Moshe P. Pasternak a, Gregory Kh. Rozenberg a, Weiming M. Xu a & R. Dean Taylor b a School of Physics and Astronomy, Tel Aviv University, 69978, Tel

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1 This article was downloaded by: [Tel Aviv University] On: 02 March 2015, At: 04:35 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK High Pressure Research: An International Journal Publication details, including instructions for authors and subscription information: Effect of very high pressure on the magnetic state of transition metal compounds Moshe P. Pasternak a, Gregory Kh. Rozenberg a, Weiming M. Xu a & R. Dean Taylor b a School of Physics and Astronomy, Tel Aviv University, 69978, Tel Aviv, Israel b MST-10, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Published online: 08 Jun To cite this article: Moshe P. Pasternak, Gregory Kh. Rozenberg, Weiming M. Xu & R. Dean Taylor (2004) Effect of very high pressure on the magnetic state of transition metal compounds, High Pressure Research: An International Journal, 24:1, 33-43, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

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3 High Pressure Research Vol. 24, No. 1, March 2004, pp EFFECT OF VERY HIGH PRESSURE ON THE MAGNETIC STATE OF TRANSITION METAL COMPOUNDS MOSHE P. PASTERNAK a, *, GREGORY KH. ROZENBERG a, WEIMING M. XU a and R. DEAN TAYLOR b a School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel; b MST-10, Los Alamos National Laboratory, Los Alamos, NM 87545, USA (Received 16 May 2003; Revised 20 August 2003; In final form 25 August 2003) By simultaneously combining the methods of X-ray diffraction for structural phase transitions and EOS measurements, 57 Fe Mössbauer spectroscopy as a site-sensitive probe, and resistivity measurements for studying insulating-metal transitions, we are able to study the effect of extreme pressures and at varying temperature on magnetic and electronic properties of transition metal compounds. Studies are carried out with specially tailored diamond anvils and diamond anvil cells, reaching pressures beyond 100 GPa. From our studies, we can investigate the most basic phenomenon of the quantum effect of magnetism in insulating antiferromagnets, the Mott insulators, such as high to low spin crossovers, quenching of the magnetic moments orbital term, and the collapse of the Mott Hubbard state. Examples of these phenomena will be given in cases of ferrous and ferric oxides, ferrous-halides and the rare-earth iron perovskites. Keywords: High-pressure; Magnetism; Transition-metal compound; Phase-transition; Correlated-system; Hyperfine-interaction INTRODUCTION The majority of transition-metal (TM) compounds are systems with strongly correlated electrons, the so-called Mott insulators [1]. These compounds, characterized by large optical gaps and large on-site magnetic moments determined by Hund s rules, may undergo the following processes with pressure increase: (i) an insulator metal transition (IMT), and=or (ii) high-spin (HS) to low-spin (LS) crossover, and=or (iii) quenching of the TM-ion magnetic moment [2]. In case of (i) pressure will affect the U=t [3] ratio by decreasing the value of the Coulomb repulsion energy U resulting in the breakdown of d d correlation and consequently an IMT transition concurrent with the collapse of the magnetic moment will take place. In case of (ii) the rapid increase of the crystal field with pressure will result in a spin crossover [4], and in case of (iii) the volume dependence of the orbital term will result in its eventual collapse to pressure. * Corresponding author. moshepa@post.tau.ac.il ISSN print; ISSN online # 2004 Taylor & Francis Ltd DOI: =

4 34 M. P. PASTERNAK et al. In this paper we examine these three fundamental phenomena using typical cases of simple Fe 2þ and Fe 3þ oxides and halides, reaching pressures up to 170 GPa and temperatures down to 5 K by means of diamond anvil cells. In order to carry out the examinations we made use of three independent experimental methods, necessary for unambiguously determining those processes: Mössbauer spectroscopy (MS) with 57 Fe which probes, in the atomic scale, the magnetic and electronic properties at and around the TM ion, resistance measurements to detect and measure the IMT, and X-ray diffraction (XRD) with synchrotron radiation, to determine the consequences of structural and volume changes on these various phenomena. Studies were carried out in hematite (Fe 2 O 3 ) in which a first-order Mott transition occurs, accompanied by a drastic volume collapse, in the orthoferrites PrFeO 3 in which an interesting HS-to-LS transition (Fe 3þ, S ¼ 5=2, 6 A 1g! S ¼ 1=2, 2 T 2g ) takes place, and FeI 2, a layered ferrous compound in which the orbit-quenching is clearly detected and followed, at higher pressures, by a Mott transition. EXPERIMENTAL Synthesis of Materials Spectroscopical pure high-quality hematite was used for all measurements. For MS, the Fe 2 O 3 sample was enriched to 25% 57 Fe. Pressure studies were carried out in a miniature opposing-plates DAC [5] developed and built at Tel Aviv University. PrFeO 3 was synthesized by a solid solid reaction in air of stoichiometric amounts of spectroscopical pure La 2 O 3 and Fe 2 O 3 (enriched to 25% 57 Fe) at 1200 C. Pressure studies were carried out in a commercial [6] miniature piston-cylinder DAC. Samples of FeI 2 were prepared in milligram quantities by metal vapor reaction of the elements in an evacuated quartz tube. [7]. For Mössbauer studies, FeI 2 isotopically enriched to 20% 57 Fe was prepared. Due to the extremely hygroscopic nature of FeI 2 the crimson-red flakes were loaded in a glove box under exceptionally dry conditions. The composition and magnetic properties of the samples at ambient pressure were confirmed by conventional powder XRD and MS. Pressure studies were performed with the opposing-plates DAC. All samples were encapsulated in 100-mm cavities drilled in stainless-steel gaskets for XRD and resistivity measurements and in a Re gasket for MS studies. Re also served as a collimator for the 14.4-keV g-rays. For the XRD and MS studies Ar was used as a pressurizing medium [8]. Ruby fluorescence served as a manometer. Measurements X-ray diffraction measurements with synchrotron radiation were carried out at 300 K in the angle dispersive mode using the monochromatic bream of the ID30 station at the European Synchrotron Radiation Facilities (ESRF). The diffraction images were collected at l ¼ Å wavelength using image plates with typical exposure times of 5 min. Data were analyzed using the FIT2D program [9]. Resistance studies in the K range were performed with miniature Pt electrodes insulated from the metal gasket by a mixture of alumina and NaCl. The DAC was mounted in a special holder device inserted into a commercial liquid He Dewar. Using a PC-controlled DC motor-drive system, the DAC was gradually lowered and cooled by the cold He gas while simultaneously recording the temperature from a calibrated Si-diode thermometer and the resistance using the four-probe method.

5 MAGNETISM UNDER PRESSURE 35 Mössbauer studies were carried out with a 57 Co(Rh) 10-mCi point source in the K temperature range using a top-loading LHe cryostat [10]. The typical collection time of a single spectrum was 24 h. All spectra were analyzed using appropriate fitting programs from which the hyperfine interaction parameters and the corresponding relative abundances of spectral components were derived. RESULTS First-Order Collapse of the Mott Hubbard State in Fe 2 O 3 Fe 2 O 3 is an antiferromagnetic insulator below T N ¼ 956 K at ambient pressure. Mössbauer spectra of Fe 2 O 3 characteristic of various pressure ranges taken at 300 K are shown in Figure 1(a) (d). Up to 45 GPa, the dominant spectral component is that of the FIGURE 1 Typical Mössbauer spectra recorded at 29, 41, 51, 65 and 82 GPa. The solid line is a curve derived from a least-squares-fit program assuming one magnetic component ((a) and (b)), magnetic and non-magnetic components ((c) and (d)) and pure quadrupole-split component with a splitting QS ¼ 2mm=s (e). At 82 GPa a spectrum similar to that at 4 K was recorded at 300 K with a smaller value of QS. The lack of a magnetic component at P > 72 GPa to the lowest temperature signals the onset of a pure metallic state.

6 36 M. P. PASTERNAK et al. LP phase ((a) and (b)) arising from the 6 A 1g high-spin state and characterized by H hf ¼ 51 T, a typical value of the hyperfine field for ionic ferric oxide bonding. In the GPa range a non-magnetic quadrupole-split component emerges (c), designated as the HP component, coexisting with the 51 T magnetic-split LP-component. In the GPa range (d) the relative abundance of the LP component keeps on decreasing, and we note that the H hf at 300 K of the magnetic component is slightly reduced, by about 10%; this magnetic phase in the mixed region is designated as an intermediate phase (IP). For P > 72 GPa, the only spectral component observed in the K range is that of the non-magnetic HP phase (e) and is characterized by a quadruple-split spectrum. The lack of a magnetic ordering down to 4 K prompted us to conclude that this single HP component reflects not a paramagnetic, but rather a non-magnetic state [11]. This signals the breakdown of the d d electronic correlation, and that will be accompanied by metallization. The pressure variation of the resistance, R(P), at 300 K is shown in Figure 2. As can be seen, a precipitous decrease occurs at the onset of the phase transition in which R(P) is reduced by more than six orders of magnitude [12]. The resistivity value at 80 GPa and 300 K was estimated to be 1.5(7) 10 6 Om, typical of a metal. To further explore the HP electronic state, we carried out R(T ) measurements at various pressures (see inset FIGURE 2 Pressure dependence of the logarithm of the resistance of Fe 2 O 3, recorded at 300 K. The solid circles () are data points recorded during the first cycle of compression, solid diamonds (r) at the successive compression cycles, and open circles () at successive decompression cycles. The large change in the resistance between the first and the successive compression cycles is explained as due to compacting of the powder sample. Note the big decrease in R of the sample, by more than six orders of magnitude until its full metallization at P > 60 GPa (see text). The vertical dotted lines set the boundaries between the insulating LP, mixed intermediate and metallic (HP) phases The inset shows the temperature dependence of the log(r) at the various phases: at 17 GPa it behaves as an insulator, at 42 and 48 as an insulator metal mixed phase and at 59 GPa as a metal with dr=dt > 0.

7 MAGNETISM UNDER PRESSURE 37 in Fig. 2). As shown, the R(T, 17 GPa) curve for the LP regime is typical of an insulator, and the R(T, 59 GPa) curve, with its positive dr=dt, is characteristic of a metallic state. In the range GPa the sample behaves as a mixture of insulating and metallic states with the metallic contribution (HP abundance) increasing with pressure. This trend is in full agreement with the MS results (see Fig. 1) provided we assign non-magnetic and magnetic components to the metallic and insulating phases, respectively. First-Order Phase Transition Concurrent with Spin-Crossover The Case of PrFeO 3 Perovskite PrFeO 3 is an antiferromagnet insulator below T N ¼ 760 K at ambient pressure. It crystallizes in the Pbnm space group, a distorted orthorhombic structure derivative of cubic perovskite. The ambient temperature V(P) dependence is shown in Figure 3, and as can be seen, it undergoes a sluggish structural phase transition; the HP phase is first detected at 30 GPa, and the transition is fully completed by 50 GPa. Within this pressure range, both the HP and LP coexist. The first-order phase transition coincides with significant changes in both magnetic and electronic properties as demonstrated by the Mössbauer results. A typical example of the evolution in the Mössbauer spectra with pressure is shown in Figure 4(a). At ambient FIGURE 3 (a) EOS of PrFeO 3. The molar volume drops by 4% at 40 GPa with no structural symmetry change. This is attributed to the partial HS > LS transition taking place at P > 40 GPa (HP1) and succeeded by a full spin crossover at P > 60 GPa (HP2). The fit to the equation of states was done with the Birch Murnaghan equation. (b) IS(P) dependence near the phase transition for PrFeO 3 is shown in the lower part. The abrupt drop in the IS near 35 GPa, corresponding to an abrupt increase in the electron-density r S (0) at the nucleus, is consistent with the crystallography findings. The pressure dependence of the relative abundance of the HP phase as deduced from the MS absorption area of PrFeO 3 is shown in the upper part. The curve implies that there is no pressure hysteresis, consistent with the XRD findings. The solid lines are to guide the eyes.

8 38 M. P. PASTERNAK et al. FIGURE 4 (a) Mössbauer spectra of PrFeO 3 below 50 GPa at RT. The smooth lines are theoretical spectra deduced from least-squares fits to the experimental data. At 6 and 31 GPa, the spectra correspond to a pure LP phase consisting of a single magnetic component typical of a HS Fe 3þ. By 38 GPa, a pressure in the coexistence regime, one observes a partial transformation of the ferric HS component into a second magnetic component with a smaller H hf, whose origin is not known, and a non-magnetic phase corresponding to the HP phase and characterized by two quadrupole-split components. At 46 GPa very little is left of the LP-phase component, and at 50 GPa, within the pure HP phase, the only components observed are the two non-magnetic ones. (b) HP phase, low temperature Mössbauer spectra of PrFeO 3. At 50 GPa, just above the coexistence region, the two QS components (see (a)) split magnetically into HS and LS components. The first is characterized by a static magnetic hyperfine interaction H hf ¼ 45 T and the second by a spin spin magnetic relaxation spectrum. With increasing pressure, a gradual spin-crossover occurs, culminating in a pure LS state at P > 70 GPa. At GPa the dotted lines are a theoretical fit to the magnetic relaxation spectra using expressions (1) and (2) and the dashed lines are a theoretical fit to a static spin-hamiltonian using expression (1). Fitting of spectra at P > 70 GPa assumes the presence of a single LS component. temperature, starting at 35 GPa, one observes a sluggish transition from the LP antiferromagnetic state into a new HP non-magnetic phase. In the pure LP phase, extending from 0to35 GPa, the deduced H hf at 300 K is 51 T, typical of a Fe 3þ high-spin (3d 5, 6 A 1g ) configuration; H hf increases slightly with pressure due to an increase in the Nèel temperature (T N ). Also, above 35 GPa one notices a new magnetic component characterized by a somewhat smaller hyperfine field (H hf )of42 T, but similar IS value. Careful inspection of the HP non-magnetic phase reveals two quadrupole split (QS) components with equal abundance. Full conversion to a non-magnetic state at 300 K is attained by 50 GPa. By following the relative HP phase abundance, based on the relative areas associated with the MS components, one finds no significant pressure hysteresis associated with this structural transformation (see Fig. 3(b)). At T > 100 K the HP-phase at its inception (>35 GPa) is composed of two equi-abundant Fe 3þ non-magnetic components, each characterized by its own values of QS and IS. The nature of these two components becomes evident upon cooling below 100 K. At low temperatures [see Fig. 4(b)] the component with the smaller quadrupole splitting (QS 1.2 mm=s, IS mm=s) splits magnetically into a well-defined sextet with

9 MAGNETISM UNDER PRESSURE 39 H hf ¼ 47 T, a value within the range of high-spin (S ¼ 5=2, 6 A 1g ) ferric ions. The component with the larger QS (QS 2.40 mm=s, IS 0.07 mm=s) shows a broad, unresolved magneticsplit spectrum. Thus the ordered state is composed of two magnetic sublattices. Another consequence of the volume contraction is the considerable drop of the magnetic ordering temperature, from well above 700 K in the LP phase to 100 K [13]. The broad spectral component is the onset of a magnetic spin relaxation of the low-spin (LS) Fe 3þ state (S ¼ 1=2, 2 T 2g ). With further pressure increases a spin-crossover of the high-spin (HS) component is clearly observed; at 61 GPa, the dominating phase is the LS ferric iron. At 70 GPa the HS sublattice is completely transformed into the LS state. FeI 2, Layered Compound in Which Orbit-Quenching Takes Place Followed, at Higher Pressures by Mott Transition FeI 2 is an antiferromagnet with T N ¼ 9.3 K at ambient pressure. Typical Mössbauer spectra at 5K(T T N ) showing the different phases evolving with increasing pressure are presented in Figure 5. Up to 18 GPa, designated as the low pressure (LP1) phase, the quadrupole interaction is comparable in strength to the magnetic interaction, and the following spin- Hamiltonian governing the nuclear excited (I* ¼ 3=2) and ground (I ¼ 1=2) states of the 14.4-keV level in 57 Fe was utilized to fit the experimental results: H(I ) ¼ m I z I H hf (y) þ e2 q zz Q 4I (2I 1) [3I z 2 I (I þ 1)] H(I) ¼ m I z H hf, I where m and m* are the ground and excited state nuclear moments, respectively, y is the angle formed between H hf and the electric field gradient eq zz, and e 2 q zz Q is the quadrupole coupling [14]. A spectrum typical of the 0 15 GPa pressure range is shown in Figure 5(a). The solid line through the experimental points is a theoretical fit using Eq. (1). Characteristic values at 5K of H hf, e 2 q zz Q=2, IS, and y corresponding to the LP1 phase are 10 T, þ1.4 mm=s, þ0.9 mm=s, and 0, respectively. Since eq zz coincides with the c-axis this fit with y ¼ 0 says that the magnetic moment also lies along the c-axis, consistent with neutron diffraction measurements at ambient pressure. Near 18 GPa we detect a second Mössbauer component with a magnetically split sextet (Fig. 5(b)) coexisting with the LP1 spectrum. The second component is a case in which mh hf e 2 q zz Q=2, and the following spin-hamiltonian applies: (1) H(I ) ¼ m I z I H hf þ e2 q zz Q(3 cos 2 y 1) 8I (2I {3Iz 2 1) I (I 1)} H(I) ¼ m I z H hf : I (2) The solid line through the experimental points of the second component is the theoretical fit based on Eq. (2) and results in characteristic MS parameters (at 5K) of H hf ¼ 32 T, the quadrupole term e 2 q zz Q(3 cos 2 y 1) ¼ 0, and IS ¼þ0.7 mm=s. The hyperfine field of this phase is substantially larger, and the reason for this large change will be clarified later; however, note that the IS barely changes. The fact that the quadrupole term is zero

10 40 M. P. PASTERNAK et al. FIGURE 5 (a) Evolution of Mössbauer spectra of FeI 2 recorded at 5 K depicting the pressure-induced phases. (a) Spectra typical of the LP1 phase in which H hf is composed primarily of the spin and orbital terms with opposing signs (see text). In this pressure regime, up to 18 GPa, the quadrupole and magnetic couplings are comparable. At 18 GPa (b) a new magnetically split component (LP2) composed of a familiar pure magnetic-split sextet appears, signaling the quenching of the orbital term. At 20 GPa (c) a non-magnetic third component (HP) appears with substantially lower QS and IS values. At P > 32 GPa (g) HP becomes the only remaining component. LP1 and LP2 both disappear in the intermediate pressure regime ((d) (f)). The lines through the experimental points are theoretical curves obtained from the least-squares fitting programs. (b) LP Mössbauer spectra of the orbital-unquenched (LP1) and orbital-quenched (LP2) phases recorded at 18 GPa. At 7 K both components are magnetically ordered; at 140 K the quadrupole-split component corresponding to the paramagnetic LP1 (T N < 140 K) and the antiferromagnetic LP2 (T N > 140 K) components is shown, and at 298 K both LP2 and LP1 components are paramagnetic. The fact that the QS and IS at 298 K of the two components are identical suggests that the LP1! LP2 transition originates from identical chemical=structural Fe local environments.

11 MAGNETISM UNDER PRESSURE 41 at T < T N is due to realignment of the moments with respect to eq zz, namely, y 55 where (3cos 2 y 7 1) ¼ 0. A third, non-magnetic component, that coexists with the preceding components, is observed upon a minor pressure increase from 18 to 20 GPa (see Fig. 5(c)). At 23 GPa the LP1 component vanishes, and the abundance of the third component (a non-magnetic doublet) increases to become easily visible in the central region of the spectrum [Fig. 2(d)]. At 30 GPa (Fig. 5(f)) the non-magnetic component is dominant, and finally at 40 GPa and beyond, the Mössbauer spectra reflect a single, quadrupole-split component (Fig. 5(g)). Up to 17 GPa where the spectral features of the first (LP1) component in the MS spectra prevail, the following continuous changes are observed with rising pressure: T N increases 16-fold to 150 K, H hf increases from 7 to 13 T, QS increases monotonically from 1.30 mm=s to a peak of 1.70 mm=s at 12 GPa followed by a decrease to 1.5 mm=s at18 GPa, IS decreases almost linearly, from 1.0 to 0.83 mm=s. At 18 GPa the abundance of LP1 declines to 75%, and a new magnetic component appears with a considerably larger H hf ( ¼ 32 T) and T N ( ¼ 260 K) albeit with comparable QS and IS values; this component is governed by the spin-hamiltonian expressed in Eq. (2). Since the IS and QS are unchanged, implying the same local electron density and symmetry surrounding the Fe ion, we assign this component to the low-pressure phase and call it LP2, the intermediate phase. Mössbauer spectra recorded at 18 GPa at various temperatures are shown in Figure 4 and clearly demonstrate the enhanced values of H hf and T N associated with LP2. At 20 GPa a non-magnetic third component with 10% abundance appears while each of the magnetic components contributes 45%. This is the highest pressure at which LP1 is still present. The abundance of LP2 peaks at 23 GPa (67%) and then sluggishly decreases to vanish at 40 GPa. The IS and QS values of the LP phases decrease monotonically, yet those corresponding to the non-magnetic component phase barely change to 45 GPa, the highest pressure reached with the MS studies. The reason for the abrupt decrease in IS, i.e. increase in r s (0), of this HP component, as will be shown later, is due to the collapse of the correlated state following the first-order phase transition. The graphs of the EOS and c=a(p) are shown in Figure 6 (a) and (b). It is possible that the onset of the OQ is related to this unusual, intermediate c=a variation with P taking place in the GPa range. The pressure dependence of the FeI 2 resistance measured at 300 K is shown in the inset of Figure 7. In the 7 25 GPa range, the resistance R decreases by eight orders of magnitude and log R(P) drops off to become pressure independent above 20 GPa. The onset of metallization is clearly seen in Figure 7. At 23 GPa FeI 2 behaves like a small-band-gap semiconductor but, in fact, is a mixed metal=insulator phase. At 28 GPa and beyond, it clearly shows metallic behavior (positive dr=dt) for the major part of the temperature range. DISCUSSION The three examples given in this paper are typical of pressure-induced processes in so-called Mott insulators: antiferromagnetic insulators. The two examples of Fe 2 O 3 and PrFeO 3 involve ferric ions with S ¼ 5=2, L ¼ 0, which configuration plays a significant role in providing their large magnetic moments and T N. In the case of hematite, the collapse of correlation is so abrupt that the pressure expected path of a HS to LS transition is bypassed. Following

12 42 M. P. PASTERNAK et al. FIGURE 6 (a) EOS of FeI 2. The (), ( ), and (- - -) lines are the theoretical fit for the LP, LP1 þ LP2, and HP phases, respectively, using the Birch Murnaghan equation of state. The transition to the HP phase is accompanied by 5% volume reduction. (b) P-dependence of c=a the LP1, LP2 and HP phases. Solid lines are to guide the eyes. FIGURE 7 R(P,T )infei 2. The temperature dependence of the percolative-like resistance at 23 GPa reflects the mixed metallic HP and insulating LP phases. Pure metallic behavior is evident at 28 and 30 GPa. The inset shows the pressure dependence of the resistance at 300 K. Note the sharp continuous eight-order decrease in R from 5 to 30 GPa.

13 MAGNETISM UNDER PRESSURE 43 the 10% drop in volume, the ferric ion becomes non-magnetic concurrent with the IMT [15]. In PrFeO 3, however, the Mott transition is rather sluggish, preceded by a spin crossover in Fe 3þ within a relatively wide pressure range. Following the volume drop (with no crystallographic symmetry change [16]) and concurrent with a partial HS > LS transition, two magnetic sublattices are formed: one with LS (S ¼ 1=2) and the second with HS (S ¼ 5=2) Fe 3þ sublattices. This strongly reduces the T N, from >900 K down to 60 K and considerably weakens the exchange interaction of the S ¼ 1=2 moments as evident by the spin spin fluctuations revealed by the MS. With further pressure increase, at 70 GPa, the spin crossover is complete and at around 140 GPa metallization takes place signaling the correlation breakdown [17]. The example of ferrous iodide (d 6, S ¼ 2) is unique in the sense that at ambient pressure the orbital term contribution to the total magnetic moment is significant, about 50% of its value and with opposing sign. Thus the total moment is rather reduced resulting in a low T N. Due to the 1=hr 3 i dependence of m O, its absolute value decreases with pressure increase. As a result of pressure the crystal field also increases. Both phenomena result in quenching of the orbital term [18]. Acknowledgement This work was partially supported by grants from the Israel Science Foundation. References [1] N. F. Mott, Metal Insulator Transitions (Taylor & Francis, London=Philadelphia, 1990) and references therein. [2] An exceptional case of a temperature-dependent Mott transition has been observed in RNiO 3 (see J. B. Torrance, et al., Phys. Rev., B45, 8209 (1992)). [3] The Hubbard Hamiltonian H ¼ P t ij a þ is a js þ U P n " i n# i, is the simplest description of a Mott-insulator (J. Hubbard, Proc. Royal Soc., A277, 237 (1964)). It is characterized by a kinetic energy term t ij denoting the hopping of an electron from site i to its nearest neighbor site j and by the extra energy cost U of putting two electrons (n " i n# i ) on the same site. The bandwidth W is proportional to t. In the case of charge transfer insulator, U, the energy gap separating the empty and filled d-bands, is replaced by D, a gap between the empty d-band and filled-ligand bands (J. G. Zaanen, G. A. Sawatzky and J. W. Allen, Phys. Rev. Lett., 55, 418 (1985)). [4] It is of note that the CF splitting increases proportional to the inverse of the fifth power of the TM-ligand-bond length (D. M. Sherman, In: S. K. Saxena, (Ed.), Advances in Physical Geochemistry, Vol. 7 (Springer-Verlag, Berlin, 1988), p. 113). [5] E. Sterer, M. P. Pasternak and R. D. Taylor, Rev. Sci. Instrum., 61, 1117 (1990). [6] Purchased from D Anvils Ltd. ( [7] Pure elemental I 2 was prepared by thermal dissociation at 300 C of spectroscopical pure PdI 2 and condensation of I 2 vapor in the same evacuated tube. [8] Due to the proximity of electrodes (10 mm), there was no need for a pressure medium. [9] A. P. Hammersley, S. O. Svensson, M. Hanfland, A. N. Fitch and D. Hausermann, High Pressure Res., 14, 235 (1996). [10] G. R. Hearne, M. P. Pasternak and R. D. Taylor, Rev. Sci. Inst., 65, 3787 (1994). [11] The ground state 2 T 2g of the low-spin 5d-electrons configuration in Fe 3 þ is (t2g ")3 (t2g #)2 with a magnetic moment approximately 1=5 of that of the high-spin 6 A 1g ground state. [12] Note that following the first compression cycle (filled circle curve), R(P) upon decompression reaches a value that is lower at 5 GPa. Successive compression and decompression cycles are reproducible. This phenomenon can be explained as due to compacting of the sample during the first compression cycle. [13] The T N for each pressure was deduced from H hf (T, P) measurements and extrapolation to H hf ¼ 0. [14] The quadrupole splitting QS is half the value of the quadrupole coupling e 2 q zz Q. [15] M. P. Pasternak et al. Phys. Rev. Lett., 82, 4663 (1999). [16] G. Kh. Rozenberg, M. P. Pasternak, R. D. Taylor (to be published). [17] W. M. Xu, O. Naaman, G. Kh. Rozenberg, M. P. Pasternak and R. D. Taylor, Phys. Rev. B, 64, (2001). [18] M. P. Pasternak, W. M. Xu, G. Kh. Rozenberg, R. D. Taylor, G. R. Hearne and E. Sterer, Phys. Rev. B, 65, (2001).