- Email: [email protected]
Structural and magnetic properties of Me2[Fe(CN)6] compounds, where Me are 3d transition metals
EISEVIER
Journal of Magnetism
and Magnetic Materials 138 (1994) 281-286
Structural and magnetic properties of Me,[Fe( CN) 6] compounds, where Me are 3d transition metals S. Juszczyk a7*Y1, C. Johansson a, M. Hanson a, A. Ratuszna b, G. Malecki ’ aDepartment of Physics, Chalmers University of Technology and University of GGteborg, S-41296 Giiteborg, Sweden b Institute of Physics, University of Silesia, PL-40007 Katowice, Poland ’ Institute of Chemistry, Universib of Silesia, PL-40007 Katowice, Poland Received 21 March 1994
Abstract We have studied the structural and magnetic properties of the Me,[Fe(CN),] family. The X-ray measurements were carried out at room temperature and the magnetic studies in the temperature range 4.2-260 K using static magnetic fields up to 12 T. From the X-ray analysis results the compounds crystallize in the face-centered cubic structure with the space group Fa3m. Both the Fe and Me ions are coordinated octahedrally by six carbon atoms and six nitrogen atoms, respectively. The Fe ions are in a strong crystalline field and the Me ions in an intermediate one of cubic symmetry. From the magnetization versus temperature curves we obtain the critical temperatures, the Curie-Weiss temperatures, the Curie constants, as well as the effective moments in the paramagnetic state. From the field dependence of the magnetization we determine the saturation magnetization and the high-field susceptibility. The data suggest that only the divalent Me cations are magnetic and coupled ferromagnetically between each other. The magnetic properties of the compounds were analyzed in the framework of the mean field theory.
1. Introduction Insulating magnetic systems of low lattice dimensionality, i.e. one-dimensional (1D) or linear chain systems and two-dimensional (2D) or layer systems, have been studied extensively for at least three decades [1,2]. The majority of the interest concerns the 1D systems, since a number of special features can occur in such systems, e.g. the spin-Peierls
* Corresponding author. Present address: Institute of Physics, University of Silesia, PL-40007 Katowice, Poland. Fax: (+ 48) 32 588 431; e-mail: [email protected]. ’ On leave from the University of Silesia, Department of Applied Geology, PL-41200 Sosnowiec, Poland. 0304-8853/94/$07.00 0 1994 Elsevier Science B.V. AR rights reserved SSDI 0304-8853(94)00436-U
transition, the alternating spin chain, solitons, bound magnons, the Haldane conjecture, etc. [3]. 2D systems are also interesting from several points of view, e.g. to obtaining examples of unusual models such as 2D-XY (which may exhibit a Kosterlitz-Thouless magnetic phase transition), to study 2D magnetic phase diagrams, and to elucidate the magnetostructural correlations within and between layers. Similar problems, connected with 2D Heisenberg antiferromagnetism, can be found in the parent compounds for the layered high-T, cuprates or in the BEDT-type of organic superconductors. Among others, DeFotis and co-workers [4-61 contributed to the explanation of magnetic properties of 2D-XY and 2D-Ising magnets. They studied a family
282
S. Juszczyk er al. /Journal
of Magneiism and Magnetic Material.? 138 (1994) 281-286
of compounds with the general formula Me(SCN& (ROH),, in which Me was a divalent transition metal ion and in which various alcohols could occur. These systems were magnetically quasi-two-dimensional (Q2D), due to a bridging network of SCN--ions which were connected to metal ions in layers and hence provided the main superexchange couplings. Our interest centres on the possibility of obtaining new Q2D magnets or even an attempt to build a 3D covalent bonding network between the magnetic positions, with different degrees of spin anisotropy. The properties of the compounds depend mainly on the metal ions, and on the cyanide CN- superexchange bridges that presumably should mediate intraplanar (or interplanar) magnetic coupling in these systems. Finally, Gadet and co-workers [7] have put forward the interesting proposal of building a 3D network through mild chemistry methods. They used molecular precursors to obtain covalent bonding between the magnetic ions. They introduced one of the simplest and most convenient molecular building blocks in the shape of the hexa-cyanometallate family [Me(CN),]“-. Thus, as a result of our interest in the preparation of lattices of magnetic ions with specific dimensionality requirements, we have prepared several bimetallic complex systems in the general family of compounds Me:+[Fe’+(CN);], where Me is a divalent transition metal ion (Mn2+, Ni’+ and Cu2’ ). These complex compounds with different 3d metals exhibit different beautiful colours; the compound with Me = Cu is bronze, with Me = Ni grey-green and with Me = Mn blue. In this paper we describe the preliminary magnetometric studies of these cyanides. After a discussion of the experimental results, we postulate their magnetic dimensionality.
2. Experimental Powder samples of the Me,[Fe(CN),] compounds, with Me = Mn, Ni and Cu, were prepared (by one of the authors (G.M.)) as follows. The solution of MeCl, .2H,O (0.1 mol/dm3) was added to an aqueous solution of K,[Fe(CN),], which led to the solvation of individual ions or molecules that were used to prepare the molecular solid. Then the mixture was heated under a reflux condenser (l-3
days). The solid was filtered and washed with water until no Cl- ions were detected in the filtrate. The solid was then dried under atmospheric pressure at the temperature 90-100°C. The X-ray measurements of the lattice parameters as well as the positions of atoms, in the compounds under study, were carried out in the 28 range of lo”-70” using a D.5000 (0-20) Siemens powder diffractometer with CuK, radiation. In order to minimize statistical errors the K, lines of the highangle reflections have been changed with a step size of 0.02” and the computing continuous scan was equal to 10 s. In determining the crystal structure the positions of Fe, Me, C and N atoms were determined by using the refinement of the powder diffraction profile lines by the Rietveld method. The function minimized was R = C( 1F, 1- 1F, 1)/C 1F,, I; the final results for the Me?[Fe(CN),] compounds with Me = Cu, Ni, and Mn gave R parameter values of 3%, 6.9% and 9%, respectively. The static magnetization was measured with an Oxford Instruments vibrating sample magnetometer in magnetic fields up to 12 T [81. Each sample was pressed into a cylinder with diameter 3 mm and height 4 mm. The field-cooled magnetization, M,,, was measured during cooling from the temperature T = 260 K in a constant field of 0.2 T to T = 4.2 K. The cooling rate was 0.05 Ks~‘. At the lowest temperature the magnetization was measured in fields from 0 up to 12 T and back down to 0.
3. Results and discussion Powder X-ray analysis indicates that the compounds Me,[Fe(CN),] are isostructural; the water molecules are absent. They crystallize in the facecentred cubic structure with the space group F43m (T:) and a unit cell contains four formula units. It can be described as a cubic close-packed array of two different anions: of carbon (C) and nitrogen (N), with the metal ions located in interstitial positions in this array. The positions of the metal ions are as follows: cation (Fe) 4a (43m) 0, 0, 0; cation (Mel) 4b (43m) l/2, l/2, l/2;
S. Juszczyk et al. /Journal
of Magnetism and Magnetic Materials 138 (1994) 281-286
cation (Me2) 4c (43m) l/4, l/4, l/4 (note: four Me2 type 4d positions are unoccupied); anion (C) 24f (mm) X, 0, 0; 0, x, 0; 0, 0, X; ?, 0, 0; 0, Z, 0; 0, 0, x; anion (N) 24f (mm) x, 0, 0; 0, X, 0; 0, 0, x; X, 0, 0; 0, Z, 0; 0, 0, X (the values of x for N and C are given in Table 1); with translations for a face-centred lattice; + (0, 0, 0; 0, l/2, l/2; l/2, 0, l/2; l/2, l/2, 0). Along the cube edge we have the following sequence of atoms: Fe-C=N-Me-N=C-Fe. The values of the lattice parameters a, the positions of C and N atoms (denoted previously by X) and the distances between cations and anions for the studied compounds are listed in Table 1. The interlayer d(Fe-Me2) separation is lower than the intralayer one (cf. Table 1). But in the structure we have four vacancies for the internal Me2 positions. This means that some superexchange paths Fe-CEN-Me2 are disconnected. Both Fe and Me ions are coordinated octahedrally by six carbon atoms and six nitrogen atoms, respectively. The FeC, and MeN, octahedra are symmetric (not distorted) but the MeN, octahedra are always larger than the FeC, ones. For Me = Cu the CuN, octahedron is nearly twice as large as the FeC, one. But for the other Me ions this ratio decreases and for Me = Mn the MnN, octahedron is only a little larger than the FeC, one. This means that the Fe2+ ions should be affected by the stronger crystalline field of an octahedral (cubic) symmetry than by the Me2+ ions. Fig. 1 (a, b, c) show the field-cooled magnetization, M,,, versus temperature, T, for the successive members of the family of complex compounds Me,[Fe(CN),], for Me = Mn, Ni and Cu, respectively. At higher temperatures the magnetization decreases with temperature according to a Curie-Weiss law. Because it turned out that the critical temperatures are a little below T = 4.2 K, our experimental
Table 1 Lattice parameters Compound Mn,[Fe(CN), Ni,[Fe(CN), Cu,[Fe(CN),
1 1 1
and distances
283
limit, we decided to determine their values from an asymptotic critical law of the form [9] x= T(T/T,
- 1))‘.
(1)
From the fitting of formula (1) to the experimental values of x( = p,,M/B), in the low-temperature range, we obtained the critical temperatures with an error of 4%. The results are given in Table 3. In each insert of Fig. 1 (a, b, c) the reciprocal susceptibility, x- ‘, is drawn as a function of temperature T, for Me = Mn, Ni and Cu, respectively. The Curie-Weiss law is fulfilled above 50 K for all investigated samples. From the fitting of x-l = (T - 0,)/C we obtained the values of the Curie constant C and the Curie-Weiss temperature, Oc,. These data are listed in Table 2. Taking into account the following relation [ 10,111: C = NA~0 p&3k,M’,
(2)
where NA is Avogadro’s number, p. the permeability of vacuum, p the density of the material, k, Boltzmann’s constant and M’ the molecular weight, we can calculate the so-called effective magnetic moment in a paramagnetic state, p,. These results are given in Table 2. The cubic crystalline field strongly affects the 3d shell of electrons of the iron-group ions. This is due partly to the large radius of the 3d shell and partly to the lack of any outer electronic shells to screen the 3d shell. The orbital moment is quenched by the crystal field, leading to M, = 0. Thus, the magnetic properties of these ions are governed only by their spins. In general, the multiplicity of the ground state of an ion will depend on the strength of the field set up by the ligands. In some cases there are strong or weak ligand fields and sometimes intermediate situations. These directly influence the resulting spin value of an ion. From the spectrochemical series results the CNions usually create a strong ligand field and the
d between anions and cations in the compounds
under study (parameters
are expressed
in &
a
d(Fe-C)
d(Me-N)
d(Fe-N)
d(C-N)
d(Fe-Mel)
d(Fe-Me21
10.061 + 0.004 10.079 f 0.005 10.014 + 0.003
1.816 1.585 1.105
1.964 1.814 1.982
3.066 3.226 3.024
1.255 1.641 1.922
5.030 5.039 5.007
4.326 4.334 4.306
284
S. Juszcqk
et al. /Journal
of Magnetism and Magnetic Materials
138 (1994) 281-286
After taking into account the fact that the [Fe’+(CN),14complex is diamagnetic we assume that the magnetic properties of the investigated bimetallic complex compounds are governed only by the Me’+ cations. Hence, we can calculated the effective value of the LandC factor, g, from the relation for the effective Bohr magneton number [2,11]:
[Fe”(CN),]“complex does not exhibit a magnetic moment. This means that the Fe2+ ions are low spin dh, S = 0 [12]. In the octahedral complexes we assume the d’sp” hybridization of a central ion. Because of symmetry, we take it that among the 3d orbitals of iron in the creation of the hybridized orbitals, type g take partly the 3d: and 3dt_, orbitals, which are the eg orbitals. From the result of an overlapping of the six hybridized orbitals of the central ion with the six ligand orbitals, we obtain six bonding localized orbitals type (T and six antibonding ones, type a*. In the [Fe2+(CN),14complex the molecular orbitals, together with the t,, orbitals, contain six valence electrons of the central ion and 12 electrons from ligands. Hence in the low-spin [Fe(CN),14- complex all the bonding orbitals (a(CN))12 are occupied. From the energetical point of view this means that the further six paired electrons should be placed in three orbitals of tza. The energy difference between the t,, and cr* orbitals (the crystal field energy) is A = 33800 cm - ’ and the bonding energy between the electrons with antiparallel coupled spins is II = 18000 cm-’ (1000 cm-’ = 12 kJ/mol) [13,14]. The stabilization energy of this octahedral complex is equal to 2.44 - 3 17 = 27120 cm-‘. Because the MeN, octahedra are larger than the FeC, for every Me cation, the Me ions can be affected by the intermediate ligand field. This means that the Me = Mn2+ (3d5), NiZf (3d*) and Cu2+ (3d9) ions should exhibit spins S = 3/2, 1 and l/2, respectively. In the case of Mn2+ three unpaired electrons will occupy an antibonding (T* orbital. In the case of NiZf two unpaired electrons and in Cu2+ ion one electron will occupy the U* orbital.
P, = g[S,,(S,,,
+
111 ‘Y
(3)
where Seff is the effective spin value of the different Me cations in the formula unit, defined as: Seff = 2s Mr, because in the formula unit we have two magnetic ions. The resulting g factor values are listed in Table 3; they are (typically) within the range of literature values for the Me2+ ions in dilute salts of the 3d metals [15]. This means that the superexchange interactions between the Me*+ ions via cyanide ions are weak because of the large distance between them. In our fee structure the interlayer Me-Me distance, as a function o,f the cube edge a, is equal to d, = 0.7~ (> 5 A) and the interlayer one is d, = 0.43a. Assuming a parallel coupling between the Me spins and taking into account the calculated values of the g factors, we can estimate the full saturation moment, pu,, in Bohr magnetons for the formula unit of the Me?[Fe(CN),] compounds, at T = 0 K. We calculated according to the formula: P, = nMegMcSMe. The estimation yields CL, values of 1.6~,, 2.6~~ and 4.95~, for the compounds with Me = Cu, Ni and Mn, respectively. These values agree well with the experimental data for T = 4.2 K listed in Table 2. The small values of 8,. and Tc indicate the presence of weak ferromagnetic exchange interactions. Taking each Me-CN-Me linkage as the su-
Table 2 The set of experimental data from magnetic studies for the Me2[Fe(CN),] compounds CM, is the saturation magnetization at T = 4.2 K, F\ the maximal number of Bohr magnetons per formula unit at T= 4.2 K, xht the high-field susceptibility at T = 4.2 K. @I,., the paramagnetic Curie-Weiss temperature, C the Curie constant, and P,,, the effective magnetic moment) Compound
M, (A m’/kg)
cr,(PEJ
Xhf (IO-“)
@,.,
Mn2[Fe(CN),l Ni2[Fe(CN),,l
81 41
4.83 2.41
5.6 4.0
24
I .50
5.0
1.8 3.0 1.1
Cu,[Fe(CN),
1
(K)
C (K)
Pm (I%)
0.144 0.040 0.012
5.7 3.2 2.3
S. Juszczyk et al. /Journal 14
,
_
/
I
I
I
2.0
a)
12
of Magnetism and Magnetic Materials I38 (1994) 281-286
I
~ 1.5 0 t
10
-I
/p
c - 0.5 L
1. B = 0.2 T
2 0
0 kLY!IIJ 0 100
200
Table 3 The set of calculated parameters for the Me,[Fe(CN),] compounds Tc is the Curie temperature, /.L~the saturation magnetization at T = 0 K, g the Land& factor, (Y the molecular field constant, and zJ/ k, the interlayer effective exchange constant between Me cations) Compound
Tc (K)
I-% (~a)
g
Mn,[Fe(CN),] Ni,[Fe(CN),]
3.1 2.9 3.2
4.95 2.60 1.60
1.65 1.30 1.62
I
Cu,[Fe(CN), 1 50
150
100
&‘/kg)
(“K: kB
0.014 0.964 0.200
0.22 0.43 2.42
300
TIKI
\
0
285
200
250
300
I
I
T[Kl
5
,
I
1 pi
4
I
I 7
b)
6 li::
4 &..a
c’s
Tc = [2&,&r
;l
3;
-
2
1
21,
0
1 _\
0
0.8
100 200 TIKI
150 TlKl
100
I
1.0 I,
1;
0
h
200
250
,
I
;
g
1
:I
2-
0
300
I
3-l
c)
S
.::.z
200
100
-I
1
50
100
150
200
(4)
300 100
-y--y---.,_... ......._...__/_ 0
+ I) J/3ka,
where Tc is the Curie temperature. Using the experimental values of T, from the fitting to an asymptotic critical law (Eq. cl)), we calculated the effective exchange constants Jz/k,. The data are listed in Table 3. The small values of the exchange constants indicate that the interlayer exchange is weak. This is a result of a structural disorder because of the Me vacancies. Thus we can suppose that we do not have an ideal 3D magnetic structure but only a Q3D one. Taking into account the relation Oc, = pCa, we have also calculated the effective molecular constant, CX.The results are also given in Table 3. Fig. 2 shows the specific (mass) magnetization, A4 versus applied field, B, at the lowest measuring
TtKI
B = 0.2 T
0
300
B = 0.2-r
50
0
account the effective spin. In the framework of the mean field theory for ferromagnets we have [16]
250
TlKl
I
2
4
I
,
I
8
10
12
80
300
Fig. 1. Field-cooled magnetization versus temperature for polycrystalline Me,[Fe(CN),] compounds: (a) Me = Mn; (b) Me = Ni; (c) Me = Cu. The insert of each figure shows the inverse molar susceptibility versus temperature.
/
rM $
60
Z
40 20
perexchange and interlayer path, we have z = 4 interlayer effective interactions between neighbouring spins of the Me cations. For the simple estimation of the superexchange interactions, we also take into
0
6
BITI Fig. 2. The mass magnetization versus applied field for Me,[Fe(CN),] compounds: (a) Me = Mn; (b) Me = Ni: (c) Me = cu.
S. Juszczyk et al. /Journal
286
of Magnetism and Magnetic Materials 138 (1994) 281-286
temperatures for the compounds under study. Only the magnetization curve for Me = Cu does not exhibit full saturation at the measuring temperature. From the high-field range, where M depends linearly on B, we calculated the high-field susceptibility, defined as the dimensionless quantity Xhf
=
/-%I pdM/dB.
the diluted salts of the 3d metals. account the respective values of LandC factors we calculated the saturation moment per formula obtain good agreement between results and calculations we intend surements.
After taking into the spins and the values of the full unit. In order to the experimental to use EPR mea-
(5)
The data of MS or CM,,,) and ,yhf are listed in Table 2. Well above Tc the susceptibility data, x(T), can be analyzed including the effects of axial and rhombic crystal field distortions, represented by: D[Sl 5/4] and E[Sz - $1, for Mn2+ (S = 3/2), D[(Sz 2/3] and E[Sz - s,:], for Nil (S = 1) as well as for cu2+ (S = l/2), with the exchange interactions incorporated in a mean field approximation. But, it turns out [6,9] that the same plot can fit a few sets of the exchange and crystal field parameters. It can be omitted in studies of single crystals or enhancing the data with EPR results. For two reasons, first that we examine polycrystalline materials and second that our preliminary asssumptions about the strong as well as intermediate crystalline fields effective on the cations, should be tested, we have decided to make the EPR measurements.
4. Conclusions We have studied the structural and magnetic properties of the Me;‘[Fe+(CN);] family of compounds, where Me = Mn, Ni and Cu. From the X-ray analysis the compounds crystallize in the fee cubic structure with the space group F43m CT:). Both the Fe and Me ions are coordinated octahedrally by six carbon atoms and six nitrogen atoms, respectively. The MeN, octahedra are larger than the FeC, ones, so that a crystalline field should affect more strongly the Fe ions than the Me ions. From the magnetization versus temperature curves we obtained the critical temperatures, Z’c, the Curie-Weiss temperatures, %v Tthe Curie constants, C, as well as the effective magnetic moments, pm in the paramagnetic range. Because the Fe2+ (d6) ions are low-spin, S = 0, only the Me ions contribute to the magnetic properties of the studied bimetallic compounds. The calculated effective values of the Land& factors are typical for
Acknowledgements This work was supported mainly by the Swedish Institute and in some part by the Polish Committee for Scientific Research.
References [I] L.J. de Jongh and A.R. Miedema, Adv. Phys. 23 (1974) 1. [2] R.L. Carlin, Magnetochemistry (Springer-Verlag, Berlin, 19861. [3] See, e.g., Organic and Inorganic Low-Dimensional Crystalline Materials, vol. 168 of NATO ASI, Ser. B: Phyics, eds P. Delhaes and M. Drillon (Plenum. New York, 1987). [4] G.C. DeFotis, E.D. Remy and C.W. Scherer, Phys. Rev. B 41 (1990) 9074. [5] G.C. DeFotis, R.V. Chamberlain and W.R.A. Jarris, J. Magn. Magn. Mater. 104-107 (1992) 187. [6] G.C. DeFotis, K.D. Dell, D.J. Krovitch and W.W. Brubalcer, J. Appl. Phys. 73 (19931 5386. [7] V. Fadet, I. Mallah, 1. Castro, M. Verdraguer and P. Veillet, J. Am. Chem. Sot. 114 (19921 9213. k31C. Johansson and M. Hanson, IEEE Trans. Magn. 30 (1994) 1064. [91 G.C. DeFotis, E.W. Harlem, E.D. Remy and K.D. Dell, J, Appl. Phys. 69 (1991) 6004. HOI E.A. Boudreaux and L.N. Mulay, Theory and Applications of Molecular Paramagnetism, (Wiley Interscience. New York, 1976) Ch. 1. illI E. Kijnig and G. Kiinig, Magnetic Properties of Coordination and Organometallic Transition Metal Compounds, Suppl. 4 to vol. 2 (1971, 1972). Landolt-Bornstein, new series, eds. K.H. Hellwege and A.M. Hellwege, vol. II/11 (Springer, Berlin, 1981); sub. vol. a: Magnetic Susceptibilities. 1121 A. Bielahski, General and Inorganic Chemistry (Polish Scientific Publishers, Warsaw, 1973). Introduction a la [I31 M. Van Meerssche and J. Feneau-DuPont, Cristallographie et a la Chimie Structurale (Oyez, Leuven, 1976), p, 144. D41 R. McWeeny, Coulson’s Valence (Oxford University Press, 1979). [151 H.P. Myers (cdl. Introduction to Solid State Physics (Taylor and Francis, London, 1990). of Physics, (Springer, New York, [161 F. Keffer, Encyclopaedia 1966) vol. XVIII/2, Sec. B.
Journal of Magnetism
and Magnetic Materials 138 (1994) 281-286
Structural and magnetic properties of Me,[Fe( CN) 6] compounds, where Me are 3d transition metals S. Juszczyk a7*Y1, C. Johansson a, M. Hanson a, A. Ratuszna b, G. Malecki ’ aDepartment of Physics, Chalmers University of Technology and University of GGteborg, S-41296 Giiteborg, Sweden b Institute of Physics, University of Silesia, PL-40007 Katowice, Poland ’ Institute of Chemistry, Universib of Silesia, PL-40007 Katowice, Poland Received 21 March 1994
Abstract We have studied the structural and magnetic properties of the Me,[Fe(CN),] family. The X-ray measurements were carried out at room temperature and the magnetic studies in the temperature range 4.2-260 K using static magnetic fields up to 12 T. From the X-ray analysis results the compounds crystallize in the face-centered cubic structure with the space group Fa3m. Both the Fe and Me ions are coordinated octahedrally by six carbon atoms and six nitrogen atoms, respectively. The Fe ions are in a strong crystalline field and the Me ions in an intermediate one of cubic symmetry. From the magnetization versus temperature curves we obtain the critical temperatures, the Curie-Weiss temperatures, the Curie constants, as well as the effective moments in the paramagnetic state. From the field dependence of the magnetization we determine the saturation magnetization and the high-field susceptibility. The data suggest that only the divalent Me cations are magnetic and coupled ferromagnetically between each other. The magnetic properties of the compounds were analyzed in the framework of the mean field theory.
1. Introduction Insulating magnetic systems of low lattice dimensionality, i.e. one-dimensional (1D) or linear chain systems and two-dimensional (2D) or layer systems, have been studied extensively for at least three decades [1,2]. The majority of the interest concerns the 1D systems, since a number of special features can occur in such systems, e.g. the spin-Peierls
* Corresponding author. Present address: Institute of Physics, University of Silesia, PL-40007 Katowice, Poland. Fax: (+ 48) 32 588 431; e-mail: [email protected]. ’ On leave from the University of Silesia, Department of Applied Geology, PL-41200 Sosnowiec, Poland. 0304-8853/94/$07.00 0 1994 Elsevier Science B.V. AR rights reserved SSDI 0304-8853(94)00436-U
transition, the alternating spin chain, solitons, bound magnons, the Haldane conjecture, etc. [3]. 2D systems are also interesting from several points of view, e.g. to obtaining examples of unusual models such as 2D-XY (which may exhibit a Kosterlitz-Thouless magnetic phase transition), to study 2D magnetic phase diagrams, and to elucidate the magnetostructural correlations within and between layers. Similar problems, connected with 2D Heisenberg antiferromagnetism, can be found in the parent compounds for the layered high-T, cuprates or in the BEDT-type of organic superconductors. Among others, DeFotis and co-workers [4-61 contributed to the explanation of magnetic properties of 2D-XY and 2D-Ising magnets. They studied a family
282
S. Juszczyk er al. /Journal
of Magneiism and Magnetic Material.? 138 (1994) 281-286
of compounds with the general formula Me(SCN& (ROH),, in which Me was a divalent transition metal ion and in which various alcohols could occur. These systems were magnetically quasi-two-dimensional (Q2D), due to a bridging network of SCN--ions which were connected to metal ions in layers and hence provided the main superexchange couplings. Our interest centres on the possibility of obtaining new Q2D magnets or even an attempt to build a 3D covalent bonding network between the magnetic positions, with different degrees of spin anisotropy. The properties of the compounds depend mainly on the metal ions, and on the cyanide CN- superexchange bridges that presumably should mediate intraplanar (or interplanar) magnetic coupling in these systems. Finally, Gadet and co-workers [7] have put forward the interesting proposal of building a 3D network through mild chemistry methods. They used molecular precursors to obtain covalent bonding between the magnetic ions. They introduced one of the simplest and most convenient molecular building blocks in the shape of the hexa-cyanometallate family [Me(CN),]“-. Thus, as a result of our interest in the preparation of lattices of magnetic ions with specific dimensionality requirements, we have prepared several bimetallic complex systems in the general family of compounds Me:+[Fe’+(CN);], where Me is a divalent transition metal ion (Mn2+, Ni’+ and Cu2’ ). These complex compounds with different 3d metals exhibit different beautiful colours; the compound with Me = Cu is bronze, with Me = Ni grey-green and with Me = Mn blue. In this paper we describe the preliminary magnetometric studies of these cyanides. After a discussion of the experimental results, we postulate their magnetic dimensionality.
2. Experimental Powder samples of the Me,[Fe(CN),] compounds, with Me = Mn, Ni and Cu, were prepared (by one of the authors (G.M.)) as follows. The solution of MeCl, .2H,O (0.1 mol/dm3) was added to an aqueous solution of K,[Fe(CN),], which led to the solvation of individual ions or molecules that were used to prepare the molecular solid. Then the mixture was heated under a reflux condenser (l-3
days). The solid was filtered and washed with water until no Cl- ions were detected in the filtrate. The solid was then dried under atmospheric pressure at the temperature 90-100°C. The X-ray measurements of the lattice parameters as well as the positions of atoms, in the compounds under study, were carried out in the 28 range of lo”-70” using a D.5000 (0-20) Siemens powder diffractometer with CuK, radiation. In order to minimize statistical errors the K, lines of the highangle reflections have been changed with a step size of 0.02” and the computing continuous scan was equal to 10 s. In determining the crystal structure the positions of Fe, Me, C and N atoms were determined by using the refinement of the powder diffraction profile lines by the Rietveld method. The function minimized was R = C( 1F, 1- 1F, 1)/C 1F,, I; the final results for the Me?[Fe(CN),] compounds with Me = Cu, Ni, and Mn gave R parameter values of 3%, 6.9% and 9%, respectively. The static magnetization was measured with an Oxford Instruments vibrating sample magnetometer in magnetic fields up to 12 T [81. Each sample was pressed into a cylinder with diameter 3 mm and height 4 mm. The field-cooled magnetization, M,,, was measured during cooling from the temperature T = 260 K in a constant field of 0.2 T to T = 4.2 K. The cooling rate was 0.05 Ks~‘. At the lowest temperature the magnetization was measured in fields from 0 up to 12 T and back down to 0.
3. Results and discussion Powder X-ray analysis indicates that the compounds Me,[Fe(CN),] are isostructural; the water molecules are absent. They crystallize in the facecentred cubic structure with the space group F43m (T:) and a unit cell contains four formula units. It can be described as a cubic close-packed array of two different anions: of carbon (C) and nitrogen (N), with the metal ions located in interstitial positions in this array. The positions of the metal ions are as follows: cation (Fe) 4a (43m) 0, 0, 0; cation (Mel) 4b (43m) l/2, l/2, l/2;
S. Juszczyk et al. /Journal
of Magnetism and Magnetic Materials 138 (1994) 281-286
cation (Me2) 4c (43m) l/4, l/4, l/4 (note: four Me2 type 4d positions are unoccupied); anion (C) 24f (mm) X, 0, 0; 0, x, 0; 0, 0, X; ?, 0, 0; 0, Z, 0; 0, 0, x; anion (N) 24f (mm) x, 0, 0; 0, X, 0; 0, 0, x; X, 0, 0; 0, Z, 0; 0, 0, X (the values of x for N and C are given in Table 1); with translations for a face-centred lattice; + (0, 0, 0; 0, l/2, l/2; l/2, 0, l/2; l/2, l/2, 0). Along the cube edge we have the following sequence of atoms: Fe-C=N-Me-N=C-Fe. The values of the lattice parameters a, the positions of C and N atoms (denoted previously by X) and the distances between cations and anions for the studied compounds are listed in Table 1. The interlayer d(Fe-Me2) separation is lower than the intralayer one (cf. Table 1). But in the structure we have four vacancies for the internal Me2 positions. This means that some superexchange paths Fe-CEN-Me2 are disconnected. Both Fe and Me ions are coordinated octahedrally by six carbon atoms and six nitrogen atoms, respectively. The FeC, and MeN, octahedra are symmetric (not distorted) but the MeN, octahedra are always larger than the FeC, ones. For Me = Cu the CuN, octahedron is nearly twice as large as the FeC, one. But for the other Me ions this ratio decreases and for Me = Mn the MnN, octahedron is only a little larger than the FeC, one. This means that the Fe2+ ions should be affected by the stronger crystalline field of an octahedral (cubic) symmetry than by the Me2+ ions. Fig. 1 (a, b, c) show the field-cooled magnetization, M,,, versus temperature, T, for the successive members of the family of complex compounds Me,[Fe(CN),], for Me = Mn, Ni and Cu, respectively. At higher temperatures the magnetization decreases with temperature according to a Curie-Weiss law. Because it turned out that the critical temperatures are a little below T = 4.2 K, our experimental
Table 1 Lattice parameters Compound Mn,[Fe(CN), Ni,[Fe(CN), Cu,[Fe(CN),
1 1 1
and distances
283
limit, we decided to determine their values from an asymptotic critical law of the form [9] x= T(T/T,
- 1))‘.
(1)
From the fitting of formula (1) to the experimental values of x( = p,,M/B), in the low-temperature range, we obtained the critical temperatures with an error of 4%. The results are given in Table 3. In each insert of Fig. 1 (a, b, c) the reciprocal susceptibility, x- ‘, is drawn as a function of temperature T, for Me = Mn, Ni and Cu, respectively. The Curie-Weiss law is fulfilled above 50 K for all investigated samples. From the fitting of x-l = (T - 0,)/C we obtained the values of the Curie constant C and the Curie-Weiss temperature, Oc,. These data are listed in Table 2. Taking into account the following relation [ 10,111: C = NA~0 p&3k,M’,
(2)
where NA is Avogadro’s number, p. the permeability of vacuum, p the density of the material, k, Boltzmann’s constant and M’ the molecular weight, we can calculate the so-called effective magnetic moment in a paramagnetic state, p,. These results are given in Table 2. The cubic crystalline field strongly affects the 3d shell of electrons of the iron-group ions. This is due partly to the large radius of the 3d shell and partly to the lack of any outer electronic shells to screen the 3d shell. The orbital moment is quenched by the crystal field, leading to M, = 0. Thus, the magnetic properties of these ions are governed only by their spins. In general, the multiplicity of the ground state of an ion will depend on the strength of the field set up by the ligands. In some cases there are strong or weak ligand fields and sometimes intermediate situations. These directly influence the resulting spin value of an ion. From the spectrochemical series results the CNions usually create a strong ligand field and the
d between anions and cations in the compounds
under study (parameters
are expressed
in &
a
d(Fe-C)
d(Me-N)
d(Fe-N)
d(C-N)
d(Fe-Mel)
d(Fe-Me21
10.061 + 0.004 10.079 f 0.005 10.014 + 0.003
1.816 1.585 1.105
1.964 1.814 1.982
3.066 3.226 3.024
1.255 1.641 1.922
5.030 5.039 5.007
4.326 4.334 4.306
284
S. Juszcqk
et al. /Journal
of Magnetism and Magnetic Materials
138 (1994) 281-286
After taking into account the fact that the [Fe’+(CN),14complex is diamagnetic we assume that the magnetic properties of the investigated bimetallic complex compounds are governed only by the Me’+ cations. Hence, we can calculated the effective value of the LandC factor, g, from the relation for the effective Bohr magneton number [2,11]:
[Fe”(CN),]“complex does not exhibit a magnetic moment. This means that the Fe2+ ions are low spin dh, S = 0 [12]. In the octahedral complexes we assume the d’sp” hybridization of a central ion. Because of symmetry, we take it that among the 3d orbitals of iron in the creation of the hybridized orbitals, type g take partly the 3d: and 3dt_, orbitals, which are the eg orbitals. From the result of an overlapping of the six hybridized orbitals of the central ion with the six ligand orbitals, we obtain six bonding localized orbitals type (T and six antibonding ones, type a*. In the [Fe2+(CN),14complex the molecular orbitals, together with the t,, orbitals, contain six valence electrons of the central ion and 12 electrons from ligands. Hence in the low-spin [Fe(CN),14- complex all the bonding orbitals (a(CN))12 are occupied. From the energetical point of view this means that the further six paired electrons should be placed in three orbitals of tza. The energy difference between the t,, and cr* orbitals (the crystal field energy) is A = 33800 cm - ’ and the bonding energy between the electrons with antiparallel coupled spins is II = 18000 cm-’ (1000 cm-’ = 12 kJ/mol) [13,14]. The stabilization energy of this octahedral complex is equal to 2.44 - 3 17 = 27120 cm-‘. Because the MeN, octahedra are larger than the FeC, for every Me cation, the Me ions can be affected by the intermediate ligand field. This means that the Me = Mn2+ (3d5), NiZf (3d*) and Cu2+ (3d9) ions should exhibit spins S = 3/2, 1 and l/2, respectively. In the case of Mn2+ three unpaired electrons will occupy an antibonding (T* orbital. In the case of NiZf two unpaired electrons and in Cu2+ ion one electron will occupy the U* orbital.
P, = g[S,,(S,,,
+
111 ‘Y
(3)
where Seff is the effective spin value of the different Me cations in the formula unit, defined as: Seff = 2s Mr, because in the formula unit we have two magnetic ions. The resulting g factor values are listed in Table 3; they are (typically) within the range of literature values for the Me2+ ions in dilute salts of the 3d metals [15]. This means that the superexchange interactions between the Me*+ ions via cyanide ions are weak because of the large distance between them. In our fee structure the interlayer Me-Me distance, as a function o,f the cube edge a, is equal to d, = 0.7~ (> 5 A) and the interlayer one is d, = 0.43a. Assuming a parallel coupling between the Me spins and taking into account the calculated values of the g factors, we can estimate the full saturation moment, pu,, in Bohr magnetons for the formula unit of the Me?[Fe(CN),] compounds, at T = 0 K. We calculated according to the formula: P, = nMegMcSMe. The estimation yields CL, values of 1.6~,, 2.6~~ and 4.95~, for the compounds with Me = Cu, Ni and Mn, respectively. These values agree well with the experimental data for T = 4.2 K listed in Table 2. The small values of 8,. and Tc indicate the presence of weak ferromagnetic exchange interactions. Taking each Me-CN-Me linkage as the su-
Table 2 The set of experimental data from magnetic studies for the Me2[Fe(CN),] compounds CM, is the saturation magnetization at T = 4.2 K, F\ the maximal number of Bohr magnetons per formula unit at T= 4.2 K, xht the high-field susceptibility at T = 4.2 K. @I,., the paramagnetic Curie-Weiss temperature, C the Curie constant, and P,,, the effective magnetic moment) Compound
M, (A m’/kg)
cr,(PEJ
Xhf (IO-“)
@,.,
Mn2[Fe(CN),l Ni2[Fe(CN),,l
81 41
4.83 2.41
5.6 4.0
24
I .50
5.0
1.8 3.0 1.1
Cu,[Fe(CN),
1
(K)
C (K)
Pm (I%)
0.144 0.040 0.012
5.7 3.2 2.3
S. Juszczyk et al. /Journal 14
,
_
/
I
I
I
2.0
a)
12
of Magnetism and Magnetic Materials I38 (1994) 281-286
I
~ 1.5 0 t
10
-I
/p
c - 0.5 L
1. B = 0.2 T
2 0
0 kLY!IIJ 0 100
200
Table 3 The set of calculated parameters for the Me,[Fe(CN),] compounds Tc is the Curie temperature, /.L~the saturation magnetization at T = 0 K, g the Land& factor, (Y the molecular field constant, and zJ/ k, the interlayer effective exchange constant between Me cations) Compound
Tc (K)
I-% (~a)
g
Mn,[Fe(CN),] Ni,[Fe(CN),]
3.1 2.9 3.2
4.95 2.60 1.60
1.65 1.30 1.62
I
Cu,[Fe(CN), 1 50
150
100
&‘/kg)
(“K: kB
0.014 0.964 0.200
0.22 0.43 2.42
300
TIKI
\
0
285
200
250
300
I
I
T[Kl
5
,
I
1 pi
4
I
I 7
b)
6 li::
4 &..a
c’s
Tc = [2&,&r
;l
3;
-
2
1
21,
0
1 _\
0
0.8
100 200 TIKI
150 TlKl
100
I
1.0 I,
1;
0
h
200
250
,
I
;
g
1
:I
2-
0
300
I
3-l
c)
S
.::.z
200
100
-I
1
50
100
150
200
(4)
300 100
-y--y---.,_... ......._...__/_ 0
+ I) J/3ka,
where Tc is the Curie temperature. Using the experimental values of T, from the fitting to an asymptotic critical law (Eq. cl)), we calculated the effective exchange constants Jz/k,. The data are listed in Table 3. The small values of the exchange constants indicate that the interlayer exchange is weak. This is a result of a structural disorder because of the Me vacancies. Thus we can suppose that we do not have an ideal 3D magnetic structure but only a Q3D one. Taking into account the relation Oc, = pCa, we have also calculated the effective molecular constant, CX.The results are also given in Table 3. Fig. 2 shows the specific (mass) magnetization, A4 versus applied field, B, at the lowest measuring
TtKI
B = 0.2 T
0
300
B = 0.2-r
50
0
account the effective spin. In the framework of the mean field theory for ferromagnets we have [16]
250
TlKl
I
2
4
I
,
I
8
10
12
80
300
Fig. 1. Field-cooled magnetization versus temperature for polycrystalline Me,[Fe(CN),] compounds: (a) Me = Mn; (b) Me = Ni; (c) Me = Cu. The insert of each figure shows the inverse molar susceptibility versus temperature.
/
rM $
60
Z
40 20
perexchange and interlayer path, we have z = 4 interlayer effective interactions between neighbouring spins of the Me cations. For the simple estimation of the superexchange interactions, we also take into
0
6
BITI Fig. 2. The mass magnetization versus applied field for Me,[Fe(CN),] compounds: (a) Me = Mn; (b) Me = Ni: (c) Me = cu.
S. Juszczyk et al. /Journal
286
of Magnetism and Magnetic Materials 138 (1994) 281-286
temperatures for the compounds under study. Only the magnetization curve for Me = Cu does not exhibit full saturation at the measuring temperature. From the high-field range, where M depends linearly on B, we calculated the high-field susceptibility, defined as the dimensionless quantity Xhf
=
/-%I pdM/dB.
the diluted salts of the 3d metals. account the respective values of LandC factors we calculated the saturation moment per formula obtain good agreement between results and calculations we intend surements.
After taking into the spins and the values of the full unit. In order to the experimental to use EPR mea-
(5)
The data of MS or CM,,,) and ,yhf are listed in Table 2. Well above Tc the susceptibility data, x(T), can be analyzed including the effects of axial and rhombic crystal field distortions, represented by: D[Sl 5/4] and E[Sz - $1, for Mn2+ (S = 3/2), D[(Sz 2/3] and E[Sz - s,:], for Nil (S = 1) as well as for cu2+ (S = l/2), with the exchange interactions incorporated in a mean field approximation. But, it turns out [6,9] that the same plot can fit a few sets of the exchange and crystal field parameters. It can be omitted in studies of single crystals or enhancing the data with EPR results. For two reasons, first that we examine polycrystalline materials and second that our preliminary asssumptions about the strong as well as intermediate crystalline fields effective on the cations, should be tested, we have decided to make the EPR measurements.
4. Conclusions We have studied the structural and magnetic properties of the Me;‘[Fe+(CN);] family of compounds, where Me = Mn, Ni and Cu. From the X-ray analysis the compounds crystallize in the fee cubic structure with the space group F43m CT:). Both the Fe and Me ions are coordinated octahedrally by six carbon atoms and six nitrogen atoms, respectively. The MeN, octahedra are larger than the FeC, ones, so that a crystalline field should affect more strongly the Fe ions than the Me ions. From the magnetization versus temperature curves we obtained the critical temperatures, Z’c, the Curie-Weiss temperatures, %v Tthe Curie constants, C, as well as the effective magnetic moments, pm in the paramagnetic range. Because the Fe2+ (d6) ions are low-spin, S = 0, only the Me ions contribute to the magnetic properties of the studied bimetallic compounds. The calculated effective values of the Land& factors are typical for
Acknowledgements This work was supported mainly by the Swedish Institute and in some part by the Polish Committee for Scientific Research.
References [I] L.J. de Jongh and A.R. Miedema, Adv. Phys. 23 (1974) 1. [2] R.L. Carlin, Magnetochemistry (Springer-Verlag, Berlin, 19861. [3] See, e.g., Organic and Inorganic Low-Dimensional Crystalline Materials, vol. 168 of NATO ASI, Ser. B: Phyics, eds P. Delhaes and M. Drillon (Plenum. New York, 1987). [4] G.C. DeFotis, E.D. Remy and C.W. Scherer, Phys. Rev. B 41 (1990) 9074. [5] G.C. DeFotis, R.V. Chamberlain and W.R.A. Jarris, J. Magn. Magn. Mater. 104-107 (1992) 187. [6] G.C. DeFotis, K.D. Dell, D.J. Krovitch and W.W. Brubalcer, J. Appl. Phys. 73 (19931 5386. [7] V. Fadet, I. Mallah, 1. Castro, M. Verdraguer and P. Veillet, J. Am. Chem. Sot. 114 (19921 9213. k31C. Johansson and M. Hanson, IEEE Trans. Magn. 30 (1994) 1064. [91 G.C. DeFotis, E.W. Harlem, E.D. Remy and K.D. Dell, J, Appl. Phys. 69 (1991) 6004. HOI E.A. Boudreaux and L.N. Mulay, Theory and Applications of Molecular Paramagnetism, (Wiley Interscience. New York, 1976) Ch. 1. illI E. Kijnig and G. Kiinig, Magnetic Properties of Coordination and Organometallic Transition Metal Compounds, Suppl. 4 to vol. 2 (1971, 1972). Landolt-Bornstein, new series, eds. K.H. Hellwege and A.M. Hellwege, vol. II/11 (Springer, Berlin, 1981); sub. vol. a: Magnetic Susceptibilities. 1121 A. Bielahski, General and Inorganic Chemistry (Polish Scientific Publishers, Warsaw, 1973). Introduction a la [I31 M. Van Meerssche and J. Feneau-DuPont, Cristallographie et a la Chimie Structurale (Oyez, Leuven, 1976), p, 144. D41 R. McWeeny, Coulson’s Valence (Oxford University Press, 1979). [151 H.P. Myers (cdl. Introduction to Solid State Physics (Taylor and Francis, London, 1990). of Physics, (Springer, New York, [161 F. Keffer, Encyclopaedia 1966) vol. XVIII/2, Sec. B.