CsFe4−δSe4: A Compound Closely Related to Alkali-Intercalated

5 days ago - Carbon nanotube net could extend battery lifetimes. As scientists try to boost the lifetime and capacity of lithium-ion batteries, they'v...
27 downloads 9 Views 4MB Size
Download PDF
Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

CsFe4−δSe4: A Compound Closely Related to Alkali-Intercalated FeSe Superconductors Kunkun Li,†,‡ Qing-Zhen Huang,§ Qinghua Zhang,† Zewen Xiao,∥ Toshio Kamiya,∥,⊥ Hideo Hosono,∥,⊥ Duanduan Yuan,†,‡ Jiangang Guo,*,† and Xiaolong Chen*,†,#,∇ †

Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States ∥ Materials Research Center for Element Strategy, Tokyo Institute of Technology, Yokohama 226-8503, Japan ⊥ Laboratory for Materials and Structures, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan # School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China ∇ Collaborative Innovation Center of Quantum Matter, Beijing 100084, China ABSTRACT: We report the synthesis and characterizations of a new FeSe-based compound CsFe4−δSe4, which is closely related to alkali intercalated FeSe superconductors while exhibits distinct features. It does not undergo phase separation and antiferromagnetic transition. Powder neutron diffractions, electron microscopy and high-angle annular-dark-field images confirm that CsFe4−δSe4 possesses an ordered Cs arrangement as √2 × √2 superstructure, evidencing a B-centered orthorhombic lattice with a space group of Bmmm. The temperature-dependent powder neutron diffractions indicate no structural and magnetic transition from 320 to 5 K. In contrast to the symmetry-breaking in FeSe, this phase naturally possesses the orthorhombic symmetry even at room temperature. DFT calculations and transport measurements reveal a novel Fermi surface geometry with two electron-like sheets centered on Γ point and intermediate density of states at the Fermi level comparing with the value of FeSe and the superconducting AxFe2Se2.

■

INTRODUCTION

Therefore, the explorations of new compounds containing less alkali metal content in these systems are of importance. Theoretical and experimental results demonstrate that the content of alkali metal in AxFe2Se2 is closely related to the phase stability.20,21 The first-principles calculations show the Coulomb attraction between K ion layers and negatively charged FeSe layers, and the accumulation of negative charge in FeSe layers dominated the stability for the alkali metal intercalated 122 phase.20 KxFe2Se2 phase without iron vacancy can only exists when 0.25 < x < 0.6. With lower potassium content x < 0.25, the structure will collapse due to the dynamic instability. However, no pure phase with alkali metals in the compositional range 0.25 < x < 0.6 has been synthesized due to phase separations up to now. Low alkali metal phases were only found in the cointercalated A x (NH 3 ) y Fe 2 Se 2 , 2 2 − 2 5 Ax(En)yFe2Se2,26 and (Li0.8Fe0.2)OHFeSe27 obtained at low temperatures. It should be noted that Tc is either about 30 K or

A0.8Fe2Se2 (A = K, Rb, Cs, Tl/Rb, and Tl/K) are a class of superconductors that exhibit intriguing properties different from their FeAs-based counterparts and have been attracting intense research interests ever since their discovery in later 2010.1−5 The superconductivity occurs at around 30 K in a mesoscopic scale phase, which always intergrows with an insulting phase A2Fe4Se5.6−12 Angular resolution photoemission spectroscopy confimed that they are heavily electrondoped with the hole-like pockets sinking far below the Fermi energy,13−15 suggesting carriers are well beyond the optimal concentration. Further increase in the alkali metals will lead to Fe vacancies, forming compounds like A 2 Fe 4 Se 5 and A2Fe3Se4.16−18 In contrast, it is thought that in these superconducting phases no Fe vacancy is present11,19 implying the valence of Fe is substantially below +2. It is expected that the transition temperature in this class of superconductors can be enhanced in a phase with higher Fe’s valences. This means that the alkali metals should be less than 0.8 as in A0.8Fe2Se2. © XXXX American Chemical Society

Received: January 23, 2018

A

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (a, b) Powder X-ray diffraction of Cs0.5Fe2−δSe2 at room temperature with structure peaks indexed with I4/mmm and Bmmm, respectively. (c) Powder neutron diffraction and Rietveld refinement profiles of CsFe4−δSe4 at room temperature. aberration. The magnetic susceptibilities were measured using a vibrating sample magnetometer (VSM, Quantum Design). The electrical resistivity (ρ), Hall coefficient (RH), and heat capacity (CP) measurements were carried out on a Physical Property Measurement System (PPMS, Quantum Design). Electrical resistivity measurements and Hall coefficient measurements were performed by a standard fourprobe method. The heat capacity measurements were performed by a thermal relaxation method using a square-shaped sample plate. Variedtemperature powder neutron diffraction measurements were performed at NIST. Density functional theory (DFT) calculations were performed using the projector-augmented wave (PAW) method, as implemented in the VASP 5.4 code.30 The generalized gradient approximation (GGA) Perdew−Burke−Ernzerhof (PBE)31 functional was chosen as the exchange-correlation functional. The plane-wave cutoff energy was set to 500 eV. A Γ-centered k-point mesh with k-spacing of ∼0.2 Å−1 was employed for sampling the Brillouin zone. The lattice parameters and atomic positions were fully relaxed until the force on each atom was smaller than 0.01 eV/Å.

about 45 K, regardless of the NH3 or other organic molecules in these cointercalated phases. It is inferred that alkali metal vacancy ordering is responsible for stabilizing these phases and hence determining Tc, but this still needs experimental evidence to support this speculation. In this study, we report a ternary compound CsFe4−δSe4 synthesized at moderate temperatures. The content of Cs is much lower than that in previously reported AxFe2Se2, while the structure is stable. Cs atoms form a one-dimensional chain along the [100] direction and a √2 × √2 ordering ensues. This unique distribution leads its symmetry of crystal structure to a B-centered orthorhombic lattice at room temperature, unlike the tetragonal-orthorhombic (C4−C2) transition in FeSe. DFT calculations and transport measurements reveal a novel Fermi surface configuration with two electron-like pockets around Γ point and intermediate density of states at the Fermi level in comparison with FeSe and the superconducting AxFe2Se2. The lack of superconductivity is due to the existence of a small amount of Fe vacancies in the FeSe sheets.

■

■

RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction pattern as well as the neutron diffraction pattern of polycrystalline Cs0.5Fe2−δSe2 sample collected at room temperature. Assuming the compound adopts a ThCr2Si2 structure, most of the reflections could be indexed by the space group I4/mmm (No. 139) with lattice parameters a = 3.8594(1) Å and c = 15.641(1) Å, as shown in Figure 1a. Compared with the reported lattice parameters a = 3.9601(2) Å and c = 15.2846(11) Å in Cs0.8(FeSe0.98)2,3 the lattice parameter a of Cs0.5Fe2−δSe2 shrinks by 2.6%, while c expands by 2.3%. In addition, we note that additional peaks at 2θ = 15−20° do exist, which can not be indexed by the tetragonal cell. Taking these peaks into account, reindexing reveals that Cs0.5Fe2−δSe2 possesses an orthorhombic cell with lattice parameters a = 5.458(3) Å, b = 15.6394(4) Å, and c = 5.457(3) Å. According to the extinction conditions, the possible space group is Cmmm (No. 65). In order to give a better comparison with other FeSe-based compounds, Bmmm is adopted with lattice parameters a = 5.458(3) Å, b = 5.457(3) Å, and c = 15.6394(4) Å, which would make FeSe layer within the ab plane, as shown in Figure 1b. The new lattice parameters a and b are nearly √2 × 3.8594 Å, indicating a possible superstructure induced by some atom ordering. Considering that the Cs content in Cs0.5Fe2−δSe2 is

EXPERIMENTAL METHODS

Polycrystalline samples of Cs0.5Fe2Se2 were prepared via conventional solid-state method using alkali metal Cs, Fe powder, and Se powder (Alfa, 99.95, 99.999, and 99.99%, respectively) as starting materials. FeSe precursors were prepared via the reaction of Fe and Se powder at 950 K for 24 h in sealed quartz tubes.28 The resultant reground FeSe powder together with stoichiometric amount of Cs was loaded into an alumina crucible and then sealed into an evacuated quartz tube under vacuum. Due to the extremely sensitivity to air and moisture of Cs, all manipulations were performed in an argon-filled glovebox. The tube was subsequently annealed at 873 K for 24 h. After that, the obtained sample was pulverized, pressed into a pellet, sealed in a quartz tube with Ar gas, and then heated and kept at 923 K for 72 h. The obtained polycrystalline samples were dark black and air-sensitive, so great care was taken in specimen preparation for characterization. Room temperature powder X-ray diffraction (PXRD) data were collected using a PANalytical X’Pert PRO diffractometer (Cu Kα radiation) with a graphite monochromator in a reflection mode (2θ = 10−130°, step = 0.017° (2θ)). Rietveld refinements were performed with the FULLPROF package.29 The high-angle annular-dark-field (HAADF) images were obtained using an ARM-200F (JEOL, Tokyo, Japan) scanning transmission electron microscope (STEM) operated at 200 kV with a CEOS Cs corrector (CEOS GmbH, Heidelberg, Germany) to cope with the probeforming objective spherical B

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. (a) Schematic crystal structure of CsFe4−δSe4. (b) Crystal structure of CsFe4−δSe4 viewed along [001] direction. (c) Crystal structure of reported Cs0.8(FeSe0.98)23 viewed along [001] direction. (d) SAED pattern taken along the [001] zone axis. The evident satellite spots can be characterized by modulation wave vectors q(1/2, 1/2, 0) and q(1/2, −1/2, 0). (e) HAADF images taken along the [001] zone axis. The atom images show different contrasts.

Figure 3. (a, b) Powder neutron diffraction pattern of CsFe4−δSe4 measured at 320 and 5 K, respectively. (c) Lattice parameter dependence of temperature. The error bars for parameter c are within the solid circles. (d) c/a ratio and unit cell volume as a function of temperature. The dashed lines are guide to eyes.

amount of impurity FeSe (∼3.1 wt %) and Fe (∼3.2 wt %). The refined composition is CsFe3.67(1)Se4, indicating the existence of ∼8.5% Fe vacancies. No superstructure peaks related to Fe vacancy ordering can be seen from our data. The schematic crystal structure of CsFe4−δSe4 is shown in Figure 2a. Figure 2b shows the crystal structure viewed along [001] direction. It has a √2 × √2 unit cell compared with the reported ThCr2Si2 structure of CsFe2Se2, as illustrated in Figure 2c. We noted that there are some reports about low alkali-metal

half the composition in ThCr2Si2 structure, we proposed a new crystal structure with Cs vacancy ordering the [100] direction. Rietveld refinements against the powder neutron diffraction data were then performed by adopting the Cs vacancy ordering structure as an initial model. As shown in Figure 1c, the refinements smoothly converged to Rp = 4.05%, Rwp = 5.37%, and Rexp = 4.22%, respectively. To reflect the Cs vacancy ordering, we will use the CsFe4−δSe4 formula to represent our sample hereafter. It is noted that the sample contains a small C

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 4. (a) FeSe layers extracted from the crystal structure. (b) FeSe bond length dependence of temperature. (c) Se−Fe−Se bond angle dependence of temperature. Hollow spots are the Se−Fe−Se bond angle of FeSe from ref 36. (d) Variation of anion height with temperature. The dashed lines are guide to eyes.

intercalated FeSe phases with K/Cs-vacancy √2 × √2 ordering accompanying a different K/Cs-ordered structure,7,32,33 and unfortunately, they are also observed in phaseseparated samples. In order to check the crystal structure of the Cs vacancy ordering, we performed the selected area electron diffraction (SAED) as well as the HAADF STEM. Figure 2d shows the typical SAED pattern taken along the [001] zone-axis direction. The main diffraction spots can be easily indexed by the reduced tetragonal cell. However, weak bright spots do exist. Comparing with the reported ThCr2Si2 structure of CsFe2Se2, the satellite spots can be characterized by modulation wave vectors q(1/2, 1/2, 0) and q(1/2, −1/2, 0). Figure 2e shows a HAADF image taken through the top view of the FeSe layer. As the contrast of HAADF image exhibits a Z1.7-dependent relation, where Z is the atomic number, different kinds of atom columns can be directly distinguished. As we can see, the weak contrast spots represent the Fe columns, while the strong contrast spots Cs/ Se columns. Spots with an intermediate contrast should be due to the Se atom columns, which result from the Cs atom ordering along a-axis. To be clear, we superimpose the deduced atom arrangement on the FeSe plane, which is consistent with the proposed crystal structure shown in Figure 2b, suggesting that CsFe4−δSe4 features an ordering of Cs vacancies. To be clear, the superimposed purple dashed lines represent the Cs/ Se columns while the greed dashed lines represent the Se columns. In Cs0.8Fe2−ySe2, an antiferromagnetic ordering at TN = 471 K arising from the Fe vacancy order and an order−disorder transition at TS = 500 K have been observed, respectively.9 In order to investigate the magnetic or structural phase transition in this compound, we measured a series of powder neutron

diffraction patterns at temperatures from 320 to 5 K. Figure 3a,b shows the comparison of the diffraction patterns measured at 320 and 5 K, respectively. As can be seen, both patterns can be well-indexed with the space group Bmmm (No. 65). No sign of additional magnetic peaks or structural phase transition can be seen. Figure 3c shows the lattice parameters refined at different temperatures. As the temperature changes from 320 to 5 K, the lattice parameter a smoothly decreases from 5.4594(10) to 5.430(1) Å while c decreases from 15.6397(4) to 15.4869(4) Å without any sign of structural phase transition. The c/a ratio as well as the unit cell volume of CsFe4−δSe4 also decreases monotonically, as shown in Figure 3d. The absence of magnetic or structural phase transition in this compound probably has a connection with the content of Fe vacancy, which is too low to form Fe vacancy ordering. As reported in Fe1−xSe, the observed lowest content Fe vacancy ordering has at least 10% Fe vacancies with a √10 × √10 superstructure.34 Besides, in K2Fe7Se8, a √8 × √10 superstructure with Fe vacancy ordering is reported;16 thus, the slight excess of Fe relative to the nominal value of CsFe3.5Se4 should also be responsible for the absence of Fe vacancy ordering. The physical properties of the iron-based superconductors are known to be closely related to the geometry of FeAs4 or FeSe4 tetrahedra. Thus, we carried out the Rietveld refinements for all the neutron diffraction patterns at different temperatures and extracted the structural parameters of FeSe4 tetrahedra. Figure 4a shows the geometry configuration of the FeSe layer of CsFe4−δSe4. Since the space group is converted to a Bcentered lattice, there are two different Se atom positions in a unit cell, leading to two different FeSe bond lengths, anion heights to the Fe atom plane (h1 and h2) and bond angles of Se−Fe−Se, as shown in Figure 4b−d, respectively. The bond D

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 5. (a) Temperature dependence of dc magnetic susceptibility for CsFe4−δSe4 sample. (b) Temperature dependence of conductivity for CsFe4−δSe4 sample. Fits of data using the thermal activation (TA) model and variable range hopping (VRH) model are shown as solid curves with red and blue color, respectively. (c) Field dependence of Hall resistivity measured at different temperature. (d) Temperature dependence of the Hall coefficient RH defined as ρxy/μ0H at 9 T; the dotted straight line represents RH equals zero. (e) Heat capacity CP divided by temperature T versus T2 (solid blue symbols) and linear fits (solid red curves) to the lowest-Temperature data by CP/T = γ + βT2.

the high-temperature range and the low-temperature range, respectively.40 This means that electrons may undergo localized states at low temperatures. A small energy gap of about 28 meV is estimated by fitting the data of high temperature. Figure 5c shows the magnetic field dependence of the Hall resistivity at different temperature ranging from 10 to 300 K. The Hall resistivity shows a good linear relation to the magnetic field. The Hall coefficient (RH), determined by the slope of the field dependence of the Hall resistivity, shown in Figure 5d, is positive above 200 K and negative below 150 K, indicating a sign-change of dominated carrier-type and a multiband system. The magnitude of RH derived from CsFe4−δSe4 is within the range of 6 × 10−8 m3/C, comparable to that in FeSe.41 The positive RH at high temperature indicates a hole-pocket carrier contribution at the Γ point, while the sign change at lower temperature can be attributed to the electron pocket carrier contribution, which is observed and explained in FeSe.41,42 The similar Hall coefficient behavior in FeSe indicates the doping level of CsFe4−δSe4 is less than that in K0.8Fe2Se2, which is heavily electron doped and shows a negative RH in whole temperature range.43 To determine the Sommerfeld coefficient γ of electronic specific heat and the Debye temperature θD, we plot the lowest-temperature CP/T data versus T2 in Figure 5e and fit the data by CP/T = γ + βT2. The values of γ and β can be obtained. The Debye temperature θD can be calculated from the value of β using θD = (12π4nR/5β)1/3, where R is the gas constant, n is the atom number per formula. The obtained θD is 173 K, and γ = 16.2 mJ mol−1 K−2. We carried out the DFT calculations to examine the electronic structure of CsFe4Se4. The existing Fe vacancies are not considered in our calculations considering the relatively low vacancy content. From the calculated band structure shown in Figure 6a, the Fermi level is crossed by two energy bands that consist mainly of Fe 3dxz/yz/xy states. The energy bands are very flat along the directions that are parallel to the FeSe layers, i.e., the Γ−Z and T−Y directions, indicating a more two-

length of Fe−Se shrinks a little as the temperature decreases, which might be due to the shrinking of the unit cell. In Figure 4c, one of the Se−Fe−Se bond angles α is ∼105.1°, while the other one β is ∼111.7°. Both values remain almost unchanged as temperature varies. Compared to the Se−Fe−Se angle of 109.3° in reported superconductor Cs0.8Fe2Se2,35 the FeSe layer in our sample is elongated along the stacking direction while compressed along the FeSe plane, resulting in a more distorted FeSe4 tetrahedra. The binary FeSe possesses Se−Fe−Se angles of 104.5 and 112.1°, and increases significantly to 103.2 and 114.5° with increasing pressure to 9.0 GPa.36 The hse is demonstrated to closely relate to the superconductivity. It was illustrated that in FeAs-based superconductors, the closer anion height is to 1.38 Å, the higher the Tc.37 The anion height of our sample is 1.48 and 1.50 Å, both are in the range of superconducting A0.8Fe2Se2 compounds. The superconducting transition temperatures of binary FeSe compound under pressure also follow this trend.38 However, in the alkali metal or (Li0.8Fe0.2)OH intercalated FeSe compounds, Tc increases from 31 to 44 K as the anion height elongating from 1.46 to1.53 Å.39 The temperature dependence of magnetism, conductivity, Hall effect, and heat capacity measurements of the CsFe4−δSe4 are plotted in Figure 5. Figure 5a shows the temperature dependence of the dc susceptibility for CsFe4−δSe4 under 1 T measured from 3 to 850 K. No clear magnetic transition can be seen in the whole temperature range, agreeing with the results based on the neutron diffractions. The distinct trend of lowtemperature susceptibility does not obey Curie−Weiss paramagnetic behavior. The reason has been not clear. The conductivity data in Figure 5b clearly shows the semiconducting behavior. A simple thermally activated conduction model σ = exp(−Eg/2kBT), where Eg is the band gap energy and kB is Boltzmann’s constant, failed to fit the data in the entire temperature region. Thus, the thermal activation and variable range hopping conductions are used to fit our data in E

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Fe atom and 1.8 states eV−1 per Fe atom, respectively, the intermediate DOS between FeSe and CsFe2Se2 is strikingly consistent with the calculated band structure as well as the intermediate doping level considering the less Cs content. In KxFe2Se2, an insulating K2Fe4Se5 phase with a large band gap of 500 meV can be found with a √5 × √5 Fe-vacancy ordering followed by an antiferromagnetic ordering.9,10 Besides, a semiconducting 234 phase with stripe-type magnetic ordering and rhombus iron vacancy ordering is also observed.17,47,48 It has a similar electronic structure to KxFe2Se2 superconductor and a deduced band gap of 40 meV. Moreover, in heavily Kdoped FeSe thin film, an insulating phase shows up as superconductivity is suppressed.49 The overdoped electrons induce an enhancement of electron correlation strength, making the FeSe thin film become an insulator. In the present case, although a Fermi surface reconstruction around R point is present possibly due to the ordered Cs arrangement, it still shows an intermediate doping level between FeSe and CsFe2Se2. The intermediate DOS indicates a metallic behavior in our compound, which is consistent with the finite electronic specific heat. Considering the relatively small Fermi surface pocket, the observed semiconducting behavior might be attributed to the existing disordered Fe vacancies, which could make the electronic states change from extended states to localized states. Besides, the larger distortion of FeSe 4 tetrahedra would result in a larger Se−Fe−Se angle, which would increase the correlation strength, leading to larger effective mass and possibly pushing the compound to the semiconducting region.50 As reported in Li1−xFex(OH)Fe1−ySe, superconductivity only appears when the content of Fe vacancy is less than 5%.51 Thus, the content of Fe vacancy is thought to be responsible for the absence of superconductivity in our case. Giving the high conductivity as well as such a small Eg of 28 meV, superconductivity might be easily achieved through doping extra Fe atoms into CsFe4−δSe4. In conclusion, we discover a novel alkali metal intercalated FeSe compound with a much lower Cs content than previously reported in AxFe2Se2. The structural analyses unravel a new structure type of B-centered orthorhombic lattice, in which Cs atoms forms a one-dimensional chain along the (100) direction accompanying a √2 × √2 ordering along with ∼8.5% disordered Fe vacancies. No structural and magnetic transitions can be confirmed by physical property measurements and lowtemperature powder neutron diffractions. DFT calculations and experimental data reveal a novel Fermi surface geometry and intermediate densities of states between FeSe and the superconducting AxFe2Se2. We contribute the lack of superconductivity to the existence of a small amount of Fe vacancies in the FeSe sheets based on our theoretical and experimental data.

Figure 6. (a) Band structure of CsFe4Se4. (b, c) Fermi surfaces of CsFe4Se4 of bands 33 and 34, which are marked by red and blue lines in (a), respectively. (d) Total and orbital projected DOS of CsFe4Se4.

dimensional character compared to CsFe2Se2,44,45 which is consistent with the layered structure of CsFe4Se4 and the larger interlayer separation. Around the Γ point, the two hole-like bands shift to higher energies comparing to the band structure of FeSe,46 leaving the relatively small hole-like pockets at the Γ point. Unlike the sinking of hole bands below the Fermi level in AFe2Se2 (A = K, Rb, and Cs),44 the band structure of CsFe4Se4 presents an intermediate doping level between binary FeSe and CsFe2Se2, which is reasonable considering the less Cs content in our compound. Figure 6b,c show the derived Fermi surfaces of CsFe4Se4. It shows a remarkable difference comparing to those reported for FeSe and CsFe2Se2, which usually have electron pockets centered at R point. In CsFe4Se4, one inner closed-hole pocket and one outer-cylinder hole pocket can be clearly seen, which is similar to the case of FeSe.46 Besides, two electron-pockets, forming two cylinders centered at the Γ point are found, and the nesting between these two nearly identical cylinders is very small. The reconstruction of Fermi surface might be due to the distortion of FeSe4 tetrahedra, giving the unique Cs atom occupation converts its symmetry of crystal structure to a B-centered orthorhombic lattice at room temperature. The total and orbital projected densities of states (DOSs) of CsFe4Se4 are shown in Figure 6d. The states near the Fermi energy (EF) consist mainly of the 3dxz/yz/xy states, slightly hybridized with Se 4p states. The total DOS is 1.2 states eV−1 per Fe atom, giving a theoretical electronic specific heat coefficient γ′ = 11.3 mJ mol−1 K−2, which matches the γ obtained from our CP data. Compared with FeSe and CsFe2Se2,45,46 which have DOS at EF of 0.95 states eV−1 per

■

ASSOCIATED CONTENT

Accession Codes

CCDC 1831632 contains 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_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. F

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry *E-mail: [email protected].

(11) Li, W.; Ding, H.; Deng, P.; Chang, K.; Song, C. L.; He, K.; Wang, L. L.; Ma, X. C.; Hu, J. P.; Chen, X.; Xue, Q. K. Phase separation and magnetic order in K-doped iron selenide superconductor. Nat. Phys. 2012, 8, 126. (12) Shermadini, Z.; Krzton-Maziopa, A.; Bendele, M.; Khasanov, R.; Luetkens, H.; Conder, K.; Pomjakushina, E.; Weyeneth, S.; Pomjakushin, V.; Bossen, O.; Amato, A. Coexistence of magnetism and superconductivity in the iron-based compound Cs0.8(FeSe0.98)2. Phys. Rev. Lett. 2011, 106, 117602. (13) Zhang, Y.; Yang, L. X.; Xu, M.; Ye, Z. R.; Chen, F.; He, C.; Xu, H. C.; Jiang, J.; Xie, B. P.; Ying, J. J.; Wang, X. F.; Chen, X. H.; Hu, J. P.; Matsunami, M.; Kimura, S.; Feng, D. L. Nodeless superconducting gap in AxFe2Se2 (A = K, Cs) revealed by angle-resolved photoemission spectroscopy. Nat. Mater. 2011, 10, 273. (14) Qian, T.; Wang, X. P.; Jin, W. C.; Zhang, P.; Richard, P.; Xu, G.; Dai, X.; Fang, Z.; Guo, J. G.; Chen, X. L.; Ding, H. Absence of a holelike Fermi surface for the iron-based K0.8Fe1.7Se2 superconductor revealed by angle-resolved photoemission spectroscopy. Phys. Rev. Lett. 2011, 106, 187001. (15) Zhao, L.; Mou, D.; Liu, S.; Jia, X.; He, J.; Peng, Y.; Yu, L.; Liu, X.; Liu, G.; He, S.; Dong, X.; Zhang, J.; He, J. B.; Wang, D. M.; Chen, G. F.; Guo, J. G.; Chen, X. L.; Wang, X.; Peng, Q.; Wang, Z.; Zhang, S.; Yang, F.; Xu, Z.; Chen, C.; Zhou, X. J. Common Fermi-surface topology and nodeless superconducting gap of K0.68Fe1.79Se2 and (Tl0.45K0.34)Fe1.84Se2 superconductors revealed via angle-resolved photoemission. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 140508. (16) Ding, X. X.; Fang, D. L.; Wang, Z. Y.; Yang, H.; Liu, J. Z.; Deng, Q.; Ma, G. B.; Meng, C.; Hu, Y. H.; Wen, H. H. Influence of microstructure on superconductivity in KxFe2‑ySe2 and evidence for a new parent phase K2Fe7Se8. Nat. Commun. 2013, 4, 1897. (17) Zhao, J.; Cao, H.; Bourret-Courchesne, E.; Lee, D. H.; Birgeneau, R. J. Neutron-diffraction measurements of an antiferromagnetic semiconducting phase in the vicinity of the high-temperature superconducting state of KxFe2‑ySe2. Phys. Rev. Lett. 2012, 109, 267003. (18) Sabrowsky, H.; Rosenberg, M.; Welz, D.; Deppe, P.; Schäfer, W. A neutron and Mössbauer study of TlFexS2 compounds. J. Magn. Magn. Mater. 1986, 54−57, 1497−1498. (19) Wang, Z. W.; Wang, Z.; Song, Y. J.; Ma, C.; Cai, Y.; Chen, Z.; Tian, H. F.; Yang, H. X.; Chen, G. F.; Li, J. Q. Structural phase separation in K0.8Fe1.6+xSe2 superconductors. J. Phys. Chem. C 2012, 116, 17847−17852. (20) Liu, Y.; Wang, G.; Ying, T. P.; Lai, X. F.; Jin, S. F.; Liu, N.; Hu, J. P.; Chen, X. L. Understanding doping, vacancy, lattice stability, and superconductivity in KxFe2‑ySe2. Adv. Sci. 2016, 3, 1600098. (21) Ying, T. P.; Chen, X. L.; Wang, G.; Jin, S. F.; Lai, X. F.; Zhou, T. T.; Zhang, H.; Shen, S. J.; Wang, W. Y. Superconducting phases in potassium-intercalated iron selenides. J. Am. Chem. Soc. 2013, 135, 2951−2954. (22) Ying, T. P.; Chen, X. L.; Wang, G.; Jin, S. F.; Zhou, T. T.; Lai, X. F.; Zhang, H.; Wang, W. Y. Observation of superconductivity at 30 ∼ 46K in AxFe2Se2 (A = Li, Na, Ba, Sr, Ca, Yb, and Eu). Sci. Rep. 2012, 2, 426. (23) Guo, J. G.; Lei, H. C.; Hayashi, F.; Hosono, H. Superconductivity and phase instability of NH3-free Na-intercalated FeSe1‑zSz. Nat. Commun. 2014, 5, 4756. (24) Burrard-Lucas, M.; Free, D. G.; Sedlmaier, S. J.; Wright, J. D.; Cassidy, S. J.; Hara, Y.; Corkett, A. J.; Lancaster, T.; Baker, P. J.; Blundell, S. J.; Clarke, S. J. Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer. Nat. Mater. 2013, 12, 15. (25) Zheng, L.; Izumi, M.; Sakai, Y.; Eguchi, R.; Goto, H.; Takabayashi, Y.; Kambe, T.; Onji, T.; Araki, S.; Kobayashi, T. C.; Kim, J.; Fujiwara, A.; Kubozono, Y. Superconductivity in (NH3)yCs0.4FeSe. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 094521. (26) Jin, S. F.; Fan, X.; Wu, X. Z.; Sun, R. J.; Wu, H.; Huang, Q. Z.; Shi, C. L.; Xi, X. K.; Li, Z. L.; Chen, X. L. High-Tc superconducting phases in organic molecular intercalated iron selenides: synthesis and

ORCID

Zewen Xiao: 0000-0002-4911-1399 Xiaolong Chen: 0000-0001-8455-2117 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Z. X., T. K. and H. H. acknowledge the funding from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through the Element Strategy Initiative to Form Core Research Center. This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 51532010, 91422303, 51672306, 51772322), the National Key Research and Development Program of China (2016YFA0300604), Beijing municipal Science & Technology Commission (Grant No. Z161100002116018), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB07020100), and the Chinese Academy of Sciences (Grant No. QYZDJ-SSWSLH013).

■

REFERENCES

(1) Guo, J. G.; Jin, S. F.; Wang, G.; Wang, S. C.; Zhu, K. X.; Zhou, T. T.; He, M.; Chen, X. L. Superconductivity in the iron selenide KxFe2Se2 (0 ≤ x ≤ 1). Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 180520. (2) Wang, A. F.; Ying, J. J.; Yan, Y. J.; Liu, R. H.; Luo, X. G.; Li, Z. Y.; Wang, X. F.; Zhang, M.; Ye, G. J.; Cheng, P.; Xiang, Z. J.; Chen, X. H. Superconductivity at 32 K in single-crystalline KxFe2‑ySe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 060512. (3) Krzton-Maziopa, A.; Shermadini, Z.; Pomjakushina, E.; Pomjakushin, V.; Bendele, M.; Amato, A.; Khasanov, R.; Luetkens, H.; Conder, K. Synthesis and crystal growth of Cs0.8(FeSe0.98)2: a new iron-based superconductor with Tc = 27 K. J. Phys.: Condens. Matter 2011, 23, 052203. (4) Wang, H. D.; Dong, C. H.; Li, Z. J.; Mao, Q. H.; Zhu, S. S.; Feng, C. M.; Yuan, H. Q.; Fang, M. H. Superconductivity at 32 K and anisotropy in Tl0.58Rb0.42Fe1.72Se2 crystals. Europhys. Lett. 2011, 93, 47004. (5) Fang, M. H.; Wang, H. D.; Dong, C. H.; Li, Z. J.; Feng, C. M.; Chen, J.; Yuan, H. Q. Fe-based superconductivity with Tc = 31 K bordering an antiferromagnetic insulator in (Tl,K)FexSe2. Europhys. Lett. 2011, 94, 27009. (6) Shoemaker, D. P.; Chung, D. Y.; Claus, H.; Francisco, M. C.; Avci, S.; Llobet, A.; Kanatzidis, M. G. Phase relations in KxFe2‑ySe2 and the structure of superconducting KxFe2Se2 via high-resolution synchrotron diffraction. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 86, 184511. (7) Wang, Z.; Song, Y. J.; Shi, H. L.; Wang, Z. W.; Chen, Z.; Tian, H. F.; Chen, G. F.; Guo, J. G.; Yang, H. X.; Li, J. Q. Microstructure and ordering of iron vacancies in the superconductor system KyFexSe2 as seen via transmission electron microscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 140505. (8) Liu, R. H.; Luo, X. G.; Zhang, M.; Wang, A. F.; Ying, J. J.; Wang, X. F.; Yan, Y. J.; Xiang, Z. J.; Cheng, P.; Ye, G. J.; Li, Z. Y.; Chen, X. H. Coexistence of superconductivity and antiferromagnetism in single crystals A0.8Fe2‑ySe2 (A = K, Rb, Cs, Tl/K and Tl/Rb): Evidence from magnetization and resistivity. Europhys. Lett. 2011, 94, 27008. (9) Ye, F.; Chi, S.; Bao, W.; Wang, X. F.; Ying, J. J.; Chen, X. H.; Wang, H. D.; Dong, C. H.; Fang, M. H. Common crystalline and magnetic structure of superconducting A2Fe4Se5 (A = K, Rb, Cs, Tl) single crystals measured using neutron diffraction. Phys. Rev. Lett. 2011, 107, 137003. (10) Bao, W.; Huang, Q. Z.; Chen, G. F.; Green, M. A.; Wang, D. M.; He, J. B.; Qiu, Y. M. A novel large moment antiferromagnetic order in K0.8Fe1.6Se2 superconductor. Chin. Phys. Lett. 2011, 28, 086104. G

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry crystal structures. Chem. Commun. (Cambridge, U. K.) 2017, 53, 9729− 9732. (27) Lu, X. F.; Wang, N. Z.; Wu, H.; Wu, Y. P.; Zhao, D.; Zeng, X. Z.; Luo, X. G.; Wu, T.; Bao, W.; Zhang, G. H.; Huang, F. Q.; Huang, Q. Z.; Chen, X. H. Coexistence of superconductivity and antiferromagnetism in (Li0.8Fe0.2)OHFeSe. Nat. Mater. 2015, 14, 325. (28) Pomjakushina, E.; Conder, K.; Pomjakushin, V.; Bendele, M.; Khasanov, R. Synthesis, crystal structure, and chemical stability of the superconductor FeSe1‑x. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 024517. (29) Rodríguez-Carvajal, J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys. B (Amsterdam, Neth.) 1993, 192, 55−69. (30) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (32) Porter, D. G.; Cemal, E.; Voneshen, D. J.; Refson, K.; Gutmann, M. J.; Bombardi, A.; Boothroyd, A. T.; Krzton-Maziopa, A.; Pomjakushina, E.; Conder, K.; Goff, J. P. Two-dimensional Csvacancy superstructure in iron-based superconductor Cs0.8Fe1.6Se2. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 144114. (33) Taddei, K. M.; Sturza, M.; Chung, D. Y.; Cao, H. B.; Claus, H.; Kanatzidis, M. G.; Osborn, R.; Rosenkranz, S.; Chmaissem, O. Cesium vacancy ordering in phase-separated CsxFe2‑ySe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 094505. (34) Chen, T. K.; Chang, C. C.; Chang, H. H.; Fang, A. H.; Wang, C. H.; Chao, W. H.; Tseng, C. M.; Lee, Y. C.; Wu, Y. R.; Wen, M. H.; Tang, H. Y.; Chen, F. R.; Wang, M. J.; Wu, M. K.; Van Dyck, D. Fevacancy order and superconductivity in tetragonal β-Fe1‑xSe. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 63−68. (35) Zavalij, P.; Bao, W.; Wang, X. F.; Ying, J. J.; Chen, X. H.; Wang, D. M.; He, J. B.; Wang, X. Q.; Chen, G. F.; Hsieh, P. Y.; Huang, Q.; Green, M. A. Structure of vacancy-ordered single-crystalline superconducting potassium iron selenide. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 132509. (36) Margadonna, S.; Takabayashi, Y.; Ohishi, Y.; Mizuguchi, Y.; Takano, Y.; Kagayama, T.; Nakagawa, T.; Takata, M.; Prassides, K. Pressure evolution of the low-temperature crystal structure and bonding of the superconductor FeSe (Tc = 37 K). Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80, 064506. (37) Mizuguchi, Y.; Hara, Y.; Deguchi, K.; Tsuda, S.; Yamaguchi, T.; Takeda, K.; Kotegawa, H.; Tou, H.; Takano, Y. Anion height dependence of Tc for the Fe-based superconductor. Supercond. Sci. Technol. 2010, 23, 054013. (38) Okabe, H.; Takeshita, N.; Horigane, K.; Muranaka, T.; Akimitsu, J. Pressure-induced high-Tc superconducting phase in FeSe: Correlation between anion height and Tc. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 205119. (39) Lu, X. F.; Wang, N. Z.; Zhang, G. H.; Luo, X. G.; Ma, Z. M.; Lei, B.; Huang, F. Q.; Chen, X. H. Superconductivity in LiFeO2Fe2Se2 with anti-PbO-type spacer layers. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 020507. (40) Hibino, H.; Tanabe, S.; Mizuno, S.; Kageshima, H. Growth and electronic transport properties of epitaxial graphene on SiC. J. Phys. D: Appl. Phys. 2012, 45, 154008. (41) Sun, J. P.; Ye, G. Z.; Shahi, P.; Yan, J. Q.; Matsuura, K.; Kontani, H.; Zhang, G. M.; Zhou, Q.; Sales, B. C.; Shibauchi, T.; Uwatoko, Y.; Singh, D. J.; Cheng, J. G. High-Tc superconductivity in FeSe at high pressure: dominant hole carriers and enhanced spin fluctuations. Phys. Rev. Lett. 2017, 118, 147004. (42) Watson, M. D.; Yamashita, T.; Kasahara, S.; Knafo, W.; Nardone, M.; Béard, J.; Hardy, F.; McCollam, A.; Narayanan, A.; Blake, S. F.; Wolf, T.; Haghighirad, A. A.; Meingast, C.; Schofield, A. J.; v. Löhneysen, H.; Matsuda, Y.; Coldea, A. I.; Shibauchi, T. Dichotomy between the hole and electron behavior in multiband superconductor FeSe probed by ultrahigh magnetic fields. Phys. Rev. Lett. 2015, 115, 027006.

(43) Ding, X. X.; Pan, Y. M.; Yang, H.; Wen, H. H. Strong and nonmonotonic temperature dependence of Hall coefficient in superconducting KxFe2‑ySe2 single crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 89, 224515. (44) Yan, X. W.; Gao, M.; Lu, Z. Y.; Xiang, T. Electronic and magnetic structures of the ternary iron selenides AFe2Se2 (A = Cs, Rb, K, or Tl). Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 054502. (45) Nekrasov, I. A.; Sadovskii, M. V. Electronic structure, topological phase transitions and superconductivity in (K,Cs)xFe2Se2. JETP Lett. 2011, 93, 166. (46) Subedi, A.; Zhang, L. J.; Singh, D. J.; Du, M. H. Density functional study of FeS, FeSe, and FeTe: Electronic structure, magnetism, phonons, and superconductivity. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 134514. (47) Chen, F.; Xu, M.; Ge, Q. Q.; Zhang, Y.; Ye, Z. R.; Yang, L. X.; Jiang, J.; Xie, B. P.; Che, R. C.; Zhang, M.; Wang, A. F.; Chen, X. H.; Shen, D. W.; Hu, J. P.; Feng, D. L. Electronic identification of the parental phases and mesoscopic phase separation of KxFe2‑ySe2 superconductors. Phys. Rev. X 2011, 1, 021020. (48) Chen, Z. G.; Yuan, R. H.; Dong, T.; Xu, G.; Shi, Y. G.; Zheng, P.; Luo, J. L.; Guo, J. G.; Chen, X. L.; Wang, N. L. Infrared spectrum and its implications for the electronic structure of the semiconducting iron selenide K0.83Fe1.53Se2. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 220507. (49) Wen, C. H. P.; Xu, H. C.; Chen, C.; Huang, Z. C.; Lou, X.; Pu, Y. J.; Song, Q.; Xie, B. P.; Abdel-Hafiez, M.; Chareev, D. A.; Vasiliev, A. N.; Peng, R.; Feng, D. L. Anomalous correlation effects and unique phase diagram of electron-doped FeSe revealed by photoemission spectroscopy. Nat. Commun. 2016, 7, 10840. (50) Yin, Z. P.; Haule, K.; Kotliar, G. Kinetic frustration and the nature of the magnetic and paramagnetic states in iron pnictides and iron chalcogenides. Nat. Mater. 2011, 10, 932. (51) Sun, H.; Woodruff, D. N.; Cassidy, S. J.; Allcroft, G. M.; Sedlmaier, S. J.; Thompson, A. L.; Bingham, P. A.; Forder, S. D.; Cartenet, S.; Mary, N.; Ramos, S.; Foronda, F. R.; Williams, B. H.; Li, X.; Blundell, S. J.; Clarke, S. J. Soft Chemical Control of Superconductivity in Lithium Iron Selenide Hydroxides Li1−xFex(OH)Fe1−ySe. Inorg. Chem. 2015, 54, 1958−1964.

H

DOI: 10.1021/acs.inorgchem.8b00179 Inorg. Chem. XXXX, XXX, XXX−XXX