4f Materials

Feb 28, 2014 - ... Behavior of Lanthanide-based Coordination Polymers: Synthesis, Structure, Absorption-Emission and Magnetic Properties. Prabu Mani ,...
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Challenges in the Search for Magnetic Coupling in 3d/4f Materials: Syntheses, Structures, and Magnetic Properties of the Lanthanide Copper Heterobimetallic Compounds, RE2Cu(TeO3)2(SO4)2 Jian Lin,†,‡ Ping Chai,† Kariem Diefenbach,† Michael Shatruk,† and Thomas E. Albrecht-Schmitt*,† †

Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana, 46556, United States

‡

S Supporting Information *

ABSTRACT: Twelve new lanthanide copper heterobimetallic compounds, RE2Cu(TeO3)2(SO4)2 (RE = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), with two different structural topologies, have been prepared by hydrothermal treatment. Both structure types crystallize in the triclinic space group, P1̅, but the unit cell parameters and structures are quite different. The earlier RE2Cu(TeO3)2(SO4)2 (RE = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm) share a common structural motif consisting of edge-sharing LnO 8 chains and [Cu(TeO3)2(SO4)2]6− units. The later lanthanide version (Yb and Lu) is composed of edge-sharing LnO7 dimers bridged by similar [Cu(TeO3)2(SO4)2]6− units. The change in the structure type can be attributed to the decreasing ionic radii of the lanthanides. The compounds containing RE3+ ions with diamagnetic ground states (Y3+ and Eu3+) exhibit antiferromagnetic ordering at 12.5 K and 15 K, respectively, owing to the magnetic exchange between Cu2+ moments. No magnetic phase transition was observed in all the other phases. The lack of magnetic ordering is attributed to the competing magnetic interactions caused by the presence of paramagnetic RE3+ ions. The magnetism data suggests that substantial 3d−4f coupling only occurs in the Yb analogue.

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INTRODUCTION Structure−property correlations in 3d/4f heterobimetallic materials are being heavily investigated owing in part to their potentially useful applications in single molecule magnets (SMMs),1 ion exchange,2 gas storage,3 fluorescence,4 and optical sensing.5 The magnetic properties of lanthanide compounds have been widely studied due to their large magnetic moments and anisotropy.6 Despite the systematic decrease in the ionic radius across the 4f series, magnetic coupling is more difficult to predict. 7 Materials with extraordinary magnetic properties, for example the strongest permanent magnets known, SmCo5 and Nd2Fe14B, have been realized by combining the anisotropy of the lanthanide ions with the itinerant ferromagnetism of transition metals.8 The inclusion of a transition metal also allows for electrical conduction and thermochromism.9 Spin−orbit coupling effects in lanthanides are much stronger compared to those of 3d transition metals; thus, the total angular momentum must be taken into consideration for the 4f elements whereas the spinorbit coupling is typically treated only as perturbation within transition metal compounds.10 Furthermore, the local magnetic anisotropy of the lanthanide is strongly correlated to crystal field effects.11 The structural chemistry of lanthanides is rich owing to high coordination numbers and a general lack of geometric © 2014 American Chemical Society

preferences. Coordination numbers of seven to ten are common, yielding a wide range of coordination geometries that include the pentagonal bipyramid, square antiprism, hexagonal bipyramid, trigonal dodecahedra, tricapped trigonal prism, capped triangular cupola, etc.12 The diversity of coordination environments exhibited by lanthanides enables different bonding geometries between metal ions, resulting in a variety of magnetic exchange pathways.13 The lanthanide contraction describes the increase in effective nuclear charge and, correspondingly, a decrease in ionic radii of lanthanide ions due to the addition of electrons to the poorly shielding 4f orbitals. Hughes et al.’s study suggests that change of atomic unit cell volume, due to the lanthanide contraction, plays a distinct role in determining magnetic properties.14 Few systematic investigations of 3d/4f heterobimetallic materials for the whole series of lanthanides have been documented.15 We have recently undertaken a study of the solid-state chemistry of transition metals, lanthanides, and actinides using tellurite as a ligand via hydrothermal synthetic methods. Tellurites possess a stereochemically active lone-pair of electrons on the Te4+ centers, and tellurite anions can be Received: January 22, 2014 Revised: February 27, 2014 Published: February 28, 2014 2187

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and Yb) were measured on polycrystalline samples using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL). Nd2Cu(TeO3)2(SO4)2, Tb2Cu(TeO3)2(SO4)2, and Lu2Cu(TeO3)2(SO4)2 were not included in this study owing to low yields and/or the presence of impurities. The DC magnetization was measured in an applied field of 0.1 T in the 1.8− 300 K temperature range, and the field-dependent magnetization was evaluated at 1.8 K with the applied field varying from 0 to 7 T. The data were corrected for diamagnetic contributions using Pascal’s constants.22 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 350 to 1100 nm. The exposure time was auto-optimized by the Craic software. Scanning Electron Microscopy and Energy-Dispersive X-ray Spectrometry (SEM-EDS) Analysis. SEM-EDS images and data were collected using a LEO EVO 50 with an Oxford INCA energydispersive X-ray spectrometer. The energy of the electron beam was set at 29.02 kV, and the spectrum acquisition time was 120 s. All of the data were calibrated with standards, and all EDS results are provided in the Supporting Information. Powder X-ray Diffraction. Powder patterns were collected in the range of 10° to 80°, with a step of 0.5°, and the data collection time was 0.5 s per step using a Panalytical X’PERT Pro powder diffractometer.

interconnected to form polymeric structures, which results in a large variability of coordination environments and overall geometries in this family of compounds.16 Furthermore, the combination of tellurite with other anions in one compound enables novel structures with the potential for creating multifunctional materials. For example, mixing second-order Jahn−Teller distorted d0 (Mo6+, V5+, Ti4+) oxoanions with tellurites can lead to formation of structures with permanent polarization when the asymmetric building units are properly aligned, resulting in strong second harmonic generation properties.17 The first lanthanide/actinide-containing tellurite sulfates, Pu(TeO3)(SO4) and Ce2(Te2O5)(SO4)2, with two distinct structures were recently reported by us.18 Applying this tellurite−sulfate system to address the preparation of 3d/4f heterobimetallic materials is a promising route for isolating solids with atypical properties. The synthesis of 3d/4f heterobimetallic materials itself has proven to be quite challenging as the 3d and 4f metals compete for the chelation by the ligand. The inclusion of mixed anions in 3d/4f heterobimetallic compounds complicates the syntheses further because it can result in multiple homometallic compounds rather than a single heterobimetallic material.19 The majority of 3d/4f heterobimetallic coordination compounds are metal− organic materials.1,6c,9e,20 Herein we describe the preparation, structure elucidation, and magnetic properties of a large family of lanthanide copper tellurite sulfates.

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RESULTS AND DISCUSSION Synthesis. RE2Cu(TeO3)2(SO4)2 (RE = Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu) were obtained from the reactions of RE2O3 and CuSO4 with TeO2 in sulfuric acid under mild hydrothermal conditions.

EXPERIMENTAL SECTION

Synthesis. Y2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb, Dy2O3, Ho2O3, Er2O3, Yb2O3, Tm2O3, Lu2O3 (99.9%, Alfa-Aesar), CuSO4 (98%, Alfa-Aesar), CuO (99.99%, Sigma-Aldrich), TeO2 (99.99%, Alfa-Aesar), and concentrated H2SO4 (98%, Alfa-Aesar) were all used as received. Reactions were run in PTFE-lined Parr 4749 autoclaves with 23 mL internal volume. Distilled and Millipore filtered water with resistance of 18.2 MΩ·cm was used in all reactions. RE2Cu(TeO3)2(SO4)2 (RE = Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu). RE2O3 (1 mmol), CuSO4 (1 mmol, 0.0795 g), TeO2 (2 mmol, 0.3192 g), 1 M H2SO4 (1 mmol, 1 mL), and 1 mL of water were loaded into a 23 mL PTFE-lined autoclave. Tb2 Cu(TeO3)2(SO4)2: Tb metal (2 mmol, 0.3179 g) was reacted with 1 M H2SO4 (3 mmol, 3 mL) in an autoclave liner under room temperature. Then CuO (1 mmol, 0.0795 g) and TeO2 (2 mmol, 0.3192 g) were loaded into the autoclave liner. The autoclave was sealed and heated to 230 °C for 3 days followed by slow cooling to room temperature at a rate of 5 °C/h. The products were washed with DI water to remove soluble solids, followed by rinsing with methanol. Crystallographic Studies. Single crystals of RE 2 Cu(TeO3)2(SO4)2 were mounted on CryoLoops with Krytox oil and optically aligned on a Bruker APEXII Quazar X-ray diffractometer using a digital camera. Initial intensity measurements were performed using an IμS X-ray source and a 30 W microfocused sealed tube (Mo Kα, λ = 0.71073 Å) with high-brilliance and high-performance focusing Quazar multilayer optics. Standard APEXII software was used for 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 to 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 the program SADABS or TWINABS.21 Selected crystallographic information is listed in Table S1, Supporting Information. Atomic coordinates and additional structural information are provided in the Supporting Information (CIFs). Magnetic Property Measurements. The magnetic susceptibilities of RE2Cu(TeO3)2(SO4)2 (RE = Y, Sm, Eu, Gd, Dy, Ho, Er, Tm,

RE 2O3(s) + CuSO4 (s) + 2TeO2 (s) + H 2SO4 (aq) → RE 2Cu(TeO3)2 (SO4 )2 (s) + H 2O(l)

The products consisted of RE2Cu(TeO3)2(SO4)2 crystals with 70−90% yields. Unreacted TeO2 is also present in some reactions but can be removed by repeated suspension in methanol. Tb2Cu(TeO3)2(SO4)2 was synthesized using Tb metal as the lanthanide source, and the desired compound was isolated in 60% yield. Ho2Cu(TeO3)2(SO4)2 is unusual with respect to the rest of the series; it adopts a pink color while the others are green under standard fluorescence room lighting. A reversible thermochromic transition and the Alexandrite effect observed for Ho2Cu(TeO3)2(SO4)2 were recently reported by us.23 Structures and Topological Descriptions. Single crystal X-ray diffraction experiments on RE2Cu(TeO3)2(SO4)2 (RE = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm) (RECuTeSO-1) reveal that they are isomorphous and crystallize in the triclinic space group P1̅. The Ho example has been reported in our previous study for which detailed structural information is available, but the remaining analogues are a result of this work. As shown in Figure 1a, RECuTeSO-1 is constructed from REO8 polyhedra, distorted CuO6 octahedra, trigonal pyramidal TeO32−, and SO42− tetrahedra, forming a three-dimensional (3D) framework. The RE3+ cations are chelated by TeO32− and SO42− anions. Each RE3+ ion is bound to three TeO32− anions and three SO42− anions. The RE polyhedra edge-share to form dimers that are in turn linked by the oxoanions to create onedimensional chains that extend along the b axis. Eightcoordinate metal centers can adopt four different coordination environments: cubic (Oh), trigonal dodecahedra (D2d), bicapped trigonal prisms (C2v), and square antiprisms (D4d).24 2188

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Figure 2. (a) Structural view of RE2Cu(TeO3)2(SO4)2 (RE = Yb and Lu) (RECuTeSO-2) extending down the a axis. (b) Representation of the pentagonal bipyramid coordination environment of RE3+. (c) [Cu(TeO3)2(SO4)2]6− unit. RE3+ polyhedra are shown in green, Cu2+ polyhedra in dark blue, TeO32− polyhedra in gray, SO42− tetrahedra in purple, and oxygen atoms in red.

Figure 1. (a) View of the structure of RE2Cu(TeO3)2(SO4)2 (RE = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm) (RECuTeSO-1) extending down the a axis. (b) Representation of the trigonal dodecahedron coordination environment of RE3+ ions. (c) [Cu(TeO3)2(SO4)2]6− unit. RE3+ polyhedra are shown in cyan, Cu2+ polyhedra in dark blue, TeO32− polyhedra in gray, SO42− tetrahedra in purple, and oxygen atoms in red.

neptunyl and uranyl and sometimes can be found in Th.26 However, they are quite rare in lanthanide compounds.12a The average RE−O bond distances are 2.286(3) Å for YbO7 and 2.282(3) Å for LuO7, respectively (cf., Table S2, Supporting Information). Similarly, a [Cu(TeO3)2(SO4)2]6− unit is observed in Yb and Lu analogues (Figure 2c). The lanthanide contraction is clearly observed in the RE2Cu(TeO3)2(SO4)2 series. The effect of lanthanide contraction is reflected in the decreasing volume of the unit cells (cf., Table S1, Supporting Information), in the decreasing RE− O bonding distance (cf., Table S2, Supporting Information), in the decreasing coordination number of the lanthanides, and more dramatically in the change in structure type from RECuTeSO-1 to RECuTeSO-2. Raymond’s study suggests that the inverse of lanthanide ionic radii as a function of f electron number gives a very good linear fit for a variety of lanthanide complexes.27 The ionic radii of Ln3+ in RE2Cu(TeO3)2(SO4)2 were obtained by subtracting the ionic radius of O2− ions from the Ln−O distances. Almost linear trends relating lanthanide ionic radii to the numbers of f electrons were observed for RECuTeSO-1 (cf., Figure 3). The Y3+ ion with an ionic radius intermediate between the radii of Ho3+ and Er3+ ions follows the structure of RECuTeSO-1.28 To adopt the same structure as RECuTeSO-1, ionic radii close to 1.093 Å and 1.082 Å are needed for Yb and Lu, respectively, based on the equation y = 0.0087x + 0.802. The crystal lattice is experiencing increasing strain with decreasing ionic radius, and Yb and Lu ions prefer to stay in a state with lower coordination number and smaller ionic radii. Magnetic Properties. Y2Cu(TeO3)2(SO4)2. We begin the discussion of magnetic properties by examining the behavior of the Y-containing phase, which serves as a model compound with the diamagnetic Y3+ ion and allows the evaluation of magnetic properties of only Cu2+ ion in the absence of 3d−4f magnetic exchange. Above 50 K, the temperature (T)

Distinguishing between the latter three can be difficult because most commonly the metal center is on a general crystallographic position. Shape8 calculations demonstrate that the geometry for the LnO8 units are best described as a distorted trigonal dodecahedron with approximate D2d symmetry.25 The average RE−O bond distances decrease from 2.47(8) Å for NdO8 to 2.36(10) Å for TmO8 (cf., Table S2, Supporting Information). This trend is consistent with the increasing localization and decreased shielding of the 4f orbitals due to lanthanide contraction. While at first glance the tetragonal distortion of the environment around the Cu2+ centers might be ascribed to the Jahn−Teller effect, the six O atoms bound to Cu2+ are donated from four TeO32− anions, which provide the equatorial O atoms, and two SO42− anions, which provide the axial O atoms (cf., Figure 1c), forming a [Cu(TeO3)2(SO4)2]6− unit. The axial Cu−O bonds are 1.943(3) Å and 1.960(4) Å compared with that of equatorial Cu−O bonds ranging from 2.030(4) Å to 2.312(3) Å. It is, therefore, difficult to know whether the distortion is caused by having different ligands in the axial and equatorial positions, or by the traditionally observed Jahn−Teller distortions of a d9 ion, or perhaps due to a combination of the two. RE2Cu(TeO3)2(SO4)2 (RE = Yb and Lu) (RECuTeSO-2) exhibit the same space group P1̅ as the yttrium and earlier lanthanides analogues, but the unit cell parameters and structures are quite different (cf., Table S1, Supporting Information). Instead of forming 1-D RE−oxo chains along the b axis, each Yb3+/Lu3+ polyhedron only shares one edge with another Yb3+/Lu3+, existing as a dimer (cf., Figure 2a). The coordination environment of Yb3+/Lu3+ ions in RECuTeSO-2 is quite different compared to that of RECuTeSO-1. Yb and Lu ions feature a slightly distorted pentagonal bipyramid geometry as shown in Figure 2b. Such geometries are very common for 2189

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Sm2Cu(TeO3)2(SO4)2 and Eu2Cu(TeO3)2(SO4)2. The Sm3+ and Eu3+ ions are well-known to show Van Vleck paramagnetism due to the presence of low-lying excited states. Consequently, the inverse susceptibility plots for both Sm2Cu(TeO3)2(SO4)2 and Eu2Cu(TeO3)2(SO4)2 exhibit strong deviation from the Curie−Weiss law (Figure S2, Supporting Information). As the samples are cooled, the depopulation of excited states leads to the continuous decrease in χT that approaches zero at lower temperatures (Figure 5a,b). Because the ground state of the Eu3+ ion is diamagnetic (7F0), one can expect the low-temperature behavior of Eu2Cu(TeO3)2(SO4)2 to be similar to that of the isostructural Y-containing analogue. Indeed, the peak observed in the magnetic susceptibility plot of the Eu-containing sample (Figure 5d) indicates antiferromagnetic ordering of the Cu2+ moments at TN = 15 K, close to the Néel temperature of Y2Cu(TeO3)2(SO4)2. In contrast, no signature of magnetic phase transition was observed for Sm2Cu(TeO3)2(SO4)2 down to 1.8 K (Figure 5c). In the latter, besides the Cu2+−Cu2+ antiferromagnetic coupling, other interactions may be present due to the paramagnetic ground state of the Sm3+ ions (6H5/2). We suggest that the competition between different exchange pathways prevents the emergence of the magnetic ordering for Sm2Cu(TeO3)2(SO4)2 in the investigated temperature range. Nevertheless, the field-dependent magnetization measured at 1.8 K shows linear behavior with relatively small maximum values for both Sm 2 Cu(TeO3)2(SO4)2 (0.31 μB) and Eu2Cu(TeO3)2(SO4)2 (0.24 μB) (Figure 5e,f). This finding suggests the Sm-containing compound is close to antiferromagnetic ordering that might occur at temperatures slightly lower than that available in our experiment. Dy2Cu(TeO3)2(SO4)2. This compound shows the linear Curie−Weiss behavior in the χ−1 vs T data nearly in the entire 1.8−300 K temperature range (Figure 6a, inset). To determine the effective moment per Dy3+ ion, the contribution from the Cu2+ ion (determined for the Y-containing analogue) was subtracted from the total moment. The effective moment obtained, 10.47 μB per Dy3+ ion, is in good agreement with the theoretical value of 10.63 μB. The Weiss temperature is only slightly positive, θ = 0.09(5) K, indicating negligible magnetic interactions between Cu2+ and Dy3+ moments. The small dip followed by the sharp increase in the χT value around 15 K (Figure 6a) is indicative of short-range antiferromagnetic correlations between the low Cu2+ magnetic moments (S = 1/2) and the high Dy3+ magnetic moments (6H15/2, J = 15/2). The field-dependent magnetization measured at 1.8 K shows the initial increase that follows the sum of Brillouin functions for two Dy3+ and one Cu2+ ions in the absence of magnetic

Figure 3. Plot of the inverse of the ionic radii vs the number of the f electrons for RECuTeSO-1 (blue) and RECuTeSO-2 (green).

dependence of inverse susceptibility (χ−1) obeys the modified Curie−Weiss law, 1/χ = 1/[χTIP + C/(T − θ)] (Figure 4a, inset), where χTIP, C, and θ represent the temperatureindependent paramagnetic (TIP) contribution, the Curie constant, and the Weiss temperature, respectively. The effective moment evaluated from the Curie constant, C = Nμeff2μB2/3kB, where N, μB, and kB are Avogadro’s number, Bohr magneton, and Boltzmann constant, respectively, is equal to 1.74 μB, which matches the spin-only theoretical moment of the Cu2+ ion (1.73 μB for S = 1/2), consistent with the expected diamagnetism of Y3+ ions. The small negative Weiss temperature, θ = −1.1(1) K, indicates weak antiferromagnetic interactions between the Cu2+ ions, which also agrees with the gradual drop in the χT value below 50 K (Figure 4b). An examination of the χ vs T dependence at low temperatures reveals a small hump around 15 K (Figure 4a), which suggests an occurrence of an antiferromagnetic phase transition. Indeed, the 1.8 K field dependence of magnetization is nearly linear (Figure 4c) and exhibits the maximum value of 0.14 μB at 7 T, which is significantly lower than 1 μB expected for the S = 1/2 Cu2+ ion. Thus, the weak magnetic exchange coupling between the Cu2+ ions results in antiferromagnetic ordering in Y2Cu(TeO3)2(SO4)2 at TN = 12.5 K (the more accurate temperature of the phase transition was established from the first derivative curve, Figure S1, Supporting Information).

Figure 4. Temperature dependence of magnetic susceptibility χ (a), inverse susceptibility (inset), and the χT product (b) for Y2Cu(TeO3)2(SO4)2. (c) Field-dependent magnetization measured at 1.8 K. The solid red line in part a shows the theoretical fit to the modified Curie−Weiss law. 2190

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Figure 5. Temperature dependence of χT (a and b), χ (c and d), and field dependence of magnetization at 1.8 K (e and f) for Sm2Cu(TeO3)2(SO4)2 and Eu2Cu(TeO3)2(SO4)2, respectively.

Figure 6. Temperature dependence of χT (a) and χ−1 (inset) and the field dependence of magnetization at 1.8 K (b) for Dy2Cu(TeO3)2(SO4)2. The solid red line in part a show the fit to the modified Curie−Weiss law, while the solid black line in part b represents the magnetization calculated as the sum of Brillouin functions for two Dy3+ and one Cu2+ ions.

Table 1. Magnetic Data for RE2Cu(TeO3)2(SO4)2 (RE = Y, Sm, Eu, Gd, Dy, Ho, Er, Tm, and Yb) Curie−Weiss fitting theor χmT at 300 K (emu K/mol) compd

2RE3+

Y Sm Eu Gd Dy Ho Er Tm Yb

0 0.176 0 15.761 28.249 28.09 22.992 14.326 5.153

μeff (RE3+, μB)

Cu2+

total

exptl χmT at 300 K (emu K/mol) total

C (emu K/mol)

θ (K)

from C

theor

0.375

0.375 0.551 0.375 16.136 28.624 28.465 23.367 14.701 5.528

0.667 1.19 3.31 16.01 27.61 27.37 22.41 13.87 5.22

0.388(2) − − 16.094(6) 27.76(1) 27.68(4) 22.95(4) 14.14(4) 5.98(1)

−1.1(1) − − −0.78(4) 0.09(5) −3.5(2) −6.8(2) −8.8(4) −44.1(7)

1.74(Cu2+) − − 7.93 10.47 10.45 9.50 7.58 4.73

1.73(Cu2+) 0.84 0 7.93 10.63 10.60 9.59 7.57 4.54

exchange (Figure 6b). Nonetheless, at fields above 0.3 T the magnetization increases much slower than expected from the Brillouin function, which can be explained by the population of low-lying excited states. Both these observations are in agreement with the weak exchange coupling between the Dy3+ and Cu2+ ions. No clear signature of magnetic ordering was observed for this phase.

RE2Cu(TeO3)2(SO4)2 (RE= Gd, Ho, Er, Tm). For all these compounds, the temperature dependence of χ−1 follows the Curie−Weiss law in the 1.8−300 K range (Figure S3, Supporting Information). The effective magnetic moments per RE3+ ion match well the theoretical values, while the negative Weiss temperatures indicate prevailing antiferromagnetic interactions between the nearest-neighbor ions (Table 1). 2191

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Figure 7. Temperature dependence of χT and 1.8 K field dependence of magnetization (insets) for RE2Cu(TeO3)2(SO4)2, where RE = Gd (a), Ho (b), Er (c), and Tm (d). The solid lines in the insets represent the magnetization calculated as the sum of Brillouin functions for two RE3+ and one Cu2+ ions.

The antiferromagnetic exchange is also supported by the decrease in the χT value at lower temperatures and the suppressed magnetization values as compared to the sum of Brillouin functions for two RE3+ and one Cu2+ ions at 1.8 K (Figure 7). A systematic increase in the strength of antiferromagnetic exchange is observed as the RE3+ ionic radius decreases, as indicated by pronouncedly more negative Weiss temperatures (−0.78(4) K for Gd3+, −3.5(2) K for Ho3+, −6.8(2) K for Er3+, and −8.8(4) K for Tm3+) (Table 1). It is difficult, however, to establish whether this effect stems from the stronger 3d−4f exchange or just from the stronger antiferromagnetic interactions between the Cu2+ ions as the lattice gets contracted (or from both). It was reported that the RE3+−Cu2+ interactions favor ferromagnetic exchange for the late half of the lanthanide series.29 Therefore, for RE2Cu(TeO3)2(SO4)2 (RE= Gd, Ho, Er, Tm), the RE3+−Cu2+ coupling may be ferromagnetic but overcome by antiferromagnetic Cu2+−Cu2+ and RE3+−RE3+ interactions.30 A systematic study of structural analogues that do not contain 3d magnetic moments (e.g., structures with Zn2+ ion) will help shed light on this question. Yb2Cu(TeO3)2(SO4)2. This compound behaves essentially similar to the four analogues just described, despite its different structure. Both the χT vs T and M vs H plots support the presence of antiferromagnetic exchange (Figure 8). The only notable difference is the significantly more negative Weiss temperature, θ = −44 K, which indicates that the antiferromangetic interactions in Yb2Cu(TeO3)2(SO4)2 are stronger than those observed in the four compounds above. This increased antiferromagnetic exchange cannot be explained by the change in the separation between the Cu2+ ions, as the nearest Cu−Cu distance in Yb2Cu(TeO3)2(SO4)2 is significantly larger than those in the other RE2Cu(TeO3)2(SO4)2 structures (Table 2). On the other hand, the RE−Cu distance in the former is the shortest in the series. Thus, a more probable cause for the stronger antiferromagnetic interactions may be the different structural type of Yb2Cu(TeO3)2(SO4)2

Figure 8. Temperature dependence of χT and 1.8 K field dependence of magnetization (inset) for Yb2Cu(TeO3)2(SO4)2. The solid line represents the magnetization calculated as the sum of Brillouin functions for two RE3+ and one Cu2+ ions.

Table 2. The Closest RE−RE, RE−Cu, and Cu−Cu Distances for All These Compounds RE

RE−RE (Å)

RE−Cu (Å)

Cu−Cu (Å)

Y Sm Eu Gd Dy Ho Er Tm Yb

3.7902(6) 3.8671(6) 3.8566(8) 3.8310(7) 3.7997(8) 3.797(3) 3.7804(5) 3.7708(6) 3.7623(9)

3.3823(4) 3.4341(4) 3.4303(7) 3.4018(5) 3.3890(7) 3.391(2) 3.3750(4) 3.3716(5) 3.2802(7)

5.1645(4) 5.2140(6) 5.2216(9) 5.1886(7) 5.171(1) 5.179(4) 5.1661(7) 5.1529(7) 5.328(2)

that promotes antiferromagnetic 3d−4f coupling. Proving this assumption, however, is nontrivial due to the other competing magnetic exchange pathways present in this crystal structure. UV−vis−NIR Spectroscopy. The solid-state UV−vis−NIR absorbance spectra of RE2Cu(TeO3)2(SO4)2 were obtained from single crystals and are shown in Figure 9. All the 2192

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phase transition was observed for any of the materials with paramagnetic RE3+ ions, which might indicate the presence of competing magnetic interactions (ferro- vs antiferromagnetic) that prevent the stabilization of a magnetically ordered ground state. Nevertheless, a clear increase in the strength of shortrange antiferromagnetic correlations was observed with the decrease in the RE3+ ionic radius, consistent with the shortening of distances between magnetically coupled metal ions in RE2Cu(TeO3)2(SO4)2.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

X-ray crystallographic files in CIF, Figure S1, Figure S2, Figure S3, Figure S4, crystal images, table of crystallographic data, table for selected bond distances, SEM-EDS data, and powder XRD data. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Materials Science of Actinides, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001089. This work was supported by a Chinese Scholarship Council Graduate Fellowship to J.L. The work on the magnetism of rare-earth-containing materials was supported in part by the National Science Foundation CAREER award DMR-0955353 to M.S.

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Figure 9. Normalized UV−vis−NIR spectra of RE2Cu(TeO3)2(SO4)2 (RE = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu).

REFERENCES

(1) (a) Mishra, A.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. J. Am. Chem. Soc. 2004, 126, 15648−15649. (b) Mori, F.; Nyui, T.; Ishida, T.; Nogami, T.; Choi, K.-Y.; Nojiri, H. J. Am. Chem. Soc. 2006, 128, 1440−1441. (c) Osa, S.; Kido, T.; Matsumoto, N.; Re, N.; Pochaba, A.; Mrozinski, J. J. Am. Chem. Soc. 2003, 126, 420−421. (d) Peng, J.-B.; Zhang, Q.-C.; Kong, X.-J.; Zheng, Y.-Z.; Ren, Y.-P.; Long, L.-S.; Huang, R.-B.; Zheng, L.-S.; Zheng, Z. J. Am. Chem. Soc. 2012, 134, 3314−3317. (2) Ma, J.-x.; Huang, X.-f.; Song, Y.; Song, X.-q.; Liu, W.-s. Inorg. Chem. 2009, 48, 6326−6328. (3) Wang, Y.; Fang, M.; Li, Y.; Liang, J.; Shi, W.; Chen, J.; Cheng, P. Int. J. Hydrogen Energy 2010, 35, 8166−8170. (4) (a) Yue, S.-T.; Wei, Z.-Q.; Wang, N.; Liu, W.-J.; Zhao, X.; Chang, L.-M.; Liu, Y.-L.; Mo, H.-H.; Cai, Y.-P. Inorg. Chem. Commun. 2011, 14, 1396−1399. (b) Bo, Q.-B.; Sun, Z.-X.; Forsling, W. CrystEngComm 2008, 10, 232−238. (5) Glover, P. B.; Ashton, P. R.; Childs, L. J.; Rodger, A.; Kercher, M.; Williams, R. M.; De Cola, L.; Pikramenou, Z. J. Am. Chem. Soc. 2003, 125, 9918−9919. (6) (a) Benelli, C.; Caneschi, A.; Gatteschi, D.; Sessoli, R. Adv. Mater. 1992, 4, 504−505. (b) Guillou, O.; Bergerat, P.; Kahn, O.; Bakalbassis, E.; Boubekeur, K.; Batail, P.; Guillot, M. Inorg. Chem. 1992, 31, 110− 114. (c) Bencini, A.; Benelli, C.; Caneschi, A.; Carlin, R. L.; Dei, A.; Gatteschi, D. J. Am. Chem. Soc. 1985, 107, 8128−8136. (7) (a) Habib, F.; Murugesu, M. Chem. Soc. Rev. 2013, 42, 3278− 3288. (b) Kovnir, K.; Thompson, C. M.; Garlea, V. O.; Haskel, D.; Polyanskii, A. A.; Zhou, H.; Shatruk, M. Phys. Rev. B 2013, 88, 104429. (8) (a) Sankar, S. G.; Rao, V. U. S.; Segal, E.; Wallace, W. E.; Frederick, W. G. D.; Garrett, H. J. Phys. Rev. B 1975, 11, 435−439.

compounds show broad absorbance from approximately 600 to 900 nm, which can be assigned to a combination of ligand-toCu2+ charge transfer and d−d transitions of Cu2+ ions.31 The band widths are related to the wide variety of Cu−O bonding distances, ranging from 1.943(3) Å to 2.312(3) Å. The electronic transitions of 4f elements were assigned decades ago, and the signature peaks of trivalent lanthanides are displayed in their respective spectra.32

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CONCLUSIONS In conclusion, RE2Cu(TeO3)2(SO4)2 (RE = Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) provide well-characterized examples of purely inorganic 3d/4f heterobimetallic compounds containing mixed anions. Owing to the lanthanide contraction, the early lanthanide analogues RECuTeSO-1 (RE = Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm) adopt a structure type different from that of the late analogues RECuTeSO-2 (RE = Yb and Lu). More specifically, the Ln−oxo bonding geometries change from 1-D chain (RECuTeSO-1) to isolated dimers (RECuTeSO-2). The coordination numbers of Ln3+ decrease from eight to seven, resulting in a coordination geometry transition from trigonal dodecahedron to pentagonal bipyramid. The phases with diamagnetic ground states of RE3+ ions exhibit antiferromagnetic ordering due to the exchange interactions between the Cu2+ ions. In contrast, no magnetic 2193

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Article

(b) Herbst, J. F.; Croat, J. J.; Pinkerton, F. E.; Yelon, W. B. Phys. Rev. B 1984, 29, 4176−4178. (9) (a) Morita, Y.; Suzuki, S.; Fukui, K.; Nakazawa, S.; Kitagawa, H.; Kishida, H.; Okamoto, H.; Naito, A.; Sekine, A.; Ohashi, Y.; Shiro, M.; Sasaki, K.; Shiomi, D.; Sato, K.; Takui, T.; Nakasuji, K. Nat. Mater. 2008, 7, 48−51. (b) Kennedy, B. P.; Lever, A. B. P. J. Am. Chem. Soc. 1973, 95, 6907−6913. (c) Willett, R. D.; Haugen, J. A.; Lebsack, J.; Morrey, J. Inorg. Chem. 1974, 13, 2510−2513. (d) Setifi, F.; Benmansour, S.; Marchivie, M.; Dupouy, G. l.; Triki, S.; Sala-Pala, J.; Salaun, J.-Y.; Gomez-García, C. J.; Pillet, S. b.; Lecomte, C.; Ruiz, E. Inorg. Chem. 2009, 48, 1269−1271. (e) Andruh, M.; Ramade, I.; Codjovi, E.; Guillou, O.; Kahn, O.; Trombe, J. C. J. Am. Chem. Soc. 1993, 115, 1822−1829. (10) Rinehart, J. D.; Long, J. R. Chem. Sci. 2011, 2, 2078−2085. (11) Long, J.; Habib, F.; Lin, P.-H.; Korobkov, I.; Enright, G.; Ungur, L.; Wernsdorfer, W.; Chibotaru, L. F.; Murugesu, M. J. Am. Chem. Soc. 2011, 133, 5319−5328. (12) (a) Lin, J.; Cross, J. N.; Diwu, J.; Polinski, M. J.; Villa, E. M.; Albrecht-Schmitt, T. E. Inorg. Chem. 2012, 51, 11949−11954. (b) Zehnder, R. A.; Wilson, C. S.; Christy, H. T.; Harris, K. S.; Chauhan, V.; Schutz, V.; Sullivan, M.; Zeller, M.; Fronczek, F. R.; Myers, J. A.; Dammann, K.; Duck, J.; Smith, P. M.; Okuma, A.; Johnson, K.; Sovesky, R.; Stroudt, C.; Renn, R. A. Inorg. Chem. 2011, 50, 836−846. (c) Lin, J.; Diwu, J.; Cross, J. N.; Villa, E. M.; AlbrechtSchmitt, T. E. Inorg. Chem. 2012, 51, 10083−10085. (d) Wang, C.-M.; Wu, Y.-Y.; Hou, C.-H.; Chen, C.-C.; Lii, K.-H. Inorg. Chem. 2009, 48, 1519−1523. (e) Baggio, R.; Garland, M. T.; Moreno, Y.; Pena, O.; Perec, M.; Spodine, E. J. Chem. Soc., Dalton Trans. 2000, 2061−2066. (f) Polinski, M. J.; Alekseev, E. V.; Darling, V. R.; Cross, J. N.; Depmeier, W.; Albrecht-Schmitt, T. E. Inorg. Chem. 2013, 52, 1965− 1975. (13) Sorace, L.; Benelli, C.; Gatteschi, D. Chem. Soc. Rev. 2011, 40, 3092−3104. (14) Hughes, I. D.; Dane, M.; Ernst, A.; Hergert, W.; Luders, M.; Poulter, J.; Staunton, J. B.; Svane, A.; Szotek, Z.; Temmerman, W. M. Nature 2007, 446, 650−653. (15) Peng, H.-M.; Jin, H.-G.; Gu, Z.-G.; Hong, X.-J.; Wang, M.-F.; Jia, H.-Y.; Xu, S.-H.; Cai, Y.-P. Eur. J. Inorg. Chem. 2012, 2012, 5562− 5570. (16) Mao, J.-G.; Jiang, H.-L.; Kong, F. Inorg. Chem. 2008, 47, 8498− 8510. (17) (a) Jiang, H.-L.; Mao, J.-G. Inorg. Chem. 2005, 45, 717−721. (b) Kong, F.; Hu, C.-L.; Hu, T.; Zhou, Y.; Mao, J.-G. Dalton Trans. 2009, 4962−4970. (c) Kong, F.; Xu, X.; Mao, J.-G. Inorg. Chem. 2010, 49, 11573−11580. (d) Jiang, H.-L.; Xie, Z.; Mao, J.-G. Inorg. Chem. 2007, 46, 6495−6501. (18) Lin, J.; Cross, J. N.; Diwu, J.; Meredith, N. A.; Albrecht-Schmitt, T. E. Inorg. Chem. 2013, 52, 4277−4281. (19) Zhao, B.; Cheng, P.; Chen, X.; Cheng, C.; Shi, W.; Liao, D.; Yan, S.; Jiang, Z. J. Am. Chem. Soc. 2004, 126, 3012−3013. (20) Mitchell, K.; Ibers, J. A. Chem. Rev. 2002, 102, 1929−1952. (21) (a) Bruker. SADABS, TWINABS; Madison, WI, 2001. (b) Bruker. SAINT; Madison, WI, 2007. (22) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532. (23) Lin, J.; Diefenbach, K.; Cross, J. N.; Babo, J.-M.; AlbrechtSchmitt, T. E. Inorg. Chem. 2013, 52, 13278−13281. (24) (a) Cross, J. N.; Villa, E. M.; Wang, S.; Diwu, J.; Polinski, M. J.; Albrecht-Schmitt, T. E. Inorg. Chem. 2012, 51, 8419−8424. (b) Cross, J. N.; Duncan, P. M.; Villa, E. M.; Polinski, M. J.; Babo, J.-M.; Alekseev, E. V.; Booth, C. H.; Albrecht-Schmitt, T. E. J. Am. Chem. Soc. 2013, 135, 2769−2775. (c) Diwu, J.; Nelson, A.-G. D.; Wang, S.; Campana, C. F.; Albrecht-Schmitt, T. E. Inorg. Chem. 2010, 49, 3337−3342. (d) Diwu, J.; Wang, S.; Good, J. J.; DiStefano, V. H.; Albrecht-Schmitt, T. E. Inorg. Chem. 2011, 50, 4842−4850. (e) Diwu, J.; Wang, S.; Liao, Z.; Burns, P. C.; Albrecht-Schmitt, T. E. Inorg. Chem. 2010, 49, 10074−10080. (25) Gorden, A. E. V.; Xu, J.; Raymond, K. N.; Durbin, P. Chem. Rev. 2003, 103, 4207−4282.

(26) (a) Volkringer, C.; Henry, N.; Grandjean, S.; Loiseau, T. J. Am. Chem. Soc. 2012, 134, 1275−1283. (b) Jin, G. B.; Skanthakumar, S.; Soderholm, L. Inorg. Chem. 2012, 51, 3220−3230. (c) Ling, J.; Ward, M.; Burns, P. C. J. Solid State Chem. 2011, 184, 401−404. (d) Daly, S. R.; Girolami, G. S. Inorg. Chem. 2010, 49, 4578−4585. (e) Salvadó, M. A.; Pertierra, P.; Castro, G. R.; Trobajo, C.; García, J. R. Inorg. Chem. 2008, 47, 1246−1248. (f) Forbes, T. Z.; Burns, P. C. J. Solid State Chem. 2007, 180, 106−112. (g) Forbes, T. Z.; Burns, P. C. J. Solid State Chem. 2005, 178, 3445−3452. (27) Raymond, K. N.; Wellman, D. L.; Sgarlata, C.; Hill, A. P. C. R. Chim. 2010, 13, 849−852. (28) Carlin, R. L. Magnetochemistry; Springer-Verlag: New York, 1986. (29) (a) Koner, R.; Lin, H.-H.; Wei, H.-H.; Mohanta, S. Inorg. Chem. 2005, 44, 3524−3536. (b) Kahn, M. L.; Mathonière, C.; Kahn, O. Inorg. Chem. 1999, 38, 3692−3697. (c) Costes, J.-P.; Dahan, F.; Dupuis, A.; Laurent, J.-P. Chem.Eur. J. 1998, 4, 1616−1620. (30) Feng, W.; Zhang, Y.; Zhang, Z.; Lü, X.; Liu, H.; Shi, G.; Zou, D.; Song, J.; Fan, D.; Wong, W.-K.; Jones, R. A. Inorg. Chem. 2012, 51, 11377−11386. (31) (a) Shirai, T.; Nakagaki, T.; Nakai, Y.; Sugar, J.; Ishii, K.; Mori, K. J. Phys. Chem. Ref. Data 1991, 20, 1−81. (b) Sugar, J.; Musgrove, A. J. Phys. Chem. Ref. Data 1990, 19, 527−616. (32) (a) Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4450−4455. (b) Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4447−4449. (c) Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4412−4423. (d) Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4424−4442.

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