Spectral and magnetic properties of impurity Tm3+ ions in YF3 Текст научной статьи по специальности «Физика»

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aänetic Resonance in Solids

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Volume 19, Issue 2 Paper No 17202, 1-7 pages 2017

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Yoshio Kitaoka (Osaka University,

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Boris Malkin (KFU, Kazan) Alexander Shengelaya (Tbilisi State University, Tbilisi) Jörg Sichelschmidt (Max Planck Institute for Chemical Physics of Solids, Dresden) Haruhiko Suzuki (Kanazawa University, Kanazava) Murat Tagirov (KFU, Kazan) Dmitrii Tayurskii (KFU, Kazan) Valentine Zhikharev (KNRTU,

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* In Kazan University the Electron Paramagnetic Resonance (EPR) was discovered by Zavoisky E.K. in 1944.

Short cite this: Magn. Reson. Solids 19, 17202 (2017)

Spectral and magnetic properties of impurity Tm3+ ions in YF3

A.V. Savinkov1*, G.S. Shakurov2, I.E. Mumdzhi1, B.Z. Malkin1, S.I. Nikitin1, S.L. Korableva1, M.S. Tagirov13

1 Kazan Federal University, Kremlevskaya 18, Kazan 420008, Russia

2 Kazan Physical-Technical Institute, Sibirskiy trakt 10/7, Kazan 420029, Russia

3 Institute of Perspective Research, TAS, L. Bulachnaya 36a, Kazan 420111, Russia

*E-mail: Andrey.Savinkov@gmail.com

(Received November 27, 2017; accepted December 5, 2017)

Stark structure of 3H6, 3H5, 3H4, 3F4, 3F3, 3F2 and 1G4 multiplets of impurity non-Kramers Tm3+ ions in the orthorhombic YF3 crystal has been determined from luminescence studies. High frequency electron paramagnetic resonance (EPR) spectra (~ 207 GHz) of Tm3+ ions have been measured at temperature 4.2 K in external magnetic field applied perpendicular to the 6-axis of YF3:Tm3+ single crystal. The results of measurements are interpreted in the frameworks of the crystal field theory. The set of crystal field parameters related to the crystallographic system of coordinates of the YF3 lattice has been obtained and used to reproduce satisfactory the crystal field energies and the EPR spectra.

PACS: 71.70.Ch, 71.70.Ej.

Keywords: crystal field parameters, exchange charge model, EPR, trifluorides

1. Introduction

The yttrium fluoride YF3 crystals doped with heavy rare-earth ions (Sm, ..., Tm and Yb) are investigated rather intensively because they may be suitable as potential solid-state laser materials and scintillators. Optical spectra of YF3:R3+ and isostructural RF3 (R = Eu, Tb, Dy, Er, Ho and Yb) crystals have been measured in Refs. [1-5]. Moreover, at present the rare-earth trifluorides attract more and more attention due to synthesis and investigation of magnetic and optical properties of the hollow fullerene-like nanoparticles RF3 (R = La, Pr, .. ) and nanoparticles of different shapes [6-11] which can have widespread potential for practical applications in high resolution displays, electroluminescent devices and markers for biomolecules.

Fluoride TmF3 and thulium doped YF3 fluoride have been the subject of quite a few studies in the past. As a result, scanty information exists on magnetic properties of non-Kramers Tm3+ ions with the ground electronic configuration 4f12 in both concentrated, TmF3, and dilute, YF3:Tm3+, compounds. Results of dc-magnetometry and NMR 19F studies in TmF3 powder as well as the high frequency electron paramagnetic resonance (EPR) measurements in YF3:Tm3+ single crystal [12] gave unambiguous evidence that TmF3 is a Van Vleck paramagnet with the gap A ~ 6.5 cm 1 between the ground and the nearest excited energy levels of Tm3+ ions.

In present article we report the results of systematic studies of crystal field and magnetic properties of Tm3+ ions in YF3:Tm3+ single crystal. The angular dependence of the high frequency EPR spectrum of Tm3+ ions was measured in the collinear constant (B0) and alternative (B1) magnetic fields lying in the crystallographic ac-plane (B0 || B1 ± b). The Stark structure of electronic multiplets of Tm3+ ions was defined by means of the laser-selective spectroscopy. Crystal field parameters (CFP) were estimated in the frameworks of the semi-phenomenological exchange charge model [13] and then corrected by making use of our experimental data. The obtained set of CFP allowed us to reproduce satisfactory the high frequency EPR spectra in YF3:Tm3+ single crystal.

2. Experimental data

The YF3 : 0.5 at % Tm3+ single crystals for optical and EPR studies were grown by the Bridgman method in carbon crucibles in the atmosphere of the high purity argon at a pulling rate of 1 mm/h.

High purity TmF3 and YF3 powders (99,99% grade) were used as starting materials. Additionally, the growth atmosphere was fluorinated by thermal decomposition of the tetra-fluorine-ethylene.

2.1 Site-selective laser spectroscopy

Fluorescence of YF3:Tm3+ crystal was excited with either a pulsed tunable dye laser (Littrow type oscillator and amplifier, linewidth of about 1 A) pumped by the second or third harmonic of a Nd-YAG laser (LQ129, Solar LS) in the visible spectral range or a Ti:Sapphire tunable laser with the linewidth of about 0.4 A (LX325, Solar LS) pumped by the second harmonic of a Nd-YAG laser (LQ829, Solar LS) in the near-IR range. The spectra were analyzed with an MDR-23 monochromator. The fluorescence signal was detected by a cooled photomultiplier (PMT-106 or PMT-83) in the photon-counting mode. The studied YF3:Tm3+ crystal was kept in the helium vapor at a temperature of

4.2 K. In order to investigate fluorescence spectra at temperatures about 2 K liquid helium bath cryostat with the vapor pumping was used.

We studied the fluorescence of the YF3:Tm3+ crystal corresponding to the radiative transitions from the metastable states of the Tm3+ ions, namely, from the lowest crystal field sublevels of the 1G4 and 3F4 multiplets, to the low-lying multiplets 3H6, 3H5 and 3H4 (^4 ^ 3H6, ^4 ^ 3H5, ^4 ^ 3H4 and 3F4 ^ 3H6). Energies of the crystal field sublevels of the ground (3H6) and low lying excited (3H5 and 3H4) multiplets were determined from the selectively excited luminescence spectra (see luminescence spectra for !G4 ^ 3H6 and 3F4 ^ 3H6 in Fig. 1a).

Energies of crystal field sublevels of the 3F4, 3F3, 3F2 and 1G4 multiplets were determined from the excitation spectra of Tm3+ ions in YF3 host. The first excited sublevel of the ground multiplet 3H6 (see Table 1) is separated from the ground sublevel by the energy of 6.5 cm"1. At temperature T = 4.2 K both sublevels are populated. Hence, excitation spectra arise due to transitions from the both lowest

Figure 1. (a) Luminescence spectra of Tm3+ ions measured in YF3:Tm3+ at T = 4.2 K. The emission from the lowest sublevels of 3F4 (lower spectrum) and 1G4 (upper spectrum) multiplets into the sublevels of the ground 3H6 multiplet was induced by radiation with the wave numbers of 12959.1 cm"1 and 21424.7 cm"1, respectively. (b) The excitation spectrum of the Tm3+ ions in YF3:Tm3+ corresponding to transitions from the lowest states of 3H6 into the 1G4 at 4.2 K. The inset shows the difference between the excitation spectra measured at T = 4.2 K (black curve) and T = 2.3 K (red curve) due to redistribution of populations of the ground quasi-doublet sublevels.

A. V. Savinkov, G.S. Shakurov, I.E. Mumdzhi et al.

sublevels of the 3H6 multiplet to sublevels of the 3F4, 3F3, 3F2 and :G4 multiplets. As a result, spectral lines registered at T = 4.2 K are split by the energy 6.5 cm"1 (see the transition 3H6 ^ :G4 in Fig. lb). In order to determine the crystal field energies of the 1G4 and 3F2 multiplets, the excitation spectra of Tm3+ ions corresponding to the transitions from the ground state into the 1G4 and 3F2 multiplets at lower temperature T = 2.3 K were measured (see inset in Fig. lb). Crystal field energies for 3H6, 3H5, 3H4, 3F4, 3F3, 3F2 and 1G4 multiplets of Tm3+ ions obtained from the analysis of the experimental data are presented in Tab. 1.

Table 1. Crystal field energies (cm-1) of the Tm3+ ions in YF3 host.

2s + 1 t Lj N Experiment Calculation

1 0 0

2 6.5 6.5

3 212 211

4 262 265

5 274 277

6 326 327

3H6 7 356 354

8 - 367

9 381 398

10 417 420

11 - 435

12 475 473

13 499 482

1 5800 5803

2 5825 5834

3 5860 5856

4 5886 5884

3H4 5 5904 5904

6 - 5910

7 5960 5967

8 5981 5981

9 - 6065

1 8263 8257

2 8270 8264

3 8448 8446

4 8468 8478

5 8510 8510

3H5 6 8557 8553

7 - 8581

8 8603 8589

9 8643 8592

10 8650 8615

11 8659 8624

2s + 1 t Lj N Experiment Calculation

1 12623 12622

2 12684 12683

3 - 12764

4 12792 12794

3F4 5 - 12835

6 12853 12852

7 12871 12873

8 12940 12932

9 12959 12952

1 14555 14558

2 14625 14619

3 14629 14628

3F3 4 14649 14646

5 14654 14650

6 14685 14666

7 14752 14717

1 15208 15184

2 15227 15216

3f2 3 15237 15232

4 15329 15330

5 - 15355

1 21273 21261

2 21325 21291

3 21336 21349

4 21353 21353

1G4 5 21395 21390

6 21425 21428

7 21527 21530

8 21568 21549

9 21660 21652

2.2 High _ frequency EPR in YF3:Tm3+

The EPR spectra of Tm3+ ions in oriented YF3:Tm3+ (0.5% Tm) single crystal were taken with high-frequency tunable EPR spectrometer [14] at frequency of 207 GHz in magnetic fields from 0 to 900 mT at temperature 4.2 K. The microwave magnetic field Bi was applied parallel to static magnetic field B0, i.e. Bi || B0. The hyperfine structure of the EPR spectra consisting of two resonant lines was observed (Fig. 2a). This observation gives evidence for resonance transitions induced by the microwave field in Tm + ions (169Tm, I = 1/2, natural abundance 100%).

High-frequency EPR spectra measured in the YF3:Tm3+ single crystal allowed us to determine the energy gap of 6.5 cm1 between the two lowest crystal field singlets. The magnetic moments corresponding to this quasi-doublet are determined by the g-factor ~13.5 and lie in the ac-plane along the directions declined by the angle ~ ±23° from the crystallographic c-axis [12]. The angular dependence of the EPR spectrum was measured by rotation of a sample around the crystallographic ¿-axis so that the external magnetic field was applied in the ac-plane of the YF3 crystal lattice, the angular step in the ac-plane was 5°. Two magnetically non-equivalent sites of Tm3+ ions were found. EPR spectra in the magnetic fields lying in the ab-plane were not observed because the resonant field was out of the magnetic field range for our magnet. Some of EPR lines are distorted, split, and have weak satellites. Probable reasons for these effects were discussed in Ref. [12].

Figure 2. The high frequency EPR (207 GHz) of Tm3+ ions measured in YF3:Tm3+ single crystal: (a) a sample spectrum of the Tm3+ EPR, measured at B0Aa = 48°; (b) the angular dependence measured in external magnetic field applied in the ac-plane of YF3 crystal structure. Open circles and solid lines represent the experimental data and the calculated resonant magnetic fields, respectively.

3. Discussion

The single crystals YF3:Tm3+ studied in this work have an orthorhombic structure with the space group Pnma (DH) [15]. The lattice constants are a = 0.63537(7) nm, b = 0.68545(7) nm, c = 0.43953(5) nm [16]. The unit cell contains four formula units. The coordinates of fluorine F1 ions in 4c positions and F2 ions in 8d positions are determined by parameters U1, U2, S2, V1, V2 and equal ± (U1, 1/4, V1), ± (U1 - 1/2, 1/4, 1/2 - V1) for F1, ± (U2, S2, v), ± (U2, 1/2 - S2, V2), ± (U2 - 1/2, S2, 1/2 - V2), ± (U2 - 1/2, 1/2 - S2, 1/2 - V2) for F2; the yttrium ions in 4c positions with the point symmetry Cs have the coordinates ± (u, 1/4, v), ± (u -1/2, 1/4, 1/2 - v). The fractional parameters U1, V1, U2, S2, V2, u, v (in units of the lattice constants) in YF3 [16] that is used as a host matrix for Tm3+ ions are represented in Tab. 2.

To describe the results of EPR and optical measurements, we consider the effective Hamiltonian of a single Tm3+ ion in the j-sublattice, operating in the total basis of 182 electron-nuclear states of the electronic 4f 12 configuration, in the external magnetic field B0:

A. V. Savinkov, G.S. Shakurov, I.E. Mumdzhi et al Table 2. Fractional atomic coordinates of the ions in Pnma structure of YF3 [16].

Y F1 F2

u V Ul Vi U2 S2 V2

0.3673(4) 0.0591(5) 0.5227(5) 0.5910(8) 0.1652(4) 0.0643(3) 0.3755(5)

Hj = H0 + Hcf, j + Hhf + Hz, j • (!)

Here Ho is the free ion Hamiltonian defined by Slater parameters F2 = 102586 cm1, F4 = 72072 cm1, F6 = 51572 cm1 of electrostatic interaction between 4f-electrons, spin-orbit coupling constant £ = 2634.5 cm"1; two-particle parameters of the configuration interaction, parameters of correlated spin-orbit and spin-spin interactions were taken from Ref. [17]. The term Hhf is the energy of the magnetic hyperfine interaction:

Hhf = A£{lJ + V3/2PC02,) (3^ -s„I)/ V6 +

n

+( + c"2)n)(,-)-i(C22) -C® )(, + )- (2)

-(( - c"2,n )( + ^ A ) ( + c"2,n)(,Iy + ^ ,i2 )]}•

Here A = 2pBjTmris the hyperfine coupling constant, where /J& is the Bohr magneton,

ftm/2n = 3.52 MHz/T is the gyromagnetic ratio for 169Tm nuclei, ln and Sn are the orbital and spin moments, respectively, of 4f-electrons; C® are the electronic spherical tensor operators and I is

the nuclear spin moment. The average value of 1/r3 for 4f-electrons defines the effective hyperfine constant that is taken as A = -0.016 cm"1. The sum is taken over all 4f-electrons.

The electronic Zeeman energy is Hzj = -^B0,j , where ^ = -MBIn(ln +2sn) is the electronic

magnetic moment of the Tm3+ ion. The Tm3+ ions in the sublattices j = 1 and 2, 3 and 4 are magnetically equivalent in pairs. The crystal field Hamiltonian

Hcf,j = I I Bk (j)£ , (3)

k=2,4,6 q=0:k n

in the Cartesian system of coordinates defined so that x|| c, y || b and z|| a, is determined by a single set of 15 crystal field parameters (CFP) Bkq (1) (Bkq (1) = Bkq (2) = (-1)kBkq (3) = (-1)kBkq (4)), Oqn k are linear

combinations of spherical tensors defined in Ref. [18].

Initial values of CFP for the Tm3+ ions in YF3 (column "Calculated" in Tab. 3) were calculated in the framework of the exchange charge model [13] by making use of the corresponding lattice structure constants and atomic coordinates presented above in Tab. 2:

Bkq =I e 2[-Zl (1 -ok )< rk > + Si1* RkLSk (Rl )](-1) qCk](3L ,p)/Rk+1 . (4)

L 7

In the expression (4), the sum is taken over lattice ions L with charges eZL and spherical coordinates (Rl, Sl, (pL) relative to the rare earth ion at the origin, ok are the shielding constants, <rk > are the moments of the 4f-electron charge density, the exchange charges are defined by the overlap integrals between the wave functions of the rare earth ions 14f, and ligand ions 11, l^j (we take into account only the outer closed 2s2 and 2p6 electronic shells of F- ions) [13]:

Sk (R) = GSS2(RL)+GX(RL)+nGX(RL), (5)

where SS = (4f0|2s0), So = (4f0|2p0), Sn = (4fl|2pl) and y2 = -6 =3/2, y4 = 1/3.

Calculations were carried out with 02 = 0.545, 04 = 0.088, 06 = -0.043 [19], <r2> = 0.646, (r4> = 1.076, (r6> = 3.647 (atomic units) [20], the dependences of the overlap integrals on the distance R (in Angstrems) between the ions were approximated by functions S0 exp(-bRd ) with the parameters S0 = 0.33789, 0.19678, 2.93065; b = 1.52924, 1.48051, 0.8624; d = 1.05503, 0.81048, 2.93065 for 5, oand nbonds, respectively [21]. The values of the model parameters were assigned as Gs = Go = Gn = 11.5. Hereafter, the calculated values of CFP (see column "Calculated" in Tab. 3) were corrected to fit the measured electronic energies of

Tm3+ ions in YF3. The final set of the CFP is Table 3. The crystal field parameters (in cm-1) represented in Tab. 3 (column "Adjusted").

of the Tm ions in YF3 crystal lattice.

The calculated crystal field energies of the 3H6, 3H5, 3H4, 3F2, 3F3, 3F4, 1G4 multiplets of Tm3+ ion in YF3 host are compared with the experimental data in Tab. 1. The total splittings of the excited multiplets and energies of the most sublevels are well reproduced by calculations (see Tab. 1, column "Calculation"). The standard deviation of the calculated values of crystal field energies from those experimentally defined is STD ~ 14 cm-1.

The obtained set of CFP allowed us to reproduce satisfactory the angular dependence of the Tm3+ EPR spectrum (Fig. 2b). The calculated principal value of the g-factor for the lowest quasidoublet equals g = 13.59, the corresponding principal direction lies in the ac-plane and has the angle q> of ±22.0° with the c-axis of YF3 crystal. These calculated values are close to experimental data, g = 13.5 and <p= ±23.0°.

Bq' Calculated Adjusted

B02 103 117

B12 838 802

B22 -264 -306

B04 -1.76 -3.45

B:4 -711 -584

B24 -11.9 -3.46

B34 289 210

B44 97.3 83.4

B06 -0.90 -1.0

B:6 -309 -261

B26 -303 -185

B36 -43.4 -32.4

B46 -77.5 -65.3

B56 -923 -896

B66 265 237

4. Conclusion

The crystal field energies of the impurity Tm3+ ions in orthorhombic crystal YF3 have been determined for 3H6, 3H5, 3H4, 3F4, 3F3, 3F2, and 1G4 multiplets from the optical absorption and luminescence studies. The initial set of crystal field parameters for the Tm3+ ions in the YF3 host has been calculated in the framework of the semi-phenomenological exchange charge model and then varied to fit the experimental data. The obtained set of CFPs allowed us to reproduce well the electronic energies and the angular dependence of EPR spectra measured in YF3:Tm3+ single crystal in external magnetic fields applied in the ac-plane. The calculated value and the direction of the magnetic moment of the Tm3+ ion in the ground quasi-doublet state agree well with the results of EPR measurements.

Acknowledgments

This work was supported by the RFBR grant №15-02-06990_a. References

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