Far infrared study of YBa2Cu3O6

Solid State Communications, Vol. 68, No. 7, pp. 677-680, 1988. Printed in Great Britain.

0038-1098/88 $3.00 + .00 Pergamon Press plc

FAR I N F R A R E D STUDY OF YBa2Cu306 S. Jandl and M. Banville D6partment de physique et Centre de recherche en physique du solide, Universit6 de Sherbrooke, Qu6bec, Canada J1K 2R1 and C. P6pin D6partement de chimie, Universit6 de Sherbrooke, Qu6bec, Canada J 1K 2RI

(Received 30 June 1988 by R. Barrie) We present far infrared reflectance spectra of YBa2Cu306and observe 15 and 20 oscillators respectively at 300 and 17K. Our results are compared to other published optical phonon frequencies and to the infrared phonons of the superconductor YBa2Cu307 x. IN T H E following paper we report on recent improvements in the measurements and the analysis of YBazCu306 far infrared reflectance. As determined from the X-ray diffraction data [1, 2], YBa2Cu307 changes from an orthorhombic configuration (D~h) for x = 0 to a tetragonal configuration (D~7 for x -- 1. The structural transformation occurs reversibly while the critical temperature T~ decreases with the removal of oxygen atoms from the Cu-O chains [3]. For x > 0.5, the superconductivity disappears and the material becomes a semiconductor [4, 51. Infrared and Raman phonons of YBa2Cu30 6 have recently been studied at room temperature by

M. Stavola et al. [6] in the frequency range co > 400cm '. They observed by infrared transmission and Raman scattering three and four active phonons respectively and developed a force constant model that assigned the features in the spectra. They also opted between two possible tetragonal structures with the oxygen empty sites either along the z axis (0(I) sites) or along the x axis (0(4) sites). The 470cm Raman band was associated with the Ag symmetric stretching motion of the bridging 0(1) atoms since no infrared band was observed at this frequency. Subsequently, they located the oxygen vacancies at the 0(4) sites. That statement disagrees with infrared measurements of Cardona et al. [7] who observed an infrared

06

4O i

E

3o W 0.4

>I--

CD Z

W

u_

O2 i

W

O0 0

I

I

200

400

FREQUENCY (a)

I ~'600

,

---I--800

,

, I00(

I

I

200

400

,

I

600

800

FREQUENCY (Cm -1)

(Cm-1)

(b)

Fig. 1. (a) Infrared reflectance spectrum of YBazCu306 at 300 K. Broken line is experimental and full line is the model prediction. (b) The infrared conductivity calculated from the factorized model with the parameters of Table 1 (full line) and the conductivity obtained from the data with the Kramers-Kronig inversion (broken line). 677

678

F A R I N F R A R E D S T U D Y OF YBa2Cu306

Vol. 68, No. 7

:50

°5L

25

'

O4

E o

~ 2o

hi 03 Z

I----

I-W O2 J b_ LU

123 Z 0 ~ 5

Ol

O.0

i

0

200

400 FREQUENCY

600

800

0

I000

O

i

,

I

200

I 400

I

I 600

~

I 800

I000

(Cm -1)

F R E Q U E N C Y (Cm -I) (a) (b) Fig. 2. (a) Infrared reflectance spectrum of YBa2Cu306 at 17 K. Broken line is experimental and full line is the model prediction. (b) The infrared conductivity calculated from the factorized model with the parameters of Table 2 (full line) and the conductivity obtained from the data with the Kramers-Kronig inversion (broken line).

active phonon at 474 cm ~located at one maximum of their reflectance spectrum. A recent room temperature infrared reflectance study of YBa2Cu306 was published by G. Burns et al. [8]. They observed ten oscillators and attributed them to phonons since group theory predicts eleven infrared active modes [9]. They also concluded by comparing their measurements to the infrared reflectance of the superconductor YBa2Cu207 x that a clear one to one correspondence is absent. In this communication far infrared reflectance

spectra of YBa2Cu306 between 75 and 900cm ~ at 300 and 17 K are presented and fitted with phonon oscillator characteristics. The original sample was a pressed pellet ofYBa2Cu307 x with x = 0.1 subjected to three successive three hour heat treatments under an argon atmosphere at 750°C. At each stage, the sample was weighed in order to monitor the oxygen

(J~TO

OJLO

7

S

Table 1. Room temperature phonon parameters (in cm ' except S)

85 106 ll6 146 172 185 203 213 233 243 312 355 380 418 447 490 516 602 659 858

92 109 123 156 261 181 201 207 222 239 320 366 404 420 458 491 545 616 670 879

13.3 8 9.3 37.3 40.4 12.2 5.8 12 16.5 11.4 50 32.7 69.3 39.1 48.1 6.8 102.6 39.4 110.1 273.2

2.54 0.64 0.92 1.34 1.21 0.33 0.05 0.17 0.20 0.11 0.16 0.26 0.21 0.01 0.09 0.00 0.11 0.07 0.04 0.08

COTo

O) LO

7

S

107 115 146 167 182 207 237 303 348 389 409 445 505 591 660

108 122 155 262 179 200 218 328 379 392 415 449 539 603 668

4.3 9.5 32.4 39.5 17.1 14 30 67 47 21.1 27.8 31.1 190 42 183.2

0.54 1.38 1.52 1.32 0.01 0.17 0.41 0.40 0.18 0.01 0.03 0.03 0.18 0.04 0.02

e~

1.94

Table 2. Phonon parameters at 17 K (in cm 1except S)

e.~

2.23

Vol. 68, No. 7

F A R I N F R A R E D S T U D Y OF YBa2Cu306

679

Table 3. Comparison of this work's transverse frequencies with YBa2Cu307 x and VBa2Cu306 optical phonon published frequencies YBa2Cu307 x YBa2Cu306 Ref [ 12] I.R. This work Ref. [6] (T = 17 K) I.R. (T=17K) I.R. R (T=295K) 85 106 116 146

110 121 140 155 179 196 204 219

Ref. [9] I.R. (T=296K) 108 119

108 118 152

172 185 203 213 233 243

242 276 309 341 367 398 423 444

162 192 218

602

147 164 190

Ref. [7] I.R. ( r = 10K)

R (T--80K)

106 136 149

158

183 209

214

217 233 249 286 317

396

355 380 418 447 490 516

217

249

312

472 515 576 600 627 680 748 839

360

344

456

520

470 495

592 636

590 625

659

303 337

360 386 410

390 453

466 474 530

500

478

592

600

593

602

639

656

648

663

858

content. It was observed that the oxygen content initially decreased rapidly and tended to level off at x ~ 1. The sample tetragonal structure was checked by X-rays, its semiconducting character established, and then mounted in a continuous flow regulated temperature helium cryostat. Infrared reflectance spectra were taken with a Fourier transform spectrometer (Bomem DA3.002 model) using a Globar source, a Ge-bolometer and Mylar 12G and 100G beam splitters. The analysis was done by fitting the reflectance with the dielectric function factorized model [10], 8(0))

R (T=85K)

Ref. [8] I.R. (T=295K)

8(O0)

[ ~ (D2LO -- 0)2 -- iTj0)

/=1110)~m

0)2

ivj0)'

where c ( ~ ) is the high frequency dielectric constant, 0)jvo, 0)jLo and 7j are respectively the frequency of the

transverse phonon, the longitudinal phonon and their damping. The fitting oscillator parameters are obtained by minimizing the chi-square function (X2) measuring the departure of the theoretical model predictions from the experimental reflectance. In order to check whether the correct parameters are obtained, a comparison is made between the conductivity calculated from the model and that obtained from the Kramers-Kronig inversion of the reflectance spectrum thus ensuring consistency. Both the fit to the reflectance spectrum and to the conductivity at 300 and 17K are shown in Figs. l(a, b) and 2(a, b) respectively. In Tables 1 and 2 we give the corresponding set of oscillator parameters. The oscillator strength Sj of each oscillator is derived from the generalized Lyddane-Sachs-Teller relation

680

FAR I N F R A R E D STUDY OF YBa2Cu306

[ll],

=

L\ OjTo

,] I1 ( Obo

O)To)"

The detection of almost all the infrared active phonons is valuable for the refinement of theoretical force models. In our study 15 oscillators are detected at 300 K and 20 oscillators at 17 K. The observation of more modes than the eleven phonons predicted by group theory is not surprising since many defects are expected in such materials. We therefore conclude that the ten modes reported in [8] do not necessarily represent the ten infrared active phonons of YBa2Cu306 but probably include some local modes. A general normal mode assignment could be associated with Cu-O stretching modes around 500cm ~ and bond bending Cu2-O2 and C u 2 - O 3 vibrations around 300cm ~. Below 2 0 0 c m * the vibrations of Y and Ba atoms along the x, y and z axes generate the far infrared phonons. The 474cm ' phonon reported in [7] is not observed, confirming the tetragonal structure assignment of M. Stavola et al. [6]. In Table 3 we colligate published Raman and infrared (COso) phonon frequencies of YBa2Cu306 and compare them to recent infrared measurements of YBa2Cu307 .~ [12]. Our measurements establish an evident correspondence between COvo phonon frequencies of the superconductor YBa2Cu307 ~ and those of the semiconductor YBa2Cu306 thus confirming, the strong similarity of their structure and their force constants. Finally as shown in Table 3, the infrared frequencies are slightly different from the Raman shifts and can be grouped in Davydov pairs as expected for a centrosymmetric tetragonal structure.

Professor A. Cabana for the use of his high resolution infrared facilities, to R. Gagnon for the treatment of YBa2CuaO7 x samples and to J. Rousseau for his technical assistance in the infrared measurements. REFERENCES 1. 2. 3.

4, 5,

6.

7. 8. 9. 10. 11. 12.

Acknowledgements -

The authors are grateful to

Vol. 68, No. 7

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