Valence and Magnetic Investigations of Alkali Metal-Doped Europium

Article pubs.acs.org/cm

Valence and Magnetic Investigations of Alkali Metal-Doped Europium Sulfide William L. Boncher,† Edward A. Görlich,

‡

Krzysztof Tomala,

‡

Julie L. Bitter,§ and Sarah L. Stoll*,†

†

Department of Chemistry, Georgetown University, Washington, D.C. 20057, United States M. Smoluchowski Institute of Physics, Jagellonian University, Krakow, Poland § Department of Chemistry, Johns Hopkins University, Baltimore, Maryland 21218, United States ‡

S Supporting Information *

ABSTRACT: Using europium sesquioxide nanowires as a starting material for the synthesis of europium sulfides, we have discovered alkali metal doping Eu1−xNaxS for the first time. Alkali metal doping stabilizes the NaCl structure type of EuS at surprisingly low temperatures. We investigated the europium valence through XPS and Mössbauer spectroscopy as a function of dopant in Eu1−xNaxS for x = 0−0.5. In addition, based on magnetic studies, the ferromagnetic ordering is diminished by the presence of the nonmagnetic dopant, causing a decrease in the ordering temperature. KEYWORDS: europium sulfide, sodium-doping, XPS, 151Eu Mössbauer

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to sulfides of copper, silver, antimony, and bismuth,18 maintaining the morphology of the parent structure. Conversion chemistry for ‘anion exchange’ has also been demonstrated for the reaction of oxides with nitrogen and sulfur sources to form metal nitrides19−23 and sulfides.24,25 Frequently anion exchange is associated with the formation of hollow nanomaterials, depending on the diffusivity of the cation versus anion.13 It is also possible to form core-oxide/shellsulfide structures from nanowires of metal oxides.26 The conversion of oxide nanowires to sulfides was initially explored as an approach to prepare sulfur analogs of carbon nanotubes, where layers of a given material such as MoS2, WS2, or NbS2 (for example) compose the concentric graphene sheets. Another advantage of anion conversion is that nanowire arrays of metal oxides, which are easily prepared such as for ZnO, can be converted to sulfide nanowires of materials such as ZnS, which are quite difficult to prepare. Here, we have studied the dependence of temperature on the anion exchange process for Eu2O3 to EuS and discovered that the reduced reaction temperatures for chemical conversion of nanostructures has allowed for the formation of alkali metal doped europium sulfide, Eu1−xNaxS for x = 0−0.5. Alkali metal doping of europium sulfide appears to stabilize the rock salt structure of EuS at surprisingly low temperatures (350 °C) and causes significant changes in cell constants and the magnetic properties. We have explored the synthesis and characterization

INTRODUCTION There has been a strong interest in the synthesis of nanoscale europium sulfide (EuS) due to its properties as an intrinsic magnetic semiconductor.1−5 This stems from the search for novel luminescent,4,5 magnetic,6−8 and photomagnetic properties observed in this class of materials. EuS has a strong magneto-optical Kerr effect,9 colossal magnetoresistive effects,9 and demonstrated spin-filtering effects with potential applications in spintronics.10,11 The most common method for synthesizing EuS nanoparticles is by thermal decomposition of dithiocarbamate complexes in coordinating solvents.2 However, this results in ligand capped nanoparticles, which can hinder some applications by creating a barrier against charge or ion transport when incorporated into devices.12 By utilizing preformed Eu2O3 nanowires, we have investigated the oxide as a templating precursor to form uncapped nanostructured EuS at reduced temperatures through sulfurization reactions, a combination of anion replacement and europium reduction. Chemical transformation of nanostructures can be an effective synthetic tool and is likely to broaden the range of materials synthesized with morphological control.13,14 Many synthetic transformations have been demonstrated, including galvanic replacement, oxidation, and diffusion, but perhaps the most successful technique is cation exchange.15 One of the earliest examples is the remarkable exchange of cadmium in CdSe with silver to form Ag2Se, which is reversible at room temperature.15 Nanoparticles of CdS have been undergone cation-exchange with copper and lead,16 as well as platinum and palladium.17 The effect has also been demonstrated in nanowires, for example ZnS nanowires have been transformed © 2012 American Chemical Society

Received: August 10, 2012 Revised: October 16, 2012 Published: October 18, 2012 4390

dx.doi.org/10.1021/cm3025507 | Chem. Mater. 2012, 24, 4390−4396

Chemistry of Materials

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mortar and pestle to increase surface area. Percent yield: 88%. Powder X-ray diffraction patterns matched that of Eu(OH)3, PDF 00-017-0781 (ICDD, 1979).28 Eu2O3 Nanowires. Eu(OH)3 nanowires (200 mg) were heated in a quartz tube in a Lindberg/Blue M horizontal tube furnace at 500 °C for 3 h under dynamic vacuum. Percent yield: 83%. Powder X-ray diffraction patterns matched that of the cubic phase of Eu2O3, PDF 00034-0392 (ICDD, 1984).29 Sodium-Doped Europium Sulfide. Eu2O3 (either nanowires as synthesized, or bulk material) was ground with a mortar and pestle to increase surface area and homogenize the sodium distribution. Initially, nanowires containing remnant sodium hydroxide were used; later, more thoroughly washed nanowires were used, and sodium hydroxide or chloride was manually loaded and ground with the material. Sample material was heated on a graphite boat in a quartz tube in a Lindberg/ Blue M horizontal tube furnace at 200 °C for 1 h under dynamic vacuum. A mixture of 5% hydrogen sulfide and 95% nitrogen gas was then flowed through the quartz tube over the material, and the furnace was set to a temperature between 350 and 900 °C, which varied with the experiment, for 2 h. After heating, the tube was removed from the furnace, and once cooled to room temperature, the stopcocks and valves were closed, and the flow of gas was ceased. Obtained powder X-ray diffraction patterns could be indexed using EuS, PDF 00-0261419 (ICDD, 1974),30 with differences in peak position and intensity due to a shift in the lattice constant and europium occupancy. Alkali and Alkaline Earth Metal-Doped Europium Sulfides. Eu2O3 powder was ground with a mortar and pestle with lithium hydroxide or calcium chloride. Sample material was then heated on a graphite boat in a quartz tube in a Lindberg/Blue M horizontal tube furnace at 200 °C for 1 h under dynamic vacuum. A mixture of 5% hydrogen sulfide, 95% nitrogen gas was flowed through the quartz tube over the material, and the furnace was at a temperature of either 650 or 800 °C for 2 h. After heating, the tube was removed from the furnace, and once cooled to room temperature, the stopcocks and valves were closed, and the flow of gas was ceased. Obtained powder X-ray diffraction patterns were similar to that of EuS, PDF 00-026-1419 (ICDD, 1974),30 with differences in peak position and intensity due to a shift in the lattice constant and europium occupancy in materials which were successfully doped. (Eu3+)x(Na+)x(Eu2+)1−2xS Reduction. In a nitrogen atmosphere glovebox, 10 mg Eu1−xNaxS nanocrystals were placed on top of 100 mg Eu foil (0.1 mm thick) on a graphite boat in a quartz tube. The tube was taken out of the glovebox and heated in a Lindberg/Blue M horizontal tube furnace at 700 °C for 3 h under dynamic vacuum. Percent yield: 78%. A powder X-ray diffraction pattern was matched with that of EuS, PDF 00-026-1419 (ICDD, 1974).30

of these doped materials using X-ray powder diffraction, XPS, 151 Eu Mössbauer spectroscopy, and magnetic susceptibility.

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EXPERIMENTAL SECTION

General Information. Europium nitrate hexahydrate and europium oxide were obtained from Strem. Sodium hydroxide, sodium chloride, and calcium chloride were obtained from Sigma. Lithium hydroxide was obtained from Fisher. Europium metal was obtained from Alfa Aesar. Chemicals were used as received. X-ray powder diffraction patterns were obtained using a Rigaku Ultima IV X-ray powder diffractometer with Cu Kα radiation at 40 kV/44 mA with a D/teX silicon strip detector. Selected area electron diffraction was performed with a JEOL JEM 2100F field emission gun transmission electron microscope, at 200 kV. Scanning electron microscopy (SEM) images were taken with a Zeiss SUPRA 55-VP scanning electron microscope, at an acceleration voltage of 20 kV with an in-lens detector. Magnetic Measurements. Magnetic measurements were made on a Quantum Design MPMS SQUID from 5 K to 300 K in a 10 000 Oe field. Crystalline, powdered samples containing ca. 10 mg of compound were loaded into a gelatin capsule. The sample was positioned within a plastic straw for analysis. The diamagnetic correction of the sample was calculated using χd = (−184.03/2)10−6 emu/mol and subtracted. The paramagnetic Curie temperature, θ, was determined using 1/χ versus T plots. The plots were fit using a linear regression. Arrott plots were obtained by calculating isotherms from 13 to 23 K. For each field, 500, 1000, 2000, and 5000 Oe, the magnetization was squared (emu2/mol2) and plotted as a function of H/M (Oe g/mol). The values of MS2 where H/M is zero (for each isotherm) were then graphed as a function of temperature. The MS2 vs T plot where MS2 goes to zero was used to determine the Curie temperature, TC, based on a linear regression. X-ray Photoelectron Spectroscopy (XPS). XPS analysis was performed using a PHI 5400 XPS system (base pressure 1000 °C.32 Unlike most lanthanide sulfides which form Ln2S3 (in various polytypes), the purely trivalent europium does not form a binary sulfide. The most stable phase with the soft sulfur, is the soft divalent europium, resulting in the monochalcogenide, EuS. There is a mixed valent binary compound, Eu3S4, which has a normal spinel-type structure, A2+(B3+)2(X2‑)4 where the A cation is Eu2+, and the B cation is Eu3+.33 In our reactions we found that increasing temperatures destroys the morphology of the oxide nanowires resulting in highly sintered agglomerated materials, shown in Figure 1. Optimistically, we explored the effect of decreasing temperature, to determine at what point EuS forms and whether the morphology could be improved. 4391



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experimental, specifically with regard to the peak intensities. We used the refined europium occupancy to determine the sodium content, as the EDX and XPS are qualitative. Europium sulfide has been doped with lanthanides,35 but to our knowledge, there are no examples of doping with alkali or alkaline earth metals. By contrast, binary alkaline earth sulfides (MgS, CaS, SrS, BaS) with the same rock salt structure have been doped with Eu2+ (or Ce3+) and are well-known to luminesce. These materials have been extensively studied for thin film electroluminescent phosphors.36 Beyond doped materials, there are very few ternary alkali metal lanthanide sulfide (A−Ln−S) phases known. The most commonly reported material has the composition ALnS2 (A = alkali metal and Ln = lanthanide), in which the lanthanide is in the trivalent oxidation state. Lanthanide reactions in alkali metal− polychalcogenide fluxes expanded this to include NaLnS3 for Ln = La and Ce, which is relatively modest given the wide number of interesting new materials discovered in this solvent system.37 Another class of compounds of interest for display materials are phosphors based on the lanthanide oxysulfides, Ln2O2S, which are difficult to prepare and frequently have poor nanostructured morphology.38−40 Interestingly, Eu2O2S can be prepared using the same precursors for EuS nanoparticles (dithiocarbamate complexes) but under an oxygen, rather than inert, atmosphere.41 Recently, it was discovered that welldefined nanostructures of lanthanide oxysulfides could be prepared as long as sodium doping was present.42 Regardless of morphology, the authors were unable to form Ln2O2S nanomaterials unless sodium was incorporated. The role of Na+ doping was ascribed to differences in the hard/soft nature of the cations in forming either the hard oxide (La2O3) or ‘soft’ oxysulfide (La2O2S) material. The high doping level (∼10%) results in the formation of anion vacancies. We were interested in the systematic variation in doping level with temperature, as seen in Figure 3. At temperatures above

Figure 1. SEM images of Eu2O3 nanowires (left) and resulting EuS material (right).

By reducing the temperature to as low as 350 °C, we discovered that the major product based on X-ray powder diffraction, was EuS with the rock salt structure type. Unfortunately, even at this low temperature, the nanowire morphology was lost. However, the stabilization of the cubic material was surprising, and there was also an unexpected reduction in the cell lattice constant to 5.72(1) Å clearly below that reported for EuS (a = 5.969 Å). The lattice constant was determined by Rietveld refinement of powder X-ray diffraction data, using GSAS/EXPGUI.34 The Rietveld refinement also revealed the material to be electron deficient at the europium site, based on the peak intensities. By contrast, EuS formed under similar conditions but at 900 °C exhibited only a modest shift in cell constant compared to material made at lower temperature. At all synthetic temperatures (350−900 °C), a cubic rock salt material, which indexed to EuS, was formed with a systematic decrease in cell constant as the temperature decreased (see Figure 2). While small amounts of Eu2O3 or

Figure 2. EuS-like material synthesized at 450 °C. Reference lines labeled with their respective miller indices indicate where the observed EuS lines would expected to be. Red stars show small Eu2O2S impurity.

Eu2O2S impurity phases were observed, the cell constant appeared to be a function of reaction temperature with plateaus at the low and high temperatures. Although we initially ascribed the shift in cell constants to europium deficient Eu1−xS, we have determined that the change is in fact due to alkali metal doping. The presence of sodium was observed with XPS (see Supporting Information), and qualitatively the amount of sodium appears to vary with the cell constant. Adventitious NaOH, remnant from the synthesis of Eu(OH)3 nanowires (used to prepare Eu2O3 nanowires) could be removed by copious washing of the Eu(OH)3 prior to dehydration and subsequent sulfurization, and this eliminated the change in cell constant. We were also able to induce a systematic change in cell constant through the addition of NaOH or NaCl to the Eu2O3 material prior to sulfurization. The Rietveld refinements were adjusted to account for the presence of sodium doping, refining based on Eu1−xNaxS. This improved the fit of the calculated diffraction pattern to the

Figure 3. Cell constant and sodium occupancy of europium sulfide synthesized at different temperatures.

800 °C, we found that the doping level depends on the stoichiometric ratio of NaOH to Eu2O3 and the resulting product’s cell constant varies linearly (see the Supporting Information). The linear correlation between cell constant and Na occupancy suggests the solid solution, Eu1−xNaxS obeys Vegard’s law. At high temperatures, the cell constant approaches a maximum below the literature value for EuS. We believe the amount of adventitious sodium remained relatively constant from synthesis to synthesis, resulting in this lack of variation in cell constant in the high temperature data (Figure 3). 4392



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charged Eu3+ is likely to have low diffusivity. It is interesting to note that in a comparison of the cell constants for the ALnS2 series (Ln = Sm−Yb), the hexagonal a parameter linearly decreases across the lanthanide series reflecting the lanthanide contraction of the Ln3+ ions, but the cell constant for NaEuS2 is off the trend line. The increased cell constant for the europium material may reflect the presence of some divalent europium.45 By contrast, compared with the lanthanides that form the cubic materials NaLaS2 (5.868 Å), NaCeS2 (5.819 Å), NaNdS2 (5.767 Å), the cubic NaEuS2 cell constant (5.72 Å) reported here is consistent with trivalent europium. We were interested to confirm the europium oxidation state as a function of doping level. To maintain charge balance, the sodium doping level should be accompanied by commensurate increase in trivalent europium, (Eu3+)x(Na+)x(Eu2+)1−2xS, although defects or anion vacancies are also possible. Despite the presence of both divalent and trivalent europium in the cubic material, the normal spinel Eu3S4, with the mixed-valence europium, was never observed. XPS studies were performed on a series of materials with a range of doping levels to probe the relative concentration of Eu3+ to Eu2+, whose signals are well separated. Although it was difficult to quantify the exact ratio of Eu2+ to Eu3+, it is clear that decreasing sodium content (increasing temperature) corresponds to a higher Eu2+/Eu3+ ratio, shown in Figure 5.

As temperature was decreased, the sodium doping increased. Notably, at the lowest temperature (350 °C), the doping level did not vary with starting stoichiometry. At 350 °C, Eu0.5Na0.5S was always formed, with excess europium resulting in Eu2O3. The stabilization of the cubic rock salt structure with a distinct composition at low temperatures can be understood by the limited europium reduction. At 350 °C without sodium, the europium sesquioxide is not reduced; only Eu2O3 is observed in the X-ray powder diffraction suggesting that Eu2+ is not thermodynamically favored at this temperature. In the presence of sodium, the cubic rock salt material is formed, but always as Eu0.5Na0.5S, even under a range of NaOH/Eu2O3 ratios. The oxidation state of the europium in the 1:1 metal ratio is purely trivalent, again suggesting the conditions are not reducing enough. The excess europium appears as Eu2O3 even under repeated sulfurization or extended reaction times. Material prepared at any temperature below 450 °C only forms Eu0.5Na0.5S, as seen where the graph flattens at low temperatures in Figure 3, with the ∼5.72(1) Å cell constant. For materials synthesized at temperatures between the two extremes, intermediate doping levels are observed, based on the increased reduction of Eu3+ to Eu2+, as the synthetic temperature is increased. The valence appears to be trivalent for the 1:1 composition. When Eu0.5Na0.5S, with a small lattice constant, was reacted with europium metal, the resulting X-ray diffraction pattern showed that the lattice constant of the material returned to the literature value (5.969 Å) for stoichiometric EuS and the intensities of the material were also near the expected values. This suggests that fully reduced europium in EuS does not support alkali metal substitution. The X-ray diffraction patterns before and after europium reduction are seen in Figure 4.

Figure 5. XPS data for the 4d Eu band; showing increase Eu2+/Eu3+ ratios at increased synthesis temperatures.

One complication in quantifying the ratio is the observed increase in oxygen levels at low doping values. This creates ambiguity of the source of Eu3+ as due to alkali metal doping versus the presence of small amounts of Eu2O3 and Eu2O2S impurities in these materials as seen in the X-ray pattern. The volume difference of Eu2+ (ionic radii for 6 coordinate cation is 1.31 Å) and Eu3+ ions (versus 1.02 Å), is often reflected in changes in lattice parameters, but this is complicated by the presence of Na+, which is slightly larger than Eu3+ (1.16 Å).45 To distinguish between europium valence states resulting from differences in a local Eu environment, even if randomly distributed over the sample, the Mössbauer effect being a local method is well suited. It is particularly feasible with 151Eu spectroscopy where the isomer shift differences for Eu2+ and Eu3+ states are distinctly larger than the resonance line width. The shift occurs as a consequence of the change of the 4f state population (4f 7 → 4f 6, respectively), which influences the

Figure 4. Powder X-ray diffraction patterns of Eu1−xNaxS before (green) and after (blue) reduction with europium metal.

The material we have identified as Eu0.5Na0.5S, is the same composition as the previously reported NaEuS2.43 The ALnS2 material can exhibit two structure types: a cubic rock salt with disordered cations or a hexagonal α-NaFeO2 material. The αNaFeO2 structure has the same anion packing as the rock salt but with segregation of the monovalent and trivalent cations into alternating layers. Cation ordering is frequently observed when the charge between the two cations differs by more than one unit.44 Previously, NaEuS2 has been reported only in the cation-ordered hexagonal structure and not the disordered cubic structure, as we observe here. It seems reasonable that this is due to the low reaction temperatures, where the highly 4393



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the dynamic process of fluctuation between the two valence states at the rate larger than set by the observation method, on the order of 108 s−1 (see Figure 6). The spectrum of the sample prepared at 350 °C, which based on X-ray powder diffraction is Eu0.5Na0.5S, consists of a single absorption peak originating from trivalent europium ions, based on the peak position. This data excludes the presence of the crystallographically proper EuS phase which should contain only divalent europium. The inability to fit the data well with a single Lorentzian line, is most likely due to the presence of a small amount of Eu2O3 impurity, as revealed in the X-ray powder diffraction. The uppermost spectrum in Figure 6 shows the result of fitting with an additional separate component accounting for the europium oxide impurity (at the level of about 10%). The parameters of this impurity line were constrained to the literature values.46 It is important to note that, in general, due to limitations set by the resolution, it is not possible to identify different phases, which contain Eu3+ ions. The higher temperature of synthesis (650 °C), with midrange dopant, Eu1−xNaxS for x between 0.5 and 0, clearly exhibits divalent europium as expected for EuS phase. In addition, the Mössbauer spectrum reveals the presence of a large amount of trivalent europium in addition to the expected divalent component, shown in the middle spectrum in Figure 6. The absorption line related to Eu3+ contributes to 42(2)% of the total absorption spectrum area and is described by the values of parameters that may be compared to those for the Lorentz line obtained for the spectrum of the Eu0.5Na0.5S (formed at 350 °C). While the value of line-width parameter, fwhm = 3.2(1) mm/s, agrees quite well with the value obtained in the case of the Eu0.5Na0.5S (350 °C sample), the line position, IS = −0.02(10), differs beyond the quoted statistical errors (see Table 1). It should be noted that the description of the absorption profile with the single Lorentz line should be considered as ineffective, due to inability to resolve contributions from individual local environments of the europium ions (which, nevertheless preserve their valence state with 4f 6 occupancy). The Mössbauer spectrum of the sample synthesized with lower doping values clearly demonstrates that the most dominant fraction (85%) represents divalent europium, but still, some amount of trivalent ions can be observed (bottom spectrum in Figure 6). It is possible that there is Eu3+ present as oxidized material that is not crystalline enough to be observed by X-ray powder diffraction. Magnetic studies were performed on materials synthesized at different temperatures, determining the Curie−Weiss constant, Θ, and the strength of the magnetic moment, μeff, using Curie− Weiss 1/χ vs T plots, and when possible, the Curie temperature, TC, using Arrott plots. The results are shown in Table 2. These studies provide insight into both the europium valence (based on the magnetic moment, μeff) as well as the ferromagnetic coupling. (See the Supporting Information for a complete tabulation of results.) Using the Curie−Weiss plots, it was found that the reduced magnetic moment (μeff) decreases from the highest value 7.39 to 4.3, generally with increasing sodium doping level. The high μeff value found in stoichiometric EuS is consistent with the spin only value (μeff is 7.94 for S = 7/2) found for Eu2+.31 As the sodium level increases, the concentration of Eu3+ (with S = 3) increases, causing a reduction in μeff. Magnetic studies also indicate that the ferromagnetic coupling in Eu1−xNaxS decreases with increasing doping levels (reduced paramagnetic

screening, thus resulting in the change of s(p) electron density at the nucleus. The Mössbauer spectra obtained for the representative samples of the investigated materials are shown in Figure 6, and

Figure 6. 151Eu Mössbauer spectra of Eu1−xNaxS materials synthesized at 350 °C, 650 °C, and 800 °C.

Table 1. Results of Eu3+

Eu2+

Eu Mössbauer Studiesa

151

sample (°C)

IS (mm/s)

fwhm (mm/s)

contr. (%)

800 650 350 800 650

−0.29(6) −0.02(10) −0.25(6) −12.4(1) −12.1(1)

3.6(2) 3.2(1) 3.2(1) 3.2(1) 3.3(1)

15(2) 42(2) 100 85(2) 58(2)

Values of parameters resulting for a single Lorentzian fit of the absorption peaks in the 151Eu Mössbauer spectra. Relative contributions (contr.) of divalent and trivalent components are calculated as a respective fraction of the surface (area) under the resonance lines. The value may be used as an estimation of the abundance of europium in a given valence state if one neglects the differences in the magnitudes of the Mössbauer−Lamb factors. a

Table 1 collects the results of the theoretical analysis of the experimental data. The spectra of samples prepared with different doping levels clearly show the presence of absorption peaks at positions either characteristic to a trivalent europium with the isomer shift close to zero or to a divalent Eu state with very large negative value (∼ −12 mm/s) of the isomer shift, or both, but not ‘in-between’. Intermediate values would have indicated an intermediate-valence state, which is understood as 4394



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Notes

Table 2. Magnetic Properties of Eu1−xNaxS Samples with Various Doping Levels a (Å)

x (Na)

Θ (K)

5.713(1) 5.710(1) 5.738(2) 5.756(1) 5.821(1) 5.898(1) 5.899(0) 5.959(0)

0.54(2) 0.53(2) 0.53(2) 0.46(1) 0.32(1) 0.19(1) 0.17(1) 0.09(1)

−248.32 −169.88 −133.8 −97.43 −12.4 15.27 14.5 25.09

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the NSF Award No. CHE112387, and X-ray powder diffraction was obtained using an instrument purchased from the MRI program at NSF (CHE0959546). We acknowledge the support of the Maryland NanoCenter and its NispLab. The NispLab is supported in part by the NSF as a MRSEC Shared Experimental Facility. We thank Gordon Yee at Virginia Tech for his assistance with some of the magnetic measurements.

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temperatures, Θ, and lower TC). The loss of magnetic ordering is consistent with the replacement of the magnetic europium by nonmagnetic sodium, resulting in disruption of the europium− europium magnetic communication. What is interesting is the strength of the antiferromagnetic coupling, as observed in the large negative Weiss constants for high sodium doping levels. One would expect the nonmagnetic ion to disrupt any coupling. The antiferromagnetism is a through bond superexchange mechanism, while the ferromagnetism is due to coupling of the magnetic moment with the conduction electrons and may be strongly influenced by the oxidation state of europium.

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CONCLUSIONS In conclusion, we report the first example of sodium-doped europium sulfide Eu1−xNaxS and discovered that the presence of sodium stabilizes the rock salt cubic structure at surprisingly low temperatures. Unlike previous reports of the 1:1 material, Eu0.5Na0.5S or NaEuS2, which were found to exhibit a hexagonal structure, we find that the sodium and europium are randomly distributed on the cation site resulting in a cubic rock salt structure type. Temperature plays an important role in forming materials with high sodium concentration, and we believe this is due to the extent to which europium is reduced. The conditions become more reducing as the temperatures is increased, requiring high temperatures (above 700 °C) to achieve a divalent state. The variation of europium oxidation through trivalent (x = 0.5) to divalent (x = 0) was determined using XPS and 151Eu Mössbauer spectroscopy. The consequences for the magnetic coupling were determined using magnetic susceptibility measurements as a function of temperature and show that the ferromagnetic ordering is weakened by incorporation of the nonmagnetic sodium ions.

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ASSOCIATED CONTENT

S Supporting Information *

Electron diffraction images at different synthetic temperatures, SEM images before and after sulfurization, correlation between cell lattice and doping level, expanded magnetic information, and the sodium region of the XPS survey scan. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. 4395



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Valence and Magnetic Investigations of Alkali Metal-Doped Europium

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