Hydrothermal Synthesis and Magnetic Characterization of the

Oct 21, 2016 - •S Supporting Information. ABSTRACT: The new quaternary layered oxide CoMo2Sb2O10 was ... prepared by hydrothermal synthesis was used...
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Hydrothermal Synthesis and Magnetic Characterization of the Quaternary Oxide CoMo2Sb2O10 Sk Imran Ali,† Reinhard K. Kremer,‡ and Mats Johnsson*,† †

Department of Materials and Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70568 Stuttgart, Germany

‡

S Supporting Information *

ABSTRACT: The new quaternary layered oxide CoMo2Sb2O10 was synthesized by hydrothermal synthesis techniques, and its structure was determined from single-crystal X-ray diffraction data. CoMo2Sb2O10 crystallizes in the monoclinic space group C2/c with one Sb3+, Mo6+, and Co2+ atom site per unit cell, respectively. The crystal structure contains building units consisting of [Co2O8]n, [Mo2O8]n, and [SbO2]n chains. These are connected through corner sharing to form chargeneutral [CoMo2Sb2O10]n layers. Thermal decomposition of CoMo2Sb2O10 starts at 550 °C. The magnetic susceptibility follows a Curie−Weiss law above 50 K with a Curie constant of C = 3.46 emu·K· mol−1 corresponding to an effective moment of μeff = 5.26 μB per cobalt atom and a Curie−Weiss temperature θ = −13.2 K. Short-range antiferromagnetic ordering dominates below 5 K. Magnetic susceptibility and heat capacity data can be successfully modeled by the predictions from an Ising linear chain with an intrachain spin exchange of ca. −7.8 K.

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INTRODUCTION A large variety of ternary oxides in the Co−Sb−O system has previously been described in literature. The oxidation state of the Sb cations plays an important role when forming different kinds of crystal structures. Sb5+ cations allow octahedral coordination, and example compounds are the CoSb2O6 crystallizing with the trirutile structure,1,2 the pyrochlore-type Co2Sb2O7,3 or the spinel-type compound Co2.5Sb0.5O4.4 Because of the presence of the stereochemically active lonepair, Sb3+ exhibits one-sided coordination, and a previously described example is CoSb2O4 crystallizing with the schafarzikite-type structure.5 In contrast to the set of ternary compounds there are comparatively fewer quaternary compounds investigated, so far. However, several quaternary M− Co−Sb5+−O (M = Ag, Na, Sr, Ba) oxides have been synthesized and investigated where the M atom often is an alkaline- or an alkaline earth metal cation. Some of these compounds exhibit interesting magnetic properties showing, for example, spin-glass transitions at low temperatures. Examples are the layered phases Ag3Co2SbO6 and Na3Co2SbO6,6 the distorted perovskites Sr3CoSb2O9 (insulator), Sr2CoSbO6, and Sr2CoSbO5.63 (semiconductors),7 and the phases Ba3CoSb2O9 and Ba2Co1.4Sb0.6O6−y.8 Also quasi-ternary Co-substituted compounds have been described, for example, the spinel Zn2.33−xCoxSb0.67O4 (0.23 ≤ x ≤ 2.33)9 and the inverse spinel Zn7−xCoxSb2O12 (x = 5−7).10 The compounds in the Co−Sb−O and M−Co−Sb−O systems quoted before were synthesized by solid-state reaction that involves heating the constituents in a silica ampoule to high or very high temperatures. In the present study we utilize a © XXXX American Chemical Society

low-temperature hydrothermal synthesis technique to grow single crystals of a new quaternary phase with composition CoMo2Sb2O10. The crystals were of very good quality and enabled a full structure determination by X-ray single-crystal diffraction. It established CoMo2Sb2O10 as the first stoichiometric quaternary oxide that simultaneously contains Co2+ and Sb3+ cations. A larger phase-pure polycrystalline sample also prepared by hydrothermal synthesis was used to investigate the thermal stability and the low-temperature thermal and magnetic properties of CoMo2Sb2O10. We found predominant antiferromagnetic exchange between the Co2+ cations and strong evidence for Ising linear chain behavior at low temperatures. Indication for long-range anti-ferromagnetic ordering was not observed.

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EXPERIMENTAL SECTION

A mixture of CoF2:MoO3:Sb2O3 = 1:2:1 in 1.5 mL of deionized water and four droplets of HF (∼0.5 cm3) were sealed in a 18 mL Teflonlined steel autoclave and heated to 230 °C at a rate of 1.6 °C/min. The plateau temperature was maintained for 4 d, and thereafter the temperature was lowered to 30 °C with the same rate as in the heating cycle. The following starting chemicals were used: Sb2O3 (99.97%, Sigma-Aldrich), CoF2 (99.8%, Sigma-Aldrich), and MoO3 (99.5%, Sigma-Aldrich), HF (48%, Sigma-Aldrich). The hydrothermal synthesis yielded green single crystals of CoMo2Sb2O10 (∼80% by weight) that were washed several times using water and ethanol followed by drying at room temperature. Synthesis experiments performed with increased water level did not yield phase-pure material. Received: August 23, 2016

A

DOI: 10.1021/acs.inorgchem.6b02031 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Single-crystal X-ray data were collected using a Bruker D8 Venture diffractometer equipped with a PHOTON 100 detector. Data integration, including the application of a correction for oblique incidence, was performed with the software package SAINT.11 Absorption correction was performed by the computer program SADABS.12 The crystal structure was solved using the program Superflip,13 and the precise atom positions were subsequently refined using the program JANA200614 by assuming anisotropic thermal displacement factors for all atoms. All crystallographic parameters are summarized in Table 1.

Brentano geometry with Cu Kα radiation (λ = 1.540 60 Å). A fast scanning mode from 4° to 70° in 2Θ with a step size of 0.0131° was employed for the data collection. All observed reflections could be indexed on the basis of the crystal structure refined from the singlecrystal data. The full powder diffraction pattern was fitted using the program Jana2006. The refined data agree well with the data obtained from the single-crystal structure refinement. Chemical compositions were obtained by EDS using a Hitachi M3000 Table top scanning electron microscope and a JEOL JSB7000F (see Supporting Information). Field-cooled and zero-field cooled magnetic susceptibilities of a polycrystalline sample of CoMo2Sb2O10 (∼6 mg) were measured with an MPMS SQUID magnetometer (Quantum Design, 6325 Lusk Boulevard, San Diego) in the temperature range of 1.9−300 K. Heat capacities were determined on a powder sample (∼3 mg) intimately mixed with Apiezon N vacuum grease to improve the thermal contact employing a PPMS system (Quantum Design, 6325 Lusk Boulevard, San Diego) in the temperature range of 0.4−20 K. The heat capacities of the sample holder and the vacuum grease were determined in a preceding measurement cycle and subtracted from the total heat capacities. Thermal gravimetric analyses (TG) were performed using a TA Instruments Discovery equipment. The measurements were performed in N2 atmosphere with a heating rate of 5 °C/min up to 900 °C, starting with ∼4.5 mg of sample.

Table 1. Crystallographic Data for CoMo2Sb2O10 chemical formula formula weight/g mol−1 temperature/K crystal system space group a/Å b/Å c/Å β/deg V /Å3 ρ/g·cm−3 Z crystal size/mm3 radiation type wavelength/Å indices range

No. of reflections measured/unique observed [I > 3σ(I)] Rint (sin θ/λ)max/Å−1 RF/wRF [F > 3σ(F)] all reflections (%) goodness of fit (all)

CoMo2Sb2O10 654.30 293 monoclinic C2/c (No. 15) 27.392 (6) 5.7735 (12) 5.0015 (10) 94.214 (5) 788.8 (3) 5.509 4 0.50 × 0.1 × 0.08 Mo Kα 0.710 69 −36 ≤ h ≤ 35 −7 ≤ k ≤ 7 −6 ≤ l ≤ 6 6575/1012 791 0.083 0.67 3.67/4.36 1.67

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RESULTS AND DISCUSSIONS Crystal Structure. The new compound CoMo2Sb2O10 crystallizes in the monoclinic space group C2/c with unit cell parameters a = 27.392(6) Å, b = 5.774(1) Å, c = 5.002(1) Å, and β = 94.214(5)°. All the crystallographic parameters are summarized in Table 1. The asymmetric unit of CoMo2Sb2O10 is shown in Figure 1. The crystal structure contains one crystallographic independent atom of the heavy elements Sb3+, Mo6+, and Co2+. The oxidation state of these ions is supported by bond−valence−sum (BVS) calculations;15 see Table 2. The Co2+ cation has a distorted octahedral coordination with Co−O distances varying from 2.050(5) to 2.176(5) Å. The [CoO6] octahedra are edge sharing to each other to form zigzag [Co2O8]n chains that extend along [001]; see Figure 2a. The Mo6+ cation has also a distorted octahedral [MoO 6 ] coordination. The Mo−O distances vary from 1.702(6) to 2.209(5) Å. Similarly, the [MoO6] octahedral unit is edge

To check for phase purity of the polycrystalline samples subsequently used in the magnetic susceptibility and heat capacity studies X-ray powder diffraction patterns were collected employing a PANalytical X’Pert PRO X-ray powder diffractometer in Bragg−

Figure 1. Asymmetric unit and selected equivalents of CoMo2Sb2O10. There is one crystallographic independent atom of each for the Sb, Mo, and Co atoms. Co and Mo show distorted octahedral coordination, whereas Sb has a trigonal pyramidal coordination. Symmetry codes: (i) x, 1 − y, −0.5 + z; (ii) −x, 1 − y, 0.5 + z; (iii) −x, y, −1.5 + z; (iv) −x, y, −0.5 − z; (v) x, −y, −0.5 + z; (vi) −x, −y, −1 − z; (vii) x, y, −1 + z; (viii) x, 1 − y, 0.5 + z. B

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Inorganic Chemistry Table 2. Bond Valence Sum Calculations for CoMo2Sb2O10a atoms

BVS

Sb1 Mo1 Co1 O1 O2 O3 O4 O5

2.9 6.0 2.0 2.1 2.0 2.0 2.1 1.8

shared by another [MoO6] to form another zigzag chain [Mo2O8]n extending along [001]; see Figure 2b. The Sb3+ ions are bonded to three oxygen atoms at distances ranging from 1.957(6) to 2.016(6) Å to form a typical one-sided [SbO3] trigonal pyramid due to presence of the stereochemically active lone pair. The [SbO3] units are connected to each other by corner-sharing through Sb−O(4)−Sb bridges to form [SbO2]n chains along [001]; see Figure 2c. Each [Co2O8]n chain connects to four [Mo2O8]n chains by corner sharing through Co(1)−O(2)−Mo(1) bridges, and each [Mo2O8]n also connects to one [SbO2]n chain via corner sharing to form layers having the composition [CoMo2Sb2O10]n parallel to (011); see Figure 2d. The lone pairs of the Sb atoms

a

Employing R0(Sb−O) = 1.973 Å, R0(Mo−O) = 1.907 Å, R0(Co−O) = 1.692 Å, and b = 0.37 Å.

Figure 2. (a) Edge-sharing [CoO6] octahedra and (b) edge-sharing [MoO6] octahedra make up [Co2O8]n zigzag chains and [Mo2O8]n zigzag chains, respectively, along [001]. (c) Corner-sharing [SbO3] units build [SbO2]n chains that also extend along [001]. The dotted line denotes that each [SbO2]n chain connects to a similar unit in the next layer by weak interactions. (d) Overview of the layered crystal structure of CoMo2Sb2O10. [MoO6] blue, [CoO6] pink, Sb atoms gray, O atoms red. C

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The inverse magnetic susceptibilities, χmol(T) = Mmol(T,H)/ H, measured in magnetic fields of 0.5, 1, and 5 T versus magnetic field are displayed in Figure 4 (upper inset). Above

protrude from the layers, and only weak interactions connect the layers via long (2.821(6) Å) Sb···O(4) contacts complete a seesaw coordination around Sb3+. According to the empirical limit (2.76 Å) proposed by Brown16 this distance is too long to be considered to belong to the primary coordination sphere of Sb3+; see Figure 2c,d. The Schafarzikite CoSb2O4 structure (containing Sb3+) shows similar kinds of distorted [CoO6] octahedra and [SbO3] trigonal pyramidal units.5 The crystal structure of CoSb2O4 contains chains made of edge-sharing [CoO6] octahedra and chains of corner-sharing [SbO3] units that are connected to each other by corner sharing. The Co−O and Sb−O distances vary in the ranges of 2.0710−2.1513 and 1.9357−1.9903 Å, respectively. A trirutile structure type is found in CoSb2O6 (containing Sb5+), which is made of [CoO6] and [SbO6] octahedra that are connected by edge sharing.2 The Co−O and Sb−O distances are in the ranges of 2.0265−2.0683 and 1.9775−1.9916 Å, respectively. The distortion of the coordination polyhedra in CoSb2O6 is less pronounced compared to CoSb2O4. However, a significant distortion of the coordination polyhedra is seen in the layered compound Sb2MoO6 (containing Sb3+).17 Therein double layers of [SbO3] units alternate with [MoO6] slabs. The Mo−O and Sb−O distances vary in the ranges of 1.739−2.267 and 1.940−2.304 Å, respectively. These distances are slightly longer than those observed in the present compound CoMo2Sb2O10. Magnetic Properties. The sample used for the magnetic susceptibility and heat capacity experiments was characterized by X-ray powder diffraction; see Figure 3. The unit cell was

Figure 4. Magnetic susceptibility data of CoMo2Sb2O10. (upper inset) Data obtained at different magnetic fields as indicated. The central panel shows the data obtained after a Honda−Owen extrapolation to infinite magnetic field. The solid red line is a fit to a Curie−Weiss law with parameters given in the text. (lower inset) Magnetic susceptibility data collected with an applied magnetic field of 0.01 T. A slight splitting between zero-field and field-cooled susceptibility branch is seen at lowest temperatures. A comparison of these data with theory is shown in Figure 6.

∼50 K the magnetic susceptibilities show slight field dependence with the susceptibility decreasing toward higher fields indicating saturation of a minute ferromagnetic impurity with higher magnetic fields. A Honda−Owen extrapolation18 to H → ∞ was performed, and the extrapolated susceptibilities are given in the mainframe in Figure 4. The susceptibilities above 50 K follow an extended Curie−Weiss law according to χ (T ) =

C + χ0 T − ΘCW

(1)

where C is the Curie constant given by C = 0.125 051 cm 3K/mol(μeff /μB )2 =

NAg 2μB2 J (̃ J ̃ + 1) 3kB (2)

and NA is Avogadro’s constant, g is the g-factor, and μB is the Bohr magneton. J ̃ is the angular moment of a Co2+ ion arising from the electronic configuration (3d7), kB is the Boltzmann constant, and ΘCW is the Curie−Weiss temperature. The magnetic moment of Co2+ in a distorted octahedral ligand field comprise substantial orbital contribution. The ground state is characterized by an effective angular moment J ̃ = 1/2 with a gfactor composed of spin and angular contributions. Large gfactors and large effective momenta and uniaxial anisotropy is often encountered with Co2+ cations leading to a description in terms of the Ising model (see below).19,20 The temperatureindependent susceptibility term χ0 comprises diamagnetic contributions from the electrons in the closed shells and temperature-independent paramagnetic van Vleck susceptibility contributions, χ0 = χdia + χVV; the latter can be assumed to the order of 2 × 10−4 cm3/mol.20 The diamagnetic contribution from the closed electronic shell can be estimated from Pascal’s increments to Co2+: −12 × 10−6 cm3/mol, Mo6+: −7 × 10−6 cm3/mol, Sb3+: −17 × 10−6 cm3/mol, and O2−: −12 × 10−6 cm3/mol.21 With the appropriate number of atoms per formula unit the diamagnetic increments add to χdia = −180 × 10−6

Figure 3. Comparison of the measured and calculated X-ray powder diffraction pattern of CoMo2Sb2O10. The calculated pattern is based on the single-crystal X-ray determination of the crystal structure. The measured diffraction pattern is shown in green, and the calculated pattern is in red. The difference between observed and calculated patterns is represented by the black line below the red vertical bars marking the positions of the Bragg reflections used to calculate the pattern.

found to be a = 27.4077(7) Å, b = 5.7770(1) Å, c = 5.0059(2) Å, and β = 94.2579(34)°, which is very close to the unit cell obtained from the single-crystal X-ray data; see Table 1. The Rietveld profile refinement of the powder diffraction pattern resulted in a reliability factor Rp = 12.98 and a goodness of fit (GOF) = 1.14. D

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Inorganic Chemistry cm3/mol, which almost balances the van Vleck contribution. Consequently, χ0 ≈ 0 was assumed in the refinements. The inverse susceptibility was fitted to (eq 1) resulting in an effective momentum, μeff = 5.26(2) μB, in good agreement with typical effective momenta found for Co2+.20 The fitted effective momentum indicates an average g-factor according to (eq 2) of g ≈ 6. The Curie−Weiss temperature was refined to ΘCW = −13.2(4) K, indicating predominant anti-ferromagnetic exchange interaction. However, no clear indication for long-range anti-ferromagnetic ordering was observed down to ∼0.4 K. The magnetic susceptibility rather exhibits a broad maximum typical for short-range anti-ferromagnetic ordering effects. Figure 4 (lower inset) shows the low-temperature magnetic susceptibility, and Figure 5 shows the total heat capacity of

resulted in an anti-ferromagnetic intrachain exchange of Jintra = −7.8(2) K and F ≈ 0.82, indicating the experimental data to be somewhat lower than theoretically expected. This slight discrepancy might be due to the rather small mass of the sample (∼3 mg) and possible losses in the preparation when mixing the polycrystalline sample with the Apiezon grease. A description of the low-temperature magnetic susceptibility of a polycrystalline sample in terms of an Ising chain model must include the susceptibilities obtained with field applied parallel χ∥ and perpendicular χ⊥ to the z-direction as calculated by Fisher (S = 1/2) or Suzuki and Katsura for general spin values,23,24 here cited after Carlin.20 χ (T ) =

NA g 2μB2 2Jintra

(Jintra /2kBT )

1 cosh2(Jintra /2kBT )

(6)

and χ⊥ (T ) =

⎞ NA g ⊥2 μB2 ⎛ 1 ⎜tanh(J /2kBT ) + (J /2kBT ) ⎟⎟ intra intra 4Jintra ⎜⎝ cosh2(Jintra /2kBT ) ⎠

(7)

To fit the susceptibility of the polycrystalline sample we have averaged the parallel χ∥, and the perpendicular susceptibilities, χ⊥ according to 1 2 χpwd (T ) = χ (T ) + χ⊥ (T ) (8) 3 3 Reasonable fits are obtained with g∥ ≈ g⊥ ≈ 5.5 and an intrachain exchange constant close to that obtained from the fits of the heat capacity data. Figure 6 displays the susceptibility

Figure 5. Total heat capacity (□) of CoMo2Sb2O10. The solid blue line represents the lattice contribution to the heat capacity constructed as described in the text. The circles show the magnetic contribution to the heat capacity, and the solid red line shows the result of a fit of the heat capacity of a spin S = 1/2 Ising chain to the data with parameters discussed in the text.

CoMo2Sb2O10 including lattice and magnetic contributions. The heat capacity is characterized by a broad feature centered at ∼3.4 K and an increase toward higher temperatures; the latter being due to lattice contributions Clat(T) to the heat capacity. These have been estimated by fitting a polynomial according to 5

C lat(T ) ≈

∑ a2i+ 1T 2i+ 1 i=1

Figure 6. Low-temperature magnetic susceptibility of CoMo2Sb2O10 taken with a magnetic field of 0.01 T together with fits to the S = 1/2 linear chain Ising (red solid line) and Heisenberg (green solid line) models with parameters quoted in the text.

(3)

where the coefficients a2i+1 are adjusted to approximate the total heat capacity toward high temperatures. The lattice contribution is represented by the red solid line in Figure 5. The magnetic contribution to the heat capacity, Cmag(T), was obtained by subtracting the lattice contribution. It can be wellapproximated by the heat capacity of a spin =1/2 Ising chain22 defined by the Hamiltonian H = − 2Jintra ∑ Jĩz Jĩz+ 1 i

data collected with an external field of 0.01 T together with fits to eqs 6−8. We also show a fit to the heat capacity of a spin-1/2 Heisenberg chain with anti-ferromagnetic nearest-neighbor intrachain spin exchange interaction.25 Both fits result in intrachain exchange constants, Jintra = (−7.6) ÷ (−7.8) K, in agreement with the value obtained from the heat capacity fits. The g-factor is somewhat smaller than the value obtained from the analysis of the high-temperature susceptibility data. In summary, CoMo2Sb2O10 does not exhibit long-range magnetic ordering down to ∼0.4 K. Our heat capacity and magnetic susceptibility data rather indicate CoMo2Sb2O10 to behave as a magnetic linear chain system consistent with the crystal structure. Co2+ appears to behave as an Ising system implying large uniaxial anisotropy of the Co moments. The g-

(4)

which is given by ⎛ J ⎞2 1 Cmag(T ) = F × R ⎜ intra ⎟ 2 ⎝ 2kBT ⎠ cosh (Jintra /2kBT )

(5)

where Jintra is the intrachain exchange, F is a fudge factor (∼1), and R is the molar gas constant. The fit of the experimental data E

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Figure 7. Weight loss on heating CoMo2Sb2O10 under N2 atmosphere.

factor obtained from the magnetic susceptibility data is typical for Co2+ cations in distorted octahedral environment. Thermal Stability. CoMo2Sb2O10 decomposes in one step in the temperature range of 530−830 °C; see Figure 7. The suggested decomposition route is

for long-range magnetic ordering is not found down to temperatures of ∼0.4 K.

CoMo2Sb2 O10 (s) → CoMoO4 (s) + Sb2 O3(g)+ MoO3(g)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02031. Listings of EDS spot analysis. (PDF) X-ray crystallographic information. (CIF)

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suggesting that 33.4 wt % remain as residue in good agreement with measured value of ∼35 wt %. The small amount of sample used for the thermogravimetric analysis did not allow for a characterization of the decomposition product by X-ray powder diffraction. Further decomposition of CoMoO4 into CoO and MoO3 may occur at higher temperatures according to CoMoO4 (s) → CoO(s) + MoO3(g)

ASSOCIATED CONTENT

S Supporting Information *

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46-8-162169. Fax: +46-8-152187.

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Notes

The authors declare no competing financial interest. Supplementary crystallographic material has been sent to Fachinformationzentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany (fax +49−7247−808− 666; e-mail: crysdata@fiz-karlsruhe.de), and can be obtained on quoting the deposit numbers CSD-431504 for the compound CoMo2Sb2O10.

which, however, was not observed in the temperature range covered by the TG experiment.

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CONCLUSIONS Single crystals of the new quaternary oxide CoMo2Sb2O10 were grown by hydrothermal synthesis. The new compound crystallizes in the monoclinic space group C2/c with unit cell parameters a = 27.392(6) Å, b = 5.774(1) Å, c = 5.002(1) Å, β = 94.214(5)°, and Z = 4. It consists of charge-neutral [CoMo2Sb2O10]n layers, where each layer is made of cornersharing chains of [Co2O8]n, [Mo2O8]n, and [SbO2]n. The layers connect to each other through weak Sb···O interactions. The compound is stable below 530 °C. The magnetic susceptibility follows the Curie−Weiss law above 50 K with a short-range anti-ferromagnetic transition below ca. 5 K. Co2+ behave as a spin S = 1/2 Ising system implying large uniaxial anisotropy of the Co components, which is consistent with the heat capacity and magnetic susceptibility found for CoMo2Sb2O10. Indication

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ACKNOWLEDGMENTS The work has in part been performed with financial support from Stiftelsen Olle Engkvist Byggmästare and the Swedish Research Council. Expert experimental assistance by E. Brücher and G. Siegle is gratefully acknowledged.

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REFERENCES

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