Raman active modes in Nd2BaCu3Oz compound

Physica C 338 Ž2000. 151–156 www.elsevier.nlrlocaterphysc

Raman active modes in Nd 2 BaCu 3 Oz compound Valery Petrykin a,) , Masato Kakihana a , Pedro Berastegui b a

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan b Inorganic Chemistry Arrhenius Laboratory, Stockholm UniÕersity, Stockholm, Sweden

Abstract In the present paper, group theoretical analysis was carried out for the insulating phase Nd 2 BaCu 3O z ŽNd213.. Raman spectra were collected for different polarization geometry using bulk sample and micro setup. The polarization analysis of Raman active lattice modes was performed and the origin of additional mode in oxygen-deficient samples was discussed. q 2000 Elsevier Science B.V. All rights reserved. PACS: 74.72.Jt; 74.25.Kc Keywords: Nd 1q x Ba 2yx CuOz solid solutions; Symmetry analysis; Raman spectroscopy

1. Introduction NdBa 2 Cu 3 O 7 ŽNd123. based high-temperature superconducting materials attract a lot of attention due to their high transition temperature in the optimally doped state and better stability in a strong magnetic field compared to YBa 2 Cu 3 O 7 compound due to the Jc –H peak effect w1x. It was suggested that the peak effect appears as a result of periodic compositional variations in Nd 1q x Ba 2yx Cu 3 Oz solid solutions from stoichiometric NdBa 2 Cu 3 O 7 to Nd 2 BaCu 3 Oz ŽNd213. w2x. Nd 2 BaCu 3 Oz was later prepared in the single-phase form and its crystal structure was determined using powder XRD data ŽFig. 1. Ref. w3x. The most prominent feature of its unit cell ŽB2mm. is the absence of inversion center, which together with the non-superconducting nature

) Corresponding author. Tel.: q81-45-924-5310; fax: q81-45924-5309. E-mail address: [email protected] ŽV. Petrykin..

of this compound makes it extremely interesting for the investigations by Raman spectroscopy. In this case, new modes, normally forbidden for the NdBa 2 Cu 3 O 7 , should appear, while being an insulating compound, Nd213 should have much higher Raman scattering cross-section compared to Nd123 and would produce a very strong signal w4x. These two features could be used to monitor compositional variations in the Nd123 based materials using Raman spectroscopy. The Raman spectroscopic studies of the Nd213 were carried out w5x and it was found that observed Raman spectra for different polarizations do not correlate well with the predictions of the group theoretical analysis for the structure on Fig. 1. The authors assumed that the distortions leading to the B2mm space group are not significant and hence a lot of new expected modes have very low intensity. It was suggested that more symmetric structure should be considered to find a reasonable explanation for the experimental results. We also reported w6x the mismatch between published crystal structure w3x and observed Raman,

0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 Ž 0 0 . 0 0 2 1 8 - 5

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san Raman spectrometer equipped with =90 lenses, which allows the laser beam to be focused onto a spot of f 1–2 mm. The 514.5-nm line of the Ar laser was used for the excitation; the scattered light was analyzed by the triple spectrometer and the spectra were recorded by a liquid nitrogen cooled CCD detector. The data were collected at room temperature in air using different polarization of incident and scattered light.

3. Results and discussions Crystal structure of Nd 2 BaCu 3 O 7 is presented in Fig. 2 w7x. Neglecting the possible distortions due to the oxygen non-stoichiometry, the unit cell should be considered as face-centered, belonging to the Ammm space group ŽCmmm in the standard settings.. Atomic positions together with the site symmetry are sum-

Fig. 1. Unit cell of Nd 2 BaCu 3 O 7 determined from powder XRD data in Ref. w3x.

IR-spectra of Nd213. In addition we pointed out that observed variable oxygen stoichiometry is not allowed in the proposed structural model w3x presented on Fig. 1. The correct structure was determined and refined recently from the combined set of XRD and TOF data w7x ŽFig. 2.. In this paper we present our own results of the polarized Raman study of Nd 2 BaCu 3 Oz and group theoretical analysis for the correct structure.

2. Experimental Bulk samples of Nd 2 BaCu 3 Oz were prepared by polymerized complex method w8x. The details of sample preparation and crystal structure refinement were described in Ref. w7x. The sample obtained was investigated by micro-Raman spectroscopy. The spectra were acquired by a Jobin IvonrAtago Bus-

Fig. 2. Unit cell of Nd 2 BaCu 3 Oz determined and refined using joint set of XRD and TOF neutron powder diffraction data in Ref. w7x.

V. Petrykin et al.r Physica C 338 (2000) 151–156

marized in the Table 1. The crystallographic unit cell ˚ b s 7.7724 A, ˚ c s 22.9862 ŽAmmm a s 3.8565 A, ˚ . contains Z s 4 formula units. The primitive cell A ˚ b s 12.1323 A, ˚ cs is monoclinic Ž a s 12.1323 A, ˚ g s 142.63588. with Z s 2 and two times 3.8565 A, smaller volume. The total number of phonon modes for N s 27 atoms in the primitive unit cell should be 3 N s 81. The group theoretical analysis for the Nd 2 BaCu 3 Oz structure at k ( 0 can be carried out using the nuclear site group approach w9x, which was shown to be valid for the cases of non-symmorphic space groups and when the crystallographic unit cell is not the primitive one. The analysis gives irreducible representation of the lattice modes in the form: G s 13A g q 3A u q 5B1g q 14 B1u q 8B 2 g q 14B 2 u q 13B 3g q 11B 3 u where B1u q B 2u q B 3u are acoustic modes, 13A g q 5B1g q 8B 2g q 13B 3g are Raman active, 13B1u q 13B 2u q 10B 3u-IR-active and 3A u are silent modes. Discussing the experimental results one should pay attention to the variable oxygen content, since it may be responsible for the appearance of forbidden modes due to the local break of symmetry. In addition different types of twin domains, which exist in the bulk sample w7x can result in the polarization leakage and observations of Raman-active modes for the polarization geometry where they should be absent under ideal conditions.

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The observed Raman spectra for different polarizations are presented on Fig. 3. The scattering geometry for each observation is given in Porto notation. Crystal axes are chosen in the non-standard setting of the space group ŽAmmm., when the longest unit cell parameter coincides with z axis Žcommonly considered for high-Tc superconductors.. We understood that it would be impossible to distinguish a and b axes of small crystallites purely from their habitues so we collected several spectra from different crystallites in the polarization geometry, which would correspond to B 2g and B 3g phonons selection rules trying to obtain groups of spectra with the pronounced differences. It turned out that all acquired spectra had only minor differences Žtwo typical spectra are presented on Fig. 3b denoted as ‘‘ y Ž xz . y’’ and ‘‘ x Ž yz . x’’.. This led us conclude that both B 2g and B 3g phonons are present on the same spectra probably because of twinning which cannot be avoided in this compound. The frequencies of normal modes are listed in the Table 2. One may find 12 normal modes of A g symmetry, almost as expected, while the number of B 1g , B 2g and B 3g modes is much smaller than deduced from the group theoretical analysis. It may happen due to the overlap of Raman peaks with close frequencies or some of modes may have low intensity to be observed under the specified conditions. As we pointed out, modes normally forbidden for a specific geometry may appear due to the twinning so it is possible that some modes in Table 2 originate from the polarization

Table 1 Atomic parameters, site symmetry and irreducible representations of lattice normal modes of Nd 2 BaCu 3 Oz . Space group Ammm ŽNo. 65., ˚ b s 7.7724 A, ˚ c s 22.9862 A˚ a s 3.8565 A, Atom

Site symmetry

x

y

z

Representation of normal modes

BaŽ1. NdŽ1. NdrBaŽ2. CuŽ1. CuŽ2. OŽ1. OŽ2. OŽ3. OŽ4. OŽ5. OŽ6.

C2z Õ C2z Õ C2z Õ C2yÕ C sy z C2yÕ C sy z C sy z C2z Õ C2z Õ D2 h

0 0 0 1r2 1r2 0 1r2 0 1r2 1r2 1r2

0 0 0 0.2824 0.2502 0.3076 0.2952 0.2565 0 0 0

0.08446 0.24712 0.40428 0 0.17474 0 0.08032 0.18328 0.1818 0.3144 0

A g q B1u q B 2g q B 2u q B 3g q B 3u A g q B1u q B 2g q B 2u q B 3g q B 3u A g q B1u q B 2g q B 2u q B 3g q B 3u A g q B1g q B1u q B 2u q B 3g q B 3u 2A g q A u q B1g q 2B1u q B 2g q 2B 2u q 2B 3g q B 3u A g q B1g q B1u q B 2u q B 3g q B 3u 2A g q A u q B1g q 2B1u q B 2g q 2B 2u q 2B 3g q B 3u 2A g q A u q B1g q 2B1u q B 2g q 2B 2u q 2B 3g q B 3u A g q B1u q B 2g q B 2u q B 3g q B 3u A g q B1u q B 2g q B 2u q B 3g q B 3u B1u q B 2u q B 3u

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to diagonal elements of Raman tensor Žthis is often observed for the Y123 type compounds.. On the other hand, the crystal structure of Nd213 is rather complex and it can be anticipated that different phonons may have similar frequencies so we did include phonons with the similar frequencies but different polarization configuration into the Table 2. The amusing similarity of zz-polarized spectra of Nd213 and Nd123 was mentioned previously w5,6x. The evolution of Nd 1q x Ba 2yx Cu 3 Oz solid solutions Raman spectra w5,6x allows one to assign the strongest mode at 560 cmy1 to the vibrations of apical oxygen OŽ2. ŽFig. 3a.. Similarly, Raman active modes at 132, 147 and 440 cmy1 ŽFig. 3a. could be attributed to the vibrations of BarNdŽ2., CuŽ2. and CuŽ2. – OŽ3–5. atoms respectively. It is interesting to note that phonon at 293 cmy1 has tetragonal-like B 1g symmetry similar to Y123 case ŽFig. 3c.. One may attribute it to the vibrations of CuŽ2. and OŽ3–5. in the CuO 2 planes based on the data available for Nd123 solid solutions; however, more conclusive experiments are required to elucidate its nature. It is important to stress that although we guessed the nature of several normal modes, our preliminary results on lattice dynamics calculations demonstrate

Table 2 Observed Raman active modes for Nd 2 BaCu 3 Oz ceramic sample. Irreducible representations and selection rules for the observed Raman-active phonons A g Ž x 2, y2, z 2 .

B1g Ž xy .

B 2g Ž xz .qB 3g Ž yz .

79 112 132 147 176 267 298 420 440 510 554 560

54 212 440 550

132 144 155 185 293 440 a 560 a 630

Expected

13A g

5B1g

Observed

12

4

8B 2g Žfor B 2g Ž xz ..; 13B 3g Žfor B 3g Ž yz .. 8

Fig. 3. Observed micro-Raman spectra for different scattering geometry. Experiment geometry is indicated using Porto notation; Ža. A g-like phonons; Žb. B 2g qB 3g ,B 1g-like phonons; Žc. tetragoX X nal-like B1g mode; x and y correspond to the x q458 and y q458 orientation of polarization vector of incident and scattered beams.

leakage. Comparison of the spectra on Fig. 3a and b help conclude that such polarization leakage is significant for the cross-polarized configuration and completely absent for the polarization corresponding

a

The observed modes may be due to the polarization leakage or imperfect orientation of the crystallites towards laser beam.

V. Petrykin et al.r Physica C 338 (2000) 151–156

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Fig. 4. Macro Raman spectrum collected for an oxygen deficient Nd 2 BaCu 3 Oz sample. Extra mode appears at 580 cmy1 .

that in many cases the amplitudes of several atoms are comparable and a particular normal mode can be hardly described in terms of single atom displacement. It should be mentioned that we did not find any peak at 580 cm – 1 observed in Ref. w5x in the properly oxidized samples. However, it is usually present in the oxygen-deficient Nd213 compound ŽFig. 4.. The different intensity of this mode compared to the 560 cmy1 modes in our oxidized, oxygen-deficient samples and in the samples used in Ref. w5x let us conclude that it appears as a result of oxygen disordering in the CuOz planes ŽCuŽ1. –OŽ1,6... It is interesting that IR spectrum of Nd213 does not contain this mode either w7x and one may probably assign it to the silent A u mode, which becomes Raman-active due to the local structure distortion. Another interesting observation that should be mentioned is the scattering by continuum at room temperature in xx, yy and xX yX configurations, which correspond to A g symmetry ŽFig. 3.. Due to the structure complexity the phonon assignment cannot be carried out purely from experimental data since a normal mode includes vibrations of different atomic groups simultaneously. The lat-

tice dynamics calculations, which will help to solve this problem, are in progress. 4. Summary The polarized Raman spectra for oxidized Nd 2 BaCu 3 O 7.3 polycrystalline sample were collected. The origin of Raman active phonons in different scattering geometry was discussed based on the group theoretical analysis carried out for the crystal structure recently determined from joint set of neutron and X-ray diffraction data w7x. Scattering by continuum at room temperature was observed for the xx, yy and xX yX configurations. Acknowledgements We are thankful to Dr. Minoru Osada for reading and useful comments on this manuscript. References w1x M. Murakami, S.I. Yoo, T. Higuchi, Jpn. J. Appl. Phys. 33 Ž1994. L715, Part 2.

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