Structure Evolution and Spin-Glass Transition of Layered Compounds

Oct 9, 2017 - The elongated Na–Se bond length would destabilize the ThCr2Si2-type structure in NaLiFeSe2, suggesting that NaxFe2–ySe2 lies at the ...
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Structure Evolution and Spin-Glass Transition of Layered Compounds ALiFeSe2 (A = Na, K, Rb) Duanduan Yuan,†,‡ Ning Liu,†,‡ Kunkun Li,†,‡ Shifeng Jin,†,§ Jiangang Guo,† and Xiaolong Chen*,†,§,∥ †

Research & Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 101408, People’s Republic of China ∥ Collaborative Innovation Center of Quantum Matter, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: Three new layered compounds, namely NaLiFeSe2, KLiFeSe2, and RbLiFeSe2, have been discovered. NaLiFeSe2 adopts a trigonal CaAl2Si2-type structure with space group P3̅m1, while the other two possess a tetragonal ThCr2Si2-type structure with space group I4/mmm. Structural refinements reveal that Li and Fe atoms randomly occupy the same sites in all these compounds without ordering. It is found that the radius of the alkali metals plays a vital role in determining the symmetry of this series of compounds. The substitution of Li at the Fe site shortens the layer spacing and elongates the A−Se bond length in the ThCr2Si2-type structure. The elongated Na−Se bond length would destabilize the ThCr2Si2-type structure in NaLiFeSe2, suggesting that NaxFe2−ySe2 lies at the border of ThCr2Si2-type and CaAl2Si2-type structures. Magnetic and resistivity measurements demonstrate that these compounds exhibit anisotropic spin-glass and narrow-band-gap semiconducting characteristics. First-principles calculations indicate that the introduction of Li enhances strong localization and weakens the correlation of the 3d electrons of Fe, which are responsible for the observed spin-glass transition and semiconducting conductions.

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INTRODUCTION It is known that the intercalation of alkali metals between [Fe2Se2] layers enhances TC from 8 K in binary β-FeSe to ∼30 K in ternary AxFe2−ySe2 (A = Na, K, Rb, Cs, Tl).1−5 Since [Fe2Ch2] (Ch = S, Se, Te) layers are electrically neutral, ternary compounds AxFe2−ySe2 are usually chemically nonstoichiometric. The existence of vacancies in A sites and Fe sites is a common feature, resulting in considerable influence on crystal structure, magnetic interactions, and superconductivity.6−11 On the other hand, Ag+ and Cu+ prefer to enter into the [Fe2Ch2] layers by substituting Fe, forming new compounds that exhibit multifarious properties.12−16 It has been reported that KFe 1.05 Ag 0.88 Te 2 features spin-glass behavior, while KFe0.85Ag1.15Te2 and KCuFeTe2 feature long-range antiferromagnetic order, demonstrating that the magnetic ground state is very sensitive to subtle changes in nonmagnetic cations, Fe cations, and defects.12,15,16 Unlike these cations with +1 valence, Li atoms can not only intercalate between [Fe2Ch2] layers, as in LiCuFeS217 and LiCuFeSe2,18 but also substitute Fe, as in LixFe7Se8 (0 ≤ x ≤ 1).19 It is evidenced that doping Li into Fe7Se8 destroys the superstructure of Fe7Se8 and hence suppresses its spin ordering, resulting in a disorder-induced metal−insulator transition.19 Since Li orbitals often sink down © XXXX American Chemical Society

from the Fermi level, Li does mimic an Fe vacancy in the electronic structure. From the point of view of charge equilibrium, substitution of Fe by Li provides an approach to obtain new vacancy-free compounds. Thus, the emergent magnetic properties and insulating states may appear in Fe vacancy-free ALiFeSe2 materials. In this contribution, we report the discovery of three compounds: namely, NaLiFeSe2 with a trigonal CaAl2Si2-type structure and KLiFeSe2 and RbLiFeSe2 with a tetragonal ThCr2Si2-type structure. The introduction of Li can exclusively enter into the Fe sites, without substituting the alkali metals. The ionic radius of alkali metals plays a vital role in determining the transition from trigonal to tetragonal. Resistivity and magnetic measurements on single crystals and powder samples suggest semiconducting and spin-glass ground states in these compounds. They are semiconductors with a band gap of 0.1− 0.26 eV. The localized spins and expanded lattice cells with increasing ionic radius are thought to be responsible for their conduction behavior. Received: August 1, 2017

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DOI: 10.1021/acs.inorgchem.7b01937 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a) XRD patterns of KLiFeSe2 (left) and RbLiFeSe2 (right) single crystals, respectively. The insets show photographs of the crystals (length scale 2 mm). (b−d) Powder X-ray diffraction and Rietveld refinement profiles of KLiFeSe2, RbLiFeSe2, and NaLiFeSe2 at room temperature, respectively. The insets show the schematic crystal structures.

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EXPERIMENTAL DETAILS

RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of KLiFeSe2 (left) and RbLiFeSe2 (right) single crystals, respectively. The peaks are sharp and well-defined, indicating the good crystalline quality. Only (00l) (l = 2n) reflections were recognized, suggesting that the crystallographic c axis is perpendicular to the plane of the single crystal. Figure 1b shows the PXRD pattern of KLiFeSe2. All of the reflections can be well indexed on the basis of a tetragonal cell with lattice parameters a = 4.1044(2) Å and c = 13.7194(9) Å. No peaks due to impurities were detected. Examination of diffraction extinction reveals that KLiFeSe2 crystallizes in a body-centered lattice with the probable space group I4/mmm, implying that it adopts a ThCr2Si2-type structure, isostructural with KxFe2−ySe21. Rietveld refinements with Li+ atoms occupying K+ sites failed to give reliable results. Rietveld refinements were then performed against the experimental data by taking KxFe2−ySe21 as the starting structural model with Li+ and Fe2+ random occupation of the 4d site. The refinements smoothly converged to Rp = 2.31%, Rwp = 3.19%, and χ2 = 2.38%. A summary of the crystallgraphic data is complied in Table S1 in the Supporting Information. The crystal structure of KLiFeSe2, as shown schematically in the inset of Figure 1b, consists of alternate stacking of antifluorite-type Se-Fe/Li-Se layers and spacer layers: namely, K cations. In comparison with the structural data for KxFe2−ySe21, the intralayer Fe/Li−Fe/Li distance is stretched by 4.8%, as evidenced by lattice constant a increasing from 3.9136(1) Å to 4.1044(2) Å. This is due to the longer Li−Se bond length (2.596 Å) in comparison to that of Fe−Se (2.441 Å) in KxFe2−ySe21, indicating a more ionic character for Li−Se bonds. The interlayer spacing of two neighboring Fe/Li−Fe/Li layers is compressed by 2.3%, from 7.0184(4) Å to 6.8597(5) Å, which can be explained by stronger Coulomb forces between the stacking layers due to the −1 valence state of [LiFeSe2] layers through doping of Li+ at Fe2+ sites. Figure 1c shows the PXRD pattern of RbLiFeSe2. Similar to the case for KLiFeSe2, it is reasonable that RbLiFeSe2 is

Polycrystalline samples of ALiFeSe2 (A = Na, K, Rb) were synthesized via conventional solid-state methods. Stoichiometric amounts of powder Fe, powder Se, and small lumps of alkalis with preprepared Li2Se were mixed and loaded into Al2O3 crucibles. Then, the alumina crucibles were sealed within evacuated quartz tubes back-filled with 0.2 atm of argon. All manipulations were conducted in a glovebox. The quartz tubes were slowly heated to 773 K, kept at this temperature for 10 h, and then cooled to room temperature. The obtained samples were pulverized, pressed into a pellet, sealed in a quartz tube with Ar gas, and then heated and kept at 873 K for 40 h. The obtained polycrystalline samples were dark black and air-sensitive; therefore, great care was taken in specimen preparation for characterization. Single crystals of KLiFeSe2 and RbLiFeSe2 were grown by a self-flux method as described in ref 1. The KLiFeSe2 and RbLiFeSe2 samples were slowly heated to 1193 and 1253 K, respectively, kept at the temperature for 2 h, and then cooled to 1073 K at a rate of 3 K/h. Platelike crystals with size up to 4 × 4 × 0.5 mm3 were obtained. 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.20 The magnetic susceptibilities were measured using a vibrating sample magnetometer (VSM, Quantum Design). Electrical resistivity measurements were performed by a standard four-probe method on a physical property measurement system (PPMS, Quantum Design). First-principles calculations were performed using the CASTEP program code21 with plane-wave pseudopotential method, on the basis of density functional theory. We adopted the generalized gradient approximation with Perdew−Burke−Ernzerhof formula for the exchange-correlation potentials.22 The ultrasoft pseudopotential with a plane-wave cutoff energy of 410 eV and a Monkhorst−Pack k-point separation of 0.04 Å−1 in the reciprocal space were used for the calculations.23 The self-consistent field was set as 5 × 10−7 eV/atom. Structural optimization was performed by the BFGS method24 with the convergence standard given as follows: energy change less than 5 × 10−6 eV/atom, residual force less than 0.01 eV/Å, and displacement of atom less than 5 × 10−4 Å. Only atomic positions were optimized, while lattice parameters were taken from experimental data. B

DOI: 10.1021/acs.inorgchem.7b01937 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Structural evolution of AxFe2−yCh2 and ALiFeSe2 (A = K, Na; Ch = S, Se). The schematic structure of KxFe2−ySe21 is the average structure without considering vacancy ordering. The structure parameters of FeSe, NaxFe2−ySe2, and NaFe1.6S2 are adopted from refs 26, 27, and 25, respectively.

NaLiFeSe2 adopts a CaAl2Si2-type structure on the grounds of the same mechanism for NaFe1.6S2. A detailed comparison of the structure parameters of AxFe2−yCh2 and ALiFeSe2 is given in Figure 2. To simplify the problem, the average structure of KxFe2−ySe21 is shown without considering vacancy ordering. The structural parameters of NaxFe2−ySe2 are adopted from ref 27, where the superconducting phase is obtained by evaporating NH3 from Na0.80(NH3)0.6Fe1.86Se2. It can be seen that both KxFe2−ySe21 and KLiFeSe2 adopt a ThCr2Si2-type structure. The layer distance is compressed from 4.1015(2) Å in KxFe2−ySe21 to 3.9869 (3) Å in KLiFeSe2 due to stronger Coulomb forces between [LiFeSe2]− layers and K+ layers. The a axis of KLiFeSe2 is 4.1044(1) Å, larger than the 3.9136(1) Å in KxFe2−ySe21, resulting in a greater K−Se distance, 3.5209(2) Å, in KLiFeSe2 in comparison to 3.4443(4) Å in KxFe2−ySe21. Meanwhile, NaxFe2−ySe227 adopts a ThCr2Si2-type structure, while NaLiFeSe2 and NaFe1.6S225 adopt a CaAl2Si2-type structure. Such structure evolution is accompanied by shortening of the Na−Ch distance from 3.2446(3) Å in NaxFe2−ySe227 to 3.0445(1) Å in NaLiFeSe2 and 2.8603 Å in NaFe1.6S2.25 Since r(Na+) = 1.02 Å, r(K+) = 1.38 Å, r(S2−) = 1.84 Å, and r(Se2−) = 1.98 Å, the A−Ch distances are close to the sum of r(A+) and r(Ch2−), resulting in strong Coulomb forces. Otherwise, supposing that NaLiFeSe2 attained a ThCr2Si2-type structure, the Na−Se distance would be too large to maintain Coulomb forces strong enough to stabilize the structure. This provides a simple and visualized way to explain the structural evolution of ALiFeSe2. It is reported that the ThCr2Si2-type structure strongly competes with the CaAl2Si2-type structure, where the CaAl2Si2type structure is preferred by smaller alkali-metal intercalation and the ThCr2Si2-type structure is preferred by larger alkali-

isostructural with KLiFeSe2 and also adopts a ThCr2Si2-type structure, as shown in the inset of Figure 1c. The final agreement factors converge to Rp = 2.42%, Rwp = 3.39%, and χ2 = 2.43%. The refined lattice parameters are a = 4.1609(1) Å and c = 14.0420(5) Å, which are reasonably larger than those of KLiFeSe2, due to the larger size of Rb+ (r = 1.52 Å) in comparison to K+ (r = 1.38 Å). In comparison with RbxFe2−ySe24, the value of the a axis parameter is elongated while the c axis parameter is shortened. Although Na0.8Fe1.6Se2 is isostructural with K0.8Fe1.6Se2 and crystallizes in a tetragonal structure with ordering of Fe vacancies within the ab plane,5 the room-temperature PXRD pattern of NaLiFeSe2 cannot be indexed on the basis of a tetragonal cell but rather on a trigonal cell. It probably adopts space group P3̅m1, i.e. a CaAl2Si2-type structure, isostructural with NaFe1.6S2.25 The structure of NaFe1.6S2 was used as a starting model for Rietveld refinements against the raw data, along with ∼4 mol % of Na3Fe2Se4 as an impurity phase. Shown in Figure 1d are the final Rietveld refinement profiles, with agreement factors Rp = 2.45%, Rwp = 3.18%, and χ2 = 1.92%, respectively. The refined lattice parameters are a = 4.16089(7) Å and c = 7.0710(1) Å, larger than those of NaFe1.6S2 (a = 3.8557(3) Å and c = 6.7928(7) Å), due to the larger size of Se2− in comparison to S2−. It was previously demonstrated that the ThCr2Si2-type structure for NaFe1.6S2 is dynamically unstable due to the large displacement of Na atoms. Instead, there is more overlap between Na and S orbitals if it adopts the CaAl2Si2-type structure, which will stabilize the structure.25 Similarly, supposing NaLiFeSe2 adopted a ThCr2Si2-type structure, the Na−Se bond length would be too large to maintain such a structure. This will lead to a larger displacement of Na atoms and less overlap between Na and Se orbitals, finally resulting in structural instability.25 We infer that C

DOI: 10.1021/acs.inorgchem.7b01937 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry metal intercalation.28 As previously reported, ThCr2Si2-type NaxFe2−ySe2 can be synthesized through a solid-state reaction5 or by evacuating NH3 from Nax(NH3)zFe2−ySe2.27 In the present case, it is found that the substitution of Li at Fe sites shortens the layer spacing and elongates A−Se bond length in the ThCr2Si2-type structure. The elongated Na−Se bond length would destabilize the ThCr2Si2-type structure in NaLiFeSe2. We can infer that NaxFe2−ySe2 is at the border of ThCr2Si2-type and CaAl2Si2-type structures, which explains why NaxFe2−ySe2 is synthesized at lower temperature in comparison to KxFe2−ySe2 and the superconducting phase NaxFe2−ySe2 is difficult to obtain through a solid-state reaction.5 Figure 3a shows the temperature dependence of FC and ZFC magnetization for single crystals of KLiFeSe2 with

isothermal M(H) curves under the H∥c axis. At 300 K, the M− H loop is linear and no hysteresis is observed. However, a small hysteresis loop is observed at 10 K and there is no sign of saturation up to 5 T. Figure 3c displays the variation of the real part of ac susceptibility χ′(T) for KLiFeSe2 at several frequencies as a function of temperature. As the frequency increases, the peak of χ′(T) shifts toward higher temperature, which is a typical behavior of a spin glass. By fitting the frequency dependence of the peak shift using K = ΔTf/(Tf Δ log f), we obtained K = 0.0257(5), as shown in the inset of Figure 3c. This is in agreement with values (0.0045 ≤ K ≤ 0.08) found in canonical SG systems.29 The magnetic susceptibility in the ab plane is larger than that along the c axis, and the freezing temperature, 9.5 K, is much lower than that along the c axis. This feature is similar to the observed magnetic anisotropy in KMnAgSe2,30 suggesting a moderate anisotropy in spin freezing. The inset in Figure 3b shows the isothermal M(H) curves for the H∥ab plane. No hysteresis loop is observed at 10 and 300 K, consistent with the anisotropy shown in ZFC and FC curves. The magnetic properties of RbLiFeSe2 are similar to those of KLiFeSe 2, as shown in Figure S1 in the Supporting Information. The freezing temperatures along the c axis and ab plane are 38.2 and 8.6 K, respectively. An applied field of 1 T along the c axis wipes out the sharpness of this transition. An obvious hysteresis loop is observed at 10 K, and no hysteresis loop at 300 K is seen along the c axis, verifying a spin-glass-like behavior, as shown in the inset of Figure S1a. Figure S1c shows the temperature dependence of FC and ZFC magnetization curves for a NaLiFeSe2 powder sample at 0.1 and 1 T, respectively. It can be seen that there is a pronounced FC− ZFC irreversibility and a cusp in the ZFC curve at 38.5 K and a large applied field smears out the cusp to a broad maximum. A small but clear shift of FC beginning at ∼180 K downward is observed, probably due to the impurity phase Na3Fe2Se4. Similarly, there is an obvious hysteresis loop at 10 K and no hysteresis loop at 300 K, as shown in the inset of Figure S1c. Figure 4 shows the resistivity versus temperature (ρ−T) curves of ALiFeSe2 measured under zero field. As the temperature decreases, ρ(T) increases rapidly, indicating that all three compounds exhibit semiconducting-like behavior in the measured temperature range. The electrical resistivity at lower temperatures is beyond the measurement limit of the instrument. The in-plane room-temperature resistivity ρab(300 K) of KLiFeSe2 is around 108 Ω cm, higher than those of KFe1.05Ag0.88Te2 (ρab(300 K) = 1 Ω cm)12 and KxFe2−yS2 (ρab(300 K) = 0.1 Ω cm).31 The ρab(T) data from 165 to 400 K can be fitted by the thermal activation (TA) model ρ = ρ0 exp(Ea/kBT), where ρ0 is a prefactor, Ea is the activation energy, and kB is Boltzmann’s constant. The fitted activation energy Ea is 209(1) meV for KLiFeSe2, which is 1 order of magnitude larger than those of KFe1.05Ag0.88Te2 (Ea = 43(2) meV) and KxFe2−yS2 (Ea = 51.8(2) meV). Such a semiconducting behavior might be ascribed to the fact that all anions and cations have full electron shells, and no extra carriers are present in KLiFeSe2. Similarly, for RbLiFeSe2, ρab(300 K) = 122 Ω cm and Ea = 260.6(8) meV on the basis of TA model fitting of resistivity data from 180 to 400 K. For NaLiFeSe2, ρ(300 K) = 12.1 Ω cm and Ea = 107.8(4) meV on the basis of TA model fitting of resistivity data from 100 to 400 K. It can be noted that all three compounds are narrow band gap semiconductors. The conductivity of the three compounds decreases as the lattice cell becomes larger from NaLiFeSe2 to

Figure 3. Magnetic properties of KLiFeSe2. (a) Zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibilities for H∥c axis at H = 0.1 and 1 T, respectively. Inset: M−H loops for H∥c axis at 10 and 300 K. (b) Temperature dependence of M(T) for H∥ab plane at H = 1 T. Inset: M−H loops for H∥ab plane at 10 and 300 K. (c) Temperature dependence of χ′(T) measured at several fixed frequencies for KLiFeSe2 powder. Inset: frequency dependence of Tf. The solid line is the linear fit to the Tf data.

different dc magnetic fields applied along the c axis. A pronounced FC−ZFC irreversibility and a cusp in the ZFC curve at Tf ≈ 47.5 K are observed. A large applied field smears out the cusp to a broad maximum. Below the Tf value, the magnitude of χZFC drops steeply while χFC tends to level off with decreasing temperature. The inset in Figure 3a shows the D

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localization, namely schemes iii and v, indicating that the spin localization is a prominent factor while the spin orientations have little influence on the electronic structure. It is speculated that KLiFeSe2 is a semiconductor with strong localization of 3d electrons of Fe. First-principles calculation results indicate that KLiFeSe2 is a semiconductor with calculated Eg = 1.01 eV, as shown in Figure 5. The valence band is contributed by Se 4p

Figure 5. Calculated total/partial DOS of randomly spin polarized KLiFeSe2 with LDA + U correction (U = 2.5 eV).

Figure 4. Temperature dependence of (a) in-plane resistivity ρab(T) of KLiFeSe2 single crystal, (b) in-plane resistivity ρab(T) of RbLiFeSe2 single crystal, and (c) ρ(T) of polycrystalline NaLiFeSe2. Insets: ln ρ vs 1/T. The solid lines are the fitting curves using thermal activation model fitting.

orbitals and Fe 3d orbitals, while the conduction band is mainly from the Fe 3d orbitals. Similar results were also obtained in NaLiFeSe2 and RbLiFeSe2, as shown in Figure S3 in the Supporting Information. The overestimation of band gap in comparison with the experimental value might be related to the inaccurate U in the LDA + U model.16,32 Cu and Ag are the most commonly used candidates to substitute Fe in compounds AFe2Ch2, as summarized in Table 1. It is found that both tellurides containing Cu and Ag show lower room-temperature resistivity and activation energies, which might be due to the contribution of Te 5p orbitals in the valence band.16 When ALiFeSe2 is compared with ACuFeS2,33 the combination of less contribution of Li 1s orbitals but more contribution of Se 4p orbitals also supports this point. In addition, the electron correlation effect is nontrivial in the FeSe-based family due to the significant Fe−Fe interaction. Typical Fe-based Mott insulators are the iron oxychalcogenides La2O2Fe2O(Se,S)2, which contain an Fe square lattice with an expanded unit cell, resulting in enhanced correlation effects through band narrowing.35 KCuFeTe2 was reported to be a Mott insulator with strong localization of the 3d electrons of Fe/Cu.16 In contrast, the electron Coulomb interactions in ALiFeSe2 due to the more ionic characters between Li and Se weaken the Fe spin correlation while they enhance the band gap widening. These effects should be responsible for the observed spin-glass transition and semiconducting conduction behaviors. The band narrowing of ALiFeSe2 being expanded

RbLiFeSe2. For the thermal activation type conduction, there is a positive correlation between conductivity and the hopping opportunity of localized carriers jumping to the nearest lattice sites. As the lattice cells become larger, the hopping opportunity becomes smaller, resulting in larger resistivity. The electronic structure of KLiFeSe2 was calculated on the basis of first-principles calculations. A 2 × 2 supercell with random distribution of Fe and Li atoms was adopted to describe the Fe/Li mixture, as shown in Figure S2a in the Supporting Information. Since both spin orientation and spin localization might influence the electronic structure, we performed calculations on the basis of five schemes: (i) a nonmagnetic calculation without consideration of spin localization, (ii) a ferromagnetic calculation without consideration of spin localization, (iii) a ferromagnetic calculation with U = 2.5 eV for 3d electrons, (iv) a magnetic calculation with a randomly spin polarized model without consideration of spin localization, and (v) a magnetic calculation with randomly spin polarized model with U = 2.5 eV for 3d electrons. The obtained DOSs (Figure S2) show a metal feature for the three models without spin localization, namely schemes i, ii, and iv, and a semiconducting feature for the other two models with spin E

DOI: 10.1021/acs.inorgchem.7b01937 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Comparison of Lattice Parameters, Magnetism (MT), and Electrical Properties of ALiFeSe2 and Related Compounds

a

material

space group

a (Å)

c (Å)

MT

KxFe2−yS231 KCuFeS233 RbCuFeS233 CsCuFeS233 KCuFeSe234 NaLiFeSe2a KLiFeSe2a RbLiFeSe2a KFe1.05Ag0.88Te212 KFe0.85Ag1.15Te215 KCuFeTe216

I4/m I4/mmm I4/mmm I4/mmm I4/mmm P3m ̅ 1 I4/mmm I4/mmm I4/mmm I4/mmm I4/mmm

8.3984(5) 3.8548(7) 3.8858(7) 3.9046(4)

13.599(1) 13.178(3) 13.673(4) 14.418(2)

4.16089(7) 4.1044(2) 4.1609(1) 4.336(2) 4.3707(9) 4.2148(4)

7.0710(1) 13.7194(9) 14.0420(5) 15.019(2) 14.9540(8) 14.729(1)

SGb SG SG SG SG SG SG SG SG AFMc AFM

ρ(300 K) (Ω cm) 100 290 2700 170

Ea (meV) 51.8(2) 200 200 100 123.3 107.8(4) 209(1) 260.6(8) 43(2) 96(2) 116

12.1 108 122 1 2.7 0.1

The data of ALiFeSe2 are from this work. bSG = spin glass. cAFM = antiferromagnetism.

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with increasing ionic radius of alkali metals can further explain the enhanced resistivity in these compounds. The substitution of Fe by Li also occurs in LiFeO2Fe2Se236 and (Li0.8Fe0.2)OHFeSe.37 It is believed that these substitutions bring about the carrier doping in the [Fe2Se2] layers and induce the superconductivity. However, LixFe7Se819 shows no superconductivity with mixing Li/Fe occupancy in [Fe2Se2] layers. Thus, this shows that the suppression of superconductivity in ALiFeSe2 should be attributed to the substitution in [Fe2Se2] layers. Finally, we must point out that narrow solid solutions exist in the vicinity of Li:Fe = 1:1 for these compounds. The solubility needs to be determined in the future. CsLiFeSe2 is found to crystallize in orthorhombic symmetry. Its structure and properties deserve a separate study.

*E-mail for X.C.: [email protected]. ORCID

Xiaolong Chen: 0000-0001-8455-2117 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51532010, 91422303, and 51472266) and the National Key Research and Development Program of China (Grant No. 2016YFA0300600).

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CONCLUSIONS In summary, we report the discovery of three new layered compounds: namely, NaLiFeSe2 with a CaAl2Si2-type structure and KLiFeSe2 and RbLiFeSe2 with a ThCr2Si2-type structure. The introduction of Li at Fe sites results in homogeneous structure and composition in these materials. The substitution of Li at Fe sites compresses the layer spacing and elongates A− Se bond length in the ThCr2Si2-type structure. The elongated Na−Se bond length would destabilize the ThCr2Si2-type structure in NaLiFeSe2. We speculate that NaxFe2−ySe2 is at the border of ThCr2Si2-type and CaAl2Si2-type structures. Magnetic and resistivity measurements suggest an anisotropic spin-glass and narrow band gap semiconductor ground state. The electrical conductivity behavior is well fitted by the thermal activation model. The semiconducting behavior can be attributed to enhanced correlation effects. The discovery of these compounds would be instructive in further investigations of electronic correlations and magnetic structures in AxFe2−ySe2 materials.

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AUTHOR INFORMATION

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REFERENCES

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01937. Summary of Rietveld refinement parameters of ALiFeSe2, magnetic properties of RbLiFeSe2 single crystal and NaLiFeSe2 powder samples, calculated total/partial DOS, and the structure model of randomly spin-polarized KLiFeSe2, NaLiFeSe2, and RbLiFeSe2 used in the firstprinciples calculations (PDF) F

DOI: 10.1021/acs.inorgchem.7b01937 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01937 Inorg. Chem. XXXX, XXX, XXX−XXX