Magnetic properties of multi-metal Prussian Blue analogue Co0.75Ni0.75[Fe(CN)6]·6.8H2O

ARTICLE IN PRESS

Physica B 362 (2005) 278–285 www.elsevier.com/locate/physb

Magnetic properties of multi-metal Prussian Blue analogue Co0.75Ni0.75[Fe(CN)6]
6.8H2O Amit Kumar, S.M. Yusuf Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 30 September 2004; received in revised form 21 February 2005; accepted 21 February 2005

Abstract We have studied magnetic properties of the ternary metal Prussian Blue analogue Co0.75Ni0.75[Fe(CN)6]
6.8H2O. Room temperature Mo¨ssbauer and IR spectra give the evidence of two FeIII sites in the compound. It undergoes a paramagnetic to ferromagnetic phase transition at 15.9 K. Field dependent magnetization data correspond to a ferromagnetic ordering of the spin only moments of FeIII, CoII and NiII ions. Approximately 87.45% FeIII ions are in low spin (S ¼ 12) and 12.55% FeIII ions are in high spin (S ¼ 52) state. On the other hand, 100% CoII ions are in low spin (S ¼ 12) and 100% NiII ions are in (t2g)6(eg)2 spin state (S ¼ 1). The observed very large coercive field at lower temperatures is an indication of the presence of magnetic hardness in the compound. Non-saturation of magnetization at 1.5 K even under 100 kOe field and a bifurcation in the field cooled and zero field cooled magnetization curves indicate the presence of some spin glass type component in the compound. r 2005 Elsevier B.V. All rights reserved. PACS: 75.50.Xx; 61.10.i; 75.60.d Keywords: Molecular magnets- Prussian Blue analogues; X-ray diffraction; DC magnetization

1. Introduction Prussian Blue analogues [1–5] having general formula Ax[B(CN)6]y
zH2O, where A and B are transition metal ions, have been studied extenCorresponding author. Tel.: +91 22 25595383; fax: +91 22 25505151. E-mail address: [email protected] (S.M. Yusuf).

sively in last decade in search of three-dimensional molecular magnetic materials with high transition temperature [6–8]. Very recently, Ohkoshi et al. have demonstrated a host of novel functionalities e.g. mixed ferro-ferrimagnet [9,10], exhibition of two compensation temperatures [11,12], photoinduced magnetic pole reversal [13], etc. in multimetal Prussian Blue analogues by incorporating various types of transition metal cations as spin centres. Their cubic structure [14] allows an easy

0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.02.024

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2. Experimental Polycrystalline Co0.75Ni0.75[Fe(CN)6]
xH2O was prepared by co-precipitation method where a mixture of 75 ml 0.1 M Co(NO3)2 and 75 ml 0.1 M NiSO4 aqueous solutions was first reacted with 75 ml 0.1 M K3Fe(CN)6 aqueous solution. Dark brown precipitate, so obtained, was filtered, washed many times with demineralized water and finally dried under IR lamp for 30 min. X-ray powder diffraction of the prepared polycrystalline sample was recorded at room temperature in 2y range 10–751 using a Cu-Ka radiation. Room temperature Mo¨ssbauer spectrum was recorded using a constant acceleration derive unit coupled with multichannel analyser operated in time mode. The IR spectrum was recorded by incorporating the sample in a KBr pellet on a Bomen MB-102 FT–IR spectrometer. DC magnetization measurements were performed down to 1.5 K and magnetic field up to 100 kOe using an Oxford Instruments make Vibrating Sample Magnetometer. Temperature dependent magnetization measurements were carried out in field-cooled (FC) and zero field-cooled (ZFC) conditions down to 2 K. In ZFC condition, first, the sample was cooled in the absence of magnetic field from room temperature down to 2 K, and then magnetization was measured as a function of temperature under the application of magnetic field in the heating cycle. In FC condition, the sample was cooled down to 2 K in the presence of same magnetic field as used as a measuring field in the ZFC case and magnetization was measured (keeping the field on)

as a function of temperature in the heating cycle. Hysteresis curves were recorded at several temperatures over 7100 kOe field.

3. Results and discussion Mo¨ssbauer spectrum of Co0.75Ni0.75[Fe(CN)6]
xH2O at room temperature is shown in Fig. 1. The observed spectrum has been fitted with two doublets of relative intensities (RI) 87.45% and 12.55% and chemical shift (CS) of 0.168 and 0.365 mm/sec, respectively. Quadrupole splitting was 0.432 and 0.625 mm/sec for the two doublets, respectively. We know that increasing electron density at the Fe57 nucleus produces a negative CS. N atom is more electronegative than C atom and as a result of that electron density is more at N atom compared to C atom. When FeIII ion is bonded to C atom, as in FeIII–CN–NiII/CoII chain, electron density tends to flow over C atoms and the shielding of FeIII ion by 3d electrons is less effective. This results in strong attraction of electron cloud by nuclear charge and hence increasing nuclear electronic charge density. In case of FeIII–NC–NiII/CoII chain, FeIII ion is X106 1.47 1.44 Counts (arb. units)

modification of their spin structures and consequently their magnetic properties, which are otherwise difficult to observe in conventional metal oxide magnets due to various kinds of magnetic interactions involved [9]. Magnetic properties like transition temperature, saturation magnetization, coercive field, etc. of these multi-metal Prussian Blue analogues can be tuned by changing the composition of different transition metal cations. In this context, we have prepared Co0.75Ni0.75[Fe(CN)6]
xH2O and the magnetic properties of this compound have been studied.

279

1.41 1.38 T = 297 K

1.35 1.32 -3

-2

-1

0

Velocity (mm

1

2

3

s-1)

Fig. 1. Room temperature Mo¨ssbauer spectrum of Co0.75Ni0.75[Fe(CN)6]
6.8H2O. Open circles show the observed data and the thick solid line is the least square fitted curve. Two thin solid lines represent the components of the least square fitted curve.

ARTICLE IN PRESS A. Kumar, S.M. Yusuf / Physica B 362 (2005) 278–285 80

Transmittance (arb. unit)

strongly shielded by 3d electrons and, thus, resulting in lower nuclear electronic charge density. The observed doublet with CS 0.168 mm/sec and RI 87.45%, therefore, correspond to low spin FeIII (S ¼ 12) state in FeIII–CN–(CoII/NiII) configuration and other doublet with CS 0.365 mm/sec and RI 12.55% corresponds to high spin FeIII (S ¼ 52) state in FeIII–NC–(CoII/NiII) configuration [15,16]. The occurrence of FeIII–NC–(CoII/NiII) structure may be due to the heating of the sample under IR lamp during the sample preparation and similar results have already been reported for Cu1.5[Fe(CN)6]
6H2O [15] and K0.2Co1.4[Fe (CN)6]
7H2O [16]. It is, therefore, evident from the Mo¨ssbauer spectrum that FeIII ion, connected to carbon end of the cyanide group, is in low spin (S ¼ 12) state and FeIII ion, connected to nitrogen end of the cyanide group, is in high spin (S ¼ 52) state. For NiII (3d8) ion, there is only one possible electronic configuration (t2g)6(eg)2 with S ¼ 1: For CoII (3d7) ion, on the other hand, there may be two configurations: one with high spin (t2g)5(eg)2, S ¼ 32 and another with low spin (t2g)6(eg)1, S ¼ 12: The actual spin state of cobalt depends upon the strength of ligand field around the cobalt ion. Strong ligand field favors low spin configuration and weak ligand field favors high spin configuration [14]. We will return to this discussion later on. To confirm the two types of atomic chains in this compound, we have recorded the IR absorption spectrum of the compound in the 3000–500 cm1 range. The recorded spectrum is shown in Fig. 2. The observed IR spectrum has two absorption frequencies in this range with wave numbers n1 ¼ 2165 cm1 and n2 ¼ 2101 cm1 ; respectively. A broad absorption is also observed at 1608 cm1. Compounds having CN functional group are easily identified by their CN stretching frequencies in 2200–2000 cm1 range [17]. The CN stretching frequencies of these hexacyanides depend, mainly, on electronegativity, oxidation number and the coordination number of the metal ion [17]. When metal ion is coordinated with CN ions, CN ions act as a s donor by donating electrons to the metal. Thus, higher the electronegativity of the metal ion, higher will be the s donation that, in turn, results in increase in the CN stretching frequency. We also know that the

70 ν1 = 2165 cm-1 60

ν2 = 2101 cm-1 ν1

50

ν3

ν3 = 1608 cm-1

ν2 40 3000

2500

2000

1500

1000

500

Wavenumber (cm-1)

Fig. 2. Fourier transform infrared (FTIR) spectrum of Co0.75Ni0.75[Fe(CN)6]
6.8H2O at room temperature.

1600

Counts (arb. units)

280

OBS CAL DIFF PEAK

1200

800

400

0 15

30 45 60 Scattering Angle (deg.)

75

Fig. 3. Room temperature X-ray diffraction pattern of Co0.75Ni0.75[Fe(CN)6]
6.8H2O. Open circles show the observed data, solid line represents the Rietveld fitted pattern and vertical short lines indicate the position of Bragg peaks. The difference pattern between the observed and calculated pattern is also shown at the bottom of the fitted curve.

electronegativity of FeIII is higher than that of CoII/NiII. Therefore, the observed absorption frequencies with wave numbers n1 ¼ 2165 cm1 and n2 ¼ 2101 cm1 can be assigned to CN stretching frequencies of the chains of FeIII–CN– (CoII/NiII) and (CoII/NiII)–CN–FeIII, respectively. The broad peak observed at 1608 cm1 is due to crystalline water [15,16]. Fig. 3 shows the X-ray diffraction pattern for Co0.75Ni0.75[Fe(CN)6]
xH2O at room temperature.

ARTICLE IN PRESS A. Kumar, S.M. Yusuf / Physica B 362 (2005) 278–285

The observed diffraction pattern confirms that prepared polycrystalline sample is in single-phase. The crystal structure was refined with Fm3 m space group using the Rietveld refinement technique. Two sites for FeIII and CoII/NiII ions, consistent with Mo¨ssbauer and IR studies, have been incorporated in the structural model. Occupancies of CoII and NiII ions in two sites have been refined separately. Here, we would like to mention that X-ray diffraction technique is not very sensitive to determine the ratio of CoII and NiII ions at a given site because of very close atomic numbers (hence almost same scattering intensity). Water molecules are identified by the positions of oxygen atoms. Same values of isotropic temperature factors (B) for cobalt, nickel and iron have been taken. Similarly same values of B have been taken for carbon, nitrogen and coordinated oxygen. The results of the refinement are given in Table 1. The lattice constant, a ¼ 10.206(3) A˚, is slightly lower than that for the parent compound, Ni1.5[Fe(CN)6]
xH2O {a ¼ 10.229(5)} [18]. Refinement yielded approximately 6.8 water molecules per formula unit. The structure contains FeIII–CN–(CoII/NiII)–NC–FeIII (87.45%) and FeIII–NC–(CoII/NiII)–CN–FeIII (12.55%) types of straight chains along the cube edges. When FeIII ion is connected to nitrogen end of a cyanide ion, carbon end of the cyanide ion is connected to

281

either CoII or NiII ion. On the other hand, when FeIII ion is connected to carbon end of a cyanide ion, nitrogen end of the cyanide ion is connected to either CoII or NiII ion. It is evident from the refined structure that there is a deficiency of [Fe(CN)6] radicals in the unit cell of Co0.75Ni0.75[Fe(CN)6]
6.8H2O. Approximately 3.4 water molecules per formula unit, located at (0.244(4), 0, 0), are coordinated to either Ni or Co ions at empty [Fe(CN)6] site. Remaining 3.4 water molecules are uncoordinated and reside at interstitial sites (14; 14; 14) and (0.291(3), 0.291(3), 0.291(3)) [19]. Strong ligand field of the cyanide group can induce low spin and high spin structures in these 3d ions. Fig. 4(a) shows the field cooled (FC) magnetization (M) vs. temperature (T) curve under 1 kOe field (H) for Co0.75Ni0.75[Fe(CN)6]
6.8H2O. Magnetic transition temperature ( ¼ 15.9 K) of the compound was estimated from minima of dM/dT vs. T curve, which corresponds to the steepest rise of magnetization with decreasing temperature. The magnetic ordering temperature ( ¼ 15.9 K) found for the present Co substituted compound is lower than that for the parent compound, Ni1.5[Fe (CN)6]
xH2O (TC ¼ 23.6 K) [18]. After deriving DC susceptibility (w ¼ M=H) using Fig. 4(a), wT vs. T curve is plotted in Fig. 4(b). The curve rises slowly till the temperature is lowered from 65 to 25 K after that it shoots up sharply, showing

Table 1 Structural parameters of Co0.75Ni0.75[Fe(CN)6]
6.8H2O Atom

x

Co1 Co2 Ni1 Ni2 Fe1 Fe2 C N O1 O2 O3

0 1 2

Rp ¼ 8.75%

z

Occupancy

B (A˚2)

0

0

1 2

1 2

0.67(1) 0.08(1) 0.66(1) 0.09(1) 0.125 0.875 6 6 3.40(7) 2.24(9) 1.16(13)

0.6(1) 0.6(1) 0.6(1) 0.6(1) 0.6(1) 0.6(1) 2.9(2) 2.9(2) 2.9(2) 14.0(9) 14.0(9)

y

0

0

0

1 2

1 2

1 2

0

0

0

1 2

1 2

1 2

0.310(3) 0.184(2) 0.244(4)

0 0 0

0 0 0

1 4

1 4

1 4

0.291(3)

0.291(3)

0.291(3)

Rwp ¼ 12.1%

Rexp ¼ 10.42%

Space group ¼ Fm3 m, a ¼ 10.206(3) A˚.

ARTICLE IN PRESS A. Kumar, S.M. Yusuf / Physica B 362 (2005) 278–285

282

Curie–Weiss law

Magnetization (emu g-1)

18

w¼ 12

6

(a)

0

χdc.T (emu K g-1 Oe-1)

0.16

0.12

0.08

0.04 (b) 0.00

-1 (10-3 emu g-1 Oe-1)-1 χdc

8

6

4

2 (c) 0 0

14

28 42 Temperature (K)

56

70

Fig. 4. (a) Field cooled magnetization curve at 1 kOe field. (b) wT vs. T curve in the temperature range 2–65 K. (c) Curie–Weiss fit of w1 vs. T curve in the temperature range 25–65 K.

maximum around 12 K and decreases as temperature is decreased further. This kind of behaviour is a characteristic of a ferromagnet [20]. For a ferrimagnetic compound wT vs. T curve undergoes to a minima before rising around magnetic ordering temperature [20]. Fig. 4(c) shows temperature dependence of w1 in the temperature range 2–65 K. Straight line is the fit to the curve in the temperature range 25–65 K according to

C . T Y

Fitting yielded the Curie constant C ¼ 2.5970.02 emu.K/mole/Oe and paramagnetic curie temperature Y ¼ 17:15  0:03 K: The positive value of Y indicates the presence of positive exchange correlations in this compound. The effective paramagnetic moment meff ¼ ð3CkB = N A m2B Þ1=2 ð8CÞ1=2 is found to be 4.55(1)mB per formula unit. Considering 87.45% FeIII ions in low spin (S ¼ 12) and 12.55% FeIII ions in high spin (S ¼ 52) as obtained from the Mo¨ssbauer study and assuming CoII ions in low spin (S ¼ 12), the theoretically expected spin only value may p of meff be calculated using the formula ½Sng2 SðS þ 1Þ mB ; where n is number of atoms per formula unit and gð¼ 2Þ is gyromagnetic ratio. Therefore, p meff ¼ ½0:1255 4 52 72 þ 0:8745 4 12 32 þ 0:75 4 ð12 32 þ 1 2Þ mB ¼ 3:91mB per formula unit. Here for NiII ions, S ¼ 1 has been used. Slight difference between the calculated and theoretical meff may arise due to weak spin orbit coupling for CoII ions as observed in KCo[Fe (CN)6] [21]. In order to understand further regarding the nature of magnetic ordering, a plot of M2 vs. H/M (Arrot plot) is shown in Fig. 5. The curves depart from straight line and bend towards the origin with decreasing value of H/M. The extrapolation of the straight line fit of M2 vs. H/M curve for higher values of H/M gives positive intercept at M2 axis with spontaneous magnetizations of 30.89, 26.88, 19.48 and 7.65 emu/g at 1.5, 5, 10 and 15 K, respectively. The presence of spontaneous magnetization at low temperatures together with the observed positive value of Y and behaviour of wT vs. T curve, therefore, indicate a ferromagnetic type of ordering in Co0.75Ni0.75[Fe(CN)6]
6.8H2O below 15.9 K (TC). Hysteresis curve at 1.5 K, over 7100 kOe field is shown in Fig. 6. Large hysteresis with a coercive field (HC) of 2297 Oe and remanence of 15.99 emu/g ( ¼ 1.21mB/f.u.), is evident. The observed value of HC is comparable with the HC value for the parent compound, Ni1.5[Fe(CN)6]
xH2O (HC ¼ 2.5 kOe at 4.4 K). But this value of HC is much

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1600 5K 10 K

1200

800

15 K

M2 (emu g-1)2

400

0 1600 1.5 K 1200 800 400 0 0

400

800

1200

1600

H/M (Oe g emu-1 ) Fig. 5. M2 vs. H/M curves at 1.5, 5, 10 and 15 K derived from virgin M vs. H data. Solid lines are the extrapolated lines of the linear fit to the higher H/M data.

higher than that for other members of the Prussian Blue analogues family. For example, the observed value of HC for CsNi[Cr(CN)6]
2H2O [6] is 71 Oe (at 3 K), for V[Cr(CN)6]0.86
2.8H2O [8] is 25 Oe (at 10 K) and for Cu1.5[Fe(CN)6]
6H2O is 240 Oe, (at 4 K) [15]. Temperature dependence of HC is shown in bottom inset of Fig. 6. HC is found to increase with decreasing temperature. This increase in HC with decreasing temperature is an indication of increasing magnetic hardness at lower temperatures. The observed value of magnetization at 1.5 K under 100 kOe field is 3.40mB/f.u. The theoretically expected value of saturation magnetization (mS ) for ferromagnetic ordering of the spin only moments of FeIII, CoII and NiII ions ¼ mS ¼ SgSn ¼ ½0:1255 2 52 þ 0:8745 2 12 þ 0:75 2 ð12 þ 1Þ ¼ 3:752mB =f:u: Here, we have considered that 87.45% FeIII ions are in low spin state (S ¼ 12), 12.55% FeIII ions are in high spin state (S ¼ 52), 100% CoII ions are in low spin state

283

(S ¼ 12) and 100% NiII ions are in (t2g)6(eg)2 spin state (S ¼ 1), as used for the calculation of meff : We recall that at 1.5 K under 100 kOe field, the observed magnetization was far from saturation. The field dependent magnetization study, therefore, concludes the ferromagnetic magnetic ordering in Co0.75Ni0.75[Fe(CN)6]
6.8H2O. On the other hand, if we assume high spin state for CoII (S ¼ 32) ion, the theoretically expected value of saturation magnetization (mS ) for ferromagnetic ordering of the spin only moments of FeIII, CoII and NiII ions would be ¼ mS ¼ SgSn ¼ ½0:1255 2 52 þ 0:8745 2 12 þ 0:75 2 ð32 þ 1Þ ¼ 5:252mB =f:u: This value is quite larger than observed (3.40 mB/f.u.). Therefore, our DC magnetization data support low spin state for CoII ion. FC and zero field cooled (ZFC) magnetization curves at various fields are shown in Fig. 7. A clear branching between the FC and ZFC magnetization curves is evident up to 2 kOe field. The variation of branching temperature, Tirr (temperature below which FC and ZFC magnetization curves separates out) and Tmax (temperature at which a peak in ZFC magnetization is found) with magnetic field are shown in the upper inset of Fig. 7. Both Tirr and Tmax are found to decrease linearly with increasing magnetic field. This kind of branching between FC and ZFC magnetization curves has been found for KCo[Fe(CN)6] [21] and KxCo[Fe(CN)6]y
zH2O [22] compounds where it was interpreted due to spin glass and cluster spin glass ordering, respectively. For the present compound, the observed branching between the FC and ZFC magnetization curves and non saturating nature of M upto 100 kOe field indicate some signature of spin glass behaviour in this ‘‘ferromagnetic’’ compound.

4. Summary and conclusions We have studied the magnetic properties of Co0.75Ni0.75[Fe(CN)6]
6.8H2O. It possesses a face centred cubic structure with space group Fm3 m. Room temperature IR, X-ray and Mo¨ssbauer studies show the presence of Fe–CN–Co/Ni–NC– Fe (87.45%) and Fe–NC–Co/Ni–CN–Fe (12.55%) linear chains in the compound. Approximately

ARTICLE IN PRESS A. Kumar, S.M. Yusuf / Physica B 362 (2005) 278–285

284

3.79

2.27

Magnetization per f.u. (µB)

1.14

1.89

1.5 K

0.00 -1.14 -2.27

-9

-5

0

5

9 Coercive Field (kOe)

0.00

-1.89

2.3 1.5 0.8 0.0 0

-3.79 -100

-50

0

5

10 15 Temperature (K)

50

20

100

Magnetic Field (kOe) Fig. 6. Hysteresis curve at 1.5 K. Top inset shows the expanded portion of the hysteresis curve. Bottom inset shows the temperature dependence of coercive field.

12 Temperature (K)

14

9

Magnetization (emu g-1)

6

Tirr Tmax Linear fit

12 10 8 6

3

0

500

200 Oe 0 20

1000

1500

2000

Field (Oe)

20

1.5 kOe

2 kOe

15

15

FC

10

ZFC

10

5 1 kOe

87.45% FeIII ions are in low spin (S ¼ 12) and 12.55% FeIII ions are in high spin (S ¼ 52) state. It undergoes a paramagnetic to ferromagnetic transition at 15.9 K. The higher value of observed effective paramagnetic moment (meff ¼ 4.55(1)mB/ f.u.) compared to the theoretically expected spin only value (meff ¼ 3.91mB/f.u.) possibly indicates a weak spin orbit coupling in this compound. Results of DC magnetization study are consistent with the low spin state (S ¼ 12) of CoII ions and (t2g)6 (eg)2 spin state (S ¼ 12) of NiII ions. Very high coercive field is found at lower temperatures. Non saturation of magnetization even in 100 kOe field and a branching between FC and ZFC magnetization curves indicate some signature of spin glass type of behaviour in this ‘‘ferromagnetic’’ compound.

0

5

0

10

20

30

40

50

Acknowledgements

500 Oe 0 0

10

20 30 Temperature (K)

40

50

Fig. 7. Temperature dependence of field cooled and zero field cooled magnetization curves at 0.2, 0.5, 1.0, 1.5 and 2.0 kOe fields. Top inset shows the field dependence of Tirr and Tmax and solid lines represent linear fit to the curves.

Authors acknowledge the help of Mr. Sher Singh in recording the Mo¨ssbauer spectrum. Authors are grateful to Dr. M. Ramanadham, Dr. J.V. Yakhmi, Dr. S.K. Kulshreshtha and Dr. V.C. Sahni for their encouragement and support in this work.

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