Antiferromagnetism in semiconducting KFe0.85Ag1.15Te2 single crystals

The discovery of superconductivity in LaFeAsO1−xFx has stimulated substantial interest in iron-based high-temperature superconductors (Fe-HTSs). Until now, several Fe-HTSs were discovered. They can be divided into two classes. The first class is iron pnictide materials.1−3 They contain two-dimensional FePn (Pn = pnictogens) tetrahedron layers and atomic sheets (e.g., Ba, K) or complex blocks [e.g., La-O(F)] along the c axis. Another class is binary iron chalcogenides FeCh (Ch = chalcogens, FeCh-11 type).4−6 In contrast to the diversity of iron pnictide superconductors, FeCh-11–type materials do not have any atomic or complex layers between puckered FeCh sheets. Very recently, the discovery of AxFe2−ySe2 (A = K, Rb, Cs, and Tl, FeCh-122 type) with Tc ≈ 30 K raised Tc in Fe-HTS by introducing alkali metal atomic layers between FeCh sheets.7−10 Further studies indicate that in the new superconductors the Tc gets enhanced when compared to FeCh-11 materials, but there is also a set of distinctive physical properties. FeCh-122 materials are close to the metal-semiconducting crossover and antiferromagnetic (AFM) order.7−10 This is in contrast to other superconductors which are in close proximity to the spin-density wave state.11 The Fermi surface in FeCh-122–type Fe-HTSs contains only electronlike sheets without the nesting features found in most other Fe-HTS.12 On the other hand, superconductivity in FeCh-11 materials is quite robust with respect to anion change, as seen on the example of FeSe1−x , FeTe1−xSex , and FeTe1−xSx .4−6 However, in FeCh-122 compounds, superconductivity is only observed in AxFe2−ySe2 or KxFe2−ySe2−zSz, while pure KxFe2−yS2 is a semiconductor with spin-glass transition at low temperature.14 Moreover, the theoretical calculation indicates that the hypothetical KFe2Te2, if synthesized, would have higher Tc than KxFe2−ySe2. Therefore, synthesis and examination of physical properties of FeCh-122 materials containing FeTe layers could be very instructive. In this Rapid Communication we report discovery of K1.00(3)Fe0.85(2)Ag1.15(2)Te2.0(1) single crystals. The resistivity and magnetic measurements indicate that this compound has the semiconducting long-range AFM order at low temperature, with no superconductivity down to 1.9 K. Single crystals of K(Fe,Ag)2Te2 were grown by the selfflux method reported elsewhere in detail,14,16 with nominal composition K:Fe:Ag:Te = 1:1:1:2. Single crystals with typical size 5×5×2 mm3 can be grown. Powder x-ray diffraction (XRD) data were collected at 300 K using 0.3184-Å wavelength radiation (38.94 keV) at the X7B beamline of the National Synchrotron Light Source. The average stoichiometry was determined by examination of multiple points using an energy-dispersive x-ray spectroscopy (EDX) in a JEOL JSM-6500 scanning electron microscope. Electrical transport measurements were performed using a four-probe configuration on rectangular-shaped polished single crystals with current flowing in the ab plane of tetragonal structure. Thin Pt wires were attached to electrical contacts made of silver paste. Electrical transport, heat capacity, and magnetization measurements were carried out in Quantum Design PPMS-9 and MPMS-XL5. Figure 1(a) shows powder XRD results and structural refinements of K(Fe,Ag)2Te2 using a general structure analysis system (GSAS).17,18 It can be seen that all reflections can be indexed in the I4/mmm space group. The refined structure parameters are listed in Table I. The determined lattice parameters are a = 4.3707(9) Å and c = 14.9540(8) Å, which are reasonably smaller than those of CsFexAg2−xTe2 [a = 4.5058(4) Å and c = 15.4587(8) Å],19 but much larger than those of KxFe2−ySe2 and KxFe2−yS2, due to the smaller ionic size of K+ than Cs+ and larger size of Ag+ and Te2− than Fe2+ and Se2−(S2−). On the other hand, a larger a-axis lattice parameter indicates that the Fe plane is stretched in K(Fe,Ag)2Te2 when compared to FeTe.20 The crystal structure of K(Fe,Ag)2Te2 is shown in Fig. 1(b), where antifluorite-type Fe/Ag-Te layers and K cation layers are stacked alternatively along the c axis. The XRD pattern of a single crystal [Fig. 1(c)] reveals that the crystal surface is normal to the c axis with the plate-shaped surface parallel to the ab plane. Figure 1(d) presents the EDX spectrum of a single crystal, which confirms the presence of the K, Fe, Ag, and Te. The average atomic ratios determined from EDX are K:Fe:Ag:Te = 1.00(3):0.85(2):1.15(2):2.0(1). The value of Fe/(Ag + Fe) determined from XRD fitting (0.38) is close to that obtained from EDX (0.43), which suggests that Te compound prefers to contain more Ag. This might explain why pure KFe2Te2 cannot form, since large Ag+ ions have to be introduced in order to match the rather large Te2− anions and keep the stability of the structure. On the other hand, it should be noted that there are no K or Fe/Ag deficiencies in K(Fe,Ag)2Te2. This is rather different from KxFe2−ySe2 and KxFe2−yS2. Moreover, synchrotron powder X-ray