EPR spectra of Cu2+ in KH2PO4 single crystals

EPR Spectra of Cu2+ in KH2PO4 Single Crystals

* Recep Biyika and Recep Tapramazb

aTürkiye Atom Energy Institution, Sarayköy Nuclear Research and Training

Center, 06983 Kazan Ankara, Turkey

bOndokuz Mayıs University, Faculty of Arts and Sciences, Department of Physics,

55139 Samsun, Turkey

Abstract

Cu2+ doped single crystals of KH2PO4 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.

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 KH2PO4 (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, 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 FeO2 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

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 2 weight percent from CuSO4 was added into it. The solution was left for

slow evaporation. Well-developed single crystals of suitable sizes were obtained after

several days.

The crystal have tetragonal symmetry ( 42m ) at room temperature with unit cell

parameters a = 7.4529 Â and c = 6.9751 Â. The unit cell contains 4 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 5o 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

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 5o 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 65Cu2+ and 63Cu2+ isotopes. Fig. 1(b) and 1(c) show

that the EPR spectrum of Cu2+ doped KDP single crystals in the yz plane with the

magnetic field inclined by 85o and 140o 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 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

H — H e

+

+

+

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;

A — P A — P_y A — P k [ 2 (a ‘ + / ) - 4 > /3 a /] 1 -K + ---+ (g x - g e) - — 7 1A k [ 2 (a 2 + / ) + A ^ J ia /] 1 -K + ---+ (g - g ) - — 7 y 1A 3 a + 4 3 / ' ] 4 3 / (.gy - ge ) + --- (g, - g, ) \ a f - 4 3 / j 1Aa 3 a - 4 3 / ] 4 3 / (g - g . ) - — (g - g . ) A k(a + / ) 1 -K + --- + (g z - g e) - — 7 1A y a + \ 4 3 / 1Aa 3a - 4 3 / 1 (g, - g , ) + _ 1A y a + 4 3 / 3a y a ■ 4 3 / - 4 3 / (gy - ge ) (2)

The parameter k is covalency factor, k is polarization constant, a and / are mixing

coefficients for d 2 2 and d 2 2 orbitals and satisfy the normalization relation

x - y 3z - r J

a 2 + / 2 — 1. P is dipolar hyperfine parameter for metal ion given as P — kP0, where free

ion value P0 for 65Cu2+ is P0 « 388 x 10-4 cmT1 and for 63Cu2+ is P « A16 x 10-4 cmT1

[25]. These equations can be solved for k, k, a and / and the ground state wave function

of metal ion can be constructed as follows;

W k a | d 2 2 > + / | d 2

1 x 2 - y ^ 1 3 z 2 > (3)

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

H - Pe (S xH xSx + g yH ySy + g zH zSz ) + I xAxSx + I yA yS y + I zA zS_{y y y} z . ₍₄₎

Five paramagnetic Cu2+ sites are resolved and labeled as Site 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 ref. [14] and [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 An = 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 tt 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

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 Eqns. (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 occupying 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

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Table and Figure Captions

Table 1 Principal values, direction cosines of the g and A tensors of the Cu2+ doped

KDP. (Magnetic field is measured within the error AH = ±0.5 mT)

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

Fig. 1. EPR spectrums of Cu2+ doped KDP single crystal with the magnetic field (a)

parallel to z axes, (b) inclined by 85o and (c) 140o 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

Table 1

Site _g Direction cosiness A (mT) Direction cosiness

x _y z x _y z I gx x=2.105 0.711 0.193 0.675 Ax x=1.4 0.819 -0.358 0.447 gy y=2.047 0.254 -0.883 0.194 Ay y=5.9 -0.198 0.909 0.365 gz z=2.388 0.559 0.426 -0.711 Az z=13.5 -0.537 0.426 -0.711 II gx x=2.096 -0.654 0.388 0.648 Ax x=3.6 -0.692 0.364 0.622 gy y=2.061 0.506 -0.862 0.006 SOII 0.478 0.878 0.017 gz z=2.416 0.561 -0.323 -0.761 Az z=13.7 0.540 -0.309 0.721 III gx x=2.110 -0.726 0.388 0.598 Ax x=3.0 0.906 0.256 -0.335 gy y=2.022 -0.344 0.932 0.110 Ay y=6.6 -0.132 0.927 0.349 gz z=2.380 0.595 -0.125 -0.793 Az z=13.5 0.401 -0.272 0.874 IV gx x=2.123 0.754 0.163 -0.635 Ax x=2.7 0.909 0.383 0.159 gy y=2.061 0.201 -0.979 -0.013 Ay y=5.4 -0.369 0.922 -0.108 gz z=2.352 0.624 -0.118 0.772 Az z=12.9 -0.188 0.039 0.981 V gx x=2.124 -0.706 0.040 0.707 Ax x=5.7 0.991 -0.015 0.127 gy y=2.069 -0.013 0.997 -0.070 SOII a\ -0.001 0.991 0.133 gz z=2.314 0.708 0.059 0.703 Az z=13.3 -0.128 -0.132 0.982

Powder spectrum values

gx x=2.102 gy y=2.064 gz z=2.373 Ax x=3.0 Ay y=5.4 Az z=13.3 Table 2 Sit e k a P K 0.8 0.9 0.1 0.4 I 8 8 6 0 0.9 0.9 0.1 0.3 II 4 9 5 9 0.9 0.9 0.2 0.3 III 1 7 1 8 0.8 0.9 0.1 0.3 IV 8 8 9 9 0.8 0.9 0.2 0.3 V 5 7 3 7