Polarized Raman and IR spectra of oriented Cd0.9577Gd0.0282□0.0141MoO4 and Cd0.9346Dy0.0436□0.0218MoO4 single crystals where □ denotes the cationic vacancies

Polarized Raman and IR spectra of oriented Cd0.9577Gd0.0282□0.0141MoO4 and Cd0.9346Dy0.0436□0.0218MoO4 single crystals where □ denotes the cationic vacancies

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 255–259 Contents lists available at ScienceDirect Spectrochimica Acta...

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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 255–259

Contents lists available at ScienceDirect

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa

Polarized Raman and IR spectra of oriented Cd0.9577Gd0.0282h0.0141MoO4 and Cd0.9346Dy0.0436h0.0218MoO4 single crystals where h denotes the cationic vacancies L. Macalik a, E. Tomaszewicz b, M. Ptak a,⇑, J. Hanuza a,c, M. Berkowski d, P. Ropuszynska-Robak c a

Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, 2 Okólna Str., 50-422 Wrocław, Poland Department of Inorganic and Analytical Chemistry, West Pomeranian University of Technology, Al. Piastów 42, 71-0065 Szczecin, Poland Institute of Chemistry and Food Technology, Faculty of Engineering and Economics, Wrocław University of Economics, 118/120 Komandorska Str., Wrocław, Poland d Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warszawa, Poland b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Cd0.9577Gd0.0282h0.0141MoO4 and

Raman and infrared spectra of CdMoO4.

Cd0.9346Dy0.0436h0.0218MoO4 crystals were grown.  Polarized Raman and IR reflectance spectra were measured.  Influence of the cationic vacancies on the structure and properties was discussed.

a r t i c l e

i n f o

Article history: Received 16 January 2015 Received in revised form 18 March 2015 Accepted 27 March 2015 Available online 13 April 2015 Keywords: CdMoO4 solid solution doped with Gd3+ and Dy3+ ions Polarized IR and Raman spectra Factor group analysis

a b s t r a c t Polarized Fourier Transform IR and Raman spectra of Cd0.9577Gd0.0282h0.0141MoO4 and Cd0.9346Dy0.0436h0.0218MoO4 oriented single crystals have been recorded and analyzed using the factor group approach (h denotes the cationic vacancies). The tetragonal I41/a (C64h) space group with Z = 2 has been applied in the discussion. The influence of the structural changes induced by the defects in the CdMoO4 host lattice on the vibrational symmetry rules has been analyzed. The assignment of the observed bands to the internal and external modes has been proposed. Ó 2015 Elsevier B.V. All rights reserved.

Introduction MIIMoO4 molybdates have been widely used as host crystals for rare-earth ions in solid state lasers, scintillators and ionic conductors [1]. They crystallize at ambient conditions in the tetragonal scheelite-type structure: space group I41/a, No. 88, Z = 4 [2]. In ⇑ Corresponding author. E-mail address: [email protected] (M. Ptak). http://dx.doi.org/10.1016/j.saa.2015.03.122 1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

their unit cell each molybdenum atom coordinates four equivalent oxygen atoms in the tetrahedral symmetry. MII atoms join eight adjacent MoO4 units, forming MIIO8 bisdisphenoids. Among other compounds of this class, CdMoO4 plays a special role because its arrangement is very close to the scheelite–wolframite instability [2]. Its high-pressure X-ray diffraction studies were performed by Errandonea et al. [3] in which the wolframitetype structure (P2/c, No. 13, Z = 2) was proposed for the highpressure phase. The pressure-induced phase transitions of this

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compound were reported by Jayaraman et al. [4] on the basis of the Raman spectra measurements. In their work two high-pressure wolframite-type phases were identified. For the above reasons, the behavior of this crystal upon doping of M3+ ions for Cd2+ ions is an important problem, especially when it is used as the host crystal for RE ions. The solid solutions that can be obtained in such substitution were studied by us in our previous work [5]. We studied the correlation between the crystallographic and spectroscopic data for Cd13xGd2xhxMoO4 solid solutions in terms of their structural changes induced by the defects in the CdMoO4 host lattice. It was established that for the Cd:Gd ratio higher than 50% of gadolinium the resulting mixture consists of two coexisting phases: Gd2(MoO4)3 and Cd13xGd2xhxMoO4 solid solution. For 0 < 2x 6 0.5, the structure of the solid solution originates from the tetragonal scheelite I41/a (C64h) space group. Also, the existence of clear relationships between the amount of the cationic vacancies and integral intensity and wavenumbers of the bands corresponding to the MoO4 unit was reported. Analyzing these results a new scientific problem could be derived: how far the changes induced by the doped ions and resulting defects can disturb the vibrational selection rules obeyed in the IR and Raman spectra of the oriented single crystals. This problem can be explained by measurements of polarized IR and Raman spectra of the oriented single crystals synthesized for the compositions Cd0.9577Gd0.0282h0.0141MoO4 and Cd0.9346Dy0.0436h0.0218MoO4 (abbreviated as CdREhMo) Therefore the aim of the present work is to answer this question.

Experimental details

monocrystalline samples of CdREhMo single crystals in a dilute aqueous hydrochloric acid solution. The concentration in Cd13xGd2xhxMoO4 was determined to be 1.63 wt% (0.47 at%) and it corresponds to the following formula of a solid solutions Cd0.9577Gd0.0282h0.0141MoO4. In a similar way the content of dysprosium ions (2.62 wt%, 0.73 at%) was determined giving the formula of the second studied crystal: Cd0.9346Dy0.0436h0.0218MoO4. Dielectric and magnetic properties of Cd0.9577Gd0.0282h0.0141MoO4 single crystal have been studied in [6].

IR and Raman studies FT Raman spectra in the 1000–80 cm1 range were measured using a Bruker FT-Raman RFS 100/S spectrometer and the 1064 nm excitation. The measurements were performed by the right angle scattering technique. The spectral resolution was 2 cm1. These spectra were compared to those recorded on Renishaw InVia Raman spectrometer equipped with confocal DM 2500 Leica optical microscope, a thermoelectrically cooled Ren Cam CCD as a detector and Ar+ ion laser operating at 488 nm. The mid- and far-IR spectra of the polycrystalline samples were measured with a Biorad 575C FT-IR spectrometer with the 2 cm1 resolution. The spectra were recorded using KBr disc and Nujol mulls techniques. The orientation of the crystallographic axes were accomplished using X-ray method. The orientation of the x || a, y || b and z || c axes were used for the unit cell I41/a: a = 5.1569, b = 5.1569 and c = 11.1888 Å [6].

Crystal growth Symmetry-based vibrational characteristics Single crystals of CdREhMo solid solutions have been successfully grown by the Czochralski method in an inductively heated platinum crucible in an ambient air atmosphere (Fig. 1). The following metal oxides were the starting materials for crystallization process: Gd2O3 (or Dy2O3) (99.99%, Alfa Aesar), CdO (99.998%, Alfa Aesar), and MoO3 (99.95%, Alfa Aesar). The oxides were dried at 300 °C, weighed in appropriate amounts, mixed and pressed into pellets. The pellets were heated in a furnace at 800 °C for 4 h and next they were loaded into a proper platinum crucible and melted. The single crystals were grown on (0 0 1) oriented seed prepared from a single crystal of pure CdMoO4, at the pulling rate of 3 mm/h and the rotation rate of 4 or 10 rpm. The as-grown crystals were dark-blue almost black in color. The concentration of gadolinium and dysprosium ions was determined by ICP-MS method (IY ULTRACE 238 spectrometer) after dissolving

The studied CdREhMo crystals are isostructural with the CdMoO4 molybdate crystallizing at ambient temperature and pressure in the tetragonal scheelite-type structure I41/a (No. 88) (C64h) and two formula units per primitive cell [2]. In this unit Cd2+ and Mo6+ ions occupy the 4b and 4a sites of the S4 symmetry, whereas oxygen ions are at 16f sites of the C1 symmetry. The tetragonal C64h unit cell comprises 12 atoms that have 36 zonecenter degrees of freedom distributed among the irreducible representation: CN = 3Ag + 5Au + 5Bg + 3Bu + 5Eg + 5Eu. The representation: CT = Au + Eu describes the acoustic modes and: CO = 3Ag + 4Au + 5Bg + 30 Bu + 5Eg + 4Eu, the optical phonons. Their distribution among the internal and external modes was presented in our previous work [5]. The vibrational representation of 33 optical phonons can be further distributed into:

Fig. 1. Images of CdMoO4 solid solution single crystals doped with Gd3+ (left) and Dy3+ (right) grown by the Czochralski method.

L. Macalik et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 255–259

 Translational modes:  Rotational modes:  Internal modes:

T0 (Cd2+/RE3+/Mo6+) = Au + 2Bg + 2Eg + Eu R(MoO4) = Ag + Bu + Eg + Eu symmetric stretching ms(MoO4) = Ag + Bu asymmetric stretching mas(MoO4) = Au + Bg + Eg + Eu symmetric bending ds(MoO4) = Ag + Au + Bg + Bu asymmetric bending das(MoO4) = Au + Bg + Eg + Eu

Table 1 presents the results of the factor group analysis performed for the studied crystal. The Ag, Bg, Eg vibrations are Raman-active, whereas the Au and Eu modes are IR active only. The Bu modes are inactive both in Raman and IR spectra. The following polarized Raman spectra were measured:  = A modes, cðbbÞc = A + B modes, cðabÞc = B modes, bðccÞb g g g g  = Eg modes. aðbcÞa

Table 1 Factor group analysis of the unit cell vibrations for tetragonal space group (I41/a, C64h); Z = 2 (N – unit cell phonons, T – acoustic phonons, T0 – translations, L – librations, i – internal modes). Symmetry

Ag Au Bg Bu Eg Eu Dimension

T

T0 (MoO4)

L(MoO4)

3 5 5 3 5 5

0 1 0 0 0 1

0 1 2 0 2 1

1 0 0 1 1 1

2 3 3 2 2 2

36

3

9

6

18

N

i

Activity RS

IR

x2 + y2, z2 – x2  y2, xy – xz, yz –

– z – – – x + iy x  iy

257

The IR spectra were measured for the powder sample and for oriented crystal with the || a  Eu modes, || b  Eu modes and || c  Au modes. Polarized Raman and IR spectra Polarized Raman spectra recorded of the Cd0.9577Gd0.0282h0.0141MoO4 crystal are shown in Fig. 2. According to the results of the factor group analysis the Raman active Ag pho experiment, for which only nons could be obtained from the bðccÞb Ag modes should be observed. In this spectrum three ms(MoO4), ds(MoO4) and R(MoO4) phonons should be active, and they are observed at 865, 310 and 179 cm1 (see Fig. 2). Apart from these bands additional three very weak bands appear at 406, 760 and 910 cm1. Two former lines appear due to polarization leakage of the Eg modes. The band at 910 cm1 arises probably from the combination of the Eg 760 + 152 wavenumbers. In the polarized cðabÞc Raman spectrum five bands should be observed, all corresponding to the Bg modes. These are two translatory lattice T0 modes and three internal modes ds(MoO4), das(MoO4) and mas(MoO4). The bands corresponding to these vibrations are observed at 134, 192, 306, 399 and 822 cm1, respectively. Moreover, three weak bands originating from the Eg modes are also observed at 760, 865 and 910 cm1. Both Ag and Bg modes should be active when the cðbbÞc recording geometry is applied. Therefore eight bands are expected in the respective Raman spectrum but only six are observed. These are 134, 192, 399 and 822 cm1 bands corresponding to the Bg modes, 865 cm1 band of Ag symmetry and 310 cm1 band that corresponds to both Ag and Bg symmetry modes. The one missing Ag  polarization, is too weak band, observed at 179 cm1 in the bðccÞb to be observed in the cðbbÞc polarization.  spectrum should show five bands The polarized aðbcÞa corresponding to the Eg modes. The spectrum shows five intense bands at 760 cm1 – mas(MoO4), 406 cm1 – das(MoO4), 278 cm1 – R(MoO4), 152 cm1 – T0 (Cd3+) and 104 cm1 – T0 (MoO4) that

Fig. 2. Polarized Raman spectra of Cd0.9577Gd0.0282h0.0141MoO4 single crystal.

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L. Macalik et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 148 (2015) 255–259 Table 2 Wavenumbers (xTO, xLO) of the TO and LO modes and oscillator strengths DeTO obtained from the fitting of the reflectance spectra of Cd0.9577Gd0.0282h0.0141MoO4 crystal.

Fig. 3. Polarized reflectance spectra of Cd0.9577Gd0.0282h0.0141MoO4 (solid lines) and Cd0.9346Dy0.0436h0.0218MoO4 (dash lines) single crystals.

Polarization

xTO (cm1)

xLO (cm1)

DeTO

E || b (Eu)

152.5 297.5 747.5

185.1 329.1 893.7

6.682 2.104 3.001

E || c (Au)

123.8 224.6 436.9 717.6

143.8 273.4 447.9 865.1

7.257 5.204 0.559 3.272

complex dielectric constant (Fig. 4d) obtained by fitting of the reflectance spectra by the four-parameter model [7]. The calculated wavenumbers of the TO and LO modes and oscillator strengths DeTO for Cd0.9577Gd0.0282h0.0141MoO4 crystal are presented in Table 2. Table 3 lists the wavenumbers of the all Raman and IR bands obtained from the polarized measurements and their assignment to the respective normal modes. The LO–TO splitting occurs in ionic solids for polar modes. The long-range electric fields associated with long-wave longitudinal phonons are responsible for this phenomenon. Analyzing the results obtained from the fitting of the reflectance IR spectra, unexpectedly large LO–TO splitting should be commented. It probably results from the fact that in the studied here materials the softest TO mode involves the largest mode effective charge that can strongly couple with the electric fields. Similar effects were observed as giant LO–TO splitting in perovskite ferroelectrics [8]. Studied here crystals are characterized by high tetragonal symmetry, ionic nature and occurring of the defects in their structure. Their centrosymmetric nature can be broken by appearing of the cationic vacancies and some phonons may be observed both in the IR and Raman spectra which takes place for the studied here crystals. Influence of the cationic vacancies on the IR and Raman spectra of the studied crystal

Fig. 4. IR spectrum of polycrystalline Cd0.9577Gd0.0282h0.0141MoO4 sample (a); its polarized Au and Eu spectra: reflectivity (b), absorption coefficient (c) and imagine part of 1/e. The experimental reflectivity spectra are drawn using thick line and fitted spectra are presented by thin line.

can be attributed to the Eg modes. The remaining bands originate from polarization leakage of the Ag (310 and 865 cm1) and Bg (192 and 822 cm1) modes. Polarized reflectance IR spectra of Cd0.9577Gd0.0282h0.0141MoO4 and Cd0.9346Dy0.0436 h0.0218MoO4 single crystals are shown in Fig. 3. Fig. 4 compares the absorption spectrum of the polycrystalline sample reported in our previous work [5] with the polarized reflectance spectra of Cd0.9577Gd0.0282h0.0141MoO4. Fig. 4 also shows the absorption coefficient (Fig. 4c) and imaginary part of

The cationic deficiency is a common phenomenon in the solid state chemistry and physics. The spectroscopic methods are widely used in the studies of the materials in which the cationic vacancies define the physicochemical properties [9–13]. Studying of the influence of the cation deficiency on IR spectra, magnetization and XRD patterns of Fe3O4 magnetite, the correlation between the variation of the physicochemical parameters and the amount of the vacancies was found [9]. In particular, IR reflectivity spectra were recognized as an effective tool in the studies of variation of the cation and vacancy distribution in defect chalcopyrite crystal, and this tool is more useful than the Raman spectra [10]. On the other hand, the shift of the m(Mo@O) vibrational wavenumber in the Raman spectra of the vanadomolybdate solid solutions was announced as the measure of the cation vacancy content [11]. Also the luminescence spectroscopy was considered as an effective tool in the studies of the correlations between the concentration of the cationic vacancies and emission band position and intensity [12] as well the lifetime of the luminescence [13]. The studied crystal was synthesized by the Czochralski method under strictly defined conditions to obtain the material of high optical quality. Precise orientation of the samples was made using the X-ray diffraction method. Our data show that the polarized spectra contain strong bands fulfilling the theoretical predictions for crystal symmetry. However, several weak bands are also observed that are not allowed for this symmetry. This behavior can be attributed to creation of cationic vacancies when Gd3+ ions are substituted for Cd2+ ions in CdMoO4. The influence of this effect

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Table 3 Wavenumber (cm1), intensity (vs, s, m, w and vw denote very strong, strong, medium, weak and very weak, respectively), symmetry and assignment of Raman and IR bands. ms, mas, ds, das, L and T0 denote symmetric stretching, asymmetric stretching, symmetric bending, asymmetric bending, librational and translational mode, respectively. Raman-active modes

IR-active modes

Ag

Bg

Eg

910 vw 865 vs

910 vw

910 vw

Au

Assignment Eu

833 vs 822 vs 779 vs 760 vs 437 w 406 w 308 vs

399 vw 308 m

308 w 278 m 245 w

192 vw 179 vw 164 w 152 w 134 vw

134 w 104 w

Combination of 760 and 152 cm1 modes ms(MoO4) mas(MoO4) mas(MoO4) mas(MoO4) mas(MoO4) das(MoO4) das(MoO4) das(MoO4) ds(MoO4) R(MoO4) ds(MoO4) T(MoO4) R(MoO4) R(MoO4) T0 (Cd2+) T0 (MoO4) T0 (MoO4)

contours originating from the respective normal vibrations are spit showing several components and clear shoulders. The obtained single crystal polarized spectra are also influenced by the cationic defects. In the spectra presented in Figs. 2–4 several additional bands not allowed by the symmetry rules for this polarization and orientation of the crystal are clearly seen. The group theory used in the discussion of vibrational spectra usually predicts the number of expected IR and Raman bands. Deviation from these predictions occurs when LO–TO splitting appears for the crystal without the center of inversion, when two-mode behavior is observed in a solid solution or when local structural distortions appear in a studied material. Therefore, it can be postulated that relaxation of the symmetry rules indicates that the studied crystal contain large number of the cationic vacancies. Presence of large amount of vacancies is further supported by observation that the IR and Raman bands of Cd0.9577Gd0.0282h0.0141MoO4 are significantly broader when compared to the bands observed in pure CdMoO4 (see Fig. 5). In conclusion, the obtained in the present work results indicate that due to the unusual phonon properties of the Gd- and Dydoped CdMoO4 crystals they can be good candidates for using them as Raman lasers. The preliminary experiments performed with the use of the femtosecond Nd:YAG excitation are very promising. References Fig. 5. Comparison of IR and Raman spectra of the powder samples and single crystal of Cd0.9577Gd0.0282h0.0141MoO4.

on the spectra of the powder samples was discussed in our previous work [5]. Presence of such vacancies was seen in the dependence of wavenumber and integral intensity of some electron absorptions, IR and Raman bands on increasing concentration of the point defects in the studied materials. The discussion of the obtained results was made taking into account the presence of two types of ions in the unit cell: MoO4 tetrahedra of the S4 symmetry and distorted tetrahedra with the cationic vacancy near one or two tetrahedron’s corners. The structure of the later units was described by the C3v symmetry for the samples with low Gdcontent or C2v symmetry for the tetrahedron that have two neighboring cationic vacancies. Because three different symmetries characterize the structure of the molybdate units, the spectral

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