- Structural refinements on intermediate phases -Автор неизвестен - страница 1

Страницы:
1  2  3  4 

ВІСНИК ЛЬВІВ. УН-ТУ Серіяхім. 2003. Bvn.43. С.2І-32

V1SNYK LVIV UNIV. Ser. Шт. 2003. No 43. P J1-32

УДК 546:736

STRUCTURAL REFINEMENTS ON INTERMEDIATE PHASES Tl-1223 - Tl-2223 O. Shcherban1, Th. Hopfinger2, R. Gladyshevskii1, Ph. Galez2, J.L. Jorda2

Fonriation of the intermediate phase Tl4Ba6Ca6Cu90is was observed in the Tl-Ba-Ca-Cu-0 system. Its crystal structure was determined by the powder method (Wmmm, 0=3.8463, c=98.45A, Z=2). It can be considered as an intergrowlh of Tl-1223 and Tl-2223 fragments in the ratio 2:1. Structural models of other intermediate phases, the formation of which is also possible during the transformation from Tl-2223 to Tl-1223 by stepwise deintercalation of ПО layers, are presented.

Key words: high-T^ superconductors, Tl-Ba-Ca-Cu-0 system, intergrowlh structure,

T]Ba2Ca2Cu3CW (unsubstituted Tl-1223) and (Т1,РЬ)(8гіВа)2СагСи309ч/ (substituted Tl-1223) are known to be promising materials for high-current applications due to their high irreversibility fields. However, the transport properties in Tl-1223 wires and tapes are dominated by weak links between the grains, resulting mainly from the grain morphology. Large plate-like grains, commonly observed in Bi-based superconductors, cannot be produ­ced by conventional powder metallurgy and the size of the brick-wall-like ТІ-1223 grains rarely exceeds 5-10 um. Fast formation kinetics impedes suitable grain growth and recrys-tallization is limited by the lack of liquid phase. It has been observed that the pathways for the formation of substituted and unsubstituted Tl-1223 phases are different: in the first case it involves the Tl-1212 phase and in the second case the Tl-2223 phase [1]. The double ТЇО layer compound crystallizes with more pronounced plate-like grains and an optimized synthesis of unsubstituted Tl-1223 could lead to improved grain morphology.

In the course of a general study on phase formation, crystal structure and super­conducting properties of ТІВагСагСи-іОу.,/, aiming at improving the transport properties, we report, in this article, the results of X-ray diffraction, AC susceptibility and scanning electron microscopy measurements. They are discussed considering the formation path of ТІВагСагСизО^/, which will be described in details elsewhere.

Samples were synthesized in two steps. Tl-free precursors were first prepared from mixtures of Ba02 (95% purity), CaO and CuO at 950 °С for 12 hours in air. After addition of Т120з, the samples were wrapped in Au foils and treated at temperatures ranging from 890 to 920 °С in sealed quartz tubes (p02 = 0.5 bar). Details of the syntheses are reported in Table 1. Sample TB910 was submitted to several treatments with intermediate grindings and Tl oxide addition before the second and fourth treatments. The final Tt content was estimated from the weight loss.

Diffraction data were collected on a Philips PW1820 powder diffractometer with Bragg-Brentano geometry (Ni-fittered CuKa radiation, room temperature, 4<29<90°, 0.03° 26 step width, 10 s step time).

'Ivan Franko National University of L 'viv, Kyryla & Me/odiyaStr. 6, UA-79005L'viv, Ukraine

Universite de Savoie, BP240, F- 74942 Annecy Cedex, France

О Shcherban O., Hopfinger Th., Gladyshevskii R. et al., 2003

Table 1

Thermal treatments

Sample

1 Tl content nominal

] Tl content final

1 Thermal treatment

TB890

1.2

0.97

890°C, 12 h

TB900

1.2

0,95

900°C, 12 h

TB910

3.0

1.9

800°C, 10 h in air

 

3.0

1.6

875°C, 15 min

 

 

 

+ 800°C, 12 h in air

 

1.6

0.95

910°C,12 h

 

1.05

0.9

910°C,3 h

TB920

1.2

0.87

920°C,12 h

The crystal structures were refined by the Rietveld method using the DBWS-9807 program [2]. The samples were found to contain Tl-1223 as major phase. Small amounts of BaCu02 and Ca2Cu03 were detected. The occurrence of broad peaks at angles lower than the first Tl-1223 peak in the diffraction patterns indicated that Tl-2223 and one or several intergrowlh structures, combinations of Tl-1223 and Tl-2223, were formed, as mentioned in earlier works [3,4].

Starting models for the structure refinements of Tl-1223 and Tl-2223 were taken from [5]. Models for possible intergrowlh structures were built of Tl-1223 and Tl-2223 stacking units, consisting of a BaO-Cu02-Ca-Cu02-Ca-CuOrBaO block followed by a single or a double TIO layer, respectively. Starting translation periods of the intergrowlh structures were calculated from the formula: Сіп1=П'Сті-шз+гп*сті-rm^, where n and m are the numbers of Tl-1223 and Tl-2223 stacking units in the intergrowlh, respectively.

For the Tl-1223 phase all atomic positional parameters of the metal and oxygen atoms were refined. Due to peak broadness and the low weight content, some parameters of the other phases were constrained or fixed. For Ti-2223 and the intergrowth phases, the positional parameters of the metal sites were refined, whereas the parameters of the oxygen positions were constrained with respect to the metal sites. Isotropic displacement parameters were fixed at 2.0 A2 for the metal atoms and 1.0 A2 for oxygen in Tl-2223 and the intergrowlh phase. The partial occupation of the Ca site by Tl was constrained to be equal in all phases. The unit cell parameter in the direction of the stacking of atom layers was refined for all phases. The unit cell parameter in the direction parallel to the atom layers was constrained for the intergrowlh phase to be equal to that of the Tl-1223 unit cell. The occupancies of the Tl sites were refined separately in all phases. All oxygen sites were assumed to be fully occupied. The Tl and О positions in the TIO layers in Tl-1223 were split. The Tl-1223 phase showed preferred orientation in the 001 direction with a parameter of the March-Dollase function of about 0.5.

In the refinements of the intergrowlh structures, all BaO-CuOj-Ca-CuCb-Ca-CuOr BaO blocks were assumed to be identical and local mirror planes were introduced at the central Cu02 planes. The refinements of these models were carried out in several stages. First the expansion of the BaO-CuO2-Ca-CuO2-Ca-CuO2-Ba0 block and the position of the single TIO layer were refined, and then the individual positions of the layers with respect to the local mirror planes were refined. The positions of the double TIO layers were refined independently.

The atomic scattering factors and correlation terms for anomalous dispersion were those used by the DBWS-9807 program. The refinements were based on diffraction data in

 [he range 8<29<90°, containing 91 reflections of Tl-1223, 103 of Tl-2223 and 275 of Tl-1223/1223/2223, excluding the low-angle region 4<29<8°.

The X-ray diffraction study indicates that all four samples mainly contain superconducting phases. This was confirmed by magnetic measurements. Traces of BaCuOj and Ca2Cu03 and some unidentified phases were detected, but we deduce that their overall amounts do not exceed 5%. In addition to Tl-1223, all samples were found to contain significant amounts of Tl-2223. This is in agreement with earlier works, where the formation path of unsubstituted Tl-1223 is considered to go through Tl-2223 [1]. From the diffraction patterns of samples prepared at different temperatures (Fig. 1) it can be seen that the complete transformation of Tl-2223 to Tl-1223 has taken place at temperatures higher than 920°C after a 12 h treatment in sealed tube. The completeness of the transformation can be achieved at lower temperatures by applying several subsequent treatments with intermediate grindings (See TB910). The low-angle diffraction peak of the Tl-2223 phase shown in Fig. 1 is broad, due to the grain morphology and an inhomogeneous content of Tl. A detailed profile study of the low-angle diffraction peaks and the Rietveld refinement lead to the conclusion that the peak broadness is caused by the existence of average structures (intergrowlh) between Т1-І223 and Tl-2223, which can be intermediate in the course of the formation of the Tl-1223 phase. Other intense diffraction peaks also belong to the intergrowlh phase: 29.0, 32.8, 34.7, 36.6° of CuATa, With increasing temperature and/or number of treatment stages the amount of intergrowth phase decreases tike that of Tl-2223. The refinement of the Tl-2223 phase is significantly improved when the presence of an intergrowth structure is taken into consideration.

3J

і   ■   r         і  ■■  і ■■ -t-i'-i-.       I   I   I   I   I   M   і   '   ■   '   ■   I   I   ,   I   І   I   .   <   ,.....J

1. Вв 5.6.вв 7.OB   Cu Kk

Fig. I. Diffraction pattern in the region 4-8° of 29 CuKa for samples treated at different temperatures

Table 2

Structural characteristics of Tl-1223, Tl-2223 and possible intergrowths

Phase

Composition

Tl content, at.%

Pearson code Space group

а, с, A

Main peak, 20 CuXa (НИ)

Tl-1223

Т1Ва2Са2Си}0<,

5.882

tPW PAimmm

3.8463(2) 15.8584(5)

5.568 (001)

Tl-1223/1223/2223

UtBa^CaeCueOa

7.547

(Л06 lAimmin

3.8463(2) 98.45(2)

5,382 (006)

Tl-1223/2223

nJBa1CaiCu6019

8.333

tni

(4/mmm

3.8463 67.31

5.247 (004)

Tl-1223/2223/2223 Tl-2223

TIjBaiCaiCiidOjg T!2Ba2Ca2CujOio

9.091 10.526

/Я55 Pi/mmm

tm

Wmmm

3,8463 51.81

3.844(3) 35.909(7)

5.113 (003) 4.918(002)

Tl-1223 and Tl-2223 are members of the Tl-based family of high-Tc superconduc­tors. The Tl-12(n-l)n and Tl-22(n-l)n structure series differ in the number of TIO layers in the structure, which consists of a stacking of BaO-Cu02-[Ca-Cu02]n.i-BaO perovskite-deri-ved blocks and single or double TIO rocksalt layers. The crystal structures of the inter­growth phases are also built of such BaO-Cu02-[Ca-Cu02]n.i-BaO blocks separated by TIO or TIO-TIO layers. We have tested three possible simple models of intergrowth with the ratios of Tl-1223 to Tl-2223 stacking units 2:1, 1:1 and 1:2 {Tl-1223/1223/2223, Tl-1223/ 2223 and Tl-1223/2223/2223), other models are considered in [6]. Structural characteristics are given in Table 2. Depending on the parity of the number of stacking layers, the space group of the intergrowth structure is primitive or body-centered tetragonal. The Tl content increases with increasing number of double layers TIO. The composition is calculated assu­ming full occupancy of the Tl site, but double TIO layers are in general partially occupied, which can be explained by TIO deintercalation during the formation of Tl-1223 from Tl-2223. The structures of the intergrowth phases mentioned here are presented in Fig. 2. The schematic drawings show the sequence of Tl-1223 and Tl-2223 (half unit cell) stacking units. Calculated diffraction patterns of the same intergrowth phases are presented in Fig.3, During the refinement of the intergrowth structures, local mirror planes were assumed through the middle Cu02 and single TIO layers. The introduction of a second TIO layer causes a shift of the BaO-Cu02-Ca-Cu02-Ca-Cu02-BaO block in the neighboring stacking unit by lA lA parallel to the atom layers. The opposite shift must take place during the TIO layer deintercalation in order to transform Tl-2223 to Tl-1223. Atom sites for Tl-1223, Tl-2223 and Tl-1223/1223/2223 are given in Table 3. Refinements considering two other models of intergrowth led to higher reliability factors, inconsistent Tl-oceupattons and interlayer distances.

Weight fractions, refined unit cell parameters, fractional atomic coordinates, isotropic displacement parameters and site occupancies of Tl-1223, Tl-2223 and Tl-1223/1223/2223 are given in Tables 4, 6 and 8, Interatomic distances and metal-metal interlayer distances are given in Tables 5, 7 and 9.

The unit cell parameters of Tl-1223 and Tl-2223 are in agreement with literature data. The a-parameter of Tl-2223 is slightly smaller than that of Tl-1223, but within the limits of esd. The а-рататеїег of the intergrowth phase is close to that of Tl-1223 and was

I IM P

-0@-0-®0™>:

• ■ а і pc

Г^2ЄЧ

■<   ф ф ф™

* *c'

- : © t-uU,

-'".■©■; І   фе pit"

«У«ГЮ

0®К""'^їьО «>, Э I *Твс

-сн«н;х«ом' b

CX®0®K>"": # І й і іЬ

ОФІСНО™1: •      • по

О®П®0«-9 і з : тс,

0®0®0^

а

• • • *

L     • *

о or

G®0®0

-ОфОфО * і й і ss

• О • бер

• •

• •

Г ®Г:

J® г

0*0

г -

ч< № О

офоФо

-0®-0

■і і О ■ 9

у W У

М

С

Ova

- • •

- r>®>■■

ОйьГ

Страницы:
1  2  3  4 


Похожие статьи

- Неизлечимая болезнь

- Страдания и беды

- Обращение

- Глосарій

- Structural refinements on intermediate phases -Автор неизвестен