends in structural transitions, the electronic structure of A E X during the approach to a valence-conduction band overlap, and f - d mixing in REX. A more detailed account of the various aspects is given elsewhere [1, 2, 12, 16-19].

References

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PHASE

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the B1 phase, one again observes an isolated reflection band. This observation is interpreted as a r e v e r s a l of the valence transition and the reappearance of the low energy f to d excitation peak. Apparently, the difference in crystal field splitting acting on the d-like conduction band states in octahedral and cubal symmetry is such that the energy of the bottom of the d-band region relative to a more localized 4f state increases at the B 1 - B 2 transition. Upon further raising the pressure in the B2 phase, the d band edge is lowered again and at about 350 kbar the increasing near-IR reflectivity provides evidence for a degree of f - d mixing in the B2 phase, which is similar to that in the B1 phase just below 200kbar. The results for EuS suggest that a decrease of the mean valence may be a general phenomenon during B 1 - B 2 transitions in IV REX.

I 1 PHOTON

1

I 2 ENERGY

3 (eV)

Fig. 9. Reflection spectra of EuS at different pressures.

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K. Syassen / Ionic monochalcogenides under pressure

[8] T.A. Grzybowsky and A.L. Ruoff, Phys. Rev. Lett. 53 (1984) 489. [9] M.S.T. Bukowinski and J. Hauser, Geophys. Research Lett. 7 (1980) 689. [10] M.S.T. Bukowinski, J. Geophys. Research 87 (1982) 303. [11] A.E. Carlsson and J.W. Wilkins, Phys. Rev. B29 (1984) 5836. [12] K. Syassen, H. Winzen, K. Fischer and J. Evers, to be published. [13] A. Jayaraman, in: Handbook on the Physics and Chemistry of the Rare Earths, Vol. 2, eds., K.A. Gshneidner and L. Eyring (North-Holland, Amsterdam, 1979) p. 575, and references therein. [14] J.M. Lawrence, P.S. Riseborough and R.D. Parks, Rep. Prog. Phys. 44 (1981) 1. [15] B. Roden, E. Miiller-Hartmann-and D. Wohlleben, eds., Proc. Int. Conf. on Valence Fluctuations, K61n 1984, J. Magn. Magn. Mat. 47&48 (1985).

283

[16] H.G. Zimmer, Diplomarbeit, Univ. Diisseldorf (1983); H.G. Zimmer, H. Winzen and K. Syassen, to be published. [17] K. Syassen, J. de Phys. (Colloque) 45 (1984) C8-123. [18] K. Syassen, H. Winzen, H.G. Zimmer, H. Tups and J.M. Leger, Phys. Rev. B, in print, and references therein. [19] G. Schmiester and K. Syassen, unpublished work. [20] K.J. Chang and M.L. Cohen, Phys. Rev. B30 (1984) 4774. [21] J.R. Chelikowsky and J.C. Phillips, Phys. Rev. B17 (1978) 2453; M. O'Keeffe and A. Navrotsky, eds., Structure and Bonding in Crystals (Academic, New York, 1981). [22] A. Jayaraman, A.K. Singh, A. Chatterjee and S.U. Devi, Phys. Rev. B9 (1974) 2513. [23] Y. Kanako, K. Morimoto and T. Koda, J. Phys. Soc. Japan 52 (1983) 4385. [24] O, Wachter, see ref. [13], p. 507.