Article pubs.acs.org/cm
Drastic Change of Magnetic Interactions and Hysteresis through SitePreferential Ru/Ir Substitution in Sc2FeRu5−xIrxB2 Martin Hermus,† Minghui Yang,‡ Daniel Grüner,§ Francis J. DiSalvo,‡ and Boniface P. T. Fokwa*,† †
RWTH Aachen University, Institute of Inorganic Chemistry, 52056 Aachen, Germany Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States § Institute of Energy and Climate Research (IEK), Forschungszentrum Jülich, 52425 Jülich, Germany ‡
S Supporting Information *
ABSTRACT: The quinary members of the complex boride series Sc2FeRu5−xIrxB2 were synthesized by arc melting the elements and characterized by powder and single-crystal X-ray diffraction as well as metallographic and energy-dispersive Xray analyses. The use of a 4d/5d mixture allows distinguishing these elements with X-ray diffraction methods, thus enabling the study of site preference and its influence on the magnetic properties. The magnetic measurements reveal several changes of magnetic ordering within the series: from antiferromagnetism (Sc2FeRu5B2) to ferromagnetism (Sc2FeRuIr4B2) and finally to metamagnetism (Sc2FeIr5B2). Within the quinary series, the magnetic moments continuously increase with increasing amounts of Ir in one (8j) of two possible Wyckoff sites. The members with x = 2 and 3 represent the first hard magnetic borides of transition metals.
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INTRODUCTION Metal borides, without any rare earth element, have been the focus of sustained research in the realm of materials science due to their outstanding physical properties, especially their mechanical hardness.1a As for their magnetic properties, they have been outclassed by the rare earth based borides, which includes the best permanent magnet at room temperature, Nd2Fe14B.1b Recent investigations have shown, however, that the former group of borides can also exhibit outstanding itinerant magnetic properties when a 3d magnetically active element is present.1c−g The tetragonal Ti3Co5B2 is a very versatile structure type,2 with more than 50 phases known to date, which has produced many phases with interesting magnetic properties.3−11 Its crystal structure, also adopted by the ternary variants A3T5B2, can be described as face connected trigonal, tetragonal and pentagonal prisms of Co- or T-atom building channels in which the other atoms reside; Ti or A occupies the tetragonal and pentagonal prisms whereas boron sits in the trigonal prisms. In the ordered quaternary variants, A2MT5B2, the centers of the tetragonal prisms are occupied by the magnetically active M elements. These M-atoms build within the above-mentioned channels well separated chains with intrachain and interchain distances of ca. 3.0 and 6.5 Å, respectively. Earlier experimental investigations of the magnetic properties of these quaternary phases identified magnetic ordering in some of these phases; antiferromagnetism (Sc 2 FeRu 5 B 2 ), 3 ferromagnetism (Sc2FeRh5B2)4 and metamagnetism (Sc2FeIr5B2).6 These magnetic properties have also been rationalized theoretically, as phases having 65 valence electrons (VE) were found to be © 2014 American Chemical Society
ferromagnets whereas those with 62 VE were identified as antiferromagnets.5 A few years ago, we started a VE-dependent investigation of the magnetic properties of these phases. Our investigations led to two quinary series Sc2FeRu5−nRhnB2 (n = 0−5, VE = 60− 65)7a and Ti2FeRu5−nRhnB2 (n = 1−5, VE = 63−67)8 in which drastic changes of the magnetic properties were observed. It was found experimentally7a and theoretically confirmed7b that the transition between antiferromagnetism and ferromagnetism lies between 62 and 63 VE in the Sc-based series. Furthermore, decreasing antiferromagnetic interactions followed by increasing ferromagnetic ones were observed, when the number of VE was increased between 60 and 65. Also, the ferromagnetic phases in this Sc-based series all have coercive fields (Hc) below 1 kAm−1. All members of the second series, Ti2FeRu5−nRhnB2, were found to order ferromagnetically below their respective Curie temperatures.8 Interestingly, a strong variation of the coercive field was also observed: with decreased VE between 67 and 63, increased Hc values from 0.9 to 23.9 kAm−1 were observed, thus leading to the first semihard magnetic borides of the transition metals. The use of X-ray diffraction for the structural investigation of these phases could not allow for a site preferential study of the Ru/Rh sites. We started the examination of a Ru/Ir mixture and, indeed, a strong site preference was found for the two Ru/ Ir mixed sites (8j and 2c) in the three series Ti3−xRu5−yIryB2+x,9 Received: January 21, 2014 Revised: February 13, 2014 Published: February 14, 2014 1967
dx.doi.org/10.1021/cm500237h | Chem. Mater. 2014, 26, 1967−1974
Chemistry of Materials
Article
Ti2FeRu5−xIrxB2 and Zr2Fe1−δRu5−x+δIrxB2,10 as Ir was found to prefer the 8j site whereas Ru prefers the 2c site (see Figure 1).
structure type.7−10 The trend of a (larger than c) is dominant, and it therefore dictates the trend of the unit cell volume, which also increases with increasing iridium amount (see Figure 2b). EDX analysis on the measured single crystals and on bulk materials (metallography) confirmed the compositions of the samples. For example, for Sc2FeRu4IrB2, atomic percentages of 23% Sc, 14% Fe, 48% Ru and 15% Ir have been obtained, which are within a 2% standard deviation, in good agreement with the single-crystal results (see Table 1) as well as the loaded compositions. For all syntheses, single crystals could be found and the resulting refinements (see Table 1) confirmed the isotypism of all structures with the Ti3Co5B2-type. Ruthenium and iridium are found together on the 2c and 8j sites, but with a clear site preference: while ruthenium prefers the 2c site, iridium is preferentially found on site 8j (see Table 2). For example, in Sc2FeRu4.05(8)Ir0.95(8)B2, 93(2)% ruthenium is found on site 2c while only 78(2)% ruthenium occupies site 8j. Even in the iridium-rich phase, Sc2FeRu1.2(1)Ir3.8(1)B2, more than twice the amount of ruthenium of site 8j [19(2)%] is found on site 2c [42(2)%]. No superstructure reflections have been observed, either in the powder or in the single crystal diffraction data, which could indicate an ordering on the mixed sites. The same Ru/Ir site preference was observed in the isotypic series M2Fe1−δRu5−x+δIrxB2 (M = Ti, Zr)10 and Ti3−xRu5−yIryB2+x (0 ≤ x ≤ 1, 1 < y < 3).9 In the series Sc2FeRu5−nRhnB2 and Ti2FeRu5−nRhnB2, however, no Ru/Rh site preference could be studied, because ruthenium and rhodium cannot be distinguish by X-ray diffraction methods. Nevertheless, a recent theoretical analysis11b of the Ru/Ir and Ru/Rh site preferences predicts the same type of site preference for both. Neutron diffraction may therefore be necessary to study the Ru/Rh site preference. The strong site preference observed experimentally in all Ru/Irbased phases can be explained mainly by size factors, as the polyhedrons around site 8j are larger than those around site 2c. A detailed analysis of this behavior was given for Ti3−xRu5−yIryB2+x,9 Ti2FeRu5−xIrxB2 and Zr2Fe1−δRu5−x+δIrxB2,10 series. In the isotypic Zr-based series, Zr2Fe1−δRu5−x+δIrxB2, a subsequent Fe/Ru mixing was observed for the 2a site.10 Likewise, a possible Fe/Ru mixing was also checked in the new Sc-based series. Several single crystals of each composition were checked, and although a small incorporation of the heavier transition metal in this site could be detected, the refined occupancy was always lower than thrice the standard deviation. Consequently, the 2a site is, in all cases, fully occupied by iron within an error margin below 3σ. The refined formulas of all four compositions are within standard deviation, in good agreement with the initial stoichiometric weight (see Table 1);
Figure 1. Perspective view along [001] of the crystal structure of the Sc2FeRu5−xIrxB2 series.
Recently, theoretical calculations on M2Fe(Ru0.8T0.2)5B2 (M = Sc, Ti, Zr; T = Rh, Ir) phases also confirmed these findings.11 Furthermore, this theoretical study also predicts that substituting Rh or Ir preferentially at the 8j site increases the calculated magnetic moments on the Fe atoms. In the Ti2FeRu5−xIrxB2 and Zr2Fe1−δRu5−x+δIrxB2 series, however, the lack of high quality powder samples has prohibited the study of their magnetic properties. As mentioned above, the two quaternary members of the Sc-related series are already known; antiferromagnetic Sc2FeRu5B22 and metamagnetic Sc2FeIr5B2.4 The present work is, therefore, focused on the synthesis of the new members of this Sc-series, their detail structure analysis and the study of their magnetic characteristics as a function of valence electron count and Ru/Ir site preference.
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RESULTS AND DISCUSSION Phase Analysis, Structure Refinement and Crystal Chemistry. Powder X-ray diffractometry showed that the quinary members of the Sc2FeRu5−xRhxB2 (x = 1−4) series, adopting the Ti 3 Co 5 B 2 -type structure, were the main components of two phase products. The intermetallic Ru1−xFex (x < 0.3) phase12 in the Ru-rich side or the Ru1−x−yIryFex, (x, y < 0.3) phase in the Ir-rich side were identified and confirmed by EDX and metallographic analyses as the second phase. A Rietveld refinement estimated the amount of each of these minor phases to be under 8% (see Table S1 and Figure S1 in the Supporting Information). Along the series, the lattice parameters vary in opposite directions: a increases with increasing iridium content, while c decreases (see Figure 2a). This behavior is typical to compound series of the Ti3Co5B2
Figure 2. Lattice parameters (a), unit cell volume (b) and iridium content on Wyckoff sites 8j and 2c as function of the number of valence electrons (VE) in the series Sc2FeRu5−xIrxB2. 1968
dx.doi.org/10.1021/cm500237h | Chem. Mater. 2014, 26, 1967−1974
Chemistry of Materials
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Table 1. Crystallographic and Single-Crystal Structure Data for the Sc2Fe1Ru5−xIrxB2 Series formula
Sc2FeRu4.05(8)Ir0.95(8)B2
space group, Z valence electrons (VE) formula weight (g/mol) crystal size (mm3)/F(000) a (Å) c (Å) V (Å3) calcd density (g/cm3) abs. coefficient (mm−1) θ range (deg) hkl range
Sc2FeRu3.2(1)Ir1.8(1)B2
60.9 759.77 0.06 × 0.02 × 0.01/659 9.329(3) 3.0103(9) 262.0(2) 9.63 40.4 4.89−34.93 −15 ≤ h ≤ 15 −15 ≤ k ≤ 11 −4 ≤ l ≤ 4 2628, 0.0777 351
no. of reflns, Rint indep reflns extinction coefficient no. parameters R1, wR2 (all I) diff peak/hole (e·Å−3) CSD numbera
Sc2FeRu2.04(7)Ir2.96(7)B2
P4/mbm (no. 127), 2 61.8 63.0 837.69 942.03 0.04 × 0.01 × 0.01/715 0.02 × 0.01 × 0.01/799 9.345(3) 9.380(2) 3.0038(9) 3.0020(6) 262.3(2) 264.1(1) 10.60 11.84 59.6 84.7 4.88−34.94 4.86−35.61 −6 ≤ h ≤ 15 −12 ≤ h ≤ 15 −15 ≤ k ≤ 14 −15 ≤ k ≤ 15 −4 ≤ l ≤ 4 −4 ≤ l ≤ 4 2617, 0.0810 2717, 0.0552 352 371 0.0034(2) 20 21 0.0601, 0.0853 0.0348; 0.0408 3.459/−3.811 2.032/−2.713 427261 427263
20 0.0544, 0.0817 2.587/−3.832 427262
Sc2FeRu1.2(1)Ir3.8(1)B2 63.8 1019.95 0.04 × 0.01 × 0.01/857 9.398(3) 2.9962(9) 264.7(2) 12.80 103.58 4.85−34.94 −15 ≤ h ≤ 10 −15 ≤ k ≤ 12 −4 ≤ l ≤ 4 2697, 0.0861 354 0.0030(4) 21 0.0433, 0.0612 2.906/−3.803 427264
a
More details on the structure determination may be obtained from the Fachinformationszentrum Karlsruhe (e-mail address: crysdata@fiz-karlsruhe. de), D-76344 Eggenstein-Leopoldshafen, Germany, on quoting the given CSD depository numbers.
Table 2. Atomic Coordinates, Site Occupation Factors (sof) and Displacement Parameters for the Sc2FeRu5−xIrxB2 Seriesa name
site
Sc2FeRu4.05(8)Ir0.95(8)B2 Ru1 8j Ir1 8j Ru2 2c Ir2 2c Sc3 4g Fe4 2a B5 4g Sc2FeRu3.2(1)Ir1.8(1)B2 Ru1 8j Ir1 8j Ru2 2c Ir2 2c Sc3 4g Fe4 2a B5 4g Sc2FeRu2.04(7)Ir2.96(7)B2 Ru1 8j Ir1 8j Ru2 2c Ir2 2c Sc3 4g Fe4 2a B5 4g Sc2FeRu1.2(1)Ir3.8(1)B2 Ru1 8j Ir1 8j Ru2 2c Ir2 2c Sc3 4g Fe4 2a B5 4g a
x
y
z
sof
Ueq
U11
U22
U33
0.07099(8) 0.07099(8) 0 0 0.3252(2) 0 0.123(2)
0.21428(7) 0.21428(7) 0.5 0.5 0.1748(2) 0 0.377(2)
0.5 0.5 0.5 0.5 0 0 0
0.78(2) 0.22(2) 0.93(2) 0.07(2) 1 1 1
0.0084(2) 0.0084(2) 0.0069(5) 0.0069(5) 0.0108(7) 0.0077(7) 0.014(3)
0.0076(3) 0.0076(3) 0.0064(5) 0.0064(5) 0.0112(9) 0.0057(9)
0.0070(3) 0.0070(3) U11 U11 U11 U11
0.0104(3) 0.0104(3) 0.0078(7) 0.0078(7) 0.010(2) 0.012(2)
0.07094(8) 0.07094(8) 0 0 0.3251(3) 0 0.124(2)
0.21473(8) 0.21473(8) 0.5 0.5 0.1749(3) 0 0.376(2)
0.5 0.5 0.5 0.5 0 0 0
0.60(2) 0.40(2) 0.80(2) 0.20(2) 1 1 1
0.0067(2) 0.0067(2) 0.0066(5) 0.0066(5) 0.0098(9) 0.0081(9) 0.016(5)
0.0063(3) 0.0063(3) 0.0055(6) 0.0055(6) 0.009(1) 0.005(1)
0.0050(3) 0.0050(3) U11 U11 U11 U11
0.0088(3) 0.0088(3) 0.0087(8) 0.0087(8) 0.011(2) 0.014(2)
0.07057(4) 0.07057(4) 0 0 0.3244(2) 0 0.125(2)
0.21590(4) 0.21590(4) 0.5 0.5 0.1756(2) 0 0.375(2)
0.5 0.5 0.5 0.5 0 0 0
0.35(2) 0.65(2) 0.65(2) 0.35(2) 1 1 1
0.0056(2) 0.0056(2) 0.0040(3) 0.0040(3) 0.0073(5) 0.0054(6) 0.010(3)
0.0054(2) 0.0054(2) 0.0040(3) 0.0040(3) 0.0076(7) 0.0046(8)
0.0051(2) 0.0051(2) U11 U11 U11 U11
0.063(2) 0.063(2) 0.0041(4) 0.0041(4) 0.0068(9) 0.007(2)
0.06972(6) 0.06972(6) 0 0 0.3240(3) 0 0.125(2)
0.21620(6) 0.21620(6) 0.5 0.5 0.1760(3) 0 0.375(2)
0.5 0.5 0.5 0.5 0 0 0
0.19(2) 0.81(2) 0.42(2) 0.58(2) 1 1 1
0.0056(2) 0.0055(2) 0.0046(4) 0.0046(4) 0.0079(8) 0.0033(8) 0.010(4)
0.0057(3) 0.0057(3) 0.0044(4) 0.0044(4) 0.008(1) 0.002(1)
0.0044(3) 0.0044(3) U11 U11 U11 U11
0.0063(3) 0.0063(3) 0.0052(5) 0.0052(5) 0.0069(2) 0.006(2)
U13, U23 = 0, U12 is 0 within standard deviation. For boron, Ueq = Uiso.
1969
dx.doi.org/10.1021/cm500237h | Chem. Mater. 2014, 26, 1967−1974
Chemistry of Materials
Article
Table 3. Selected Interatomic Distances for the Sc2FeRu5−xIrxB2 Series within pentagonal prisms Sc3 Ru2/Ir2 Sc3 Ru1/Ir1 within tetragonal prisms Fe4 Ru1/Ir1 within trigonal prisms B5 Ru1/Ir1 B5 Ru2/Ir2 network of prisms (in [001] plane) Ru2/Ir2 Ru1/Ir1 Ru1/Ir1 Ru1/Ir1 (along [001] axis) Fe4 Fe4
Sc2FeRu4.05(8)Ir0.95(8)B2
Sc2FeRu3.2(1)Ir1.8(1)B2
2.754(3) 2.833(2)−2.932(2)
2.756(3) 2.835(3)−2.932(3)
2.771(2) 2.840(2)−2.936(2)
2.778(3) 2.846(3)−2.933(3)
2.5885(8)
2.5927(8)
2.6062(5)
2.6081(7)
2.19(2) 2.22(2)
2.18(2) 2.22(2)
2.179(9) 2.233(9)
2.18 (2) 2.23(2)
2.747(2) 2.833(2)−2.978(2)
2.747(2) 2.833(2)−2.989(2)
2.7458(7) 2.8325(7)−3.0131(7)
2.747(1) 2.845(1)−3.019(2)
3.0103(9)
3.0038(9)
3.0020(6)
2.9962(9)
Sc2FeRu1.2(1)Ir3.8(1)B2
0.3) phase in the Ir-rich side of the new quinary series. We have also successfully synthesized and analyzed magnetically Ru1−xFex for x = 0.25, which was found to be Pauli paramagnetic in accordance with the reported susceptibilty measurements on Ru1−xFex (x < 0.5) alloys.13 Therefore, the magnetic orderings (see Figures 3−6) observed in these samples should be attributed to the main phases only (Table 4).
therefore, in the following, we will designate the phases with their ideal formulas. The two mixed Ru/Ir sites build in the crystal structures, trigonal, tetragonal, and pentagonal prisms, in the centers of which the other atoms are found. The small boron atoms occupy the centers of the trigonal prisms (Wyckoff site 4g). Iron is found inside the tetragonal prisms (Wyckoff site 2a), whereas the larger Scandium is found in the pentagonal prisms (Wyckoff site 4g). Figure 1 shows a projection of the crystal structure along [001]. The distances found in these new quinary compounds (see Table 3) are comparable to those found in the isotypic series, for example, in M2Fe1−δRu5−x+δIrxB2 (M = Ti, Zr). The iron atoms build chains along the [001] axis with intrachain Fe−Fe distances, which vary between 2.996(1) and 3.010(1) throughout the series. Although no strong bonding interactions are expected between the iron atoms, these distances are in the perfect range for inducing strong magnetic interactions in these compounds, as already demonstrated in many isotypic compounds and series.1c
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Sc2FeRu2.04(7)Ir2.96(7)B2
MAGNETISM
In the following section, we will discuss the magnetic properties of the new quinary members of the Sc2FeRu5−xIrxB2 series, compare them with those of the quaternary members Sc2FeRu5B2 and Sc2FeIr5B2 and discuss the similarities and differences with the Sc2FeRu5−nRhnB2 and Ti2FeRu5−nRhnB2 series. Magnetic Properties of the Known Sc2FeRu5B2 and Sc2FeIr5B2. At high temperatures, the magnetic susceptibilities of the previously characterized Sc2FeRu5B2 (60 VE)3,7a obey the Curie−Weiss law χm = C/(T − θ). The experimentally obtained Curie constant C = 2.4 × 10 −5 m 3 ·K·mol −1 corresponds to the paramagnetic moment μ = 3.9 μB per formula unit and a Weiss constant θ = −995 K, indicating strong antiferromagnetic Fe−Fe interactions. The intermetallic phase Sc2FeIr5B2 (n = 5 with 65 VE) orders metamagnetically at TN = 190 K, but above TN, the observed Curie−Weiss behavior indicates very strong ferromagnetic interactions with a Weiss constant θ = +241 K and a paramagnetic moment μ = 4.5 μB per formula unit.6 In the following, we will discuss the new quinary phases with intermediate VE counts (61, 62, 63 and 64). As mentioned above already, all phases are contaminated by the minor (amount < 6%) intermetallic Ru1−xFex (x < 0.3) phase in the Ru-rich side or the minor (amount < 9%) Ru1−x−yIryFex, (x, y
