CHAPTER IV

SYNTHESIS A N D CHARACTERISATION OF REBa,HfO 5.5 [RE=Y, Gd, Sm] - A NEW CLASS OF CERAMIC COMPOUNDS

IV.1 Introduction

In Chapter III it is established that the addition of HfO, in YBCO

superconductor has resulted in the formation of a secondary phase YBa,HfO,,,.

By replacing Y by Gd and Sm, we could develop a new group of complex

~erovskite REB~,H~O, , where RE-Y, Gd and Sm having the general formula

A,(BBt)O, [where A-Ba, B-Y, Gd and Sm and B'-Hfl which also could be

used as substrate materials for YBCO film.

In this chapter we present the synthesis and sintering of REBa,HfO,,*

as single phase material. The crystal structure, sintered density, resistivity

measurements, dielectric properties, surface morphology and their chemical

compatibility with YBCO are studied in detail and the results are presented.

IV.2 Preparation of REBa,HfO,,,

REBa,HfO,,, was prepared by mixing stoichiornetric amounts of high

* I'atented in the United Stares ofAmerica. 13rmpe mid in India.

purity [ 9 9 . 9 O / 0 ] rare earth oxides p,O,, Gd,O,, Sm,O,] BaCO, and HfO, in an

agate mortar with acetone as the mixing medium. The wet ~ o w d e r was dried

in an electric oven at 150°C for 2 h, followed by calcination a t 1200°C in air

for 20 h in an alumina crucible with two intermediate grindings. The calcined

material was powdered thoroughly and pessed in the form of pellets having

diameter l l m m and thickness 1.5 mm by uniaxial pressing at a pressure of

5 ton/cm2. These pellets were sintered at temperature 1450°C for 12 h. This

critical sintering condition was standardised from the results obtained in a

systematic study on the variation of physical properties like sintered density,

electrical resistivity, dielectric constant and dissipation factor of REBa,HfO,,,

samples processed at different temperatures.

IV.3 Crystal Structure of REBa,HfO,,

The crystal structure of REBa,HfO,," was studied by x-ray powder

diffraction method. The x-ray diffraction pattern taken on YBa,HfO,,,

GdBa,HfO,, and SmBa,HfO,, samples sintered at 1450°C for 28 values

between 5" and 90" are shown in Fig.IV.1. The computerised XRD data for

~hcsc materials arc given in Tables IV.1, IV.2 and IV.3 rcspcctivcly. l'hc XRD

patterns and the corresponding data clearly show that all these materials have

* The XI</) doto ond a brief sirmmorj. ofthe sjnthesis of Y H a A m , , . GdHo.HfU,,o~id SrnHo/ffU,, hove beeri accepted in JCPDSjjle prrblished by the Notioriol jisritirte qf Scierice ond l i . chno lo~ , United Srates Deporrrrienr o/Co~rrrrrcrce. Notiorla1 lristi/irte oJSrorlclorrl.s and lechnology, and will nppeor in one ofits jbrthcor~iing edrriorr.

Fig IV.1 X.ray diffraction patterns of single phase (A) YUa,NfO,, (U) GdUa,I-IlO,,, ( C ) SmUa,fIfO,,

* Table 1V.I X-ray diffraction data of sintered YBa,HfO,,,

S.No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

1/10

4

6

100

4

3

24

4

5

27

3

10

9

4

9

28

18.310

21.310

30.260

35.620

37.300

43.380

47.440

48.660

53.820

57.710

63.100

71.540

79.480

87.420

hkl

111

200

220

311

222

400

331

420

422

333

440

620

444

642

d (calculated)

4.833

4.186

2.959

2.524

2.416

2.093

1.921

1.872

1.708

1.61 1

1.479

1.329

1.208

1.118

Width

0.285

0.435

0.750

0.270

0.240

0.510

0.212

0.405

0.570

0.345

0.540

0.524

0.285

0.360

d (observed)

4.841

4.166

2.951

2.518

2.408

2.084

1.915

1.870

1.702

1.596

1.472

1.318

1.205

1.115

Table IV.2 X-ray diffraction data of sintered GdBa,Hf06,6 *

-

hkl

111

200

220

311

222

400

331

420

422

440

620

444

642

111,

7

8

100

7

7

33

17

12

38

18

18

16

18

No.

1

2

3

4

5

6

7

8

9

10

11

12

13

Width

0.138

0.135

0.450

0.128

0.139

0.390

0.240

0.143

0.285

0.105

0.195

0.195

0.120

20

18.380

22.220

30.190

35.600

37.200

43.280

47.390

48.630

53.570

62.720

71.340

79.490

87.300

d (observed)

4.823

4.183

2.958

2.520

2.41 5

2.089

1.917

1.871

1.704

1.480

1.321

1.205

1.116

d (calculated)

4.824

4.178

2.954

2.519

2.412

2.089

1.916

1.868

1.705

1.477

1.321

1.206

1.116

Table IV.3 X-ray diffraction data of sintered SmBa,HfO,,, *

-

hkl

111

200

220

311

222

400

331

420

422

333

440

620

444

642

1/10

13

14

100

10

12

31

12

8

28

11

18

15

10

16

-

S.No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

d (observed)

5.202

4.227

2.899

2.635

2.413

2.099

1.945

1.932

1.706

1.538

1.481

1.321

1.205

1.119

d (calculated)

4.847

4.198

2.968

2.531

2.423

2.099

1.926

1.877

1.713

1.615

1.484

1.327

1.211

1.121

-

28

17.030

21.000

30.820

34.000

36.020

43.060

46.670

47.000

53.690

60.110

62.690

71.370

79.500

87.000

Width

0.315

0.300

0.555

0.234

0.280

0.660

0.315

0.324

0.645

0.270

0.375

0.270

0.198

0.134

the same structure as judged by the similarity in their intensities and positions

of the lines on the x-ray powder diffraction patterns. The crystal structure

diagram of REBa,HfO,, is shown in Fig. IV.2. REBa,HfO,, were also found to be

@RE @Ba OHf 00

Fig IV.2. Crystal structure diagram of KEBa,I-If05,

isostructural with other rare-earth cubic perovskites will1 the general forrnula

A,(BB1)Ob such as EuBa,NbOb and YBa,SbOb(l-2) reported in the JCI'DS file,

in which doubling of the basic perovskite unit cell is observed. This doubling

78

is due to the ordering of B and B' on the octahedral sites. In a substitutional

solid solution BB', there is random arrangement of B and B' atoms in equivalent

positions in the crystal structure. If, upon suitable heat treatlnent, the random

solid solution rearranges into a structure in which the B and B' occupy the

same set of positions but in a regular way, the structure is described as a

superstructure (3). In the superstructure, the positions occupied by B and B'

are no longer equivalent and this is exhibited in the XRD pattern by the

presence of superstructure lines. The presence of superstructure lines in the

XRD patterns shown in Fig. IV.1 thus indicates, the ordering of thc basic

ABO, perovskite unit cell in the REBa,HfO,, materials. The XRD peaks

including the minor peaks for REBa,HfO,,, are now indexed for a complex

cubic perovskite s t ructure. T h e lattice constant values calculated for

YBa,HfO,,, GdBa,HfO,, and SmBa,HfO,, con~pounds were a-8.372, 8.364

and 8.396 A respectively.

IV.4 Sintered Density and Resistivity Measurements

The theoretical densities of REBa,HfO,, were calculated from atolnic

masses and crystal structure. The cubic perovskite cell of REha,IliO,,, contains

four unit formulae of atoms. The theoretical densities based on the number of

atolns pcr unit cell were calculated from the l a ~ t i c c constant values.

Experimentally the densities of sintered REBa,IlfO,,, pellets were measured

by Archimedes method and each was found to be about 98% of their theoretical

79

density values. The values of lattice constant, theoretical density and sintered

density are given in Table IV.4. The properties of polycrystalline materials

are highly dependent on their bulk density and hence the sintered density of

the specimen material should be approximately equal to the theoretical density.

The d-c resistivities of REBa,HfO,,, were measured using a d-c

Table IV.4 Lattice constant, theoretical density, sintered density and d-c resistivity of REBa2Hf06,,

electrometer (Keithley 602) and were found to be highly dependant on the

Material

YBa,HfO,,

GdBa2Hf0,,

SmBa,HfO,,

sintering temperatures. The resistivity values at room temperature were found

to bc of the order of 101OR cm for each of REBa,I-IfO,,, pellets and are also

Lattice constant [a:

(4

8.372

8.364

8.396

givcn in l'able IV.4 along with the sintcring temperatures.

IV.5 Measurement of Dielectric Properties

Theoretical density

( gem.')

7.129

7.944

7.759

Dielectric properties give the response of the material medium to various

electric fields passing through them. They are highly dependant on crystal

Sintered density

(gem.')

6.986

7.745

7.588

dc resisistivity

( ~ 1 ~ r n )

5x1 OtO

8x1 OjO

9x1 010

structures and polarisability of the material. In the case of polycrystalline

materials the dielectric properties, in addition, depend on the grain size, grain

boundaries, their distribution and nature of the impurity phases present. I-Ience

it is clear that the dielectric properties measured on polycrystalline materials

with inhomogeneties and other impurities are not the actual dielectric

properties of the material but, a modulated picture of all the effects. O n the

other hand, materials in its single crystal form give more accurate values of

dielectric properties.

Dielectric properties of polycrystalline REBa,MfO,,, were studied in

the frequency range of 30 H z to 13 MHz at room tenlperature and at liquid

nitrogen temperature by using an im~edence analyser. A brief description of

the instrument and the method of measurement of dielectric constant and

dielectric loss factor have been given in chapter 11. The variation of dielectric

constant (E') for sintered REBa,HfO,., pellets with frequency measured at room

temperature is given in Fig. IV.3 and the variation of dissipation factor (tan6)

is given in Fig. IV.4. The values of the dielectric constant were found to be

constant in the KHz to MHz region and the dissipation factor of the materials

were considerably reduced in these frequency ranges for each REBa,HfO,,,

samples. The values of E' in the H z to K H z region were slightly higher and

Fig JY.3 Variation of dielectric constant (E') with frequency for (A) YBa,lIfO,, (D) GdBa,HfO,,(C) SmnBa,HfO,, measured at 300K

z 2 4 - w C

t (IJ w 2 20- 0 0 0 .-

16- 0 a, - a, .- a

12

8

-

I "I

- l a c :

:A

-

I I

2 3 4 5 6 7

Log f

Log f

Fig IV.4 Variation of loss factor (tan 8 ) with frequency for (A) YBa,I-IfO,, (I)) C;cl1%a,II1O5,, (C) StnBa,I IfO,, tncasurcd nt 300K

83

decreased as the frequency increased to MHz region. This slightly high value

of E' at lower frequencies can be attributed to the space charge polarisation

taking place when a time varying electric field passes through the samples.

This space charge polarisation becomes inoperative as the frequency increases.

Since the value of E' is steady from KHz to MHz region the magnitude of the

orientation polarisation and ionisation polarisation is very low. At MHz and

higher frequency region only electronic polarisation mechanism is operative

and its magnitude is relatively large. The dielectric loss factor is a measure of

loss of electrical energy in the dielectric medium. Since the tan6 values for

REBa,IlfO,, samples are very low, these compounds can be considered as

good dielectric materials.

In view of their applications as substrate for YBCO superconducting

films, we have also conducted low temperature dielectric measurements in

REBa,HfO,,, samples. Figure IV.5 gives the variation of dielectric constant

values with frequencies measured at 77K. Figure IV.6 gives the variation of

dielectric loss factor with frequencies at the same temperature. The measured

values of E' at this temperature were almost same as those obtained at room

temperature. In MHz region the tan6 values of REBa,I-IfO,, measured at

77K were found to be relatively lower than that taken at room temperature.

Fig IV.5 Varia~ion of dielectric constant (z') xvirll Ircquc~lcy for (A) Ylla,IITO,, (U) GdUa,I-IIO,, (C) S I~U~, I - I~O, , , measured at 771L

C a 2 4 - w w r= 'sl w cn r 2 0 - 0 0 0 . - L

5 16- a, - a, . - n

12

8

-

- ------ -I,

-

-

c--*-L--.-.----.-L.-. c - @--~.-*. A -

- I I I

2 3 4 5 6 7 Log f

Log f

Fig IV.6 Variation of loss factor (tan 6 ) with frequency for (A) YDa,IIfO 5.5

(I%) Gd~a,I I fO, , (C) SmUa,I IfO,,, mea~ured at 77I<

For a comparitive study, the dielectric constant and loss factor values for

REBa,I-IfO,, measured at 10 MHz at room temperature and at liquid nitrogen

temperatures are given in Table IV.5.

Table IV.5 Dielectric constant (E') and loss factor (tan6)of REBa,HfO,,,

IV.6 Surface Morphology of REBa,HfO,.,

Microstructure of polycrystalline materials are very important to assess

their measured properties. Optical micrographs of sintered and highly polished

samples of REBa,HfO,,, are shown in Fig.IV.7 [A, B, C]. YBa,IlfO,,,

GdBa,I-IfO,,, and SmBa,HfO,,, samples form unifrom patterns with no

Material

YBa,HfO,,

GdBa,HfO,,

SmBa,HfO,,

additional phases. A few white spots in each of the micrographs represent

pores, which agree with the sintered density measurements. Since there are

very few pores combined with uniform distribution of grains as observed in

the micrographs, it gives approximately the true values of material properties

measured on these samples.

tan6 (at 10MHz-77K)

-lxlo4

-3x1 O4

-2x104

d (at lOMHz-300K)

-1 0

-1 9

-1 4

tan6 (at lOMHz300K)

-8x1 O4

-5x1 OJ

-4x1 OJ

E' (at 10MHz-77K)

-9

-1 9

-1 3

Fig. N.7 Optical micrographs of (A) YBa,HfO,, (B) GdBa,HfO,, (C) SmBa,HfO,, (Magnification 800)

IV.7 Chemical Compatibility of REBa,HfO,, with YBa,Cu,O,,

One of the most important criteria for the selection of any material as

substrate for YBCO superconductors is the chemical non-reactivity between

the substrate and the film at the processing temperature(4-9). In order to see

whether REBa,HfO,, is chemically compatible with YBCO, the chemical

reactivity of REBa,HfO,, withYBCO was studied at temperatures upto 950°C.

Superconducting YBCO powder was mixed with REBa,HfO,,, in 1:l volume

ratio and pressed in the form of pellets. The pellets were then annealed a t

950°C for 15 h and cooled slowly. If YBCO reacted with REBa,HfO,,, a t

such annealing conditions, new additional phases besides YBCO and

REBa,HfO,,, could be observed in the x-ray diffraction patterns. O n the other

hand, if YBCO does not react with REBa,HfO,,, the crystalline phases after

annealing will be just two phases of YBCO and REBa,HfO,,,. Powder

diffraction patterns of annealed samples of 1:l volume mixtures of YBCO

and REBa,HfO,, are shown in Fig.IV.8 [A-Dl. X-ray diffraction patterns

of the two phases in the annealed samples [Figs. IV.8(B-D)] were

compared with those of pure YBCO [Fig.IV.g(A)] and pure REBa-I-IfO,,

(Fig.IV.l). It is clear f rom these figures [IV.8(B-D)] that no new

additional phase was formed (within the precision of the XRD

technique) besides YBCO and REBa,HfO,,, in the YBCO-REBa,IlfO,,,

composites. This indicates that there is no detectable chemical reaction

8(r

Fig rV.8 X-ray diffraction patterns of (A) pure YBCO (B) 1:l volume mixture of YBCO and YBa,HfO,, (C) 1:l volume mixture of YBCO and GdBa,IIfO,,, (D) 1:l volume mixture of YBCO and SmBa,IIfO,,,

taking place between YBCO and REBa,HfO,,, when under severe heat

treatment.

T h e effect of REBa,HfO,,, addi t ion on t h e superconduct ing

properties of YBCO was studied by temperature-resistivity measurements.

Figure IV.9 [A-C]. shows temperature versus resistivity curves for the YBCO-

REBa,HfO,, composite system containing 50 vol% of REBa,HfO,, annealed at

950°C for 15 hours.

Fig. IV.9. Temperature-resistivity curves for YBCO-REBa,HfO,,,compositcs

containing 50 vol. O/O of (A) YBa,HfO,,, (B) GdBa,HfO,,, (C) SmBa,HfO,,,

a1

A superconducting transition temperature of 92K in all these composite

samples indicates that a substantial addition of REBa,HfO,, in YBCO did not

have any detrimental effect on the superconducting transition temperature of

YBCO even after severe heat treatment.

IV.8 Discussion

Three new compounds YBa,HfO,,,", GdBa,I-IfO,,," a l ~ d SmBa,HfO,,

have been synthesised, characterised and sintered as single phase materials by

solid state reaction method. X-ray diffraction analysis shows that these

materials are isostructural and have an ordered cubic perovskite structure of

type A,(BB1)06. The presence of superstructure lines in the XRD patterns of

REBa,HfO,,, indicates the ordering of the basic ABO, perovskite unit cell.

Though REBa,HfO,,, materials have the A,(BB1)06 structure, taking into

account the valency of Hf at 4+, the chemical formulae for the compounds are

written as YBa,HfO,,j, GdBa,HfO,, and SmBa,HfO,,. The electrical resistivity

measurements show that these compounds are good insulators. The dielectric

constant and loss factor values of REBa,HfO,, samples are found to be in a

range suitable for their use in microelectronic applications when compared to

those of some conventional ceramic dielectric materials like MgO, SrTiO,

'"~lrblished in Mater. Letters. 11, 278 (1992), 21, 301 (1771) Norh ifollaiid

" Published in Supercond. Sci. Technol. 8,121 (1991) Uti

92

and A1,0, which arc commonly used as substrate materials for the

iabrication of YBCO thick and thin films(l0-17). The IZEBa,HfO,,, was

found t o be chemically compatible with YBCO. A substantial addition of

these tnaterials in YBCO did not show a detrimental effect on the superconducting

properties of YBCO even after a prolonged and severe heat treatment upto 950°C.

O n e of the most unique characteristics which make these materials of

tremendous technological importance is their chemical non-reactivity with

YBCO superconductors even under severe processing conditions, making these

materials suitable as substrates for YBCO superconductors. REBa,HfO,,,

materials are highly stable under atmospheric conditions and no degradation

was observed in the stability of the samples, even if they were kept in boiling

water for 1 h. The samples were mechanically strong and could be sliced into

pieces of 0.5 mm thickness with a diamond cutter. Good reflecting surfaces

were obtained by mechanical polishing. Organic solvents such as alcohol,

acetone and carbon tetrachloride could be used as effective cleaning agents.

I n addition to chemical conlpatibility and iavourable dielectric

properties and phase stability, the lattice matching of REBa,HfO,,, with YBCO

is also an important factor for the epitaxial growth of YBCO films on single

crystal substrates. REBa,HfO,,, has a complex cubic perovskite structure with

lattice constants between 8.364 and 8.396A [Table IV.41. Even though the

lattice matching of these materials with YBCO is not perfect, the latticc

constant values of REBa,HfO,, based on the doubling of the simple perovskite

unit cell are in a range comparable to that of MgO [a-4.208K] which is an

extensively used substrate for epitaxial of YBCO films. As single

crystals are needed for substrate applications in the fabrication of YBCO thin

films, melting experiments were carried out to examine whether REBa,HfO,,,

materials melt congruently for possible single crystal growth from melt. Single

phase REBa,HfO,,, was placed in a platinum crucible and the material was

melted completely in air at a temperature of 1750 3C using a muffle furnace.

The melted samples were withdrawn from the furnace and quenched in air to

the room temperature. The quenched samples were then examined by x-ray

diffraction. The XRD patterns were found to be identical tothose of sinterd

samples indicating that REBa,HfO,,, materials melt congruently and could be

grown from the melt as single crystals.

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