Effect of chemical environment on L subshell fluorescence yields using synchrotron radiation

Accepted Manuscript Effect of Chemical environment on L subshell fluorescence yields using Synchrotron radiation Krishnananda, Santosh Mirji, N.M. Badiger, M.K. Tiwari PII:

S0925-8388(15)31205-6

DOI:

10.1016/j.jallcom.2015.09.228

Reference:

JALCOM 35503

To appear in:

Journal of Alloys and Compounds

Received Date: 30 August 2015 Revised Date:

24 September 2015

Accepted Date: 25 September 2015

Please cite this article as: Krishnananda, S. Mirji, N.M. Badiger, M.K. Tiwari, Effect of Chemical environment on L subshell fluorescence yields using Synchrotron radiation, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.09.228. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Effect of Chemical environment on L subshell fluorescence yields using Synchrotron radiation Krishnanandaa, Santosh Mirjia, N. M. Badigera*, M. K. Tiwarib *Correspondence to: N. M. Badiger, Department of Studies in Physics, Karnatak

a

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University Dharwad - 58003, India, E-mail: [email protected] Department of Studies in Physics, Karnatak University Dharwad - 58003, India

b

X-ray Optics Section, Indus Synchrotron Utilization Division, Raja Ramanna Centre for

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Advanced Technology, Indore - 452013, India ABSTRACT

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The L1, L2 and L3 subshell fluorescence yields have been measured for some rare earth elements such as Gd, Tb, Ho and compounds such as Gd2(CO3)3, Tb2O3 and Ho2O3 using Indus-2 synchrotron radiation. By adopting reflection geometry, the elemental and compound targets are excited with 10 keV and 11 keV synchrotron radiations in order to generate the characteristic L x-ray photons. These energies of the characteristic L x-ray photons have been measured with a silicon drift detector, which has a high energy

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resolution of 130 eV at 5.9 keV. By measuring the intensities of L x-rays photons, the L subshell fluorescence yields have been determined for some rare earth elements and compounds and found to be dependent on chemical environment.

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Keywords: Rare earth compounds, Vacancy formation, L shell parameter, L subshell fluorescence yield, Chemical environment, Synchrotron radiation

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1. Introduction

The x-ray emission spectroscopy has been one of the best tools for the study

of trace elements in the samples such as environmental sample, biological sample and engineering sample [1-3]. It is well known that the K x-rays (Kα and Kβ) are produced when vacancies in the K shell created by incident radiation or particles are filled by electrons from higher shells through radiative process. Similarly, the L x-rays such as (Lα, Lβ and Lγ) are also produced due to filling of the L shell vacancies by higher shell electrons through radiative process. The L shell fluorescence yield is the ratio of number of L x-ray photons produced to the number vacancies created in L shell. As there are three subshells 1

ACCEPTED MANUSCRIPT in L shell, the corresponding fluorescence yields are ω1, ω2 and ω3. It is well known that when an electron is added or removed from the atom, the K shell binding energy and consequently K x-rays are affected [4]. Lindgren [4] has shown using theoretical calculation that the shift in K shell binding energy is about 10 eV to 20 eV and the K x-ray energy is about 1 eV to 2 eV for 3d elements. When an atom is in chemical environment,

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the type of chemical bonding, the oxidation state and coordination number would shift binding energy as well as x-ray emission lines of the atom. Niranjana et al. [5-6] have studied the effect of chemical environment on K shell binding energy of Ag and Sn, and Ho and its compounds using bremsstrahlung radiation. They have shown that the shift in K

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shell binding energy depends on crystal structure. Kavcic et al. [7] have measured the shift in Kβ line of sulfur in sulfide, sulfite and sulfate compounds using high resolution x-ray spectroscopy. They have found that the shift in K x-rays depends on the oxidation state.

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Mirji et al. [8] have studied the dependence of K shell fluorescence parameters on crystal structure. Similarly L shell binding energies and L x-rays are also influenced when the atom is in chemical environment. Several investigators have measured L shell fluorescence parameters by exciting the targets with synchrotron radiation [9-12] and gamma radiations [13-14, 19-23]. Barrea et al. [9] have measured L subshell Coster-Kronig and fluorescence

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yields of Er using synchrotron radiation. They have observed a good agreement between experimental and theoretical values. Bonzi et al. [12] have determined Ll, Lα, Lβ and Lγ xray fluorescence cross-sections for the elements with 45 ≤ Z ≤ 50 at 10 keV synchrotron radiation. Porikli [13] has measured FWHM as well as intensity ratios of L x-ray photons 241

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of Dy, Ho, Er elemental and compound targets by exciting with 59.5 keV gamma rays from Am. They concluded that the presence of the foreign atoms in the compound, the

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electronic structure of the compound is different from the metal. Kacal et al. [14] have determined L subshell fluorescence yield for U, Th, Bi, Pb, Tl, Hg, Au, Pt, Os, W, Ta, Lu, Yb and Er by exciting the targets with 22.6 keV gamma photons from

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Cd. They

observed that ω1 values agree with the theoretical values of Krause [15] and Campbell [1617] but higher than DHS values [18]. Durdu and Kucukonder [21] have measured L x-ray fluorescence cross-sections, intensity ratios and fluorescence parameters of Sm and Eu in halogen compounds by exciting the target with 59.54 keV gamma radiations. They have concluded that the above parameters depend on the quantity of unpaired 4f electrons in the atom. Turhan et al. [22] have measured L x-ray fluorescence parameters of Ho, Lu, W, Hg and Bi using EDXRF technique. They found good agreement between theory and experiment. Recently, Aylikci et al. [23] have determined empirical and semi-empirical 2

ACCEPTED MANUSCRIPT interpolated L x-ray fluorescence parameters for elements in the atomic range 50 ≤ Z ≤ 92 by exciting the targets with gamma photons. The measured data agree with theoretical and other experimental values. Very recently, Dogan et al. [24] have measured the L x-ray intensity ratios, the production cross section and L3 subshell fluorescence yield for Pb in phthalocyanine complexes.

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From the above data, we understand that the x-ray fluorescence parameters depend on chemical environment such as type of chemical bonding, oxidation state, coordination number, electronegativity as well as crystal structure. If K shell fluorescence parameters depend on the crystal structure and chemical environment around the target atom, the L

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shell fluorescence parameter should also depend on chemical environment. Therefore, in the present experiment our objective is to look for the dependence of L subshell fluorescence yields of some rare earth elements such as Gd, Tb and Ho on chemical

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environment. The atomic radii of Gd, Tb and Ho are 180.2 pm, 178.2 pm and 176.6 pm respectively. We have deliberately selected the rare earth compounds which have same oxidation states (+3), almost same effective ionic radius (1.0 Å) and same coordination number (8) but different crystal structures and chemical bonding. As the synchrotron radiation is monoenergetic, linearly polarized and high intensed, we have excited the

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elements and compounds using synchrotron radiation from Indus-2. With best knowledge of the authors the effect of chemical environment on L subshell fluorescence yields using synchrotron radiation has not been studied so far.

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2. Experimental Details

The experimental arrangement used in the present investigations is shown in Fig. 1.

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The experiment was carried out at XRF BL-16, Indus-2 synchrotron light source, Indore, India [25]. The polychromatic beam produced by 2.5 GeV electrons in Indus-2 storage ring was monochromatized by using DCM with Si(111) symmetric and anti symmetric crystals (mounted side by side). The beam is focused using Kirkpatrick-Baez (KB) focusing optics. The slits S1 and S2 are used for shaping of x-ray beam dimensions. The S1 is water cooled four-blade slit and S2 is an uncooled four-blade slit. The beam is focused to have about 2 mm × 2 mm size. The photon flux is about 2x108 photons/sec in focused beam mode and is about 1x109 photons/sec/mm2 in the collimated beam mode (at 100 mA ring current). The L1, L2 and L3 binding energies of Gd are 8.381 keV, 7.931 keV and 7.243 keV respectively and we have used 10 keV synchrotron radiation to excite pure element Gd and its 3

ACCEPTED MANUSCRIPT compound Gd2(CO3)3. The L1, L2 and L3 binding energies of Tb are 8.717 keV, 8.252 keV and 7.515 keV respectively and we have used 10 keV synchrotron radiation to excite pure element Tb and its compound Tb2O3. The L1, L2 and L3 binding energies of Ho are 9.399 keV, 8.916 keV and 8.067 keV respectively and we have used 11 keV synchrotron radiation to excite pure element Ho and its compound Ho2O3. Such synchrotron radiation

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can create vacancies in L subshells and can produce characteristic L x-ray photons namely Ll, Lα, Lβ1, Lβ2, Lγ1 and (Lγ2 + Lγ3). The characteristic L x-ray photons are detected with a Vortex 90EX® make silicon drift detector (SII Nano Technology, USA) by keeping the detector at 90o to the beam axis. The detector, which has an energy resolution of 130 eV at

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5.9 keV, is coupled to MCA. The detector has been calibrated using characteristic K x-ray photons of pure elemental targets and the calibration constant is found to be 10.024 eV/Ch. The spectrum of characteristic L x-ray photons is shown in Fig. 2. From the figure we

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notice that, all the characteristic L x-ray photons namely Ll, Lα, Lβ1, Lβ2, Lγ1 and (Lγ2 + Lγ3) are resolved, showing the capability of the energy resolution of the detector. We have fitted each characteristic L x-ray photons using PeakFit [26] program to determine the intensity of x-ray photons. From these L x-ray photon intensities the L subshell fluorescence yields

3. Data Analysis

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(ω1, ω2 and ω3) have been determined for elements as well as compounds.

The L subshell fluorescence yields have been determined experimentally using the following equations [11]


()

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ω1 =
( )( 

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ω2 ′ =

(1)

   )










[
 ( )  
 ( )] 

ω2″ = [
( )






 
 ( )] 






    ]
 ( ) 
 ( )] (    )

ω3′ = [
( )[ 



ω3″ = [
( )[ 








    ]
 ( ) 
 ( )] 

(2) (3) (4) (5)

where σLc (c = γ2+γ3, γ1, β1, α and β2) is the experimental L x-ray production cross section,  ( ) (i=1, 2 and 3) is the theoretical L subshell photoionization cross section of given 

element at excitation energy Ei, fr(r =12, 13 and 23) is the Coster-Kronig transition probabilities and F is the fractional x-ray emission rate. The L subshell fluorescence yield

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Ic ( G ϵ ) β t

(6)

where Ic is area under the peak c (= γ2+γ3, γ1, β1, α and β2), Io is the intensity of the incident radiation falling on the target, G is the geometrical factor,  is the detector efficiency, t is

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the thickness of the target (in g/cm2) and β is the self-absorption factor for the target. The factor I0G is the total incident flux and is determined by measuring Kα and Kβ x-ray photons of various targets. We have used elemental targets Fe, Ni, Cu and Zn and compound targets NaVO3.H2O and KCr(SO4)2.12H2O to generate Kα and Kβ x-ray photons.

I0G = "

 ! !# !$

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Using these K x-ray photons, I0G has been determined by using the following equation (7)

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where IKi(i = α, β ) is the net counts under the K x-ray peaks, σKi is K x-ray fluorescence cross-section and βKi is the self-absorption factor. The factor β is given by β=

µ! µ  - .$] +!,θ +!,θ µ µ * !  - .$ +!,θ +!,θ

%&'() [&*

(8)

where µi and µe are the total mass attenuation coefficients of the target material at the incident synchrotron radiation and emitted x-ray energy, θ1 and θ2 are the angle of incident

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radiation and emitted x-ray photon with respect to target and equal to 45˚ in the present experimental setup. The theoretical values of σKi is given by σKα = σKP(Ei) ωK FKα (9)

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σKβ = σKP(Ei) ωK FKβ

where σKP(Ei) is the K shell photo-ionization cross-section of the target at excitation energy Ei, ωK is the K shell fluorescence yield, FKα and FKβ is the fractional x-ray emission rates

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for Kα and Kβ . These fractional x-ray emission rates are given by, 

FKα = [1 +  / ]-1 0



FKβ = [1 +  0 ]-1 /

(10)

where IKβ/IKα is the intensity ratio corresponding to Kβ and Kα. The σKP(Ei) values are taken from Scofield [27], the ωK values from Bambynek et al. [28] and IKβ/IKα values from Scofield [29]. Using Eqn. (7) I0G  has been determined and the plot of I0G v/s K x-ray energy is shown in Fig. 3. Using the fitted curve, we have calculated I0G values for our required L x-ray energy. The I0G and other parameters are substituted in Eqn. (6) and we have determined experimental L x-ray production cross section values, σLc. From these 5

ACCEPTED MANUSCRIPT values ω1, ω2 and ω3 are determined using Eqns. (1)-(5). In the determination of ω1, ω2 and ω3 we have taken Coster-Kronig transition probabilities from Puri et al. [30] and the fractional x-ray emission rates from Scofield [29]. We have taken weighted average values for ω2 (weighted average values of ω2′ and ω2″) and ω3 (weighted average values of ω3′ and ω3″). Such measured L subshell fluorescence yields ω1, ω2 and ω3 of compound targets are

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given in Table 1.

4. Results and Discussion

The rare earth atoms such as Gd, Tb and Ho have 7, 9 and 11 electrons in the 4f

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state respectively. The electrons in 4f state are not involved in chemical bonding and they are shielded by 5d and 6s electrons. The effective ionic radius of rare earth elements decreases with increase in atomic number due to lanthanide contraction. Using Indus-2

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synchrotron radiation, we have measured L subshell fluorescence yield namely ω1, ω2 and ω3 for Gd and its compound, Tb and its compound and Ho and its compound and they are given in Table 1.

From Table 1 we notice that the L subshell fluorescence yields of elemental foil of Gd, closely agree with theoretical and other experimental values. It is need to mention that

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the Gd element has hexagonal structure and metallic bonding. One may conclude that the L subshell fluorescence yields of Gd element can be determined accurately using synchrotron radiation. The compound Gd2(CO3)3 has the oxidation number +3, the effective ionic radius 1.06 Å and coordination number 8, but the chemical bonding is ionic type and

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crystal structure is orthorhombic. It is interesting to note that the ω1, ω2 and ω3 values for Gd2(CO3)3 are lower than the values of Gd; the difference probably due to the type

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bonding and crystal structure in Gd2(CO3)3. We notice that from Table 1, the L subshell fluorescence yields (ω1, ω2 and ω3) of

elemental foil of Tb closely agree with theoretical and other experimental values. We wish to mention that the Tb element has hexagonal structure and metallic bonding. The compound Tb2O3 has the oxidation number +3, the effective ionic radius 1.04 Å and coordination number 8, but the chemical bonding is covalent type and crystal structure is cubic. It is interesting to note that the ω1, ω2 and ω3 values for Tb2O3 are lower than the values of Tb. This difference might be due to the type bonding and crystal structure in Tb2O3. The L subshell fluorescence yields (ω1, ω2 and ω3) of elemental foil of Ho closely agree with theoretical and other experimental values. It is important to be noted that the Ho 6

ACCEPTED MANUSCRIPT element has hexagonal structure and metallic bonding. The compound Ho2O3 has the oxidation number +3, the effective ionic radius 1.02 Å and coordination number 8, but the chemical bonding is covalent type and crystal structure is cubic. It is interesting to note that the ω1, ω2 and ω3 values for Ho2O3 are lower than the values of Ho. The type chemical bonding and crystal structure in Ho2O3 may be the reason behind this difference.

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The maximum uncertainties in the measured ω1, ω2 and ω3 are 6%, 5% and 5% respectively. The uncertainties in the measured L subshell fluorescence yields are due to evaluation of area under the L x-ray peaks (0.5%-1%), I0G factor (3%-4%), mass thickness of the target (0.2%-0.4%) and self attenuation factor (0.4%-0.8%). The χ2 value

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for fitted Ll, Lα, Lβ1, Lβ2, Lγ1 and (Lγ2 + Lγ3) x-ray peaks are about 0.99 and the second order polynomial is fitted for the I0G data points as shown in Fig. 3.

From above data we may conclude that using synchrotron radiation L subshell

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fluorescence yields of some rare earth elements can be determined accurately. Using same synchrotron radiation L subshell fluorescence yields of some rare earth compounds have also been measured and found that measured the ω1, ω2 and ω3 values for compounds are lower than that for elements. We have shown that this difference is not due to oxidation state, coordination number and ionic radius, but may depend on the type of bonding and

5. Conclusion

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crystal structure. To verify this effect, more data is required.

L subshell fluorescence yields of some rare earth elements and compounds can be

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measured accurately using synchrotron radiation. For compounds, the L subshell fluorescence yields are lower than that for elements indicating the chemical environment

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such as type of bonding and type of crystal structure may play significant role in x-ray emission.

Acknowledgements

Mr. Krishnananda is grateful to University Grant Commission New Delhi, Government of India, for award of junior research fellowship for meritorious student under the RFSMS scheme.

References

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ACCEPTED MANUSCRIPT 11. Bonzi EV, Badiger NM. Measurement of L subshell fluorescence yields of elements in the range 45 ≤ Z ≤ 50 using synchrotron radiation. Nucl Instr Meth Res B 2006; 248: 242-246. 12. Bonzi EV, Grad GB, Barrea EA. Experimental determination of L X-ray fluorescence cross sections for elements with 45 ≤ Z ≤ 50 at 10 keV. Pap Phys

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Coster-Kronig transition probabilities for the elements with 25 ≤ Z ≤ 96. X-ray Spectrom 1983; 22: 358-361.

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Table 1: The L subshell fluorescence yields of some rare earth elements and compounds.

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Gd2(CO3)3 Tb

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Tb2O3 Ho

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+3

Effective Ionic Radii (Å) 1.06

Crystal structure Hexagonal

Chemical bonding Metallic

1.04

Orthorhombic Hexagonal

Ionic Metallic

1.02

Cubic Hexagonal

8

8

Covalent Metallic

Ho2O3 +3 Present experiment

a

f

Oz et al. [19]

8

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+3

CN*

ω1

ω2

Cubic

Covalent Chen et al. [18]

b

c

g

h

Krause [15]

Aylikci et al. [23]

Ozdemir and Durak [20]

*CN-Coordination number based on crystal structures.

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ω3

0.089 ± 0.004a 0.176 ± 0.008 a 0.167 ± 0.007 a b b 0.079 0.158 0.155b c c 0.083 0.175 0.167c 0.09d 0.175d 0.167d e f 0.102 0.156 ± 0.006 0.158 ± 0.006f f g 0.078 ± 0.003 0.1596 ± 0.0081 0.1562 ± 0.008g g 0.0778 ± 0.004 0.057 ± 0.003a 0.137 ± 0.006 a 0.126 ± 0.006 a 0.176 ± 0.007 a 0.076 ± 0.004a 0.186 ± 0.008 a b b 0.083 0.186 0.164b c c 0.087 0.186 0.167c d d 0.1 0.186 0.175d e f 0.107 0.174 ± 0.005 0.169 ± 0.005f 0.084 ± 0.003f 0.1793 ± 0.0091g 0.1656 ± 0.0084g g h 0.0806 ± 0.0041 0.168 ± 0.013 0.163 ± 0.007h h 0.080 ± 0.006 0.068 ± 0.004a 0.168 ± 0.008 a 0.156 ± 0.007 a a a 0.091 ± 0.005 0.209 ± 0.009 0.188 ± 0.008 a b b 0.094 0.189 0.182b c c 0.095 0.208 0.193c 0.11d 0.208d 0.193d e f 0.116 0.189 ± 0.015 0.181 ± 0.015f f g 0.096 ± 0.008 0.1885 ± 0.0096 0.1831 ± 0.0093g g h 0.0928 ± 0.0047 0.194 ± 0.02 0.184 ± 0.01h h i 0.087 ± 0.009 0.199 ± 0.021 0.172 ± 0.01i i 0.081 ± 0.011 0.068±0.003a 0.161±0.008a 0.143±0.005 a d e Campbell [16] Campbell [17]

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Gd

Oxidation State

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Z

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Sample

i

Turhan at al.[22]

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Figure Captions: Fig. 1. Experimental setup. Fig. 2. The Characteristic L x-ray photons of some rare earth elements and compounds.

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Fig. 3. I0Gє versus K x-ray energy.

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Fig. 1. Experimental setup.

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Fig. 2. The characteristic L x-ray photons of some rare earth elements and compounds.

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Fig. 3. I0Gє versus K x-ray energy.

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Highlights •

With the best knowledge of the authors, the effect of chemical environment on L subshell fluorescence yields of some rare earth elements have been measured for the first time using synchrotron radiation. We have deliberately selected the compounds having same oxidation state, effective ionic

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•

radius and coordination number but different chemical bonding and crystal structures. In the present work we have shown that the measurement of L subshell fluorescence

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yields depend on chemical bonding and crystal structure.

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•