Article pubs.acs.org/IC
Successive Phase Transitions in Fe2+ Ladder Compounds Sr2Fe3Ch2O3 (Ch = S, Se) Kwing To Lai,*,†,‡ Peter Adler,† Yurii Prots,† Zhiwei Hu,† Chang-Yang Kuo,† Tun-Wen Pi,§ and Martin Valldor†,∥ †
Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Strasse 40, 01187 Dresden, Germany Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong § National Synchrotron Radiation Research Centre, Hsinchu 30076, Taiwan ∥ Leibniz Institute for Solid State and Materials Research, Helmholtz Strasse 20, 01069 Dresden, Germany ‡
S Supporting Information *
ABSTRACT: Small single crystals of Sr2Fe3Ch2O3 (Ch = S, Se) have been synthesized by flux methods, and bulk materials have been obtained by solid state reactions. Both compounds are isostructural to the compound Sr2Co3S2O3 (space group Pbam), which contains a novel hybrid spin ladder: a combination of a 2leg rectangular ladder and a necklace ladder. The 2-leg ladder acts as a well-defined magnetic entity, while intimate magnetic coupling to the necklace ladder induces three successive phase transitions in the range of 40−120 K in each composition (Ch = S or Se), as revealed by Mössbauer spectroscopy, thermodynamics, and magnetometry. The complex magnetic behaviors can be explained by the unique spin−lattice topology.
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INTRODUCTION Divalent iron (Fe2+) in combination with heavier chalcogenides is drawing more attention since the discovery of hightemperature superconductivity in FeSe-based superconductors with layered crystal structures.1−3 Similarly to the case of the well-studied cuprates, high-Tc superconductivity in iron-based systems frequently occurs in proximity to antiferromagnetic states, and therefore understanding the magnetism in nonsuperconducting relatives is an important issue.4−7 While the magnetism in many pnictides is rather itinerant, larger magnetic moments indicate more localized magnetism in some chalcogenides. The vicinity of superconductivity to magnetism is particularly evident for the spin-ladder compound BaFe2S3,8 which at ambient pressure is a Mott-type magnetic insulator, but becomes superconducting near 10 GPa. Similar spin-ladder compounds with mixed oxide−chalcogenide ladders like AFe2Ch2O (A = Ba, Sr; Ch = S, Se)9−12 clearly show localized magnetism due to strong Fe−O−Fe exchange interactions. Hence, exploring closely related chemistry in search of novel compounds with low dimensional magnetic lattices, e.g., spin ladders, is rewarding and will provide insights into the factors governing the magnetism in such compounds. Spin ladders not only superconduct under high pressure, like Sr14−xCaxCu24O4113 and BaFe2S3, but also exhibit a wide variety of spin ground states at ambient pressure. For instance, the above-mentioned AFe2Ch2O (A = Ba, Sr; Ch = S, Se) show non-Curie−Weiss behaviors at high temperatures, indicating highly anisotropic magnetic interactions. Another ladder © 2017 American Chemical Society
compound, BaMn2O3, shows the lifting of frustration between the Mn−O ladders due to the structural distortion at low temperatures, resulting in a long-range antiferromagnetic ordering.14 Magnetic field induced successive phase transitions are also reported in, e.g., a frustrated spin-ladder compound BiCu2PO6.15 Recently a novel ladder compound Sr2Co3S2O3 (space group Pbam) was discovered,16 which has a new structure type with two extraordinary features: (1) a unique hybrid spin ladder combining 2-leg rectangular ladders and necklace ladders and (2) meridional heteroleptic octahedral coordination of Co2+ by three O2− and three S2− ions. Magnetic susceptibility and specific heat measurements suggest the coexistence of spin ordering and effective spin-half dimers. By replacing Co2+ with Fe2+, two isostructural compounds Sr2Fe3Ch2O3 (Ch = S, Se) form, which enable us to study the magnetism of the unique Fe2+ hybrid spin ladder. Through thermodynamic and spectroscopic measurements, two magnetic Fe2+ sites can be distinguished, and their complex interactions result in successive phase transitions.
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EXPERIMENTAL SECTION
Synthesis. Single crystals of Sr2Fe3Ch2O3 (Ch = S, Se) were synthesized in a CsCl flux. Powder of SrO, Fe (Alfa Aesar 99.9+%), αFe2O3 (Alfa Aesar 99.99%), and S (Alfa Aesar 99.95%) or Se (Alfa Received: August 10, 2017 Published: October 3, 2017 12606
DOI: 10.1021/acs.inorgchem.7b02042 Inorg. Chem. 2017, 56, 12606−12614
Inorganic Chemistry
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Aesar 99.999%) were used as starting materials. SrO was prepared inhouse by heating SrCO3 (Aldrich 99.9+%) at 1080 °C under high vacuum ( 350 K, a furnace was inserted into the MPMS. As there is a systematic temperature-independent shift in the data measured with the furnace, the high-temperature data were shifted relative to the data measured without the furnace at 350 K. Specific heat measurements were performed in a physical property measurement system (Quantum Design PPMS) with the standard nonadiabatic thermal relaxation technique. Spectroscopic Measurements of Polycrystals. The soft X-ray absorption spectroscopy (XAS) at the Fe-L2,3 edges was measured at the BL08B beamline of the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan, with an energy resolution of about 0.2 eV. Clean sample areas were obtained by cleaving the crystals in situ at pressures in the 1 × 10−9 mbar range. The Fe-L2,3 XAS spectrum of polycrystalline Sr2Fe3S2O3 was recorded using the total electron yield method at room temperature. α-Fe2O3 single crystals was measured simultaneously to serve as energy reference. 57 Fe Mössbauer spectra were collected between 5 and 290 K with a standard WissEl spectrometer equipped with a 57Co/Rh source, operating in the constant acceleration mode. Powdered samples corresponding to 13 mg of Fe were mixed with boron nitride homogeneously and were filled into a Plexiglas sample container (inner diameter = 13 mm). Spectra were obtained at different temperatures controlled by a Janis-SHI-850-5 closed cycle refrigerator (CCR). The isomer shifts were given relative to α-Fe. The data were evaluated with MossWinn21 using the thin absorber approximation. For some spectra hyperfine field distributions were extracted using the Hesse−Rübartsch method22 implemented in MossWinn.
Article
RESULTS Crystal Structure Refinement. Single crystal and powder synchrotron X-ray diffraction reveal that Sr2Fe3S2O3 and Sr2Fe3Se2O3 are isostructural to Sr2Co3S2O3 with a space group of Pbam (No. 55).16 The corresponding unit cell is illustrated in Figure 1a. The atomic parameters refined from the
Figure 1. Schematic drawings of crystal structure of Sr2Fe3Ch2O3 (Ch = S, Se): (a) The unit cell with a surplus content. (b) The hybrid spin ladder. The Fe−O network forms rectangular 2-leg spin ladders (SL) and necklace ladders (NL) as alternating layers along the ac-plane. (c) The trans-FeS4O2 octahedral coordination in the Fe1 site and (d) the mer-FeS3O3 octahedral coordination in the Fe2 site. The interatomic distances are in Å. The values in parentheses represent the data for Ch = Se.
single crystal X-ray diffraction are presented in Table 1, while further refinement details can be found in Tables S1−S3. Refinements on powder diffraction patterns of both compounds at room temperature and 75 K (see Figures S1 and S2) agree well with the models obtained from the single crystal refinements. In the polycrystalline samples, SrS (∼2 mol %) and SrSe (∼1 mol %), respectively, were the only detectable secondary phases. Similar to Sr2Co3S2O3, the title compounds contain the characteristic Fe−O hybrid spin ladder, as illustrated in Figure 1b, in which rectangular 2-leg ladders and necklace ladders combine by sharing common legs alternatively across the acplane. There are two distinct Fe2+ sites with clearly different crystal fields: The meridional (mer-) heteroleptical octahedrally coordinated Fe2 site (Figure 1d) locates within the 2-leg ladders’ part, while the trans-octahedrally coordinated Fe1 site (Figure 1c) serves as the central ion with the necklace ladders’ 12607
DOI: 10.1021/acs.inorgchem.7b02042 Inorg. Chem. 2017, 56, 12606−12614
Article
Inorganic Chemistry
(c) Mg0.94Fe0.04O, but is shifted by 1.6 eV to lower energies with respect to Fe3+ reference (d) α-Fe2O3. This confirms an Fe2+ valence state of Sr2Fe3S2O3, agreeing with the expectations from the chemical compositions. The multiplet spectral structure of Sr2Fe3S2O3 is different from that of Mg0.94Fe0.04O with octahedral Fe2+ coordination as well as from that of CaFeSeO with tetrahedral Fe2+ coordination, reflecting the rare mer-coordination of the Fe2+ in Sr2Fe3S2O3 with its locally low symmetry compared to the octahedron in the rock-salt structure Mg0.94Fe0.04O. Magnetic Properties and Specific Heat Capacity. Temperature Dependence of Magnetic Susceptibility and Specific Heat Capacity. To properly present the following results, Fe2+ is stated as high-spin (S = 2) in Sr2Fe3Ch2O3 (Ch = S, Se), which is later proven by Mössbauer spectroscopy. Figure 3 summarizes the temperature dependences of magnetic susceptibilities χ(T) and specific heat capacities Cp(T) of Sr2Fe3Ch2O3. The key features at low temperatures (T < 150 K) are three successive phase transitions at low fields for both Ch = S and Se observed in χ(T) accompanied by peaks in Cp(T). The characters of the anomalies in χ(T) can be denoted as a broad hump (TN1), an antiferromagnetic-like peak (TN2), and a sharp edge (T′). The corresponding temperatures, at which they are observed, are tabulated in Table 2. In overview, T′ is shifted more than TN1 and TN2 between Ch = S and Se. Note that all the observed anomalies correspond to secondorder phase transitions, as estimated from the peaks observed in Cp(T). The high-field χ(T) curves of Sr2Fe3Ch2O3 (Figures 3a and 3d) are different from the low-field curves; TN1 in Ch = S is not observable, while χ(T) in Ch = Se becomes strongly field dependent below TN2 (∼50 K). The field dependent behavior is further confirmed by Cp(T) measurements at 3 T (Figure 4), where T′ becomes too weak to be detected, TN2 becomes broader, and TN1 seems unaffected by the applied field. At higher temperatures (T > 150 K), both compounds show no clear Curie−Weiss behavior, as illustrated in the insets in Figures 3a and 3d. Moreover, all high-temperature Cp(T) data agree with the Dulong−Petit law and saturate close to 3NR ∼ 250 J mol−1 K−1, where N = 10 is the number of independent atoms in the unit cell and R = 8.314 J mol−1 K−1 is the gas constant. The fact that all three transitions in the title compounds are revealed with strong features in Cp(T) data suggests that all magnetic phenomena are intrinsic, as both samples are better than 95% phase pure. Field Dependent Magnetization. The field dependent magnetizations M(H) for Ch = S at 2 and 300 K in Figure 5a can be described with a minor saturating ferromagnetic impurity in combination with either antiferromagnetic or paramagnetic state, respectively. By extrapolating the highfield region down to zero field, the impurity corresponds to ∼0.6% of Fe. The M(H) curves for Ch = Se, as shown in Figures 5b−e, are more intricate. At T = 2 K, the curve shows a step-like feature at ∼0.6 and 2 T. After the transition at 2 T, M(H) continuously increases and nearly saturates at H = 5 T. The M(H) hysteresis and the relatively small saturated magnetic moment suggest a ferrimagnetic-like phase at H > 2 T. At T = 5 K, the high-field transition decreases to H ∼ 1 T while the hysteresis is getting smaller, as expected. At T = 40 K, the hysteresis effect becomes negligible. The field-induced transition persists up to T ∼ 45 K and vanishes at higher temperatures, which supports the field dependence on χ(T) below TN2 (Figure 4). On further
Table 1. Atomic Coordinates and Displacement Parameters (in Å2) in the Crystal Structures of Sr2Fe3S2O3 and Sr2Fe3Se2O3 (Space Group Pbam) atom
site
x
Sr Fe1 Fe2 S O1 O2
4g 2a 4h 4h 2d 4g
0.09378(6) 0 0.2578(1) 0.4011(2) 0 0.2321(5)
Sr Fe1 Fe2 Se O1 O2
4g 2a 4h 4h 2d 4g
y
Sr2Fe3S2O3 0.34369(6) 0 0.09056(9) 0.3392(2) 1/2 0.0911(4) Sr2Fe3Se2O3 0.09174(5) 0.34894(4) 0 0 0.25893(7) 0.08808(6) 0.40124(5) 0.33536(4) 0 1/2 0.2328(4) 0.0835(3)
z
Ueq/isoa
0 0 1/2 1/2 1/2 0
0.0080(1) 0.0096(3) 0.0091(2) 0.0084(3) 0.006(1) 0.0065(7)
0 0 1/2 1/2 1/2 0
0.00800(8) 0.0087(2) 0.0077(1) 0.00687(8) 0.0088(6) 0.0099(5)
a
Anisotropic displacement parameters for Sr, Fe, and S (Se) are listed in Table S2. O1 and O2 positions were refined with isotropic displacement parameters for both structures.
part. The trans-octahedra (Figure 1c) in Sr2Fe3S2O3 have similar bond lengths compared to those in Sr2Co3S2O3. However, the mer-octahedra (Figure 1d) in Sr2Fe3S2O3 are more distorted than those in Sr2Co3S2O3. Similar behaviors can be observed in Sr2Fe3Se2O3, but the Fe−Se bond lengths are on average 0.1 Å larger than the Fe−S bond in Ch = S, owing to the larger ionic radii of Se2−. It is worthy noting that the shortest Fe−Fe distance, across face-sharing octahedra in the necklace ladders, is about 2.9 Å, which is too long for any direct magnetic interactions. Elemental Analysis. Elemental compositions of polycrystalline samples were confirmed by EDX. By averaging the results of the 10 measurements, the compositions are Sr1.9(1) Fe 3.0(1)S 1.7(1) O 3.3(1) and Sr2.0(1) Fe3.2(1) Se 1.9(1) O 2.8(1) . These values are fairly close to the nominal stoichiometries. X-ray Absorption Spectroscopy. XAS at the 3d transition metal (TM) L2,3 edges is a very sensitive technique to establish their valence states, spin states, and local symmetries in the solid state matter.23−25 Figure 2 shows that the Fe-L2,3 XAS spectrum of (a) Sr2Fe3S2O3 has its main weight at the same energy position as that of an Fe2+ reference (b) CaFeSeO26 and
Figure 2. Room temperature Fe-L2,3 XAS spectra of (a) Sr2Fe3S2O3, (b) CaFeSeO, (c) Mg0.94Fe0.04O, and (d) α-Fe2O3. Spectra b and c serve as an Fe2+ reference while spectrum d serves as an Fe3+ reference. 12608
DOI: 10.1021/acs.inorgchem.7b02042 Inorg. Chem. 2017, 56, 12606−12614
Article
Inorganic Chemistry
Figure 3. Temperature dependence of magnetic susceptibility χ(T) and specific heat Cp(T) of Sr2Fe3Ch2O3 (Ch = S, Se). (a, b, d, e) The χ(T) data for Ch = S, Se at 3 and 0.1 T, respectively. The insets in panels a and d are the plot of inverse magnetic susceptibility χ−1(T) at high temperatures for Ch = S, Se, respectively. The blue dots represents the data obtained with the installation of the furnace in MPMS. The inset in panel b is the first derivative of magnetic susceptibility dχ/dT around T′ for Ch = S. (c, f) The Cp(T) data for Ch = S, Se at zero field, respectively. The dashed lines are the guide for eyes to compare the phase transition temperatures between χ(T) and Cp(T).
Table 2. Summary of the Phase Transition Temperatures of Sr2Fe3Ch2O3 (Ch = S, Se) at Low Fields transition temp (K) Ch
TN1
TN2
T′
S Se
116 112
52 47
75 40
Figure 4. Temperature dependence of specific heat Cp(T) of Sr2Fe3Se2O3 at μ0H = 0 and 3 T. The inset shows the Cp(T) data around T = TN1. Figure 5. Field dependence of magnetization M(H) of (a) Sr2Fe3S2O3 at 2 and 300 K, respectively, and (b−e) Sr2Fe3Se2O3 at various temperatures. The dotted line in panel b represents the virgin curve.
increasing the temperature, M(H) shows a linear behavior, confirming the antiferromagnetism below TN1 and the paramagnetism above TN1. Note that there is also small saturating ferromagnetic feature at T = 300 K, suggesting ∼0.02% Fe impurity.
Mössbauer Spectroscopy. 57Fe Mössbauer spectroscopy provides detailed insights into the nature of the phase transitions detected by the magnetization and heat capacity 12609
DOI: 10.1021/acs.inorgchem.7b02042 Inorg. Chem. 2017, 56, 12606−12614
Article
Inorganic Chemistry
quadrupole splittings and thus the valence contributions to the efg are only weakly temperature-dependent, which indicates that the splitting of the t2g levels due to the strongly distorted coordination environments is larger than the thermal energy. In the spectra below T = 120 K the minority quadrupole doublet persists whereas the majority component features a broad magnetic hyperfine pattern which successively sharpens on cooling. Accordingly, the phase transition seen at TN1 = 116 K in the Cp(T) data and the hump in the χ(T) data near 120 K are attributed to antiferromagnetic ordering of the ladder sublattice, but the necklace sublattice still remains in the paramagnetic state, which is in agreement with the pronounced increase in magnetic susceptibility below ∼110 K (Figure 3b). The quadrupole splitting of the Fe1 sites remains essentially unchanged, which verifies that the local environment of Fe1 is not influenced by the magnetic phase transition of Fe2. Most remarkably, two magnetically inequivalent sites are apparent in spectra obtained in the temperature range T′ ≤ T ≤ TN1 (see the 80 and 90 K spectra in Figure 6). In contrast, spectra collected at T = 60 (not shown) and 70 K (
