Crystallographic structure and elastic measurements of La2−xSrxCuO4 single crystal

~__~ Solid State Communications, Vol. 77, No. 12, pp. 945-948, 1991. Printed in Great Britain.

0038-1098/9153.00+.00 Pergamon Press pie

CRYSTAI.LOGRAPIIIC $I-RIJCI'URE AND ELASTIC MEASUREMENTS SINGLE CIU't'STA L

OF La2_xSrxCuO4

M.Slaski*, O.Steinsvollk E.J Samuelsen, O-M.Nes, Wu Ting, T.Lzegreid, K.Fossheim, Y.llidaka 2 Division of Physics, The Nolweegian Institute of Teclmology, at~d Divisiou of Applied Physics SINTEF, N-7034 Trondheim, Norway I1FE, N--2007 Kjeller, Norway 2Nippon Telegraph and "I'elcphone Corporation, Tokai, lbaraki 319-1 I, Japan ( Received

October

i0,

1990

by

G.

GOntherodt

)

A single crystal of lmL.a~Sro.uCuO4 wa:; studied by means of X-ray and neutron difflaction experituents and elastic measurements. The obtained data show that this COml~ound undergoes a tetragonal to orthothotnbic phase transition at around 260 K. The order parameter expouent value I~ = 0.34 _+ 0.02 and the elastic constant exponeut p -- 0.5 wele deduced from the neutron diffraction and souml velocity data respectively. Both wdues agree well with theoretical predictio~s. We do not observe any anomalies in the inteusity of the supethutice peak vs. temperature ctuves il~ the range from IO K to 260 K iu either X ray or neutron diffraction tueasutements.

1. Introduction

the powdered sample was examined at room temperature by X - r a y powder diffraction using an automated Phillips PW 1700 goniometer with graphite monochromator and Cu-K0q radiation. The single-crystal neutron diffraction studies have been performed at IFE, Kjeller using k = 2.45 A wavelength in the temperature range 10 - 300 K. A time---of-flight method using standard Matec equipment has been employed to perform ultrasonic measurements7, 9 in the temperature range from 60 K to 300 K. The sample for this experiment consisted of two plates each 7x7x2 mm 3 glued together by Nonaq. Longitudinal ultrasound (fr = 13 MHz) was propagated along the a (or b) direction. The resonance frequency of a 7x3x0.3 mm 3 reed was measured t't using a vibrating reed system.

The crystallographic structure of the system La2_xSrxCuO4 is the simplest one among copper based high temperature superconductors. It contains isolated sheets of CuO2 which are believed to be responsible for establishing superconductivity. The superconducting La2_xSrxCuO4 phase has tetragonal K2NiF4 - type structure (I4/mmm space group) at high temperatures. With decreasing temperature this system undergoes a tetragonal to orthorhombic (space group Cmca) phase transition. The transition temperature depends strongly on the strontium content 1-2 x. There are several reports 3--4 on the influence of Sr - doping on the transport and magnetic properties of La2_xSrxCuO4. The isostructural La2_xBaxCuO4 system was found to have a second, low-temperature structure (tetragonal symmetry, P42/ncm space group) 5-6. Keeping in mind the similarities between these two systems it is interesting to investigate such a possibility in La2_xSrxCuO4 as well. There is already experimental evidence from elastic measurements 7-n indicating the presence of low-temperature structural instabilities in La2_xSrxCuO4. We have extended our previous studies 7. 9,12 performed on a single crystal with nominal composition Lal.g6SroAaCuO 4 and report here X - r a y and neutron diffraction measurements along with the elastic experiments.

3. Results and Discussion The normal and superconducting properties of the La2-xSrxCuO4 system depend strongly on the stoichiomerry of the compound so it is essential to determine oxygen and strontium content rather accurately. A nominal Sr - doping x = 0.14 is too high if we compare the observed temperature of the tetragonal to orthorhombic phase transition (To = 260 K - see Tab. 1) with those reported previously in the literature2,ts. According to these data we deduce that in our crystal the strontium content is approximately equal to 0.12. This is consistent with the temperature of the transition to the superconducting state (T¢ = 12 K as found from ac susceptibility measurements). Such a low value of T¢, in comparison to the polycrystalline compound with the same strontium concentration, is probably due to oxygen vacancies 15. An iodometric method used to determine the oxygen content in our single crystal was not sensitive enough to detect such small changes and gave an oxygen content close to 4. It was already found 16 that the as-grown crystals are oxygen deficient and this may explain the lower To. The X--ray powder diffraction pattern (see Fig.l) taken at room temperature confirms the .single-phase character oF our sample. There are no traces of any impurities and the least squares method gives the lattice para-

2. Experimental A CuO flux method was used to grow single crystals of La2_xSrxCuO4 system. The growth technique is fully described in ref. 13. Our sample was denoted as NTT-33. The X - r a y diffraction measurements on the single crystal of Lat.86Sr0.14CuO4 (nominal composition) were performed with a 2-axis diffractometer using graphite monochromatized M o - K o q radiation and a low temperature camera. After pulverizing part of the single crystal *Permanent address: Institute of Physics, P.K. 30059 Krakow, Poland 945

946

Vol. 77, No. 12

ELASTIC MEASUREMENTS OF La2_xSrxCuO 4 SINGLE CRYSTAL

TABLE I. Lattice parameters deduced from X - ray data taken at 293 K, transition temperatures and critical exponent as obtained from the fit to the intensity of (0,4,1) peak of a single crystal of L a ] . 8 8 S r o . 12CUO4 • T~ denotes the transition temperature to the superconducting state as measured by ac susceptibility method, Toc, Ton, Toy, and Toe denote the tetragonal to orthorhombic phase transition temperatures as obtained from X - ray, neutron diffraction, vibrating reed and elastic measurements respectively.

a (,~)

c (,~)

T: (K)

3.7829(3)

13.219(2)

12+2

Toe (K)

Ton (K)

To, (K)

Toe (K)

260&-_10

260-$-_10

252+5

255-+5

12000

x18 3 8.88

.34+.02

-

a)

8.48 4.8e 4.28

C

• ...

ol

8000

(o,-'m,s:

i

3.68 3.08

121

2.48 1.88

~ 4000 O3 Z IM I--Z

1.88 8.68

48.8

28.8

188.888,8] 88,8 48.8 88.8

L

68.8

88.8

188.8

"lToc

128.8 0

C LAI .SSRS.2CUO4 SUPERCOID ' UCTO_R 16

......... 0

i ......... I O0

i ......... 200

~O

TEMPERATURE (K)

l, 28.8

,Jl 48.8

.... I 68.8

,,,

.. . . .

~ ,, 88.8

188.8

188.8

8000

Fig. 1. The X-ray powder diffraction pattern of powderized Lal. 88Sr0.12CuO 4 single crystal. At the bottom the reference pattern (LaL8Sr0.2CuO 4 from "Powder Diffraction", 2 (1987) 196) is shown. meters shown in Tab. 1 along with other data. Our lattice constants are in good agreement with those previously reported 3-4 and confirm that the Sr - content is close to 0.12 rather than to the nominal value (x = 0.14). Temperature dependent X-ray studies were performed on the single crystal. Single crystals can be characterized by rocking curves. Assuming a Gaussian line shape we can estimated the FWHM (full width at half maximum) to be: Fmo8 = .46 o. The well--known high temperature tetragonal (I-ITT) to medium temperature orthorhombic (MTO) phase transition takes place at To = 260 K as deduced from the temperature dependence of the intensity of (0,-16,3) and (i,4,0) superstructural reflections from the X-ray and neutron diffraction measurements respectively (see Fig. 2). The temperature dependence of the superlattice peak intensity of the (1,4,0) reflection is well described between 200 and 255 K by a power law (To - T)213 with ~ = 0.34 + 0.02. It indicates that the orthorhombic to tetragonal phase transition is of the second order with a value of the order parameter exponent close to those found in 3D Heisenberg or XY magnetic systemstT. In the orthorhombic phase there is a distortion by alternate tilting of the CuO6 octahedra along the [110] direction of the tetragonal unit cell. Analysis of our data assuming a maximum tilt angle TM 0 = 3.0 o leads to the conclusion that the order parameter (proportional to 0) vs. temperature curve does not deviate significantly (by only 3% at low temperatures) from the square root of the corrected intensity. Analyzing further the intensity vs. temperature curves we conclude that the orthorhombic to tetragonal phase transition is rather sharp and we attribute the observed

b)

•% . .

"~"~';~',,,, ,e**

..~6000"

oQ o

E

(1,4,0)

toQ

_d L

,~0 7

0 4000

°

", Ton

Z2000. LtJ F-z

-I Q

l s o o ~

200~

0.00

.............................

@I

211~00 2 ~ 0 0 2zooo 2,,o:00 z~o~ TEMPERATURE (K)

100.00

200.00

TEMPERATURE

(K)

300.00

Fig. 2. Superlattice peak intensities in La].88Sr0j2CuO4 from: a)X-ray diffraction - (0,-16,3) reflection, b)neutron diffraction - (1,4,0) reflection. The inset shows the high temperature part of the neutron diffraction data with the solid line representing the power law I = lo(To-T)21} with I] = 0.34. rounding mainly to critical scattering. The observed rounding (< 15 K) corresponds to a spread in Sr concentration in the crystal of Ax < 0.007 if we assume the equation dT/dx = 2100 K/(formula unit) to hold tg. It is important however to note that the uncertaintity in establishing the strontium content using such an equation is quite large. After a proper absorption and sample motion correction of the present X-ray data we do not observe any pro-

Vol. 77, No. 12

ELASTIC

nounced changes in intensity around 80 K (as we reported previouslyn). The neutron diffraction data also show no anomalies in this temperature range. Measurements of sound velocity Vs and attenuation ¢tc were done 7 both on cooling and heating and are shown in Fig. 3 (cooling run). Around Toe = 255 K a sharp maximum in the attenuation and a pronounced minimum of the sound velocity are clearly visible• They undoubtedly correspond to the ~ ~ MTO phase transition. The sharpness of this transition observed on both curves means that the spread in strontium and oxygen concentration within our crystal is relatively small. The data were comparedT, 9 to the prediction for the elastic constant exponent ~t from scaling. A value of ~t approximately equal to 0.5 was obtained in both cases. The fact that the attenuation peak appears about 5 K above the velocity minimum may indicate additional softening from domain wall motion• The maximum softening of the lattice at Toe is about 6% and persists at lower temperatures. A bump observed at about 110 K may indicate lattice instabilities. Below 80 K there is a tendency of stiffening which may be related to other structural changes. However in this temperature range the ultrasonic pulsetraln deteriorated so the measurements of vs are uncertain here. Further measurements of Young modulus in Lal.ssSr0.mCuO4 are shown in Fig.4. In accordance with the ultrasonic data a minimum in the vibrating reed resonance frequency is observed at Toy = 252 K corresponding to the H I T ~ MTO phase transition. At around 108 K a small bump is detected in fr similarly to the sound velocity vs. temperature curve. The reason for this feature in not known.

5700-

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/

L5,

133 £n

~5600-

~

5500

15

\\/,

t)

05400 > 5300-

tO

/' 52005O

Z

0 FZ_ hl

h.J

160

12~

Toe

i~0 2~0 250 TEMPERATURE (K)

300

947

OF La2_xSrxCuO 4 SINGLE CRYSTAL

MEASUREMENTS

350

Fig. 3. Ultrasonic sound velocity and attenuation in Lal.ssSr0.12CuO4 measured with longitudinal waves propagating along the a (or b) axis at 13 MHz.

8000~

~75oo Z W

".T~ I

o

~7000-

650C

~ ° ° .% .o

160

160 260 260 TEMPERATURE (K)

360

3 i0

Fig. 4. Vibrating reed measurement of resonance frequency, fr, in Lal.ssSr0.;2CuO4, measured on cooling. Young's modulus is proportional to fr2. 4. Conclusions By means of X-ray and neutron diffraction and elastic measurements the single crystal of La 1.ssSr0.uCuO 4 was studied. The H I T --~ MTO phase transition at around 260 K is observed through pronounced anomalies in the elastic quantities Vs, ctc and fr as well as in the X--ray and neutron orthorhombic superlattice intensities• The order parameter exponent value extracted (13 = 0.34) from the neutron diffraction experiment agrees well with other, reported data~. Theoretical studies within the context of Landau theory and series expansion analyses give almost this 13 value (13 = 0.35 for 3D XY model21 and I~ = 0.37 for 3D Heisenberg modepT). The precise theoretical value for 3D XY system with cubic anisotropy is not known to the authors. The possible anomalies observed in the vicinity of 110 K by ultrasound and vibrating reed measurements have not been seen in the X--ray and neutron diffraction patterns, where only a smooth variation of the intensity with temperature was detected. Recent elastic datas indicate that possible structural anomalies in the La2-xSrxCuO4 system may occur below 10 K, i.e. below the accessible range in our X - ray and neutron diffraction measurements. Acknowledgements - The support from NAVF and NORSK HYDRO MS is acknowledged. One of us (M.S.) is grateful for a fellowship from NTNF.

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Vol. 77, No. 12

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