3d Materials

Article pubs.acs.org/crystal

Structure−Property Correlations in the Heterobimetallic 4f/3d Materials Ln2M(TeO3)2(SO4) (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; M = Co or Zn) Jian Lin, Kariem Diefenbach, Mark A. Silver, Naresh S. Dalal,* and Thomas E. Albrecht-Schmitt* Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States

Downloaded by UNIV OF CAMBRIDGE on August 23, 2015 | http://pubs.acs.org Publication Date (Web): August 20, 2015 | doi: 10.1021/acs.cgd.5b00860

S Supporting Information *

ABSTRACT: Eighteen new lanthanide transition metal heterobimetallic compounds, Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) and Ln2Zn(TeO3)2(SO4)2 (Ln = Sm, Gd, Dy, Ho, Er, or Yb), have been prepared. They crystallize in triclinic space group P1̅ with two different structural topologies occurring because of a reduction in the Ln3+ coordination number from eight to seven with the smallest lanthanides, Yb3+ and Lu3+. Magnetic susceptibility studies of compounds with diamagnetic lanthanides and lanthanide-like ions suggest that antiferromagnetic interactions occur between the Co2+ ions. Similarly, the replacement of Co2+ with Zn2+ yields Ln2Zn(TeO3)2(SO4)2 (Ln = Gd, Dy, Ho, or Er), and these materials allow for the resolution of the nature of the interactions between lanthanide ions. The data suggest that the short-range Ln3+···Ln3+ interactions are ferromagnetic. However, a wide range of ferroand antiferromagnetic interactions occur between the Ln3+ and the Co2+ cations, with several compounds exhibiting short-range magnetic correlations below 25 K. The results are discussed and contrasted with those recently reported for the related Ln2Cu(TeO3)2(SO4)2 family.

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properties.12 Tellurites are particularly rich for a variety of reasons, including variable coordination numbers, the presence of a stereochemically active lone pair of electrons, and the ability to form polymeric anionic networks.13 The combination of tellurite with sulfate promotes oligomerization,14 stabilizes low-valent actinides,15 and leads to large families of lanthanide tellurite sulfates.12b Applying the aforementioned tellurite sulfate system to the study of lanthanide materials has been proven to be a promising route for creating new 3d/4f heterobimetallic compounds with atypical physicochemical properties.12a,16 We recently reported the structure and properties of the lanthanide copper tellurite sulfates, Ln2Cu(TeO3)2(SO4)2.16 The Cu2+ compounds containing Ln3+ ions with diamagnetic ground states (Y3+ and Eu3+) exhibit antiferromagnetic correlations because of magnetic exchange between the Cu2+ moments. Additionally, Ho2Cu(TeO3)2(SO4)2 exhibits both thermochromism and the Alexandrite effect.12a Thus, it is of interest to examine how such effects would propagate in other transition metals. Herein, we extend the 3d/4f tellurite sulfate system to Co2+ because among 3d transition metals, Co2+ (3d7) is unique because of the variability of its spin states and orbital contributions. Co2+ exhibits high-spin, low-spin, and even intermediate-spin states, depending on its ligand field and coordination geometry.17 Spin interactions between Ln3+ and

INTRODUCTION Heterometallic coordination compounds, particularly those containing 3d/4f metals, have received an increased level of attention in recent years because of their potential applications in single-molecule magnets (SMMs),1 ion exchange,2 gas storage,3 fluorescence,4 and optical sensing.5 The importance of mixed 3d/4f materials is exemplified by their extraordinary magnetic properties as shown by the strongest permanent magnets known, SmCo5 and Nd2Fe14B. These latter materials combine the anisotropy of certain lanthanide ions with the itinerant ferromagnetism of transition metals.6 The introduction of different spin carriers within the same structure type can lead to a vast array of magnetic interactions in heterobimetallic compounds.7 The diversity of coordination environments exhibited by lanthanides allows a different arrangement between the metal ions, resulting in a variety of magnetic exchange pathways.8 Orthogonal magnetic orbitals of two different spin carriers can more readily lead to ferromagnetic interactions than in homometallic systems.9 In addition, a large number of 3d/4f heteronuclear clusters have been reported with magnetic properties that differ greatly from those of homometallic materials.10 Via investigation of these 3d/4f heterobimetallic systems, important features of molecular magnetism, including the magnetocaloric effect (MCE),1d quantum tunneling,1c and slow magnetization relaxation,11 have been realized. We have recently explored the reactions of oxoanions with large bond hyperpolarizabilities with lanthanides to probe structural discontinuities and atypical optical and magnetic © XXXX American Chemical Society

Received: June 19, 2015 Revised: August 13, 2015

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DOI: 10.1021/acs.cgd.5b00860 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Co2+ can further add to the complexities. The Ising-type magnetic anisotropy of Co2+ could support SMM behavior.18 Diamagnetic lanthanide ions and yttrium allow us to probe the Co2+···Co2+ interactions without lanthanide magnetic contributions. Furthermore, to isolate the effects between Re3+ ions, a systematic study of Zn2+ structural analogues was also conducted. These measurements help shed light on the magnetic interactions in Ln2Co(TeO3)2(SO4)2. The syntheses, crystal structures, optical properties, and magnetism of the lanthanide cobalt/zinc tellurite sulfates Ln2Co(TeO3)2(SO4)2 and Ln2Zn(TeO3)2(SO4)2 are presented herein.


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Article

RESULTS AND DISCUSSION Synthesis. Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu) were obtained from the reactions of Ln2O3 and CoO/CoCl2·6H2O/CoSO4·6H2O/CoAc2·4H2O with TeO2 in sulfuric acid. The products consisted of Ln2Co(TeO3)2(SO4)2 crystals with 50−90% yields. The crystal habits for all analogues are similar, and representative photographs of the crystals are shown in Figure 1. Nd2Co-

EXPERIMENTAL SECTION

Synthesis. Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb4O7, Dy2O3, Ho2O3, Er2O3, Tm2O3, Yb2O3, Lu2O3 (99.9%, Alfa-Aesar), TeO2 (99.99%, Alfa-Aesar), CoO (95%, Alfa-Aesar), CoCl2·6H2O (99.99%, AlfaAesar), CoSO4·6H2O (99%, Alfa-Aesar), CoAc2·4H2O (98%, AlfaAesar), ZnAc2·2H2O (97%, Alfa-Aesar), and concentrated H2SO4 (98%, Alfa-Aesar) were all used as received. Reactions were conducted in PTFE-lined Parr 4749 autoclaves with 23 mL internal volume autoclaves. Distilled and Millipore filtered water with a resistance of 18.2 MΩ cm was used in all reactions. Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu). Ln2O3 (0.5 mmol)/Tb4O7 (0.25 mmol, 0.1869 g), TeO2 (1 mmol, 0.1596 g), CoO/CoCl2·6H2O/CoSO4·6H2O/CoAc2· 4H2O (0.5 mmol), 1 M H2SO4 (1 mmol, 1 mL), and 1 mL of H2O were loaded into a 23 mL PTFE-lined autoclave liner. Ln2Zn(TeO3)2(SO4)2 (Ln = Sm, Gd, Dy, Ho, Er, or Yb). Ln2O3 (0.5 mmol), TeO2 (1 mmol, 0.1596 g), Zn(O2CCH3)2·2H2O (0.5 mmol, 0.1098 g), 1 M H2SO4 (1 mmol, 1 mL), and 1 mL of H2O were loaded into a 23 mL PTFE-lined autoclave liner. The autoclave was sealed and heated to 230 °C for 3 days and then slowly cooled to room temperature at a rate of 5 °C/h. The products were washed with deionized water to remove soluble solids, followed by rinsing with methanol. Crystallographic Studies. Single crystals of all Ln 2 M(TeO3)2(SO4)2 compounds were mounted on Mitogen mounts with viscous Krytox oil and optically aligned on a Bruker D8 Quest X-ray diffractometer using a digital camera. Initial intensity measurements were performed using an IμS X-ray source, a 50 W microfocused sealed tube (MoKα, λ = 0.71073 Å) with high-brilliance and highperformance focusing APEXII multilayer optics. Standard Quest software was used for the determination of the unit cells and data collection control. The intensities of reflections of a sphere were collected by a combination of four sets of exposures (frames). Each set had a different φ angle for the crystal, and each exposure covered a range of 0.5° in ω. A total of 1464 frames were collected with an exposure time per frame of 10−60 s, depending on the crystal. The SAINT software was used for data integration, including Lorentz and polarization corrections. Semiempirical absorption corrections were applied using SADABS or TWINABS.19 Selected crystallographic information is listed in Tables S1−S4. Atomic coordinates and additional structural information are provided in the Supporting Information (CIFs). Magnetic Susceptibility Measurements. The magnetic susceptibilities of Ln2M(TeO3)2(SO4)2 compounds were measured on powder samples using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL). The DC magnetic susceptibility was measured in an applied field of 0.1 T in the temperature range of 1.8−300 K. Magnetization was also measured with the magnetic field varying between 0 and 5.5 T at 1.8 K. The data were corrected for diamagnetic contributions using Pascal’s constants.20 UV−Vis−NIR Spectroscopy. UV−vis−NIR data were acquired from single crystals using a Craic Technologies microspectrophotometer. Crystals were placed on quartz slides under Krytox oil, and the data were collected from 200 to 900 nm. The exposure time was auto-optimized using Craic software.

Figure 1. Photographs of crystals of Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu).

(TeO3)2(SO4)2 is unusual with respect to the rest of the series in that it adopts a purple coloration (typical of many Nd3+ compounds) while the others are pink. Unreacted TeO2 is also present in some reactions but can be removed by repeated suspension and ethanol rinses. Ln2Zn(TeO3)2(SO4)2 (Ln = Sm, Gd, Dy, Ho, Er, or Yb) were obtained from the reactions using Zn(O2CCH3)2(H2O)2 as the starting material under the same condition that was used for the Co analogues. The product yields are between 50 and 70%. Structural and Topological Descriptions. Single-crystal X-ray diffraction experiments with Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) reveal that they crystallize into two different structures and that the transition occurs between Tm (Figure 2a) and Yb (Figure 2b) because of the lanthanide contraction, confirming the earlier reported trend in the copper analogues. 16 Ln 2 Zn(TeO3)2(SO4)2 (Ln = Sm, Gd, Dy, Ho, Er, or Yb) are isomorphous with their corresponding Co analogues. Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, or Tm) (LnCoTeSO-1) and Ln2Co(TeO3)2(SO4)2 (RE = Yb or Lu) (LnCoTeSO-2) crystallize in the same space group, P1̅, but their unit cell parameters and structures are slightly different (cf. Table S1). Both of the structures are composed of Ln-oxo polyhedra, CoO6 octahedra, trigonal pyramidal TeO32−, and SO42− tetrahedra, forming a three-dimensional framework. However, there are two significant topological differences between LnCoTeSO-1 and LnCoTeSO-2. First, the Ln polyhedra in LnCoTeSO-1 edge-share, forming one-dimensional chains that extend along the b axis (cf. Figure 2a); Yb3+/ Lu3+ ions in LnCoTeSO-2 share only one edge with another crystallographically equivalent cation creating dimers (cf. Figure 2b). Second, the coordination geometries of Ln3+ ions in these structures are quite different. The structural chemistry of B




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Figure 2. (a) View of the structure of Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, or Tm) (LnCoTeSO-1). (b) View of the structure of Ln2Co(TeO3)2(SO4)2 (Ln = Yb or Lu) (LnCoTeSO-2) extending down the a axis.

Figure 3. Normalized UV−vis−NIR spectra of Ln2Co(TeO3)2(SO4)2 (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, or Yb).

elongated or compressed along the 4-fold axis.16 This distortion is caused by having different ligands in the axial and equatorial positions or the result of the traditionally observed Jahn−Teller distortions of a d7 ion, or even a combination of the two. UV−Vis−NIR Spectroscopy. The solid-state UV−vis−NIR absorbance spectra of Ln2Co(TeO3)2(SO4)2 were obtained from single crystals and are shown in Figure 3. All of the compounds show broad peak with λmax between ∼500 and ∼525 nm that can be assigned to the d−d transitions of the Co2+ ions.23 The absorption feature in the UV is ascribed to the charge-transfer bands from the tellurite units based on comparisons with TeO2 crystals, although sulfate features are certainly buried beneath these features, as well. The electronic transitions of 4f elements were assigned decades ago, and the signature peaks of trivalent lanthanides are displayed in their respective spectra.24 These sharp peaks remain largely unaltered because of their corelike nature. The purple color of Nd2Co(TeO3)2(SO4)2, which is normal for Nd3+ compounds, is attributed the f−f transitions actually being more intense than the d−d transitions, and they therefore dominate the coloration. All the Co2+ ions in Ln2Co(TeO3)2(SO4)2 have

lanthanides is rich because of the high coordination numbers and a general lack of geometric preferences. Coordination numbers of 6−10 are well-represented, with eight- and ninecoordinate lanthanides being the most common.21 Shape8 calculations demonstrate that the geometry for the LnO8 units in LnCoTeSO-1 can be best described as a distorted trigonal dodecahedron with approximate D2d symmetry.22 The reduction in coordination number occurs with the two smallest lanthanides, and LnO7 polyhedra with slightly distorted pentagonal bipyramid geometries are observed in LnCoTeSO-2. The lanthanide contraction is clearly delineated in the Ln2Co(TeO3)2(SO4)2 series as provided by the decreasing volume of the unit cells (cf. Table S1), decreasing Ln−O bond distances (cf. Table S2), reduction in the coordination number of the lanthanides, and more dramatically the change in structure type from LnCoTeSO-1 to LnCoTeSO-2. The average Ln−O bond distance decreases from 2.470(3) Å for Nd to 2.373(5) Å for the Er analogue. The average Ln−O bond distances are 2.296(4) Å for Yb and 2.285(4) Å for LuO7 (cf. Table S2). The octahedrally coordinated Co2+ can undergo a tetragonal distortion in which the octahedra are typically C




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Figure 4. Temperature dependence of magnetic susceptibility χ, χT product with inverse susceptibility as an inset for Y2Co(TeO3)2(SO4)2 (a and b), Eu2Co(TeO3)2(SO4)2 (c and d), and Lu2Co(TeO3)2(SO4)2 (e and f). The solid red lines in the insets show the fitting to the Curie−Weiss law in terms of inverse susceptibility.

tions of Ln3+, four zinc analogues Ln2Zn(TeO3)2(SO4)2 (Ln = Gd, Dy, Ho, or Er) were isolated and measured, within which lanthanide ions are the sole contributors to the magnetic moments. Y2Co(TeO3)2(SO4)2, Eu2Co(TeO3)2(SO4)2, and Lu2Co(TeO3)2(SO4)2. Y- and Lu-containing phases serve as model compounds with diamagnetic Y3+/Lu3+ ions and allow the evaluation of magnetic contributions due exclusively to Co2+ ion contributions in the absence of 3d/4f magnetic exchange. Because the ground state of the Eu3+ ion is diamagnetic, 7F0, one can expect the low-temperature behavior of Eu2Co(TeO3)2(SO4)2 to be similar to that of Y2Co(TeO3)2(SO4)2. The temperature dependences of the magnetic susceptibility data for Y2Co(TeO3)2(SO4)2, Eu2Co(TeO3)2(SO4)2, and Lu2Co(TeO3)2(SO4)2 measured under an applied field of 1000 Oe are shown in panels a, c, and e of Figure 4, respectively. An examination of the χ versus T dependence at low temperatures reveals a Curie tail of ∼3 K for these three compounds. This feature is attributed to the intrinsic anisotropy of the Co2+ ions because it is unlikely that all of these independently prepared complexes would contain similar impurities. The magnetic anisotropy arises from the orbital

similar Jahn−Teller distorted octahedral coordination geometries. However, a close examination of the Co−O bond distances reveals that the Co−O(1) distance is longer for Nd2Co(TeO3)2(SO4)2 and Co−O(2) is the shorter than in any of the other analogues (cf. Table S2). This slight distortion of the Co2+ coordination may be playing a role in the coloration, as well. Magnetic Properties. The magnetic properties of all Ln2Co(TeO3)2(SO4)2 materials were investigated except those of Sm2Co(TeO3)2(SO4)2 and Tm2Co(TeO3)2(SO4)2, because of their relatively low yields and the presence of impurities. Our data agree with theoretical values when cobalt is assumed to be high-spin and exhibit strong spin−orbit coupling as observed in other studies.25 The effective magnetic moment obtained by Buschow and Boer for a Co2+ ion using μeff = g[L(L + 1) + 4S(S + 1)]1/2 yields 5.21 μB, which is in good agreement with our value of 5.21(1) μB obtained from the linear Curie−Weiss fitting of Lu2Co(TeO3)2(SO4)2, discussed below.26 Thus, electron Zeeman factors are not equal to the free electrons (g = 2), and spin−orbit interactions have been accounted for in our Brillouin functions for both lanthanide and cobalt contributions.25 For comparisons and isolating the contribuD



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momentum contributions of the spin−orbit-coupled Co2+ unpaired electrons within the nonspherical octahedral crystal field environment. The similarities in magnetic susceptibility between Y2Co(TeO3)2(SO4)2 and Eu2Co(TeO3)2(SO4)2 are not surprising because of the diamagnetic ground states of Y and Eu within the same structure type. Although Lu2Co(TeO3)2(SO4)2 has a structure different from those of the early lanthanide compounds, similar behavior is observed that can be attributed to the comparable Co2+···Co2+ distances between the nearest spin carriers compared with that of Eu2Co(TeO3)2(SO4)2 (Table 1). Above 100 K, the temperature (T) dependence of

value decreases exponentially with decreasing temperature, attributed to antiferromagnetic interactions. The observed value of μeff at 300 K is 7.20 μB and is relatively close to the calculated value of 7.30, obtained by μeff = gμB[J(J + 1)]1/2, as expected for two Nd3+ ions and one Co2+ ion for the corresponding ground states 4I9/2 and 2F9/2, respectively.27 The slight difference of 0.1 μB could be due to nearest-neighbor antiferromagnetic interactions (θ = −32.6 K), which was obtained via a Curie− Weiss fitting to the χ−1 plot between 75 and 300 K. Ln2Co(TeO3)2(SO4)2 (Ln = Gd, Tb, or Dy). For these compounds, the temperature dependence of χ−1 was leastsquares fit to the Curie−Weiss law in the range of 50−300 K (Figure 6a,c,e). The Curie constant was used to calculate the effective magnetic moment by following μeff = [(3kC)/NμB2]1/2, where N is Avogadro’s number and k is the Boltzmann constant, yielding μeff values that match well with the theoretical values for two Ln3+ ions and one Co2+ ion (vide infra) (Table 2). The negative Weiss temperatures of −2.14(8) K for the Gd and −2.94(4) K for the Dy compounds may indicate weak antiferromagnetic interactions between the nearest-neighbor ions. The Tb analogue exhibits a positive Weiss temperature of 1.8(1) K, suggesting a weak ferromagnetic relationship between closest-neighbor spins above 50 K. It is difficult, however, to establish whether this divergence stems from the stronger 3d/4f exchange or just from the interactions between Ln3+ ions (or from the combination of both). A study of Ln2Zn(TeO3)2(SO4)2 (Ln = Gd or Dy) analogues suggests ferromagnetic correlations between the lanthanide centers below 25 K (Figure S1). These results match well with the χT values of Gd and Dy analogues (panels b and f of Figure 6, respectively), which decrease with a decrease in temperature followed by the sharp increase in the χT values around 25 K. Nonetheless, at temperatures below 3 K, the χT values decrease abruptly, which can be attributed to the Co2+ ions. Tb2Zn(TeO3)2(SO4)2 was not included in this study because the same synthetic method failed to generate the isotypic analogue; however, it was reported that the Tb3+···Tb3+ interactions favor ferromagnetic exchange.12b In addition, the Tb analogue displays the same increase in χT around 25 K as observed for the Gd and Dy analogues (Figure 6d). The Tb analogue differs slightly in that it displays slight ferromagnetic interactions throughout the measured temperature range, which is clearly seen in the increasing χT versus T susceptibility (Figure 6d). Therefore, for Ln2Co(TeO3)2(SO4)2 (Ln = Gd, Tb, or Dy), Ln3+···Ln3+ interactions exhibit weak ferromagnetic correlations that are overcome by weak Co2+···Co2+ interactions at 3 K. The


Table 1. Closest Ln3+···Ln3+, Ln3+···Co2+, and Co2+···Co2+ Distances in Ln2Co(TeO3)2(SO4)2 Ln

Ln3+−Ln3+ (Å)

Ln3+−Co2+ (Å)

Co2+−Co2+ (Å)

Y Nd Sm Eu Gd Tb Dy Ho Er Yb Lu

3.7990(9) 3.9393(6) 3.8904(7) 3.873(1) 3.8653(6) 3.8188(1) 3.8259(6) 3.811(1) 3.7906(6) 3.7526(6) 3.7393(5)

3.4260 (5) 3.4815(5) 3.4631 (4) 3.462(1) 3.4593(5) 3.4410(4) 3.4400(4) 3.430(1) 3.4183(4) 3.3280(4) 3.3178(3)

5.2216(4) 5.3553(7) 5.3120(4) 5.302(2) 5.2897(7) 5.2762(7) 5.2609(4) 5.245(1) 5.2334(2) 5.4004(6) 5.3893(5)

inverse susceptibility (χ−1) of Y2Co(TeO3)2(SO4)2 and Lu2Co(TeO3)2(SO4)2 obeys the Curie−Weiss law [1/χ = (T − θ)/C] (Figure 4b,f, inset), where C and θ represent the Curie constant and the Weiss temperature, respectively. The negative Weiss temperatures [θ = −17.1(4) and −3.1(5) K] indicate weak antiferromagnetic interactions occurring between the nearestneighbor Co2+ ions that also agree with the gradual drop in the χT value below 100 K (Figure 4b,f). The Eu3+ ion is known to show Van Vleck paramagnetism because of the presence of lowlying excited states.27 Consequently, Eu2Cu(TeO3)2(SO4)2 exhibits strong deviations from the Curie−Weiss law. As Eu2Co(TeO3)2(SO4)2 is cooled, the depopulation of excited states leads to decrease in χT sharper than those of Y2Co(TeO3)2(SO4)2 and Lu2Co(TeO3)2(SO4)2 (Figure 4b, d, f). Nd2Co(TeO3)2(SO4)2. The plots of χ, χT, and 1/χ versus temperature for Nd2Co(TeO3)2(SO4)2 are given in panel a, panel b, and the panel a inset of Figure 5, respectively. The χT

Figure 5. Temperature dependence of (a) magnetic susceptibility, χ, inverse magnetic susceptibility (inset), and the (b) the χT product for Nd2Co(TeO3)2(SO4)2. E




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Figure 6. Temperature dependence of magnetic susceptibility χ, inverse susceptibility (inset), and the χT product for (a and b) Gd2Co(TeO3)2(SO4)2, (c and d) Tb2Co(TeO3)2(SO4)2, and (e and f) Dy2Co(TeO3)2(SO4)2.

Table 2. Summary of Magnetic Data for Ln2Co(TeO3)2(SO4)2 (Ln = Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, or Yb) Curie−Weiss fitting parameters theoretical χT at 300 K (emu K mol−1) 3+

Ln

2Ln

Y Nd Eu Gd Tb Dy Ho Er Yb Lu

0 3.28 0 15.77 23.63 28.26 28.1 23 5.15 0

experimental χT at 300 K (emu K mol−1)

μeff (2Ln3+ and Co2+) (μB) −1

2+

total

total

C (emu K mol )

θ (K)

experimental

calculated

3.39 3.39 3.39 3.39 3.39 3.39 3.39 3.39 3.39 3.39

3.39 6.67 3.39 19.16 27.02 31.65 31.49 26.39 8.55 3.39

3.56 6.49 6.84 20.73 27.7 33.56 31.89 27.35 8.09 3.35

3.759(6) 7.199(2) − 20.84(1) 27.57(1) 33.901(7) 33.84(5) 27.75(3) 8.739(9) 3.394(9)

−17.1(4) −32.55(6) − −2.14(8) 1.8(1) −2.94(4) −1.8(2) −4.8(2) −24.2(2) −3.1(5)

5.48 7.59 − 12.91 14.85 16.47 16.45 14.9 8.36 5.21

5.21 7.3 5.21 12.38 14.7 15.91 15.87 14.53 8.27 5.21

Co

compounds show typical Curie−Weiss behavior in the χ−1 versus T data in the temperature range of 100−300 K (Figure S2). The Weiss temperatures are 13.6(2) and 9.7(5) K for Ho2Zn(TeO3)2(SO4)2 and Er2Zn(TeO3)2(SO4)2, respectively, indicating ferromagnetic interactions between nearest-neighbor Ln3+ ions. The ferromagnetic exchange is also supported by the

different magnitudes of these two interactions result in diverse magnetic behavior in the Gd, Dy, and Tb phases. Ln2Co/Zn(TeO3)2(SO4)2 (Ln = Ho or Er). The Zncontaining phases, with diamagnetic Zn2+ ions, allow for the evaluation of magnetic properties arising because of Ho3+/Er3+ ions in the absence of any 3d···4f magnetic exchange. These F




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Figure 7. Temperature dependence of magnetic susceptibility, χ, inverse susceptibility (inset), and the χT product for (a and b) Ho2Co(TeO3)2(SO4)2 and Ho2Zn(TeO3)2(SO4)2 and (c and d) Er2Co(TeO3)2(SO4)2 and Er2Zn(TeO3)2(SO4)2.

Figure 8. Temperature dependence of magnetic susceptibility, χ, inverse susceptibility (inset), and the χT product for Yb2Co(TeO3)2(SO4)2.

increase in the χT value with a decrease in temperature from 300 to 5 K for the holmium analogue (Figure 7b) and from 300 to 15 K for erbium (Figure 7d). Thereafter, the χT values decrease abruptly near 5 K for Ho and 15 K for the Er analogues, which could be attributed to the depopulation of the mJ levels of the Ho3+ and Er3+ ions at low temperatures.1d As shown in panels a and c of Figure 7, Ho2Co(TeO3)2(SO4)2 and Er2Co(TeO3)2(SO4)2 follow the Curie− Weiss law with slightly negative Weiss temperatures [−1.8(2) K for Ho and −4.8(2) K for the Er phase]. The effective moment is equal to 16.45 μB for Ho2Co(TeO3)2(SO4)2 and 14.90 μB for Er2Co(TeO3)2(SO4)2, which are slightly higher but within reasonable agreement with the theoretical values of 15.87 and 14.53 μB, respectively. The field-dependent magnetization measured at 1.8 K shows suppressed magnetization values as compared to the sum of Brillouin functions for two Ln3+ ions and one Co2+ ion in the absence of magnetic exchange (Figure S3). Both the decreasing χT upon cooling and suppressed magnetization relative to that of paramagnetic ions suggest weak antiferromagnetic interactions between the Ln3+ and Co2+ ions as well as between Co2+ ions, overshadowing the weak

ferromagnetic correlations between Ln3+ ions. No clear signature of magnetic ordering was observed for these phases. Yb2Co(TeO3)2(SO4)2. Yb2Co(TeO3)2(SO4)2 is isomorphous with Lu2Co(TeO3)2(SO4)2 and exhibits the LnCoTeSO-2 structure type. Both the 1/χ versus T (Figure 8a, inset) and χT versus T (Figure 8b) plots support the presence of antiferromagnetic exchange interactions with a negative Weiss temperature (θ = −44 K). The field-dependent magnetization measured at 1.8 K shows linear behavior with a very small maximal value of 0.26 μB (Figure S4). This further enforces the fact that the Yb-containing compound has antiferromagnetic interactions. Magnetization Studies. When the Ln3+ cation in Ln2Co(TeO3)2(SO4)2 is diamagnetic, the field dependence of magnetization at 1.8 K allows the probing of the cobalt ground terms. Theories of the paramagnetic properties of Co2+ ions have been discussed in great detail where anisotropy and spin− orbit coupling play a large role and must be accounted for to understand experimental results.28 Data for the yttrium analogue are provided in Figure 9a and show a saturation of magnetization at approximately 2.5 μB. G




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Figure 9. Magnetization plots of (a) Y2Co(TeO3)2(SO4)2, (b) Gd2Co(TeO3)2(SO4)2, (c) Tb2Co(TeO3)2(SO4)2, and (d) Dy2Co(TeO3)2(SO4)2. The solid red line represents the theoretical Brillouin fitting that accounts for noninteracting magnetic spins with spin−orbit coupling.

well with the computational results for two paramagnetic lanthanide ions with a high-spin cobalt ion. The Weiss constants represent how the nearest neighbors are interacting. While structural characterizations yield a deviation in structure between thulium and ytterbium, no such deviation is evident in terms of magnetic properties, further hinting at the paramagnetic behavior of the magnetic ions. By substituting diamagnetic ions for Ln3+ in Ln2Co(TeO3)2(SO4)2, we observe antiferromagnetic correlations between cobalt ions. Tb2Co(TeO3)2(SO4)2 compounds display ferromagnetic correlations at higher temperatures, while the rest are antiferromagnetic. Careful analysis of the zinc analogues (Ln = Gd, Dy, Ho, or Er) yields ferromagnetic interactions occurring between the lanthanide ions that are cloaked in Ho2Co(TeO3)2(SO4)2 and Er2Co(TeO3)2(SO4)2. No long-range magnetic ordering is observed.

The calculated effective magnetic moments are much higher, even excluding orbital contributions; a high-spin Co2+ ion yields 3.87 μB (S = 3/2), while accounting for orbital contributions yields a magnetic moment of 5.20 μB (S = 3/2, and L = 3). Most observed values of Co2+ ions in analogues environments typically saturate between 4.5 and 5.2 μB as expected,29 although some complexes reported by Sacco and Cotton saturate much lower, for instance, 1.82, 1.89, and 1.92 μB for [Co(CH3NC)4][CdX4] (X = Br or I), Co(MeNC)4X2 (X = Cl, Br, or I), and [Co darsine3](CLO4)2, respectively.28 The low magnetic saturation values exhibited by Ln2Co(TeO3)2(SO4)2 are thus not surprising considering the aforementioned Co2+ ions in octahedral environments that saturate far below the theoretical values calculated from μeff = g[L(L + 1) + 4S(S + 1)]1/2 for noninteracting Co2+ ions. The Gd3+, Tb3+, and Dy3+ cases will be briefly compared because of the diversity displayed by their field-dependent magnetizations. Gd3+ has the largest spin moment (S = 7/2) and possesses electronically isotropic f orbitals. Consequently, the magnetization of Gd2Co(TeO3)2(SO4)2 is in good agreement with the sum of Brillouin functions for two free Gd3+ ions and one Co2+ ion (Figure 9b) until higher applied fields are reached, after which it saturates below the expected value because of the effects of the Co2+ ion. The Tb analogue exceeds the theoretical Brillouin function up to 0.5 T, which can be explained by ferromagnetic correlations, and then saturates at a value lower than the calculated Brillouin function due to orbital contributions from the Tb3+ ions (L = 3) and Co2+ effects that are shown in Figure 9c. The Dy analogue (Figure 9d) tracks well with the Brillouin function up to 0.3 T and then deviates because of the population of low-lying excited states as observed for our previously reported dysprosium analogue.16 Summary of Magnetic Data. At this point, it seems worthwhile to summarize the magnetic data described above. The main results are presented in Table 2. Via comparison of the last two columns of Table 2, the experimental results match

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CONCLUSIONS We report the syntheses, crystal growth, structural characterization, and electronic absorption spectra for 12 new compounds in the Ln2Co(TeO3)2(SO4)2 (Ln = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) family constituting well-characterized examples of purely inorganic 3d/4f heterobimetallic compounds. In particular, while Ln 2 Co(TeO3)2(SO4)2 are isomorphous with Ln2Cu(TeO3)2(SO4)2, they exhibit distinctly different magnetic properties; in particular, competing ferromagnetic and antiferromagnetic correlations in the cobalt analogues make them more interesting and difficult to interpret. Variable-temperature and variable-field magnetic susceptibility measurements reveal paramagnetic behavior above 25 K, but below this temperature, the Ln2Co(TeO3)2(SO4)2 analogues exhibit diverse short-range magnetic correlations ranging from ferromagnetic to antiferromagnetic. Fairly definitive analysis is made possible by measurements of specifically substituted diamagnetic analogues. The different magnitudes of these interactions between Ln3+··· H



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Ln3+, Ln3+···Co2+, and Co2+···Co2+ entities result in diverse magnetic behavior in these phases. These data form a new basis for the theoretical modeling of the electronic properties of these new and well-characterized 3d/4f materials.

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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.cgd.5b00860. Figures S1−S4, a table of crystallographic data, and a table of selected bond distances (PDF) X-ray crystallographic files (CIF)

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AUTHOR INFORMATION

Corresponding Authors


* E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Heavy Elements Chemistry Program, via Award DEFG02-13ER16414. This work was also supported by a Chinese Scholarship Council Graduate Fellowship to J.L.

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3d Materials

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