A Potential Multiferroic Co Bearing Member of the ... - ACS Publications

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Pb3TeCo3V2O14: A Potential Multiferroic Co Bearing Member of the Dugganite Series H. J. Silverstein,*,1 K. Cruz-Kan,2 A. M. Hallas,2 H. D. Zhou,3 R. L. Donaberger,4 B. C. Hernden,1 M. Bieringer,1 E. S. Choi,3 J. M. Hwang,3 A. S. Wills,5 and C. R. Wiebe1,2 1

Department of Chemistry, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada Department of Chemistry, University of Winnipeg, Winnipeg, Manitoba R3B 2E9, Canada 3 National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306-4005, United States 4 National Research Council, Chalk River Laboratories, Chalk River, Ontario K0J 1J0, Canada 5 Department of Chemistry, University College London, London WC1H 0AJ, United Kingdom 2

ABSTRACT: Polycrystalline Pb3TeCo3V2O14, a structural analogue of the multiferroic Ba3NbFe3Si2O14, was synthesized and characterized using X-ray diffraction, magnetic susceptibility, specific heat, dielectric constant, and neutron diffraction. Magnetic susceptibility, specific heat capacity measurements, and bond valence analysis confirmed that the V5+ ion is nonmagnetic, while Co2+ is in its high spin state (S = 3/2). Two magnetic transitions were seen at TN1 = 8.6 K and TN2 = 6.0 K where the spins first adopt a magnetic structure with propagation vector k⃗ = (0.752,0,1/2) and reorder into a commensurate structure with propagation vector k⃗ = (5/6,5/6,1/2). Changes in the dielectric constant at both magnetic phase transitions suggest that magnetoelectric coupling exists in Pb3TeCo3V2O14. KEYWORDS: multiferroic, magnetoelectric coupling, frustrated magnets, neutron scattering, langasite, property measurements

1. INTRODUCTION Multiferroic materials are an interesting class of compounds as they can maintain both a spontaneous electric and magnetic polarization.1−3 These materials hold promising potential applications, fueling research toward the development of new transition metal and rare-earth oxides such as RMnO3 (R = Ho, Y, Lu), 4 , 5 EuTiO 3 , 6 CuO, 7 Ba 2 Mg 2 Fe 1 2 O 2 2 , 8 , 9 and Sr3Co2Fe24O41.10The cause of multiferroic behavior is still a matter of debate in some materials, but geometric frustration is believed to play a role in many.14−12 Geometric frustration tends to prevent a compound with antiferromagnetically coupled spins from ordering resulting in exotic ground states with interesting and useful properties including, but not limited to, multiferroicity.13,14 There are two classes of multiferroic compounds. Ferroelectricity in “proper” multiferroic compounds arises from nonmagnetic lone pairs that distort to break symmetry in the lattice.15 Consequently, magnetoelectric coupling is weak in these materials.16−18 Conversely, “improper” multiferroics show strong magnetoelectric coupling as the electric polarization has a magnetic origin.15,19,20 The multiferroic behavior of these materials has been attributed to complex magnetic ordering; most typically based on spiral spin structures.7,8,20 Geometrically frustrated compounds are ideal improper multiferroic candidates because of their tendency to order in unconventional magnetic ground states such as long-range incommensurate spiral structures.15 The best studied geometrically frustrated multiferroics are those with 2D stacked frustrated spin sublattices such as © 2012 American Chemical Society

YMnO34,5 and Ba3NbFe3Si2O14.21−24 The latter contains isolated 2-dimensional (2D) stacks of Fe3+ (S = 5/2) trimers. The Fe3+ moments tend to order at TN = 26 K into an incommensurate spiral with propagation vector k⃗ = (0,0,τ) where τ ≈ 1/7.24 Only one chirality of this helix exists, prompting much interest in the magnetic properties of Ba3NbFe3Si2O14 and other related Fe containing compounds.25,26 To date, not much attention has focused on substituting the Fe3+ ion for other magnetic ions in the hopes of making new multiferroics. The problem lies in that these magnetic ions much prefer the octahedral site occupied by Nb5+ rather than the Fe3+ tetrahedral site. This results in the breakdown of the 2D sublattice. However, Te6+ is only known to occupy the octahedral site; using Te6+ instead of Nb5+ forces other magnetic ions like Co2+ and Mn2+ into the tetrahedral site.27,28 Additionally, it has been shown that electronegativity plays a role in increasing the multiferroic transition temperature in langasites.26 Substituting Nb5+ for the more electronegative Sb5+ raises the multiferroic transition temperature from 26 to 35 K. Te6+ is also more electronegative than Nb5+ and raises the possibility of greater Te−O hybridization leading to higher multiferroic transition temperatures in these Pb compounds. We were able to synthesize polycrystalline Pb3TeCo3V2O14, a derivative of the naturally occurring mineral dugganite. This compound is isostructural to Ba3NbFe3Si2O14 with Co2+ Received: August 22, 2011 Revised: January 24, 2012 Published: January 27, 2012 664

dx.doi.org/10.1021/cm202502p | Chem. Mater. 2012, 24, 664−670

Chemistry of Materials

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Figure 1. (a and b) The chemical structure of Pb3TeCo3V2O14 as viewed in the ab and ac planes, respectively. The Co2+ trimers are easily identifiable in (a) and the stacking of those trimers along the c-axis is readily observed in (b). The unit cell is outlined in black; (c) Rietveld refinement of powder Pb3TeCo3V2O14 using Cu Kα1,2 X-rays. River Laboratories. Measurements were made using two sets of conditions. For the first set, a wavelength of 1.3284 Å was used at 298 and 4.3 K using both the low and the high angle detector banks to obtain a full diffraction pattern counting for two hours each. The second set of conditions used a wavelength of 2.3696 Å over a temperature range of 4.3 to 10.25 K in 0.25 K steps. Additional measurements were taken at 15 K, 20 K, 25 K, and 40 K. The FullProf Suite30 was used to complete a joint X-ray/neutron refinement with Xray agreement factors Rp = 2.46%, Rwp = 3.36%, and χ2 = 4.16 and neutron agreement factors Rp = 5.50%, Rwp = 7.23%, and χ2 = 8.48. Coarse collimation, short data collection time, and small impurities that appear on the neutron powder diffraction pattern raise the χ2 of the refinement. To better the quality, the refinement was weighted 70% in favor of the neutron data to obtain the oxygen atomic coordinates and thermal parameters. Afterward, these values were fixed in a second joint refinement weighted 80% in favor of the X-ray data to obtain the cobalt and lead atomic coordinates and thermal parameters. The V−O1 atomic coordinates were fixed to values obtained from a Rietveld refinement of a single crystal sample of the natural dugganite mineral.31 This was due to the difficulty in determining where the electron density was situated along the V−O1 bond positioned along the c-axis.

occupying the tetrahedral sites oriented in the isolated 2D stacked trimers. Although Ivanov et al.29 and Mill27 detail the first synthesis of this material, no refinements were offered to verify their purity. We have studied the structure of this compound extensively and have characterized the magnetic susceptibility, the magnetic heat capacity, the low temperature magnetic structure and the temperature dependence of the dielectric constant with the goal of discovering a potential new class of geometrically frustrated multiferroics.

2. EXPERIMENTAL SECTION Polycrystalline Pb3TeCo3V2O14 was prepared using a standard solidstate reaction. Stoichiometric amounts of PbO, TeO2, Co3O4, and V2O5 were ground together and pressed into 1 cm diameter pellets under 10 000 kPa of pressure. The pellets were sintered in air between 650 and 800 °C for 15−24 h. Samples were annealed at 600 °C for up to 72 h with intermittent grindings. All oxides were of high purity (99.99%) and purchased from Sigma-Aldrich. DC susceptibility measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer purchased from Quantum Design using an applied field of 0.1 T. Specific heat measurements were measured using a Physical Property Measurement System (PPMS) also purchased from Quantum Design. Both DC susceptibility and specific heat measurements were made down to 4.3 K. Dielectric constant measurements were made by creating electrical contacts on two opposite surfaces of a thin plate sample using Ag paste. An automated capacitance (Andeen-Hagerling AH-2700A) bridge operating at a frequency of 10 kHz was used. Capacitance was converted to the dielectric constant by approximating the sample as an infinite parallel capacitor. All magnetic characterization measurements were conducted at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida. Room temperature powder X-ray diffraction (XRD) data were collected on a PANalytical X’pert Pro System with Cu Kα1,2 radiation equipped with a diffracted beam Ni-filter and an X’Celerator detector. Using a zero background sample holder, the angular range 10° ≤ 2θ ≤ 120° was measured in 0.0083° steps over 24 h. Neutron powder diffraction (NPD) measurements were performed using the C2 diffractometer at the Canadian Neutron Beam Centre, NRC, Chalk

3. RESULTS AND DISCUSSION Figure 1 shows the room temperature XRD pattern of Pb3TeCo3V2O14. The sample has small impurities consisting of mainly Pb2O3, TeO3, and Co3O4 on the order of 1% by mass.32−34 This compound has a hexagonal noncentrosymmetric unit cell with P321 symmetry. As shown in Figure 1a, the Co2+ atoms form isolated triangles in the ab plane stacked along the c direction in a manner similar to the Fe3+ ions in the isostructural multiferroic compound Ba3NbFe3Si2O14.21 The crystallographic parameters are listed in Table 1. Problems occurred when refining the XRD or NPD data alone. First, since V5+ and O1 share site symmetry in the x and y axes, their electron density must be positioned properly along the z axis. Refinement of XRD data alone would not allow us to properly refine O1 due to its relative lack of electron density 665



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(crystal radius = 0.885 Å) making disorder at the Co2+ site unlikely. No structural distortions were detected upon cooling from 40 to 4.3 K using neutron powder diffraction data. These results are consistent with those obtained using X-rays in Ba3NbFe3Si2O14, although our measurements had lower resolution.26 Figure 2a shows the inverse DC susceptibility fitted to a Curie−Weiss law with Weiss temperature of θ = −18.3 ± 0.2 K

Table 1. Pb3TeCo3V2O14 Crystallographic Parameters at Room Temperature with Lattice Constants a = 8.5595(4) Å and c = 5.2167(3) Å atom

site

x

y

z

Biso (Å2)

Pb Te Co V O1 O2b O3b

3e 1a 3f 2d 2d 6g 6g

0.59984(11) 0 0.2373(6) 1/3 1/3 0.4583(16) 0.1261(12)

0 0 0 2/3 2/3 0.1866(13) 0.2127(13)

0 0 1/2 0.5294(6)a 0.239(4)a 0.3297(20) 0.226(2)

2.53(3) 2.36(8) 2.9(2) 2.5(2) 2.82(1) 2.82(1) 2.82(1)

a

Values fixed to those found in the work by Lam et al.31 bValues fixed after a joint X-ray/neutron refinement weighted 30%/70%.

while refinement of the NPD data alone would not allow us to refine V5+ data due to its small coherent cross-section. Even refining the XRD and NPD data together did not give realistic relative positions of V5+ and O1. Consequently, we used values for As5+ and O3 for the naturally occurring dugganite mineral.31 Table 2 lists the calculated bond valences and bond distances. Pb2+ are 8-fold coordinated in distorted Thompson Table 2. Bond Valence, Coordination, and Distances As Deduced by a Joint X-ray/Neutron Refinement central

bond valence

coordination

ligand

distance (Å)

multiplicity

Pb2+

1.75

8

Te6+ Co2+

5.12 2.19

6 4

V5+

5.61

4

O1 O2 O2 O3 O3 O2 O3 O1 O2

2.896(11) 2.731(15) 2.985(2) 2.388(12) 1.976(10) 1.974(18) 1.865(11) 1.52(4) 1.75(3)

2 2 2 2 6 2 2 1 3

O12‑ O22‑ O32‑

2.23 1.90 1.95

cubes with RPb−O = 2.388−2.985 Å. Te6+ octahedra (RTe−O = 1.976 Å) separate each Co2+ trimer in the c direction. The Te− O distances are slightly larger than those calculated by Brown and Altermatt35 (R = 1.917 Å). However, Te6+−O distances can easily distort due to the lack of lone pairs on the central atom. The Te−O distances seen in this compound are well within the natural variance for this coordination state; normal Te6+−O distances are found to be somewhere between 1.85 and 2.05 Å.28 Pb2+ may also contribute to the anomalously short V−O1 bond distance. O1 sits between three lead sites and would experience repulsion between the lead lone-pairs, which would push O1 toward V5+. Bond valence analysis on Co2+ and V5+ indicates that Co2+ (V = 2.19) is paramagnetic while V5+ (V = 5.61) is diamagnetic. The high V5+ bond valence is directly related to the distorted V−O1 bonds, although this distortion is real and is seen in analogous bonds in the naturally occurring mineral.31,36 The bond valence of Te6+ (V = 5.12) is far less than the expected value of 6, but this is more likely attributed to errors in the bond valence method than incorrect Te−O distances.28,37 Although Te6+ and V5+ have similar crystal radii, the possibility of site disorder is unlikely as Te6+ is only known to exist in octahedral coordination.28,38 More important is the difference in charge and size of V5+ and Co2+

Figure 2. (a) Inverse magnetic susceptibility fit to the Curie−Weiss law. Inset: The first derivative of the magnetic susceptibility shows two distinct transition temperatures; (b) the magnetic component of the specific heat capacity. Inset: Specific heat capacity of Pb3TeCo3V2O14 (red) and Pb3TeZn3V2O14 (black) used as a lattice standard; (c) entropy released through magnetic ordering. Blue lines represent the theoretical maximum (top) and entropy released before TN1 (bottom).

fitted to room temperature. This gives Pb3TeCo3V2O14 a small frustration index of f = 2.3. The calculated effective magnetic moment for Co2+ is μeff = 4.5 ± 0.1 μB, which corresponds to a high spin state (S = 3/2) with a partially quenched orbital contribution; further verification that Co2+ is the only paramagnetic ion in this sample.39 The anomalies seen in the DC-susceptibility are extremely weak. Therefore, the inset shows the first derivative of the magnetic susceptibility of this sample where two transitions are clearly seen at TN1 = 8.6 K and at TN2 = 6.0 K. A former experiment by Ivanov et al.29 showed that this compound has one antiferromagnetic transition at 11 K with Co2+ μeff = 4.5 μB and a Weiss temperature of θ = −13 K. Although their value for the effective moment agrees with our value, the transition temperature and 666



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Figure 3. (a) Contour plot of neutron powder diffraction scans showing the magnetic transitions at TN1 = 8.6 K and TN2 = 6.0 K using a wavelength of 2.37 Å near the (100), (001), and (110) peaks. Diffuse scattering can also be seen surrounding the (100) peak starting at 8.0 K and increasing in intensity with temperature. Inset: A closeup of the (100) peak; (b) neutron powder diffraction scans taken at select temperatures to highlight the magnetic phase transition. Rietveld refinement of the magnetic phases are shown for (c) T = 6.75 K and (d) T = 6.0 K.

different mechanism. This is evidence for a two-tiered ordering of the magnetic moments; the moments fluctuate to a partially ordered state at 8.6 K and lock into a final structure at 6.0 K. Also worth noting are the particularly strong anomalies in the heat capacity seen at both transitions as compared to the DC susceptibility. Low temperature neutron diffraction experiments were performed at Chalk River Laboratories using the C2 instrument. No magnetic ordering due to the Co3O4 impurity was observed. The contour map in Figure 3a shows the appearance of magnetic Bragg peaks below TN1 and new, different magnetic Bragg peaks below TN2. Significant diffuse scattering is seen about the (100) and (001) peaks around 7 K and persisting until at least 40 K (not shown). This suggests that short-range order plays a role in the origin of the magnetic entropy release. Diffuse scattering is seen at the (100) peak in Ba3NbFe3Si2O14 well above TN.26 Two distinct magnetic phases can be seen in Figure 3a,b. The SARAh package was used for solving the magnetic structure using representational analysis, the details of which can be found in the literature (program available from www. ccp14.ac.uk).40−42 In Ba3NbFe3Si2O14 the moments are oriented solely in the ab plane with a stacking periodicity of approximately 7 unit cells. Pb3TeCo3V2O14 is the first in the dugganite/langasite series containing a sole magnetic ion other than iron to show long-range ordering. Although the first magnetic structure will be mentioned, the discussion will be limited to the second. Figure 3c displays the first magnetic structure with propagation vector k⃗ = (0.752,0,1/2). A second transition occurs at TN2 = 6.0 K as seen in Figure 3. The magnetic contribution to the powder neutron diffractogram of Pb3TeCo3V2O14 below 6 K can be well described with an ordering of the Co2+ moments according to the irreducible

Weiss temperature are not within error probably due to differences in how the measurements were obtained or impurities within their sample. Ivanov et al. fit the region of the susceptibility to the Curie−Weiss law using a different temperature regime. Moreover, they measured TN as the point at which a broad feature can be seen in the magnetic susceptibility, whereas in this experiment, the derivative of the susceptibility is used to determine the number and location of the transition temperatures. Evidence from specific heat measurements and neutron scattering experiments support the results obtained by our methods. The magnetic component of the specific heat capacity, Cmag, was obtained using a lattice standard with no magnetic ions (plotted as Cmag/T vs T in Figure 2b). Pb3TeZn3V2O14 was synthesized in the same manner as the compound of interest using both PbO and PbO2 powders in order to conserve oxygen stoichiometry. Both transitions observed in the magnetic susceptibility data can also be seen in the specific heat data. Figure 2c shows the total calculated magnetic entropy S = 10.7 J/(mol K) is about 93% of the theoretical value of 11.5 J/(mol K). However, the magnetic component begins to rise after 40 K resulting in a considerable entropy release (1 − [S(T > TN1)/S] = 0.286 = 28.6%) before the first transition temperature. A similar phenomenon occurs in Ba3NbFe3Si2O14 suggesting the formation of short-range order or the presence of strong spin fluctuations above TN.21 It should be noted that the release in entropy before the transition is much higher in Ba 3NbFe 3Si 2O14 than in Pb3TeCo3V2O14. This could be due to the former having a Weiss temperature 10 times larger than the latter, suggesting the interactions between Fe3+ spins are stronger than those for Co2+. It is also possible that the additional entropy in our compound is due to novel spin fluctuations arising from a 667



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Figure 4. View depicting how the Co2+ moments change within each trimer at T = 6.0 K in the (a) ab plane and the (b) c direction. The black line depicts the boundaries of the nuclear unit cell. (c) Model depicting how the spin vector changes in the a direction. Moment size fluctuates between 1.6 and 3.1 μB from site to site in the periodic chain.

representation (IR) Γ(1) of the little group Gx formed from the space group P321 and the propagation vector k, restricted to the Hilbert space defined by Ψ(1), Ψ(2), and Ψ(3).40−43 We find that the moments refined, using the program SARAh, into a complicated structure with propagation vector k⃗ = (5/6,5/6,1/2) that would correspond to an enormous magnetic unit cell with 72 times the volume of the chemical unit cell. The refinement of this phase is shown in Figure 3d while a picture depicting this magnetic phase is shown in Figure 4. The refinement of the magnetic phase was extremely unstable when both Co2+ moments were refined separately; as a result, the coefficients of moment orientation vectors for both Co2+ were refined to be the same. The magnetic moments have a ccomponent to them and adopt an unusual conformation within each trimer. Figure 4c shows that the magnitude of the magnetic moment changes over six unit cells. The behavior in Pb3TeCo3V2O14 is markedly different from Ba3NbFe3Si2O14. As a result, the exchange pathways will change from those in Ba3NbFe3Si2O14. Multiferroics containing Co as the sole magnetic ion are very limited but include Co3V2O8: a system where the Co moments also exhibit complex ordering into many distinct magnetic phases. 44,45 Coupling between ferromagnetic ordering and the dielectric constant was observed.46,47 The dielectric constant was measured and is plotted in Figure 5 along with the first derivative of the curve. Three features can be seen in this plot. The first two are kinks in the trend around both transition temperatures. The third anomaly is a broad dip in the dielectric constant seen after TN2 = 6.0 K. All of these features are observed in many multiferroic compounds, including Ba3NbFe3Si2O14.2,14,11,21,23,26 Dielectric measurements on polycrystalline Ba3NbFe3Si2O14 show a similar trend with a dielectric anomaly on the same order of magnitude as our compound.26 No electric polarization measurements were made on this sample as powder averaging would most likely diminish any hint of a polarization−electric field (P-E) loop. However, the presence of a P-E loop does not necessarily

Figure 5. Dielectric constant changes with temperature in a manner similar to Ba3NbFe3Si2O14. Both transition temperatures are labeled. Inset: First derivative of the curve in the main figure. Each point in this curve is an average of the first derivate of one point in the outset figure and the first derivative of the four points that follow it. This was done to better see some of the features; this artificially decreased the temperature at which local maxima occur.

entail multiferroic behavior.48−50 A single crystal greatly enhances features seen on both dielectric temperature dependence and P-E curves as evidenced with Ba3NbFe3Si2O14.21 Growth of Pb containing single crystal samples has proved challenging due to the high volatility of PbO, TeO3, and V2O5, though recent techniques using an image furnace are showing promise.51 Co3V2O8 was grown using the floating zone technique.52 A flux growth might also be possible as was the case for Co2V2O7.53 For Ba3NbFe3Si2O14, the multiferroic behavior is thought to derive directly from its chiral magnetic structure.12 Lee et al. were able to calculate the ferroelectric polarization using the exchange parameters J from a model that includes five nearest neighbors and found their model was in good agreement with the experimental value found by Zhou et al.21 Using the Weiss 668



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temperature, θ, we have calculated the average exchange parameter, |J| = 0.6 K, with θ = 2[S(S + 1)/3k b] ∑ z iJi

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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(1)

ACKNOWLEDGMENTS This work was supported by NSERC, the ACS Petroleum Fund, CFI, The University of Manitoba, NHMFL, and NSF Grant DMR-0654118. H.J.S. would like to thank Paul Sarte and Brenden Van Wyk for useful discussions. The NHMFL is operated under a cooperative agreement with Florida State University and the NSF under DMR-0654118. We are greatly appreciative of the staff and support of the National Research Council at Chalk River Laboratories.

where S is the spin number on each site and zi is the number of nearest neighbors connected by the exchange parameter. This is much lower than the average exchange parameter of |J| = 6.3 K for Ba3NbFe3Si2O14 or |J| = 2.3 K for Ba3TaFe3Si2O14, another multiferroic langasite with a chiral spin structure.54 Although the exchange pathways of Pb3TeCo3V2O14 are different from those of Ba3NbFe3Si2O14, the map for the former should be easily made starting from the model for the latter. Clearly, the dugganite system is more complex than the langasite system as evidenced by moment interactions within the ab plane. It seems that slight changes in bond lengths and angles greatly change the interactions between magnetic moments.12 It is interesting to draw correlations between the dugganite system and other known Co−V systems. Single crystalline Co2V2O7 has monoclinic symmetry where Co2+ occupies an octahedral site and displays drastically different behavior than the powder form.53,55 While the powder sample indicates signs of ferromagnetic ordering of Co2+ (S = 1/2), the single crystal form shows behavior indicative of high-spin Co2+ with a large orbital contribution and weak net antiferromagnetic interactions. Furthermore, the single crystal form has two magnetic transition temperatures at TN1 = 13.3 K and TN2 = 6.0 K with hints of short-range order above TN. Magnetoelectric coupling is observed around the transition temperature that is thought to arise from Co2+ spin anisotropy.56 The nature of the ordering in Co2V2O7 is different from that seen in the multiferroic Co3V2O8 with orthorhombic symmetry.44,45 In the dugganites, it would seem that the mere presence of Co2+ and V5+ plays a much larger role in determining the behavior of these systems than structural considerations. To observe the magnetoelectric effect, inversion symmetry has to be broken. In the langasite, chiral magnetic ordering induces an electric polarization within the sample. In the dugganite system, the chemical structure likely plays a role since no chirality is observed in the magnetic structures. This is consistent with the space group of the crystal lattice (P321) being noncentrosymmetric. The system may have an antiferroelectric component that helps drive the massive reordering of Co2+ spins. For example, an electric component could arise from heavily distorted V5+ tetrahedra or, possibly, distortions at the subangstrom level. In either case, a deeper investigation of this matter is warranted.

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REFERENCES

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4. CONCLUSION The synthesis of Pb3TeCo3V2O14 showcases that using Te6+ instead of Nb5+ is an effective method in replacing Fe3+ with other magnetic ions within this structural motif. This compound, a derivative of the dugganite structure, is isostructural to the multiferroic langasite Ba3NbFe3Si2O14. The lattice is frustrated; Co2+ sits on the vertices of isolated triangles in the ab plane stacked in the c direction. It was shown that this dugganite has two magnetic transitions at TN1 = 8.6 K and TN2 = 6.0 K. The dugganite system, like other Co−V systems, displays magnetoelectric coupling which might entail multiferroic behavior. Inelastic neutron scattering experiments and single crystal growth are two obvious directions for future study. 669



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