interaction of yba2cu3o6.8with atmospheric moisture during low-temperature annealing

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ISSN 10637834, Physics of the Solid State, 2014, Vol. 56, No. 8, pp. 1536–1541. © Pleiades Publishing, Ltd., 2014. Original Russian Text © I.B. Bobylev, Yu.S. Ponosov, N.A. Zyuzeva, 2014, published in Fizika Tverdogo Tela, 2014, Vol. 56, No. 8, pp. 1486–1491. 1536 1 INTRODUCTION It is known that YBa 2 Cu 3 O y compounds are unsta ble with respect to water [1, 2]. The interaction of an YBa 2 Cu 3 O y compound with water at nearroom tem perature leads to the amorphization of nearboundary grain regions and to the formation of stacking faults being structural fragments of the 124type compound, which, in addition to oxygen, presumably contain hydroxyl ions [1]. It was shown in [3–6] that, at T = 200–300°C, the 123type compounds nonstoichio metric in oxygen also interact with water vapors con tained in the annealing atmosphere. In this case, the water enters into the structure of the compound and its absorption is the stronger, the smaller is the oxygen index of this compound [5]. The water absorption leads to a change in the structure of YBa2Cu3Oy, i.e., to its transition from the tetragonal or orthorhombic modification to a pseudocubic state (a = b c/3 = 3.863 Å) [5, 7, 8] or, according to [9], to the tetragonal lattice with parameters a = 3.883 Å and c = 27.17 Å. In addition, the electron microscopy and Xray diffrac tion studies [10–12] showed that YBa 2 Cu 3 O y nonsto ichiometric in oxygen decomposes into phases with different oxygen contents during annealing at T = 200–300°C in the presence of moisture. It was found in [13–17] that the longtime anneal ing of YBa 2 Cu 3 O 6.9 at T = 200°C in a humid atmo sphere deteriorates its superconducting properties up to the transition of the material to the paramagnetic state. However, subsequent shorttime annealing at T = 930°C after oxidation at T = 400°C recovers the superconductivity. Moreover, under certain condi tions, such double heat treatment leads to a significant improvement of the critical parameters of the 123 compounds as compared to the initial state, including those in strong magnetic fields [13, 14, 16]. It was assumed that planar stacking faults formed during interaction with water are partially retained after shorttime hightemperature annealing and are pin ning centers of magnetic vortexes. In this case, they initiate a correlated pinning [15], which is most effec tive when external magnetic field is applied perpendic ularly to axis c. The aim of this work was to study the effect of water absorbed by YBa 2 Cu 3 O 6.8 on the phase state, structure, and electrophysical properties of the compound. It was of interest to reveal mechanisms leading to a change in the 123 structure upon interaction with water and appearance of residual defects, which favor an improvement of the critical parameters after low temperature treatment in a humid atmosphere. To do this, we used Raman spectroscopy providing informa tion on local features of the structure of a material. 2. SAMPLES AND EXPERIMENTAL TECHNIQUE We studied a hightextured YBa 2 Cu 3 O 6.9 ceramics synthesized by melttextured growth (MTG) method [18] at the Bochvar HighTechnology Scientific Research Institute for Inorganic Materials (VNIINM). The samples contained the Y 2 BaCuO 5 phase (30 wt %). According to the data obtained from measurements of the magnetic susceptibility, the critical temperature SUPERCONDUCTIVITY Interaction of YBa 2 Cu 3 O 6.8 with Atmospheric Moisture during LowTemperature Annealing I. B. Bobylev*, Yu. S. Ponosov, and N. A. Zyuzeva Institute of Metal Physics, Ural Branch of the Russian Academy of Sciences, ul. Sofii Kovalevskoi 18, Yekaterinburg, 620990 Russia * email: [email protected] Received January 17, 2014 Abstract—The interaction of YBa 2 Cu 3 O 6.8 (123) with water vapors at T = 200°C and the water influence on the structure and electrophysical properties of the compound have been studied using Raman spectroscopy, magnetometry, and Xray diffraction. It has been found that the penetration of water into the 123 structure leads to its transition to a hydride–oxyhydroxide H 2x z YBa 2 Cu 3 O y + x z (OH) z containing fragments of the 124type structure and exhibiting the spectrum of twomagnon scattering characteristic of antiferromagnetic 123 á compositions; in this case, the superconducting properties of the material as a whole are conserved. After shorttime recovery annealing and subsequent oxidation, the water is removed from the compound structure, which leads to the disappearance of the spectrum of spin fluctuations. A possible mechanism of change in the 123 structure upon hydrogen and water intercalation has been discussed. DOI: 10.1134/S1063783414080058

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Page 1: Interaction of YBa2Cu3O6.8with atmospheric moisture during low-temperature annealing

ISSN 1063�7834, Physics of the Solid State, 2014, Vol. 56, No. 8, pp. 1536–1541. © Pleiades Publishing, Ltd., 2014.Original Russian Text © I.B. Bobylev, Yu.S. Ponosov, N.A. Zyuzeva, 2014, published in Fizika Tverdogo Tela, 2014, Vol. 56, No. 8, pp. 1486–1491.

1536

1 INTRODUCTION

It is known that YBa2Cu3Oy compounds are unsta�ble with respect to water [1, 2]. The interaction of anYBa2Cu3Oy compound with water at near�room tem�perature leads to the amorphization of near�boundarygrain regions and to the formation of stacking faultsbeing structural fragments of the 124�type compound,which, in addition to oxygen, presumably containhydroxyl ions [1]. It was shown in [3–6] that, at T =200–300°C, the 123�type compounds nonstoichio�metric in oxygen also interact with water vapors con�tained in the annealing atmosphere. In this case, thewater enters into the structure of the compound and itsabsorption is the stronger, the smaller is the oxygenindex of this compound [5]. The water absorptionleads to a change in the structure of YBa2Cu3Oy, i.e.,to its transition from the tetragonal or orthorhombicmodification to a pseudocubic state (a = b ≅ c/3 =3.863 Å) [5, 7, 8] or, according to [9], to the tetragonallattice with parameters a = 3.883 Å and c = 27.17 Å. Inaddition, the electron microscopy and X�ray diffrac�tion studies [10–12] showed that YBa2Cu3Oy nonsto�ichiometric in oxygen decomposes into phases withdifferent oxygen contents during annealing at T =200–300°C in the presence of moisture.

It was found in [13–17] that the long�time anneal�ing of YBa2Cu3O6.9 at T = 200°C in a humid atmo�sphere deteriorates its superconducting properties upto the transition of the material to the paramagneticstate. However, subsequent short�time annealing atT = 930°C after oxidation at T = 400°C recovers thesuperconductivity. Moreover, under certain condi�

tions, such double heat treatment leads to a significantimprovement of the critical parameters of the 123compounds as compared to the initial state, includingthose in strong magnetic fields [13, 14, 16]. It wasassumed that planar stacking faults formed duringinteraction with water are partially retained aftershort�time high�temperature annealing and are pin�ning centers of magnetic vortexes. In this case, theyinitiate a correlated pinning [15], which is most effec�tive when external magnetic field is applied perpendic�ularly to axis c.

The aim of this work was to study the effect of waterabsorbed by YBa2Cu3O6.8 on the phase state, structure,and electrophysical properties of the compound. Itwas of interest to reveal mechanisms leading to achange in the 123 structure upon interaction withwater and appearance of residual defects, which favoran improvement of the critical parameters after low�temperature treatment in a humid atmosphere. To dothis, we used Raman spectroscopy providing informa�tion on local features of the structure of a material.

2. SAMPLES AND EXPERIMENTAL TECHNIQUE

We studied a high�textured YBa2Cu3O6.9 ceramicssynthesized by melt�textured growth (MTG) method[18] at the Bochvar High�Technology ScientificResearch Institute for Inorganic Materials (VNIINM).The samples contained the Y2BaCuO5 phase (30 wt %).According to the data obtained from measurements ofthe magnetic susceptibility, the critical temperature

SUPERCONDUCTIVITY

Interaction of YBa2Cu3O6.8 with Atmospheric Moisture during Low�Temperature Annealing

I. B. Bobylev*, Yu. S. Ponosov, and N. A. ZyuzevaInstitute of Metal Physics, Ural Branch of the Russian Academy of Sciences,

ul. Sofii Kovalevskoi 18, Yekaterinburg, 620990 Russia * e�mail: [email protected]

Received January 17, 2014

Abstract—The interaction of YBa2Cu3O6.8 (123) with water vapors at T = 200°C and the water influence onthe structure and electrophysical properties of the compound have been studied using Raman spectroscopy,magnetometry, and X�ray diffraction. It has been found that the penetration of water into the 123 structureleads to its transition to a hydride–oxyhydroxide H2x – zYBa2Cu3Oy + x – z(OH)z containing fragments of the124�type structure and exhibiting the spectrum of two�magnon scattering characteristic of antiferromagnetic123 á compositions; in this case, the superconducting properties of the material as a whole are conserved.After short�time recovery annealing and subsequent oxidation, the water is removed from the compoundstructure, which leads to the disappearance of the spectrum of spin fluctuations. A possible mechanism ofchange in the 123 structure upon hydrogen and water intercalation has been discussed.

DOI: 10.1134/S1063783414080058

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INTERACTION OF YBa2Cu3O6.8 WITH ATMOSPHERIC MOISTURE 1537

(Tc) of the samples was 90 K. The X�ray diffractionanalysis showed that only 001�type lines are reflectedfrom plane ab, which demonstrates high texture of thematerial.

In this work, we studied the samples with the oxy�gen content y = 6.8. This content was achieved atT = 500°C for 24 h. The low�temperature treatmentwas carried out at T = 200°C for ≥100 h in both a dryatmosphere and air saturated with water vapors. Thewater content was controlled by the gravimetrymethod. The mass was measured accurate to ±0.005%.We studied the samples absorbed 0.29 wt % water(~0.1 mol H2O per mol 123) and 1.2 wt % water(~0.4 mol H2O per mol 123). The recovery annealingwas performed at T = 930°C (3 h) followed by oxida�tion at T = 400°C in an oxygen atmosphere for 24 h toreach oxygen index ~7.

The Raman spectra were excited in a quasi�inversegeometry by line 514 nm (2.41 eV) of an argon ioniclaser with a power to 0.5 mW focused into a spot on asample ~2 μm in diameter. The local heating of thesample under such conditions was negligible. Toexclude surface effects, the measurements were car�ried out on as�cleaved surfaces. Despite the existenceof inclusions of the green phase (211), the sampleswere twinned single crystals, which allowed us to per�form full�valued polarization measurements fromboth the basal plane and a plane containing axis c ofthe crystal. The scattered radiation was measured on aRenishaw RM 1000 spectrometer equipped with edgefilters to exclude a low�frequency Rayleigh scatteringand thermoelectrically cooled registration systembased on a device with charge coupling. The spectralresolution was ~3 cm–1.

Temperature dependences of the magnetic suscep�tibility were measured on a Quantum D MPMS�XL�5�type SQUID magnetometer at a frequency of 80 Hzwith an ac field amplitude of 4 Oe. The X�ray diffrac�tion studies were performed on a DRON�3M�typediffractometer in a copper radiation.

3. RESULTS AND DISCUSSION

Figure 1 depicts the Raman spectra of the samplessubjected to various heat treatments in the XX and ZZpolarizations to 800 cm–1 (X and Z are the directionsof the light polarization in the basal plane and alongaxis c, respectively). According to [19], in polarizationXX in the initial state, there are four modes from fivemodes allowed by the selection rules; the modes cor�respond to vibration of Ba (110 cm–1), Cu (147 cm–1),plane oxygen O2.3 (338 cm–1, and apical oxygen O4

(496 cm–1). In addition, at least, there are still threepeaks at 185, 233, and 585 cm–1. Their appearance isrelated to the violation of the selection rules due tononstoichiometry in oxygen leading to an activity offorbidden vibrations of the chain oxygen O1 [20]. In

polarization ZZ, along with vibrations of Ba and Cu,there are only modes corresponding to vibrations ofthe plane (436 cm–1) and apical (499 cm–1) oxygen.

The spectrum is almost unchanged after treatmentat T = 200°C in dry air even as the treatment timeincreased to 200 h (Fig. 1, curves 2). This fact showsthat the decomposition into phases with different oxy�gen contents occurs very slowly under these condi�tions, according to [21].

Similar treatment in an atmosphere saturated withwater vapors substantially changes the spectrum. Inpolarization XX, the ratio of intensities of the modesresponsible for the vibrations of Ba and Cu is changed(Fig. 1a). Such ratio of these mode intensities is char�acteristic of the tetragonal phase with y ~ 6 [20, 22,23]. In this case, the chain lines substantially shift from586 to 597 cm–1. In addition, the intensity of the line(448 cm–1) corresponding to the plane oxygen vibra�

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Fig. 1. Raman spectrum in the phonon region ofYBa2Cu3O6.8 subjected to various heat treatments inpolarizations (a) XX and (b) ZZ: (1) initial state, (2) aftertreatment at T = 200°C for 200 h in a dry air, (3) after waterintercalation to 0.29 wt % H2O, (4) after water intercala�tion to 1.2 wt % H2O, and (5) after recovery at T = 930°Cfor 3 h and oxidation at T = 400°C.

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BOBYLEV et al.

tions increases. According to the X�ray diffractiondata (Fig. 2), when water is absorbed, a phase transi�tion occurs, which is demonstrated by approach andsequent confluence of the lines 200–020–006 [5].

In polarization ZZ, with increasing water absorp�tion, the plane and apical oxygen modes significantlyshift to higher frequencies. In addition, the strong lineappears at 605 cm–1, and also new lines appears at 207and 246 cm–1. According to [24, 25], one of them canbelong to the Cu1 (246 cm–1) vibrations in Cu–Ochains, which, as well as line at 605 cm–1, is related tothe appearance of the 124�type fragments in the struc�ture [9].

The treatment in a dry atmosphere does not alsochange the spectrum in the higher�frequency region(Fig. 3); however, annealing in an atmosphere satu�rated with water vapors brings about the appearance,in polarization XX, of a wide band at ~2950 cm–1 char�acteristic of the two�magnon scattering by spin fluctu�ations in the tetragonal phase with low oxygen content(y ~ 6). Only in polarization ZZ, a weak line appears at3450 cm–1 corresponding to valence vibrations of theOH– group (Fig. 3b, curves 3 and 4). According to [3–6], this fact demonstrates the entrance of water in the123 structure. The absence of line at 3450 cm–1 inpolarization XX indicates that the O–H bond isdirected along axis c in the crystal.

It was supposed in [26] that the O–H bonds are notobserved in the inelastic neutron scattering spectrabecause of short proton lifetime near an oxygen ion,because proton is predominantly in the interstitialsites. It can be supposed that, during hydration, OH–

groups dissociate partially into proton and O2– and, asa result, the OH– group fraction is small, and proton

enters in its proper positions. This assumption is sup�ported by the fact that the hydration of 123 [8], simi�larly to the hydration [5], leads the same phase transi�tion.

Therefore, the described shift of the apical line tohigher frequencies seems to indicate the bondstrengthening [27], which can occur due to entranceof additional O2– ions formed during dissociation ofOH– groups in O1 vacancies in the Cu–O chains, andthis really increases the oxygen content in the system.

After the recovery annealing at T = 930°C andsequent oxidation, the spectra become similar to cor�responding spectra of a sample with oxygen indexy ~ 7.0 [28] (Figs. 1 and 3). However, unlike of them,a weak mode of chain oxygen (596 cm–1) is retained inpolarization XX, which can indicate that some struc�tural disorder is conserved. It should be noted that it isprecisely the doubled heat treatment, namely, low�temperature annealing and short�time recovery

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Fig. 2. X�ray diffraction patterns of YBa2Cu3O6.8: (1) ini�tial state, (2) after water intercalation to 0.29 wt % H2O,and (3) after water intercalation to 1.2 wt % H2O.

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Fig. 3. Raman spectra of YBa2Cu3O6.8 in the high�fre�quency region: (1) initial state, (2) after treatment at T =200°C for 200 h in a dry air, (3) after water intercalation to0.29 wt % H2O, (4) after water intercalation to 1.2 wt %H2O, and (5) after recovery at T = 930°C for 3 h and oxi�dation at T = 400°C.

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INTERACTION OF YBa2Cu3O6.8 WITH ATMOSPHERIC MOISTURE 1539

annealing at 930°C followed by oxidation that sub�stantially increases jc, particularly, in high magneticfields applied perpendicularly to axis c [13, 14].

We performed spectroscopic studies along with thestudy of magnetic properties. Figure 4 depicts the tem�perature dependences of the real χ' and imaginary χ''parts of the magnetic susceptibility, which show thatthe low�temperature treatment with absorption of0.29 wt % water increases Tc from 82 to 88 K and thisfact can indicate that the oxygen content in the super�conducting phase increases. This conclusion is alsoconfirmed by the shift of the maximum magnetizationreversal loss to higher temperatures and its narrowing(Fig. 4b). Similar result was obtained on the ceramicsamples too [5, 29]. The increase in Tc can be due tothe phase decomposition which leads to the formationof the oxygen�enriched matrix phase. In this case, theCu(III) concentration in the phase increases as com�pared to that in the initial state (y = 6.8). On the otherhand, the dissociation of hydroxyl�ions into protonand O2– ion can also increase the oxygen concentra�tion. The O2– ion is fitted into O1 vacancies withoutchanges in the Cu(III) concentration. An indirectproof of the dissociation of the OH– groups is that pro�ton is quasi�free upon hydration [26], although it mustbe added to oxygen ion and form OH– group, which islikely disadvantageous thermodynamically.

An increase in Tc in the case of low degree of hydro�genation (0.29 wt % H2O) agrees well with the shift ofthe mode responsible for vibrations of the apical oxy�gen to a high�frequency region. Further increase in thewater content (1.2 wt %) decreases the critical temper�ature. Despite the variations of Tc with absorbingwater, the diamagnetic response decreases successively(Fig. 4a), which can be due to deterioration of theconductivity at grain boundaries because of thehydrolysis (boundary amorphization [1]) and also adecrease in the fraction of the superconductor phase asa result of the formation of the 124�type defects andrelated stresses in the superconducting matrix. Afterthe recovery annealing (930°C + oxidation), the tran�sition to the superconducting phase becomes narrow,and Tc corresponds to the transition characteristic ofthe 123 compounds with oxygen index y = 6.9 (Fig. 4,curve 4).

Thus, the water�intercalated samples can simulta�neously exhibit the superconductivity with Tc charac�teristic of the 123 compounds with y ~ 7, and the anti�ferromagnetic short�range fluctuations, which is dem�onstrated by the appearance of the spectrum of spinfluctuations. This combination of the antiferromag�netic correlations and the superconductivity in thesame phase is observed in low�doped 123�type com�pounds [30]; however, this is not characteristic of the123 compounds with high oxygen content in theabsence of water.

It can be assumed that the interaction of the 123compound with water occurs as follows. The adsorbedwater molecules are dissociated at the crystallite sur�face [1]. The formed OH– groups penetrate into the123 structure and dissociate into proton and O2–.

Thus, the 123 compound becomes a hydride–oxyhy�droxide H2x – zYBa2Cu3Oy + x – z(OH)z.

Different process occurs during hydrogenation:

This is, whether water or air is intercalated, the sameequilibrium shifted to the side of the formation of pro�ton and O2– is stated.

O12–

�5 H2O+ + 2OH– 2H

+ 2O2–

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Fig. 4. Temperature dependences of (a) the real χ' and (2)imaginary χ'' parts of the magnetic susceptibility ofYBa2Cu3O6.8: (1) initial state, (2) after water intercalationto 0.29 wt % H2O, (3) after water intercalation to 1.2 wt %H2O, and (4) after recovery at T = 930°C for 3 h and oxi�dation at T = 400°C.

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BOBYLEV et al.

In both the cases, the 123 compound transits to anew pseudo�124 phase due to splitting the Cu–Ochains [9, 26].

In this case, spin fluctuations appear in planes,which lead to the simultaneous coexistence of theantiferromagnetism and the superconductivity in thesystem in certain range of water concentrations. Thesuperconductivity disappears after the saturation withwater and completion of the reaction [3, 5].

A significant resemblance of the structure and theelectrophysical properties of materials subjected tohydration and hydrogenation noted in [9] allows us topropose that, independent on the method of interca�lating hydrogen�containing compound, the proceed�ing topotactic process is the same. Unlike [1], in thecase of the hydrogenation, the process is a splitting ofthe Cu–O chains, according to reaction,

where proton enters instead of vacant each the secondpositions of copper. The difference between the hydra�tion and the hydrogenation is as follows. In the firstcase, the Cu–O chains form with a significant oxygendeficit, while it is substantially smaller upon waterintercalation, since oxygen enters in the system alongwith water. It is like, because of this, the hydrated 123compound is capable of absorbing oxygen as is shownin [26]. Thus, hydrogen introduced to the 123 struc�ture in one way or another does not occupy interstitialsites and forms positions “for itself” by splitting of theCu–O chains, which leads to the transition of the 123to pseudo�124 structure In this case, proton plays a roleof Cu+ and, likely, is two�coordinated, as well as univa�lent copper in YBa2Cu3Oy with low oxygen content.

The recovery annealing removes water, and the 123compound transits from the hydride–oxyhydroxide tothe oxide form. During annealing at T = 400°C, theceramics is saturated with oxygen and the oxygen isordered. If the annealing at T = 930°C is short�term,the formed structural defects are partially retained, butwater is absent in the compound structure, and, as aresult, antiferromagnetic results disappear. Theretained structural defects complement the centers ofmagnetic vortex pinning existing primordially in theinitial material, thus improving the current�carryingability in high magnetic field due to the sinergeticeffect [31].

4. CONCLUSIONS

(1) The implantation of OH– ions into the 123structure is confirmed by Raman spectroscopy; it isfound that the O–H bond is directed parallel to axis cof the crystal. During heat treatment at T = 200°C in ahumid atmosphere, water entering into theYBa2Cu3O6.8 structure causes its transition tohydride–oxyhydroxide H2x – zYBa2Cu3Oy + x – z(OH)z.Upon water intercalation, the OH– groups partially

CuO H2O HCuO OH( ),+

dissociate into protons and O2–, which increases thelength of antiferromagnetic fluctuations in CuO2

planes and leads to the appearance of the spectrum oftwo�magnon scattering with conserving the supercon�ducting properties of the material as a whole.

(2) After short�term recovery annealing and subse�quent oxidation, water is removed from the compoundstructure, and the Raman two�magnon spectrum dis�appears. In this case, structural defects are partiallyconserved in the 123 structure, which is manifested insome peculiarities in the Raman spectra. This state ischaracterized by substantial improvement of electro�physical properties of the 123�type materials, particu�larly in field applied perpendicularly to axis c.

ACKNOWLEDGMENTS

We are grateful to A.V. Korolev, V.A. Sazonova, andM.V. Chuntonova, collaborators from the CollectiveUsage Center at the Institute of Metal Physics, UralBranch of the Russian Academy of Sciences (Yekater�inburg, Russia) for performing the magnetometric andX�ray diffraction measurements of the studied sam�ples.

This study was supported by RNF (ONG) (projectno. 14�12�00603).

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Translated by Yu. Ryzhkov