Intercalation of halogens into tetragonal YBa2Cu3O7−x

Intercalation of halogens into tetragonal YBa2Cu3O7−x

~ Solid State Communications, Vol. 72, No. I, pp. 107-112, Printed in Great Britain. 1989. 0038-1098/8953.00+.00 Pergamon Press plc INTERCALATION ...

553KB Sizes 35 Downloads 52 Views

~

Solid State Communications, Vol. 72, No. I, pp. 107-112, Printed in Great Britain.

1989.

0038-1098/8953.00+.00 Pergamon Press plc

INTERCALATION OF IIALOGENS IETO TETP~AGONAL YBa2Cu307_ x Yu.T.Pavlyukhin, TA.P.l~emudry, 1~.G.Khainovsky and V.V.Boldyrev Institute of Solid State Chemistry,

630091Novosibirsk,

USSR

(Received 8 June 1989 by C.N.R. Ran)

Intercalation of halogens (C!, Br, I) into tetragonal modification of YBa2Cu307_ x is realized. The new halogencontaining, compounds are the high-te~r~perature superconductors. ~ossbauer's spectroscopy sho~s that in general the halogens are localized in oxygen vacancies of the Cu(1) layer, a1~ the intercalation is accompanied by the Charge transfer from the Cu(2) layers on to the halogens. So, the high-temperature superconductivity (HTSC) is determined by the certain concentration of holes in the Cu(2) layers which arises as a result of intercalation of acceptors of different types into the Cu(1) layers. ~%e possible versions of explanation of appearance of HTSC in the given compounds are considered.

lity of the intercalation process in YBa2Cu307_ x was dravn% in [8] as a result o~ reversible incorporation of oxygen into the Cu(1) layer with the formation of oxygen anions in the oxydation state less than 2-. It was of interest to elucidate the question whether the existence of electron transfer into the Cu(1) layer is sufficient for the occurrence of superconductivity, or the chemical nature of the Cu(1) layer is of importance as well. Intercalation of some other guests acting as acceptors into the Cu(1) layer of the nonsuperconducting tetragonal modification of YBa2Cu307_ x can contribute to answering this question. It should be noted that in the literature the partial substitution of oxygen by other anions arises considerable interest. So, Ovshinsky and c ~ r k ers have reported [9] that the inclusion of fluorine into YBa2Cu~O 7 x by solid state reaction results ~i~- a multiphase system which shows superconductivity at temperatures of 155-168°K. Besides the reaction of YBa2Cu307_ x with gaseous NF~ leads to the formation of compounds ~with different contents of fluorine and a critical temperature (Tc) of 155°K [10] . The results of Ovshinsky were confirmed in [11] , but in [12, 13] the treatment with fluorine has shown to lead in one case to a Tc of 80°K and in the other case to a Tc o f 96°K. The substitution of oxygen by chlorine has recently been reported in [14] . Thus, in spite of the conflicting literature data the partial substitution of oxygen in YBa2Cu307_ x is of

An important problem in the study of the nature of superconductivity in YBa2Cu307_ x is to determine the charge state of ions (primarily the Cu ions). In this connection the discovery of antiferromagnetic ordering in the Cu(2) layers has been of prime importance. It was first demonstrated in muon spin rotation experiments [1] . Determination of the Neel temperature by this method gives the following estimates: T N ~ 4 5 0 n K (x = 1.0) and T N L 2.5 ° K (x = 0 . 6 ) [2~ . These data were confirmed by the results of neutron scattering [3] : TN = 400~I0°K (x = 0.85) and T u L 500°K (x = 1.0). A series of works o~ neutron diffraction was further carried out [4-6] . Similar results were derived by ~ossbauer spectroscopy [7] • The discovery of antiferromagnetic ordering allows the fundamental conclusion to be made that in YBa2Cu307_ x different crystallographic positions Cu(1) and Cu(2) are occupied by cations with different oxidatio~ states. For example, for x ~ 1.0, ~u ]+ is in the Cu(1) position and Cu z+ in the Cu(2) position. Decreasing x leads to appearance of electron holes in the Cu(2) layers and antiferromagnetic ordering is destroyed at the moment of occurren~ of superconductivity in this compound. This suggests that the electron structure of the Cu(2) layer is responsible for the occurrence of s u p e r c o n d u c t i v i t ~ The role of the Gu(1) layer upon the incorporation of oxygen into it reduces to electron transfer from the Cu(2) layer to the Cu(1) layer. The conclusion about the possibi107

108

Vol. 72, No. 1

INTERCALATION OF HALOGENS INTO TETRAGONAL YBa2Cu307_ x

some interest from the viewpoint of a possible increase of Tc and for the study of the superconductivity mechanism in YBa2Cu307_ x. This paper presents the results of our study de~ling with the substitution of oxygen in certain structural positions by halogens (I, Br, C1) with the use of the "soft chemistry" technique. ~ne influence of halogens intercalation into YBa2Cu307_ x on its high-temperature superconductivity (HTSC) is discussed. The present study makes it possible to define more exactly the role of the Cu(1) and Cu(2) layers in the HTSC mechanism.

E E

02, (ezt.) ZOO

j

fO0

d

7bo aOO ~bo

¢~o

ebo

r,~

E

JO0

/

,~O0

rot in x is _+0.1. The interaction of halogens with power samples o f t he HTSC modification was carried ou~ in vacuumsealed off ampoules (P ~ 10 -m mm Hg) and in membrane zero manometers. The bromine content of the reaction products was determined by coulometric and argentometric titrations. The error in the determination of bromine percentage content in + 0.30. The iodine content of the samples was determined by ionometric analysis using an iodineselective chalcogenide glass electrode and by argentometric titrationso The error in the determination of the iodine percentage content is +0°05. X-ray phase analysis was perforr~ed on a DRON-3 diffractometer with a graphite monochromator usir~ CuK~ -radiation. T h ~ p r o c e d ure of Mossbauer studies of J~Fe-doped samples is described in [16,17] ; note that the chemical shift is given relative to ~ - F e . The chemica½ shifts for ~ossbauer experiments on I 9i_dope d samples a~e given relative to MgTe04o

~oo

~/ (&t.)

Experimental, To obtain the tetragonical HTSC modification of YBa2Cu307_ x the orthorhombic phase samples prepared as described in [1~ were heated in dynamic vacuum (P ~ 10 -2 mm Hg) for one hour and then rapidly quenched. The oxygen content was determined by the formula

~0o

/

// /,/ /1"

/00

E

300

0a c~ 200

Results and Discussion. According to our data obtained from pressure measurements of halogen vapours over the HTSC modification (Fig. 1) in the system being at thez~al equilibrium, the interaction of I, Br and C1 with YBa2CU3OT_ x ( x ~ O . 5 ) becomes noticeable at temperatures close to 473 °, 453 ° and 323°K, respectively. To find the phase composition of the reaction products the interactions of halogens with the HTSC modification were performed in ~npoules for 5-6 hours

/

too

/ /

mo

zoo

sbo

~o

~o

T,K

Figure 1. The temperature dependence of the quasiequilibrium pressure of halogen vapours above YBa2Cu307_ x.

Vol. 72, No. 1

INTERCALATION

OF HALOGENS

INTO TETRAGONAL YBa2Cu307_ x

at different temperatures (in an isothermal regime). X-ray phase analysis data show that the absorption of I, C1 and Br by the sample of YBa2Cu307_ x (x70.5) during this period up to temperatures of 523 ° , 493 ° and 393°K, respectively, is accompanied by transition of the tetragonical HTSC structure to the orthorhombic one. Table I contains the results of chemical analysis for the products of interaction of YBa2Cu307_ x (x ~0.5) with halogens, their unit cell parameters calculated from the positions of (200), (020) and (006) reflections, and the temperature of transition to the superconductil%g state (Tq) determined from magnetic measuremenws. The data for samples with the maximum oxygen content are given for comparison. It is worth n o t i ~ that the products of interaction of YBa2Cu307_ x with I2, Br 2 are monophas~ For chlorine containing samples, in contrast to the data of [14] , the monophase composition was not achieved (the samples contain admixtures of the s t a r t i ~ reactant). The interaction of halogens with the HTSC modification at higher temperatures (523-773, 493-573 and 393-473°K for I2, Br 2 and C12, respectively) for 5-8 hours leads to the appearence on the X-ray diffraction patterns of additional hardly identified reflections and to a decrease in the intensity of the diffraction maxima of the orthorhombic phase. As the reaction temperature further increases, the gaseous oxygen occurs in the system, the reflections of the orthorhombic HTSC modification completely disappear, and a mix-

109

ture of complex composition forms which appears to contain various halogenides and oxyhalogenides, and iodates in the case of interaction with ~odine. The interaction of I 9I with YBa~CuaO7 x at 523°K for 24 hours resulks fn'~ complete decomposition of the starting compound. The Nossbauer spectrum of the products shows the formation of a mixture of iodine compounds in which iodine is in three oxidation states: I-, 17+ and 15+ (Fig. 2). The relative intensities of the components, the chemical shifts (mm/sec) and the constants of quadrupole splittings e2qQ129(n~/sec) are, correspondingly, (86%, 3.13-3.8), (8%, 0.34, 0.00) and (6%, 5.63, 16.8).

ID(~.'~""

I "'~

~

?I',

o

" " " ~"



2:2 2:

LU ~9~ >

~ ~ I

,

T=4K I

-8

-4

0

~,

i

4

8

Figure 2. The Mossbauer spectrum for the products of interaction between YBa2Cu307_ x and 129I at 4 K.

Table 1

No. of the sample

Compound

Unit cell parameters

ao

bo 3.83

11.74

1.5

3.85

11.69

60

I

YBa2Cu306o 3

3.83

2

YBa2Cu306.3Ioo2.O. 4

3.89

,

3

T c , K*

C o

,

YBa2Cu306.3BrI.O.I. 8

3.87

3.85

11.63 .

90 .

.

.

.

.

L

4

YBa2Cu 06.3C1 x

3.87

3.85

11.63

86

5

YBa2Cu306.94

3.89

3.83

11.67

93

* The results of Ref.

[18].

INTERCALATION OF HALOGENS INTO TETRAGONAL YBa Cu O Vol. 72, No. 1 2 3 7-x It is seen that the proportion of structural Cu(1) position with coordithese forms corresponds to a high degree nation numbers 2 (external doublet) of accuracy to that in the disproportand more than 2 (internal doublet). ionation reaction. The third state Fe(III) was assigned to It should be noted that the curve the Cu(2) position. The validity of of the quasiequilibrium pressure of hathis assignment is supported by the logen vapours over the tetragonal modifact that antiferromagnetic ordering of fication of YBa~Cu~O~ shows that lowthe Cu(2) layer leads to the transitL ~ i-x temperature interactmon products have ion of the Fe(llI) doublet to e Zeeman sextet and has no influence on the rather narrow temperature ranges of states Fe(1) and ~'e(II). Therefore, ~he stability or these are metastable comincrease in the intensity of the I~(II) pounds. ~-hrthermore, the strengthening doublet directly evidences the i n c r e a ~ of oxidation properties (electron affied amount of the Pe states coordinated nity) and the decrease of the radius with anions in the Cu(1) positions... of halogens in the series I, Br and C1 A sample without oxygen, the Mossis likely to be responsible for the inbauer spectrum of which contains the creased rate of halogenation for sampmagnetic-ordered Fe(III) component ~as les of the tetragonal YBa2Cu307_ x mochosen as the starting sample for halodification. As a consequence, the progenation. The highest intensity of the blem of monophase composition there~e(I) state is the consequence of alwith arises for chlorine containing most complete absence of oxygen in the samples having the orthorhombic strucCu(1) layer. ture: these contain either an admixtu~lere is only the Fe(ll) component re of the starting reactant, or oxyin the spectlunn of the bromine-containclorides and chlorides of Y, Ba and Cu. ing HTSC compound. After bromine treatThe difference in the degree of disperment of the s~nples the uncoordinated sion, in order of texture and oxygen iron cations in the Cu(1) plane are conte~ts of the TBa2Cu307_ x samples practically ~bsent. ~lere are merely some traces of the Fe(I) state (Fig. 3) also have a marked effect on the kinetin the spectrum. Specti~un 3 in Fig. 3 ics of the formation and on the composshows the change in the proportion of ition of halogen-containing HTSC comthe Fe(1) and Fe(II) states under extpounds° reme conditions of oxygen saturation Thus, the interaction of 12, Br 2

ii0

and C12 with YBa2Cu307_ x ( x ~ 0 . 5 ) has two characteristic temperature ranges. Of oarticular interest is the low-temperature range in which the formation of compounds which exhibit HTSC t~kes place. To determine the structural position of halogens in HTSC compounds studies were made of YBa~Cu~Osamples doped with 1-3~ of W~Fe cations (Table 2) using the Nossbauer spectroscopy. ~ e interpreta$io~ was based on the results of [7] where two doublets (Fe(I) and ~e(II)) with chemical shift close to zero were assigned to the

(P02~ 10 arm).

~lus the changes in the proportion of the Fe(I) and Fe(II) components, which occur in the Mossbauer spectra of the samples after treatment with bromine is basically localized in the Cu(1) layer. It should be noted that the interaction of the starting s~mple with bromine gives rise to the disappearance of the characteristic sextet. In corporation of iodine into the sample also results in the transition of the magnetic-ordered component to a paramagnetic doublet (cf. spectra I and 2 in Fig. 3)- ~o marked increase of the

Table 2 Compound

No. of the sample

Unit cell parameters o

T O , K~

A ao

I i

bo

Co

6

YBa2(Ouo.97Feo.03)306.3

3.85

3.85

11.74

7

YBa2(Cuo.97Feo.03)306.310.2

3.86

3.86

11.70

i

8

YBa2 (Cuo. 97Feo. 03 )306.98

3.86

3.88

11.66

9

YBa (Cu

3.85

3.85

11.56

2

0o97 Fe 0.03 )30 6 . ~ . 8

88

Vol. 72, No. I

INTERCALATION OF HALOGENS INTO TETRAGONAL YBa2Cu307_ x

:

,,/

\

OIJ

7f . O ~ co

co I-

l.a_l r-Y

f.O~ 1

.

0

I

l

-2

-I

~

VV

Figure 3. I~Sssbauer starting sanJple with netic-ordered Cu(1) either iodine 2 , bromine 4 content, at 300 I(.

I

1

o

+I

i

+2MMI$

spectra for the the antiferromaglayer I , with or oxygen 3 or which were taken

ordinary chemical bond Fe-I and, hence, to noticeable changes in the M~ssbauer parameters of iron in the Cu(1~ layer. Static restrictions for bromine in comparison with iodine are considerably weakened, and its chemical state is likely to be closer to the ionic form and, as a consequence, the Mossbauer spectra are substantially modified. The results obtained indicate two important facts. First, the nature of an electron acceptor in the Cu(]) layer is of no significance for the occurrence of superconductivity. This enables the assumption to be made that superconductivity is related only to the Cu(2) planes, so the concentration of holes in the Cu(2) layers is the determinant. It is probable that for HTSC in YBa2Cu307_ x the conditions are realized when physicochemical factors contribute to the creation of a certain concentration of holes in the Cu(II) layer which is near the optimum concentration required to obtain high T c. It is the reason for which the thez~odynamically equilibrium concentration of holes is attained by different number of acceptors with different electron affinity. Thus, in the case of YBa2Cu307_ x the concentration of a c c ~ tots (0) in the Cu(1) layer reaches 50% of the number of possible structural positions for oxygen, and in the case of the bromine-containing modification, acceptors occupy all possible vacant positions in this layer. On the other hand, the optimum concentration of holes in the Cu(2) layer required to achieve high Tc can be estimated from the dependence of T c on x for the systems Ba1_xBixCuO 3 and La2_xSrxCu04"

Fe(II) component in this case is obsezved, i.e. there is no increase in the average coordination ntunber of the Fe cations in the Cu(1) layer. At the same time the average charge state of Cu in the Cu(2) layer changes, ~lich is evidenced by the disappearance of the magnetic-ordered state. This is probably associated with electron transfer from the conduction band of the YBa2(Cuo.97Feo.o3)307_x matrix to iodine and with the formation of the state 16- with a relatively small value of 5. Steric reasons seem basic here. The size of even a neutral iodine atom is close to that of the anion vacancy in the Cu(1) layer, and further increase of the iodine radius with increasing the negative charge on it is difficult. ~&e coordination of the iron cations by iodine atoms the chemical state of which is close to the neutral does not, therefore, lead to the formation of the

iii

This value

is about

10 - 20%. The question therefore arises ~ly a hole concentration in the Cu(2) layer of the order of 0.1-0.2 is ultimate for YBa2Cu307_ x in the formation of electronic subsystem. This is caused by two mechanisms previously discussed. The first is the formation of a polaron - the bonded state of a hole and the lattice deformation near the hole due to the polarization of the nearest surruondings. From the chemical point of view, the gain in energy is therewith associated with the energy of rearrangement of the surround ings. According to [19] , in this case a gain in energy is-possible w h e n polarons slightly influence each other. The typical ultimate concentration is shown to be 10% [19] • To reveal the second, we may take advantage of the theory of alloys. It is well known that as the concentration of electrons in an alloy changes, i.e. the Fermi energy changes, the structur-

112

INTERCALATION OF HALOGENS INTO TETRAGONAL YBa2Cu307_ x

a! phase transitions are possible if the Fermi energy reaches the boundary of the Brillouin zone (the Yum-Rozery rules)° For two-dimensional electronic system with a pronounced directivity of the bond in the Cu(2) plane, the Fe~ui surface considerably differs from a spherical one, and it is, therefore, quite possible for the ~ermi surface to reach the Brillouin zone at this hole concentration. Furthermore, by analogy, the subsequent phase transition in the Cu(2) plane, i.e. deformation, becomes impossible because of the stabilizing of the rest of the lattice, and further increase of the hole concentration becomes unprofitable for energetic reasons. The interaction of holes with the

Vol. 72, No. 1

lattice is of significance for both mechanisms. In addition, the ultimate lm!e concentration is the limiting, in stability, range of existence of the starting electronic structure. It is therefore quite probable that in this case a strong interaction of charge carriers and lattice deformations take place. Note also that this interaction is related to a much less degree to the equilibrium distribution function of phonons as it is typical of the ordinary electron-phonon interaction in BCS. On the whole, the results obtained allow the consideration of new aspects of the problem on the relationship between the chemical bond fo~mation and the phenomenon of high-temperature superconductivity.

References I. ~.Nishida, II.~iyatake, D.Sh£mada et al. Japn.J.Appl.Phys. 26(1987)L1856. 2. G.M.Luke, R.F.Kieff, J.H.Brewer et al. Physica 0. 153-155 (1988) 759. 3. J.~.Tranquada, D.E.Cox, W.Kunnmann et al. Phys.Rev.Letters 60 (1988) 156. 4. II.Kadowaki, M.Nishi, Y.Yamada et al. Phys.Rev.B., 37 (1988) 7932. 5. }~.Sato, S.Shamoto, J.~.Tranquada et al. Phys.Rev.Letters 61 (1988) 1317. 6. J.M.Tranquada, A.H.I~Ioudden, A,I.Go]dman et al. ?hys.Rev. 38 (1988) 2477. 7. Yu.T.Pavlyukhin, N.G.Hainovsky, Y.Y.I~edikov, A.I.Rykov. Pramana J.Phys. 31 (1988) L445. 8. Yu.T°Pavlyukhin, N.G°Hainovsky, A.I.Rykov. Pramana-J.Phys. 31 (1988) L433. 9. Ovsinsky S.R. et ~i. Phys.Rev.Lett. 58 (1987) 2579.

lO.S.R.Ovsinsky et al° Rev.So]id St. Sc~j

I(1987) 2o7. 11. R.N.Bhargara et al. Phys.Rev.Lett. 59 (1987) 1468. 12.R.C.Yu et al. Rev.Solid St. Sci. I (1987) 181. 13.X.N.Civillo et al. Solid St.Commun.

66 (1988) 1237. 14.Yu.A.Osipyan et al. Pisma v Zh.Eksp. Teor.Fiz. 48 (1988) 225. 15°Yu.T°Pavlyukhin et al. izv.Akad.~auk SSSR, SeroF~him.Nauk 17 (1988) 114. 16.Yu.T.Pavlyukhin et al. Ibid, p. 101. 17.Yu. ToPavlyukhin et al. Ibid, p. 107. 18.A.G°Klimenko, V°I.Kusnetsov, Ya.Ya. ~ednikov et al. Preprint IICh 88-24, Novosibirsk 1988. 19. H.Mott and E.Davis~ Electronic processes in noncrystalline substances Vol. I, Mir, 1982.