Synthesis and characterization of overdoped Hg-1234 and Hg-1245 phases; the universal behavior of Tc

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1 Ž. Physia C Synthesis and haraterization of overdoped Hg-1234 and Hg-1245 phases; the universal behavior of T variation in the HgBa 2Ca ny1cu no 2 nq2qd series K.A. Lokshin a, D.A. Pavlov a, M.L. Kovba a, E.V. Antipov a,), I.G. Kuzemskaya b, L.F. Kulikova b, V.V. Davydov b, I.V. Morozov b, E.S. Itskevih b,1 a Department of Chemistry, Mosow State UniÕersity, , Mosow, Russian Federation b Institute of High Pressure Physis RAS, , Troitsk, Mosow, Russian Federation Reeived 10 Deember 1997; aepted 14 January 1998 Abstrat HgBa2Ca3Cu4O10qd and HgBa 2Ca 4Cu5O12qd were prepared by high pressurerhigh temperature tehnique. The phase omposition in the Hg Ba Ca Cu O system was shown to be very sensitive to synthesis onditions. A variation of the oxygen ontent in the starting oxide mixture by hanging the BaOrBaO2 ratio allowed the aquisition of Hg-1234 and Hg-1245 in the overdoped state. The heat treatments Ž nitrogen and oxygen flows, oxygen pressure 100 bar and 2000 bar. resulted in a hange of T s from initial overdoped Ž 116 and 100 K for Hg-1234 and Hg-1245, respetively. to underdoped Ž 95 and 85 K. and, finally, to optimally-doped states Ž 125 and 111 K.. The dependenies of T vs. a-parameter Ž and, onsequently, oxygen ontent. for these phases exhibit a upola-shaped behavior, as it was previously found for the lower Ž nf3. members of the HgBa Ca Cu O series. q 1998 Elsevier Siene B.V. 2 ny1 n 2 nq2qd Keywords: Hg-1234; Hg-1245; High pressure synthesis; Heat treatments 1. Introdution Hg-based superonduting Cu mixed oxides family, HgBa 2Ca ny1cu no2 nq2qd with ns1 6, is a very attrative subjet in the study of the fundamental aspets of high T superondutivity in layered uprates. It is well-known that there are two main parameters influening T in this family: a width of a ) Corresponding author. Fax: q ; antipov@inorg.hem.msu.su. 1 Also orresponding author. Fax: q ; itskev@ns.hppi.troitsk.ru. perovskite slab Ž n. and a onentration Ž d. of the extra oxygen loated in the Hg layers. In the latter ase, the dependene of T vs. extra oxygen ontent Ž d. exhibits a upola-shaped behavior as it was found for Hg-1201, Hg-1212, and reently for Hg-1223 w1 3 x. The origin of this phenomenon is a hange of a arrier onentration in a onduting band from underdoped to overdoped states, followed by a T variation with a maximum orresponding to an optimal hole doping. It should be mentioned that the larger width of the perovskite slab in the struture, the more diffiult to obtain the overdoped Hg-based superondutor. For instane, a heat treatment under 100 bar oxygen pressure de r98r$19.00 q 1998 Elsevier Siene B.V. All rights reserved. Ž. PII S

2 72 ( ) K.A. Lokshin et al.rphysia C reases T for the first two members, while no signifiant hanges in T has been found for the third member w2,4,5 x. Reently, we have shown that the overdoped Hg-1223 samples with the muh smaller T s118 K than the optimal value Ž 135 K. ould be prepared under high pressure using the initial oxide mixture with a high oxygen ontent wx 3. In ontrast to these phases Ž the lower members., the overdoped state has not yet been reported for the higher memwx 6 and bers with n G 4. Earlier, Loureiro et al. Capponi et al. wx 7 showed that the T s for the under- doped phases with ns4 and 5 ould be enhaned by a heat treatment under 1300 bar oxygen pressure up to 125 and 110 K, respetively, but a derease of T due to overdoping was not observed. The seond strutural parameter influening T in this series is a width of the perovskite-like slab. The highest reported value of T for eah member in- reases from the first Ž HgBa CuO, T s97 K. 2 4qd up to the third member ŽHgBa 2Ca 2Cu 3O 8qd, T s135 K. and then dereases to 107 K for HgBa 2Ca5- Cu O w7,8 x 6 14qd. Thus, a dependene of T vs. n exhibits a upola-shaped behavior too. The reason for this phenomenon is still unknown. The probable explanation might be an insuffiient arrier onentration for the higher members of the series with ng4 or speifi strutural features of these phases. Therefore, it would be interesting to prepare Hg-1234 and Hg-1245 samples in overdoped states to aept or rejet the first assumption. Earlier, we showed that a variation of the BaO2rBaO ratio in the initial oxide mixture was an important parameter, whih influened not only the extra oxygen ontent in the phase, but a phase formation as well wx 3. For instane, the variation of x from 10.2 to 10.6 in the initial oxide mixture HgBa 2Ca 3Cu 4O x resulted in a drasti hange of the phase omposition of the as-prepared samples. A high fration of Hg-1234 was obtained when the synthesis was performed under 2 GPa at 9008C for 2 h and x was equal to 10.2, while no traes of this phase were deteted in the sample prepared under the same onditions with xs10.6. The latter sample ontained mainly overdoped Hg The formation of the HgBa Ca Cu O ompounds in the 2 ny1 n 2 nq2qd Hg Ba Ca Cu O system is influened by other parameters like temperature, pressure, and ation omposition. Therefore, a variation of these parameters might allow the synthesis of the high members in overdoped states. The goal of the present work was to obtain overdoped Hg-1234 and Hg-1245 phases and to larify the relationship between T and extra oxygen ontent variation for these ompounds. 2. Experimental The high pressurerhigh temperature synthesis used in the present study was desribed in detail in Ref. wx 3. The stoihiometri mixtures of HgO Ž 99.9%., BaO, BaO, CaO and CuO Ž 99.9%. 2 were used as the starting materials. BaO was prepared by a deomposition of BaCO Ž 99.9%. 3 in vauum at 11008C for 72 h. CaO was synthesized by a similar way from CaCO Ž 99.9%. 3 annealed at 9008C. BaO2 was obtained by heating BaO at C in purified oxygen. The initial mixtures HgBa 2Ca 3Cu 4O x and HgBa 2Ca 4Cu5O x were prepared from the starting reagents with a different oxygen onentration Ž x.. The variation of oxygen ontent was made by hanging the BaOrBaO2 ratio. All preliminary operations were arried out in a glove box filled with nitrogen, and NaOH and P2O5 were used for a purifiation from CO2 and water vapor. The syntheses were performed under high pressure Ž 2 4 GPa. in gold apsules Žvolume of a apsule was about 0.04 ml. at the temperature range C for 2 h. Subsequent heat treatments of the obtained samples were performed at 3008C in purified oxygen and nitrogen flows. Several annealings were arried out under high oxygen pressure of 100 and 2000 bar at 3008C. X-ray powder diffration study of the prepared samples was performed using a Guinier fousing monohromati amera FR-552 Ž Cu K a radiation. 1 with Ge as an internal standard. The lattie parameters of the Hg-based superondutors were determined using the same set of non-overlapping strong refletions for all samples. Frations of different phases were estimated visually using intensities of the main refletions. The temperature dependene of AC suseptibility was measured by an indution method with an external field amplitude of 1 Oe at a modulation frequeny of 27 Hz.

3 ( ) K.A. Lokshin et al.rphysia C Results and disussion 3.1. High pressure synthesis For the preparation of overdoped Hg-1234 and Hg-1245 superondutors, we deided to use two different ways: Ž. i to vary temperature and pressure for annealing the samples with the fixed initial stoihiometry HgBa Ca Cu O and Ž ii to vary oxygen ontent in the starting mixture HgBa 2Ca 4- Cu O annealed under fixed onditions Ž 5 x 2 GPa, 9508C, 2 h.. Earlier, we have obtained optimally doped Hg with T s 125 K from the initial mixture HgBa 2Ca 3Cu 4O x with xf10.4, while the samples with x s 10.6 ontained only overdoped Hg-1223 superondutor wx 3. So, it was logial to use the initial oxide mixture with an intermediate oxygen ontent: HgBa 2Ca 3Cu 4O The results of the high pressure synthesis are presented in Table 1. The phase formation in the Hg Ba Ca Cu O system was found to be very sensitive to a temperature variation. An inreasing of synthesis temperature Ž under the same pressure 2 GPa. results in a preparation of the higher members of the homologous series. For instane, Hg-1212 was obtained at 9008C Žsam- ple a1. as a main phase, while at 11008C, sample a4 ontained mainly Hg-1245 and a minority of Hg An inrease of pressure from 2 GPa to 4 GPa did not allow the preparation of Hg-1234, and, moreover, sample a3 ontained Hg-1212 as an im- Table 2 Charateristis of the samples prepared from the starting mixture HgBa Ca Cu O under 2 GPa at T s9508c for 2 h x N x Phase omposition, % Lattie parameters T, K a, A, A Hg-1234 Ž Ž Ž Hg-1245 Ž Ž Ž CaO Ž. 5, CuO Ž. 5 Hg-1245 Ž Ž Ž Hg-1234 Ž Ž Ž CuO Ž. 5, CaO Ž. 5 Hg-1245 Ž Ž Ž Hg-1234 Ž Ž Ž CuO Ž. 5, CaO Ž. 5 purity phase instead of Hg-1234 in sample a2. The ell onstants of the Hg-based superondutors and their T s are in agreement with those reported earlier w6,7 x. Thus, we ould not obtain the overdoped samples of Hg-1234 or Hg-1245 phases using starting omposition HgBa 2Ca 3Cu 4O The seond way, whih was based on a variation of an oxygen ontent, was more suessful. Earlier, we have prepared overdoped Hg-1223 from the mixture with a starting omposition HgBa Ca Cu O x where x was equal to Now we deided to use the starting mixture HgBa Ca Cu O with x x lying in the range The results of these experiments are summarized in Table 2. The fration of Hg-1245 inreases with the inrease of x. Samples a5 and a7 ontain predominantly Hg-1234 or Table 1 Charateristis of the samples prepared from the starting mixture HgBa2Ca 3Cu 4O10.5 N P, GPa T,8C Phase omposition, % Lattie parameters T, K a, A, A Hg-1212 Ž Ž Ž Hg-1223 Ž. 5 y y 132 CuO Ž. 5, CaO Ž. 5, BaCuO Ž Hg-1223 Ž Ž Ž Hg-1234 Ž. 5 y y y CaO Ž. 5 Hg-1223 Ž Ž Ž Hg-1212 Ž 10. y y 95 CuO Ž. 5, CaO Ž 10., BaCuO Ž Hg-1245 Ž Ž Ž Hg-1234 Ž 20. y y 122 CaO Ž. 5, CuO Ž 2 5., Hg-1223 Ž 2 5.

4 74 ( ) K.A. Lokshin et al.rphysia C reported earlier for these optimally doped ompounds. Taking into onsideration the small values of their a-parameter Žand, onsequently, the in-plane Cu O bond distanes whih are very sensitive to the hole onentration variation., we suggested that these superondutors were indeed overdoped. However, the latter observation gave us an indiret support of this supposition only. Therefore, to larify this problem the different subsequent heat treatments of samples a5 and 7 were performed. Fig. 1. Temperature dependene of the AC suseptibility of as-prepared Hg-1234 Ž solid square., and after heat treatments: in nitrogen flow Ž triangle., in oxygen flow Ž irle. and under 2000 bar oxygen pressure Ž open square.. Hg-1245, respetively, and their frations in the samples are not smaller than 80%, while the amount of superonduting impurities ŽHg-1245 and Hg- 1234, respetively. is not higher than 10%. The x Ž T. measurements of these as-prepared samples Ž Figs. 1 and 2. revealed two well-defined transitions on eah urve, and the main slopes on these urves orresponded to the dominating superondutor in the sample. It should be noted that the as-prepared Hg-1234 and Hg-1245 phases exhibit a relatively low T Ž 116 and 100 K, respetively. in omparison with those 3.2. T Õariation in Hg-1234 and Hg-1245 aused by heat treatments Samples a5 and 7 were subsequently heat treated under the following onditions: Ž C, oxygen flow, 50 h; Ž C, nitrogen flow, 20 h; Ž C, oxygen flow, 50 h; Ž C, PŽ O. 2 s 100 bar, 30 h; Ž C, PŽ O. 2 s2000 bar, 10 h. The lattie onstants of the Hg-1234 and Hg-1245 phases and their T s are presented in Table 3, and the x Ž T. urves are shown in Figs. 1 and 2. Both ompounds exhibit a similar behavior ŽT and a- parameter variations. aused by heat treatments. The first annealing in an oxygen flow did not hange the harateristis of the ompounds in omparison with those of the as-prepared samples. However, a redution in nitrogen flow led to a notieable inrease of the a-parameters and a hange of T s. It should be mentioned that the presene of a small amount of Table 3 Lattie onstants and T of Hg-1234 Ž first line. and Hg-1245 Ž seond line. after subsequent heat treatments onset Treatment T,K a, A, A a As-prepared Ž Ž Ž Ž. 5 Nitrogen flow, 3008C, 20 h Ž Ž Ž Ž. 4 b Oxygen flow, 3008C, 50 h Ž Ž Ž Ž. 6 PŽ O. s2000 bar, 3008C, 10 h Ž Ž Ž Ž. 4 Fig. 2. Temperature dependene of the AC suseptibility of as-prepared Hg-1245 Ž solid square., and after heat treatments: in nitrogen flow Ž triangle., in oxygen flow Ž irle., and under 2000 bar oxygen pressure Ž open square.. a The heat treatment in oxygen flow at 3008C for 50 h did not hange T s and lattie parameters. b The subsequent heat treatment under PŽ O. 2 s100 bar at 3008C for 30 h did not hange neither T s nor lattie parameters of the phases.

5 ( ) K.A. Lokshin et al.rphysia C impurities in the studied samples did not allow the performane of a hemial analysis, but a variation of the a-parameter gives us an indiation of an alteration of the extra oxygen ontent in the Hg-based superondutor. The subsequent oxidation in an oxygen flow resulted in both a-parameter dereasing and T s inreasing Ž up to 116 and 106 K for Hg-1234 and Hg-1245, respetively.. In ontrast to the lower members of the series Ž nf3., annealing in an oxygen flow does not allow the aquisition of optimal T s for Hg-1234 and Hg Moreover, annealing under 100 bar oxygen pressure did not hange lattie onstants and T s. Therefore, a heat treatment under the higher PŽ O. was neessary to ahieve an opti- 2 mal hole onentration, like what was done in Refs. w6,7 x. Indeed, the oxidation under 2000 bar oxygen pressure resulted in a signifiant inrease of T s for both phases Ž 125 and 110 K, respetively., while the a-parameters dereased. These values are lose to those reported earlier for these optimally-doped omw6,7 x. It should be mentioned that these treat- pounds ments resulted in hanging T of impurity phases also, and moreover, their values agree very well with those desribed before for the orresponding ompounds. 4. Conlusion Summarizing these experiments, we an onlude that the heat treatments hanged a arrier onentration in the same Hg-1234 and Hg-1245 samples from the overdoped state for the initial as-prepared ompounds to the underdoped phases Žafter treatments in nitrogen and then in oxygen flows. and, finally, to the optimally-doped phases Žafter a treatment under 2000 bar oxygen pressure.. In Fig. 3 we plotted the dependenies of T vs. the a-parameter for the HgBa 2Ca ny1cu no2 nq2qd superondutors with n s2 5. We would like to note that the only experimental data for the same as-prepared and heat-treated Hg-1234 and Hg-1245 samples were used. It an be learly seen that all these dependenies demonstrate a upola-shaped behavior, as well as one for the Hg-1201 phase w1,2 x. Therefore, we an onlude that all HgBaCa ny1cu no2 nq2qd superondutors have the same type of T variation aused by a hange of the extra oxygen ontent Ž d., beause of Fig. 3. The dependenies of T s vs. a-parameter for the Hg-1212, Hg-1223 Ždata taken from Refs. w1,3 x., Hg-1234 and Hg-1245 Ž present work.. The dotted lines are guides to the eye. the similar struture arrangements of these phases. However, to ahieve the optimal hole onentration in the higher members, a treatment under high PŽ O 2. is neessary. From a hemial point of view, this fat looks quite surprising beause it means that the stabilization of the similar opper valene in the losely related ompounds require different oxygen pressures. We think that the origin of this phenomenon has a mainly strutural reason. An interalation of an extra oxygen in the Hg-layers auses a stati displaement of the surrounding ations Ž4 Hg ations and 2 Ba ones. toward the inserted anion due to a hemial interation between them. However, an insertion of the larger amount of an extra oxygen in the struture of the higher members Žwhih is neessary to ahieve the same hole onentration as in the lower members. ould be less energetially profitable. It requires the higher PŽ O. 2, beause some of this extra oxygen should be plaed in the mesh formed by the 4 Hg ations already shifted towards the oxygen in another ell. The higher members have the smaller a-parameters ompared with those for the lower members. The suessive interalation of additional Ž CaCuO 2. slabs into the strutures of Hg-based superondutors is aompanied by ompressing the in-plane parameters for the next members of this series, thus providing the stronger in-plane interations. This an be a reason why the T variation for the lower members

6 76 ( ) K.A. Lokshin et al.rphysia C is aompanied by the larger variation of the a- parameter, while the T Ž a-parameter. dependenies beome narrower for the higher members. Moreover, the a-parameters have pratially the same values for the higher members Žtreated under the same onditions. in ontrast to the first three members where the differene in their values is signifiant. It an be learly seen from Fig. 3 that the Hg-1223, Hg-1234 and Hg-1245 phases with the same a- parameter values have signifiantly different T s, whih derease from the first to the final phase. On the left side of eah dotted urve, the superondutors are in an overdoped state Žwith the higher than optimal arrier onentration.. Therefore, we should rejet the simple idea of dereasing T due to under- doping. We an suggest that the origin of this phenomenon indeed has strutural reasons, whih an be found by a further preise strutural study of these materials. Aknowledgements The authors would like to thank Dr. P.E. Kazin for the magneti measurements and Dr. N.A. Nikolaev for the tehnial assistane. The researh is supported by the Russian Sientifi Program on Superondutivity Ž Projets Poisk and and INTAS Ž ext.. K.A.L. and D.A.V. are grateful to the ISSEP for the finanial support Žgrants a861 and s Referenes wx 1 E.V. Antipov, S.N. Putilin, E.M. Kopnin, J.J. Capponi, C. Chaillout, S.M. Loureiro, M. Marezio, A. Santoro, Physia C Ž wx 2 A. Fukuoka, A. Tokiwa-Yamamoto, M. Itoh, R. Usami, S. Adahi, K. Tanabe, Phys. Rev. B 55 Ž wx 3 K.A. Lokshin, I.G. Kuzemskaya, L.F. Kulikova, E.V. Antipov, E.S. Itskevih, Physia C 279 Ž wx 4 B. Morosin, E.L. Venturini, J.E. Shirber, P.P. Newomer, Physia C 226 Ž wx 5 S. Adahi, A. Tokiwa-Yamamoto, M. Itoh, K. Isawa, H. Yamauhi, Physia C 214 Ž wx 6 S.M. Loureiro, E.V. Antipov, E.M. Kopnin, M. Brunner, J.J. Capponi, M. Marezio, Physia C 257 Ž wx 7 J.J. Capponi, E.M. Kopnin, S.M. Loureiro, E.V. Antipov, E. Gautier, C. Chaillout, B. Souletie, M. Brunner, J.L. Tholene, M. Marezio, Physia C 256 Ž wx 8 B.A. Sott, E.Y. Suard, C.C. Tsuei, D.B. Mitzi, T.R. MGuire, B.-H. Chen, D. Walker, Physia C 230 Ž