EPR laboratory, Department of Physics, University of Allahabad, Allahabad , India (Received November 20, 2009)

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1 CHINESE JOURNAL OF PHYSICS VOL. 48, NO. 5 OCTOBER 2010 EPR and Optical Absorption Studies of Mn 2+ Doped Diglycine Calcium Chloride Tetrahydrate Ram Kripal and Manisha Bajpai EPR laboratory, Department of Physics, University of Allahabad, Allahabad , India (Received November 20, 2009) Electron paramagnetc resonance (EPR) studies of Mn 2+ doped diglycine calcium chloride tetrahydrate were carried out at liquid nitrogen temperature. The values of the spin Hamiltonian parameters that give a good fit to the observed EPR spectra were obtained. The values of the spectroscopic splitting factor (g), hyperfine structure constants (A and B), axial zero field splitting parameter (D), rhombic zero field splitting parameter (E), and cubic field splitting parameter (a) are: g = ± , A = (106 ± 2) 10 4 cm 1, B = (93 ± 2) 10 4 cm 1, D = (276 ± 2) 10 4 cm 1, E = (58 ± 2) 10 4 cm 1, and a = (20 ± 1) 10 4 cm 1 for Site I and g = ± , A = (107 ± 2) 10 4 cm 1, B = (93 ± 2) 10 4 cm 1, D = (275 ± 2) 10 4 cm 1, E = (59 ± 2) 10 4 cm 1, and a = (22 ± 1) 10 4 cm 1 for Site II. The percentage of covalency of the metal-ligand bond has also been determined. The optical absorption study was performed at room temperature. The observed optical bands are assigned as transitions from the 6 A 1g (S) ground state to various excited quartet levels of a Mn 2+ ion in a cubic crystalline field. The electron repulsion parameters (B and C) and crystal field splitting parameter (D q ) were obtained as: B = 770 cm 1, C = 2322 cm 1, and D q = 716 cm 1. PACS numbers: v, q, Me I. INTRODUCTION Electron paramagnetic resonance (EPR) is a spectroscopic technique used to obtain microscopic, chemical, and physical information about a molecule. EPR studies on an Mn 2+ ion in a variety of host lattices have been reported [1 3] earlier. The optical study provides the energy level ordering of the different orbital levels of the paramagnetic ion and crystalline field strength in the host lattice [4]. EPR and optical absorption have been used as an investigative tool for the study of transition metal ions and radicals in solid materials to obtain information about the symmetry of the crystalline electric field and the associated distortion in the lattice [5 8]. The complexes of amino acids play an important role in biology and occur in living systems. Glycine compounds have therapeutic values. Hence, the addition compound of glycine, the simplest amino acid, with calcium chloride is taken up. Divalent manganese is of interest among the paramagnetic complexes of the iron group [9 13]. The ground state is 6 S. The crystalline electric field can affect the electron spins only through high order interactions, so that the spins are almost completely free to orient themselves in an external magnetic field [14]. In the present study the EPR and optical study of Mn 2+ doped diglycine calcium chloride tetrahydrate (DGCCT) are reported c 2010 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA

2 672 EPR AND OPTICAL ABSORPTION STUDIES OF... VOL. 48 on to obtain information as to whether an Mn 2+ ion enters the lattice substitutionally or interstitially and to obtain the structure of the energy levels of the Mn 2+ ion. Furthermore, the data obtained are used to get information about the nature of the bonding of metal ion with its different ligands. II. CRYSTAL STRUCTURE DGCCT, [(NH 2 CH 2 COOH) 2.CaCl 2.4H 2 O] single crystals are monoclinic, space group P2 1 /n with Z = 4 [15]. The dimensions of the unit cell are a = 13.01, b = 6.79, c = Å, β = The calcium atom is coordinated to seven oxygen atoms, three of them belonging to water molecules and the rest to the carboxyl groups of the glycine molecules. The Ca-O distances range from 2.34 to 2.54 Å. No chlorine atom coordinates to calcium. The glycine molecules have the normal bond distances. The hydrogen bonds are between the nitrogen, oxygen, and chlorine atoms. III. THE EXPERIMENT Single crystals of DGCCT were grown by slow evaporation at room temperature of a saturated aqueous solution containing stoichiometric amounts of glycine and calcium chloride in distilled water. For Mn 2+ doped crystals 0.1wt% of manganese chloride is added to the mixture. Good transparent crystals grow in about 25 days. The EPR spectra of Mn 2+ doped DGCCT were recorded at liquid nitrogen temperature (LNT) on a Varian X-Band E-112 (9.1 GHz) reflection type EPR spectrometer. The DGCCT crystal is monoclinic so we choose the a, b, and c* axis system for measurements. In this system c* is orthogonal to both a and b. The spectra were recorded along these three mutually perpendicular crystallographic axes at the interval of 10 each. Tetracene negative (TCNE) (g = ) was used as a field marker. The optical spectrum of the crystal was recorded on a Unicam spectrophotometer in the nm region at room temperature. IV. RESULTS AND DISCUSSION The EPR spectra of Mn 2+ doped DGCCT at liquid nitrogen temperature (LNT) show two distinct sites each having five sets with six lines in each set. The spectrum is characteristic of a system with S = 5/2 and I = 5/2. A typical EPR spectrum recorded, when an applied magnetic field B is parallel to the a axis, is shown in Fig. 1(a). The corresponding simulated spectrum, using EasySpin [16] and the estimated spin Hamiltonian parameters g = ± , A = (106 ± 2) 10 4 cm 1, B = (93 ± 2) 10 4 cm 1, D = (276±2) 10 4 cm 1, and E = (58±2) 10 4 cm 1 for Site I and g = ±0.0002, A = (107 ± 2) 10 4 cm 1, B = (93 ± 2) 10 4 cm 1, D = (275 ± 2) 10 4 cm 1, and E = (59 ± 2) 10 4 cm 1 for Site II are given in Fig. 1 (b).

3 VOL. 48 RAM KRIPAL AND MANISHA BAJPAI 673 FIG. 1: (a) EPR spectra of Mn 2+ doped DGCCT single crystal for a magnetic field B parallel to the a axis. (b) Simulated EPR spectrum of Mn 2+ doped DGCCT single crystal for a magnetic field B parallel to the a axis (Microwave Freq. 9.1 GHz). Magnetic Field (Gauss) c* b Angle (Degree) Magnetic Field (Gauss) a c* Angle (Degree) Magnetic Field (Gauss) a b Angle (Degree) Fig. 2(a) Fig. 2(b) Fig. 2(c) FIG. 2: Angular variation of the fine structure of Mn 2+ doped DGCCT single crystal in the planes (solid lines and symbols represent theoretical and experimental resonance fields, respectively): (a) c*b, (b) ac*, (c) ab. The spin Hamiltonian for a spin multiplet due to second order effects and other

4 674 EPR AND OPTICAL ABSORPTION STUDIES OF... VOL. 48 zero-field terms, is given by the following expression [17 20]: H = gµ B B.S + D[S 2 z 1 3 S(S + 1)] + E(S2 x S 2 y) + a 6 [S4 x + S 4 y + S 4 z 1 5 S(S + 1)(3S2 + 3S 1)] + F 180 {35S4 z 30S(S + 1)S 2 z + 25S 2 z 6S(S + 1) + 3S 2 (S + 1) 2 } + K 4 [{7S2 z S(S + 1) 5}(S S 2 ) + (S S 2 ){7S 2 z S(S + 1) 5}] +AS z I z + B(S x I x + S y I y ), (1) where µ B is the Bohr magneton, B is the external magnetic field, g is the spectroscopic splitting factor, and S is the effective spin vector. The parameters a, D, and E are the cubic, axial, and rhombic zero-field splitting (ZFS) parameters, respectively. The first term represents the electronic Zeeman interaction, the second and third terms represent the axial and rhombic parts of the ZFS, the fourth term represents the fourth-rank cubic ZFS term [21], and the fifth and sixth terms represent axial and rhombic fourth-rank ZFS terms, respectively; the seventh and eighth terms are the hyperfine interaction terms (I = 5/2). The fifth and sixth terms in Eq. (1) have been omitted here, as their effect is small [20, 22]. Due to this, there may be a small error in the value of a [23]. In the absence of an applied magnetic field, the ground state of the Mn 2+ ion 6 S 5/2 splits into three Kramer doublets with separations of 4D and 2D due to the electronic magnetic interaction. These doublets split further by application of an external magnetic field into six levels with successive separations gµ B B+4D, gµ B B+2D, gµ B B, gµ B B 2D, gµ B B 4D [17]. Transitions between these levels will give rise to five equally spaced lines, each of which further splits into a sextet due to the hyperfine interaction resulting from the nuclear spin of I = 5/2. Hence, a pattern of overall thirty hyperfine lines is expected. The direction of the maximum overall splitting of the EPR spectrum is taken as the z axis and that of the minimum as the x axis [24]. The (x,y,z) system is parallel to the crystallographic axes. The local site symmetry axes, i.e., the symmetry adapted axes (SAA) in the present case, are the nearly orthogonal directions of metal-ligand bonds [15]. The Z-axis of the SAA is coincident with the crystal a-axis, and the other two axes (X, Y ) lie in the c b plane. The allowed transitions and the field B at which they occur when the Zeeman inter-

5 VOL. 48 RAM KRIPAL AND MANISHA BAJPAI 675 action is dominating are given by [17, 18] M = + 5 ( D ; B = B 0 2D(3cos 2 2) ( D θ 1) 32 sin 2 θ cos 2 2) θ + sin 4 θ 2pa, B 0 B 0 M = + 3 ( D ; B = B 0 D(3cos 2 2) ( D θ 1) + 4 sin 2 θ cos 2 2 ) θ 5 sin 4 θ + 5 B 0 4B 0 2 pa, M = + 1 ( D 2) ( D ; B = B sin 2 θ cos 2 2) θ 2 sin 4 θ, (2) B 0 B 0 M = 1 ( D ; B = B 0 + D(3cos 2 2) ( D θ 1) + 4 sin 2 θ cos 2 2 ) θ 5 sin 4 θ 5 B 0 4B 0 2 pa, M = 3 ( D ; B = B 0 + 2D(3cos 2 2) θ 1) 32 sin 2 θ cos 2 θ + B 0 ( D 2 B 0 ) sin 4 θ + 2pa. TABLE I: Spin Hamiltonian parameters for Mn 2+ in DGCCT together with other host lattices. Host g A B D E a Reference DGCCT Site I ± ±2 93±2 276±2 58±2 20±1 Present Site II ± ±2 93±2 275±2 59±2 22±1 work TMATC-Zn [1] AOM [2] SHOD Site I [3] Site II A, B, D, E and a are all in units of 10 4 cm 1. where B 0 = hν/gµ B is the field at which a line would occur if all the fine structure terms are zero. θ is the angle of rotation. The parameter p due to the cubic field is given by the expression p = (1 5φ), where φ = (l 2 m 2 + m 2 n 2 + n 2 l 2 ); (l,m,n) being the direction cosines of B with respect to the axes of the cubic crystal field. The values of g, A, B, D, E and a for Mn 2+ in DGCCT obtained using a computer are given in Table I together with other host lattices. The signs of the parameters A and B are taken to be negative [18]. D and a have opposite signs [18], this gives a as being negative. The position of the centre of five groups each of six lines (taken as midway between the central lines of each group) and the calculated B from Eq. (2) in three orthogonal planes c b, ac and ab are plotted and are shown in Fig. 2(a c) (fine structure plot). From Fig. 2, the collapse of the fine structure near θ =55 is verified [17]. The principal axes of the ZFS D tensor are determined by searching extrema in the fine structure spreads of the EPR spectra along three orthogonal crystal directions [10]. The direction of the greatest separation is considered as the Z-axis, the direction of the next greatest separation is defined as the Y -axis and the third as the X-axis of the ZFS tensor. The maximum separation of the resonance fields due to the D

6 676 EPR AND OPTICAL ABSORPTION STUDIES OF... VOL. 48 tensor was recorded along a direction in the ac and ab planes (Fig. 2) of the crystal, and this direction was assigned as the Z-axis of the D tensor. From the angular dependence of the resonance fields on the c b plane (Fig. 2), the two other principal axes X and Y of the ZFS tensor are determined to be about b and c, respectively. Direction cosines of the distortion axis have been calculated [25] and are given in Table II along with the direction cosines of the different bonds estimated from the crystal structure data. The percentage of covalency of the Mn-ligand bond is calculated from Matumura s plot [25, 26]. The covalency of the bond between manganese and its ligand will also affect the value of the isotropic hyperfine coupling constant [27]. The covalency C of a bond between the atoms P and Q is approximately related to their electronegativities χ P and χ Q [28] by the relation C = {1 0.16(χ P χ Q ) 0.035(χ P χ Q ) 2 }/n, (3) where n is the number of neighbour bond atoms. Using the values of χ Mn = 1.4 and χ N = 3.0 and χ O =3.5, the percentage of covalency is obtained to be 9. The value of the hyperfine splitting constant cm 1 predicted from the graph [26] agrees reasonably well with the observed value of cm 1, [(A + 2B)/3] for both sites. V. OPTICAL ABSORPTION ANALYSIS TABLE II: Distances, direction cosines of different bonds, and the distortion axis of Mn 2+ derived from EPR. Bonds Ionic distances (nm) Direction cosines a b c* Ca-O(11) ± ± ± Ca-O(21) ± ± ± Ca-O(12) ± ± ± Ca-O(22) ± ± ± Ca-N(11) ± ± ± Ca-N(12) ± ± ± Distortion axis Site I Site II In a strong cubic crystalline field Mn 2+ 3d 5 electrons are distributed in the t 2g and e g orbitals. Thus the ground state configuration is written as (t 2g ) 3 e 2 g. This configuration gives the electronic states 6 A 1g, 4 A 1g, 4 E g, 4 T 1g, 4 T 2g, 4 A 2g, 4 A 2g (F), 4 T 1g (F) and a number of doublet states. Of these 6 A 1g lies lowest. The other electronic configurations like (t 2g ) 4 e g, (t 2g ) 2 e 3 g, and t 2ge 4 g give rise to several doublet and quartet states. Thus, all the absorption bands of high-spin Mn 2+ result from spin forbidden transitions.

7 VOL. 48 RAM KRIPAL AND MANISHA BAJPAI 677 Fig. 3(a) Fig. 3(b) FIG. 3: Optical absorption spectrum of Mn 2+ doped DGCCT single crystal at room temperature in the wavelength range (a) nm, (b) nm. The observed optical absorption spectrum at room temperature is shown in Fig. 3(a b). The spectrum consists of ten main bands located at 13889, 15385, 19417, 20833, 21739, 23529, 25157, 28986, 32216, and cm 1. In addition to the above, one weak band at cm 1 is observed. Among the bands observed in the present study, the bands at 19417, 20833, and cm 1, are found to be sharp. Ligand field bands are sharp when the energy expressions for the transitions are independent of D q, because the number of t 2g electrons is the same in both the ground and excited states [29]. The two states 4 A 1g (G) and 4 E g (G) are normally degenerate, but the covalency in the crystal often lifts their degeneracy [30]. Therefore the bands at and cm 1 are attributed to the 4 A 1g (G) and 4 E g (G) states, respectively. The third sharp band at cm 1 is assigned to the transition 6 A 1g (S) 4 T 1g (P). With the help of the Tanabe-Sugano diagram [31] the bands at 13889, 15385, 21739, 23529, 25157, 32216, 38986, and cm 1 are assigned to the 4 T 1g (G), 4 T 2g (G), 4 T 2g (D), 4 E g (D), 4 T 1g (F), and 4 A 2g (F) states, respectively. The wave numbers of the bands are given in Table III together with their assignments. The energy levels are calculated using the Racah parameters (B and C), the crystal field splitting parameter (D q ), and the trees correction (α). The correction term is relatively small, and so it is arbitrarily fixed at the free ion value of 76 cm 1. The energy matrices including the trees correction have been given by Mehra [32]. The electrostatic parameters B and C are evaluated from the energy states 4 E g (G) and 4 E g (D), which are independent of D q.

8 678 EPR AND OPTICAL ABSORPTION STUDIES OF... VOL A 2g (F) Energy(cm -1 ) T 1g (F) 4 T 1g (P) 4 E g (D) 4 T 2g (D) 4 T 2g (G) 4 E g (G) 4 A 1g (G) 4 T 1g (G) Dq (cm -1 ) 6 A 1g (S) Fig. 4 FIG. 4: The energy level diagram of Mn 2+ in DGCCT single crystal showing the variation of the levels with D q for B = 770 cm 1 and C = 2322 cm 1 (the circles show the experimental energies). Taking the energy matrix for 4 E g (G,D) and substituting E = E(GS)+T = 35B+T, we get 13B + 5C + 12α T ( 2B + 4α) 3 ( 2B + 4α) = 0, (4) 3 14B + 5C + 14α T which on solving gives T = 1/2[(27B + 10C + 26α) ± (49B 2 188Bα + 196α 2 )]. (5) The solution for the above equation is T 1 = 1/2[(27B + 10C + 26α) (49B 2 188α + 196α 2 )], (6) T 2 = 1/2[(27B + 10C + 26α) + (49B 2 188α + 196α 2 )]. Taking only the positive value of the square root {49(T 2 T 1 ) 2-768α 2 }, we obtain B = [94α + {49(T 2 T 1 ) 2 768α 2 }]/49. (7)

9 VOL. 48 RAM KRIPAL AND MANISHA BAJPAI 679 TABLE III: The experimental and calculated energy values of Mn 2+ ions in DGCCT. Transition from 6 A 1g (S) Observed (cm 1 ) Calculated (cm 1 ) 4 T 1g (G) 13889(10) (10) 4 A 1g (G) 19417(9) E g (G) 20833(8) T 2g (G) 21739(6) T 2g (D) 23529(5) E g (D) 25157(7) T 1g (P) 28986(11) T 1g (F) 32216(12) A 2g (F) 38986(13) (15) B = 770 cm 1, C = 2322 cm 1, D q = 716 cm 1 and α =76 cm 1. Uncertainties are given in brackets. Assuming in Eq. (5) we obtain T = 1/2[(27B + 10C + 26α) ± (7B 14)α], (8) T 1 = 10B + 5C + 20α = 4 E g (G), Thus, T 2 = 17B + 5C + 6α = 4 E g (D). (9) C = (T 1 + T 2 27B 26α)/10. (10) We have taken T 1 as 6 A 1g (S) 4 A 1g (G), 4 E g (G), since the energies 4 A 1g (G), 4 E g (G) are normally degenerate, T 2 as 6 A 1g (S) 4 E g (D), and obtained B and C from Eqs. (7) and (10) as B=770 cm 1, C=2322 cm 1. The energy values for the quartet electronic states have been calculated [32] for different values of D q with B = 770 cm 1, C = 2322 cm 1, and α = 76 cm 1 and are plotted in Fig. 4. A good fit of the experimentally observed band positions is obtained for D q = 716 cm 1, as is seen from the graph in Fig. 4. The free ion value of the Racah parameters B and C are 960 and 3325 cm 1, respectively [22, 33]. In the present study, we obtain the values of B = 770 cm 1, C = 2322 cm 1, respectively. The considerable decrease in the value of the Racah electronic repulsion parameters (from 960 to 770 cm 1 and from 3325 to 2322 cm 1 ) indicates that there is strong covalent bonding between the central metal ion and the ligand.

10 680 EPR AND OPTICAL ABSORPTION STUDIES OF... VOL. 48 If one assumes that the Mn 2+ ion enters the DGCCT lattice substitutionally in the Ca 2+ position, one can expect from the crystal structure data [15] at least two magnetically distinct sites per unit cell. Thus, two sets of allowed Mn 2+ hyperfine lines are expected in any plane. In the present study, as two sets of allowed hyperfine lines are observed in all three planes, the Mn 2+ ion is expected to enter the lattice substitutionally. For identifying the distortion axis, we first rotate the crystal about the three mutually perpendicular axes, namely a, b, c*, and the maximum value for the fine structure splitting is calculated in each of these rotations. The maximum splitting in each plane i for the extreme lines is 4D (3cos 2 θ i 1). The value of D can be obtained from a powder spectrum by measuring the separation between an extreme set of sextets. The calculation of θ 1, θ 2, and θ 3 gives the angles between the magnetic field and the distortion axis in each rotation when the axis of rotation, the distortion axis, and the magnetic field are coplanar. Therefore, the angles the distortion axis makes with the orthogonal set of rotation axes are simply (90 θ i ) [25]. This will give only a rough idea of the distortion axis and for further correlation, this should be corroborated with the X-ray structure of the host lattice. From Table II, the direction cosines of the distortion axis nearly coincides with the direction cosines of Ca-N(12) and Ca-O(21) bonds calculated from the crystal structure data [15]. This shows a distorted octahedral substitutional site for the Mn 2+ ion in the DGCCT lattice. In addition, the ionic radius of Mn 2+ (0.80 Å) is less than the ionic radius of Ca 2+ (0.99 Å) [34]. Thus, the Mn 2+ ion can fit well at the place of Ca 2+. This supports the conclusion drawn on the basis of direction cosines. The above impurity site is further supported by an optical absorption study. As Mn 2+ is expected to replace Ca 2+ in DGCCT, the site symmetry should be rhombic. The 4 E levels are associated with the half-filled strong-field configuration (t 2g ) 3 e 2 g, and hence cannot show first-order crystal field splittings. The degeneracy of the 4 E g (G), 4 A 1g (G) level is expected to be lifted in rhombic distortion [35]. The band assigned to 6 A 1g (S) 4 E g (G), 4 A 1g (G) is sharp and exhibits splitting (Fig. 3), thus suggesting the Mn 2+ ion to be in a rhombically distorted octahedral site. This is in agreement with the EPR investigations for the manganese ion in struvite and zinc-struvite [36].The g-value is close to the free spin value of The observed deviations g = g , are of either sign, the larger one being positive. The negative shift is observed in compounds where a fair amount of covalent bonding would be expected. Watanabe [37] has elaborated that in the presence of covalent bonding excited sextets of symmetry 6 T 1g are present, whose single electron states will tend to be full or empty as the electrons are transferred to or from the central ion by the action of bonding. A second-order shift is then possible in the g-value, whose sign depends on the direction of electron transfer. The deviation g = g = for Site I and for Site II indicates that the electrons are transferred from the central ion to the ligand. The g-value obtained in the present study is consistent with the results obtained by earlier workers [38 40].

11 VOL. 48 RAM KRIPAL AND MANISHA BAJPAI 681 VI. CONCLUSIONS The ESR study of Mn 2+ doped DGCCT has been done at LNT. The spin Hamiltonian parameters g, A, B, D, E, and a have been determined. The results indicate that Mn 2+ ion substitutes in place of Ca 2+ ion. The covalency has also been determined, which gives good agreement with the observed hyperfine structure constant. The optical absorption study has been done at room temperature, and the bands observed have been assigned to the transitions from the 6 A 1g (S) state to various excited levels of Mn 2+ ion. The observed band positions have been fitted with Racah parameters (B and C) and the crystal field splitting parameter (D q ). The data of B and C indicated a strong covalent bonding between the central metal and the ligand. Acknowledgment The authors are thankful to Dr. T. K. Gundu Rao, Senior Scientific Officer, Sophisticated Analytical Instrument Facility (SAIF), I.I.T., Powai, Mumbai for providing the facility of the EPR spectrometer. References Electronic address: ram_kripal2001@rediffmail.com [1] R. Kripal and M. Maurya, Mat. Chem. Phys. 108, 257 (2008). [2] R. Kripal and V. Mishra, Solid State Commun. 134, 699 (2005). [3] R. Kripal and D. K. Singh, Spectrochim. Acta A 69, 889 (2008). [4] R. Kripal and V. Mishra, J. Magn. Reson. 172, 201 (2005). [5] H. A. Kuska and M. T. Rogers, Radical Ions, ed. E. T. Kaiser and L. Kevan (Interscience, New York, 1968). [6] J. A. Weil, J. R. Bolton, and J. E. Wertz, Electron Paramagnetic Resonance: Elementary Theory and Practical Applications (Wiley, New York, 1994). S. Guner, F. Yildiz, B. Rameev, and B. Aktas, J. Phys.: Condens. Matter 17, 3943 (2005).. [7] B. Aktas et al., J. Magn. Magn. Mat. 258, 409 (2003). [8] N. O. Gopal, K. V. Narsimhulu, and J. L. Rao, J. Phys. Chem. Solids 63, 295 (2002). [9] S. K. Misra in: Handbook of ESR (Vol.2), eds. C. P. Poole Jr., H. A. Farach (Springer, New York, 1999), Chapter IX, p [10] H. Anandlakshmi, K. Velavan, I. Sougandi, R. Venkatesan, and P. S. Rao, Pramana 62, 77 (2004). [11] P. S. Rao, Spectrochim. Acta A 49, 897 (1993). [12] S. K. Misra, Physica B 203, 193 (1994). [13] B. R. McGarvey, Electron Spin Resonance of Transition Metal Complexes ( Transition Metal Chemistry, Vol. 3), ed. R. L. Carlin (Marcel Dekker, New York, 1966). [14] S. Natarajan and J. K. Mohana Rao, Curr. Sci. 45, 490 (1976). [15] S. Stoll and A. Schweiger, J. Magn. Reson. 170, 42 (2006). [16] B. Bleaney and D. J. E. Ingram, Proc. Roy. Soc. A 205, 336 (1951). [17] A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of Transition Ions (Clarendon,

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