Linear Trimer Formation with Antiferromagnetic Ordering in 1T-CrSe2

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Linear Trimer Formation with Antiferromagnetic Ordering in 1T‑CrSe2 Originating from Peierls-like Instabilities and Interlayer Se−Se Interactions Shintaro Kobayashi,*,†,∇ Naoyuki Katayama,† Taishun Manjo,† Hiroaki Ueda,‡ Chishiro Michioka,‡ Jun Sugiyama,§,□ Yasmine Sassa,∥,○ Ola Kenji Forslund,⊥ Martin Månsson,⊥ Kazuyoshi Yoshimura,‡,# and Hiroshi Sawa† Downloaded via UNIV OF MELBOURNE on April 11, 2019 at 02:15:55 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Department of Applied Physics, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan § Toyota Central Research & Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan ∥ Department of Physics & Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden ⊥ Department of Applied Physics, KTH Royal Institute of Technology, Electrum 229, SE-16440 Stockholm Kista, Sweden # Research Center for Low Temperature and Material Sciences, Kyoto University, Kyoto 606-8501, Japan ○ Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden ‡

S Supporting Information *

ABSTRACT: Anomalous successive structural transitions in layered 1TCrSe2 with an unusual Cr4+ valency were investigated by synchrotron Xray diffraction. 1T-CrSe2 exhibits dramatic structural changes in in-plane Cr−Cr and interlayer Se−Se distances, which originate from two interactions: (i) in-plane Cr−Cr interactions derived from Peierls-like trimerization instabilities on the orbitally assisted one-dimensional chains and (ii) interlayer Se−Se interactions through p−p hybridization. As a result, 1T-CrSe2 has the unexpected ground state of an antiferromagnetic metal with multiple Cr linear trimers with three-center−two-electron σ bonds. Interestingly, partial substitution of Se for S atoms in 1T-CrSe2 changes the ground state from an antiferromagnetic metal to an insulator without long-range magnetic ordering, which is due to the weakening of interlayer interactions between anions. The unique lowtemperature structures and electronic states of this system are determined by the competition and cooperation of in-plane Cr− Cr and interlayer Se−Se interactions.

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INTRODUCTION

causes partial chalcogen to metal electron transfer; it has been proposed that these compounds exhibit molecular orbital formations with three-center−two-electron (3c-2e) σ bonds which form in systems with noninteger numbers of d4/3 electrons.14−16 The cooperation and/or competition between in-plane metal−metal interactions and weak interlayer anion− anion interactions provide new possibilities for producing novel structural phase transitions in layered 1T-MX 2 compounds that can result in interesting physical properties. 1T-CrSe2 is expected to show strong in-plane Cr−Cr and interlayer Se−Se interactions for the following reasons. CrSe2 has an anomalous Cr4+ valence assuming the formal Se valence of −2. CrSe2 belongs to a family of d2-type compounds, which often form orbital molecules, as proposed by some experimental and theoretical reports.16−23 In addition, the Cr4+ state has electronic instabilities owing to the lower energy

Materials with layered structures have received considerable attention in fundamental and applied research owing to their wide variety of structures and physical properties.1−4 Among these materials, 1T-MX2 compounds (M = transition metals, X = chalcogen atoms), with stacking via van der Waals interactions, have been intensely studied due to their rich low-dimensional physical properties, such as charge density wave (CDW) transitions and related superconductivities.5−9 In contrast, MTe2 compounds exhibit both strong in-plane interactions between metal atoms and interlayer interactions between Te atoms owing to the large ionic radius and small electronegativity of Te2− ions. The interlayer interactions often induce novel structural and physical properties different from those of simple two-dimensional systems. For example, a temperature-induced polymerization transition with the formation of Te−Te bonds has been proposed in IrTe2, which exhibits superconductivity by Pt substitution.10−13 The p−p hybridization between anions in VTe2, NbTe2, and TaTe2 © XXXX American Chemical Society

Received: January 21, 2019

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DOI: 10.1021/acs.inorgchem.9b00186 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

of SPring-8 (λ = 0.41324 Å).31 Rietveld analyses of powder XRD patterns were conducted using the RIETAN-FP program.32 The obtained structural parameters are provided in the Supporting Information. Synchrotron XRD data for the single crystals were collected using a charge-coupled device detector at the BL02B1 beamline of SPring-8 (λ = 0.3560 and 0.3904 Å). To determine the lattice constants of the single crystals of S-substituted samples, synchrotron powder XRD measurements of crushed single crystals were conducted at the BL5S2 beamline of the Aichi Synchrotron Radiation Center (λ = 0.6522 Å). XAFS Experiment. XAFS spectra, including X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) at the Cr K-edge, were acquired in transmission mode at the BL11S2 beamline of the Aichi Synchrotron Radiation Center. Energy calibration was carried out using the spectra of Cu metal foil. The Fourier transforms were performed on k2-weighted EXAFS over the k = 3−13.5 Å−1 wavenumber range. The obtained data were analyzed using the Athena and Artemis software packages, and theoretical phase shifts and scattering amplitude functions were calculated using the FEFF6 program.33 Characterization of Physical Properties. dc magnetization measurements were conducted using a superconducting quantum interference device magnetometer (Quantum Design MPMS-XL system). The electrical resistivities of single crystals were measured perpendicular to the c axis using a conventional four-probe method. The absolute values of electrical resistivity were determined from the average values of about 10 measurements using different single crystals. The Seebeck coefficient of single crystals was measured perpendicular to the c axis using a conventional two-probe method. Positive muon spin rotation and relaxation (μ+SR) spectra were acquired in zero magnetic field (ZF) on the M20 surface-muon beamline using the LAMPF spectrometer at TRIUMF, Canada’s particle accelerator centre. Powder samples (∼100 mg) were placed in 1 × 1 cm square envelopes made of 0.05 mm thick aluminized Mylar tape to minimize the signal from the envelope. The experimental techniques are described in more detail elsewhere.34,35

level of Cr d bands. As a result, electron transfer from Se to Cr bands occurs when interlayer Se−Se interactions are strengthened via p−p hybridization, as described in detail below. Thus, CrSe2 is expected to exhibit exotic structural and physical properties originating from the interplay between inplane and interlayer interactions. CrSe2 has shown successive structural transitions at transition temperatures of Tt1 ≈ 190 K and Tt2 ≈ 170 K,24,25 although the low-temperature structures have not yet been clarified. Contrary to the expectation that successive structural transitions often originate from simple CDW transitions in MX2 systems, CrSe2 exhibits anomalous temperature dependence of the magnetic susceptibility and antiferromagnetic ordering below Tt2,25,26 which cannot be explained by simply considering CDW or molecular orbital formations. Interestingly, in addition to temperature variations, low concentrations of elemental substitution in CrSe2 induce various electronic states in CrSe2.27−29 For example, slight Ti or V substitution of the Cr atoms in CrSe2 suppresses structural transitions and induces other types of magnetic states.28 S substitution of only approximately 4% at the Se site suppresses the successive structural transitions observed in CrSe2 and induces another type of structural transition indicative of a different ground state.27 These drastic changes by slight elemental substitution indicate that the system has several competing interactions. In this paper, we clarify the low-temperature crystal structures of 1T-CrSe2 by synchrotron X-ray diffraction (XRD) and Cr K-edge X-ray absorption fine structure (XAFS) analysis. We found that CrSe2 has the characteristic ground state with coexisting antiferromagnetic ordering and orbital molecules. The origin of the characteristic ground state of CrSe2 was clarified by comparison with its S-substituted systems, the details of which are discussed here.

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RESULTS AND DISCUSSION Crystal Structure of CrSe2 at 300 K. The crystal structure of CrSe2 is compared with that of the LiCrSe2 reference sample. Figure 1 shows the powder XRD profiles of CrSe2 and LiCrSe2 at 300 K. Both XRD profiles were well reproduced

EXPERIMENTAL SECTION

Synthesis. Powder samples of 1T-Cr(Se1−xSx)2 were prepared using a soft-chemical method followed by a solid-state reaction. The soft-chemical method is similar to that reported previously by us.25,27 Starting materials Cr2(Se1−4x/3S4x/3)3 were synthesized by heating stoichiometric mixtures of the pure elements. Precursor samples of K0.9Cr(Se1−xSx)2 were obtained by the reactions 0.9KN3 + 1/2Cr2(Se1 − 4x /3S4x /3)3 + 1/2Se → K 0.9Cr(Se1 − xSx )2 + 2.7/2N2 Appropriate amounts of starting material were mixed, pelletized, and sealed in an evacuated silica tube. The quartz tube was then gradually heated to 400 °C for 24 h to prevent the rapid exothermic decomposition of KN3, kept at this temperature for 24 h, and then heated at 700 °C for 24 h. Single crystals of K0.9Cr(Se1−xSx)2 were obtained using a KBr flux method with the same starting materials. Polycrystalline samples and single crystals of 1T-Cr(Se1−xSx)2 were synthesized by the deintercalation of K from K0.9Cr(Se1−xSx)2 with iodine in acetonitrile at 50 °C for 120 h. As references, polycrystalline LiCrSe2 samples were synthesized by the direct reaction of the elements, as previously reported.30 For synchrotron-radiation singlecrystal XRD, single crystals of CrSe2 and Cr(Se0.90S0.10)2 with typical dimensions of 50 × 50 × 20 μm were prepared by the deintercalation of Li from the obtained polycrystalline samples of LiCrSe2 and LiCr(Se0.90S0.10)2, respectively, with iodine in acetonitrile at 100 °C for 24 h. Here, we denote the compositions of the samples by their nominal compositions. Synchrotron XRD Measurements. Synchrotron radiation XRD profiles of powder samples were collected using a high-resolution onedimensional solid-state detector (MYTHEN) at the BL02B2 beamline

Figure 1. Synchrotron powder XRD patterns of polycrystalline samples of (a) LiCrSe2 and (b) CrSe2 acquired at 300 K (black data points), together with the corresponding Rietveld refinement (red curves). The green ticks highlight the positions of the allowed Bragg reflections, while the blue data points highlight the differences between the acquired and calculated patterns. The insets in panels (a) and (b) show the crystal structures of LiCrSe2 and CrSe2, respectively. B


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Inorganic Chemistry using previously reported structural models with the trigonal P3̅m1 space group.24,25,30 The Rietveld refinement gave good fits to the XRD patterns at 300 K. The lattice parameters obtained at 300 K were a = 3.39307(4) Å and c = 5.91301(7) Å for CrSe2 and a = 3.65328(9) Å and c = 6.28535(8) Å for LiCrSe2. Note that CrSe2 shows a large isotropic displacement parameter, Biso, for Cr at 300 K (Biso(Cr) = 1.25(2) Å2), while that for LiCrSe2 is small (Biso(Cr) = 0.43(2) Å2). Even though both CrSe2 and LiCrSe2 are composed of CrSe2 layers established through an edge-sharing network of CrSe6 octahedra, the valence of the Cr ions is thought to be different; given the formal valence, CrSe2 and LiCrSe2 have tetravalent and trivalent Cr ions, respectively. In fact, LiCrSe2 has an effective magnetic moment corresponding to Cr3+ (spin 3/2)30 and is therefore a good reference sample for investigating the valence state of Cr ions in CrSe2. The Cr− Se distance in CrSe2 (2.4699 Å) is shorter than that in LiCrSe2 (2.5401 Å), which indicates that the valence of Cr in CrSe2 is higher than that in LiCrSe2. The difference in their valence states was also confirmed using Cr K-edge XANES measurements. Figure 2a shows normalized Cr K-edge XANES spectra χμ(E) of CrSe2 and LiCrSe2. The edge energy of CrSe2 was higher than that of LiCrSe2, which also indicates that the valence of Cr in the CrSe2 sample is higher than that in LiCrSe2. It should be noted that the edge energies of CrSe2

and LiCrSe2 were lower than those for tetravalent and trivalent chromium oxides,36 probably due to the strong covalency of Cr 3d and Se 4p states, as previously noted.37 The prominent preedge peak at ∼5990 eV was weakly observed for CrSe2. Preedge peaks are commonly observed in some chromium oxides and complexes36 and are due to the dipole-forbidden 1s to 3d transition for regular octahedral coordination. The large Biso(Cr) value at 300 K for CrSe2 suggests some atomic displacement of Cr from the average structure determined by powder XRD. To clarify the local structure around the Cr atom, Cr K-edge EXAFS data were acquired for CrSe2 and LiCrSe2. Figure 2b shows the Fourier transform magnitudes of the EXAFS oscillations of CrSe2 and LiCrSe2 at 300 K. The main peak around 2−3 Å is due to the six nearestneighbor Se atoms. The estimated Cr−Se distances were 2.470 Å for CrSe2 and 2.534 Å for LiCrSe2. These values are very close to those determined by structural analysis. The nextnearest neighbors are neighboring Cr atoms, and their contributions should appear at around 3−4 Å for CrSe2 and LiCrSe2. However, the peak corresponding to the Cr−Cr distance was clearly observed for LiCrSe2, but not for CrSe2. The very weak peak for CrSe2 is possibly related to a dynamic and/or static distribution in the nearest-neighbor Cr−Cr distance at 300 K, which indicates a large fluctuation in the Cr−Cr distance. Although structural disorder, such as lattice defects, distributes the Cr−Cr distance, this interpretation is not consistent with the EXAFS spectra acquired at 50 K, which showed clear peaks corresponding to the Cr−Cr distance, as shown in Figure 2c. In addition, weak diffuse scattering lines were observed in single-crystal XRD patterns above the structural transition temperatures (Figure 2e), which likely originates from dynamic and/or static distributions in the nearest-neighbor Cr−Cr distance in the high-temperature region. The diffuse scattering lines disappeared and superlattice reflections appeared in the low-temperature region (Figure 2f). It is difficult to conduct low-temperature structural analysis using such single-crystal XRD data because of broad peaks and the occurrence of twinning below the structural transition temperatures. Hence, we conducted structural analysis using powder XRD data, as discussed in the following section. Low-Temperature Structures of CrSe2. We previously reported that CrSe2 shows successive structural transitions at Tt1 ≈ 190 K and Tt2 ≈ 170 K and forms superlattice structures below Tt1.25 However, the structures in the intermediatetemperature phase (IT phase, Tt2 < T < Tt1) and the lowtemperature phase (LT phase, T < Tt2) were not clarified. To investigate the specific structures formed during the phase transitions, we conducted synchrotron XRD measurements using CrSe2 powder samples at various temperatures. As shown in Figure 3, superlattice reflections were clearly observed in the IT and LT phases. Although the integrated intensities of superlattice reflections at 50 and 185 K are approximately 2−3 orders of magnitude lower than those of the fundamental reflections, structural analysis is possible using the highresolution and wide-dynamic-range synchrotron XRD data collected at the BL02B2 beamline.31 Clear peak splitting was observed in the powder XRD profiles of CrSe2 in the IT phase. As a typical example, the inset of Figure 3a shows the temperature dependence of the 110 peak, which is indexed using the trigonal cell (at, bt, ct) in the high-temperature phase (HT phase, T > Tt1). Such peak splitting indicates that the IT phase is a monoclinic or triclinic

Figure 2. (a) Energy dependences of the normalized Cr K-edge XAFS oscillations χμ(E) of CrSe2, Cr(Se0.95S0.05)2, and LiCrSe2. (b−d) Fourier transform (FT) magnitudes of the EXAFS oscillations (weighted by k2) of (b) CrSe2 and LiCrSe2 at 300 K, (c) CrSe2 at 300, 175, and 50 K, and (d) Cr(Se0.95S0.05)2 at 300 and 50 K. The inset of (d) shows expanded EXAFS oscillations of Cr(Se0.95S0.05)2 at various temperatures. (e, f) Synchrotron XRD patterns of singlecrystal CrSe2 at (e) 300 K and (f) 50 K. C


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Inorganic Chemistry

absence of Bragg reflections was observed at h + k = 2n + 1 for hkl indices. On the basis of these results, the space group of the IT phase was determined to be C2/m, a subgroup of P3m1. Rietveld refinement using the C2/m space group provided good fits to the synchrotron XRD pattern obtained at 185 K (Figure 3a). The following lattice parameters were obtained at 185 K: a = 17.5232(9) Å, b = 3.39061(4) Å, c = 8.2907(4) Å, and β = 134.926(2)°. While additional superlattice reflections were observed in the LT phase, any peak splitting corresponding to monoclinic distortions was insignificant, as shown in Figure 3. All superlattice reflections were indexed to a trigonal cell with 3at × 3at × 3ct, while the Bragg reflections obeyed rhombohedral extinction rules. These results are consistent with those from our previous electron diffraction study25 and suggested that the space group in the LT phase was R3m or its subgroup with 3-fold rotation axes. However, Rietveld refinement assuming a space group such as R3m or R3 with 3-fold rotation axes did not fit the powder XRD profiles well. In addition, the peak intensities of most of the superlattice reflections appearing below Tt1 changed only slightly below Tt2, suggesting that the LT phase structure is similar to that of the IT phase. Therefore, we concluded that the LT phase is a monoclinic or a triclinic crystal system. Finally, the space group of the LT phase was determined to be monoclinic C2/m with ⎯⎯⎯→ ⎯⎯→ ⎯⎯⎯⎯→ ⎯⎯⎯→ unit cell a′ = ⎯⎯⎯a→, b′ = 3 b , and c′ = ⎯⎯⎯c→. This

Figure 3. Synchrotron powder XRD patterns of polycrystalline CrSe2 samples acquired at (a) 185 K (IT) and (b) 50 K (LT) (black data points), together with the corresponding Rietveld refinements (red curves). The green ticks highlight the positions of the allowed Bragg reflections, while the blue dots highlight the differences between the acquired and calculated patterns. The inset of (a) shows a magnification of the 110 peak indexed to the trigonal cell at 220 K (HT), 185 K (IT), and 160 K (LT). The inset in panel (b) shows expanded XRD patterns of CrSe2 at 300 K (HT), 185 K (IT), and 50 K (LT).

m

m

t

m

t

t

m

m

m

m

m

space group is a subgroup of R3m with 3at × 3at × 3ct. Rietveld refinement of the synchrotron XRD profile obtained at 50 K using this space group provided satisfactory results (see Figure 3b). The following lattice parameters were obtained at 50 K: a = 17.5683(15) Å, b = 10.1318(3) Å, c = 8.2626(6) Å, and β = 135.078(4)°. Figure 4b shows the crystal structure of CrSe2 in the IT phase. There are two crystallographic Cr sites: Cr1 and Cr2 sites with Wyckoff positions 2a and 4i, respectively. The Cr2 atoms approach the Cr1 atoms along the a axis direction across the structural transition from the HT to IT phase. The shortest Cr−Cr distance at 185 K is that between Cr1 and Cr2 (3.18 Å), which is approximately 6% shorter than that in the HT phase. The Cr1−Cr2 bond forms one-dimensional doublezigzag chains (ribbon chains) along the b axis, and the Cr2− Cr2 distance between ribbon chains increases (3.79 Å). Such a ribbon-chain structure has been observed in VTe2, NbTe2, TaTe2, and Li0.33VS2.15,38−41 In the LT phase, the ribbon-chain structure is distorted, mainly along the b axis direction, as shown in Figure 4c. There are four different crystallographic Cr sites in the structure. The Cr1 site in the IT phase splits into Cr1a and Cr1b sites with Wyckoff positions 2a and 4g in the LT phase, respectively. The Cr1b atoms are mainly shifted by about 0.23 Å toward the Cr1a atoms along the b axis direction across the structural transition from the IT to LT phases. As a result, the distance between the Cr1a and Cr1b atoms is about 6% shorter and the distance between Cr1b atoms is about 12% longer than that between Cr1 atoms in the IT phase. The Cr2 site in the IT phase splits into Cr2a and Cr2b sites with Wyckoff positions 4i and 8j, respectively. The Cr2b atoms are mainly shifted by about 0.13 Å toward the Cr2a atoms along the b axis direction accompanied by the structural transition. The distance between Cr2a and Cr2b along the b axis direction is about 4% shorter and the distance between Cr2b atoms is about 6.4% longer than that between Cr1 atoms in the IT phase. Clear

crystal system. In addition, all superlattice reflections were well → ⎯→ ⎯ indexed using the monoclinic cell, where ⎯⎯⎯a→ m = 6 a t + 3 bt , ⎯⎯→ → ⎯a − → b +→ c . Figure 4a shows the b = b , and ⎯⎯⎯c→ = −2 ⎯→ t

relationship between the trigonal cell of the HT phase and the monoclinic cell of the IT phase. In addition, the systematic

Figure 4. Schematic illustrations of the molecular structure of CrSe2. (a) In-plane relationship between the trigonal unit cell in the HT phase (at and bt, red lines) and monoclinic unit cell in the IT phase (am and bm, purple lines) and LT phase (a′m and b′m, blue lines). (b, c) Arrangement of in-plane Cr atoms in the (b) IT phase and (c) LT phase. Some Cr linear trimers within the plane are highlighted in red, green, and blue. The dashed lines indicate the unit cell. (d) Distortion of one-dimensional chains accompanied by a Peierls-like trimerization transition. (e) Description of the three molecular orbitals (antibonding, nonbonding, and bonding) originating from the formation of linear trimers with 3c-2e σ-bonds. (f) Crystal structure in the LT phase viewed from the b axis, where a short interlayer Se−Se distance is indicated. D


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Inorganic Chemistry peaks were observed at positions corresponding to the short Cr−Cr distance of this structure in the Cr K-edge EXAFS oscillation at 50 K (see Figure 2c). A similar structural distortion was observed in TaTe2.38 Even though the structure in the LT phase seems to be complex, it is easily described by considering the arrangements of Cr linear trimers with short Cr−Cr distances (see Figure 4d). Some Cr linear trimers within a plane are colored in red, green, and blue in Figure 4b,c. In the IT phase, multiple Cr linear trimers in two certain directions form an undistorted ribbon-chain structure. In the LT phase, additional Cr linear trimers are formed along the b axis direction. As a result, the structure consists of multiple Cr linear trimers in three directions, resulting in a distorted ribbon-chain structure. There are two types of in-plane Cr−Cr distances at 50 K: short Cr−Cr distances (3.04−3.36 Å) within linear trimers and long Cr−Cr distances (3.61−3.83 Å) between linear trimers. The formation of Cr linear trimers can be explained by orbital-driven Peierls-like instabilities in one dimensionally aligned t2g orbitals, as proposed by Khomskii and Mizokawa.42 The concept is essentially identical with the mechanism of ribbon-chain formation in 1T-VTe2 proposed by Whangbo and Canadell.16 In the HT phase, t2g orbitals of a Cr atom are composed of nearly degenerate dxy, dyz, and dzx orbitals, where their in-plane lobes extend toward adjacent Cr atoms. The inplane lobes aligned in the same direction connect to form onedimensional linear chains, resulting in three one-dimensional bands: the dxy, dyz, and dzx bands. When the one-dimensional bands are properly filled, a Peierls-like transition occurs due to hidden one-dimensional Fermi-surface nesting. Assuming the formal oxidation state of Cr4+ with d2electrons, each dxy, dyz, and dzx band is 2/3-filled and has an instability that results in the trimerization of Cr atoms (see Figure 4d). Two electrons on a trimer can occupy a bonding orbital with a low energy level (see Figure 4e). Hence, Cr4+ ions within a triangular lattice have instabilities that form Cr linear trimers with a 3c−2e σ-bond in three directions. The successive structural changes within the Cr layers originate from successive Peierls-like trimerization transitions. Accompanied by the phase transition from the HT phase to the IT phase, the Peierls-like transition occurs in two specific bands, where the resultant multiple Cr linear trimers form undistorted ribbon chains within a Cr layer (see Figure 4b). The single residual band has Peierls-like instability, resulting in trimerization accompanied by the phase transition from the IT phase to the LT phase. As a result, the distorted ribbon-chain structure with the in-plane supercell 3at × 3bt is stabilized in the LT phase (see Figure 4c) . The in-plane supercell is equivalent to the nesting vector in the ab plane 1 ÷÷÷◊ 1 ÷÷÷◊ (q ⃗ = 3 a* + 3 b*) according to the band calculation using the room-temperature structure.43 The successive formation of Cr linear trimers results in drastic changes in the lattice parameters. Figure 5 shows the temperature dependence of the lattice constants a, b, and c, and the unit cell volume V of CrSe2, where these values correspond to those of the trigonal cell (at, bt, ct, and Vt). In the HT phase, at was observed to monotonically decrease and ct to increase with decreasing temperature. With the phase transition from the HT phase to the IT phase, at remained almost unchanged, while bt increased only slightly (by 0.4%), despite the drastic decrease in the in-plane Cr−Cr distance at Tt1. In contrast, ct was lower by 1.2% at Tt1, resulting in a

Figure 5. Temperature dependences of the (a) unit cell volume Vt, (b) lattice constant ct, and (c) lattice constants at and bt of CrSe2 and Cr(Se0.95S0.05)2.

decrease in Vt by 0.8%. At Tt2, at was observed to increase and bt was observed to decrease, and as a result, both parameters approach almost the same value. In the LT phase, at and bt did not change significantly with temperature. In contrast, ct did not change significantly at Tt2 but ct decreased gradually with decreasing temperature in the LT phase. In the HT phase, at (corresponding to in-plane Cr−Cr distance) greatly decreases with decreasing temperature while 1 da ct increases. The linear expansion coefficient α = a dTt of t

CrSe2 is large (5.4 × 10−5 K−1), approximately 3 times larger than those of isostructural TiSe2 and VSe2 around room temperature.44,45 The steep decrease in at above the transition temperature is likely due to precursory random clustering of Cr atoms, which has been discussed in a previous report.24 This is consistent with the absence of EXAFS peaks at ∼3.4 Å, which correspond to the in-plane Cr−Cr distance in the HT phase. Given the occurrence of Cr linear trimers in the IT and LT phases, randomly arranged Cr linear trimers are likely to exist within Cr layers, even in the HT phase. This explains the increases in bt at Tt1 with decreasing temperature shown in Figure 5, which are due to the lack of formation of linear trimers along the b axis direction in the ribbon-chain structure (see Figure 4b). In other words, the breaking of linear trimers which are randomly formed along the b axis in the HT phase should increase the Cr−Cr distance along this direction at Tt1. The largest change in lattice constants at Tt1 was observed within the interlayer distance ct, suggesting enhanced p−p hybridization between interlayer Se atoms as pointed out in 1T-MTe2.14,46 At 300 K, the interlayer Se−Se distance of CrSe2 is 3.50 Å, which is smaller than the sum of the van deer Waals radius of Se (3.80 Å) and those of 1T-TiSe2 (3.58 Å), 1T-VSe2 (3.55 Å), 1T-HfSe2 (3.55 Å), and 1T-ZrSe2 (3.78 Å).45,47−50 Hence, p−p hybridization is not negligible, even in the HT phase. The interlayer Se−Se distance was observed to decrease significantly with the structural transition from the HT phase to the IT phase; the shortest interlayer Se−Se distances in CrSe2 are 3.38 Å at 185 K and 3.34 Å at 50 K. Thus, interlayer E


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Inorganic Chemistry

phase is different from those in the IT and LT phases, although the detailed crystal structures in each phase were not clarified.27 The partially S-substituted samples with x ≈ 0.03 most likely have five different phases: the HT phase (T > Tt1), the IT phase (Tt2 < T < Tt1), the LT phase (Tt3 < T < Tt2), the IT2 phase (Tt4 < T < Tt3), and the LT2 phase (T < Tt4). The system has a complex phase diagram, suggesting the occurrence of several competitive interactions. To clarify the effect of S substitution from a structural viewpoint, the low-temperature structure was investigated for highly S substituted Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 samples with ground states different from that of CrSe2. The crystal structure and valence state of a Cr(Se0.95S0.05)2 powder sample in the HT phase was investigated. Figure 7a shows an

p−p hybridization in the IT and LT phases is enhanced in comparison with that in the HT phase. Effect of S Substitution on the Crystal Structure. We have proposed that CrSe2 forms Cr linear trimers originating from successive Peierls-like transitions. Thus, CrSe2 has the characteristic ground state with coexisting antiferromagnetic ordering25,26 and orbital molecules, while most 1T-MX2 compounds with cluster formations do not exhibit magnetic ordering.4,5,21,40 This characteristic ground state may originate from p−p hybridizations between interlayer Se atoms, which was investigated by the partial substitution of Se atoms with S atoms; this can reduce interlayer p−p hybridization, as the ionic radius of S2− is much smaller than that of Se2− and the electronegativity of S2− is larger than that of Se2−. We observed that the interlayer anion−anion distance of CrSe2 at low temperatures greatly increases following S substitution, which is discussed in this section (vide infra). To further understand the S-substituted system, we first discuss the composition−temperature phase diagram of Cr(Se1−x Sx)2 that we previously reported27 (see Figure 6).

Figure 7. (a, b) Synchrotron XRD patterns of the powder sample of Cr(Se0.95S0.05)2 acquired at (a) 300 K (HT) and (b) 60 K (LT2) (black data points), together with the corresponding Rietveld refinements (red curves). The green ticks highlight the positions of the allowed Bragg reflections, while the blue dots highlight the differences between the acquired and calculated patterns. The inset in panel (a) shows the lattice parameter a of Cr(Se1−xSx)2 as a function of S content x.27 (c) Expanded XRD patterns. (d) Synchrotron singlecrystal XRD pattern of Cr(Se0.90S0.10)2 at 30 K (LT2).

XRD pattern of the Cr(Se0.95S0.05)2 powder sample at 300 K. The XRD profile was well indexed to the trigonal P3̅m1 space group, consistent with the space group of CrSe2 in the HT phase. The lattice parameters obtained at 300 K were a = 3.38364(5) Å and c = 5.89855(9) Å, which are smaller than those of CrSe2 due to the smaller ionic radius of S2− in comparison with that of Se2−. The refined structural parameters of Cr(Se0.95S0.05)2 are very close to those of CrSe2 (see the Supporting Information). As shown in Figure 2a, the Cr K-edge XANES spectra of CrSe2 and Cr(Se0.95S0.05)2 show almost the same edge energies, indicating similar valence states. Figure 5 shows the temperature dependences of at, ct, and Vt for a Cr(Se0.95S0.05)2 powder sample, which are similar to those presented in our previous report.27 In the HT phase above Tt4, at decreased and ct increased monotonically with decreasing temperature. At Tt4, at decreased by 0.9% and ct increased by 2.5%, resulting in an increase in Vt of 0.8%. Thus, the interlayer distance increases considerably at around Tt4, suggestive of

Figure 6. Composition−temperature phase diagram of Cr(Se1−xSx)2. The phase boundary was determined from magnetic susceptibility measurements during heating.27 The black circles, white diamonds, and white squares indicate the transition temperatures determined from magnetic susceptibility, electrical resistivity, and Seebeck coefficient measurements, respectively. The HT and LT2 phases with random displacement of Cr atoms are shown in dark and light pink, while the IT and LT phases with regular displacement of Cr linear trimers are shown in dark and light blue. The schematic drawings above and below the phase diagram indicate the possible bonding configurations in each phase.

CrSe2 has three phases: the HT phase (T > Tt1), the IT phase (Tt2 < T < Tt1), and the LT phase (T < Tt2). The IT and LT phases are fully suppressed at x ≳ 0.04, and the second lowtemperature phase (LT2 phase) appears below Tt4 ≈ 125 K. Highly S substituted samples with x ≳ 0.04 have two phases: the HT phase (T > Tt4) and LT2 phase (T < Tt4). Our previous study showed that the crystal structure in the LT2 F


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correspond to the Cr−Cr distance, since the peak positions are quite similar to those corresponding to the short Cr−Cr distance in CrSe2 at 50 K. We conclude that only short-range correlations between neighboring Cr atoms occur in Cr(Se0.95S0.05)2 at low temperatures, and long-range ordering is not formed even at low temperatures. The EXAFS peak intensities gradually increased with decreasing temperature, even above Tt4, indicating possible developing correlations above Tt4. The development of short-range correlations between neighboring Cr atoms suggests the clustering of Cr atoms. Considering that Cr linear trimers are likely to randomly exist within Cr layers in the HT phase, and that the average structures in the HT and LT2 phases are very similar, randomly arranged Cr linear trimers probably also exist in the LT2 phase. The clustering correlation in the LT2 phase is stronger than that in the HT phase, as at (average nearestneighbor Cr−Cr distance) in the LT2 phase is much smaller than that in the HT phase. We summarize the structural features in the HT, IT, LT, and LT2 phases by focusing on the atomic displacement of Cr atoms. Some of the structural parameters are summarized in Table 1, and schematic diagrams of the possible bonding mechanism in each phase are shown in Figure 6. The phase diagram can be divided into two main regions from the perspective of Cr atom arrangements. One region (indicated in red) refers to the HT and LT2 phases with random atomic displacements of Cr atoms, where short-range correlations develop in nearest-neighbor Cr−Cr bonds. The other region (indicated in blue) refers to the IT and LT phases with the regular formation of Cr linear trimers in two and three directions, respectively. The former and latter regions have large and small Biso(Cr) values, respectively. The other characteristic structural difference between the HT and LT2 phases is the interlayer anion−anion distance. Table 1 shows the interlayer anion−anion distances of CrSe2 and Cr(Se0.95S0.05)2 at various temperatures. While the interlayer anion−anion distance of Cr(Se0.95S0.05)2 is 3.49 Å at 300 K in the HT phase, it is 3.59 Å at 60 K in the LT2 phase, which is much longer than those of CrSe2 in the IT and LT phases. Thus, interlayer p−p hybridization becomes negligibly small in the LT2 phase, while it becomes large in the IT and LT phases. The p−p hybridization of Cr(Se1−xSx)2 becomes weaker with increasing x due to the smaller ionic radius and larger electronegativity of S2− in comparison to Se2−. The anomalous composition dependence of the lattice parameters in the Te-substituted system Cr(Se1−yTey)227 can be explained well by considering the occurrence of p−p hybridization between interlayer anions. In the system, the interlayer distance at 300 K decreases with increasing y in the 0.075 < y < 0.15 range, indicating that interlayer p−p hybridization is enhanced when Se is partially substituted with Te due to the large ionic radius and low electronegativity of Te. Interlayer interactions via p−p hybridization seem to determine the presence of 3-fold rotation axes in each phase. In the IT and LT phases, ribbon chains consisting of Cr atoms and the short interlayer Se−Se bonds cooperatively aligned one dimensionally along the b axis, resulting in the absence of 3-fold rotation axes in the LT phase (Figure 4b,c,f). In the HT and LT2 phases, interlayer interactions are small due to weak p−p hybridization, where Cr atoms do not form regularly arranged clusters but possibly form random linear trimers, resulting in the presence of 3-fold rotation axes. These results

weakening of interlayer p−p hybridization, as discussed below. In the LT2 phase below Tt4, at decreased and ct increased monotonically. Even though the discontinuous changes in at, ct, and Vt suggest a drastic structural change at Tt4, the XRD spectra acquired of the HT and LT2 phases were unexpectedly similar. Figure 7b,c show powder XRD patterns of Cr(Se0.95S0.05)2 at 60 K, where neither superlattice reflections nor peak splitting was observed. The absence of superlattice reflections was also confirmed from the single-crystal XRD pattern of Cr(Se0.90S0.10)2 at 30 K (see Figure 7d). The powder and single crystal XRD profiles were well indexed with the trigonal P3̅m1 space group (the space group of the HT phase). An anomalous structural feature in the LT2 phase is the large Biso(Cr) value of 1.75(4) Å2 at 60 K for Cr(Se0.95S0.05)2 (see Table 1), which is significantly larger than that of the Table 1. Structural Parameters of CrSe2 at 300 K (HT), 185 K (IT), and 50 K (LT) and of Cr(Se0.95S0.05)2 at 300 K (HT) and 60 K (LT2)a Cr−Cr dis (Å) Biso(Cr) (Å2)

min

max

Se−Se dis (Å) min

av

max

CrSe2 HT (300 K) IT (185 K) LT (50 K) Cr(Se0.95S0.05)2 HT (300 K) LT2 (60 K)

1.25 0.53 0.38 1.53 1.75

3.39 3.18 3.04

3.50 3.79 3.83

3.38 3.31

3.38 3.34

3.48 3.45

3.54 3.59

3.49 3.59

a

The average isotropic displacement parameters of Cr sites (Biso(Cr)), minimum (min) and maximum (max) in-plane Cr−Cr distances (Cr−Cr dis), and minimum, average (av), and maximum interlayer Se−Se distances (Se−Se dis) are summarized.

anions at 60 K (Biso(Se/S) = 0.42(1) Å2). This suggests significant atomic displacement of Cr and negligibly small displacement of Se from the average structure in the LT2 phase. In addition, Biso(Cr) at 300 K for Cr(Se0.95S0.05)2 was 1.53(4) Å, which is comparable to the value at 60 K, suggesting that significant atomic displacement of Cr occurred in all the measured temperature ranges. Note that the Biso(Cr) values are comparable to that of CrSe2 in the HT phase and larger than those of CrSe2 in the IT and LT phases with an ordered arrangement of Cr atoms. A deviation from the average structure in the LT2 phase is indicated by the presence of weak diffuse scattering lines in the single-crystal XRD profile of Cr(Se0.90S0.10)2 (see Figure 7d). To investigate the origin of the large Biso(Cr) value of Cr(Se0.95S0.05)2, we conducted Cr K-edge EXAFS spectroscopy, which is a powerful tool for clarifying short-range correlations around Cr atoms. Figure 2d shows the temperature variation of the EXAFS oscillations of Cr(Se0.95S0.05)2. Despite the small difference between Biso(Cr) values at 300 and 60 K, the EXAFS spectra showed significant temperature variations. At 300 K, the EXAFS oscillation did not show clear peaks corresponding to the Cr−Cr distance, suggesting that there is a dynamic and/ or static distribution in the nearest-neighbor Cr−Cr distance at high temperatures as proposed for CrSe2. In contrast, the EXAFS spectra of Cr(Se0.95S0.05)2 at low temperatures showed clear peaks at ∼3 Å. We considered that these peaks G


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function of temperature for the powder samples; these data have already been reported.27 Above the transition temperatures, χ decreased with decreasing temperature. While χ of CrSe2 increased at Tt1 and Tt2, χ of Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 decreased at Tt4 ≈ 125 K with decreasing temperature. The χ values of Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 at ∼50 K were ∼20% of that of CrSe2 at the same temperature. In addition, the ZF-μ+ SR measurements clarify the significant difference in the magnetic ground states of CrSe226 and Cr(Se0.90S0.10)2 (see inset of Figure 8b). The ZF-μ+SR spectrum for CrSe2 recorded at 2 K shows a clear oscillation due to the formation of antiferromagnetic ordering.26 In contrast, the spectrum for Cr(Se0.90S0.10)2 exhibits a nonoscillatory relaxation, which evidences the absence of static magnetic order in Cr(Se0.90S0.10)2, even at 2 K. Thus, the substitution of Se for S suppresses the formation of antiferromagnetic ordering in the Cr(Se,S)2 plane. Figure 8b shows the temperature dependence of the electrical resistivity ρ for single crystals of CrSe2 and the highly S substituted Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 samples. In the HT phase, the ρ value of each sample increased slightly with decreasing temperature down to each transition temperature. The ρ value of CrSe2 abruptly decreased at Tt1, drastically increased at Tt2, exhibited a maximum at ∼90 K, and then finally decreased to the lowest temperature. In contrast, with decreasing temperature, the ρ curves for Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 showed clear increases at Tt4 and subsequent rapid increases as the temperature decreased to the lowest measured temperature. This behavior is typical of insulating materials, even though CrSe2 is a metal in the ground state. Hence, a metal−insulator transition is induced by S substitution. The temperature dependence of the Seebeck coefficients of single crystals also changed significantly with S substitution, as shown in Figure 8c. The Seebeck coefficients of all samples were positive, suggesting that hole-type carriers are present (consistent with the band calculations).43 In the hightemperature region, the Seebeck coefficients of all samples slightly increased with decreasing temperature down to the transition temperatures. The Seebeck coefficient of CrSe2 abruptly decreased with decreasing temperature at Tt1, then drastically increased at Tt2, passed through a maximum at ∼90 K, and finally decreased to zero in the low-temperature region. This temperature dependence is quite similar to that of a previous report.24 The Seebeck coefficients of Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 drastically increased at Tt4, passed through a maximum at ∼70 K, and then decreased to zero with decreasing temperature. The maximum Seebeck coefficients below the transition temperatures increased with increasing x, suggesting that the hole concentration decreases with S substitution. The slightly S substituted Cr(Se0.97S0.03)2 sample showed complex temperature dependences of χ, ρ, and the Seebeck coefficient, showing values between those of CrSe2 and Cr(Se0.95S0.05)2 for almost all of the measured temperature ranges. The temperature dependence of χ and the Seebeck coefficient exhibited several anomalies, while the ρ curve showed only one clear anomaly. In addition, a characteristic hysteresis at temperatures over 60 K was observed. These results indicate that several competitive interactions occur in these materials; however, further investigation is required.

suggest that weak interlayer Se−Se bonds play important roles in the regular arrangements of linear trimers. Effect of S Substitution on Magnetic and Transport Properties. To clarify the influence of interlayer p−p hybridization on the physical properties, we investigated the magnetic and transport properties of Cr(Se1−xSx)2 by comparing the parent compound CrSe2 with the slightly S substituted Cr(Se0.97S0.03)2 sample and highly S substituted Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 samples. These three types of samples correspond to three representative regions of the phase diagram: (1) CrSe2 has three phases (HT, IT, and LT), (2) the slightly S-substituted samples likely have five phases (HT, IT, LT, IT2, and LT2), and (3) the highly S substituted samples have two phases (HT and LT2) that are temperature dependent. Here, we focus on the physical properties of the highly S substituted samples. Single crystals of Cr(Se1−xSx)2 with x = 0, 0.03, 0.05, and 0.10 were synthesized for transport measurements, and their compositions were determined using the calibration curve shown in the inset of Figure 7a. The lattice constant a is known to decrease linearly with x, as previously reported.27 Therefore, the correct values of x for the single crystals can be determined from their a values. The determined x values of the single crystals were 0.0287(55), 0.0547(58), and 0.0980(57), which are almost the same values as their nominal compositions, namely 0.03, 0.05, and 0.10, respectively. In the following discussion, the nominal values are used. The temperature dependences of the magnetic susceptibility χ of CrSe2 and the highly S substituted Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 samples were observed to be distinctly different around their transition temperatures. Figure 8a shows χ as a

Figure 8. Temperature dependence of (a) magnetic susceptibility χ, (b) electrical resistivity ρ, and (c) the Seebeck coefficient S of Cr(Se1−xSx)2. Only data acquired during heating are shown here. The magnetic susceptibility data are taken from our previous report.27 The inset in panel (b) shows the ZF-μ+SR spectra for CrSe226 and Cr(Se0.90S0.10)2 recorded at 2 K. The inset in panel (c) shows a magnified view of the temperature dependence of the Seebeck coefficient of Cr(Se0.97S0.03)2. The triangles indicate anomalous temperatures. For the Cr(Se0.97S0.03)2 sample that showed large hysteresis, the cooling process is also shown for reference. H


Article

Inorganic Chemistry Coexistence of Orbital Molecules and Magnetic Ordering in CrSe2. We demonstrated that the Cr(Se1−xSx)2 system exhibits interesting behaviors from a number of perspectives. Although it is clear that the strength of interlayer p−p hybridization between anions affects the development of the different ground states, this point requires further investigation. Here, we discuss the effect of p−p hybridization between interlayer anions by considering 1T−MTe2 compounds.14,16,46 In such compounds, the Te bands increase in energy due to interactions between Te atoms, leading to partial electron transfer from the top part of the Te bands to metal bands. Such electron transfer increases the d electron count of the metal ions in comparison to the formal oxidation state of +4. Partial electron transfer has been proposed in VTe2, NbTe2, and TaTe2,14,16 for which the number of d electrons is 1, considering each formal oxidation state, but is actually closer to 4/3. As interlayer p−p hybridization becomes stronger, the energy levels of the antibonding anion bands near the Fermi level increases, subsequently increasing the amount of electron transfer. A similar electron transfer mechanism is expected for CrSe2, originating from the unusual nature of Cr4+ ions. It is wellknown that Cr atoms with octahedral coordination favor the trivalent states, as their half-filled t32g electron configurations are highly stable. In contrast, Cr4+ is relatively unstable, and Cr d bands have a tendency to lose energy; the top part of the Se bands can extend above the Fermi energy. Thus, enhanced interlayer p−p hybridization causes electron transfer from Se to Cr bands, which increases the number of d electrons of Cr4+. Considering these electron-transfer effects, we can explain the origin of the coexistence of orbital molecules with 3c-2e σ bonds and magnetic ordering in the ground state of CrSe2. If Cr ions are in tetravalent states with d2 electron configurations, the formation of three 3c-2e σ bonds will result in a nonmagnetic system, as all of the valence electrons of Cr4+ occupy bonding orbitals. However, in the case of interlayer p− p hybridization, where partial electron transfer from Se to Cr bands occurs, d electrons can occupy both the bonding and nonbonding orbitals (Figure 4e). In general, the electrons occupying bonding orbitals contribute to the stabilization of cluster formations. In contrast, the electrons in the nonbonding orbitals can contribute to magnetic ordering. Thus, the characteristic ground state of CrSe2 is realized by the cooperative effects of orbital molecule formation and interlayer p−p hybridization. For highly S substituted Cr(Se0.95S0.05)2 and Cr(Se0.90S0.10)2 samples, interlayer p−p hybridization is negligibly small, which is fundamentally different from the behavior of CrSe2. In this case, all valence electrons occupy bonding orbitals, assuming a molecular orbital formation with 3c-2e σ bonds. Hence, such samples do not show magnetic ordering and become insulators in their ground states. These properties are consistent with those of transition-metal compounds that form orbital molecules.17−19,23,51−56 The clustering phenomena in the 1T-CrSe2 system are both similar to and different from those observed in transition-metal oxides forming orbital molecules, which often exhibit interesting clustering patterns.18,19,23,51−61 The simplest clustering patterns are the spin-singlet dimers observed in many transition-metal oxides, such as VO2, MgTi2O4, LiRh2O4, and Li2RuO3,51−56 while more complex clusters have been reported, such as the trimer in LiVO218,19 and the heptamers (or trimers and tetramers) in AlV2O4 and GaV2O4.57−59 The

formation of orbital molecules involving short metal−metal bonds in such a compound stabilizes the system and changes its magnetic and electrical resistivity. CrSe2 also forms orbital molecules with concomitant drastic changes in magnetic and electrical resistivity. Other transition-metal dichalcogenides 1T-MX2 also exhibit various cluster formations depending on their electron counts,5,6 such as the Star of David cluster in TaS2 with a d1 system,8,9 ribbon chain clusters in VTe2, NbTe2, and TaTe2 with d4/3 systems,38−41 zigzag chain clusters in MoTe2 and WTe2 with d2 systems,21,22 and diamond chain clusters in ReS2 and ReSe2 with d3 systems,62,63 whose formations can be understood on the basis of the concepts of both hidden one-dimensional Fermi-surface nesting and local chemical bondings.16,20 In addition, some oxides, such as Li2RuO3, LiRh2O4, AlV2O4, and GaV2O4, exhibit disordered orbital molecules even above structural transition temperatures,58−61 similar to the case of CrSe2, which likely exhibits precursory random clustering of Cr atoms even above its transition temperature. The main difference between CrSe2 and conventional cluster compounds is in the ground state; CrSe2 is an antiferromagnetic metal, while almost all of the transition-metal oxides with orbital molecules are nonmagnetic insulators. Furthermore, almost no 1T-MX2 compounds exhibit magnetic ordering. We believe that this difference originates from p−p hybridizations between interlayer Se atoms. Our results indicate that transition-metal chalcogenides have the potential to exhibit exotic clustering phenomena. Finally, we discuss the origin of the characteristic physical properties of CrSe2 by focusing on two effects: (i) in-plane Cr−Cr interactions originating from Peierls-like trimerization instabilities and (ii) interlayer anion−anion interactions through p−p hybridization. The anomalous temperature dependence of χ in the HT phase is well explained by the precursory random clustering of Cr atoms originating from Peierls-like instabilities. This system has a large χ at 300 K, on the order of 10−3 emu/mol, which is comparable to that estimated from the Curie law with a spin of 1 at 300 K (3.3 × 10−3 emu/mol), indicating the occurrence of almost localized d electrons. However, χ does not follow the Curie law and decreases with decreasing temperature. This characteristic temperature dependence probably originates from the development of random clustering between neighboring Cr atoms, which reduces the magnetic moments of Cr atoms with decreasing temperature. The unexpected increase in χ of CrSe2 at Tt1 accompanied by the formation of orbital molecules is well explained by the electron-transfer mechanism described above. At Tt1, interlayer p−p hybridizations between Se atoms increase due to the phase transition, resulting in electron transfer from Se to Cr bands. The increased d electron count of the Cr atoms increases χ. Electron transfer increases the concentration of holes in the Se bands, resulting in a decrease in the Seebeck coefficient and ρ at Tt1. At Tt2, χ increases again with decreasing temperature. It should be noted that both the Peierls-like trimerization transition and magnetic ordering occur at Tt2. Thus, the increase in χ is probably related to magnetic ordering patterns and/or enhanced magnetic interactions. The ρ and Seebeck coefficient values drastically increase at Tt2 with decreasing temperature, since the lowtemperature structure satisfies the in-plane nesting condition.43 In contrast to CrSe2, highly S substituted samples show decreases in χ and increases in ρ at Tt4 with decreasing temperature; this temperature dependence is typical of I


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conventional molecular cluster compounds.17,18,53−55 Associated with the phase transition from the HT phase to the LT2 phase, p−p hybridization between interlayer anions becomes negligibly weak and clustering correlation develops, resulting in a decrease in χ at Tt4. In addition, short-range clustering patterns probably satisfy the nesting condition of the hole-like Fermi surface,43 assuming randomly arranged Cr linear trimers with 3c-2e σ bonds in three directions. Hence, the ρ and Seebeck coefficient values increase drastically below Tt4, and the samples became insulators in the ground state, as all valence electrons occupy bonding orbitals. The characteristic phase transitions in this system originate from competition and cooperation of the in-plane Cr−Cr interactions derived from Peierls-like instabilities and interlayer anion−anion interactions via p−p hybridization. In the HT phase, precursory random clustering of Cr atoms and weak interlayer p−p hybridization between anions originate from these two effects. In CrSe2, the cooperative effects of these interactions result in ribbon chains consisting of Cr linear trimers and weak interlayer Se−Se bonds being aligned in one direction. However, one of the three t2g orbitals does not form orbital molecules in the IT phase, indicating that Peierls-like instability remains in one chain. Hence, an additional trimerization occurs at Tt2, although there is no energy gain through interlayer anion−anion interactions and a large amount of energy loss due to the lattice distortion. Substitution with S changes the competition between in-plane Cr−Cr and interlayer Se−Se interactions. With increasing S content, interlayer p−p hybridization becomes weaker, while the Peierls-like instabilities remain strong. Hence, highly S substituted samples probably form Cr linear trimers, resulting in a high energy gain due to orbital molecule formation, and negligibly small interlayer p−p hybridization occurs in the ground state. For slightly S substituted samples, the strong competition between interlayer Cr−Cr and in-plane Se−Se interactions probably induces complex multistep transitions. The low-temperature structures and electronic states are controlled by the interplay of in-plane Cr−Cr and interlayer anion−anion interactions in this system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00186. Refined structural parameters determined from synchrotron powder X-ray diffraction measurements at various temperatures (PDF) Accession Codes

CCDC 1888113−1888115, 1888117−1888118, and 1888120 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.K.: [email protected]. ORCID

Shintaro Kobayashi: 0000-0002-7306-8458 Naoyuki Katayama: 0000-0002-9456-2201 Present Addresses ∇

S.K.: Japan Synchrotron Radiation Research Institute, SPring8, 1-1-1 Kouto, Sayo 679-5198, Japan. □ J.S.: CROSS Neutron Science and Technology Center, Tokai, Ibaraki 319-1106, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank S. Kawaguchi, K. Sugimoto, A. Nakano, S. Kitou, and H. Amano for supporting the synchrotron XRD experiments, Prof. M. Tabuchi for supporting the XAFS experiments, and Prof. J. H. Brewer, H. Nozaki, and I. Umegaki for supporting μ+SR experiments. This work was partially carried out using the facilities at the Research Center for Low Temperature and Materials Sciences, Kyoto University, and the Institute for Solid State Physics, the University of Tokyo. This work was supported by a Grant-in-Aid from JSPS KAKENHI (grant numbers 16J04046, 16H04131, 17K17793, 18H01863, and18KK0150) and the Toukai Foundation for Technology. In Sweden, O.K.F. and M.M. were supported by the European Commission through a Marie Skłodowska-Curie Action and the Swedish Research Council-VR (Dnr. 20146426 and 2016-06955) as well as the Carl Trygger Foundation for Scientific Research (CTS-16:324). Furthermore, Y.S. was fully funded by the Swedish Research Council (VR) through a Starting Grant (Dnr. 2017-05078). The synchrotron radiation experiments were performed at the BL02B1 and BL02B2 beamlines of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (proposals 2016A1620, 2016B1437, 2016B1270, and 2017A1081) and at the BL5S2 and BL11S2 beamlines of the Aichi Synchrotron Radiation Center, Aichi Science & Technology Foundation, Aichi, Japan (proposals 201706129 and 201802042). We thank the staff of TRIUMF for help with the μ+SR experiments. The crystal structure figure was created using VESTA.64

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SUMMARY Characteristic phase transitions of CrSe2 and Cr(Se1−xSx)2 compounds were investigated using synchrotron XRD, XAFS, magnetic susceptibility, μ+SR, electrical resistivity, and Seebeck coefficient measurements. Successive structural transitions in CrSe2 originate from cooperative effects of in-plane Cr−Cr interactions derived from Peierls-like trimerization instabilities and interlayer Se−Se interactions through p−p hybridization. CrSe2 has the characteristic ground state with coexisting orbital molecules and antiferromagnetic ordering. The key to realizing this unique ground state is electron transfer from Se to Cr bands originating from strong interlayer p−p hybridization between Se atoms. The p−p hybridization is significantly weakened by partial substitution of Se atoms for S atoms, resulting in another type of structural transition involving random clustering of Cr atoms at high levels of S substitution. S substitution changes the ground state from that of an antiferromagnetic metal to an insulator without long-range magnetic ordering. The differences in the ground states of CrSe2 and the S-substituted samples originate in the strength of the p−p hybridization between interlayer anions. J


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Inorganic Chemistry

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Linear Trimer Formation with Antiferromagnetic Ordering in 1T-CrSe2

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