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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Switching on a Spin Glass: Flux Growth, Structure, and Magnetism of La11Mn13−x−yNixAlySn4−δ Intermetallics Julia V. Zaikina,†,# Van S. Griffin,‡ and Susan E. Latturner*,† †

Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States Applied Superconductivity Center, National High Magnetic Field Laboratory, Tallahassee, Florida 32310, United States

‡

S Supporting Information *

ABSTRACT: Reactions of tin and manganese in a lanthanum/ nickel eutectic melt in alumina crucibles produce La11Mn13−x−yNixAlySn4−δ (0 ≤ x ≤ 3.6; 2.5 ≤ y ≤ 4.9; 0.6 ≤ δ ≤ 1.1) phases with the stoichiometry dependent on the reactant ratio. These compounds crystallize in a new tetragonal structure type in space group P4/mbm, with a = 8.4197(1) Å, c = 19.2414(3) Å, and Z = 2 for La11Mn8.2Ni0.8Al4Sn3.3. The structure can be viewed as an intergrowth between La6Co11Ga3-type layers and Cr5B3-type La/Sn slabs. This system represents a unique playground to study the itinerant magnetism of diluted icosahedral Mn layers. The dilution of manganese sites in the Mn/Ni/Al layer with nonmagnetic elements has a significant effect on magnetic properties, with low Mn content analogues being paramagnetic and higher Mn content analogues such as La11Mn10Al3Sn3.4 exhibiting spin-glass behavior with a freezing transition at 20 K. The lack of long-range magnetic ordering is confirmed by heat capacity and resistivity measurements.

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INTRODUCTION Intermetallic compounds containing either transition metals or rare-earth metals (or both) exhibit a variety of complex magnetic characteristics.1 The networks of transition metals and magnetic anisotropy of the rare-earth ions in Nd2Fe14B and SmCo5 yield hard ferromagnetic behavior, making these compounds very useful as components in hard drives and electric motors.2,3 The interplay between the magnetism of europium and surrounding transition-metal layers in EuCo2Pn2 (Pn = P, As)which can exhibit various europium oxidation states and moments on transition metal sites, depending on temperature and pressurehas been extensively studied.4,5 Intermetallics containing rare-earth and 3d transition metals exhibit a plethora of interesting and exotic ground states, including heavy Fermion behavior, superconductivity, and Kondo effect.6−8 In addition to long-range magnetic ordering, many multinary intermetallics exhibit a variety of short-range or disordered magnetic interactions, such as formation of spin clusters or spin-glass behavior, as seen in R117M52+xX112+y (R = rare-earth metal, M = Fe, Co, and Cr, and X = Ge, Sn), Dy5Pd2, and La21Fe8Sn7C12.8−10 Characterization and understanding of these complex magnetic behaviors requires detailed information about the structure and bonding within a compound, as well as highpurity samples. Both of these requirements can be met by the growth of large single crystals. Metal flux growth has proven to be an excellent method to produce crystals of multinary intermetallic compounds.11,12 This synthetic technique allows for reactions at low temperatures, which promotes the © XXXX American Chemical Society

formation of metastable or kinetically stabilized phases as opposed to the more thermodynamically stable compounds that form at higher temperatures. Slow cooling of the melt allows products to grow as large crystals. Eutectic mixtures of early rare-earth metals with late first-row transition metals are effective solvents for the majority of elements, allowing for the isolation of complex intermetallics such as La21M8Sn7C12 (from La/Ni melt; M = Fe or Mn), Pr2Co2SiC (from Pr/Co melt), Nd8Co4Ge2C3 (from Nd/Co melt), and Ce33Fe13B18C34 (from Ce/Co melt).10,13−16 In this work, reactions of manganese and tin (and aluminum leached from the reaction crucible) in La/Ni eutectic produced a series of compounds La11Mn13−x−yNixAlySn4−δ with a new structure type. Analogues can also be grown by replacing the manganese with iron or the tin with lead. The crystal structure features layers of linked icosahedra of transition metals with partial aluminum substitution, separated by large slabs comprised of lanthanum and tin atoms. The magnetic properties of the compounds are strongly dependent on the concentration of manganese in the transition-metal layers; as Mn is substituted by Ni and Al, the magnetic behavior changes from spin-glass to paramagnetic. This offers a significant advance in the ability to tune a material across the threshold between disordered paramagnetism and spin-glass behavior; traversing this threshold may enable access to complex states of matter such as cluster glasses and spin liquids. Received: October 6, 2017

A

DOI: 10.1021/acs.inorgchem.7b02555 Inorg. Chem. XXXX, XXX, XXX−XXX

a

La11Mn8.2Ni0.8Al4Sn3.3 (II)

a = 8.4197(1) c = 19.2414(3) 1364.05(3) 6.148 24.050 1.06 to 41.50° 14 129 2527 [Rint = 0.059] 2527/51 1.00 R1 = 0.0372 wR2 = 0.0710 R1 = 0.067 wR2 = 0.085 3.34/−2.86 1567594

La11Mn6.7Ni2.9Al3.4Sn3.1 (I)

a = 8.382(2) c = 19.221(4) 1350.4(5)

6.216 24.837 2.12 to 37.97° 12 267 1911 [Rint = 0.033]

1911/0/51 1.10 R1 = 0.027 wR2 = 0.058 R1 = 0.035 wR2 = 0.059 2.52 and −1.62 1573503

M = Mn/Ni/Al or Fe/Ni/Al; E= Sn or Pb (Compounds I−VI).

volume, Å3 Z density (calc), mg/m3 absorption coeff, mm−1 θ range reflections collected independent reflections refinement method data/parameters goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) max diff peak/hole, e·Å−3 CCDC No.

refined composition temperature wavelength space group unit cell dimensions, Å

Table 1. Crystal Data and Structure Refinement for La11M13E4−δa La11Mn9.2Ni1.4Al2.4Sn3.4 (III) La11Mn10Al3Sn3.4 (IV) 295(2) K Mo Kα (λ = 0.710 73 Å) P4/mbm (No. 127) a = 8.4233(1) a = 8.4342(1) c = 19.2255(3) c = 19.2412(4) 1364.09(3) 1368.74(4) 2 6.288 6.219 24.889 24.263 3.42 to 44.82° 1.06 to 47.39° 20 331 25 384 3001 [Rint = 0.036] 3372 [Rint = 0.055] full-matrix least-squares on F2 3001/51 3372/50 1.28 1.06 R1 = 0.0270 wR2 = 0.0528 R1 = 0.0325 wR2 = 0.0672 R1 = 0.030 wR2 = 0.054 R1 = 0.051 wR2 = 0.077 3.96 and −2.13 3.43/−3.49 1572278 1567592 2968/50 1.02 R1 = 0.0357 wR2 = 0.0815 R1 = 0.049 wR2 = 0.088 4.98/−4.01 1567590

6.620 39.319 2.11 to 44.60° 20 807 2968 [Rint = 0.052]

a = 8.4401(4) c = 19.349(1) 1378.3(1)

La11Mn6.6Ni1.5Al4.9Pb3.1 (V)

3062/50 1.00 R1 = 0.0328 wR2 = 0.0559 R1 = 0.063 wR2 = 0.064 3.64/−3.58 1567593

6.159 24.526 2.11 to 45.53° 19 957 3062 [Rint = 0.063]

a = 8.3095(1) c = 19.3075(3) 1333.14(3)

La11Fe7.5Ni0.8Al4.7Sn2.96 (VI)

Inorganic Chemistry Article

B

DOI: 10.1021/acs.inorgchem.7b02555 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 2. Atomic Coordinates and Equivalent Isotropic Displacement Parameters and Site Occupancy Factors for La11Mn6.7(2)Ni2.9(1)Al3.39(3)Sn3.108(4) (I)

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atom

Wyckoff

x/a

y/b

z/c

s.o.f.

Ueq (Å2)

La(1) La(2) La(3) La(4) Sn(1) Sn(2) Mn(1)/Ni(1) Mn(2) Mn(3)/Al(3) Mn(4)/Ni(4)

2b 4e 8k 8k 4e 4h 2d 8i 8k1 8k2

0 0 0.66836(2) 0.16378(2) 0 0.12307(7) 0 0.06745(8) 0.6183(1) 0.17485(5)

0 0 0.16836(2) 0.66378(2) 0 0.62307(7) 1/2 0.20784(8) 0.1183(1) 0.67485(5)

1/2 0.13430(2) 0.38755(1) 0.23517(1) 0.31480(2) 1/2 0 0 0.11647(6) 0.07265(3)

1 1 1 1 1 0.554(2) 0.78/0.22(4) 1 0.152/0.848(7) 0.33/0.67(2)

0.0236(1) 0.00962(8) 0.01367(7) 0.01123(7) 0.01022(9) 0.0132(2) 0.0073(3) 0.0090(1) 0.0085(3) 0.0092(2)

was consecutively refined. The positions for which refinement yielded Mn occupancy higher than 100% were set as mixed occupied by Mn/ Ni (positions Mn(1) and/or Mn(4)), while positions with smaller than 100% refined occupancy were refined as mixed occupied by Mn/ Al (positions Mn(2) and Mn(3)). One of the tin positions, 4h, was found to be partially occupied, while the occupancy of another tin position, 4e, refined close to 100%. The crystal structures of analogues La11Mn6.6Ni1.5Al4.9Pb3.1 (V) and La11Fe7.5Ni0.8Al4.7Sn2.96 (VI) were determined similarly. Crystallographic information is listed in Table 1 for compounds I−VI; atomic positions and site occupancies for representative example La11Mn6.7Ni2.9Al3.4Sn3.1 (I) are listed in Table 2. Additional tables of atomic positions and bond lengths for each analogue are found in Supporting Information (Tables S1 and S2). Further issues concerning the crystal structure refinement are discussed in the Results and Discussion section. Elemental Analysis. Semiquantitative elemental analysis was performed with energy-dispersive X-ray spectroscopy (EDXS) on a JEOL 5900 scanning electron microscope equipped with PGT Prism energy dispersion spectroscopy software. Crystals were oriented with a flat face perpendicular to the beam and analyzed using a 30 kV accelerating voltage and an accumulation time of 100 s. In addition to determining the ratios of La, Mn (or Fe), and Sn (or Pb), the samples were also monitored for Ni and Al, which may be incorporated from the flux or from etching of the crucible, respectively. Magnetic Properties. The temperature and field dependencies of magnetization of La11Mn13−x−yNixAlySn4−δ samples with compositions of I−IV were measured over the temperature range of 1.8−300 K using a SQUID magnetometer (Quantum Design MPMS system). Powdered samples were contained in gelatin capsules. For anisotropy studies, single crystals (2−30 mg each) were sealed into kapton tape in specific crystallographic orientations with respect to the applied field and inserted into a plastic straw. Field-cooled (FC) and zero-fieldcooled (ZFC) measurements were performed at several applied fields (0.01, 0.02, 0.03, 0.3, and 3 T), while field-dependent isothermal measurements were done at 1.8 K. Alternating-current (AC) susceptibility data were collected from 5 to 40 K with an applied AC field of 0.0005 T at four different frequencies, 1, 10, 100, and 1000 Hz. Heat Capacity and Resistivity. Resistivity measurements were performed on large crystals of La11Mn13−x−yNixAlySn4−δ (compositions II and IV) using a conventional four-point direct-current (DC) method on a Physical Property Measurement System (PPMS) by Quantum Design. Crystals (size range: 5 × 1 × 1 mm3) were mounted on the sample holder of a 4He probe with a small amount of N-type Apiezon vacuum grease. Crystals were connected to the electrodes of the sample holder with 0.001 in. diameter gold wires using a point welder. Measurements were performed from 1.8 to 300 K, using an applied excitation current of 1 mA. The heat capacity measurement of the same crystals was performed on PPMS in zero-field and at applied fields of 0.01 and 2 T at temperatures between 2 and 200 K.

EXPERIMENTAL PROCEDURE

Synthesis. La11(Mn/Ni/Al)13Sn4−δ phases were formed from the reaction of Mn and Sn in La/Ni flux, with aluminum introduced by the etching of the alumina crucibles by the strongly reducing flux. Reactants were stored and handled in an argon-filled glovebox. Lanthanum powder (99.9%, Alfa Aesar), manganese powder (Alfa Aesar, 99.6%), and tin powder (99.9%, Fisher Chemicals) were combined in a variety of ratios, with the powders sandwiched between layers of La/Ni eutectic (88:12 wt %, Alfa Aesar 99.9%; 1.5 g used per reaction) in an alumina crucible. For example, a 1:1:1 mmol ratio of La/Mn/Sn reacts in 1.5 g of La/Ni eutectic to form La11Mn9.2Ni1.4Al2.4Sn3.4 (III). Reactions were also performed exploring deliberate addition of millimole amounts of aluminum (99%, Strem Chemicals) as a reactant. In addition, analogues containing iron instead of manganese and lead instead of tin were synthesized from reactions of La, Fe, and Sn or La, Mn, and Pb in La/Ni flux, respectively. For each flux reaction, reactants were loaded into an alumina crucible inside a glovebox; a second alumina crucible filled with quartz wool was inverted above the reaction crucible to act as a filter during centrifugation. These were placed into a silica tube, which was flamesealed under a vacuum of 1 × 10−2 torr. The ampule was then heated to 950 °C in 3 h, held at this temperature for 12 h, and then cooled to 850 °C in 10 h. The reaction mixtures were held for 48 h at 850 °C and then cooled to 600 °C in 84 h. At 600 °C the ampule was removed from the furnace, quickly inverted, and placed into a centrifuge for 60 s to decant the molten flux. Solid products were scraped out of the alumina crucible and stored in an argon-filled glovebox. X-ray Single-Crystal Structure Determination. The crystal structures of La11Mn13−x−yNixAlySn4−δ analogues (compounds I−IV) were determined by X-ray single-crystal diffraction methods. Crystal fragments cleaved from larger crystals (previously analyzed by EDXS) were mounted on glass fibers using epoxy. Diffraction data were collected at room temperature on a Bruker SMART APEX2 CCD diffractometer with a Mo Kα X-ray tube. The data were collected up to high 2θ (sin θ/λ > 0.85) values to ensure a sufficient data/parameter ratio and to facilitate refinement of the Mn/Ni ratio by enhancing the difference in scattering factors of these elements. The data frames were integrated with the Bruker SAINT software package; correction for absorption effects was performed using numerical methods.17 The data sets were indexed in the primitive tetragonal unit cell with parameters a ≈ 8.4 Å, c ≈ 19.2 Å. Examination of systematic absences in the X-ray single-crystal data set suggested three possible space groups P4/mbm, P4̅b2, and P4bm. The highest symmetry P4/mbm (No. 127) was chosen for the structure solutions. Refinement was performed using the SHELX-97 software package.18 The positions of all atoms except Al(3)/Mn(3) were found by direct methods. Siting of Al(3)/Mn(3) was determined from a combination of least-squares refinement and difference Fourier maps. The assignment of atomic positions was made based on local coordination as well as on occupancy refinement. At first stage of the refinement, all positions within the M13 layer were set as fully occupied by Mn; afterward, the occupancy of these positions C

DOI: 10.1021/acs.inorgchem.7b02555 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Structure of La11Mn13−x−yNixAlySn4−δ. La atoms represented by red spheres, Sn by blue spheres; different transition-metal sites are colorcoded: black M(1), cyan M(2), purple M(3), and green M(4), M = Mn/Ni/Al. (a) Tetragonal unit cell viewed down b-axis; M13 icosahedra shown in polyhedral mode. (b) The Mn/Ni/Al slab of linked icosahedra, viewed down c-axis. (c) An isolated M13 icosahedron with sites labeled. (d) Bicapped square antiprismatic coordination of the Sn(1) site. (e) Dimers formed by the partially occupied Sn(2) site and its symmetry equivalent, surrounded by La.

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RESULTS AND DISCUSSION Synthesis. Crystals of La11Mn13‑x‑yNixAlySn4‑δ phases were initially found as minor byproducts of reactions of Mn, Sn, and either B or C in a 1:1:1 mmol ratio in an excess of La/Ni flux, along with predominant phases Mn2B or La21Mn8Sn7C12.13 Optimization of the synthesis conditions was performed, including variation of reactant ratios and temperature profile. It is notable that no La11Mn13−x−yNixAlySn4−δ is obtained if the centrifugation temperature is raised from 600 to 700 °C or above. Thus, this phase is most likely formed in a narrow temperature range between 600 and 700 °C, slightly above the temperature of solidification of La/Ni eutectic melt (∼530 °C).19 Control of composition was complicated by the fact that aluminum was leached from the alumina crucible during the reaction, and nickel was incorporated from the La/Ni flux. Deliberate addition of aluminum metal as a reactant led to more aluminum-rich compositions; to minimize aluminum content (to target more magnetically interesting manganeserich compositions), aluminum was not deliberately added to subsequent reactions. The (La/Ni)La/Mn/Sn reactant ratios were varied according to the schematic compositional diagram in Supporting Information (Figure S1), taking into account that the composition of La11Mn13−x−yNixAlySn4−δ is situated in between that of La6Mn13−x−yNixAlySn and La5Sn3. When the starting ratios are close to the composition of the target phase La11Mn13−x−yNixAlySn4−δ, it tends to form together with the structurally related La6Mn13−x−yNixAlySn phase. This byproduct La6Mn13−x−yNixAlySn has the La6Co11Ga3 structure type and has been previously studied;20 its crystals have a similar platelike habit to those of La11Mn13−x−yNixAlySn4−δ (Figure S2). Increasing the amount of tin reactant promotes the formation of La11Mn13−x−yNixAlySn4−δ; however, crystals of La5Sn3 become a significant byproduct. The following starting La/ Sn/Mn ratios react in La/Ni flux to produce crystals with different stoichiometries: a 1.5:0.5:0.5 mmol ratio produced La11Mn8.2Ni0.8Al4Sn3.3 (II), a 1:1:1 mmol ratio produced La11Mn9.2Ni1.4Al2.4Sn3.4 (III), and a 1.2:0.7:1 mmol ratio

produced La11Mn10Al3Sn3.4 (IV). A 1:1:1 mmol ratio in only 1 g of La/Ni flux produced La11Mn6.7Ni2.9Al3.4Sn3.1 (I). Analogues can be formed substituting iron for manganese or lead for tin; production of these phases was promoted by deliberately adding aluminum. The lead analogue La11Mn6.6Ni1.5Al4.9Pb3.1 (V) was produced from the reaction ratio of La/Mn/Pb/Al = 1.7:1:0.5:0.5 mmol in 1.5 g of La/Ni eutectic. La6Fe7.5Ni0.8Al4.7Sn3.0 (VI) was produced from a flux reaction with starting composition La/Fe/Sn/Al = 1.7:1.5:0.5:0.5 mmol in 1.5 g of La/Ni eutectic. Crystal Structure and Comparison with Other Structure Types. La11Mn13−x−yNixAlyE4−δ (E = Sn or Pb) phases crystallize in tetragonal space group P4/mbm with a layered, highly anisotropic crystal structure (Figure 1). Layers comprised of linked M13 icosahedra (M = Mn/Ni/Al) are separated by nonmagnetic La/E slabs (E = Sn or Pb). The M13 icosahedra are centered by a Mn(1)/Ni(1) site on special position 2d; this is coordinated by 12 atoms in the positions M(2), M(3), and M(4). The icosahedra are linked to each other by short M(2)−M(2), M(3)−M(4), and M(2)−M(4) bonds to form two-dimensional M13 slabs. The M13 slabs are separated from each other by La/Sn layers. There are two sites occupied by tin atoms within the La/Sn block. Position 4e is fully occupied by Sn(1). This site is coordinated by 10 La atoms (Sn@La10) forming bicapped tetragonal antiprisms with La−Sn distances of 3.42−3.60 Å; see Figure 1d. These distances are very similar to those seen in other La/Ni flux grown products containing tin, including La14Sn(MnC6)3 (with Sn@La9 clusters, La−Sn distances of 3.42−3.56 Å) and La6SnNi3.67Ru0.76Al3.6 (containing Sn@La9 clusters with La− Sn distances of 3.39−3.42 Å).21,22 Another tin-occupied position Sn(2) lies on a 4h site; this forms dimers with its symmetry equivalent (dSn−Sn = 2.92−2.99 Å). This position consistently refines as partially occupied (occupancy factor 0.55−0.71). The tin dimers are surrounded by 12 La atoms as shown in Figure 1e (Sn2@La12, with dLa−Sn = 3.28−3.34 Å). The crystal structure of La11Mn13−x−yNixAlySn4−δ can be viewed as an intergrowth of two known structure types. The relationships between the title phase and other intermetallic D

DOI: 10.1021/acs.inorgchem.7b02555 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Structural relationships between compounds comprised of layers of icosahedra of transition metals (shown as yellow polyhedra) and slabs of La/Sn (red and blue spheres), ranging from transition-metal-rich end-member LaFe12Al (NaZn13 type) to transition-metal-free La5Sn3−γ.

Structural Features and Disorder. Dilution of the transition-metal layers of La11Mn13−x−yNixAlySn4−δ with nonmagnetic aluminum and nickel has a significant effect on the magnetic behavior of this phase (vide infra). The aluminum content in mixed occupied sites can be determined rather accurately from single-crystal X-ray diffraction (XRD). Aluminum preferably occupies position M(3), which is the outermost, capping site of the M13 layer (8k1 Wyckoff site). In addition to being part of the M13 icosahedra, this site is additionally coordinated by five La atoms (3.31 Å ≤ dAl−La ≤ 3.47 Å). This is similar to the siting in the La6Fe13−xAlx phase, where Al tends to occupy the capping site of the Fe13 layer.20 However, in the La6Fe13−xAlx phase the Al occupancy on this site does not exceed 35%; in La 11 Mn 13−x−y Ni x Al y Sn 4−δ compositions rich in aluminum the capping M(3) site is fully occupied by aluminum. Aluminum is also found in the 8i M(2) sites of La11Mn13−x−yNixAlySn4−δ. The aluminum content is never higher than 20% for this site in Al-rich compositions, while for Mn-rich compositions aluminum does not incorporate into this site at all. Therefore, La 11Mn13−x−yNixAlySn4−δ incorporates more aluminum into its transition-metal layers compared to the structurally related La6Fe13−xAlx. While the determination of Mn/Ni site mixing based on Xray single-crystal data is hindered by the similarity in their scattering factors, the overall Mn/Ni ratio derived from X-ray single-crystal data refinement roughly matches that determined by EDXS. If nickel is incorporated, it is found to mix with manganese on the M(1) 2d (center of icosahedron) site and the M(4) 8k2 site (see Table 2 and Table S1 in Supporting Information). The simultaneous occupancy of these positions by three types of atoms Mn/Ni/Al cannot be ruled out; however, in the structurally related phases LaFe13−xSix and La6Fe13−xAlx the incorporation of p-elements into the site in the center of the icosahedra has never been observed.29 The Sn(2) position of La11Mn13−x−yNixAlySn4−δ (4h Wyckoff site) consistently refines as partially occupied. Partial occupancy of this site is also seen in the lead analogue La11Mn6.6Ni1.5Al4.9Pb3.1 (V) (see Table S1; the Pb(2) site is 53.4(2)% occupied). The Sn(2) atom in the 4h site has a short distance of 2.92−2.99 Å to its symmetry equivalent. This produces tin dimers surrounded by 12 lanthanum cations (Sn2@La12). Similar Sn−Sn distances are found in binary La− Sn and alkali-earth metal−tin compounds (2.91 Å in La5Sn4; 2.92 Å in BaSn; 2.98 Å in Ba5Sn3).30−32 The partial occupancy of this position (ranging from 55.4(2) to 71.2(3)%) implies two statistically possible situations. According to the first

structures are shown in Figure 2. The La/Sn block has the structure of Cr5B3, with La and Sn atoms on the Cr and B positions, respectively.23 The M13 layer is the same as that found in the cubic NaZn13 structure. The NaZn13-type layer of 3d metals doped with p-elements (Al, Si, etc.) is a common structure motif found in other intermetallic compounds. For instance, cubic LaFe12Al is isostructural to NaZn13; substitution of Al on iron sites is required to stabilize this phase (LaFe13 is not known). 23 In the crystal structure of La 6 Fe 10 Al 4 (La6Co11Ga3 type) the iron−aluminum slabs Fe10Al3 are 7.5 Å apart, separated from each other by a La/Al block.20 Slabs Fe10Al3 at z = 0 and z = 1/2 are rotated 90° with respect to each other, thus producing the I-centering of the La6Fe10Al4 unit cell. The La/Al block of this structure adopts a section of the Cr5B3-structure type with La atoms in Cr sites and Al atoms taking the boron position that is surrounded by Cr atoms only. In the structure of Th4Mn13Sn5 the separation between Mn13 slabs is also ∼7.5 Å, but the Mn13 layers have identical orientation, which leads to a P-centered unit cell with a cparameter approximately half that of the I-centered La6Fe10Al4.24 The Th/Sn block, separating the Mn13 slabs, adopts the middle section of the Cr5B3 structure, but in this case Cr positions are taken by Sn, while B positions, which form pairs of symmetry equivalents, are occupied by Th atoms. In the structure of title phase La11Mn13−x−yNixAlySn4−δ, the M13 slabs (M = Mn/Ni/Al) are further separated (14.7 Å apart; Figure 2). Similar to Th 4 Mn 13 Sn 5 , the M 13 slabs of La11M13Sn4−d have uniform orientation, which results in the P-centering of the cell, but the c-parameter is greatly expanded. As the distance is increased between M13 slabs (M = Fe or Mn, doped with Ni and Al) in the series LaM13−La6Fe10Al4−Th4Mn13Sn5−La11M13Sn4, a corresponding decrease in 3d metal content occurs, with the extreme case being Cr5B3-type “La5Sn3−γ” with no M13 slab present at all. Bulk La5Sn3 actually has the W5Si3 structure type, but the La5Tt3 (Tt = Ge, Sn, Pb) phases are all susceptible to polymorphism with temperature or interstitial stabilization.25−28 La5Si3 is stable in the Cr5B3 structure type, and La5Pb3 converts to this structure if small interstitial atoms are present. It is possible that the partial occupancy of the tin atom on t he 4h site of La11Mn13−x−yNixAlySn4−δ stabilizes the La/Sn slab in the Cr5B3 structure type. Given the position of La11Mn13−x−yNixAlySn4−δ in the middle of the structural series shown in Figure 2, it is not surprising that La6M10Al3Sn and La5Sn3 are observed as byproducts from flux reactions with low and high amounts of tin, respectively. E

DOI: 10.1021/acs.inorgchem.7b02555 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry model, the majority of Sn(2) atoms have no neighboring tin in the adjacent 4h position and the rest of Sn(2) atoms (10.8− 21.2%) form Sn−Sn dimers. Another extreme situation would occur if this site is either 0 or 100% occupied, corresponding to all Sn(2) atoms being part of Sn−Sn pairs. The latter situation is less probable, because it corresponds to a considerable amount of vacancies in the crystal structure. Another issue is that this position can also be refined as mixed occupied by tin and a lighter atom (e.g., this site refines as 0.33 Sn/0.67 Ni or 0.62 Sn/0.38 Al for compound II). However, the EDXS data for this compound show a La/Sn ratio close to 11:3, which indicates a considerable amount of tin in this position. Also, 2.92−2.99 Å is typical for a Sn−Sn bond, and similar Sn2@La12 dimer units are found in the structure of La5Sn4 (Sm5Ge4-type with dSn−Sn = 2.91 Å).30 Substitution of a smaller Ni or Al on this site would lead to Ni−Sn or Al−Sn moieties, which would have shorter distances (Sn−Ni: 2.49 Å in LaNiSn and 2.55 Å in LaNi2Sn2; Sn−Al: 2.70 Å in Sr3Al2Sn2 and 2.77 Å in Ba3Al2Sn2).33−35 Nonetheless, the possibility of minor doping of Ni or Al into the 4h site of La11Mn13−x−yNixAlySn4−δ cannot be completely neglected. For instance, the doping of Ni into this site was observed in the case of La5Si3 phase (Cr5B3-type), which has the same La−Sn block as the title phase.28 But only a small amount of Ni can be incorporated into this site while retaining the same structure: x→max = 0.265 for La5Si3−xNix. Variation of Unit-Cell Parameters with Composition. The La11Mn13−x−yNixAlySn4−δ phase exhibits a wide homogeneity range (all refined compositions fall into the range 0 ≤ x ≤ 3.6; 2.5 ≤ y ≤ 4.9; 0.6 ≤ δ ≤ 1.1) associated with mixed occupation of positions within the M13 layer by Mn/Ni/Al atoms as well as with partial occupancy of 4h position with Sn atoms. The variation in composition affects interatomic distances and unit-cell size. The a-parameter increases with increase of Mn content, while the c-parameter decreases. The analysis of interatomic distances shows that the distances between M(2) and M(2) sites (Δd = 0.04 Å) follows the same trend as the dependence of the a-parameter versus Mn content (Figure S3). The elongation of this distance is greater compared to that of other interatomic distances with the same trend (M(3)−M(4)). The M(2)−M(2) bond links the M13 icosahedra to form a net in the ab-plane; thus, the change in this distance has the largest impact on the a-parameter with increasing Mn content. Magnetism. The magnetic properties of La11Mn13−x−yNixAlySn4−δ strongly depend on composition of the Mn/Ni/Al layer, particularly on Mn content. Susceptibility measurements indicate that the compounds with compositions La11Mn6.7Ni2.9Al3.4Sn3.1 (I) and La11Mn8.2Ni0.8Al4Sn3.3 (II), rich in Al and relatively low in Mn, are paramagnetic in the entire measured temperature range. Data for I are shown in Figure 3; data for II are similar (Figure S4, in Supporting Information). No magnetic ordering is observed. The high-temperature (T > 100 K) magnetic susceptibility data were fitted with the modified Curie−Weiss law, χ = χ0 + C/(T − θ) (Figure 3). The fit for compound I indicates a temperature-independent term χ0 = 0.0074(8) emu/mol, a negative Weiss constant (θ = −83(15) K), and a total magnetic moment per formula unit of 6(1) μB. This corresponds to an average of ∼2 μB per Mn atom, which is in the range of those reported for other intermetallics such as YMn6Ge6 (3.0 μB), LuMn6Sn4Ge2 (2.3 μB), LaMn4Al8 (0.6 μB), and La21Mn8Bi7C12 (1.0 μB).1,36,37,13 Increasing the Mn content, particularly in the M(3) position, promotes the appearance of cooperative magnetic phenomena.

Figure 3. Magnetic susceptibility data on powdered samples of paramagnetic La11Mn6.7Ni2.9Al3.4Sn3.1 (I) (●) (Happlied = 200 Oe) and spin-glass La11Mn9.2Ni1.4Al2.4Sn3.4 (III) (□) (Happlied = 300 Oe). Hightemperature data (T > 100 K) was fitted using modified Curie−Weiss law (orange line for I and magenta line for III). The 1/χ data for compound III is shown as blue squares.

Magnetic measurements were performed for both powdered samples of La11Mn9.2Ni1.4Al2.4Sn3.4 (III) (Figure 3) and large single crystals of La 11 Mn 9.2 Ni 1.4 Al 2.4 Sn 3.4 (III) and La11Mn10Al3Sn3.4 (IV) (Figures 4 and S5). The magnetic susceptibility for both of these compounds exhibits a cusp at ∼20 K for both zero-field-cooled (ZFC) and field-cooled (FC) data. The data exhibit ZFC-FC divergence below this cusp,

Figure 4. Magnetic susceptibility temperature dependence data for an oriented single crystal of La11Mn9.2Ni1.4Al2.4Sn3.4 (III). (top) Data collected at low field (H = 0.01 T) show a cusp at 20 K and FC/ZFC splitting in both orientations, indicative of a spin freezing transition. (bottom) Data collected at high field (μ0H = 3 T) indicates the spinglass state is eliminated. Feature at 50 K is due to presence of trace oxygen. F

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Inorganic Chemistry which is not typical for anti-ferromagnets. Moreover, the FC curve exhibits a significant increase of the magnetization below 9 K. The high-temperature susceptibility data for the powdered sample of III (Figure 3) was fit with a modified Curie−Weiss law; however, the presence of three parameters in the χ = χ0 + C/(T − θ) resulted in correlations between them. The unconstrained fitting yields a negative Weiss constant (−68(9) K), which is indicative of anti-ferromagnetic interactions between adjacent Mn atoms, a total moment of 4.4(3) μB per f.u., and considerable temperature-independent term, χ0 = 0.0482(6) emu/mol. This term is due to the Pauli paramagnetism of the conduction electrons, and it is essential to include it into the fitting, as evidenced from the 1/χ versus T dependence (Figure 3). Constraining the value of χ0 to a lower value of 0.045 emu/mol led to a larger Weiss constant (θ = −109(3)K) and total magnetic moment of 6(1) μB per f.u. The resulting average moment per Mn atom for III is ∼2 μB, similar to that of I. It appears that as the manganese content increases from La11Mn6.7Ni2.9Al3.4Sn3.1 (I) to La11Mn9.2Ni1.4Al2.4Sn3.4 (III), the strength of the anti-ferromagnetic coupling force increases. This causes a change from the paramagnetic behavior of I (and II) to the spin-glass behavior of III (and IV). Anisotropic measurements on a single crystal of La11Mn9.2Ni1.4Al2.4Sn3.4 (III) are shown in Figure 4 (data for a single crystal of IV is shown in Figure S5 and is similar). The magnetic susceptibility was higher with the field oriented parallel to the crystallographic c-axis of the sample, indicating this is the easy axis of magnetic moment alignment. Similar to the powder data shown in Figure 3, a cusp is observed at 22 K for both orientations of the crystal at 0.0100 T, and the ZFCFC curves diverge at this temperature with FC curves exhibiting an increase of the susceptibility at low T. This might be indicative of spin-glass-like behavior. The origin of such glassiness is likely the disorder in the magnetic structure due to random substitution of magnetic Mn with nonmagnetic Al or Ni. Application of a large external field of 3 T (see Figure 4, bottom) leads to the suppression of the ZFC/FC splitting, resulting in a broad ferromagnetic-like transition. This behavior can be explained by the nature of spin-glass freezing: moments freeze in random directions when the sample is cooled in the absence of magnetic field (ZFC) but will slightly align with the applied field for the FC-process. The cusp in the ZFC/FC curve corresponding to the spin freezing temperature disappears at stronger field (3 T). At the higher magnetic field, spins can overcome the frustrated anti-ferromagnetic coupling forces associated with randomly oriented magnetic moments in the disordered structure of La11Mn13−x−yNixAlySn4−δ. The field dependence of the magnetization at 1.8 K for III (Figure S6, Supporting Information) supports this, showing a change in slope above 1 T as the strength of the field overcomes the spin-glassiness, but no saturation is observed at higher fields. The preferred alignment along the c-axis is also confirmed by the field dependence data. To verify the spin-glass behavior, AC magnetization data were collected at this temperature range for La11Mn9.2Ni1.4Al2.4Sn3.4 (III) at different frequencies (Figure 5). The cusp in magnetization exhibits clear frequency dependence, further indication of spin-glass-like properties.38 Fitting of the frequency dependence of the shift in the cusp temperature (Figure S7) yields an empirical Mydosh parameter, φ = 0.011(2), where φ = (Tmax(ν1) − Tmax(ν2))/Tmax(ν1)(log ν1 − log ν2), Tmax(ν) is the temperature of the maximum in the χ′ versus T curve at the corresponding frequency. The

Figure 5. AC magnetic susceptibility data for La11Mn9.2Ni1.4Al2.4Sn3.4 (III); the frequency dependence of the cusp at 20 K indicates a spin freezing transition.

determined value of the Mydosh parameter is in the range typical for spin glasses (0.004−0.08).38 Similar analysis for the AC susceptibility data for IV (Figure S8) indicates a Mydosh parameter of 0.022(2), also consistent with spin glassiness. Heat Capacity and Resistivity. Heat capacity studies were performed to verify the absence of long-range magnetic ordering. Data were collected on a crystal of La11Mn10Al3Sn3.4 (IV), which exhibits a magnetic transition, and a crystal of La11Mn8.2Ni0.8Al4Sn3.3 (II), which does not (Figure 6). It is

Figure 6. Heat capacity data for paramagnetic La11Mn8.2Ni0.8Al4Sn3.3 (II) and spin-glass La11Mn10Al3 Sn3.4 (IV) at zero-field and at μ0H = 0.01 T for (IV).

notable that there is no feature in the heat capacity data for IV at 20 K, indicating that the cusp in the magnetic susceptibility data is not due to a long-range magnetic ordering transition. The application of magnetic field to the crystal has no impact on the temperature dependence of heat capacity, further supporting the proposed spin-glass behavior. Temperature dependence curves for both the magnetic (IV) and paramagnetic (II) analogues are similar. The low-temperature (T < 20 K) heat capacity data for compound IV was fitted to Cp/T = γ + βT 2 (Figure S9) to determine the Sommerfeld coefficient γ and the coefficient β, which is related to the Debye temperature (θD) as β = 12/5π4Rn(θD)−3, where R is the gas constant, and n is the number of atoms per formula unit. On the one hand, the resulting Debye temperature (θD = 206 K) is in the range typical of lanthanide intermetallics. 39 The Sommerfeld G

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Inorganic Chemistry coefficient γ = 315(9) mJ/mol K2 (or 32(1) mJ/mol K2 per Mn atom), on the other hand, is higher than expected. An enhanced Sommerfeld coefficient is often a sign of heavy Fermion behavior such as that resulting from correlation of conduction electrons with localized f-moments. However, similar effects can occur from interaction of conduction electrons with disordered spin clusters, as seen in spin glasses and spin liquids such as La2Fe4Sb5 (γ = 118 mJ/mol K2), Sc3Mn3Al7Si5 (γ = 80 mJ/mol K2), and LaMn4Al8 (γ = 265 mJ/mol K2).40,41,37 The electrical resistivity data for crystals of II and IV are shown in Figure 7. The lack of a distinct feature at 20 K

widely studied systems such as Y(Mn1−xAlx)2 (which goes from anti-ferromagnetic to spin glass as x increases) and β-Mn1−xAlx (which undergoes a spin liquid-to-spin-glass transition with rising x).42−44 However, compared to these relatively simple structures the title system presents a more complex playground to explore, with four Mn sites (and four La sites) amenable to substitution. It is likely that further enrichment of manganese in the transition-metal layers (targeting the hypothetical unsubstituted La11Mn13Sn4−δ) would allow for long-range ferro- or antiferromagnetic ordering. However, this is hindered by incorporation of nickel from the flux and aluminum from the crucible. Attempts to use niobium crucibles for synthesis led to formation of niobium/tin compounds. The large distance between the transition-metal slabs may also play a significant role in hindering long-range ordering; it is notable that La6Mn10Al4 (containing similar manganese layers, separated by only 7.5 Å, Figure 2) exhibits hard ferromagnetic behavior with a TC of 200 K.20

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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.7b02555. Tables of crystallographic data and bond lengths for La11Mn13−x−yNixAlySn4−δ analogues. Reaction phase space diagram, SEM images of crystals, bond length correlations, and additional magnetic susceptibility data (PDF)

Figure 7. Temperature dependence of resistivity for La11Mn8.2Ni0.8Al4Sn3.3 (II) and La11Mn10Al3Sn3.4 (IV).

Accession Codes

indicates that the spin-glass freezing transition does not affect electrical conductivity of IV. It is notable that the resistivities differ by almost an order of magnitude, with lower resistivity and metal-like temperature dependence for paramagnetic La11Mn8.2Ni0.8Al4Sn3.3 (II) and poor metal/heavily doped semiconductor behavior for spin-glass La11Mn10Al3Sn3.4 (IV). The gradual formation of disordered spin clusters in the latter compound may scatter the conduction electrons, increasing the resistivity. The compositional variation within the icosahedron slab, that is, different Mn/Ni/Al ratios, not only influences the magnetic properties but also has an impact on the conduction of electrons.

CCDC 1567590, 1567592−1567594, 1572278, and 1573503 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

CONCLUSION The La11Mn13−x−yNixAlySn4−δ (0 ≤ x ≤ 3.6; 2.5 ≤ y ≤ 4.9; 0.6 ≤ δ ≤ 1.1) phases and their iron- and lead-containing analogues add to the plethora of complex intermetallic phases grown from reactions in rare-earth/transition-metal eutectic fluxes. The magnetic characteristics of Mn-rich La11Mn13−x−yNixAlySn4−δ analogues (ZFC/FC splitting at low field and its suppression at higher field, frequency dependence of the cusp temperature, nonsaturation of field dependence of magnetization) are strong indicators of spin-glass behavior. This is supported by the enhanced Sommerfeld coefficient and the absence of longrange ordering features in the temperature dependence of heat capacity and resistivity. As more manganese is substituted by nonmagnetic nickel or aluminum in the transition-metal slabs, the compounds’ behavior changes from spin-glass to paramagnetic. The ability to control the magnetic behavior of transition-metal sites by dilution with nonmagnetic impurities places the La11Mn13−x−yNixAlySn4−δ family in the company of

ORCID

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (850) 644-4074. Fax: (850) 644-8281.

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Susan E. Latturner: 0000-0002-6146-5333 Present Address #

Department of Chemistry, Iowa State University, 2415 Osborn Drive, Ames, Iowa 50011−1021, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the NSF (Grant Nos. DMR-05-47791 and DMR-14-10214) is gratefully acknowledged. REFERENCES

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