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Single crystal EPR investigation on Mn(II) doped biomineral: cobalt potassium phosphate hexahydrate
Journal of Physics and Chemistry of Solids 66 (2005) 876–881 www.elsevier.com/locate/jpcs
Single crystal EPR investigation on Mn(II) doped biomineral: cobalt potassium phosphate hexahydrate K. Velavan, R. Venkatesan, P. Sambasiva Rao* Department of Chemistry, Pondicherry University, Pondicherry 605 014, India Received 23 September 2004; accepted 20 October 2004
Abstract Single crystal EPR study of Mn(II) doped in cobalt potassium phosphate hexahydrate has been carried out at room temperature. The impurity shows a 30 line pattern EPR spectra along a particular crystallographic axis suggesting the existence of only one type of impurity in place of Co(II) ion in the host lattice. The spin Hamiltonian parameters have been estimated as: g11Z2.011, g22Z1.998, g33Z1.991, and A11ZK8.9, A22ZK8.8, A33ZK8.4 mT and D11ZK15.2, D22ZK9.4, D33Z24.6 mT, respectively. The sign of A is designated as negative and D as positive. The covalency of metal–oxygen bond has been estimated. The relaxation times, calculated as a function of temperature, indicate spin–lattice relaxation narrowing at room temperature. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Crystal growth; D. Electron paramagnetic resonance (EPR)
1. Introduction Since the electron paramagnetic resonance technique is insightful to local symmetry and character of chemical bonds; it is used as an important probe to understand various phase transitions, structural and bonding factors by means of transition metal ions, especially first row metal ions in its magnetically diluted forms. The probability of assembling information from magnetically concentrated system is diminished due to very broad resonance, arising from dipolar–dipolar interaction. Hence, to understand the symmetry around the embedded ion, covalence, etc., generally paramagnetic ions are incorporated either in diamagnetic or paramagnetic host lattices of known symmetry. The incorporation of paramagnetic ions in paramagnetic host lattices has been discouraged by reason of ascendancy of dipolar and exchange interactions, which result in broadening of resonance lines. Even though it is challenging, it has been done due to the extra additional information like spin lattice relaxation, nature of interaction
* Corresponding author. Tel.: C91 413 265 5991. E-mail address: [email protected] (P. Sambasiva Rao). 0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.10.011
between the guest and host ions, etc., obtained from EPR studies. EPR of Mn(II) in both diamagnetic and paramagnetic host lattices have been studied [1–8] to understand the site symmetry and phase transitions. EPR study of Mn(II) in Mg(OH2)6(NO3)2 [9] shows two magnetically equivalent sites, whose z-axes make an angle of w158 with each other in zx plane. On the other hand, Mn(II) ion in a paramagnetic host, Ni(OH2)6(NO3)2, exhibits a 30 line spectrum showing that the two Ni(II) ions in the lattice are equivalent. In case of Mn(II), in another paramagnetic host, CoH6CeMo12 O42$12H2O, a large E value is observed [10], indicating a low symmetry for the substitutional site. Generally, the outcomes of these works are specific for the system under study. In addition, Cu(II) doped in a paramagnetic cobalt potassium phosphate hexahydrate [11] lattice has shown interesting results, in the sense that the ground state is not a pure dx2Ky2 but admixed with dz2, confirmed by the observation of low hyperfine coupling constant for copper nucleus. In addition, variable temperature measurements have helped in getting relaxation times that are comparable with the literature values. Hence, Mn(II), a d5 ion, which is more sensitive even to small distortions, is incorporated in this paramagnetic host lattice, i.e., cobalt potassium
K. Velavan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 876–881
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phosphate hexahydrate to understand the symmetry around the impurity, covalency of metal–ligand bond, phase transition, relaxation times etc. If the host paramagnetic ion influences the relaxation of the guest ion, the relaxation parameters can be estimated, by measuring the linewidths as a function of temperature. On the other hand, if the host ion is EPR silent down to 77 K (the limit of our temperature study), the results can be explained by assuming, as if a paramagnetic ion is incorporated in a diamagnetic lattice. With this in mind, Mn(II) ion has been incorporated in a Co(II) host lattice and the results are furnished in the present communication.
2. Experiment The preparative method of manganese doped cobalt potassium phosphate hexahydrate (CoKPO4$6H2O, abbreviated as CoPPH) single crystal essentially consists of slow evaporation of equimolar ratio of cobaltous sulphate and potassium dihydrogen orthophosphate in its aqueous solution, with manganous sulphate (0.05%) as the paramagnetic dopant. Well-defined single crystals of Mn(II)/ CoPPH are separated out after two weeks. The crystals have been examined under the polarizing microscope to identify the crystal planes and to avoid twinning, before doing crystal rotations. As mentioned later, EPR spectra are also useful in identifying the nature of the crystals, such as single or twinning or multifaced. The EPR measurements are done on JEOL JES TE100 ESR spectrometer operating at X-band frequency with 100 kHz field modulation. Crystal rotations are done for every 108 of orientation in the three mutually orthogonal planes, namely ab, ac and bc, respectively. The variable temperature measurements are done with a JEOL ES-DVT3 setup. DPPH with a g value of 2.0036 is used as an internal field marker for g factor calculations.
3. Crystal structure Cobalt potassium phosphate hexahydrate (CoKPO4$ 6H2O), here after referred as CoPPH, belongs to biomineral family, having isomorphous structure with magnesium potassium phosphate hexahydrate (MgKPO4$6H2O). Most of the biominerals have close structural parameters. Since the exact crystal structure of CoPPH is not known so far, the crystal structure of magnesium potassium phosphate hexahydrate (MPPH), a structural analogue, has been considered here, for EPR analysis. MPPH [12] has the space group Pmn21 and belongs to orthorhombic system with unit cell dimensions aZ0.6873 nm, bZ0.6160 nm and cZ1.1087 nm and there are two ions per unit cell (ZZ2). Here the six water molecules are disposed in a distorted octahedral symmetry around the magnesium ion as shown in Fig. 1. It is to be noted here that during crystal rotations,
Fig. 1. The arrangement of water oxygen atoms around the central metal ion in MPPH in ac plane (C—metal ion, B—oxygen from water molecule and the Metal—Ow1 is identified as longest bond from crystallographic data).
only one site is observed in ab plane, whereas two sites are seen in ac and bc planes. This means the two magnetically inequivalent sites become in equivalent along crystallographic c axis.
4. Result and discussion A single crystal of optimum size is taken and crystal rotations are done along the three mutually perpendicular crystallographic axes, namely a, b and c-axes. Single crystal EPR measurements of Mn(II) doped CoPPH at room temperature show a variety of complicated spectra. A typical EPR spectrum, when the applied magnetic field (B) is parallel to crystallographic c-axis, is shown in Fig. 2. In this spectrum, the transitions at high and low field regions, corresponding to jG5/2i4jG3/2i, are not well resolved at this and most of the orientations in the bc plane of rotation. In other words, the resonances corresponding to the transitions, jG3/2i4jG1/2i and jC1/2i4jK1/2i are mainly seen in the EPR spectra, during crystal orientations. This kind of observation is noticed in ac plane also. Generally, one expects the intensity of zero-field lines in Mn(II) system to be in the ratio of 5:8:9:8:5. However, in the present case, a random distribution is noticed. This immediately suggests that the E term, which represents deviation of D from axial symmetry, is non-zero (see below). It is also important to mention here that the linewidths of Mn(II) hyperfine lines are relatively broad (4.4 mT), due to dipolar interaction between the guest Mn(II) impurity and the host Co(II) ion (see below).
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Fig. 2. Single crystal EPR spectrum of Mn(II)/CoPPH at room temperature, when the applied magnetic field (B) is parallel to c-axis, while rotating the crystal along crystallographic a-axis. Here, the outermost transitions are not resolved (see text). FrequencyZ9.10867 GHz.
In addition, at few orientations during the crystal rotations, more then 30 lines are observed, due to two spatially distinct sites in the crystal lattice. The splitting of a resonance line into two due to magnetically different sites will generally have equal intensity and so is the case in the present study also. It is to be recalled that the unit cell contains two molecules. Even though two magnetically different, but chemically identical sites for Mn(II) are noticed, only one site is followed, due to the overlap of hyperfine resonances. Fig. 2 also corresponds to the maximum spread in the bc plane of rotation. The road maps are drawn in all three planes (ac, bc and ab) and angular dependence of fine structure is followed. The maximum spread is noticed at qZ08 and the spread between two extreme fine structures descends as q increases. At qZ54.78, the fine structure lines collapse. Fig. 3 shows
Fig. 4. Angular dependence of fine structure resonances when the crystal is rotated in ac plane. The solid circle corresponds to experimental points and the solid lines are calculated values using EPR-NMR program, with the data given in Table 1. FrequencyZ9.10867 GHz.
the EPR spectrum of Mn(II)/CoPPH at the indicated orientation (608 away from the c-axis). This orientation roughly corresponds to the magic angle, where D becomes zero. Further increase of q, again increases the spread and the pattern is repeated. This shows that the angular variation of fine structure follows a (3 cos2qK1) variation. In order to reduce the complex nature of the roadmap, only the zerofield transitions are considered for drawing isofrequency plots. One such road map, when the crystal is rotated along the crystallographic b-axis is given in Fig. 4. However, the hyperfine lines are also included, to get spin Hamiltonian parameters. The angular variation of the fine structure and hyperfine lines in the three orthogonal planes are fitted with the help on EPR–NMR program [13] to the spin Hamiltonian (including second order effects) [14] H ZgbBS C At ðSx Ix C Sy Iy Þ C As Sz Iz C D½S2z K 1=3SðS C 1Þ C EðS2x K S2y Þ C ða=6Þ½S4a C S4b C S4g K ð1=5ÞSðS C 1Þð3S2 C 3S K 1Þ C ðF=180Þ½35S4z K 30SðS C 1ÞS2z C 25S2z K 6SðS C 1Þ C 3S2 ðS C 1Þ2
Fig. 3. Single crystal EPR spectrum of Mn(II)/CoPPH when B is 608 away from c-axis, while rotating the crystal in bc plane. The sharp line at the center corresponds to DPPH. FrequencyZ9.10867 GHz.
ð1Þ
Here a, b and g are the cubic field axes, in the present case represents the nearly orthogonal Co–O chromophore, z is the axis of distortion. D is the zero field splitting matrix, E represents the deviation from axial symmetry, a represents fourth order cubic parameter and F represents the deviation from cubic symmetry [14]. The spin Hamiltonian parameters, thus obtained, are presented in Table 1, along with few literature results. The g and A values agree with the normally reported literature values. The g and A values
K. Velavan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 876–881 Table 1 The spin Hamiltonian parameter evaluated for Mn(II) doped CoPPH and similar lattices, uncertainty in g is G0.004, in A and D G0.1 Host lattice
g
A (mT)
NMTH
2.030 2.008 1.980 1.999
K8.4 K8.9 K9.2 K8.4 K9.0 K9.1 K8.7 K8.9 K8.8 K8.4
SASD CoNbOF5$H2O CoPPH
2.012 2.002 2.011 1.998 1.991
D (mT) 22.3
E (mT)
Ref.
6.2
[26]
where, n is the number of neighboring atoms around central metal ion and cp, cq are electronegativities of p and q. The covalency obtained is 8.5%. Again, the approximate covalency (c) is calculated from the hyperfine coupling constant (Aiso) by using the equation [17,18] Aiso Z ð2:04c K 104:5Þ !10K4 cmK1
27.1
7.1
[27]
K27.0
2.8
[28]
K15.2 K9.4 24.6
2.9
Present study
NMTH, nickel maleate tetrahydrate; SASD, sodium ammonium sulphate dihydrate; CoPPH, cobalt potassium phosphate hexahydrate.
reveal that the symmetry around the impurity is rhombic nature. The parameter E, which describes the deviation from axial symmetry has been calculated and found to be 2.9 mT. This parameter is responsible for the failure of observation of expected intensities of zero-field transitions and the unsymmetrical nature of EPR spectrum. Using the spin Hamiltonian parameters given in Table 1, the isofrequency plot has been simulated and included in Fig. 4. 4.1. Relative signs of A and D The sign of A for Mn(II) high spin complexes is always assigned as negative, since the isotropic hyperfine coupling constant arises by the use of the polarization of the inner s-electrons [15]. Then the sign of D is assigned with respect to A by following separation between the hyperfine structures from the low field to the high field. If the separation between the hyperfine lines in the low field is greater than the high field, then the ratio D/A becomes positive or D/A will be negative for the reverse case. Here, in present case, the separation between the hyperfine lines increases from lower field to higher field, which indicates that D/A is negative. Since the sign of A is designated as negative, the sign of D becomes positive.
879
(3)
The value obtained is 8.6%. This calculated value agrees well with the one calculated from electro negativity relationship, indicating normal ionic character for Mn–O bond of the complex under study. 4.3. Identification of distortion axis The recognition of distortion axis gives valuable information about the location of the guest ion. The direction cosines of the distortion axis have been evaluated, relating the maximum spreads during crystal rotation in all three planes with 4D 0 (3 cos2q 0 iK1), neglecting all other minor terms. Here, D 0 Z36.7(5) mT is zero field splitting value, obtained from powder spectrum, given in Fig. 5. The powder spectrum indicates only the central six lines corresponding to jC1/2i4jK1/2i transition only. Generally, due to large anisotropic effects in g, A and D, the other four transitions corresponding to jG5/2i4jG3/2i and jG 3/2i4jG1/2i will be weak. In addition, the distribution of sites also affects the linewidth of these transitions. It is expected that a small variation in the magnetic field value does not affect transitions occurring between energy levels with equal Ms absolute value [19,20], i.e., jC1/2i4jK1/2i. Hence the resonance lines corresponding to the jG5/2i 4jG3/2i and jG3/2i4jG1/2i are much broader when compared to jC1/2i4jK1/2i transitions. However, the broad nature of jC1/2i4jK1/2i transition itself indicates
4.2. Covalency of metal–ligand bonds Generally, the covalency of the metal-ligand bond is reflected on hyperfine coupling constant. Matumura [16] has calculated the amount of ionic character of Mn(II)–ligand bond in various salts and a linear relationship is suggested between the hyperfine coupling constant and the ionicity of the bond. In the present case, the hyperfine coupling constant (Aiso) value is 8.7 mT (average of the three A values). The covalency of the bond obtained from Matumura’s plot is about 7.8%. The covalency (c) can also be calculated from electronegativities of the metal (p) and bonding atom (q), by using the approximate relationship [17] c Z 1=n½1 K 0:16ðcp K cq Þ K 0:035ðcp K cq Þ2
(2)
Fig. 5. Polycrystalline EPR spectrum of Mn(II)/CoPPH recorded at room temperature. Only the central transitions are seen. The broadening of the Mn(II) hyperfine lines can be attributed to the Co(II) ion (see text). FrequencyZ9.39102 GHz.
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K. Velavan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 876–881
Table 2 Metal–oxygen bond direction of CoPPH assuming that CoPPH is isomorphous with MPPH Direction cosines Co–O1 Co–O3 Co–O4 Distortion axis
a
b
c
0.8834 K0.3251 K0.3213 0.9083
0.4683 0.5916 0.6347 0.4153
0.0000 K0.7378 0.7028 0.0501
the domination of dipolar interaction over the other effects. The D 0 value is obtained by recording the EPR spectrum at higher gain and modulation conditions. The calculation of q 0 1, q 0 2, q 0 3 gives the angle between the magnetic field and distortion axis in each rotation when, the axis of rotation, distortion axis and the magnetic field are coplanar. Hence, (90Kq 0 i) is the angle between the distortion axis and the orthogonal set of rotation axes. This gives information about the distortion axis [21]. Since the crystal structure of the present host lattice is not known as mentioned earlier, the distortion axis is correlated with the direction cosines of an isomorphous biomineral MPPH. The direction cosines of the distortion axis, thus calculated, is given in Table 2, along with the direction cosines of various Co–O bonds in the host lattice, obtained from the X-ray structure of MPPH. The direction cosines of the distortion axis match fairly well with Co–O1 direction, the angle between them is around 4.58. This shows that the guest paramagnetic ion has entered into the lattice substitutionally in the place of Co(II). 4.4. EPR at variable temperature Since a paramagnetic ion is incorporated in a paramagnetic host lattice, it will be interesting to study the relaxation effects in these systems. As mentioned earlier, the hyperfine lines are slightly broader, compared to Mn(II) incorporated in a diamagnetic host lattice. The single crystal EPR spectra are recorded at different temperatures from 303 to 203 K when B is parallel to c-axis and are analyzed. The zero-field splitting parameter (D) is found to increase slightly (not appreciably) on lowering the temperature. Dissimilar to the behavior of diamagnetic lattices, in the present case, the linewidth is getting increased, with a decrease in intensity, as the temperature is changed from room temperature to 203 K. Further lowering the temperature from 203 K has resulted in a straight line, due to dipolar and exchange interactions. Since, Mn(II) is doped in cobalt lattice, the dipolar broadening may be due to the interaction between the host and guest and/or between two guest ions itself. Since the impurity is doped in very small concentrations, the latter effect can be neglected and conclude that the broadening is dominated by the former one. The observation of an increase in linewidth while cooling the sample from 300 K (Fig. 6) can be explained by the spin lattice relaxation time of the host Co(II) ion. Mitsuma [22]
Fig. 6. The graph representing the temperature dependence of linewidth for Mn(II)/CoPPH single crystal.
has explained that the very fast spin lattice relaxation of the host paramagnetic ion [Co(II)] can trim down the dipolar and exchange interaction to an extent that exists between the host Co(II) ion and guest Mn(II) ion. Hence, the spin–lattice relaxation times have been calculated as a function of temperature and are shown in Fig. 7, to understand this process. When temperature is lowered, the spin–lattice relaxation of the host ion is condensed. The slow relaxation of host ion leaves path to dipolar and exchange interactions that are reflected on the linewidth of impurity by broadening, where as the reverse takes place, while
Fig. 7. Graphical representation of spin–lattice relaxation times (SLRT) as a function of temperature for Mn(II)/CoPPH.
K. Velavan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 876–881
the temperature is increased. The host relaxation time (T1) is related to the resonance linewidth by [22–24] T1 Z ð3=10Þðh=2gh bÞðDBimp =B2d Þ
(4)
where B2d Z 5:1ðgh bnÞ2 Sh ðSh C 1Þ. In the above equations, h is Planck’s constant, b is Bohr magneton, Sh is host ion effective spin and is taken as 1/2, n is number of host spin per unit volume and DBimp is the impurity linewidth. The g value for Co(II) ion is 6.54. From Figs. 6 and 7, it has been concluded that the system undergoes spin–lattice relaxation narrowing at room temperature as suggested by Mitsuma and the relaxation time at room temperature is found to agree with reported values fairly [25,26].
5. Conclusion The single crystal EPR study of a paramagnetic ion in a paramagnetic host lattice shows that the paramagnetic impurity has entered into the lattice substitutionally in the place of Co(II) ion. Angular rotations in the three orthogonal axes have been done and spin Hamiltonian parameters are evaluated. The positive sign has been assigned for D after having a close look at hyperfine structure of the impurity. The system shows considerable distortion from axial symmetry, as shown by the E parameter. The broadening of EPR resonance lines while going down from room temperature is discussed with the help of dipolar and exchange interaction and it has been explained that the spin–lattice relaxation narrowing taking place at room temperature. References [1] A. Kassiba, R. Hrabanski, D. Bonhomme, A. Hader, J. Phys. Condens. Matter 7 (1995) 3339.
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[2] C.J. O’Connor, R.L. Carlin, Inorg. Chem. 14 (1975) 297. [3] V.S.K. Anthonisamy, M. Velayutham, R. Murugasen, Physica (B) 262 (1999) 13. [4] R. Janakiraman, G.C. Upreti, J. Chem. Phys. 54 (1971) 2336. [5] G.C. Upreti, J. Magn. Reson. 14 (1974) 274. [6] V.K. Jain, V. Kapoor, V. Prekash, Solid State Commun. 97 (1996) 425. [7] N.O. Gopal, K.V. Narasimhulu, J. Lakshmana Rao, J. Phys. Chem. Solids 63 (2002) 295. [8] K.V. Narasimhulu, J. Lakshmana Rao, Physica (B) 254 (1998) 37. [9] R.S. Saraswat, G.C. Upreti, J. Phys. C: Solid State Phys. 10 (1977) 2233. [10] T. Jeyabalan, P. Sami, A. Shunmugasundaram, R. Murugesan, Spectrochim. Acta Part A 55 (1999) 2187. [11] S. Uma maragatham, K. Velavan, R. Venkatesan, P. Sambasiva Rao, submitted for publication. [12] M. Mathew, L.W. Schroeder, Acta Crystallogr. B35 (1979) 11. [13] EPR-NMR Program developed by F. Clark, R.S. Dickson, D.B. Fulton, J. Isoya, A. Lent, D.G. McGavin, M.J. Mombourquette, R.H.D. Nuttall, P.S. Rao, H. Rinnerberg, W.C. Tennant, J.A. Weil, University of Saskatchewan, Saskatoon, Canada, 1996. [14] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Metal Ions, Clarendon Press, Oxford, 1970. [15] M. Korkman, M. Dupont, B. Aktas, J. Phys. Chem. Solids 45 (1984) 465. [16] O. Matumura, J. Phys. Soc. Jpn 14 (1959) 108. [17] E. Simanek, K.A. Muller, J. Phys. Chem. Solids 31 (1970) 1027. [18] A.M.F. Benial, V. Ramakrishnan, R. Murugesan, Spectrochim. Acta A55 (1999) 2573. [19] R.F. Wenzel, Y.W. Kim, Phys. Rev. 140 (1965) 1592. [20] G. Morin, D. Bonnin, J. Magn. Reson. 136 (1999) 176. [21] Jesiamma Joseph, P. Sambasiva Rao, Spectrochim. Acta A52 (1996) 607. [22] T. Mitsuma, J. Phys. Soc. Jpn 17 (1962) 28. [23] V.K. Jain, J. Magn. Reson. 64 (1985) 512. [24] V.K. Jain, J. Chem. Phys. 84 (1985) 1944. [25] R. Hrabanski, Spectrochim. Acta 44A (1988) 647. [26] J. Lech, A. Slezak, R. Hrabanski, J. Phys. Chem. Solids 52 (1991) 685. [27] T. Bodziony, I.E. Lipinski, J. Kuriata, W. Bednarski, Physica (B) 299 (2001) 70. [28] R. Murugesan, S. Subramanian, J. Magn. Reson. 57 (1984) 385.
Single crystal EPR investigation on Mn(II) doped biomineral: cobalt potassium phosphate hexahydrate K. Velavan, R. Venkatesan, P. Sambasiva Rao* Department of Chemistry, Pondicherry University, Pondicherry 605 014, India Received 23 September 2004; accepted 20 October 2004
Abstract Single crystal EPR study of Mn(II) doped in cobalt potassium phosphate hexahydrate has been carried out at room temperature. The impurity shows a 30 line pattern EPR spectra along a particular crystallographic axis suggesting the existence of only one type of impurity in place of Co(II) ion in the host lattice. The spin Hamiltonian parameters have been estimated as: g11Z2.011, g22Z1.998, g33Z1.991, and A11ZK8.9, A22ZK8.8, A33ZK8.4 mT and D11ZK15.2, D22ZK9.4, D33Z24.6 mT, respectively. The sign of A is designated as negative and D as positive. The covalency of metal–oxygen bond has been estimated. The relaxation times, calculated as a function of temperature, indicate spin–lattice relaxation narrowing at room temperature. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Crystal growth; D. Electron paramagnetic resonance (EPR)
1. Introduction Since the electron paramagnetic resonance technique is insightful to local symmetry and character of chemical bonds; it is used as an important probe to understand various phase transitions, structural and bonding factors by means of transition metal ions, especially first row metal ions in its magnetically diluted forms. The probability of assembling information from magnetically concentrated system is diminished due to very broad resonance, arising from dipolar–dipolar interaction. Hence, to understand the symmetry around the embedded ion, covalence, etc., generally paramagnetic ions are incorporated either in diamagnetic or paramagnetic host lattices of known symmetry. The incorporation of paramagnetic ions in paramagnetic host lattices has been discouraged by reason of ascendancy of dipolar and exchange interactions, which result in broadening of resonance lines. Even though it is challenging, it has been done due to the extra additional information like spin lattice relaxation, nature of interaction
* Corresponding author. Tel.: C91 413 265 5991. E-mail address: [email protected] (P. Sambasiva Rao). 0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2004.10.011
between the guest and host ions, etc., obtained from EPR studies. EPR of Mn(II) in both diamagnetic and paramagnetic host lattices have been studied [1–8] to understand the site symmetry and phase transitions. EPR study of Mn(II) in Mg(OH2)6(NO3)2 [9] shows two magnetically equivalent sites, whose z-axes make an angle of w158 with each other in zx plane. On the other hand, Mn(II) ion in a paramagnetic host, Ni(OH2)6(NO3)2, exhibits a 30 line spectrum showing that the two Ni(II) ions in the lattice are equivalent. In case of Mn(II), in another paramagnetic host, CoH6CeMo12 O42$12H2O, a large E value is observed [10], indicating a low symmetry for the substitutional site. Generally, the outcomes of these works are specific for the system under study. In addition, Cu(II) doped in a paramagnetic cobalt potassium phosphate hexahydrate [11] lattice has shown interesting results, in the sense that the ground state is not a pure dx2Ky2 but admixed with dz2, confirmed by the observation of low hyperfine coupling constant for copper nucleus. In addition, variable temperature measurements have helped in getting relaxation times that are comparable with the literature values. Hence, Mn(II), a d5 ion, which is more sensitive even to small distortions, is incorporated in this paramagnetic host lattice, i.e., cobalt potassium
K. Velavan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 876–881
877
phosphate hexahydrate to understand the symmetry around the impurity, covalency of metal–ligand bond, phase transition, relaxation times etc. If the host paramagnetic ion influences the relaxation of the guest ion, the relaxation parameters can be estimated, by measuring the linewidths as a function of temperature. On the other hand, if the host ion is EPR silent down to 77 K (the limit of our temperature study), the results can be explained by assuming, as if a paramagnetic ion is incorporated in a diamagnetic lattice. With this in mind, Mn(II) ion has been incorporated in a Co(II) host lattice and the results are furnished in the present communication.
2. Experiment The preparative method of manganese doped cobalt potassium phosphate hexahydrate (CoKPO4$6H2O, abbreviated as CoPPH) single crystal essentially consists of slow evaporation of equimolar ratio of cobaltous sulphate and potassium dihydrogen orthophosphate in its aqueous solution, with manganous sulphate (0.05%) as the paramagnetic dopant. Well-defined single crystals of Mn(II)/ CoPPH are separated out after two weeks. The crystals have been examined under the polarizing microscope to identify the crystal planes and to avoid twinning, before doing crystal rotations. As mentioned later, EPR spectra are also useful in identifying the nature of the crystals, such as single or twinning or multifaced. The EPR measurements are done on JEOL JES TE100 ESR spectrometer operating at X-band frequency with 100 kHz field modulation. Crystal rotations are done for every 108 of orientation in the three mutually orthogonal planes, namely ab, ac and bc, respectively. The variable temperature measurements are done with a JEOL ES-DVT3 setup. DPPH with a g value of 2.0036 is used as an internal field marker for g factor calculations.
3. Crystal structure Cobalt potassium phosphate hexahydrate (CoKPO4$ 6H2O), here after referred as CoPPH, belongs to biomineral family, having isomorphous structure with magnesium potassium phosphate hexahydrate (MgKPO4$6H2O). Most of the biominerals have close structural parameters. Since the exact crystal structure of CoPPH is not known so far, the crystal structure of magnesium potassium phosphate hexahydrate (MPPH), a structural analogue, has been considered here, for EPR analysis. MPPH [12] has the space group Pmn21 and belongs to orthorhombic system with unit cell dimensions aZ0.6873 nm, bZ0.6160 nm and cZ1.1087 nm and there are two ions per unit cell (ZZ2). Here the six water molecules are disposed in a distorted octahedral symmetry around the magnesium ion as shown in Fig. 1. It is to be noted here that during crystal rotations,
Fig. 1. The arrangement of water oxygen atoms around the central metal ion in MPPH in ac plane (C—metal ion, B—oxygen from water molecule and the Metal—Ow1 is identified as longest bond from crystallographic data).
only one site is observed in ab plane, whereas two sites are seen in ac and bc planes. This means the two magnetically inequivalent sites become in equivalent along crystallographic c axis.
4. Result and discussion A single crystal of optimum size is taken and crystal rotations are done along the three mutually perpendicular crystallographic axes, namely a, b and c-axes. Single crystal EPR measurements of Mn(II) doped CoPPH at room temperature show a variety of complicated spectra. A typical EPR spectrum, when the applied magnetic field (B) is parallel to crystallographic c-axis, is shown in Fig. 2. In this spectrum, the transitions at high and low field regions, corresponding to jG5/2i4jG3/2i, are not well resolved at this and most of the orientations in the bc plane of rotation. In other words, the resonances corresponding to the transitions, jG3/2i4jG1/2i and jC1/2i4jK1/2i are mainly seen in the EPR spectra, during crystal orientations. This kind of observation is noticed in ac plane also. Generally, one expects the intensity of zero-field lines in Mn(II) system to be in the ratio of 5:8:9:8:5. However, in the present case, a random distribution is noticed. This immediately suggests that the E term, which represents deviation of D from axial symmetry, is non-zero (see below). It is also important to mention here that the linewidths of Mn(II) hyperfine lines are relatively broad (4.4 mT), due to dipolar interaction between the guest Mn(II) impurity and the host Co(II) ion (see below).
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Fig. 2. Single crystal EPR spectrum of Mn(II)/CoPPH at room temperature, when the applied magnetic field (B) is parallel to c-axis, while rotating the crystal along crystallographic a-axis. Here, the outermost transitions are not resolved (see text). FrequencyZ9.10867 GHz.
In addition, at few orientations during the crystal rotations, more then 30 lines are observed, due to two spatially distinct sites in the crystal lattice. The splitting of a resonance line into two due to magnetically different sites will generally have equal intensity and so is the case in the present study also. It is to be recalled that the unit cell contains two molecules. Even though two magnetically different, but chemically identical sites for Mn(II) are noticed, only one site is followed, due to the overlap of hyperfine resonances. Fig. 2 also corresponds to the maximum spread in the bc plane of rotation. The road maps are drawn in all three planes (ac, bc and ab) and angular dependence of fine structure is followed. The maximum spread is noticed at qZ08 and the spread between two extreme fine structures descends as q increases. At qZ54.78, the fine structure lines collapse. Fig. 3 shows
Fig. 4. Angular dependence of fine structure resonances when the crystal is rotated in ac plane. The solid circle corresponds to experimental points and the solid lines are calculated values using EPR-NMR program, with the data given in Table 1. FrequencyZ9.10867 GHz.
the EPR spectrum of Mn(II)/CoPPH at the indicated orientation (608 away from the c-axis). This orientation roughly corresponds to the magic angle, where D becomes zero. Further increase of q, again increases the spread and the pattern is repeated. This shows that the angular variation of fine structure follows a (3 cos2qK1) variation. In order to reduce the complex nature of the roadmap, only the zerofield transitions are considered for drawing isofrequency plots. One such road map, when the crystal is rotated along the crystallographic b-axis is given in Fig. 4. However, the hyperfine lines are also included, to get spin Hamiltonian parameters. The angular variation of the fine structure and hyperfine lines in the three orthogonal planes are fitted with the help on EPR–NMR program [13] to the spin Hamiltonian (including second order effects) [14] H ZgbBS C At ðSx Ix C Sy Iy Þ C As Sz Iz C D½S2z K 1=3SðS C 1Þ C EðS2x K S2y Þ C ða=6Þ½S4a C S4b C S4g K ð1=5ÞSðS C 1Þð3S2 C 3S K 1Þ C ðF=180Þ½35S4z K 30SðS C 1ÞS2z C 25S2z K 6SðS C 1Þ C 3S2 ðS C 1Þ2
Fig. 3. Single crystal EPR spectrum of Mn(II)/CoPPH when B is 608 away from c-axis, while rotating the crystal in bc plane. The sharp line at the center corresponds to DPPH. FrequencyZ9.10867 GHz.
ð1Þ
Here a, b and g are the cubic field axes, in the present case represents the nearly orthogonal Co–O chromophore, z is the axis of distortion. D is the zero field splitting matrix, E represents the deviation from axial symmetry, a represents fourth order cubic parameter and F represents the deviation from cubic symmetry [14]. The spin Hamiltonian parameters, thus obtained, are presented in Table 1, along with few literature results. The g and A values agree with the normally reported literature values. The g and A values
K. Velavan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 876–881 Table 1 The spin Hamiltonian parameter evaluated for Mn(II) doped CoPPH and similar lattices, uncertainty in g is G0.004, in A and D G0.1 Host lattice
g
A (mT)
NMTH
2.030 2.008 1.980 1.999
K8.4 K8.9 K9.2 K8.4 K9.0 K9.1 K8.7 K8.9 K8.8 K8.4
SASD CoNbOF5$H2O CoPPH
2.012 2.002 2.011 1.998 1.991
D (mT) 22.3
E (mT)
Ref.
6.2
[26]
where, n is the number of neighboring atoms around central metal ion and cp, cq are electronegativities of p and q. The covalency obtained is 8.5%. Again, the approximate covalency (c) is calculated from the hyperfine coupling constant (Aiso) by using the equation [17,18] Aiso Z ð2:04c K 104:5Þ !10K4 cmK1
27.1
7.1
[27]
K27.0
2.8
[28]
K15.2 K9.4 24.6
2.9
Present study
NMTH, nickel maleate tetrahydrate; SASD, sodium ammonium sulphate dihydrate; CoPPH, cobalt potassium phosphate hexahydrate.
reveal that the symmetry around the impurity is rhombic nature. The parameter E, which describes the deviation from axial symmetry has been calculated and found to be 2.9 mT. This parameter is responsible for the failure of observation of expected intensities of zero-field transitions and the unsymmetrical nature of EPR spectrum. Using the spin Hamiltonian parameters given in Table 1, the isofrequency plot has been simulated and included in Fig. 4. 4.1. Relative signs of A and D The sign of A for Mn(II) high spin complexes is always assigned as negative, since the isotropic hyperfine coupling constant arises by the use of the polarization of the inner s-electrons [15]. Then the sign of D is assigned with respect to A by following separation between the hyperfine structures from the low field to the high field. If the separation between the hyperfine lines in the low field is greater than the high field, then the ratio D/A becomes positive or D/A will be negative for the reverse case. Here, in present case, the separation between the hyperfine lines increases from lower field to higher field, which indicates that D/A is negative. Since the sign of A is designated as negative, the sign of D becomes positive.
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(3)
The value obtained is 8.6%. This calculated value agrees well with the one calculated from electro negativity relationship, indicating normal ionic character for Mn–O bond of the complex under study. 4.3. Identification of distortion axis The recognition of distortion axis gives valuable information about the location of the guest ion. The direction cosines of the distortion axis have been evaluated, relating the maximum spreads during crystal rotation in all three planes with 4D 0 (3 cos2q 0 iK1), neglecting all other minor terms. Here, D 0 Z36.7(5) mT is zero field splitting value, obtained from powder spectrum, given in Fig. 5. The powder spectrum indicates only the central six lines corresponding to jC1/2i4jK1/2i transition only. Generally, due to large anisotropic effects in g, A and D, the other four transitions corresponding to jG5/2i4jG3/2i and jG 3/2i4jG1/2i will be weak. In addition, the distribution of sites also affects the linewidth of these transitions. It is expected that a small variation in the magnetic field value does not affect transitions occurring between energy levels with equal Ms absolute value [19,20], i.e., jC1/2i4jK1/2i. Hence the resonance lines corresponding to the jG5/2i 4jG3/2i and jG3/2i4jG1/2i are much broader when compared to jC1/2i4jK1/2i transitions. However, the broad nature of jC1/2i4jK1/2i transition itself indicates
4.2. Covalency of metal–ligand bonds Generally, the covalency of the metal-ligand bond is reflected on hyperfine coupling constant. Matumura [16] has calculated the amount of ionic character of Mn(II)–ligand bond in various salts and a linear relationship is suggested between the hyperfine coupling constant and the ionicity of the bond. In the present case, the hyperfine coupling constant (Aiso) value is 8.7 mT (average of the three A values). The covalency of the bond obtained from Matumura’s plot is about 7.8%. The covalency (c) can also be calculated from electronegativities of the metal (p) and bonding atom (q), by using the approximate relationship [17] c Z 1=n½1 K 0:16ðcp K cq Þ K 0:035ðcp K cq Þ2
(2)
Fig. 5. Polycrystalline EPR spectrum of Mn(II)/CoPPH recorded at room temperature. Only the central transitions are seen. The broadening of the Mn(II) hyperfine lines can be attributed to the Co(II) ion (see text). FrequencyZ9.39102 GHz.
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Table 2 Metal–oxygen bond direction of CoPPH assuming that CoPPH is isomorphous with MPPH Direction cosines Co–O1 Co–O3 Co–O4 Distortion axis
a
b
c
0.8834 K0.3251 K0.3213 0.9083
0.4683 0.5916 0.6347 0.4153
0.0000 K0.7378 0.7028 0.0501
the domination of dipolar interaction over the other effects. The D 0 value is obtained by recording the EPR spectrum at higher gain and modulation conditions. The calculation of q 0 1, q 0 2, q 0 3 gives the angle between the magnetic field and distortion axis in each rotation when, the axis of rotation, distortion axis and the magnetic field are coplanar. Hence, (90Kq 0 i) is the angle between the distortion axis and the orthogonal set of rotation axes. This gives information about the distortion axis [21]. Since the crystal structure of the present host lattice is not known as mentioned earlier, the distortion axis is correlated with the direction cosines of an isomorphous biomineral MPPH. The direction cosines of the distortion axis, thus calculated, is given in Table 2, along with the direction cosines of various Co–O bonds in the host lattice, obtained from the X-ray structure of MPPH. The direction cosines of the distortion axis match fairly well with Co–O1 direction, the angle between them is around 4.58. This shows that the guest paramagnetic ion has entered into the lattice substitutionally in the place of Co(II). 4.4. EPR at variable temperature Since a paramagnetic ion is incorporated in a paramagnetic host lattice, it will be interesting to study the relaxation effects in these systems. As mentioned earlier, the hyperfine lines are slightly broader, compared to Mn(II) incorporated in a diamagnetic host lattice. The single crystal EPR spectra are recorded at different temperatures from 303 to 203 K when B is parallel to c-axis and are analyzed. The zero-field splitting parameter (D) is found to increase slightly (not appreciably) on lowering the temperature. Dissimilar to the behavior of diamagnetic lattices, in the present case, the linewidth is getting increased, with a decrease in intensity, as the temperature is changed from room temperature to 203 K. Further lowering the temperature from 203 K has resulted in a straight line, due to dipolar and exchange interactions. Since, Mn(II) is doped in cobalt lattice, the dipolar broadening may be due to the interaction between the host and guest and/or between two guest ions itself. Since the impurity is doped in very small concentrations, the latter effect can be neglected and conclude that the broadening is dominated by the former one. The observation of an increase in linewidth while cooling the sample from 300 K (Fig. 6) can be explained by the spin lattice relaxation time of the host Co(II) ion. Mitsuma [22]
Fig. 6. The graph representing the temperature dependence of linewidth for Mn(II)/CoPPH single crystal.
has explained that the very fast spin lattice relaxation of the host paramagnetic ion [Co(II)] can trim down the dipolar and exchange interaction to an extent that exists between the host Co(II) ion and guest Mn(II) ion. Hence, the spin–lattice relaxation times have been calculated as a function of temperature and are shown in Fig. 7, to understand this process. When temperature is lowered, the spin–lattice relaxation of the host ion is condensed. The slow relaxation of host ion leaves path to dipolar and exchange interactions that are reflected on the linewidth of impurity by broadening, where as the reverse takes place, while
Fig. 7. Graphical representation of spin–lattice relaxation times (SLRT) as a function of temperature for Mn(II)/CoPPH.
K. Velavan et al. / Journal of Physics and Chemistry of Solids 66 (2005) 876–881
the temperature is increased. The host relaxation time (T1) is related to the resonance linewidth by [22–24] T1 Z ð3=10Þðh=2gh bÞðDBimp =B2d Þ
(4)
where B2d Z 5:1ðgh bnÞ2 Sh ðSh C 1Þ. In the above equations, h is Planck’s constant, b is Bohr magneton, Sh is host ion effective spin and is taken as 1/2, n is number of host spin per unit volume and DBimp is the impurity linewidth. The g value for Co(II) ion is 6.54. From Figs. 6 and 7, it has been concluded that the system undergoes spin–lattice relaxation narrowing at room temperature as suggested by Mitsuma and the relaxation time at room temperature is found to agree with reported values fairly [25,26].
5. Conclusion The single crystal EPR study of a paramagnetic ion in a paramagnetic host lattice shows that the paramagnetic impurity has entered into the lattice substitutionally in the place of Co(II) ion. Angular rotations in the three orthogonal axes have been done and spin Hamiltonian parameters are evaluated. The positive sign has been assigned for D after having a close look at hyperfine structure of the impurity. The system shows considerable distortion from axial symmetry, as shown by the E parameter. The broadening of EPR resonance lines while going down from room temperature is discussed with the help of dipolar and exchange interaction and it has been explained that the spin–lattice relaxation narrowing taking place at room temperature. References [1] A. Kassiba, R. Hrabanski, D. Bonhomme, A. Hader, J. Phys. Condens. Matter 7 (1995) 3339.
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