A systematic study of the Raman spectra of Y-Ba-Cu-O superconducting
materials under different experimental conditions clarifies the controversial results given in the literature. Under the determined conditions and thanks to the micro-Raman technique, reproducible polarized Raman spectra can be obtained and a coherent assignment can be suggested for YBa2Cu307and EuBa2Cu307.For YBa2Cu3(OjF);, the introduction of fluorine atoms was found to be localized in pyramidal CuO units rather than in CuO chains

J. C. Grenier

P. Bezdicka

Pham V. Huong

English

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CROATICA CHEMICA ACTA CCACAA 61 (2) 201-212 (1988)CCA-1791YU ISSN 0011-1643UDC 543.42OriginaL Scientific PaperMicro-Raman Spectra of YBaCuO SuperconductorsPham V. Huong*Laboratoire de Spectroscopie MoLecuLaire et CristaUine (U. A. 124) Universite deBordeaux I, 351, cours de La Liberation 33405 TALEN CE, FmnceandJ. C. GTenieT and P. Bezdicka"Laboratoire de Chimie du Solule, Universitć de Bordeaux I, (U. P. 8661)351, cours de La Liberation, 33405 TALENCE, FranceReceived March 8, 1988A systematic study of the Raman spectra of Y-Ba-Cu-O su-perconducting materials under different experimental conditionsclarifies the controversial results given in the literature. Underthe determined conditions and thanks to the micro-Raman te-chnique, reproducible polarized Raman spectra can be obtainedand a coherent assignment can be suggested for YBa2Cu307andEuBa2Cu307.For YBa2Cu3(OjF);, the introduction of fluorine atomswas found to be localized in pyramidal CuO units rather than inCuO chains.1. INTRODUCTIONSince the discovery of Y-Ba-Cu-O materials exhibiting superconductivityat nearly 93 K, febrile activities have been manifested in many fields. Weattend to a burst of research works circulating in the form of preprintswhich are as much an authority as any printed papers in well establishedjournals.Chu's materials Y-Ba-Cu-01 have been easily reproduced and their macro--physical properties have been reconfirmed and extended". But things aregoing perhaps too fast. This high competition for publishing is certainly agood thing; nevertheless, we must be careful.Many papers are concerned with Raman spectroscopy but generally thereported spectra differ from one author to another+?". As Raman spectro-scopy is well known to be a very accurate technique which allows investigatingdown to the molecular and sub-molecular scales-" ", one can ask what is goingwrong.In this paper, we try to find an explanation for this apparent inconsi-stency by studying, under variable experimental conditions, the Raman spectraof YBa2Cu307_o,YBa2Cu307_oFo.land EuBa2Cu307_ousing micro-Raman spectro-a Permanent address: Institute of Inorganic Chemistry, Center of ChemicalResearch, Slovak Academy of Sciences, CS-84236,Bratislava Czechoslovakia.202 P. v. HUONG ET AL.scopy, this technique being able to give the Raman spectra of materials ofmicrometer size23,24.II. EXPERIMENTALMBa2Cu307--O(M= Y, Eu; (J = 0.2) were prepared from appropriate amountsof barium carbonate, rare-earth oxide (Y203or EU203)and copper oxide. The fineJyground mixture was heated for 20 hours in air at 950 DC,then quenched and re-ground. Pellets of 8 mm diam. were pressed and sintered at 950 DCfor 2 hours andfinally heated at 450 DCfor two days.A phase containing a small amount of fluorine was also prepared in the sameway starting from the following mixture: 1.95 BaC03, 0.05 BaF2, 1.5 Y20S and3 CuO. As barium fluoride is very stable, it can be assumed that there is no hydro-lysis and that the composition is close to YBa2Cu307-,Fo.t.X-ray diffraction powder analysis was carried out on a diffractometer. D. C.resistivity measurements versus temperature were performed using the standardfour probe method. The Raman spectra were recorded at room temperature on amicro-Raman spectrometer DILOR/OMARSusing an ion argon laser source Spectra--Physics ernitting at 514.5 nm, and a diode-array multichannel detector.III. RESULTS AND DISCUSSIONMBa2Cu307_oand the related compounds exhibit »norrnal« macrophysicalproperties.A - X-ray Diffraction AnalysisThe X-ray diffraction analysis shows quite similar patterns to thosepreviously reported and characteristic of the orthorhombic symmetry with thefollowing cell parameters:Compound a (A) b (A) c(A)YBa2Cu307--O 3.83 3.88 11.71YBa2Cu307--OFo.l 3.82 3.88 11.66EuBa:iCu307--O 3.86 3.87 11.71174173175Replacement of yttrium by europium slightly increases the unit cellvolume while a decrease is observed for the fluorinated compound, asexpected from the difference in ionic radius sizes'".B - Resistivity MeasurementsTypical resistivity measurements for YBaCuO and YBaCuOF are shownin Figure 1. The superconducting transition temperatures are, respectively,90.6 K and 89.3 K (zero-resistance Te) in very good agreement with previousresults",The introduction of a small amount of F ions does not affect significantlythe Te value but increases somewhat the resistivity of the sample above Te.This can be explained by the higher electronegativity of fluorine as comparedto oxygen, which increases the electronic localization. Higher concentrationsof fluorine lead to multiphase materials which become insulators. In thecourse of this work, Ovshinsky et al. reported Y-Ba-Cu-O-F materials withYBaCuO 20315.-----------------------------------------~R(mm10YB.1CuOF5YBaCuO°8~0~===-====~9~0----~-----1iO-0---=T~(~K~)--~110Figure 1. Resistance vs. temperature measured at a constant current of 10 mA forYBa2Cu307....•and YBa2Cu307-,Fo1.large amounts of fluorine, showing very high superconducting temperatures'";these results do not seem to be reproducible.Eu-Ba-Cu-O compound shows a superconducting state under 90 K, againin agreement with previous results'".C - Raman SpectroscopyIt should be noted, as we have already mentioned, that all the spectrapreviously reported are different although they are concerned with sup-posedly identical materials, this identity being actually supported by the factthat the macro-physical properties, i. e. resistance vs. temperature, magneticproperties, etc. are quite similar.1 - Influence of Laser Source PowerOur samples have been illuminated with different laser source powersfor obtaining Raman spectra. It was observed that the obtained Raman spectradepend strongly on the source power used. This means that the structure ofthese superconducting materials is very sensitive to the laser beam and maydrastically change. This can be due to a local increase of temperature or tophoto-induced chemical changes. To obtain reproducible Raman spectra, theilluminating laser reaching the sample must be always kept lower than 2 mW.2 - Hetero-domainsThanks to the micro-Raman technique, a spatial selectivity is possible.Instead of sending the laser beam directly onto the sample as in normalRaman investigations, the incident beam is directed through an optical micro-scope before reaching a limited area of the sample surface and then, theback-scattered beam can be collected into the monochromator of a classicalRaman spectrometer. In this way, Raman spectra of sample areas as smallas 1 IJ.m2can be obtained; the size of these areas is only limited by the wave-length of lasers commonly used (0.3-0.8 urn).r204 P. V. HUONG ET AL.When investigating Y-Ba-Cu-O superconducting materials, it was oftenobserved that different particles, or different areas of a particle of the samematerial, did not give identical Raman spectra, even when the reproducibilityof experiments was respected. Two explanations can be suggested.As the crystal symmetry of these materials is orthorhombic, the Ramanspectrum obtained for various particles may result from different orientationsof these small crystallites with respect to the incident and scattered lights.Usually, the particles are not single crystals, and, the direction of the incidentbeam does not coincide with a given crystal axis. Consequently, the obtainedspectra differ only by the relative intensities as there is rarely completeextinction of a forbidden mode.The micro-Raman spectrometer (DILOR, ModelOMARS) allows one torecord different spectra of different intensity polarizations, even when sean-ning at different areas on the same surface of the sample. These results arein agreement with recent electron microscopy investigations=-", showing thata common feature of the orthorhombic Y-Ba-Cu-O phases is the existence oftwin domains.As the spatial resolution used was 0.4 [.tm, the size of these heterodomains can be evaluated as 0.16 [.tm2 and larger.3 - HeterogeneityFor some samples, the spectra appear too different from one to the otherto be considered as issued from the same chemical entity.As the Raman spectrum is directly connected to. the local structure ofthe illuminated area or volurne, it should be admitted that some of the micro-Raman spectra do tome from different »chernicalspecies«. We must admit theexistence of heterogeneity in many samples. .This heterogeneity could come from the existence of defects involvingoxygen over- or sub-stoechiornetry in the superconducting materials.All these reasons explain the previous controversial results.When avoiding all these unconveniences, and thanks to the micro-Ramantechnique, the Raman spectra of micro-single crystals ofsuperconducting ma-terials can be obtained.4· - Micro-Raman -Specira of Micro-Single CrystalsSmall particles of Y-Ba-Cu-O, Eu-Ba-Cu-O and Y-Ba-Cu-O-F were firstselected under an optical microscope to present planar surfaces.The regularity was -then checked by Raman spectroscopy: various spotson the same surface might give the same Raman spectrum with the samerelative intensity for all bands.- As the orientation of the crystal is not known, the first configurationmay' be fixed by the total extinction of at least one Raman band; the spectrumis then recorded. The other configuration is obtained by rotation in relationto the previous one. When changing the particle, research on Raman intensityextinction of other band(s) will lead to other. and remaining configurations.YBaCuO 205~./I '.~~,JFE>-I-vlZwl-ZDBA700 600 500 400 300 200 100WAVENUMBERS (cm-1)Figure 2. Polarized Raman Spectra from different configurations of single crystalsof YBa2CuS07.206 P. v. HUONG ET AL.After a series of Raman measurements, a second record on the samerepered spot with the same polarization is made to check the stability ofthe sample and verify that no damage by laser occurred.When taking these precautions, the following Raman spectra were obtained.For YBa2Cu306.8,our polarized Raman spectra are displayed in Figure 2.Except for some published spectra which could be considered as resultingfrom damaged or heterogenous samples, the following published Ramanspectra can be accepted as correct although they are generally different onefrom the others. The reason is: each previous author gave only one or un-complete polarized configurations. For instance, the Raman spectrum of Mac-Farlane, Rosen and Seki'' and of Krol" is identical to our configuration 2C,;that of Liu-! is not of good quality but is still similar to Figure 2C, while thatof Yamanaka- coincides with our Figure 2D and that of KirbytšIs the same as2B. Again, the Raman spectra of Dai and Campion", of IqbaF, of Rosen!",of Kourouklis-! and of Chrzanowski-" are identical to configuration 2A, whilethe two polarized (J..zz and (J..xy Raman spectra given by Hemley and Ma04are identical to our configurations 2F and 2D.In Figure 2E, two identical Raman spectra of the same configuration,recorded at the beginning and at the end of the series of experiments, aresuper-imposed. This identity confirms the stability of the sample.Thus, the polarized spectra of all configurations must be taken into ac-count for a characterization of superconducting materi als. This result helpsto clarify the apparent controversial published results and restores confidenceinto the Raman spectroscopy technique in the study of new materials.The obtained Raman frequencies of YBa2Cu30·, are now listed in Table Iwith a partial and tentative assignment.TABLE IFrequencies (in cm-I) of Raman Bands of Superconducting MaterialsYBa2Cu307 EuBa2Cu307 YBa2Cu3(O!Fh Assignments632 635 635 } Chain Cu-O stretchings625592 I581 584 )513 f Pyramidal Cu-O stretchings501 501 502492I450 445337 337 }313298 Cu-O bendings220 220 230 Pyramidal O-Cu-F stretchings210150118>-I-vlZwl-ZYBaCuO 207oc700 600 50U 400 _ 1300WAVENUMBEr~S (cm J200Figure 3. Polarized Raman spectra from different configurations of single crystals ofEuBa2Cu307.For EuBa2Cu307_5,different polarized Raman spectra are obtained by themicro-Raman technique, and given in Figure 3.No strong effect was observed by comparison to the Raman spectra ofYBa2Cu307. This is coherent of the assignment (Table I) that the observedJ208>-I-VJZWzP. V. HUONG ET AL.oY Ba2Cu3(O IF)7700 GOO 50D 4UU 300-1IVAVENUMBERS (cm )200Figure 4. Polarized Raman spectra from different configurations of single crystalsof YBa2Cu307FO.l.bands are mostly due to Cu-O vibrations, as there is no oxygen in the Y or Euplanes.Note that the Raman spectrum given by Batlogg and Cava" and by Liu!'is identical to our configuration 3C; that of Cardona'" could represent an inter-mediary orientation between 3C and 3D.(YBaCuO 209For YBa2Cu307_sFo.h the introduction of some fluorine atoms seems to affectstrongly the relative intensity of the Raman bands. A new band appears at298 cm ". This could indicate that the fluorine atoms occur in the pyramidalCuO units rather than in CuO chains, as the latter vibrate at higher frequencies.IV. CONCLUSIONUnder well defined experimental conditions micro-Raman polarizationspectra can be recorded for Y-Ba-Cu-O superconducting materials.This systematical study makes it possible to explain the different Ramanspectra presented by previous workers, to eliminate some others and to pro-pose a coherent assignment.Note added in proof. - Recently, having a hornogeneous single crystal ofYBa2Cu306.5 with big size enough to allow the recording of the six Raman seat-tering configurations corresponding to an orthorhombic structure, Pmmm. We canthus identify spectra F, E, D and C in Figure 2. respectively to configurationsX(ZZ)X, Y(XX)Y, X(YY)X and Y(ZX)Y, while spectra B and A are due totwinned solid and can be considered as linear combinations of the previousspectra. A nEW band around 50 cm? in configurations X(YY)X and Z(XY)Z. (Figure5) is also observed.By us ing the exciting line 488 nm, bands at 593 and 570 crn? in polarizationZZ and at 592 and 635 cm? in polarization XX and YY are strongly enhanced(Figure 6). This resonance Raman effect also suggests the presence of 02-,. as itsstretching vibration is expected in this frequency range.Finally, wecan assign the bands at 594, 548, 474-503 cm? (the frequencyof this band depends on the amount of oxygen in the material), 154 and 117 cm!can be assigned to Ag phonon modes while the bands at 635, 586, 457, 342, 145 and50 cm? to phonon of B, symmetries.REFERENCES1. C. W. C h u, P. H. Hor, R. L. Me n s. L. G a o, Z. J. H u a n g, and Y. Q.W a n e. Phys. Rev. Lett. 58 (1987) 405.2. D. W. Mu r p h y, S. S u n s h i n e, R. B. Van D ove r, R. J. C a v a, B.Bat log g, S. M. Z ah ura k, and L. F. S c h nee m e y e r, Phys. Rev. Lett.58 (1987) 1888.3. B. Bat log s. R. J. C a v a, A. Ja y ara man, R. B. Van D ove r, G.A. K o u r o u k 1i s, S. S u n s h i n e, and D. W. Mu r p h y, Phys. Rev. Lett.58 (1987) 2333.4. R. J. He m 1e y and H. K. Ma o, Phys. Rev. Lett. 58 (1987) 2340.5. A. Y ama n a k a, F. M i nam i, K. W ata n a b e, K. I n o u e, S. Tak e-ka w a, and N. I v i, Jap. J. Appl. Phys. 26 (1987) L.1404.6. Y. F. Da i, J. S. S w i n ne a, H. Ste i n f i n k, J. B. G o ode n o u g h,A. Cam p i o n, J. Amer. Chem. Soc. 109 (1987) 5291.7. Z. I q bal, S. W. Ste i n h a u ser, A. B o s e, N. C i P o Il i n i, H. E c k-ha r d t, Phys. Rev. B. 36 (1987) 2283.8. R. M. Mac Farlane, H. Rosen, and H. Seki, Solid State Comm.63 (1987) 831.9. I. B o zov i c, D. Mit z i, M. B e a sl e y, A. Kap i t u l n i k, T. G e b a Il e,S. Per r o w i t z, G. C. Car, P. L o u, R. S u d h ars ana n, S. S. Y o m,Phys. Rev. B. 36 (1987) 4000.10. H. R o s e n, E. M. Eng.l e r, T. C. Str and, V. Y. Lee, D. Bet h u n e,Phys. Rev. B. 36 (1987) 726.11. R. Liu, R. Merlin, M. Cardona, H. Mattausch, N. Bauhofer,A. S i m o n, F. Ga r c i a - Al v ara d o, E. Mo r a n, M. ValI e t, J. M.G o n z ale z - Gal bet, M. A. Ala r i o, Solid State Comm 63 (1987) 839.12. M. Car d o n a, L. Gen z e 1, R. L i u, A. W i t t l i n, H. Mat tau s c h, F.Ga r c i a - Al v ara d o, E. Ga r c i a - G o n z ale z, Solid State Comm. 64.(1987) 727.210>-1-.VJZwl-zP. V. HUONG ET AL.>-I-100500 400 300 200WAVENUMBER/CM-1Figure 5. Raman spectra of asingle crystal of YBa2Cu306.5 excited with 514.5 nmlaser.}..=514.5nmYBa2Cup7_'&300i<--3»:::'" -- .. ,VJZwI-z~~=c=±====t=~=t~~~±=~=±~===iX(YY)X600 500 400 300 200WAVENUMBER/CM-'lY(ZX)Y YBa2CU307_b300K co514.5 nm ;: !::JI .X(ZY)XZ(XY)ZJ'"M'" '"M '".i"""c, •.."\,,.,\,A"-"''''Q'''~''''"··:700 600 100YBaCuO 211I.X(ZZ)X YBa2Cu307_~)\0 = 488 nm 300K>-I-Y(XX)YLI)MlO'"LI)II II. 'IulZwl-ZFigure 6. Raman spectra of asingle crystal of YBa2Cu306.5 excited with 488.0 nmlaser.13. D. M. Kr o I, M. S ta vol a, W. Web e r, L. E. Sc h nee m e y e r, J. V.War z cak, S. M. Z ah ura k, S. G. K o s i n s k i, Phys. Rev. B. 36 (1987)8325.14. G. A. K o u r o u k I i s, A. Ja y ara man, B. Bat log g, R. J. C a v a, M.S t a vol a, D. M. Kr o I, E. A. Rie t man, L. F. S c h nee m e y e r, Phys.Rev. B. 36 (1987) 8320.15. P. B. Kir b y, M. R. H a r r i s o n, W. G. F r e e man, I. Sam u e I, M.J. Hai n e s, Phys. Rev. B. 36 (1987) 8315.16. J. Chrzanowski, S. Gygax, J. C. Irwin, W. N. Hardy, soua.State Comm. 65 (1988) 139.17. C. Thomsen, R. Liu, A. Wittlin, L. Genzel, M. Cardona, W.K o n i g, M. V. C a b ana s, E. G arc i a, Solid State Comm. 65 (1988) 219.18. Raman Spectroscopy, Linea?' and Non Linear, .J. Las c o m b e and P. V.H u o n g (Ed.), J. Wiley, Chichester-New York 1982.19. P. V. H u o n g, M. C o u z i, Spectres Infrarouges et Raman des Perovskitesin »L'Infrarouge en Chirnie des Solides«, Masson Ed., Pari s 1973.20. P. V. H u o n g, in Vibrational Spectra and Structure, J. R. Du r i g Ed.,Elsevier, New York-Amsterdam, 1980.21. A. F reu n d I i c h, A. Le y c ura s, J. C. G r ene t, C. Ver i e, P. V.H u o n s. Appl. Phys. Lett. 51 (1987) 1352.22. P. V. H u o n g, in Lasers and Applications, H. D. B i s t and J. S. Goe 1a(Eds.), Tata-McGraw Hill, Bombay, 1982.212 P. V. HUONG ET AL.23. P. V. H u o n g, in Reviews on Analytical Chemistry, L. Niinisto (Ed.), AkademiaiKiado, Budapest 1982.24. P. V. H u o n g, in Analysis for the Development of Microelectronic Devices,M. G r a s ser b aue rEd., Elsevier Amsterdam, 1988.25. R. D. S han n o n and C. T. .p e w i t t, Acta Cryst. B25 (1969) 928.26. S. R. °V s h i n sky, R. T. You n g, D. D. All red, G. DeM a g g i o, andG. A. Van der Lee den, Phys. Rev. Lett. 58 (1987) 2579.27. B. W a 1z, H. Jen n s, G. Lee man n, V. G e i ser, S. J o s t, T. F rey,H. Bre t s c h e r, and H. J. G u n the rod t, MRS Meeting Strasbourg,France, 1987.28. Y. S. Yo n o, M. K i k u c h i, K. ° h - I s h i, K. Hi r a g a, H. Ara i, Y.Mat s u i, N. K o b a y ash i, T. S asa oka, and Y. Mu t o, Jpn. J. Appl.Phys. 26 (1987) 498.29. M. Hervieu, B. Domenges, C. Michel, G. Heger, J. Provost,and B. R ave a u, Phys. Rev. B 36 (1987) 3920.SAŽETAKlUikro-Ramanovi spektri supervodiča YBaCuOPham V. Huong, J. C. Grenier i P. BezdickaSustavno proučavanje Rarnanovih spektara supravodljiv ih materijala Y-Ba-Cu-O,snimljenih pri različitim eksperimentalnim uvjetima, omogućava objašnjenje kon-troverznih rezultata iz literature. Pri određenim uvjetima, zahvaljujući mikro-Rama-novoj tehnici, mogu se postići reproducibilni Ramanovi spektri, te se može predlo-žiti koherentna asignacija za YBa2Cu~07 i EuBa2Cu3(O/F17, pri čemu je utvrđeno dasu uvedeni atomi fluora smješteni u piramidnim jedinicama CuO, prije nego li ulancima CuO.

Micro-Raman Spectra of YBaCuO Superconductors

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CROATICA CHEMICA ACTA CCACAA 61 (2) 201-212 (1988)CCA-1791YU ISSN 0011-1643UDC 543.42OriginaL Scientific PaperMicro-Raman Spectra of YBaCuO SuperconductorsPham V. Huong*Laboratoire de Spectroscopie MoLecuLaire et CristaUine (U. A. 124) Universite deBordeaux I, 351, cours de La Liberation 33405 TALEN CE, FmnceandJ. C. GTenieT and P. Bezdicka"Laboratoire de Chimie du Solule, Universitć de Bordeaux I, (U. P. 8661)351, cours de La Liberation, 33405 TALENCE, FranceReceived March 8, 1988A systematic study of the Raman spectra of Y-Ba-Cu-O su-perconducting materials under different experimental conditionsclarifies the controversial results given in the literature. Underthe determined conditions and thanks to the micro-Raman te-chnique, reproducible polarized Raman spectra can be obtainedand a coherent assignment can be suggested for YBa2Cu307andEuBa2Cu307.For YBa2Cu3(OjF);, the introduction of fluorine atomswas found to be localized in pyramidal CuO units rather than inCuO chains.1. INTRODUCTIONSince the discovery of Y-Ba-Cu-O materials exhibiting superconductivityat nearly 93 K, febrile activities have been manifested in many fields. Weattend to a burst of research works circulating in the form of preprintswhich are as much an authority as any printed papers in well establishedjournals.Chu's materials Y-Ba-Cu-01 have been easily reproduced and their macro--physical properties have been reconfirmed and extended". But things aregoing perhaps too fast. This high competition for publishing is certainly agood thing; nevertheless, we must be careful.Many papers are concerned with Raman spectroscopy but generally thereported spectra differ from one author to another+?". As Raman spectro-scopy is well known to be a very accurate technique which allows investigatingdown to the molecular and sub-molecular scales-" ", one can ask what is goingwrong.In this paper, we try to find an explanation for this apparent inconsi-stency by studying, under variable experimental conditions, the Raman spectraof YBa2Cu307_o,YBa2Cu307_oFo.land EuBa2Cu307_ousing micro-Raman spectro-a Permanent address: Institute of Inorganic Chemistry, Center of ChemicalResearch, Slovak Academy of Sciences, CS-84236,Bratislava Czechoslovakia.202 P. v. HUONG ET AL.scopy, this technique being able to give the Raman spectra of materials ofmicrometer size23,24.II. EXPERIMENTALMBa2Cu307--O(M= Y, Eu; (J = 0.2) were prepared from appropriate amountsof barium carbonate, rare-earth oxide (Y203or EU203)and copper oxide. The fineJyground mixture was heated for 20 hours in air at 950 DC,then quenched and re-ground. Pellets of 8 mm diam. were pressed and sintered at 950 DCfor 2 hours andfinally heated at 450 DCfor two days.A phase containing a small amount of fluorine was also prepared in the sameway starting from the following mixture: 1.95 BaC03, 0.05 BaF2, 1.5 Y20S and3CuO. As barium fluoride is very stable, it can be assumed that there is no hydro-lysis and that the composition is close to YBa2Cu307-,Fo.t.X-ray diffraction powder analysis was carried out on a diffractometer. D. C.resistivity measurements versus temperature were performed using the standardfour probe method. The Raman spectra were recorded at room temperature on amicro-Raman spectrometer DILOR/OMARSusing an ion argon laser source Spectra--Physics ernitting at 514.5 nm, and a diode-array multichannel detector.III. RESULTS AND DISCUSSIONMBa2Cu307_oand the related compounds exhibit »norrnal« macrophysicalproperties.A - X-ray Diffraction AnalysisThe X-ray diffraction analysis shows quite similar patterns to thosepreviously reported and characteristic of the orthorhombic symmetry with thefollowing cell parameters:Compound a (A) b (A) c(A)YBa2Cu307--O 3.83 3.88 11.71YBa2Cu307--OFo.l 3.82 3.88 11.66EuBa:iCu307--O 3.86 3.87 11.71174173175Replacement of yttrium by europium slightly increases the unit cellvolume while a decrease is observed for the fluorinated compound, asexpected from the difference in ionic radius sizes'".B - Resistivity MeasurementsTypical resistivity measurements for YBaCuO and YBaCuOF are shownin Figure 1. The superconducting transition temperatures are, respectively,90.6 K and 89.3 K (zero-resistance Te) in very good agreement with previousresults",The introduction of a small amount of F ions does not affect significantlythe Te value but increases somewhat the resistivity of the sample above Te.This can be explained by the higher electronegativity of fluorine as comparedto oxygen, which increases the electronic localization. Higher concentrationsof fluorine lead to multiphase materials which become insulators. In thecourse of this work, Ovshinsky et al. reported Y-Ba-Cu-O-F materials withYBaCuO 20315.-----------------------------------------~R(mm10YB.1CuOF5YBaCuO°8~0~===-====~9~0----~-----1iO-0---=T~(~K~)--~110Figure 1. Resistance vs. temperature measured at a constant current of 10 mA forYBa2Cu307....•and YBa2Cu307-,Fo1.large amounts of fluorine, showing very high superconducting temperatures'";these results do not seem to be reproducible.Eu-Ba-Cu-O compound shows a superconducting state under 90 K, againin agreement with previous results'".C - Raman SpectroscopyIt should be noted, as we have already mentioned, that all the spectrapreviously reported are different although they are concerned with sup-posedly identical materials, this identity being actually supported by the factthat the macro-physical properties, i. e. resistance vs. temperature, magneticproperties, etc. are quite similar.1 - Influence of Laser Source PowerOur samples have been illuminated with different laser source powersfor obtaining Raman spectra. It was observed that the obtained Raman spectradepend strongly on the source power used. This means that the structure ofthese superconducting materials is very sensitive to the laser beam and maydrastically change. This can be due to a local increase of temperature or tophoto-induced chemical changes. To obtain reproducible Raman spectra, theilluminating laser reaching the sample must be always kept lower than 2 mW.2 - Hetero-domainsThanks to the micro-Raman technique, a spatial selectivity is possible.Instead of sending the laser beam directly onto the sample as in normalRaman investigations, the incident beam is directed through an optical micro-scope before reaching a limited area of the sample surface and then, theback-scattered beam can be collected into the monochromator of a classicalRaman spectrometer. In this way, Raman spectra of sample areas as smallas 1 IJ.m2can be obtained; the size of these areas is only limited by the wave-length of lasers commonly used (0.3-0.8 urn).r204 P. V. HUONG ET AL.When investigating Y-Ba-Cu-O superconducting materials, it was oftenobserved that different particles, or different areas of a particle of the samematerial, did not give identical Raman spectra, even when the reproducibilityof experiments was respected. Two explanations can be suggested.As the crystal symmetry of these materials is orthorhombic, the Ramanspectrum obtained for various particles may result from different orientationsof these small crystallites with respect to the incident and scattered lights.Usually, the particles are not single crystals, and, the direction of the incidentbeam does not coincide with a given crystal axis. Consequently, the obtainedspectra differ only by the relative intensities as there is rarely completeextinction of a forbidden mode.The micro-Raman spectrometer (DILOR, ModelOMARS) allows one torecord different spectra of different intensity polarizations, even when sean-ning at different areas on the same surface of the sample. These results arein agreement with recent electron microscopy investigations=-", showing thata common feature of the orthorhombic Y-Ba-Cu-O phases is the existence oftwin domains.As the spatial resolution used was 0.4 [.tm, the size of these heterodomains can be evaluated as 0.16 [.tm2 and larger.3 - HeterogeneityFor some samples, the spectra appear too different from one to the otherto be considered as issued from the same chemical entity.As the Raman spectrum is directly connected to. the local structure ofthe illuminated area or volurne, it should be admitted that some of the micro-Raman spectra do tome from different »chernicalspecies«. We must admit theexistence of heterogeneity in many samples. .This heterogeneity could come from the existence of defects involvingoxygen over- or sub-stoechiornetry in the superconducting materials.All these reasons explain the previous controversial results.When avoiding all these unconveniences, and thanks to the micro-Ramantechnique, the Raman spectra of micro-single crystals ofsuperconducting ma-terials can be obtained.4· - Micro-Raman -Specira of Micro-Single CrystalsSmall particles of Y-Ba-Cu-O, Eu-Ba-Cu-O and Y-Ba-Cu-O-F were firstselected under an optical microscope to present planar surfaces.The regularity was -then checked by Raman spectroscopy: various spotson the same surface might give the same Raman spectrum with the samerelative intensity for all bands.- As the orientation of the crystal is not known, the first configurationmay' be fixed by the total extinction of at least one Raman band; the spectrumis then recorded. The other configuration is obtained by rotation in relationto the previous one. When changing the particle, research on Raman intensityextinction of other band(s) will lead to other. and remaining configurations.YBaCuO 205~./I '.~~,JFE>-I-vlZwl-ZDBA700 600 500 400 300 200 100WAVENUMBERS (cm-1)Figure 2. Polarized Raman Spectra from different configurations of single crystalsof YBa2CuS07.206 P. v. HUONG ET AL.After a series of Raman measurements, a second record on the samerepered spot with the same polarization is made to check the stability ofthe sample and verify that no damage by laser occurred.When taking these precautions, the following Raman spectra were obtained.For YBa2Cu306.8,our polarized Raman spectra are displayed in Figure 2.Except for some published spectra which could be considered as resultingfrom damaged or heterogenous samples, the following published Ramanspectra can be accepted as correct although they are generally different onefrom the others. The reason is: each previous author gave only one or un-complete polarized configurations. For instance, the Raman spectrum of Mac-Farlane, Rosen and Seki'' and of Krol" is identical to our configuration 2C,;that of Liu-! is not of good quality but is still similar to Figure 2C, while thatof Yamanaka- coincides with our Figure 2D and that of KirbytšIs the same as2B. Again, the Raman spectra of Dai and Campion", of IqbaF, of Rosen!",of Kourouklis-! and of Chrzanowski-" are identical to configuration 2A, whilethe two polarized (J..zz and (J..xy Raman spectra given by Hemley and Ma04are identical to our configurations 2F and 2D.In Figure 2E, two identical Raman spectra of the same configuration,recorded at the beginning and at the end of the series of experiments, aresuper-imposed. This identity confirms the stability of the sample.Thus, the polarized spectra of all configurations must be taken into ac-count for a characterization of superconducting materi als. This result helpsto clarify the apparent controversial published results and restores confidenceinto the Raman spectroscopy technique in the study of new materials.The obtained Raman frequencies of YBa2Cu30·, are now listed in Table Iwith a partial and tentative assignment.TABLE IFrequencies (in cm-I) of Raman Bands of Superconducting MaterialsYBa2Cu307 EuBa2Cu307 YBa2Cu3(O!Fh Assignments632 635 635 } Chain Cu-O stretchings625592 I581 584 )513 f Pyramidal Cu-O stretchings501 501 502492I450 445337 337 }313298 Cu-O bendings220 220 230 Pyramidal O-Cu-F stretchings210150118>-I-vlZwl-ZYBaCuO 207oc700 600 50U 400 _ 1300WAVENUMBEr~S (cm J200Figure 3. Polarized Raman spectra from different configurations of single crystals ofEuBa2Cu307.For EuBa2Cu307_5,different polarized Raman spectra are obtained by themicro-Raman technique, and given in Figure 3.No strong effect was observed by comparison to the Raman spectra ofYBa2Cu307. This is coherent of the assignment (Table I) that the observedJ208>-I-VJZWzP. V. HUONG ET AL.oY Ba2Cu3(O IF)7700 GOO 50D 4UU 300-1IVAVENUMBERS (cm )200Figure 4. Polarized Raman spectra from different configurations of single crystalsof YBa2Cu307FO.l.bands are mostly due to Cu-O vibrations, as there is no oxygen in the Y or Euplanes.Note that the Raman spectrum given by Batlogg and Cava" and by Liu!'is identical to our configuration 3C; that of Cardona'" could represent an inter-mediary orientation between 3C and 3D.(YBaCuO 209For YBa2Cu307_sFo.h the introduction of some fluorine atoms seems to affectstrongly the relative intensity of the Raman bands. A new band appears at298 cm ". This could indicate that the fluorine atoms occur in the pyramidalCuO units rather than in CuO chains, as the latter vibrate at higher frequencies.IV. CONCLUSIONUnder well defined experimental conditions micro-Raman polarizationspectra can be recorded for Y-Ba-Cu-O superconducting materials.This systematical study makes it possible to explain the different Ramanspectra presented by previous workers, to eliminate some others and to pro-pose a coherent assignment.Note added in proof. - Recently, having a hornogeneous single crystal ofYBa2Cu306.5 with big size enough to allow the recording of the six Raman seat-tering configurations corresponding to an orthorhombic structure, Pmmm. We canthus identify spectra F, E, D and C in Figure 2. respectively to configurationsX(ZZ)X, Y(XX)Y, X(YY)X and Y(ZX)Y, while spectra B and A are due totwinned solid and can be considered as linear combinations of the previousspectra. A nEW band around 50 cm? in configurations X(YY)X and Z(XY)Z. (Figure5) is also observed.By us ing the exciting line 488 nm, bands at 593 and 570 crn? in polarizationZZ and at 592 and 635 cm? in polarization XX and YY are strongly enhanced(Figure 6). This resonance Raman effect also suggests the presence of 02-,. as itsstretching vibration is expected in this frequency range.Finally, wecan assign the bands at 594, 548, 474-503 cm? (the frequencyof this band depends on the amount of oxygen in the material), 154 and 117 cm!can be assigned to Ag phonon modes while the bands at 635, 586, 457, 342, 145 and50 cm? to phonon of B, symmetries.REFERENCES1. C. W. C h u, P. H. Hor, R. L. Me n s. L. G a o, Z. J. H u a n g, and Y. Q.W a n e. Phys. Rev. Lett. 58 (1987) 405.2. D. W. Mu r p h y, S. S u n s h i n e, R. B. Van D ove r, R. J. C a v a, B.Bat log g, S. M. Z ah ura k, and L. F. S c h nee m e y e r, Phys. Rev. Lett.58 (1987) 1888.3. B. Bat log s. R. J. C a v a, A. Ja y ara man, R. B. Van D ove r, G.A. K o u r o u k 1i s, S. S u n s h i n e, and D. W. Mu r p h y, Phys. Rev. Lett.58 (1987) 2333.4. R. J. He m 1e y and H. K. Ma o, Phys. Rev. Lett. 58 (1987) 2340.5. A. Y ama n a k a, F. M i nam i, K. W ata n a b e, K. I n o u e, S. Tak e-ka w a, and N. I v i, Jap. J. Appl. Phys. 26 (1987) L.1404.6. Y. F. Da i, J. S. S w i n ne a, H. Ste i n f i n k, J. B. G o ode n o u g h,A. Cam p i o n, J. Amer. Chem. Soc. 109 (1987) 5291.7. Z. I q bal, S. W. Ste i n h a u ser, A. B o s e, N. C i P o Il i n i, H. E c k-ha r d t, Phys. Rev. B. 36 (1987) 2283.8. R. M. Mac Farlane, H. Rosen, and H. Seki, Solid State Comm.63 (1987) 831.9. I. B o zov i c, D. Mit z i, M. B e a sl e y, A. Kap i t u l n i k, T. G e b a Il e,S. Per r o w i t z, G. C. Car, P. L o u, R. S u d h ars ana n, S. S. Y o m,Phys. Rev. B. 36 (1987) 4000.10. H. R o s e n, E. M. Eng.l e r, T. C. Str and, V. Y. Lee, D. Bet h u n e,Phys. Rev. B. 36 (1987) 726.11. R. Liu, R. Merlin, M. Cardona, H. Mattausch, N. Bauhofer,A. S im o n, F. Ga r c i a - Al v ara d o, E. Mo r a n, M. ValI e t, J. M.G o n z ale z - Gal bet, M. A. Ala r i o, Solid State Comm 63 (1987) 839.12. M. Car d o n a, L. Gen z e 1, R. L i u, A. W i t t l i n, H. Mat tau s c h, F.Ga r c i a - Al v ara d o, E. Ga r c i a - G o n z ale z, Solid State Comm. 64.(1987) 727.210>-1-.VJZwl-zP. V. HUONG ET AL.>-I-100500 400 300 200WAVENUMBER/CM-1Figure 5. Raman spectra of asingle crystal of YBa2Cu306.5 excited with 514.5 nmlaser.}..=514.5nmYBa2Cup7_'&300i<--3»:::'" -- .. ,VJZwI-z~~=c=±====t=~=t~~~±=~=±~===iX(YY)X600 500 400 300 200WAVENUMBER/CM-'lY(ZX)Y YBa2CU307_b300K co514.5 nm ;: !::JI .X(ZY)XZ(XY)ZJ'"M'" '"M '".i"""c, •.."\,,.,\,A"-"''''Q'''~''''"··:700 600 100YBaCuO 211I.X(ZZ)X YBa2Cu307_~)\0 = 488 nm 300K>-I-Y(XX)YLI)MlO'"LI)II II. 'IulZwl-ZFigure 6. Raman spectra of asingle crystal of YBa2Cu306.5 excited with 488.0 nmlaser.13. D. M. Kr o I, M. S ta vol a, W. Web e r, L. E. Sc h nee m e y e r, J. V.War z cak, S. M. Z ah ura k, S. G. K o s i n s k i, Phys. Rev. B. 36 (1987)8325.14. G. A. K o u r o u k I i s, A. Ja y ara man, B. Bat log g, R. J. C a v a, M.S t a vol a, D. M. Kr o I, E. A. Rie t man, L. F. S c h nee m e y e r, Phys.Rev. B. 36 (1987) 8320.15. P. B. Kir b y, M. R. H a r r i s o n, W. G. F r e e man, I. Sam u e I, M.J. Hai n e s, Phys. Rev. B. 36 (1987) 8315.16. J. Chrzanowski, S. Gygax, J. C. Irwin, W. N. Hardy, soua.State Comm. 65 (1988) 139.17. C. Thomsen, R. Liu, A. Wittlin, L. Genzel, M. Cardona, W.K o n i g, M. V. C a b ana s, E. G arc i a, Solid State Comm. 65 (1988) 219.18. Raman Spectroscopy, Linea?' and Non Linear, .J. Las c o m b e and P. V.H u o n g (Ed.), J. Wiley, Chichester-New York 1982.19. P. V. H u o n g, M. C o u z i, Spectres Infrarouges et Raman des Perovskitesin »L'Infrarouge en Chirnie des Solides«, Masson Ed., Pari s 1973.20. P. V. H u o n g, in Vibrational Spectra and Structure, J. R. Du r i g Ed.,Elsevier, New York-Amsterdam, 1980.21. A. F reu n d I i c h, A. Le y c ura s, J. C. G r ene t, C. Ver i e, P. V.H u o n s. Appl. Phys. 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Provost,and B. R ave a u, Phys. Rev. B 36 (1987) 3920.SAŽETAKlUikro-Ramanovi spektri supervodiča YBaCuOPham V. Huong, J. C. Grenier i P. BezdickaSustavno proučavanje Rarnanovih spektara supravodljiv ih materijala Y-Ba-Cu-O,snimljenih pri različitim eksperimentalnim uvjetima, omogućava objašnjenje kon-troverznih rezultata iz literature. Pri određenim uvjetima, zahvaljujući mikro-Rama-novoj tehnici, mogu se postići reproducibilni Ramanovi spektri, te se može predlo-žiti koherentna asignacija za YBa2Cu~07 i EuBa2Cu3(O/F17, pri čemu je utvrđeno dasu uvedeni atomi fluora smješteni u piramidnim jedinicama CuO, prije nego li ulancima CuO.

Micro-Raman Spectra of YBaCuO Superconductors

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