Study of the crystal field in CeF3 and CeF3:Pr3 Текст научной статьи по специальности «Физика»

ISSN 2072-5981

aänetic Resonance in Solids

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Volume 18, Issue 2 Paper No 16206, 1-5 pages 2016

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Study of the crystal field in CeF3 and CeF3:Pr3+

F.F. Shafikov1, A.V. Savinkov1*, M.S. Tagirov1 2 1 Kazan Federal University, Kremlevskaya 18, Kazan 420008, Russia 2 Institute of Perspective Research, TAS, L. Bulachnaya 36a, Kazan 420111, Russia *E-mail: Andrey.Savinkov@gmail.com (Received December 10, 2016; accepted December 18, 2016)f

The crystal field analysis based on calculations in the framework of the semi phenomenological exchange charge model was carried out. The set of crystal field parameters for Ce3+ and Pr3+ ions in the matrix CeF3 related to the crystallographic system of coordinates has been obtained and used to reproduce satisfactory the crystal field energies of Ce3+ and Pr3+ ions.

PACS: 71.70.Ch, 71.70.Ej.

Keywords: crystal field parameters, exchange charge model, trifluorides

1. Introduction

The rare-earth trifluorides CeF3 in the tysonite structure have been studied in the past as a model system for testing theories of rare-earth magnetism in insulators [1-3] and due to their prospects for applications. In recent years interest in the study of rare-earth trifluorides RF3 (R is rare-earth ion Ce, Pr,...) has returned thanks to advances in the fabrication of nanoparticles of pure rare-earth fluorides RF3 and rare-earth ions doped nanoparticles as well (see, for example, [4-8]). The great interest appeared due to potential applications of the nanosized materials in high resolution displaying, electroluminescent devices and markers for biomolecules. Knowledge of the crystal field parameters is important in describing magnetic and optical properties of trifluorides, as well as for the investigation of trifluorides by resonance methods (19F NMR, rare-earth NMR, EPR).

In the present work data about Ce3+ and Pr3+ optical energy spectra in CeF3 host [9, 10] taken from literature have been analyzed in the framework of the crystal field theory. We have found sets of the crystal field parameters (CFP) for the Ce3+ and Pr3+ ions which describe satisfactory Stark energies of the rare-earth ions in CeF3 matrix.

2. Discussion

The space group of the tysonite lattice RF3 (R = Ce, Pr) has been the subject of controversial discussions over decades. Only recently this question has been settled in favor of the microtweened trigonal D34d (P3c1) structure [11-13]. Assuming microtweened trigonal D34d structure, the R3+ ion has C2 site symmetry and there are six formula units per one cell. The parameters of the CeF3 crystal cell are a = 0.7112 nm, c = 0.7279 nm, p = 0.6602 [11, 14]. The rare-earth ion with the crystallographic coordinates (ap0 c/4). The same parameters describe the crystal fields acting on the Ce3+/Pr3+ ions at the sites (-ap -ap c/4), (0 -ap -c/4) with the local coordinate systems rotated by 2%/3 and 4%/3, respectively, around the c-axis; at the three other sites with the local coordinate systems rotated by %/3, % and 5%/3, the crystal fields are described by the same complex conjugated parameters. Atomic parameters of CeF3 (D34d structure) are presented in Table 1 [11].

The parameters of the electrostatic interaction of the open 4f-shell electron belonging to the rare-earth ion in CeF3, with the charges of the ions of the CeF3 crystal lattice, in the first approximation can be represented as a sum of terms, corresponding to the interaction of the point charges with the electric fields:

f This paper material was selected at XIX International Youth Scientific School "Actual problems of magnetic resonance and its application", Kazan, 24 - 28 October 2016. The paper was recommended to publication in our journal and it is published after additional MRSej reviewing.

Study of the crystal field in CeFs and CeFs:Pr3+

Table 1. Experimental values of atomic positions in the CeF3 in the D34d phase.

Atom X y z

Ce 0.6602 0 0.25

F1 0.3659 0.0537 0.0814

F2 0.3333 0.6667 0.1867

F3 0 0 0.25

УqL(1 -*f)(rk)(-1)qC^{dL)

b(pc)k ^ JLV k /v 7 -^ L^L' (1)

nq L.I r>k+1 ' V '

L RL

where eqL is the charge of the ligand, L; 6l and (pL are angles in a spherical coordinate system with the origin at the nucleus of the rare-earth ion; R is the distance from the rare-earth ion to the ligand L. The values of the moments of the spatial density of the Ce3+ 4f-electrons are <r2> = 1.120, <r4> = 3.455, <r6> = 21.226 a.u. [15] and of the Pr3+ 4f-electrons are <r2> = 1.086, <r4> = 2.822, <r6> = 15.726 a.u. [15]; &kn1 are the shielding factors.

In order to correct the Coulomb interaction for the spatial distribution of the ligand charges, we used the semi phenomenological exchange charge model [16]:

Bqec)k=i 22TT) • R- • sn (Rl x-1)qc-kM ), (2)

l 21 +1 Rl

Where Sn represents the bilinear forms of overlap integrals for the wave functions of the Ce3+/Pr3+ ion's 4f-valence electrons, and the wave functions of the 2s, 2p-electrons of the ligand ions (F-):

Sn = G, |sn112 + Ga \S"' I2 + Gy, \S"' I2 (3)

k si s а а I nt k n v '

Here Sn(Rl) = (n10| n"0^, S?(RL) = (n10| n"10), SnJ(RL) = {n11\ n"11), the coefficients у2 = 3/2,

Y4 = 1/3, Y6 = -3/2 for 4/-электронов (n and 1 are quantum numbers). Gs, Gaи GM are the parameters of the model with initial values Gs = 1.0, Ga = 5.0, Gn = 1.5 for Pr3+ [1] and Gs = 1.0, Ga = 4.0, Gn = 1.0 for Ce3+ ion.

Then the crystal field parameters were calculated as the sum of the inputs determined by expressions (1) and (2):

Bk = B( Pc)k + B(ec)k (4)

q q q ^ '

The calculated values for the crystal field parameters for Ce3+ ion in CeF3 are shown in Table 2 (column "CeF3 calc."), the CFP for Pr3+ ions in CeF3 host are shown in Table 2 (column "CeF3:Pr calc."). Finally, the sets of CF parameters are obtained by varying the calculated values to minimize the squared deviation of the theoretically obtained energy values of the experimentally defined energy sublevels of multiplets for Ce3+ ions in CeF3 [9] and for Pr3+ ions in CeF3:Pr3+ [10]. The experimentally defined and calculated energy levels for CeF3 and CeF3:Pr3+ are represented in Table 3 and Table 4, correspondingly. The final sets of CF parameters for Ce3+ and Pr3+ ions are represented in the Table 2 in columns "CeF3 fit." and "CeF3:Pr fit.". Also the CP parameters fitted for Pr3+ ion in PrF3 and adjusted to CeF3 by Gerlinger and Schaack [9] are represented in column "CeF3 ref."

Calculations of the Ce3+ in and Pr3+ electron energies in CeF3 were performed over a complete basis of electron states of the rare-earth ions (14 states for Ce3+ and 91 states for Pr3+), using the Hamiltonian:

F.F. Shafikov, A.V. Savinkov, M.S. Tagirov H = Hfi +X Bqk C ). (5)

k ,q

Here, the first term represents the energy of the free ion. The constants of the electrostatic and spinorbital interaction for Pr3+ ion were accepted as being relatively equal F2 = 69360 cm"1, F4 = 50626 cm"1, F6 = 32633 cm"1, £ = 726 cm"1 (initial values in Ref. [17]), the parameters of configuration interactions were taken from [17]. Spin-orbital constant for Ce3+ was taken as £ = 628.5 cm"1 [9].

Table 2. The CP parameters (in cm used to calculate the Ce3+ and Pr3+ energy levels.

k, q CeF3 ref. CeF3 calc. CeF3 fit. CeF3:Pr calc. CeF3:Pr fit.

2 0 -285 -104 -193 -103 -215

2 2 Re -121 23 -67 22 38

2 2 Im 0 5.7 0 5.2 15

4 0 755 699 637 614 855

4 2 Re 379 485 413 429 356

4 2 Im 230 131 160 115 128

4 4 Re 537 508 500 451 300

4 4 Im 410 263 660 231 182

6 0 634 721 974 620 560

6 2 Re -1316 -1103 -1581 -944 -1315

6 2 Im -70 -81 -55 -68 -75

6 4 Re -26 -156 -78 -133 -150

6 4 Im -609 -508 -922 -436 -634

6 6 Re -596 -604 -675 -517 -559

6 6 Im -827 -638 -754 -589 -500

Table 3. Experimentally defined energy levels of the Ce3+ in CeF3 [9] ("CeF3 exp.") and calculated ("CeF3 calc.") with parameter set from column "CeF3 fit." of Table 2.

2s + 1 t Lj CeF3 exp. CeF3 calc.

2F5/2 0 0

~160 154

«280 277

2161 22 7

2Fj/2 2239 2374

2640 2667

2860 2787

Study of the crystal field in CeF3 and CeF3:Pr3+

Table 4. Experimentally defined energy levels of the Pr3+ in CeF3 [10] ("CeF3:Pr exp.") and calculated ("CeF3:Pr calc.") with parameter set from column "CeF3:Pr fit." of Table 2.

2S + 1Lj CeF3:Pr exp. CeF3:Pr calc. 2S + 1Lj CeF3:Pr exp. CeF3:Pr calc.

0 0 21482 21527

49 59 21542 21553

110 112 21580 21580

184 172 21596

3H4 244 240 21611

261 269 21626 21614

324 1I6 21650

489 21693

525 21748

16818 16816 21783

16841 16841 21809

1d2 16906 16909 21999

16930 22031

17100 22701 22692

3P0 20991 20961 22743 22735

21413 21400 3P2 22754

3P1 21432 21428 22770

21450 21459 22838 22843

3. Conclusion

This paper presents a calculation of CF parameters for the Ce3+ and Pr3+ ions in CeF3 crystal, using the semi phenomenological model of exchange charges. The calculated CF parameters were in good agreement with the previously determined CF parameters for CeF3 trifluoride. Obtained parameters allow for a satisfactory description of the Ce3+ and Pr3+ energy levels in CeF3 host determined earlier. The standard deviation of the calculated energies of the Pr3+ ion from the experimental values is less than ~15 cm"1, however the standard deviation for Ce3+ ion is about 66 cm1. As it is seen, results of our calculations based on the set of the crystal field parameters which has been obtained in this study are in qualitative agreement with the experimental data.

Acknowledgments

The work is performed according to the Russian Government Program of Competitive Growth of Kazan Federal University, partly supported by the Russian Foundation for Basic Research (Project No.15-02-06990_a).

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