EPR spectra of Cu2+ in KH2PO4 single crystals

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Spectrochimica Acta Part A 69 (2008) 174–177

EPR spectra of Cu2+ in KH2PO4 single crystals Recep Biyik a,∗ , Recep Tapramaz b a

T¨urkiye Atom Energy Institution, Sarayk¨oy Nuclear Research and Training Center, 06983 Kazan, Ankara, Turkey b Ondokuz Mayıs University, Faculty of Arts and Sciences, Department of Physics, 55139 Samsun, Turkey Received 20 December 2006; accepted 18 March 2007

Abstract Cu2+ doped single crystals of KH2 PO4 were investigated using EPR technique at room temperature. The spectra of the complex contains large number of overlapping lines. Five sites are resolved and four of them are compatible with the tetragonal symmetry, and the fifth one belongs to an interstitial site. The results are discussed and compared with previous studies. Detailed investigation of the EPR spectra indicate that Cu2+ substitute with K+ ions. The principal values of the g and hyperfine tensors and the ground state wave function of Cu2+ ions are obtained. © 2007 Elsevier B.V. All rights reserved. Keywords: EPR; KDP; Cu2+ ; Ground state wave function

1. Introduction Paramagnetic Cu2+ ions are frequently used as probe in crystalline host materials reflecting the local symmetry and the structural properties of the host. Therefore, the EPR spectra of Cu2+ ion in different diamagnetic host lattices have been studied by many workers to get information about the structure, dynamics and environment of the host lattices [1–9]. Potassium di-hydrogen phosphate KH2 PO4 (KDP) and the KDP type crystals are well known crystal group for their significance in scientific and technological applications. The crystals of the family show nonlinear electromechanical behavior for acoustic applications [10]. They also show electro-optical effect. This nonlinear optical property of KDP type crystals is utilized in optics, especially for laser applications to convert the frequency of a coherent radiation to different one and to mix different frequencies (Pockels effect). They have very high optical damage thresholds and this can be exploited in intense laser beam applications [11,12]. Very large and highly perfect KDP single crystals (of the order of 60 cm wide or more) can be grown [13]. KDP undergoes a paraelectric phase transition at 122 K and the symmetry changes from tetragonal to orthorhombic. Because of its wide applications in technology, impurities in KDP crystals, including divalent and trivalent metal ions, ∗

Corresponding author. Tel.: +90 312 8154300; fax: +90 312 8154307. E-mail addresses: [email protected], [email protected] (R. Biyik). 1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2007.03.029

are introduced and investigated to see the effects on optical, electrical and other physical properties; and also the effects on the crystal growing mechanism and face morphology [14–19]. Divalent and trivalent metal ions occupy mainly different locations. Trivalent metal ions are generally adsorbed on the surface layer, but in a specific study it is seen that trivalent Fe3+ ions occupy the FeO2− site in the form of FeO4 2− by compensating the charge deficiency via nearby potassium or hydrogen vacancy [17]. Some other groups, like dyes, are also introduced into KDP to see the effects on optical properties, growth mechanism and face morphology [20,21]. Two EPR studies of Cu2+ doped KDP single crystal are reported almost simultaneously in 1968 and 1969 [14,15]. In both of the studies, the structure is assumed to be purely axial, and the resolution of the spectra made hypothetically rather then experimentally. Since the spectra of this crystal is highly complex and poorly resolvable, it would worth while to repeat the resolution of Cu2+ doped KDP crystal to report some additional properties. As will be explained, there are some similarities and some differences due probably to crystal growing conditions and the techniques employed in analysis. 2. Experimental KDP was obtained commercially. A near-saturated aqueous solution was prepared and a two weight percent from CuSO4 was added into it. The solution was left for slow evaporation.

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Well-developed single crystals of suitable sizes were obtained after several days. ¯ The crystal have tetragonal symmetry (42m) at room temper˚ and c = 6.9751 A. ˚ ature with unit cell parameters a = 7.4529 A The unit cell contains four formula units [13,15]. EPR spectra were recorded with a Varian E-109 X-band EPR spectrometer using 2 mW microwave power and a 1.2 G magnetic field modulation frequency of 100 kHz. The single crystal was glued on a quartz pillar of a goniometer graded in degrees and rotated at 5◦ intervals in three mutually perpendicular planes, respectively. The powder spectra of the samples in a quartz tube were recorded. The spectrometer frequency was corrected using the DPPH (dihenylpicrylhydrazyl) sample (g = 2.0036). Simulations of the powder spectra was made using Bruker’s WINEPR software. 3. Result and discussion EPR spectra of Cu2+ doped KDP single crystal were taken in three mutually perpendicular planes xy, zx and yz (coincides to ab, ca and bc planes of tetragonal axes where a = b) at each 5◦ . The spectra obtained obviously arises from Cu2+ with I = 3/2. When magnetic field was parallel to z axis (H//z), a quartet was obtained, Fig. 1(a). At this orientation the whole spectra superimposed onto the simple spectrum. The inner two lines at this orientation split into doublets and outer lines split into triplets. Doublets indicate that there are two structurally different paramagnetic centers with close EPR parameters and triplets arise from 65 Cu2+ and 63 Cu2+ isotopes. Fig. 1(b) and (c) show that the EPR spectrum of Cu2+ doped KDP single crystals in the yz plane with the magnetic field inclined by 85◦ and 140◦ to the z axis, respectively. In Fig. 2(c), however, a large number of unidentifiable lines are seen. Since the lines overlap almost in all orientations and hence are untraceable, the identification and the precise resolution are almost impossible on the spectra. But, if we consider the behaviors of spectra together with the plots of line positions in three planes, Fig. 2, resolution becomes easier. A super hyperfine splitting is also observable in the low field lines of Cu2+ having the splitting constant of 1.1 mT with intensities 1:2:1, which indicates the existence of two equivalent

Fig. 1. EPR spectrums of Cu2+ doped KDP single crystal with the magnetic field (a) parallel to z axes, (b) inclined by 85◦ and (c) 140◦ relative to z axis in the yz plane.

Fig. 2. Variation of the g2 values in three planes of Cu2+ doped KDP single crystal. The solid lines represent the least squares fitted values.

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Fig. 3. Powder spectrum (a) and the simulated spectrum (b) of Cu2+ doped KDP.

hydrogen atoms in the neighborhood, Fig. 2. This super hyperfine splitting is not observable in the high field lines of Cu2+ ion due to overlapping. Fig. 3 shows the powder and simulated spectrum of Cu2+ doped KDP. These components of hyperfine and g values obtained from the powder spectrum are also given in Table 1. The parameters are comparable with those of Cu2+ ion in octahedral environment with slight rhombic distortion [5,6,22–24]. The spectra can be fitted to the rhombic Hamiltonian for d9 ions given in Eq. (1): H = He + HHF + HSO + HCF

(1)

The terms represent electron Zeeman, hyperfine, spin–orbit and crystal field interactions, respectively. Nuclear Zeeman and quadrupole interactions are neglected. Each term can be expressed explicitly [25,26], and the Hamiltonian can be solved for rhombic environment giving equalities for principal hyperfine splitting constants:  √ k[2(α2 + β2 ) − 4 3αβ] + (gx − ge ), Ax = P −κ + 7   √ √  1 3α + 3β 3β √ − (gy − ge ) + (gz − ge ) , 14 α − 3β 14α  √ k[2(α2 + β2 ) + 4 3αβ] Ay = P −κ + + (gy − ge ) 7   √  √ 1 3α − 3β 3β √ − (gz − ge ) , (gx − ge ) − 14 α + 3β 14α   √  4k(α2 + β2 ) 1 3α − 3β √ Az = P −κ + + (gz − ge ) − 7 14 α + 3β   √  1 3α + 3β √ × (gx − ge ) + (gy − ge ) (2) 14 α − 3β The parameter k is covalency factor, κ is polarization constant, α and β are mixing coefficients for dx2 −y2 and d3z2 −r2 orbitals and satisfy the normalization relation α2 + β2 = 1. P is dipolar hyperfine parameter for metal ion given as P = kP0 , where free ion value P0 for 65 Cu2+ is P0 ≈ 388 × 10−4 cm−1 and for 63 Cu2+ is P0 ≈ 416 × 10−4 cm−1 [25]. These equations can be solved for k, κ, α and β and the ground state wave function of metal ion can be constructed as follows:      ψ = k α dx2 −y2 + β d3z2 −r2 (3)

Table 1 Principal values, direction cosines of the g and A tensors of the Cu2+ doped KDP (magnetic field is measured within the error H = ±0.5 mT) Site

g

Direction cosiness x

A (mT) y

z

Direction cosiness x

y

z

I

gxx = 2.105 gyy = 2.047 gzz = 2.388

0.711 0.254 0.559

0.193 −0.883 0.426

0.675 0.194 −0.711

Axx = 1.4 Ayy = 5.9 Azz = 13.5

0.819 −0.198 −0.537

−0.358 0.909 0.426

0.447 0.365 −0.711

II

gxx = 2.096 gyy = 2.061 gzz = 2.416

−0.654 0.506 0.561

0.388 −0.862 −0.323

0.648 0.006 −0.761

Axx = 3.6 Ayy = 6.4 Azz = 13.7

−0.692 0.478 0.540

0.364 0.878 −0.309

0.622 0.017 0.721

III

gxx = 2.110 gyy = 2.022 gzz = 2.380

−0.726 −0.344 0.595

0.388 0.932 −0.125

0.598 0.110 −0.793

Axx = 3.0 Ayy = 6.6 Azz = 13.5

0.906 −0.132 0.401

0.256 0.927 −0.272

−0.335 0.349 0.874

IV

gxx = 2.123 gyy = 2.061 gzz = 2.352

0.754 0.201 0.624

0.163 −0.979 −0.118

−0.635 −0.013 0.772

Axx = 2.7 Ayy = 5.4 Azz = 12.9

0.909 −0.369 −0.188

0.383 0.922 0.039

0.159 −0.108 0.981

V

gxx = 2.124 gyy = 2.069 gzz = 2.314

−0.706 −0.013 0.708

0.040 0.997 0.059

0.707 −0.070 0.703

Axx = 5.7 Ayy = 6.9 Azz = 13.3

0.991 −0.001 −0.128

−0.015 0.991 −0.132

0.127 0.133 0.982

Powder spectrum values: gxx = 2.102; gyy = 2.064; gzz = 2.373; Axx = 3.0; Ayy = 5.4; Azz = 13.3.

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In most of the applications the Hamiltonian includes only electron Zeeman and hyperfine interactions, the spin orbit and crystal field interactions are included implicitly in the anisotropy of electron Zeeman and hyperfine interactions and therefore the Hamiltonian for practical applications in rhombic environment can be taken as follows: H = βe (gx Hx Sx + gy Hy Sy + gz Hz Sz ) + Ix Ax Sx + Iy Ay Sy + Iz Az Sz

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Table 2 Ground state wave function of Cu2+ ions in KDP Site

k

α

β

κ

I II III IV V

0.88 0.94 0.91 0.88 0.85

0.98 0.99 0.97 0.98 0.97

0.16 0.15 0.21 0.19 0.23

0.40 0.39 0.38 0.39 0.37

(4)

Five paramagnetic Cu2+ sites are resolved and labeled as sites I, II, III, IV and V. The intensities of all sites are comparable with each other. The site resolved in this work and labeled as Site V, in fact, is an interstitial site and is not reported in Refs. [14,15]. Moreover, the site with weak lines in Ref. [15] is formed probably due to phosphoric acid impurity included into the solution prior to crystallization, and is not seen in the spectra of this work. The hyperfine and g value variations are found and related tensors are constructed. Table 1 gives the principal hyperfine and g values of resolved five sites. Previous studies made on KDP assume purely axial structure [14,15]. The reported hyperfine values are A ∼ = 16 mT and A⊥ ∼ = 2 mT in both studies. Parallel component corresponds Az and perpendicular components correspond to Ax or Ay values of this work. The values are comparably different from the results of this study. The basic reason is the assumption of purely axial structure; in this work, however, rhombic structure is taken into consideration. Average values, in all works are close to each other within the experimental error. The g and g⊥ values found in the previous works change between 2.34–2.39 and 2.06–2.08, respectively, and are close to the corresponding results of this work. Cu2+ ions substitute with K+ ions in the host and compensate the negative charge deficiency via oxygen atoms of (PO4 )3− groups in the ligant positions. They are relatively close and equidistant to oxygen atoms of each [PO4 ]3− group where oxygen atoms are bridged via hydrogen atoms with the corresponding oxygen atom of the next group. The first four sites in Table 1 arise from similar Cu2+ locations in tetragonal environment. The fifth site is an interstitial site. The principal hyperfine and g values of four sites given in Table 1 show this similarity within experimental error. Although the hyperfine and g tensors are close to axial symmetry, both angular variations and principal values point out appreciable difference in perpendicular components. The difference comes out of the unequal oxygen distances forming the plane of the octahedron. Therefore the environment symmetry is rhombic rather then axial. The Hamiltonian given in Eqs. (1) and (4) are valid for these complex structures. The ground state wave function coefficients, Eq. (3), are calculated using the Eq. (2) for rhombic symmetry and given in Table 2. Unpaired electron occupies mainly d9 orbital of the central Cu2+ ion with an average density of 90% as expected in this type of complexes [1,3–6]. The rest of 10% of density is in the ligand orbitals. Therefore, the density in the ligand orbitals occu-

pying the equatorial plane of the octahedron is approximately 9%, and the rest of 1% is on the oxygen atoms in the apex positions. The hydrogen hyperfine splittings measured about 1.1 mT presumes that the unpaired electron density on hydrogen atoms is about 2% and is grater then the current density in the apexes. On the other hand, although all oxygen atoms of (PO4 )3− groups have hydrogen bonds in the crystal with the nearest oxygen atoms of other (PO4 )3− in the neighborhood, only two oxygen atoms in the equatorial plane are close to Cu2+ ion. The other oxygen atoms are comparably far from metal ion. References [1] R. Kripal, S. Misra, J. Phys. Chem. Solids 65 (2003) 939. [2] E.D. Mauro, S.M. Domiciano, Physica B 304 (2004) 398. [3] Y. Yerli, S. Kazan, O. Yalc¸ın, B. Aktas¸, Spectrochim. Acta Part A 64 (2006) 642. [4] S.K. Misra, X. Li, C. Wang, J. Phys. Condens. Matter 3 (1991) 8479. [5] B. Karabulut, R. Tapramaz, A. Bulut, Z. Naturforsch. 54a (1999) 256. [6] R. Bıyık, R. Tapramaz, B. Karabulut, Z. Naturforsch. 58a (2003) 499. [7] P. Huang, H. Ping, M.G. Zhao, J. Phys. Chem. Solids 64 (2003) 523. [8] M.R.S. Kou, S. Mendioroz, P. Salerno, V. Munoz, Spectrosc. Lett. 35 (2002) 565. [9] I. Sougandi, R. Venkatesen, P.S. Rao, Spectrochim. Acta Part A 60 (2004) 2653. [10] U. Straube, H. Bige, J. Alloys Compd. 310 (2000) 181. [11] I.P. Kaminow, Introduction to Electro-Optic Devices, Academic Press, New York, 1974. [12] F. Zernike, J.E. Midwinter, Applied Nonlinear Optics, John Wiley and Sons, New York, 1973. [13] For Large KDP Crystals Growing and Applications Visit: http:// clevelandcrystals.com. [14] H. Koga, K. Hukuda, J. Phys. Soc. Jpn. 25 (1968) 630. [15] A. Otani, S. Makhisima, J. Phys. Soc. Jpn. 26 (1969) 85. [16] T.A. Eremina, N.N. Eremin, V.A. Kuznetsov, T.M. Okhrimenko, N.G. Furmanova, E.P. Efremova, V.S. Urusov, Crystallogr. Rep. 47 (Suppl. 1) (2002) 76. [17] N.Y. Garces, K.T. Stevens, L.E. Halliburton, M. Yan, N.P. Zaitseva, J.J. DeYoreo, J. Cryst. Growth 225 (2001) 435. [18] J. Podder, J. Cryst. Growth 237 (2002) 70. [19] S. Seif, K. Bhat, A.K. Bata, M.D. Aggarwal, R.B. Lal, Mater. Lett. 58 (2004) 991. [20] N.Y. Garces, K.T. Stevens, L.E. Halliburton, S.G. Demos, H.B. Radousky, N.P. Zaitseva, J. Appl. Phys. 84 (1) (2001) 47. [21] S. Hirota, H. Miki, K. Fukui, K. Maeda, J. Cryst. Growth 235 (2002) 541. [22] T.F. Yen, Electron Spin Resonance of Metal Complexes, Plenum Press, New York, 1969. [23] F. K¨oksal, I. Kartal, B. Karabulut, Z. Naturforsch. 54a (1999) 177. [24] F. K¨oksal, B. Karabulut, Y. Yerli, Int. J. Inorg. Mater. 3 (2001) 413. [25] H.N. Dong, S.Y. Wu, P. Li, Phys. Status Solidi (B) 241 (8) (2004) 1935. [26] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Metal Ions, Oxford University Press, 1970.