Temperature- and Pressure-Induced Spin Crossover in Co1+xCr2

Article pubs.acs.org/IC

Temperature- and Pressure-Induced Spin Crossover in Co1+xCr2−xSe4 (x = 0.24): A Diffraction Study V. Svitlyk,*,† D. Chernyshov,‡ Y. Mozharivskyj,§ F. Yuan,§ O. Zaharko,∥ and M. Mezouar† †

ID27 High Pressure Beamline, ESRF, CS40220, F-38043 Grenoble, France Swiss−Norwegian Beamlines, ESRF, CS40220, F-38043 Grenoble, France § Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada ∥ Laboratory for Neutron Scattering, Paul Scherrer Institute, 5232 Villigen, Switzerland ‡

S Supporting Information *

ABSTRACT: The Co2+ ions of the Co1+xCr2−xSe4 phase (Co1.24Cr1.76Se4 composition with x = 0.24, C2/m space group, Cr3S4-type structure) undergo a high- to low-spin-state transition around 230 K, as concluded from the temperature-dependent single-crystal synchrotron radiation diffraction experiments and the previously reported physical property studies. The change of the spin state is not instantaneous and goes through a wide spin-crossover (SCO) region of 75 K. A similar Co2+ high- to low-spin-state transition is suggested at a pressure of 14.5 GPa, as is evident from the pressure-dependent single-crystal synchrotron radiation diffraction experiments. The corresponding SCO region is equal to 5 GPa, and the structural behavior is different from the one observed during the temperature-dependent transition. Coupling between the spin-conversion process in Co1+xCr2−xSe4 and the concomitant changes in the physical properties opens a way for a controlled tuning of the observed physical response through compositional and structural modifications.

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INTRODUCTION A phenomenon of spin-crossover (SCO) discovered in 19311 rapidly gained vast attention because of the associated complex changes in the structural and physical properties. SCO corresponds to equilibrium between different spin states low (LS), high (HS), or intermediate (IS)which, in turn, are determined by the magnitudes of the crystal-field splitting and electron-pairing energies. This implies that the spin state of an atom is determined by its surroundings, i.e., types of ligands and their symmetry and coordination number, and by its effective oxidation state. Equilibrium between these two contributions can be achieved and controlled by external stimuli like temperature (T), pressure (P), optical activation, etc.2−7 The spin states influence various physical properties, including magnetic, transport, optical, and others. Therefore, the materials that possess a SCO can be used as highly sensitive micro- and nanoresponsive devices, including switches, data recording, and storage media.8−10 The SCO is a common property for the first-row transition metals with d4−d7 electronic configurations.11 The solid-state CoCr2Se4 system (for the sake of simplicity, we use a stoichiometric CoCr2Se4 composition in the text when referring to the Co1+xCr2−xSe4 system, including the Co1.24Cr1.76Se4 phase studied in this work), with the Co2+ ions in a d7 configuration, exhibits a diverse set of physical and structural properties. At room temperature, CoCr 2 Se 4 adopts a monoclinic Cr3S4-type structure and undergoes a first-order order−disorder transition at 1023 K to a filled CdI2-type structure.12 Upon cooling, the monoclinic CoCr2Se4 phase © XXXX American Chemical Society

undergoes an antiferromagnetic transition at TN = 230 K, which was confirmed by our previous neutron diffraction experiments.13 Together with the magnetic properties, the electrical properties also change at 230 K. For CoCr2Se4 single crystals, the electrical resistivity within the (100) plane increases until TN and then decreases with T. The resistivity in the orthogonal direction increases with T and changes the slope at TN.13 Simultaneous changes in the magnetic and electrical properties in CoCr2Se4 at 230 K can originate from the structural changes that are occurring. As was concluded from the neutron powder diffraction experiments, the symmetry of the monoclinic CoCr2Se4 phase remains unchanged down to 5 K.13 However, this does not exclude a possible isostructural spin-state transition around 230 K. This spin-state transition would be coupled with structural changes, primarily related to the environment of the Co2+ ions. In order to detect and track subtle structural changes at 230 K, we have performed detailed single-crystal T-dependent X-ray diffraction experiments using synchrotron radiation in the 80−450 K temperature range. A SCO has indeed been observed as a function of T, and a series of P-dependent single-crystal synchrotron radiation diffraction experiments up to a P value of 40 GPa have been performed. A possible P-dependent spin-state transition was observed around P = 14.5 GPa. In this manuscript, we discuss the associated anomalies in the structural parameters of CoCr2Se4 and explain Received: October 27, 2015

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DOI: 10.1021/acs.inorgchem.5b02475 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. T-dependent behavior of the a (top left), b (top right), and c (bottom left) unit cell parameters and the unit cell volume V (bottom right) of CoCr2Se4 from the single-crystal data. possible for the data collected above 5 GPa as a result of P-induced strains in the monoclinic lattice of CoCr2Se4. The experimental unit cell parameters as a function of P were extracted using the CrysAlis software,14 and a corresponding unit cell volumes and equation of state (EOS) were calculated.

their origin in terms of the crystallographic peculiarities of the Co atoms’ environment.

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EXPERIMENT

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T-dependent single-crystal synchrotron radiation diffraction experiments on CoCr2Se4 were performed at the BM01A station of the Swiss−Norwegian beamlines at the ESRF (Grenoble, France) using a KUMA6 diffractometer with a Titan CCD detector. The wavelength was set to λ = 0.6983 Å, and the scans were performed at T values of 80, 110, 140, 170, 200, 210, 220, 230, 240, 250, 260, 330, 410, and 450 K to study the T-dependent structural evolution in detail. The temperature was controlled with an Oxford Cryostream 700C N2 blower. The studied single crystal is the same as that in our previous work,12 marked as “slowly cooled”. Details of the synthesis can be found therein.12 Raw data were processed with the CrysAlis software.14 Empirical absorption corrections were performed with the SADABS package.15 The structure solution and refinement were done with the SHELXS and SHELXL97 programs, respectively.16 P-dependent single-crystal diffraction data on CoCr2Se4 were collected using a PerkinElmer flat-panel detector at the ID27 HighPressure beamline at the ESRF up to a P value of 41 GPa with a typical step of 1.1 GPa. The synchrotron radiation was generated by a pair of 23-mm-period undulators and set to a wavelength of 0.3738 Å. The CoCr2Se4 single crystal was contained in a rhenium gasket with a hole of 150 μm diameter on the 300 μm diamond in a membrane diamond anvil cell. Helium was used as a pressure-transmitting medium (PTM) because it preserves high hydrostaticity at least up to P = 50 GPa.17 The pressure was calculated from shifts of the fluorescence signals from the added ruby spheres. A reliable structural refinement was not

RESULTS AND DISCUSSION T-dependent single-crystal data revealed clear anomalies in the structural parameters of CoCr2Se4 around 230 K (Figure 1; all data in the manuscript are presented with the corresponding experimental errors). The a and c unit cell parameters undergo an additional decrease (with respect to a monotonic decrease as a result of thermal contraction) around 230 K upon cooling, while the b parameter features a jump at this temperature. The behavior of the unit cell volume is influenced primarily by the a and c parameters, and as a result, it exhibits a similar additional decrease around 230 K (Figure 1, bottom right). The corresponding effective volume compression for the CoCr2Se4 structure at 80 K is close to 1 Å3 (Figure 1, bottom right). The observed T-dependent behavior of CoCr2Se4 is consistent with a transition of the Co2+ ions from the highto low-spin state configurations around 230 K. In CoCr2Se4, the Co atoms are coordinated by the Se6 octahedra stacked in the layers parallel to the bc plane (Figure 2). Typically Co atoms occupy half of the available Se6 octahedra in an alternating fashion. However, as we have shown in our previous studies, B


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Inorganic Chemistry

Table 1. Crystal Data and Structure Refinement for CoCr2Se4 from Single-Crystal Diffraction Data at 330 K empirical formula refined formula space group lattice parameters a, Å b, Å c, Å β, deg volume, Å3 2θ range for data collection, deg index ranges reflns collected indep reflns completeness to max 2θ, % data/param GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak/hole (e/A3)

Figure 2. Structure and coordination polyhedra in the Cr3S4-type CoCr2Se4 phase.

because of the presence of a high-T (HT) order−disorder transition in CoCr2Se4, Co atoms can populate, to a certain degree, adjacent vacant Se6 octahedra.12 The magnitude of this occupancy can be controlled by cooling conditions. In the HT hexagonal-filled CdI2-type CoCr2Se4 structure, the Co atoms are distributed uniformly within the hosting layers.12 If the disorder−order transition from the HT trigonal CoCr2Se4 structure to the low-T monoclinic Cr3S4-type structure at 1023 K is induced with a high cooling rate, i.e., quenching, the Co atoms do not order completely within the hosting layers because of kinetic constraints. Up to 23% of the residual Co atoms in the adjacent octahedra was reported.12 Alternatively, if the disorder−order transition temperature is passed with a moderate cooling rate, i.e., 15 K/h as in the previously studied case, the Co atoms order completely.12 The single crystal studied in this work was obtained by a controlled and slow cooling and did not show a disorder within an error range as concluded from our previous laboratory X-ray diffraction analysis.12 However, with a help of a bright synchrotron radiation, we have reliably found that the vacant Se6 octahedra host 3.3(3) atom % of Co atoms (here we denote them as Co′). In addition, we have observed a limited mixing of Co with Cr within the Cr-containing layers, resulting in a 12(3) atom % concentration of Co atoms at the Cr sites. Similar mixing has been found in our previous work.12 The effective refined formula is therefore equal to Co1.24(6)Cr1.76(6)Se4. Refinement results and crystallographic data from the singlecrystal diffraction experiment at 330 K as an example are presented in Tables 1−3. From the experimental single-crystal data, anomalies in the environment of both the Co and Cr/Co atoms are clearly visible, mainly manifesting as a crossover between the two Tdependent Co−Se2 bonding configurations (Figure 3). The observed structural behavior is indicative of a spin-state transition from the Co2+ HS t2g5eg2 configuration to the Co2+ LS t2g6eg1 configuration. This transition results in a decrease of the effective Co2+ radius, which, in turn, translates in a decrease of the Co−Se2 distances. As can be concluded from the observed structural behavior (Figures 1 and 3), the spin-state transition at 230 K is not instantaneous and occurs in a wide SCO region of about 75 K, implying that both spin states coexist in this range. Finally, the proposed t2g6eg1 electronic configuration at lower T values is in accordance with our previous neutron diffraction experiments, where it was shown that the effective magnetic moment on the Co2+ ions at the base temperature of 5 K is equal to 1 μB.13 The effective difference in the radii of the Co atoms in the HS and LS CoCr2Se4 modifications is ca. 0.008 Å (Figure 3, left). The tabulated difference between the Co HS and LS ionic

CoCr2Se4 Co1.24(6)Cr1.76(6)Se4 C2/m = 13.1019(5) = 3.62780(10) = 6.2980(3) 117.838(5) 264.707(18) 6.92−69.36 −20 ≤ h ≤ 20, −5 ≤ k ≤ 5, −7 ≤ l ≤ 10 1686 637 [Rint = 0.0122] 94.7 637/25 1.179 R1 = 0.0322, wR2 = 0.0860 R1 = 0.0340, wR2 = 0.0869 1.642/−1.517

Table 2. Atomic and Equivalent Isotropic Temperature (U) Parameters for CoCr2Se4 from Single-Crystal Diffraction Data atom

site

occupancy

Co Co′ Cr/Co

2a 2d 4i

Se1 Se2

4i 4i

96.7(3) 3.3(3) 88(3)/ 12(3) 1 1

x/a

y/b

0 0 0.2571(1)

0 1 /2 0

0.3672(1) 0.1183(1)

0 0

z/c

U (Å2)

0 /2 0.2784(2)

0.017(1) 0.017(1) 0.015(1)

0.0287(1) 0.4515(1)

0.013(1) 0.014(1)

1

Table 3. Anisotropic Displacement (Uij, Å2) Parameters for CoCr2Se4 from Single-Crystal Diffraction Data atom

U11

U22

U33

U23

U13

U12

Co Co′ Cr/Co Se1 Se2

0.018(1) 0.018(1) 0.015(1) 0.014(1) 0.014(1)

0.019(1) 0.019(1) 0.015(1) 0.012(1) 0.013(1)

0.015(1) 0.015(1) 0.015(1) 0.013(1) 0.012(1)

0 0 0 0 0

0.007(1) 0.007(1) 0.007(1) 0.007(1) 0.005(1)

0 0 0 0 0

radii for Co2+ in a 6-fold coordination is 0.095 Å.18 This kind of discrepancy between the observed and theoretical values is expected because the Co2+ ions in CoCr2Se4 are part of a rigid extended 3D atomic network (see the corresponding discussion below). Thus, contrary to the spin-state transitions in flexible molecular complexes,3,6,19 only limited structural deformations can be induced. Similarly, the relatively big magnitude of the change in the Cr/Co−Se2 distances may seem counterintuitive, taking into account a small content of Co atoms (12%) at this site. However, we believe that, along with a purely chemical contribution (a decrease in the effective size of Co atoms), this change in the distances also reflects a geometrical response of the rigid 3D atomic network of CoCr2Se4 (Figure 3), which follows a contraction around the Co sites. From a crystallographic point of view, the order parameter responsible for the transition from Co (HS) Cr 2 Se 4 to Co(LS)Cr2Se4 is composed of simultaneous displacements of Cr and Se atoms. This isostructural process does not affect the average symmetry, and therefore the transition corresponds to a C


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Inorganic Chemistry

Figure 3. Behavior of the Co−Se2 (left) and Cr/Co−Se2 (right) distances of CoCr2Se4 as a function of T.

single Γ1+ (notation from ref 20) irreducible representation.21 The Landau expansion of free energy for the Co(HS)Cr2Se4 to Co(LS)Cr2Se4 transition contains invariants of odd order, which indicates a first-order transition or a crossover regime.22 The gradual character of the observed changes favors the latter scenario, i.e., the SCO. Because the spin states of Co atoms are in the origin of the transition, no coexistence of two different macroscopic crystallographic phases around the transition temperature is observed. The presence of a Co(HS)/Co(LS) statistical mixture merely results in a decrease of the effective Co radii with a corresponding structural response (Figure 3). However, a hypothetical ordering of the Co(HS) and Co(LS) atoms in the SCO region, resulting in a splitting of the Co 2a site, could not be excluded. This could induce a decrease in the crystallographic symmetry with a C2/m−P1− ̅ C2/m sequence of reentrant phase transitions with a corresponding coexistence of different macroscopic structures. Our experiment allows one to rule out this scenario in agreement with chemical considerations because all of the Co atoms have identical octahedral coordination. A counterintuitive anomaly in the T-dependent behavior of the b parameter is a result of the corresponding T-induced bonding optimizations by the Se atoms following a decrease in the Co radius. In the Se6 octahedra of Co atoms (Figure 4), the in-plane Se1 atoms, which are crystallographically equivalent, lie in the planes quasi-parallel to the ab plane. They form a square close-packed lattice, with the Se−Se bonding distances equal to

3.628(1) and 3.662(2) Å (distances are extracted from the single-crystal data collected at 330 K; Tables 1−3). Because the crystal radius of Se2− ions is equal to 1.84 Å,18 the in-plane Se1 atoms are not able to efficiently enhance bonding with the Co2+ ions after its HS−LS transition. As a result, the in-plane Se1 atoms move away from the central Co2+ ions predominantly along the b direction, thus permitting the off-plane Se2 atoms to increase their overlap with the Co2+ atoms in the LS configuration (Figure 4). Apparently, the observed anisotropic response in the ab plane (Figure 1, top) is a structural compromise that results in a maximal effective Co−Se overlap and is allowed by the monoclinic symmetry of CoCr2Se4. Indeed, the Co−Se1 distances do not show an additional decrease during the HS−LS transition (Figure 5, T-dependent

Figure 5. Behavior of the Co−Se1 distances of CoCr2Se4 with temperature.

data; to be compared with the T-dependent behavior of the Co−Se2 distances, Figure 3, left). Together with the in-plane Se1−Se1 distances (Figure 6), they mirror the behavior of the b structural parameters within the error range. The Co high- to low-spin-state transition described above is accompanied by an additional decrease of the unit cell volume and, therefore, can be expected to be induced by the application of external pressure. Indeed, anomalies in a CoCr2Se4 unit cell volume and its b structural parameter are present around the P value of 14.5 GPa (Figure 7), as seen from the P-dependent

Figure 4. Se6 octahedral coordination of the Co2+ ions in CoCr2Se4 and its evolution with temperature. D


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Inorganic Chemistry

Figure 6. Behavior of the adjacent (left) and opposite (right) Se1−Se1 distances of the Se6 octahedra of CoCr2Se4 with temperature.

Figure 7. Behavior of the unit cell volume V (top left), b (top right), a (bottom left), and c (bottom right) unit cell parameters as a function of P for CoCr2Se4.

of 14.5 GPa (Figure 7, top right). The a and c parameters do not show any P-dependent anomalies within the error range (Figure 7, bottom). The behavior of the unit cell volume is influenced predominantly by the b parameter and features a small additional decrease around the transition pressure (Figure 7, top left). The compressibilities of the low-P (denoted as HS)

single-crystal synchrotron radiation diffraction experiments. However, the P-dependent behavior of the structural parameters of CoCr2Se4 (Figure 7) is different from the Tdependent case (Figure 1). Contrary to the T-dependent behavior (Figure 1, top right), the b parameter shows an additional decrease around a P value E


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Inorganic Chemistry

P, even being accompanied by a different structural relaxation mechanism, could still be a reason for changes in the resistivity and magnetic susceptibility. Because the distribution of Co ions over the three independent crystallographic positions in CoCr2Se4 can be controlled by thermal treatment, the associated SCO properties are expected to be altered as well, thus opening new routes for fine-tuning a desired response.

and high-P (denoted as LS; see the discussion below) phases of CoCr2Se4 were estimated using a third-order Birch−Murnaghan EOS [eq 1; V0 is the volume at zero pressure, B0 is the bulk modulus, and B0′ is the first pressure derivative of the bulk modulus] (Figure 7, top left, red solid lines). The fitted parameters are presented in Table 4. As expected, the high-P modification of CoCr2Se4 is less compressible than the low-P one.

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3B0 ⎡⎛ V0 ⎞7/3 ⎛ V0 ⎞5/3⎤ ⎢⎜ ⎟ − ⎜ ⎟ ⎥ P(V ) = ⎝ V ⎠ ⎥⎦ 2 ⎢⎣⎝ V ⎠

S Supporting Information *

⎧ ⎤⎫ ⎡⎛ V ⎞2/3 ⎪ ⎪ 3 ⎨1 + (B0 ′ − 4)⎢⎜ 0 ⎟ − 1⎥⎬ ⎪ ⎥⎦⎪ ⎢⎣⎝ V ⎠ 4 ⎩ ⎭

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02475. X-ray crystallographic data in CIF format (CIF) All of the numerical values plotted in this paper (Tables S1−S3), which may serve as constraints for further ab initio calculations for CoCr2Se4 (PDF)

(1)

Table 4. Experimental Coefficients of the Third-Order Birch−Murnaghan EOS for the HS and LS CoCr2Se4 Phases

a

ASSOCIATED CONTENT

phase

V0, A3

B0, GPa

B0′, GPa

CoCr2Se4 HS CoCr2Se4 LS

259.3(5) 252(2)

54(3) 61(3)

4.3(8) 4.3a

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Fixed value.

Notes

The authors declare no competing financial interest.

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Although the behavior of the P-dependent structural parameters of CoCr2Se4 is different from the one with temperature, the observed anomaly at 14.5 GPa is compatible with a possible P-induced Co(HS)−Co(LS) transition. Namely, the transition at 14.5 GPa features an additional decrease in the unit cell volume induced by an additional decrease in the b structural parameter (Figure 7, top). This behavior implies a different scenario for the presumed Pdependent Co(HS)−Co(LS) transition compared to the Tdependent case (Figure 4). Certainly, the difference can stem from a different nature of the external stimulus, e.g., T versus P, which implies distinct initiation mechanisms for the Co(HS)− Co(LS) transformation. Finally, the presence/absence and nature of a PTM can also influence the observed properties of crystalline materials (ref 23 and references cited therein). Measurement of the P-dependent physical properties analogous to the T-dependent ones reported by us earlier13 and Pdependent spectroscopy studies would help to clarify the nature of the transition at 14.5 GPa.

ACKNOWLEDGMENTS We thank V. Dmitriev (Swiss−Norwegian Beamlines, ESRF) for useful discussions and comments. REFERENCES

(1) Cambi, L.; Szegö, L. Ber. Dtsch. Chem. Ges. B 1931, 64, 2591− 2598. (2) Takano, M.; Nasu, S.; Abe, T.; Yamamoto, K.; Endo, S.; Takeda, Y.; Goodenough, J. B. Phys. Rev. Lett. 1991, 67, 3267−3270. (3) Scepaniak, J. J.; Harris, T. D.; Vogel, C. S.; Sutter, J.; Meyer, K.; Smith, J. M. J. Am. Chem. Soc. 2011, 133, 3824−3827. (4) Gü tlich, P.; Goodwin, H. Spin CrossoverAn Overall Perspective. In Spin Crossover in Transition Metal Compounds I; Gütlich, P., Goodwin, H. A., Eds.; Springer: Berlin, 2004; Vol. 233, pp 1−47. (5) Gudyma, I. V.; Maksymov, A. I. Phys. B 2010, 405, 2534−2537. (6) Chernyshov, D.; Vangdal, B.; Tornroos, K. W.; Burgi, H.-B. New J. Chem. 2009, 33, 1277−1282. (7) Hostettler, M.; Törnroos, K. W.; Chernyshov, D.; Vangdal, B.; Bürgi, H.-B. Angew. Chem., Int. Ed. 2004, 43, 4589−4594. (8) Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762−1765. (9) Létard, J.-F.; Guionneau, P.; Goux-Capes, L. Towards Spin Crossover Applications. In Spin Crossover in Transition Metal Compounds III; Springer: Berlin, 2004; Vol. 235, pp 221−249. (10) Kahn, O.; Martinez, C. J. Science 1998, 279, 44−48. (11) Cotton, F. A.; Wilkinson, G.; Gaus, P. L. Basic Inorganic Chemistry, 3rd ed.; Wiley: New York, 1995. (12) Svitlyk, V.; Mozharivskyj, Y. Inorg. Chem. 2009, 48, 5296−5302. (13) Svitlyk, V.; Kolodiazhnyi, T.; Cranswick, M. D. L.; Luke, G.; Mozharivskyj, Y. Mater. Res. Express 2015, 2, 105004. (14) Rigaku Oxford Diffraction. CrysAlisPro Software System, version 1.171.38.41; Rigaku Corp.: Oxford, U.K.. 2015, (15) Sheldrick, G. M. SADABS, version 2.06; University of Goettingen: Goettingen, Germany, 2002, (16) Sheldrick, G. M. SHELXL97 and SHELXS97; University of Goettingen: Goettingen, Germany, 1997. (17) Takemura, K. J. Appl. Phys. 2001, 89, 662−668. (18) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, 32, 751.

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CONCLUSIONS With a help of T- and P-dependent synchrotron radiation diffraction experiments, we have shown that the spin-state transition in the Co2+ sublattice of CoCr2Se4 can be induced by T and, presumably, by P. From a structural point of view, the HS−LS transition is manifested as an anisotropic deformation of the Se6 octahedra around the Co2+ ions which leads to anisotropic changes in the lattice parameters. The anisotropy originates from bonding optimizations within the Se 6 octahedra, which are clearly seen from the T-dependent single-crystal diffraction data. A possible P-induced Co(HS)− Co(LS) transformation is governed by a different mechanism, which results in a different structural response at the corresponding SCO region. The microscopic nature of the Pinduced changes is still to be uncovered. The Co2+ HS to LS transition observed near 230 K is accompanied by changes in the physical properties, including magnetic and transport ones. Changes of the spin states at high F


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Temperature- and Pressure-Induced Spin Crossover in Co1+xCr2

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