ESR and optical absorption spectra of Cu2+ in LiKSO4

Volume 101A, number 2

PHYSICS LETTERS

12 March 1984

ESR AND OPTICAL ABSORPTION SPECTRA OF Cu 2+ IN LiKSO 4 S.V.J. LAKSHMAN and A. SUNDAR JACOB

Spectroscopic Laboratories, Department of Physics, S. V. University, TirupatL 517502, India Received 7 March 1983 Revised manuscript received 11 November 1983

ESR spectra of C u 2 + in LiKSO4 have been studied at different temperatures. The measured g-values suggest a rhombic field for Cu 2÷ ion in the lattice. Optical absorption spectra of the crystal have been studied at room and liquid nitrogen temperatures. From the nature and position of the observed bands, they have been attributed to Cu 2* ions in D2h symmetry. The orbital reduction parameters obtained indicate a substantial degree of covalency in the bonding of the copper ion.

1. Introduction. The Cu 2+ impurity has been well characterized in various hydrated double sulphates [ 1 - 5 ] . Lithium potassium sulphate, LiKSO 4 (herein after referred to as LPS) has been the subject o f many investigations for the past several years [ 6 - 9 ] . However, there is no information in the literature on the ESR and optical absorption spectra o f Cu 2+ in these crystals. Therefore the authors have undertaken these studies in order to determine the site symmetry o f the ion in the crystal. LPS crystal is hexagonal and its space group is C6(P63) with two formula units per unit cell of the edge lengths [6,10] a = 5.1457 A

and

c = 8.6298 A

(26°C).

The four oxygens o f the SO 4 group are situated at the four corners o f a regular tetrahedron with the sulphur atom at the centre. The lithium ions alternate with the sulphur ions. Each potassium ion is surrounded by six sulphate ions and each sulphate ion contributes one oxygen to the potassium ion forming approximately an octahedron.

2. Experimental. Crystals o f LPS doped with Cu 2+ were grown by slow evaporation at room temperature from an aqueous solution containing equimolar weights o f lithium sulphate and potassium sulphate to which 0.1 mole percent of copper sulphate was added as impurity. 0.375-9601/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

ESR measurements were carried out on powder samples on a JEOL FE 1X X-band spectrometer operating at 9.21 GHz with 100 kHz field modulation. Measurements were made at low temperatures using a Dewar provided with a variable temperature controller. Optical absorption spectra were recorded both at room and liquid nitrogen temperatures on a Cary-14 spectrophotometer with a crystal of about 1 mm thickness in the wavelength region 2 0 0 - 1 5 0 0 nm. The oscillator strengths o f the absorption bands were calculated using the expression reported by Ballhausen [ 11 ] f = 4.6 × 10 . 9 (A/bc) AVl/2 , where A is the maximum value of the absorbance o f the band, b is the thickness of the crystal, c is the concentration o f the impurity dopant and AVl/2 is the half band width.

3. Results and discussion. ESR spectra of the sample were recorded at different temperatures in the temperature range 3 0 0 - 1 2 0 K. Due to some experimental limitations, the ESR spectra were recorded only in the powder form. The powder spectrum, which is a superposition of spectra from randomly oriented single crystals, is instructive however. Powder spectra measurements enable one to compare similar spectra in different samples. F r o m these spectra one can obtain the salient features pertinent to the calculation o f 109

Volume 101A, number 2

PHYSICS LETTERS

12 March 1984

g2 300 K 120 K

/

I

"

l I

! l

t g3 I

I

L

2600

I

i

2800

I

I

3000

I

L

I

31.00

3200

H (G)

Fig. 1. Polycrystalline ESR spectra of Cu2 ÷ in LiKSO4 at 300 K and 120 K.

the spin-hamiltonian constants [ 1 2 - 1 4 ] . Furthermore, such observation provides a convenient way for studying the temperature variation of the ESR spectrum. The ESR spectra recorded at 300 K and 120 K are shown in fig. 1. The polycrystalline sample shows a typical three g-value spectrum expected for a six-coordinated copper complex with rhombic distortion [3, 15]. The ESR spectrum at room temperature consists of four lines in the low field (corresponding to g l ) , one broad line in the mid-field (corresponding to g2) and four lines in the high field (corresponding to g3)The structure o f the broad line is well resolved at low temperatures. The separations o f the hyperfine peaks at 120 K are 85, 87.5 and 85 G in the low field, 20, 22.5 and 20 G in the mid-field and 65, 70 and 67.5 G in the high field regions respectively. With decreasing temperature, the g l value increases and the g2 value decreases while the g3 value remains temperature invariant. The g-values obtained at different temperatures are presented in table 1. The observed variation o f g-factors is characteristic of a rhombic field around 110

the Cu 2+ ion [3,16]. These variations are similar to those observed for copper doped K2Zn(SO4)2.6H20 [3] and zinc(II) bis(pyridine-3-sulphonate) hydrate [16] systems in which Cu 2÷ experiences a rhombic distortion. The large value o f g 1 is typical of C u - O bonding [ 17]. The principal g and A values at 120 K

Table 1 g-values of Cu2÷ in LiKSO4 at different temperatures. Temperature (K)

gl

g2

g3

300 275 250 225 200 175 150 125 120

2.3735 2.3756 2.3756 2.3756 2.3756 2.3756 2.3756 2.3786 2.3810

2.1990 2.1935 2.1898 2.1898 2.1826 2.1808 2.1772 2.1772 2.1754

2.0341 2.0341 2.0341 2.0341 2.0341 2.0341 2.0341 2.0341 2.0341

Volume 101A, number 2

PHYSICS LETI'ERS

12 March 1984

g l = 2.3810,

A 1 = 95 × 10 - 4 cm 1 ,

g2 = 2.1754,

A 2=21×

10 - 4 c m - 1 ,

The free ion dipolar term (P), Fermi contact term ( K ) and the bonding parameter (a 2) are calculated from the following expressions reported by Kato and Abe [21]:

g3 = 2.0341,

A 3=63×

10 - 4 c m -1

P = 14(A 2

are

7

The g and A values obtained are consistent with what one would expect for Cu 2+ in rhombic symmetry. These values are comparable with other double sulphates in which the presence o f rhombic perturbation was observed [3,4,18]. From the measuredg-values, it is also possible to get the most precise information on the electronic ground state of the ion in the crystal. It has been suggested by Dudley and Hathaway [ 19] that for rhombic g-val-

a2 = g [(A 3

";E

11 6 Ag2] -A1)/P-Agl+i-4 Ag3--7-4

K=A2/P+'q2 a2

+ A g 2 _ ~ 4 Ag3

where A g 1 = ge -- g l , Ag2 = ge -- g2 and Ag 3 = ge - g3 (ge being 2.0023). The values of P, a 2 and K are found to be - 0 . 0 2 4 5 cm - 1 , 0.6517 and - 0 . 0 6 5 9 respectively. The theoretical estimate of P is 0.036 cm -1 . The value o f a 2 is close to unity for ionic bonds and becomes smaller with increasing covalent bonding. Thus a substantial amount o f covalency is indicated. The optical absorption spectra of the crystal recorded at room and liquid nitrogen temperatures are shown in fig. 2. In all, four bands have been observed at room temperature, two broad and intense bands at 9610 and 11590 cm - t , one band in the form o f a shoulder at 10280 cm - t and one weak band at 13060 cm -1

ues (with g 1 >g2 >g3), if(g2 - g 3 ) / ( g l ~-72) = R is greater than unity, then a predominantly d z ground state is present. For a dx2y2 ground state, the value o f R is expected to be less than unity. The observed R value of 0.687 for the present sample therefore suggests a predominantly dx2_yZ ground state for the copper ion [20].

l

-A3)/17(Ag 3 -- Ag2) ,

_

O,,

0.5

o~ ®

-;

o

~-

o

E

o.z, p..

0'3

O.2

o

\

0.1

I 800

I 900

1 1000 WAVELENGTH

Fig. 2. Optical absorption spectra o f

Cu 2÷

I 1100

\

I 1200

(nm)

in LiKSO4 (a) at 300 K and (b) at 77 K. Ill

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Table 2 The observed energies, oscillator strengths and assignments of the bands of Cu2+ in LiKSO4 Band positions 300 K

Oscillator strengths (f X 10s)

77 K

h (nm)

v (cm -1 )

~. (nm)

v (cm-1 )

1041 973 863 766

9610 10280 11590 13060

1038 963 838 763

9640 10390 11940 13110

On cooling the crystal to liquid nitrogen temperature, changes in intensities as well as positions of the bands have been observed. At this temperature, the bands are located at 9640, 10390, 11940 and 13110 cm -1 . For Cu 2+ with d 9 configuration, the ground state 2D is split into 2Eg and 2T2g states in the presence of an octahedral field. With tetragonal distortion, the degeneracy of the states is further removed and 2E~ is split into 2Blg and 2Alg while 2T2g is split into 2B2g and 2E. states. In a rhombic field (D2h), 2E_ is split into 2Ag(dx2 y2) and 2Ag(d 2) while 2T2g is split into 2B lg (dxy), 2B2g (dxz) and 2B3g(dyz) states. From the nature and relative positions of the observed bands, they have been attributed to the Cu 2+ ion in D2h symmetry with dx2 _y2 as ground state. In most of the copper compounds having a distorted octahedral structure, the transition dx 2 _y2 lies close in energy to the other d - d transitions, i.e., 1 0 0 0 0 - 1 4 0 0 0 cm -1 and it has been assigned with this range in a number of copper complexes [22,23]. Indeed, the spectra of some of the Tutton salts have previously been assigned on this basis [24,25]. Therefore the band observed at 9610 cm -1 has been assigned to the transition dx2 _y2 ~ dz2. The remaining three bands at 10280, 11590 and 13060 cm -1 have been assigned to the transitions dx2 _y2 ~ dxy, dx2 _y2 ~ dxz and dx2 _y2 ~ dy z, respectively, in increasing order of energy. Similar assignments have been made by several authors [26,27] for the bands of Cu 2÷ in D2h symmetry. In D2h symmetry, the transition d x 2 _y2 -+ dxy is forbidden in the z-polarization while the transition dx2_y2 ~ dy z is forbidden in the x-polarization [26]. For the present crystal, considerable decrease in intensity for the dx2_y2 ~ dxy transition in the z-polarization and the dx2 _y2 ~ dy z transi112

Assignment w.r.t, the ground state

300 K

77 K

dx2_y2 (2Ag)

4.0 2.6 5.9 1.1

4.0 1.3 4.9 1.1

dz2 (2Ag) dxy (2Btg) dxz (2B2g) dy z (~B3g)

tion in the x-polarization further confirms the assignments. These assignments yield a tentative one-electron orbital sequence as dx 2_y2 < d 2 < dxy < dxz < dy z. All of the absorption bands exhibited blue shift when the crystal was cooled to liquid nitrogen temperature. Such shifts are usually observed for a six coordinated Cu 2+ ion as theoretically predicted by Dunn [28]. The observed band positions and their oscillator strengths along with their assignments are presented in table 2. ESR and optical absorption data can be correlated to obtain the combined orbit and spin-orbit reduction parameters by the following expressions [26].

gl = 2 -- 8Xk21cos2 /3/ A E l ( d x 2_y2 ~ dxy) , g2 = 2 - 2Xk2(cos/3 + v ~ s i n 13)2/ A E 2 (dx2 _y2 ~ dxz), g3 = 2 - 2Xk2(cos 13 - X / ~ sin/3)2/

zxE3 (dx2_y2 --' dyz), where X = - 8 3 0 cm -1 for the free ion of copper and /3 is the angle related to rhombic perturbation of the axial field. From the above three expressions, one can evaluate the values of either k l , k 2 and k 3 assuming /3 to be zero or kll, k I and/3 with the approximation that k 2 ~ k 3 = k± and k 1 = kll. Since g2 and g3 as well as AE 2 and A E 3 observed in the present work are close to each other, it is assumed that k 2 ~ k 3 = k± and k 1 = kl[. The values of 13, kll and k± thus evaluated are found to be 11.75 °, 0.79 and 0.83, respectively. The values of the reduction parameters obtained agree well with those of copper Tutton salts in which the rhombic perturbation was observed [5].

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4. Conclusion. It is clear from the ESR studies that Cu 2+ is characterized by a rhombic g tensor and not axial. Moreover, the difference between gl and g2 becomes larger as the temperature is lowered while g3 is not affected. Also the g-values obtained are close to those expected from the ground state which is predominantly Ix 2 y2). From the values of the parameters P, a 2 and K which give the nature of the bonding it is found that Cu 2÷ doped in LPS is bound by a mixture of ionic and covalent bonds with the surrounding atoms [21]. For most Tutton salts, the hyperfine lines of Cu 2÷ are resolved only at low temperatures (20 K). It is found that well-resolved hyperfine lines could be obtained even at room temperature for this crystal indicating a smaller spin-lattice interaction. The large value o f g 1 and the temperature variation of g-factors are consistent with Cu 2+ in LPS situated in a rhombic field. To confirm whether Cu 2+ enters the lattice substitutionally or interstitially, a more detailed single crystal sudy is required. Since the potassium ion in the crystal is surrounded by six oxygens in an octahedral fashion and since the ionic radius of Li + (0.68 A) is less that of Cu 2+ (0.72 A), it may be assumed that Cu 2+ in the case of LPS might substitute for K + and electrostatically bond to a neighbouring Li + vacancy as has been observed for Mn 2+ in lithium potassium tartrate [29]. The authors wish to express their thanks to Professors J. Sobhanadri and S. Radhakrishna, I.I.T., Madras for permission to use their Cary-14 spectrophotometer. One of the authors (AS J) is grateful to the Department of Science and Technology, New Delhi, India for financial assistance.

References [ 1] D.M .S. Bagguley and J.H.E. Griffiths, Proc. Phys. Soc. A65 (1952) 594. [2] B. Bleaney, K.D. Bowers and D.J.E. Ingram, Proc. Roy. Soc. A228 (1955) 147.

12 March 1984

[3] B.L. Silver and D. Getz, J. Chem. Phys. 61 (1974) 638. [4] B.A. Sastry and G.S. Sastry, J. Phys. C4 (1971) L347. [5] M.A. Hitchman and T.D. Waite, lnorg. Chem. 15 (1976) 2150. [6] A. Bradley, Philos. Mag. 49 (1925) 1225. [7] J.P. Mathieu, L. Couture and H. Poulet, J. Phys. Radium 16 (1955) 781. [8] J, Hiraishi, N. Tanguchi and H. Takahashi, J. Chem. Phys. 65 (1976) 3821. [9] M.L. Bansal, S.K. Deb, A.P. Roy and V.C. Sahni, Solid State Commun. 36 (1980) 1047. [10] R.W.C. Wyckoff, in: Crystal structures, Vol. 3, 2nd Ed. (lnterscience, New York, 1965) p. 112. [11] C.J. Ballhausen, Prog. Inorg. Chem. 2 (1960) 25 l. [12] R. Borcherts and C. Kikuchi, J. Chem. Phys. 40 (1964) 2270. [13] F.K. Kneubuhl, J. Chem. Phys. 33 (1960) 1074. [14] J. Rubio O., E. Munoz P., J, Boldu O., Y. Chen and M.M. Abraham, J. Chem. Phys. 70 (1979) 633. [ 15 ] B,J. Hathaway and D.E. Billing, Coord. Chem. Rev. 5 (1970) 143. [16] B. Walsh and B.J. Hathaway, J. Chem. Soc. Dalton Trans. 4 (1980) 681. [17] V.G. Krishnan, J. Phys. C l l (1978) 3493. [18] K.D. Bowers and J. Owen, Rep. Prog. Phys. 18 (1955) 304. [19] R.J. Dudley and B.J. Hathaway, J. Chem. Soc. (A) (1970) 2799. [20] D,E. Billing, R.J. Dudley, B.J. Hathaway and A.A.G. Tomlinson, J. Chem. Soc. (A) (1971) 691. [21] T. Kato and R. Abe, J. Phys. Soc. Japan 35 (1973) 1643. [22] O.G. Holmes and D.S. McClure, J. Chem. Phys. 26 (1957) 1686. [23] R.A. Palmer, C.G. Roy and R.C. Roy, J. Chem. Soc. (A) (1971) 3084. [24] R.C. Marshall and D.W. James, J. Phys. Chem. 78 (1974) 1235. [25] R.C. Marshall and D.W. James, J. Inorg. Nuel. Chem. 32 (1970) 2543. [26] D.E. Billing and B.J. Hathaway, J, Chem. Soc. (A) (1968) 1516. [27] T.D. Waite and M.A. Hitchman, lnorg. Chem. 15 (1976) 2155. [28] T.M. Dunn, in: Modern coordination chemistry (Interscience, New York, 1967). [29] A.K. Jain, Mol. Phys. 38 (1979) 2037.

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