Article pubs.acs.org/IC
K2FeGe3Se8: A New Antiferromagnetic Iron Selenide Kai Feng,†,‡,§ Wendong Wang,∇ Ran He,†,‡,§ Lei Kang,†,‡,§ Wenlong Yin,†,‡,§ Zheshuai Lin,†,‡ Jiyong Yao,*,†,‡ Youguo Shi,*,⊥ and Yicheng Wu†,‡ †
Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Key Laboratory of Functional Crystals and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ∇ School of Science, Beijing University of Post and Telecommunication, Beijing 100876, People’s Republic of China ⊥ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China S Supporting Information *
ABSTRACT: A new iron selenide, K2FeGe3Se8, has been obtained by spontaneous crystallization. It adopts a new structure type in the noncentrosymmetric monoclinic space group P21. In the structure, FeSe4 and GeSe4 tetrahedra are connected alternately via corner-sharing to form one-dimensional (1D) 1∞[FeGeSe6]6− chains along the a-direction. These chains are further linked by sharing Ge2Se6 units to generate two-dimensional (2D) 2∞[FeGe3Se8]2− layers stacked parallel to the ac-plane and separated by K+ cations. Deduced from temperature-dependent susceptibility measurement and specific heat measurement under different magnetic fields, K2FeGe3Se8 exhibits an antiferromagnetic transition at ∼10 K. Furthermore, the magnetic property shows anisotropy between directions parallel and perpendicular to the plane of 2∞[FeGe3Se8]2− layer. The diffuse reflectance spectra measurement indicates that the band gap of K2FeGe3Se8 is ∼1.95(2) eV, consistent with the calculated values of 1.80 and 1.53 eV in the spin-up and spin-down directions, respectively. Based on electronic structure calculation, the spin of the Fe2+ cation is 1.85ℏ, which is comparable to the experimental value.
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INTRODUCTION
superconductivity in the Fe-based materials is unconventional and non-BCS-type.19−21 The fascinating properties of these Fe-based superconductors not only stimulate tremendous interest in optimizing experimental conditions to increase the Tc and in elucidating the electronic structure and superconducting mechanism, but also inspire intensive work in the exploratory synthesis of new Fe-based (oxy)pnictide and chalcogenides. Actually, the discovery of the superconductivity in the LaFePnO (Pn = pnictogen) clearly demonstrates the important role of exploratory synthesis in the development of modern materials science: without the initial synthesis and structural determination of the LaFePnO compounds in 1995,22 no superconductivity could be found in Fe-based (oxy)pnictide and chalcogenides in ∼10 years later.1 With great efforts, several new Fe-based (oxy)pnictide and chalcogenides has been synthesized, which show interesting structures and physical properties. For example, CaFe4As3 possesses an open threedimensional framework built from FeAs4 tetrahedra and exhibits metallic behavior with significant anisotropy and
Since the discovery of superconductivity in the quaternary Febased oxypnictide LaFePnO (Pn = pnictogen), there has been worldwide interest in Fe-based (oxy)pnictide and chalcogenides.1−10 Until now, various Fe-based superconductors, such as the ZrCuSiAs-type oxypnictide LnFeAsO (Ln = rare-earth elements),1,4,5 the ThCr2Si2-type AFe2As2 pnictide (A = alkaline-earth),2,6 the Fe2As-type AFeAs pnictide (A = Li, Na),11,12 the anti-PbO-type chalcogenide FeSe,3 and the intercalated chalcogenide AFe2Se2 (A = alkali-metal),8−10,13 have been discovered and extensively studied, both experimentally and theoretically. The highest reported Tc = 55 K for iron-based superconductivity was achieved in oxygen-deficient SmFeAsO prepared via high-pressure synthesis.5 These Febased superconductors exhibit fascinating interplay between the superconductivity and magnetism. A typical example is the AFe2As2 (A = alkaline-earth) pnictide system: the antiferromagnetic (AFM) order is suppressed and superconductivity is induced by either K doping in the Ba or Sr sites,2,14 or by Co and Ni doping in the Fe sites,15−17 or by applying pressure.18 The high-critical-temperature (high-Tc) values, along with the proximity of a magnetically ordered state, suggest that the © 2013 American Chemical Society
Received: November 1, 2012 Published: January 28, 2013 2022
dx.doi.org/10.1021/ic302394e | Inorg. Chem. 2013, 52, 2022−2028
Inorganic Chemistry
Article
complex magnetic behavior,23 FexPb4−xSb4Se10 represents a new class of ferromagnetic semiconductors with quasi-onedimensional (quasi-1D) ladders,24 and the RE12Fe57.5As41 (RE = La, Ce) compounds show ferromagnetic ordering at 125 and 95 K for the La and Ce compounds, respectively.25 To fully understand the structure−property relationship in solid-state chemistry and materials science, a materials database with a large number of related compounds is of great importance. In this context, we explored the A−Fe−M−Q system (A = alkali metal; M = Group IV element, Q = S, Se, Te) and discovered a new compound: K2FeGe3Se8. In this work, we report the synthesis, structure, optical, specific heat, magnetic properties, and electronic structure calculations of K2FeGe3Se8. This compound adopts a new structure type in the monoclinic space group P21 and exhibits an antiferromagnetic transition with a Neel temperature (TN) of ∼10 K. The calculated spin of Fe2+ is 1.85ℏ, consistent with the experimental value. In addition, the calculated band gap is 1.80 and 1.53 eV in spin-up and spin-down directions, which is also consistent with the experiment.
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Table 1. Crystal Data and Structure Refinements for K2FeGe3Se8 parameter
value/comment K2FeGe3Se8 983.500
chemical formula formula weight, fw uni-cell dimensions a b c β V space group Z T λ ρc μ R(F)a RW(Fo2)b
7.4088(15) Å 12.268(3) Å 34.974(7) Å 96.04(3)° 3161.3(11) Å3 P21 8 153(2) K 0.71073 Å 4.133 g/cm3 25.460 cm−1 0.0799 0.2030
R(F) = ∑ ||Fo | − |Fc||/∑|Fo| for Fo2 > 2σ(Fo2). bRw(Fo2) = {∑ [w(Fo2 − Fc2)2]/∑wFo4}1/2 for all data. w−1 = σ2(Fo2) + (z P)2, where P = (Max(Fo2, 0) + 2Fc2)/3. a
EXPERIMENTAL SECTION
Crystal Growth. A mixture of K2Se:FeSe:3GeSe2, based on molar ratio, was loaded into a fused-silica tube under an argon atmosphere in a glovebox. The tube was sealed under 10−3 Pa atmosphere and then placed in a computer-controlled furnace. The sample was heated to 1173 K in 24 h and kept for 48 h, then cooled at a slow rate of 4 K/h to 673 K, and finally cooled to room temperature. The product consisted of thin dark-red plates of K2FeGe3Se8, which were manually selected for structure characterization. Analysis of the crystal with an EDX-equipped Hitachi Model S-3500 scanning electron microscopy (SEM) system showed the presence of K, Fe, Ge, and Se in the approximate molar ratio of 2:1:3:8. The crystals are stable in air. Structure Determination. Single-crystal X-ray diffraction data were collected with the use of graphite-monochromatized Mo Kα (λ = 0.71073 Å) at a temperature of 153 K on a Rigaku Model AFC10 diffractometer equipped with a Saturn CCD detector. Crystal decay was monitored by re-collecting 50 initial frames at the end of data collection. The collection of the intensity data was carried out with the program CrystalClear.26 Cell refinement and data reduction were carried out with the use of the program CrystalClear,26 and faceindexed absorption corrections were performed numerically with the use of the program XPREP.27 The structure was solved with the direct methods program SHELXS and refined with the least-squares program SHELXL of the SHELXTL.PC suite of programs.27 Because of the strong layer tendency in the structure, the selected thin plate-shaped crystal was heavily twinned with fractional contributions of domains being 0.34, 0.33, 0.27, and 0.05. The high linear absorption coefficient (25.46 mm−1) and the crystal twinning make the absorption correction difficult, which may explain why the final R/wR2 values were relatively high. Additional experimental details are given in Table 1, and selected metric data are given in Table 2. Further information may be found in the Supporting Information. Diffuse Reflectance Spectroscopy. A Cary Model 5000 ultraviolet−visible light−near-infrared (UV-vis-NIR) spectrophotometer with a diffuse reflectance accessory was used to measure the spectrum of K2FeGe3Se8 over the range from 300 nm (4.13 eV) to 1500 nm (0.83 eV). Magnetic Susceptibility Measurements. The temperature dependence of the magnetic susceptibility was measured on single crystals of K2FeGe3Se8 (6.3 mg for H//ac and 10.8 mg for H//b) in magnetic fields of 1, 10, and 50 kOe with the use of a superconducting quantum interference device (SQUID) magnetometer (Model MPMS7T, Quantum Design). The zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities were obtained on crystals in the temperature range of 2−300 K. The field dependence of
Table 2. Selected Interatomic Distances for K2FeGe3Se8 atom pair
bond distance (Å)
atom pair
bond distance (Å)
Fe1−Se7 Fe1−Se6 Fe1−Se2 Fe1−Se5 Fe2−Se4 Fe2−Se9 Fe2−Se21 Fe2−Se22 Fe3−Se30 Fe3−Se23 Fe3−Se32 Fe3−Se27 Fe4−Se19 Fe4−Se31 Fe4−Se29 Fe4−Se26 Ge1−Se22 Ge1−Se24 Ge1−Se8 Ge1−Se10 Ge2−Se21 Ge2−Se17 Ge2−Se15 Ge2−Se16 Ge3−Se23 Ge3−Se18 Ge3−Se8 Ge3−Se10 Ge12−Se2 Ge12−Se14 Ge12−Se3 Ge12−Se28
2.425(5) 2.454(6) 2.474(5) 2.485(6) 2.379(6) 2.417(6) 2.493(5) 2.501(6) 2.416(6) 2.439(5) 2.467(6) 2.492(5) 2.455(6) 2.464(6) 2.470(6) 2.474(5) 2.300(5) 2.353(5) 2.392(4) 2.403(5) 2.291(5) 2.372(5) 2.382(6) 2.403(4) 2.274(5) 2.351(4) 2.389(5) 2.407(4) 2.307(5) 2.377(6) 2.379(5) 2.388(4)
Ge4−Se5 Ge4−Se6 Ge4−Se3 Ge4−Se1 Ge5−Se26 Ge5−Se15 Ge5−Se16 Ge5−Se13 Ge6−Se27 Ge6−Se12 Ge6−Se28 Ge6−Se14 Ge7−Se19 Ge7−Se25 Ge7−Se20 Ge7−Se11 Ge8−Se32 Ge8−Se30 Ge8−Se12 Ge8−Se18 Ge9−Se31 Ge9−Se29 Ge9−Se13 Ge9−Se25 Ge10−Se7 Ge10−Se1 Ge10−Se20 Ge10−Se11 Ge11−Se9 Ge11−Se4 Ge11−Se17 Ge11−Se24
2.308(5) 2.358(5) 2.377(5) 2.404(4) 2.295(5) 2.375(4) 2.385(6) 2.388(4) 2.322(5) 2.367(4) 2.380(6) 2.388(4) 2.292(5) 2.358(5) 2.393(6) 2.393(4) 2.307(5) 2.325(5) 2.389(5) 2.426(4) 2.297(5) 2.335(5) 2.392(5) 2.428(5) 2.312(5) 2.342(5) 2.383(4) 2.403(6) 2.280(5) 2.307(5) 2.427(5) 2.464(5)
magnetization was measured at 2, 5, 10, 30, and 300 K by changing the applied magnetic field between −50 kOe and 50 kOe. 2023
dx.doi.org/10.1021/ic302394e | Inorg. Chem. 2013, 52, 2022−2028
Inorganic Chemistry
Article
Specific Heat Measurement. Specific heat capacity (Cp) versus temperature (T) of K2FeGe3Se8 was recorded at 2−300 K (ZFC) and 0−15 K for 20, 40, 70, and 90 kOe magnetic fields by a pulse relaxation method, using a Quantum Design Model PPMS calorimeter. Electronic Structure Calculations. The first-principles calculation for K2FeGe3Se8 was performed by the plane-wave pseudopotential method based on the density functional theory (DFT).28,29 The CASTEP package30 was employed to determine the electronic structures, including total density of states (DOS) and partial DOS (PDOS). The ion−electron interactions were modeled by the optimized ultrasoft pseudo-potentials31 for elements in the compound. The adopted density functional is an additional on-site orbitaldependent correlation Hubbard U (LDA + U), and the DOS presented here are those adopting U values of 2.5 eV only for Fe2+. In addition, the magnetic moment of the crystal is determined by the spin-polarized calculations.32 The kinetic energy cutoffs of 500 eV and Monkhorst−Pack k-point meshes with the spanning of
