Emergence of Magnetic States in Pr2Fe4–xCoxSb5 - ACS Publications

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Emergence of Magnetic States in Pr2Fe4−xCoxSb5 (1 < x < 2.5)

Pilanda Watkins-Curry,† Kyle J. Pujol,† Katherine A. Benavides,† Joseph Vade Burnett,† Jenny K. Hedlund,† Julia Bykova,‡ Gregory T. McCandless,† Amy V. Walker,§ and Julia Y. Chan*,† †

Department of Chemistry & Biochemistry, §Department of Materials Science and Engineering, and ‡Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, Texas 75080, United States S Supporting Information *

ABSTRACT: Single crystals of Pr2Fe4−xCoxSb5 (1 < x < 2.5) were grown from a Bi flux and characterized by X-ray diffraction. The compounds adopt the La2Fe4Sb5 structure type (I4/mmm). The structure of Pr2Fe4−xCoxSb5 (1 < x < 2.5) contains a network of transition metals forming isosceles triangles. The x ∼ 1 analogue is metallic and exhibits a magnetic transition at T1 ≈ 25 K. The magnetic moment obtained from the Curie− Weiss fit is 11.49(4) μB, which is larger than the spin-only Pr3+ moment. The x ∼ 2 analogue orders magnetically at T1 ≈ 80 and T2 ≈ 45 K. This is the first case of the substitution of Co into the La2Fe4Sb5 structure type, evidenced by the increased concentration of dopant with decreased lattice parameters coupled with a change in the transition temperature with a change in the cobalt concentration. The added complexity in the magnetic behavior of the x ∼ 1 and 2 analogues indicates that the increased concentration of Co invokes an additional magnetic contribution of the transition metal in the sublattice. Furthermore, X-ray photoelectron spectroscopy measurements support the change in the oxidation states of transition metals with increasing cobalt concentration.

1. INTRODUCTION Materials that display magnetic frustration are of interest because of the emergence of novel magnetic phases upon cooling, such as spin glass. In two-dimensional systems, the triangular lattice is the ideal candidate to study geometric frustration because this can lead to spin-disordered states.1−3 For example, magnetic frustration can be achieved by suppressing magnetic order by the substitution of a nonmagnetic Al3+ in the MAl2S4 (M = Mn2+, Fe2+, and Co2+) phase, thereby resulting in a candidate for a two-dimensional Heisenberg spin-glass system.4 Ln2Fe4Sb5 (Ln = La−Nd, Sm), which consists of two Fe sites that form nearly equilateral triangular units, have been shown to exhibit magnetically frustrated spin-glass behavior.5 Inspired by the emergence of spin glass in Ln2Fe4Sb5 (Ln = La−Nd, Sm) and the valence instability of Pr3+, we hoped to study the effects of Co doping on the magnetic properties of Pr2Fe4Sb5. In view of this, we report the synthesis, structure, and magnetic properties of single-crystalline Pr2Fe4−xCoxSb5 (1 < x < 2.5). Rare-earth intermetallics have garnered interest because of their rich structural chemistry,6−8 complex magnetic ordering,9−12 colossal magnetoresistance,13−18 and spin-glass behavior.5,19,20 PrFeSb3 contains Pr inserted between a layer of Fecentered octahedra and a distorted square net of Sb.21 The effective magnetic moment for this compound is solely due to Pr3+, with the Fe not carrying a magnetic moment. Eu3InP3, a Zintl phase, features chains of InP4 tetrahedra and Eu2+ ions in a square-pyramidal configuration.10 Antiferromagnetic transitions at TN = 5, 10.4, and 14 K arise because of the ordering of Eu in three crystallographically unique sites. Eu14MnSb11 © 2016 American Chemical Society

features two sublattices that both contribute to the magnetic ordering of the compound.22 MnSb4 tetrahedra and the sublattice of Eu2+ ions weakly couple ferromagnetically. The ferromagnetic transition at ∼92 K is due to the Mn−Mn interaction, while the transition at ∼15 K is due to Eu ordering. Nd3Fe3Sb7 exhibits complex magnetic behavior that is due to magnetic contributions from both the Nd and Fe atoms.12 Nd3Fe3Sb7 orders magnetically at ∼360 K, and the magnetic state becomes unstable at ∼50 K. This structure features columns of Fe clusters and triangular prisms of Nd atoms separated by Sb square nets. Another example of a hightemperature magnetic transition material includes Hf3Fe4Sn4, which orders antiferromagnetically at 373(5) K. This compound has large, 10-membered Fe rings in the bc plane surrounded by highly disordered Sn atoms and separated by Hf−Sn layers.23

2. EXPERIMENTAL SECTION 2.1. Synthesis. Single crystals of Pr2Fe4−xCoxSb5 (1 < x < 2.5) were grown via an inert Bi flux, as adapted from the successful growth of Ln2Fe4Sb5 (Ln = La−Nd, Sm).5 Pr (rod, 99.9%), Fe (powder, 99.9%), Co (powder, 99.9%), Sb (ingots, 99.9%), and Bi (granules, 99.9%) were used as received. For each reaction, the elements were placed in an alumina crucible, topped with quartz wool, capped with a second crucible, and then sealed in an evacuated fused silica tube filled with ∼0.3 atm of argon. The sealed tube was then placed in a furnace. The reaction ampule was heated to 1200 °C for 24 h at a rate of 100 Received: December 21, 2015 Published: February 3, 2016 1946

DOI: 10.1021/acs.inorgchem.5b02950 Inorg. Chem. 2016, 55, 1946−1951

Article

Inorganic Chemistry °C/h and slowly cooled to 875 °C at a rate of 5 °C/h. The reaction was removed at a higher temperature (720 °C) than that of the reported parent Ln2Fe4Sb5 analogues to avoid competing binaries CoSb2 and CoSb3.5 The excess Bi flux was removed via centrifugation, and rectangular platelike single crystals of up to 0.5 mm along each direction were obtained and were etched in dilute 0.1 M HCl. An alternate temperature profile was developed for growth of the x ∼ 2.4 compound. This involved heating to 1150 °C for 24 h at a rate of 100 °C/h and cooling to 875 °C at a rate of 6 °C/h. We have successfully grown single crystals of Co-doped Pr2Fe4−xCoxSb5 for x up to 2.4. With increasing Co, the crystal size decreases. For this manuscript, we will highlight x ∼ 1, 2, and 2.4. Attempts to synthesize Pr2Fe4−xCoxSb5 (x > 2.5) were not successful using either of these temperature profiles and reactions led to polycrystalline samples of Pr2Fe4−xCoxSb5, PrFe1−xSb2, PrFeSb3, and PrSb2.21,24 Also, for x > 2.5, the concentration of the substituted metal is no longer homogeneous. We also attempted to make the Co analogue of Pr2Fe4Sb5, but this also led to the growth of PrCo1−xSb2. 2.2. Structure Determination. Phase identification and sample homogeneity were determined by powder X-ray diffraction. Data were collected using a Bruker D8 Advance powder X-ray diffractometer operating at 40 kV and 30 mA with Cu Kα (1.54184 Å) radiation and a LYNXEYE XE detector. Data were collected in the 2θ range of 10− 80° with a step size of 0.01°, and powder diffraction patterns are provided in the Supporting Information. Single crystals of Pr2Fe4−xCoxSb5 (1 < x < 2.5) were cut to appropriate sizes and mounted on a glass fiber with epoxy. The fibers were mounted on a Bruker D8 Quest Kappa single-crystal X-ray diffractometer equipped with a Mo Kα IμS microfocus source (λ = 0.71073 Å) operating at 50 kV and 1 mA, a HELIOS optics monochromator, and a CMOS detector. The collected data were corrected for absorption using the Bruker program SADABS (multi-scan method). A starting model of Pr2Fe4−xCoxSb5 (1 < x < 2.5) was obtained using SHELXS2013,25 and atomic sites were refined anisotropically using SHELXL2014.26 The models of Pr2Fe4−xCoxSb5 (1 < x < 2.5) were refined by starting with the atomic coordinates of the parent Pr2Fe4Sb5. The crystallographic parameters, atomic coordinates, occupancies, and displacement parameters are provided in Tables 1 and 2. Selected interatomic distances and angles are provided in Table 3. Pr2Fe4−xCoxSb5 consists of one Pr site, two M (M = Fe, Co) sites, and three Sb sites. The transition metal sites consist of a fully occupied M1 site (except for x ∼ 2.4) and a partially occupied M2 site. Because of the indistinguishability of Fe and Co from X-ray analysis and for the purposes of refinement, only Fe was modeled as present on the M1 site and only Co was modeled as present on the M2 site. 2.3. Characterization. Elemental analysis of Pr2Fe4−xCoxSb5 (1 < x < 2.5) single crystals was conducted via electron-dispersive spectroscopy using a Zeiss LEO model 1530 variable-pressure field effect scanning electron microscope equipped with an EDAX detector at an accelerating voltage of 19 kV. Spectra were integrated for 60 s, and the results from at least five spots were averaged and normalized to Pr to determine the atomic percentage of each element. The normalized concentration of the phases are Pr2.00(4)Fe3.24(5)Co0.89(2)Sb4.94(3), Pr2.00(5)Fe2.17(7)Co1.95(4)Sb4.70(9), and Pr2.00(6)Fe1.47(3)Co2.44(9)Sb4.67(12). Ex situ X-ray photoelectron spectroscopy (XPS) spectra were measured using a PHI VersaProbe II sanning XPS microprobe (Physical Electronic Inc.) equipped with an Al Kα X-ray source (Ep = 1486.7 eV). During measurement, the pressure in the chamber typically was lower than 6.7 × 10−10 mbar. The spectra were collected with a pass energy of 23.5 eV, an energy step of 0.2 eV, and an analysis angle of 45°. To ensure that all adventitious C or O species were removed, prior to data collection, the sample was exposed for 2 min of argon-gas cluster-ion-beam sputtering. All spectra were collected using a charge compensation system with both electron and ion beams incident on the surface. The binding energies were calibrated using the C 1s binding energy (285.0 eV). Spectra were analyzed using CasaXPS 2.3.16 (RBD Instruments, Inc., Bend, OR) and AAnalyzer 1.07. 2.4. Physical Properties. Magnetic measurements of Pr2Fe4−xCoxSb5 (x ∼ 1, 2) were collected using a Quantum Design

Table 1. Crystallographic Parameters of Pr2Fe4−xCoxSb5 (1 < x < 2.5) compound a (Å) c (Å) V (Å) Z cryst dimens (mm3) temp (K) θ range (deg) μ (mm−1) measd reflns indep reflns Rint h k l reflns/param Δρmax (e/Å3) Δρmin (e/Å3) extinction coeff GOF R1 (F)a wR2b

x∼1

x∼2

4.307(2) 25.747(8) 477.7(4) 2 0.03 × 0.04 × 0.04 298(2) 3.2−30.5 29.89 8087 268 0.031 −6 to +6 −6 to +6 −35 to +36 268/20 2.30 −1.99 0.0059(4) 1.27 0.0250 0.061

a R1 = ∑||F o | − |F c ||/∑|F o |. ∑[w(Fo2)2]]1/2.

4.299(1) 25.711(9) 475.2(3) 2 0.03 × 0.03 × 0.05 298(2) 3.2−30.5 30.79 1867 270 0.047 −4 to +6 −4 to +6 −33 to +36 270/21 2.96 −1.88 0.0011(3) 1.25 0.0340 0.090 b

x ∼ 2.4 4.301(1) 25.704(12) 475.4(4) 2 0.02 × 0.06 × 0.10 298(2) 3.17−30.5 30.824 1617 262 0.047 −4 to +4 −6 to +5 −36 to +27 262/21 2.60 −2.92 0.0024(4) 1.18 0.0366 0.096

wR2 = [∑[w(F o 2 − F c 2 )2 ]/

Magnetic Property Measurement System. The direct-current magnetic susceptibility was measured under zero-field-cooled (ZFC) and fieldcooled (FC) conditions from 3 to 300 K under an applied magnetic field of 0.1 T. Field-dependent magnetization data were collected at 3 K with applied magnetic fields up to 8 T. The mass of individual single crystals were too small for magnetic measurements; therefore, magnetic measurements were conducted on multiple fully characterized single crystals. All measurements were collected on fully characterized single crystals. For clarity, the properties of Pr2Fe4−xCoxSb5 will be reported as approximate nominal compositions of x ∼ 1 and 2. Samples of concentration x ∼ 2.4 did not have sufficient yield for physical property measurements to be performed.

3. RESULTS AND DISCUSSION 3.1. Structure of Pr2Fe4−xCoxSb5 (1 < x < 2.5). Pr2Fe4−xCoxSb5 (1 < x < 2.5), displayed in Figure 1, adopts the La2Fe4Sb5 structure type (tetragonal, I4/mmm) and was first reported by Woll.27 As presented in Figure 2a, Pr atoms are surrounded by four Sb1 and four Sb2 atoms arranged in a square antiprism, with Sb1 atoms forming square nets; this subunit is similar to the Pr environment in PrSb2,28 PrCo1−xSb2,29 PrFeSb2,24 PrFeSb3,21 and Pr2Fe4Sb5.5 Table 3 provides the Pr−Sb interatomic distances, which range from 3.2308(13) to 3.3216(1) Å, and are comparable to the Pr−Sb distances found in Pr-capped square nets in PrFeSb3 and Pr2Fe4Sb5, and are shown in Figure 2a.21,30 Parts b and c of Figure 2 show the transition metal (M = Fe, Co) sublattice. The transition metal sublattice can be described in two parts (M1−M2 and M2−Sb environments), which are shown in Figure 2b and 2c, with selected distances provided in Table 3. The M1 atoms are coordinated to eight M2 atoms that are occupationally disordered (Figure 2b), and the M2 atoms are surrounded by two Sb2 and two Sb3 atoms in a tetrahedral environment (Figure 2c). Additionally, the M2 site is coordinated to two M1 atoms, forming a distorted octahedron. 1947

DOI: 10.1021/acs.inorgchem.5b02950 Inorg. Chem. 2016, 55, 1946−1951

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Inorganic Chemistry Table 2. Atomic Positions of Pr2Fe4−xCoxSb5 (1 < x < 2.5)

a

atom

Wyckoff site

point symmetry

Pr M1 M2 Sb1 Sb2 Sb3

4e 2a 8g 4d 4e 4e

4mm 4/mmm 2mm 4̅m2 4mm 4mm

Pr M1 M2 Sb1 Sb2 Sb3

4e 2a 8g 4d 4e 4e

4mm 4/mmm 2mm 4̅m2 4mm 4mm

Pr M1 M2 Sb1 Sb2 Sb3

4e 2a 8g 4d 4e 2b

4mm 4/mmm 2mm 4̅m2 4mm 4/mmm

x

y

Pr2Fe4−xCoxSb5 (x ∼ 1) 0 0 0 0 1 0 /2 1 /2 0 0 0 0 0 Pr2Fe4−xCoxSb5 (x ∼ 2) 0 0 0 0 1 0 /2 1 /2 0 0 0 0 0 Pr2Fe4−xCoxSb5 (x ∼ 2.4) 0 0 0 0 1 0 /2 1 /2 0 0 0 0 0

z

occupancy

Ueqa (Å2)

0.34794(3) 0 0.44824(8) 1 /4 0.11020(4) 0.4917(3)

1 1 0.753(11) 1 1 0.5

0.0080(3) 0.0109(6) 0.0217(8) 0.0051(3) 0.0080(3) 0.0481(18)

0.34844(4) 0 0.44734(9) 1 /4 0.11017(5) 0.4937(6)

1 1 0.810(12) 1 1 0.5

0.0061(3) 0.0078(7) 0.0141(9) 0.0035(3) 0.0063(3) 0.027(2)

0.34888(4) 0 0.4469(1) 1 /4 0.11024(5) 1 /2

1 0.95(2) 0.837(13) 1 1 1

0.0078(4) 0.0071(14) 0.0136(9) 0.0044(4) 0.0080(4) 0.0289(7)

Ueq is defined as one-third of the trace of the orthogonalized Uij tensor. M1 and M2 represent transition metals.

Table 3. Selected Interatomic Distances (Å) and Angles (deg) of Pr2Fe4−xCoxSb5 (1 < x < 2.5) x∼1 Pr−Sb1 (×4) Pr−Sb2 (×4) M1−M2 (×8) M2−M2 (×1) M2−Sb2 (×4) M2−Sb3 (×2) M2−Sb3′ (×2) Sb1−Sb1 (×4) M2−M1−M2 M1−M2−M2 Sb1−Sb1−Sb1

x∼2

Distance (Å) 3.316(1) 3.3206(11) 3.2308(13) 3.2208(8) 2.533(1) 2.540(1) 2.666(4) 2.708(5) 2.627(2) 2.6089(16) 2.428(3) 2.458(8) 2.651(4) 2.630(9) 3.0457(12) 3.0399(7) Angle (deg) 63.50(8) 64.41(9) 58.25(4) 57.79(5) 90.0 90.0

x ∼ 2.4 3.3291(13) 3.2173(11) 2.5473(15) 2.731(4) 2.604(2) 2.547(1) 2.547(1) 3.0409(9) 64.84(10) 57.58(5) 90.0

Sb3′ is the Sb3 position generated from the mirror plane. Figure 1. Crystal structure of Pr2Fe4−xCoxSb5.

The M2−Sb3 distances are comparable to than the Fe2−Sb3 distances observed for Pr2Fe4Sb5, which are in the range of 2.342(2)−2.771(4) Å. These distances systematically increase as a function of increasing Co concentration. However, this is not the case for the M2−Sb3′ distances, which decrease as the Co concentration rises. The occupational disorder of the M2 site has occupancies in the range of 0.75−0.84. Therefore, as a function of increasing Co concentration (x), the mixed M2 site becomes closer to a fully occupied M2 site. The greater static positional disorder of the Sb3 position also leads to shorter Fe2−Sb3 interatomic distances in the Pr2Fe4Sb5 end member, consistent with elongation of the Fe2 anisotropic displacement parameters pointing in the direction of the Sb3 position. Figure 2b shows the M1−M2 contacts of the transition metal sublattice of Pr2Fe4−xCoxSb5 (x ∼ 2.4), which forms nearly equilateral triangles comprised of M1 atoms bonded to two M2 atoms at a distance of 2.5473(15) Å and a M2−M2 distance of 2.731(4) Å. The M2−M2 distance is also longer than the Fe2−

Fe2 distance [2.652(7) Å] of Pr2Fe4Sb5. The transition metal sublattice angles also change with increasing Co concentration (x). In Pr2Fe4Sb5, the M2−M1−M2 angle is 63.18(13)° and the M1−M2−M2 angle is 58.41(7)°. Upon Co substitution (x ∼ 1), the M2−M1−M2 angle is 63.50(8)° and the M1−M2− M2 angle is 58.25(4)°. With the addition of Co, the M2−M1− M2 angle becomes more obtuse, which subsequently lengthens the M2−M2 distances (Table 3). This indicates a possible reduction of the transition metal oxidation state on either metal site. Although structurally similar to Pr2Fe4Sb5, there are some important differences in the static positional disorder of the Sb3 site in Pr2Fe4−xCoxSb5 (1 < x < 2.5). The Sb3 positions for the x ∼ 1 and 2 members are positionally disordered around a mirror plane. However, the Sb3 site in x ∼ 2.4 is not positionally disordered; this site sits on the 2b Wyckoff position. Consequently, among the Pr2Fe4−xCoxSb5 (1 < x < 1948

DOI: 10.1021/acs.inorgchem.5b02950 Inorg. Chem. 2016, 55, 1946−1951

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Inorganic Chemistry

Figure 2. Local environments of (a) a Pr square antiprism, (b) a M1−M2 triangular network, and (c) M2−Sb sublattice of Pr2Fe4−xCoxSb5 (x ∼ 2.4).

2.5) compounds, the Sb3 position for x ∼ 1 is the most positionally disordered, decreasing to the least disordered Sb3 position for x ∼ 2.4. The x ∼ 1 and 2 concentrations share the positional disorder at the 4e Sb3 site with the parent compound Pr2Fe4Sb5. A plot of the c lattice parameters and volumes, including the Pr2Fe4Sb5 end member, is provided in Figure 3.

Figure 4. Fe 2p, Co 2p, Sb 3d, and Pr 3d XPS spectra for Pr2Fe4−xCoxSb5 for x ∼ 1 and 2 concentrations. The dotted lines represent the lines of the best fit to the data. To make clear the changes in the spectra, the highest peak in each spectrum was normalized to a value of 1.

Figure 3. c axis length and volume of Pr2Fe4−xCoxSb5 as a function of the Co concentration.

The c axis parameter and volume decrease as the Co concentration (x) increases until x ∼ 2. Between x ∼ 2 and 2.4, the changes in the c axis parameter and volume are not significantly different. Figure 4 displays the Fe 2p, Co 2p, and Sb XPS spectra. In the Fe 2p region for x ∼ 1, four peaks are observed. The Fe 2p3/2 binding energies at 710.3 and 714.0 eV are assigned to Fe2+/Fe3+ and Fe3+, respectively.31 Consistent with this assignment, two Fe 2p1/2 XPS peaks are observed at 723.7 eV (Fe2+/Fe3+) and 728.5 eV (Fe3+). For x ∼ 2, the spectra indicate that the oxidation state of Fe changes to Fe0 (2p3/2, EB = 706.6 eV; 2p1/2, EB = 720.0 eV) and Fe3+ (2p3/2, EB = 713.2 eV; 2p1/2, EB = 722.5 eV).31 For Co, which is also present in the transition metal sublattice, a similar trend is observed. At x ∼ 1, Co is observed in a mixed oxidation state:31 Co0 (2p3/2, EB = 778.4 eV; 2p1/2, EB = 789.0 eV), Co2+ (2p3/2, EB = 783.6 eV; 2p1/2, EB = 795.7 eV), and Co2+/Co3+ (2p3/2, EB = 802.7 eV). When x ∼ 2, the Co oxidation state changes to predominantly Co0 (2p3/2, EB = 778.0 eV; 2p1/2, EB = 793.0 eV) with a small amount of Co2+ present (2p3/2, EB = 781.0 eV; 2p1/2, EB = 796.2 eV). This change in the oxidation state in the transition

metal sublattice is consistent with the X-ray diffraction results, which show that as x increases and the c-axis parameter and cell volume decrease. As the oxidation states of the transition metals decrease, there is less charge−charge repulsion, and it is expected that the c-axis parameter and cell volume decrease. The XPS data (Figure 4) also indicate that Sb is present in the 3+ (3d5/2, EB ∼ 530.5 eV; 3d3/2, EB ∼ 540 eV) and 0 (3d5/2, EB ∼ 528 eV; 3d3/2, EB ∼ 537.5 eV) oxidation states.31 As x changes from 1 to 2, the ratio of Sb3+ to Sb0 XPS peaks changes and broadens. These observations suggest that a Sb network is present, with Sb interacting with multiple species in the structure. 3.2. Magnetic Properties. Pr2Fe4−xCoxSb5 (x ∼ 1). The field-dependent magnetization of Pr2Fe4−xCoxSb5 (x ∼ 1) at 3 and 50 K at applied fields of up to 7 T is shown in Figure 5a. The field-dependent magnetization at 50 K steadily increases with the field and reaches a magnetization of 3.7 μB/mol of Pr2Fe4−xCoxSb5 at 7 T. At 3 K, the magnetization is hysteretic below 3.5 T before reaching to 6.3 μB/mol of Pr2Fe4−xCoxSb5 at 7 T; both are above the magnetic moment solely for 2 mol of 1949

DOI: 10.1021/acs.inorgchem.5b02950 Inorg. Chem. 2016, 55, 1946−1951

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Inorganic Chemistry

Figure 5. (a) Field-dependent magnetization (M) of Pr2Fe4−xCoxSb5 (x ∼ 1) with magnetic fields of up to 7 T at 3 and 50 K. (b) Temperaturedependent magnetic susceptibility (χ) of Pr2Fe4−xCoxSb5 (x ∼ 1) at a magnetic field (H) of 0.1 T (inset) Temperature-dependent inverse magnetic susceptibility (1/χ).

Figure 6. (a) Field-dependent magnetization (M) of Pr2Fe4−xCoxSb5 (x ∼ 2) with magnetic fields of up to 7 T at 3 and 50 K (b) Temperaturedependent magnetic susceptibility (χ) of Pr2Fe4−xCoxSb5 (x ∼ 2) at a magnetic field (H) of 0.1 T.

to 2 T before a steady increase with the field accompanied by a small hysteresis. As shown in Figure 6b, the magnetic susceptibility increases with decreasing temperature. Below 180 K, the susceptibility increases until a downturn at T1 ≈ 80 K is observed, indicating spin reorientation. The magnetic susceptibility then abruptly decreases to T2 ≈ 45 K, where the ZFC and FC susceptibilities diverge. The susceptibility cannot be fitted because of the multiple transitions below 300 K.

Pr3+. The temperature-dependent magnetic susceptibility at 0.1 T is shown in Figure 5b. The magnetic susceptibility increases as a function of decreasing temperature with a spin reorientation at T1 ≈ 25 K, followed by bifurcation of the ZFC and FC susceptibilities at 15 K. Above 100 K, the magnetic susceptibility data were fit to a Curie−Weiss equation, χ(T) = C/(T − θ), where C represents the Curie constant and θ is the Weiss temperature in the paramagnetic region. A positive Weiss temperature of θ = 31.7(2) K suggests ferromagnetic interactions. The effective moment of 11.49(4) μB corresponds to two independent Pr3+ (3.58 μB/mol), and the remaining magnetic moment of 5.15 μB/mol is due to the magnetic moments of the transition metals. In agreement with this observation, the XPS data also indicate the presence of Pr3+ (Figure 4). In the Pr 3d XPS spectra, two peaks are observed at binding energies of ∼934 eV (3d5/2) and ∼954 eV (3d3/2), which are assigned to Pr in the 3+ oxidation state.32,33 It is also noted that, because there is orbital overlap between the Pr d and f orbitals, in the Pr 3d XPS spectra, there are also two satellite peaks, which are labeled s and s′ in Figure 4.32,33 Pr2Fe4−xCoxSb5 (x ∼ 2). Figure 6a shows the field-dependent magnetization of Pr2Fe4−xCoxSb5 (x ∼ 2). The magnetization at 50 K increases until ∼1 T and starts to saturate at 0.7 μB/mol of Pr2Fe4−xCoxSb5 with a field of up to 7 T. At 3 K, a stepwise increase in magnetization is observed at 0.5 T, followed by a change in the slope, indicating a metamagnetic transition at Hc = 0.5 T. A sharp increase in magnetization is also observed up

4. CONCLUSION We have successfully grown single crystals of Pr2Fe4−xCoxSb5 (1 < x < 2.5) using an inert Bi flux. Pr2Fe4−xCoxSb5 is isostructural to Ln2Fe4Sb5 (Ln = La−Nd, Sm).5 The structural motif of Sb square nets is a common feature found in antimonide intermetallics. For instance, in LnMSb3 and LnMSb2 (Ln = La−Nd, Gd; M = Cr, Fe, Cu, Ni),21,34−41 which consist of M atoms octahedrally coordinated with Sb atoms, MSb6 octahedra are separated by layers of Ln atoms in a square-antiprismatic coordination environment. Pr2Fe4−xCoxSb5 is composed of a network of Fe/Co atoms that are tetrahedrally coordinated to Sb atoms interpenetrating a triangular lattice of transition metals (M = Fe, Co) with nearest-neighbor interactions of ∼2.6 Å. The site disorder and presence of two magnetic sublattices have also led to intriguing magnetic properties. The analogue with a higher concentration of Co exhibits an additional magnetic transition. The 1950

DOI: 10.1021/acs.inorgchem.5b02950 Inorg. Chem. 2016, 55, 1946−1951

Article

Inorganic Chemistry

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occupationally disordered M2 and positionally disordered Sb3 sites vary as a function of increasing Co concentration (x). In our quest to study the effects of Co substitution in Pr 2 Fe 4 Sb 5 , we discovered complex magnetic ordering. Pr2Fe4−xCoxSb5 (x ∼ 1) obeys a Curie−Weiss law consistent with a localized magnetic moment. The susceptibility data of Pr2Fe4−xCoxSb5 (x ∼ 2) cannot be fit to the Curie−Weiss law, and the system may be itinerant. In smaller concentrations (x ∼ 1), only Pr is magnetic, but with the introduction of a higher concentration of Co, a second transition is now observed. As is evident in Pr2Fe4−xCoxSb5 (1 < x < 2.5), a combination of competing localized and itinerant magnetic spin states coupled with two or more magnetic sublattices in systems with site disorder and a structural motif, such as the Sb square nets, may facilitate enhanced transport properties. The layered nature of these materials also warrants an investigation of the anisotropic magnetic and transport properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02950. Powder X-ray diffraction and XPS data (PDF) X-ray crystallographic data in CIF format for all compounds presented in Tables 1−3 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Grants NSF-DMR-1360863 and CHE-1213546 for partial funding of this work. REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02950 Inorg. Chem. 2016, 55, 1946−1951