Polarized Raman spectra of the oriented NaY(WO4)2 and KY(WO4)2 single crystals

Journal of Molecular Structure 555 (2000) 289–297 www.elsevier.nl/locate/molstruc

Polarized Raman spectra of the oriented NaY(WO4)2 and KY(WO4)2 single crystals L. Macalik a, J. Hanuza a,*, A.A. Kaminskii b a

W. Trzebiatowski Institute for Low Temperature and Structure Research, Polish Academy of Sciences, 50-950, P.O. Box 937, Wroclaw 2, Poland b Joint Open Laboratory for Laser Crystals and Precise Laser Systems, Institute of Crystallography, Russian Academy of Sciences, Moscow, Russia Received 3 November 1999; revised 27 January 2000; accepted 27 January 2000

Abstract Polarized Raman scattering spectra of the NaY(WO4)2 (NYW) single crystal have been measured. Its structure is described in the tetragonal space group isomorphic to CaWO4 scheelite. The Ag, Bg and Eg spectra were made and discussed in terms of factor group analysis. These spectra are compared to those of monoclinic KY(WO4)2 (KYW) single crystals whose structure differs from the other crystal. The NYW unit cell comprises of the isolated WO4 tetrahedra whereas the KYW structure is built from the WO6 octahedra joined by WO2W double bonds and WOW single bridges. The vibrational characteristics of the bridge bond systems are proposed. On this basis, the role of the vibronic transitions for the KYW crystal doped with Eu 3⫹ ions is discussed. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Polarized Raman spectra; NaY(WO4)2; KY(WO4)2

1. Introduction There is still a strong interest in the double molybdates and tungstates of alkali metals and rare earth elements of the M IM III(XO4)2 stoichiometry (where M I ˆ K,Na,Li, M III ˆ Ln,Y, X ˆ W,Mo) [1–3]. The excellent optical, mechanical and thermal properties of these materials make them very promising hosts for the active dopands. The possibility of doping of these crystals not only with Cr 3⫹ ions but also with many other transition and rare earth elements allows the production of active solid state lasers with effective generating properties. Special attention is paid to the * Corresponding author. Tel.: ⫹48-71-343-5021; fax: ⫹48-71441-029. E-mail address: [email protected] (J. Hanuza).

investigation of these systems working in the visible and infrared ranges both with the crystals doped with transition elements and with colour centres. The discovery of stimulated Raman scattering in KM III (WO4)2:Ln 3⫹ expands the spectral range of potential laser generation with these crystals [4]. Recently, a new type of laser with self-pump stimulated Raman scattering has been discovered and realized [5]. In the present work, the Raman spectra were measured and the vibrational assignment is proposed. The KYW and NYW crystals are of the scheelite-type in origin, however, the former crystallizes in the monoclinic space group and the latter in the tetragonal one. The NYW unit cell comprises of the isolated WO4 tetrahedra whereas the KYW structure is built from the WO6 octahedra joined by means of bridged WO2W double bonds and WOW single bond. The

0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(00)00612-8

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Fig. 1. (a) The perspective view of the KY(WO4)2 crystal structure. The W–O bonds are shown as thick lines in order to show the octahedral coordination of W atoms and the presence of single and double oxygen bridges. (b) The perspective view of the NaY(WO4)2 crystal structure.

experimental data were collected and the vibrational characteristics of the bridge bond systems are proposed. The vibronic coupling and the role of oxygen bridge bonds in the energy transfer is discussed on the example of the laser crystal: KY(WO4)2 doped with Eu 3⫹ ions.

2. Experimental The single crystals have been grown by the Czochralski method by the Kaminskii group in the Institute of Crystallography of the Russian Academy of Sciences (Joint Open Laboratory for Laser Crystals and Precise Laser Systems, Moscow, Russia). The crystals obtained are colourless and have high optical quality; their length is up to 1.5 cm. The orientation of the crystals for the optical studies was performed by

using the X-ray method, where a, b and c axes were determined. The crystals were cut into plates with edges of plates parallel and perpendicular to the b unique axis of the monoclinic unit-cell and to the c and a axes of the tetragonal unit cell. The Raman room temperature spectra were recorded using a DFS 24 double monochromator. Excitation was performed by 488-nm line of an ILA 120 argon laser. The resolution was 2 cm ⫺1. A cooled GaAs photomultiplier with a computer photon-counting detection were used.

3. Crystal structure and vibrational characteristics The present paper deals with the scheelite-type double tungstates of the formula KY(WO4)2 and NaY(WO4)2. Their structure is derived by statistic

L. Macalik et al. / Journal of Molecular Structure 555 (2000) 289–297 Table 1 6 , Z ˆ 2 and for Factor group analysis for NaY(WO4)2 —C4h 6 KY(WO4)2 —C2h , Z ˆ 4 crystals Unit-cell modes n(N)

n(T)

Activity 0

n(T )

n(L)

n(int)

6 C4h Ag 3 Bg 5 n Eg 5 5 Au 5 3 Bu n Eu 5 5

0 0 0 0 1 0 1 1

0 2 2 2 1 0 1 1

1 0 1 1 0 1 1 1

2 3 2 2 3 2 2 2

36

3

9

6

18

17 19 17 19

0 0 1 2

2 4 4 5

3 3 0 0

12 12 12 12

72

3

15

6

48

6 C2h Ag Bg Au Bu

IR

Raman

– – –

XX ⫹ YY, ZZ XX ⫺ YY,XY XZ, YZ

Z – X, Y

– – –

– – Y X, Z

XX, YY, ZZ, XZ XY, YZ – –

291

2⫹

substitution of the Ca crystallographic sites in the CaWO4 scheelite by K ⫹ (or Na ⫹) and Y 3⫹ ions. KY(WO4)2 crystallizes in the monoclinic space 6 …Z ˆ 4† with cell parameters: a ˆ group C2=c ⬅ C2h  b ˆ 130:5⬚ [6]. Its 10:64; b ˆ 10:35; c ˆ 7:54 A; crystallographic structure is described in detail in earlier work [7]. NaY(WO4)2 crystallizes in the tetra6 …Z ˆ 2† with cell paragonal space group I41 =a ⬅ C4h  c ˆ 11:38 A  [8]. The NYW meters: a ˆ b ˆ 5:24 A; unit cell comprises the isolated WO4 tetrahedra whereas the KYW structure is built from the WO6 octahedra joined by means of WO2W and WOW bonds. The general view of these structures is presented in Fig. 1. The primitive unit-cell of the NYW crystal contains 12 atoms giving rise to 36 …k ˆ 0† fundamental vibrations, described by the irreducible representation: G ˆ 3A g ⫹ 5Bg ⫹ 5Eg ⫹ 5Au ⫹ 3Bu ⫹ 5Eu : Its distribution among n(T) acoustic, n(T 0 ) translational, n(L) librational and n(int) internal modes is presented in Table 1. The results of the factor group analysis (FGA) for

Fig. 2. Polycrystalline and polarized Raman spectra of the KY(WO4)2 crystal (the following relation between the optical x, y, z and crystallographic a, b, c directions exists. x k aⴱ ; y k b and z k c; ⬔b…a; c† ˆ 130⬚; ⬔…aⴱ ; c† ˆ 90⬚†:

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Fig. 3. Polarized Raman spectra of the NaY(WO4)2 crystal (x k a; y k b and z k c; x, y, z and a, b, c are optical and crystallographic directions, respectively).

KY(WO4)2 crystal are also listed in Table 1. The number of librational lattice modes for this crystal was calculated taking into account that the tungstate anions appear as W2 O4⫺ 8 dimers. The Raman active A and B modes were measured in x(yy)z, x(yx)z, and z(xz)x geometries, where x k aⴱ ; y k b and z k c; ⬍ b…a; c† ˆ 130⬚; ⬍ …aⴱ ; c† ˆ 90⬚. The optic modes of the tetragonal unit cell were obtained recording the Raman active A, B and E modes in x(zz)y, x(yx)y, x(zx)y and x(yz)y geometries, where x k a; y k b and z k c: The recorded spectra for the KYW crystal are presented in Fig. 2 and those of the NYW crystal in Fig. 3. The Rayleigh wings are seen in the almost all spectra presented in Figs. 2 and 3 but do not appear in the Ag -x…yx†z spectrum (Fig. 2). The intensity of the bands of the latter spectrum is significantly higher and the Rayleigh scattering is narrower in comparison with the other spectra. The frequencies of the optical Raman active modes for crystals studied are compiled in Table 2.

The discussion of the KYW spectra was presented in our previous paper [9] where the description of the external and internal modes was proposed. This analysis was based on the polarization behaviour of the bands measured for several differently oriented samples with the different amounts of rare-earth element doping. The number of modes observed for KYW is significantly smaller than predicted for this crystal but larger than expected for the simple CaWO4 scheelite [10–13]. For the NYW, the number of bands is the same as observed for pure CaWO4, which proves that the structure of both these crystals is isomorphic. The most intense line appears at 904 cm ⫺1 for potassium tungstate and at 919 cm ⫺1 for sodium derivative having higher frequency than CaWO4 scheelite [10]. It is assigned to fully symmetric vibrations of the WO2⫺ 4 ions. It is well known that the scheelite crystals, where oxy-ions form isolated tetrahedra, exhibit the energy

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293

Table 2 The frequencies of the optical Raman active modes for the NaY(WO4)2 and KY(WO4)2 crystals KY(WO4)2

NaY(WO4)2

Assignment

Ag

Bg

Ag-x(zz)y

Bg-x(yx)y

Eg-x(zx)y

Eg-x(yz)y

Assignment

n s(WO6) n s(WO6) ⫹ n (WOOW) a n as(WO6) ⫹ n as(WOW) a n (WOOW) Stretching

933 m 904 vs 808 m 763 s 750 sh 738 sh 687 w 675 sh 620 w 530 w 498 w 443 w 415 sh 406 w 396 w 375 m 350 m 345 sh 316 w 298 w

933 sh 903 vs 807 w 761 vs 746 s

919 vs

919 s

919 s

919 s

n s(WO4)

811 m

807 m,b

807 m,b

n as(WO4)

416 m

411 m

411 m

d as(WO4)

339 m

335 m

335 m

d s(WO4)

175 w 216 vw 123 w

175 w

216 m

L(WO4) T 0 (Na ⫹/Y 3⫹)

84 s 67 m

84 m

n s(WOOW) in-plane Stretching n s(WOW) v (WOOW) out-of-plane wagging d (WOOW) in-plane bending d as(WO6) d s(WO6)

d (WOOW) out-of-plane bending T 0 (K ⫹) d (WOW) bending L(WO6) T 0 (Na ⫹/Y 3⫹) T 0 (WO6)

a

236 m 224 m 178 w 149 w 121 w 109W 88 m 75 w

Abbreviations: WOOW is related to the

686 m 610 w 530 w 498 w 441 m 406 m 397 w 374 w 349 w 345 w 314 m 295 w 258 w 235 m 224 m 177 m 150 w 121 w 109 w 87 vs 75 w

335 s

224 w

84 s 67 m bridge bond, WOW to the

gap in the 450–700 cm ⫺1 region [14–17]. This is true also for the NYW crystal studied here. The stretching n s, n as and bending d as, d s vibrations are observed for this material at 919, 807–811, 411–416 and 335–339 cm ⫺1, respectively. Their symmetry can be deduced from the dichroic behaviour of the generative Raman lines. The inspection of the polarized spectra allow to assign the lines at 919, 335 and 224 cm ⫺1 to the Ag phonons, at 811, 416, 339, 216 and 67 cm ⫺1 to the Bg phonons and at 807,411, 175, 123 and 84 cm ⫺1 to the Eg phonons. Their vibrational description is proposed in Table 2. These results coincide very well with those observed by Porto and Scott for

123 w T 0 (WO4)

oxygen bond.

the simple scheelites [18]. Although the orientation of the NYW crystal was done using X-ray method and the quality of the polarizers was high, however, some of the strongest bands were not fully polarized. For the example the Ag line at 919 cm ⫺1 are seen in the Bg and Eg spectra being significantly weaker. This effect was expected for the double tungstates of the scheelite type. The M I and M III ions in such crystals are short-range ordered and their distribution deviates from statistical. The local disorder of the cationic layers forms the domain-like structure, which seems to be characteristic for scheelite-type crystals. This is the reason why some of the bands become active in all

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Table 3 Correlation diagram for vibrations of WO42⫺ in KY(WO4)2 and NaY(WO4)2 crystals. (Abbreviations: IR—infrared activity, RS—Raman activity)

polarized spectra although they should be fully polarized. The same effect was observed by us previously for NaBi(MoO4)2 crystal [19–21]. The final energy level scheme for the NaY(WO4)2 crystal is given in Table 3, where the correlation diagram for both crystals studied are compared. The results of the Raman studies for the KYW crystal were discussed in our previous work [9]. Therefore, in this paper, the discussion of their IR

and RS data will be restricted to the bridge bond vibrations. Comparing the Raman spectra of the NYW and KYW crystals, all additional lines observed for the latter at 440–700 and 230–300 cm ⫺1 regions should be related to the WOW and WOOW bonds. The vibrations of the double oxygen bridge is characterised by six normal modes, four in-plane and two out-of-plane, where quasi-plane is formed by four W–O bonds of the system. These modes

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295

Fig. 4. The vibronic part of the 7F0 ! 5D0 excitation spectrum of the 5D0 emission of Eu 3⫹ in KEu(WO4)2 (solid line) and Raman spectrum for KEu(WO4)2 crystal (dashed line).

could be given in the graphical form as follows [19,22]: in-plane:

Table 4 The relative positions of the vibronic lines in the 7F0 ! 5D0 excitation spectrum of the 5D0 emission of Eu 3⫹ in KEu(WO4)2 at 77 K. The zerophonon line is at 17 197 cm ⫺1 Dn ˆ E1 ⫺ E0 (cm ⫺1)

Energy level

l (nm)

n (cm ⫺1)

(cm ⫺1)

552.0 555.6 557.7 559.3 561.6 564.1 566.8 567.9 570.1 573.5 576.0 577.8

18 17 17 17 17 17 17 17 17 17 17 17

919 802 734 682 609 530 446 412 344 240 164 110

116 999 931 879 806 727 643 609 541 437 361 307

Assignment to the respective normal modes

ag or bg ag or bg ag ag or bg bg ag or bg ag ag ag or bg ag or bg ag or bg

n s(WO6) n s(WO6) and n as(WOW) n (WOOW) n s(WOOW) n s(WOW) d (WOOW) d as(WO6) d s(WO6) T 0 (K ⫹) and/or d (WOW) T 0 (Eu 3⫹) T 0 (WO6)

296

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and out-of-plane:

Our studies presented here allow us to assign the respective vibrational levels to these modes. They are: n A, 904 cm ⫺1; n B, 763 cm ⫺1; n C, 687 cm ⫺1; n D, 443 cm ⫺1; n E, 498 cm ⫺1 and n F 298 cm ⫺1 for Ag spectra. Another energy sequence could be proposed for the single bridge bond system WOW formed in the KYW crystal. The vibrations of this bond are described by means of three normal modes, two of them in-plane and one out-of-plane. On the basis of the present studies the following assignment could be made: in-plane vibrations:

lowest crystal field components of the ground state and the excited state occurs at 0 cm ⫺1. The assignment of the vibronic components to the respective Raman active modes is proposed in the Table 4. The present studies of the polarized Raman spectra of KYW and NYW crystals allow to describe the vibronics to the external lattice vibrations at low frequences (⬍260 cm ⫺1), vibrations of the Y–O and Eu–O bonds (120–200 cm ⫺1) and various internal vibrational components of the WO4 or WO6 groups and bridge bonds (310–1000 cm ⫺1). It should be noted that the bridge bonds modes play an important role in the electron–phonon coupling and consequently in the energy transfer in these crystals. Acknowledgements

and out-of-plane modes:

The Ag vibrational frequencies of these modes are observed in the Raman spectra at 808, 530 and 236 cm ⫺1, respectively. The vibrational characteristics proposed in the present paper are based on the polarized Raman spectra measurements performed for the oriented NYW and KYW crystals. For the complete results, the polarized IR spectra are needed. We will try to complete them in the future after getting higher dimension crystals. The KYW crystal is an attractive host material for doping with optically active RE ions. Our studies of such crystals indicate that the vibronic transitions complicate their absorption and emission spectra. However, it is possible to determine which lines are vibronic by comparison of these spectra to the crystal Raman spectra (Fig. 4). As phonons couple to different electronic levels, it is possible to observe the corresponding vibronic sideband in the absorption and emission spectra. Particularly strong electron– phonon coupling was observed for the Eu 3⫹-iondoped KYW [23]. Fig. 4 presents the excitation spectrum plotted as a function of energy vs. wavenumbers. It is shifted in such way that the transition that corresponds to the energy difference between the

This work was supported by the Polish State Committee for Scientific Research, Grant No 3T09A 057 13. A.A.K. is grateful to the Russian Foundation for Basic Research and the State Scientific Programs “Fundamental Metrology” and “Fundamental Spectroscopy” for partial financial support. The authors acknowledge cooperation with the Joint Open Laboratory for Laser Physics and Precise Laser systems. References [1] G. Blasse, Struct. Bonding 42 (1980) 1. [2] A.A. Kaminskii, N.R. Agamalyan, L.P. Kozeeva, V.F. Nesterenko, A.A. Pavlyuk, Phys. Status Solidi A 75 (1983) K1. [3] A.A. Kaminskii, Phys. Status Solidi A 148 (1995) 9. [4] A.M. Ivanyuk, V.A. Sandulenko, M.A. Ter-Pogosyan, P.A. Shachverdov, V.G. Chervinskii, A.V. Lukin, V.L. Ermolaev, Optika i Spektroskopiya 62 (1985) 961. [5] A.A. Kaminskii, Kvantovaya Elektronika 23 (1993) 457. [6] S.V. Borisov, R.F. Klevtsova, Kristallografiya 13 (1968) 517. [7] L. Macalik, J. Hanuza, B. Macalik, W. Ryba-Romanowski, S. Go;a˛b, A. Pietraszko, J. Lumin. 79 (1998) 9. [8] P.V. Klevtsov, R.F. Klevtsova, Zh. Strukt. Khim. 18 (1977) 419. [9] J. Hanuza, L. Macalik, Spectrochim. Acta A 43A (1987) 361. [10] J.F. Scott, J. Chem. Phys. 48 (1968) 874. [11] M. Nicol, J.F. Durana, J. Chem. Phys. 54 (1971) 1436. [12] V. Ramakrishnan, G. Aruldhas, Spectrochim. Acta 41A (1985) 1301. [13] V. Ramakrishnan, G. Aruldhas, Spectrochim. Acta 42A (1986) 1341. [14] N. Yamada, S. Shionoya, J. Phys. Soc. Jpn. 31 (1971) 3. [15] N. Yamada, S. Shionoya, J. Phys. Soc. Jpn. 31 (1971) 841.

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