Re-induced Raman active modes in HgBa 2 Ca n 1 Cu n O 2n 2 compounds

Transcription

1 PHYSICAL REVIEW B VOLUME 60, NUMBER 5 1 AUGUST 1999-I Re-induced Raman active modes in HgBa 2 Ca n 1 Cu n O 2n 2 compounds N. Poulakis, D. Lampakis, and E. Liarokapis Department of Physics, National Technical University, Athens , Greece Akira Yoshikawa Institute for Materials Research, Tohoku University, Sendai , Japan Jun-ich Shimoyama and Kohji Kishio Department of Superconductivity, University of Tokyo, Bunkyo-ku, Tokyo , Japan G. B. Peacock, J. P. Hodges, I. Gameson, and P. P. Edwards School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom C. Panagopoulos IRC in Superconductivity, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, United Kingdom Received 25 August 1998; revised manuscript received 12 February 1999 A comparative Raman study of Re-free and Re-doped HgBa 2 Ca n 1 Cu n O 2n 2 with n 1,3,4,5 is presented in an attempt to further clarify the structural and phononic modifications brought about by Re substitution. A number of extra high-frequency phonon peaks show up in the spectra of the Re-doped samples and are attributed to the oxygen modes of a strongly bound, almost decoupled ReO 6 octahedron. As regards the apex oxygen in the Hg site, a clear transfer of spectral weight from the 590 to the 570 cm 1 apex phonon band is observed upon Re substitution. Such a change may well be accounted for assuming increased excess oxygen content for the Re-doped samples. Another interesting result is the enhancement upon Re doping of a narrow peak probably attributed to c-axis vibrations of Ba whose frequency shows a distinctive change with the number n of the CuO 2 layers, providing an easy way to identify the various phases in a sample. S INTRODUCTION There is mounting evidence that members of the Hgbased superconducting family HgBa 2 Ca n 1 Cu n O 2n 2 with n 1 5 are of oxygen-defect structure with the excess oxygen located in the HgO layers. The excess oxygen content in the as-prepared HgBa 2 Ca n 1 Cu n O 2n 2 samples has been found to vary considerably with the number n of CuO 2 layers ranging from almost zero for Hg-1201 (n 1) to as high as 0.4 for Hg-1223 (n 3). 1 3 Further annealing in O 2 or N 2 has been shown to change in a more or less controllable way. 1,3 Interestingly, a striking feature of all members of the Hg series is the drastic change of T c with just as in the case of the Y-Ba-Cu-O superconductor. But contrary to the latter case, an extended range from underdoped to overdoped regime is easily achieved for the Hg compounds and states with well above the optimum level have been prepared which show a considerable reduction of T c. 1,3 5 Partial Re substitution in the HgBa 2 Ca n 1 Cu n O 2n 2 parent compounds with n 3 5, has been proved to considerably stabilize their chemical structure, as well as to improve their superconducting properties like flux pinning and irreversibility field strength, making them suitable for practical applications Neutron-diffraction investigations of Re-substituted HgBa 2 Ca n 1 Cu n O 2n 2 with n 3,4 have shown that Re occupies the Hg site and is fully surrounded by oxygen atoms in octahedral coordination. 9,11 The high oxidized state of Re and its smaller ionic radius compared to Hg result in a shortening of the intermediary layers, an enhancement of the interlayer coupling strength and flux pinning. 9 Raman scattering has been employed in the study of only pure, Re-free HgBa 2 Ca n 1 Cu n O 2n 2 samples with n 1 Refs or more CuO 2 planes intending mainly to identify the intrinsic phonon spectrum of this family of superconductors. This effort is hindered partly from the existence of impurities, HgO being the most commonly encountered, and partly from the multiphase nature of these compounds. A further obstacle in the Raman study of Hg s comes from their inherent oxygen-deficient structure in the HgO layer which induces a strong disorder causing the appearance in the Raman spectra of normally not allowed phonon bands. 20,21 Agreement in the assignment of the various phonon peaks seems to have been established by now as regards the strongest peaks at 592 and 570 cm 1 which are attributed to vibrations of the apex oxygen along the c axis, the former being dominant for n 1, while the latter increasing in relative intensity for n 2. 14,20 Two other broad phonon bands at 470 and 540 cm 1 are questionably attributed to the disorder-induced vibrations of the excess oxygen atoms along and perpendicular to the HgO planes. 20 Moreover, beyond any doubt is the assignment of the 216 cm 1 peak to the B 1g mode of the plane oxygen atoms of the n 4 member. However, quite unexpectedly, this mode seems to be so weak that it does not appear in the spectra of /99/60 5 / /$15.00 PRB The American Physical Society

2 PRB 60 Re-INDUCED RAMAN ACTIVE MODES IN the other members with n 2, 3, and 5, though it is not forbidden by symmetry. 21 Only by using the nm excitation wavelength could be observed in the n 3 member of the Hg family. 22 In this work we report a comparative Raman study of Re-free and Re-doped HgBa 2 Ca n 1 Cu n O 2n 2 with n 1, 3, 4, and 5 in an attempt to further clarify the structural and phononic modifications brought about by Re substitution. Extra phonon peaks appear in the high-frequency region of the spectra of the Re-doped samples, which are independent of the number of planes and are attributed to internal vibrations of the ReO 6 octahedron. Upon Re substitution, a clear redistribution of spectral weight is observed between the 590 and 570 cm 1 apex phonon bands. This is attributed to an excess of the oxygen content for the Re-doped samples. Another interesting result, brought about by Re doping, is the gain in intensity of a narrow peak at 100 cm 1, tentatively attributed to c-axis vibrations of Ba, whose frequency shows a distinctive change with the number n of the CuO 2 layers, so providing a simple way to recognize the various phases and probe a sample for a multiplicity of phases. EXPERIMENTAL Powder samples with nominal compositions HgBa 2 CuO 4,Hg 0.8 Re 0.2 Ba 2 CuO 4, HgBa 2 Ca 2 Cu 3 O 8, Hg 0.9 Re 0.1 Ba 2 Ca 2 Cu 3 O 8, Hg 0.8 Re 0.2 Ba 2 Ca 2 Cu 3 O 8, Hg 0.75 Re 0.25 Ba 2 Ca 2 Cu 3 O 8, Hg 0.75 Re 0.25 Ba 2 Ca 3 Cu 4 O 10, and Hg 0.75 Re 0.25 Ba 2 Ca 4 Cu 5 O 12 were examined in this work. In the following subsections, detailed information about preparation conditions, structural, and superconductive properties are quoted for each sample. For the Re-free HgBa 2 CuO 4 sample, a mixture of oxides HgO, BaO, and CuO in the stoichiometric quantity were pelletized and encapsulated in an air-sealed quartz tube. This was heated at 850 C for 30 min, followed by quenching to room temperature. X-ray diffraction showed no impurity peaks, and gave cell constants of a 3.879(1) Å and c 9.506(2) Å. The T c of this sample sets at 99 K, but considering the transition width it is safe to say that T c is 95 K. The preparation of the Re-doped Hg 0.8 Re 0.2 Ba 2 CuO 4 sample took place in a manner similar to the above, but with the addition of a stoichiometric amount of Re 2 O 7, and therefore carried out in a glove box since Re 2 O 7 is air-sensitive. Firing has been carried out at 800 C for 8 h. X-ray diffraction showed similarly phase purity to 99%. The T c was just as the Re-free sample. The HgBa 2 Ca 2 Cu 3 O 8 sample was prepared by a simple single-step technique and had T c K. Details can be found elsewhere. 23 For the preparation of Hg 0.8 Re 0.2 Ba 2 Ca 2 Cu 3 O 8, the HgO, Re 2 O 7, BaO, CaO, and CuO oxides were mixed in stoi- chiometric amounts in a glove box. This was then pelletized, and sealed in a space-filling double-skinned reaction vessel. 24 This was heated at 850 C for 8 h and quenched to room temperature. Note that in all cases, sealed quartz tubes were contained within steel bombs to contain the results of any explosions. Characterization by x-ray diffraction showed the phase purity of the sample to be over 90%, better than the Re-free sample. The T c of the asprepared sample was 134 K. The oxygen annealing of the sample did not affect the T c. For the crystal growth of (Hg 0.9 Re 0.1 )-1223, (Hg 0.75 Re 0.25 )-1223, (Hg 0.75 Re 0.25 )-1234, and (Hg 0.75 Re 0.25 )-1245, a partial melt-solidifying technique was attempted alternatively. Precursors were prepared by conventional solid-state reactions. High-purity powders 99.9% of BaO 2, CaCO 3, CuO, and RO 3 were first mixed in an agate mortar, calcined at 900 C for 24 h, thoroughly ground and recalcited at around 900 C for 12 h. The obtained precursors were mixed with HgO and pressed into disk-shaped pellets. These pellets were sealed in an evacuated quartz ampoule together with (Hg,R)1223 pellet and Ti metal as a reducing agent and covered by a hastelloy foil. Melt solidification was performed as follows. First, the ampoule was rapidly heated up to 960 C, which was higher than the partial melting temperature of precursor materials. After keeping at 960 C for 20 min the ampoule was cooled down to 920 C at a rate of 400 K/h and held for 18 h, and then quenched to room temperature. (Hg,Re)Ba 2 Ca n 1 Cu n O y samples (n 3,4,5) containing large crystals with a typical size of approximately m 3 were successfully obtained by adjusting the amount of Ti metal. The lattice parameter of (Hg,Re)Ba 2 Ca n 1 Cu n O y (n 3,4,5) samples were a (1) Å, and c (2) Å for n 3, a (3) Å and c (1) Å for n 4, and a (5) Å and c (3) Å for n 5 tetragonal P4/mmm space group. Powder x-ray diffractometer RIGAKU was used to analyze those lattice constants. The x-ray source was Cu K, the access voltage was 35 kv, and access current was 25 ma. The c axes of HgRe1223, 1234, and 1245 were about 0.1 Å shorter than that of the Re-free samples. The polarized Raman spectra were obtained at room temperature by means of a Jobin-Yvon T64000 triple spectrometer equipped with a CCD camera. The samples were investigated in the backscattering geometry under a microscope (objective 100) and several individual microcrystals were examined for statistical reasons and for probing of the impurities or any inhomogeneities. In order to avoid overheating the sample, attention was paid to the laser power and nm not to exceed mw/ m 2. Total acquisition time varied from about 1 h for parallel polarizations up to 4 5 h for crossed polarizations. The acquired spectra were further analyzed by fitting with simple or multiple Lorenzians. In some cases, microcrystals of impurity phases were detected, such as traces of HgO, which is frequently reported to cover the surface of the Hg crystallites. In all cases, however, the concentration of the impurities did not exceed 10% and the spectra were obtained from the crystallites without any trace of HgO. The phase of HgO is easily identified because it has characteristic phonon modes at 330 and 580 cm 1. RESULTS AND DISCUSSION Hg-1201 Figure 1 shows the zz spectra of pure and Re-doped Hg The spectrum of pure Hg-1201 Fig. 1 a is similar to those previously reported, 12 15,20,21 with one peak at 163 cm 1 and a group of high-frequency peaks out of which the

3 3246 N. POULAKIS et al. PRB 60 FIG. 1. Raman spectra of a pure and b Re-doped Hg-1201 in the zz scattering geometry. 593 cm 1 one dominates. These peaks correspond to vibrations along the z axis of the Ba and the apex oxygen, respectively, since these two atoms are the only ones, which have a site symmetry that can produce Raman-active phonon modes as predicted from group theoretical analysis for Hg Both modes exhibit almost exclusively zz Raman polarizability see Fig. 2 a, as is the case for the majority of the phonon modes in high-temperature superconductors HTSC s due to their high c-axis anisotropy. A weaker peak at 572 cm 1 is also attributed to the apex oxygen. This mode comes from apex ions which are bonded to highly oxygen-coordinated Hg. It cannot be due to traces of the HgO, since the other strong Raman-active mode of this oxide at 330 cm 1 is either completely absent or it can be barely seen in our spectra. On the other hand, its 593 cm 1 counterpart has been conjectured to be associated with Hg ions having negligible oxygen coordination in the HgO plane, i.e., a sticklike arrangement of Hg and O apex along the c axis. Such a conjecture is based on laser annealing experiments 14,20,21 and is in accordance with crystallographic results that show lengthening of the Hg-O apex bond distance upon increase of the excess oxygen content in the HgO planes. 3 In this context, the high intensity of the 593 cm 1 peak in Fig. 1 a implies that the Hg-1201 phase is highly oxygen deficient, in agreement with neutron-diffraction experiments. 1 Another weak and broader peak at 540 cm 1 has been tentatively attributed by Zhou et al. 20,21 to excess oxygen modes in the HgO planes. This peak is induced in the Raman spectra because of the disorder even though it is normally Raman forbidden. However, this assignment is not consistent with our results of the higher members as discussed below. Contrary to the simplicity of the pure Hg-1201 spectrum, a lot of changes arise upon Re substitution Fig. 1 b. A number of intense and relatively narrow new peaks show up in the high-frequency region. An interchange in relative intensities takes place between the 590 and 570 cm 1 peaks corresponding to the 593 and 572 cm 1 peaks of the pure Hg-1201 sample, respectively, and a considerable enhancement of the Ba phonon intensity appears at 163 cm 1.It should be noted that all the above-mentioned spectral features depicted in Fig. 1 b are common to all measured microcrystals of Re-doped samples. The high-frequency spectrum comprises peaks at about 636, 720, 777, 791, 803, and 855 cm 1. All these new peaks above 600 cm 1 are intrinsic of the Re-doped Hg-1201 and do not belong to impurity phases as they appear in every spectrum of a Re-doped sample but not in any of the Re-free ones. They show the same relative intensities compared to the rest of the peaks which are unambiguously attributed to Hg Especially the 636 cm 1 peak cannot be due to the BaCuO 2 as it is much narrower in width than the corresponding peak 25 at 633 cm 1 and it has a very strong zz polarization. Besides, to the best of our knowledge, none of the parent materials including Re 2 O 3, 26 ReO 2, 27 and ReO 3, which was measured in the frame of this work or the known impurity phases which may be obtained during the preparation of the Hg compounds gives spectrum similar to that of Fig. 1 b. The possibility that the high-frequency spectrum is actually due to Hg-Re disorder in the doped system can be excluded, because in the case of disorder-induced spectrum we should have very broad bands which of course comes to contradiction with the striking narrow peaks in our Raman spectra. Finally, any assumption that these Re-induced peaks originate from two-phonon scattering should be also rejected, because in this case the corresponding one-phonon peaks should be observed in the region cm 1, which of course is not our case the spectral region between 200 and 450 cm 1 is completely structureless. Besides, these new Raman peaks appearing to the Re-doped samples are quite intense to be attributed to two-phonon scattering which is usually much weaker than the first-order scattering. In the Re-substituted sample, both the 572 and 593 cm 1 apex phonon bands of the pure Hg-1201 sample Fig. 1 a shift slightly to 570 and 590 cm 1, respectively Fig. 1 b. Besides, no clear trace of the 540 cm 1 peak of the pure Hg compound was detected in the Re-doped sample. More interesting however is the characteristic spectral gain of the 570 cm 1 apex band, which takes place clearly at the expense of the 590 cm 1 peak upon Re doping. The practically absent mode at 330 cm 1 characteristic of the HgO phase excludes any connection of the strong peak at 570 cm 1 with this oxide. This may be an indication of increased excess oxygen content in the Re sample according to the above identification of the apex phonon modes. Such an increase of the excess oxygen is motivated by the smaller ionic radius of Re which, substituting Hg ions and forming strongly bound ReO 6 octahedra, induces pressure relief effects in the HgO planes and, in this way, leaves more room in these planes for occupation by extra oxygen. Figure 2 depicts the spectra of pure and Re-doped Hg samples with in-plane (xx/ yy) polarizations. Traces of the 330 cm 1 mode probably due to a very small amount of

4 PRB 60 Re-INDUCED RAMAN ACTIVE MODES IN FIG. 2. Raman spectra of a pure and b Re-doped Hg-1201 in the xx scattering geometry. HgO can be seen in this particular spectrum. The broad band at 580 cm 1 of pure Hg-1201 is probably also related with the HgO phase and provides an estimate of the very small contribution if any of this phase to the 570 cm 1 mode. From the aforementioned high-frequency peaks only these at 803 and 855 cm 1 keep a considerable Raman intensity, while all others seem to have only zz polarizations. One should notice that this is particularly true with the strong mode at 636 cm 1 which is completely suppressed in the xx spectra following an analogous behavior to that of the cm 1 apex phonon zone. In the following, the present results will be compared with those of the higher members of the Hg family. Hg-1223 Figure 3 depicts typical spectra of pure and Re-substituted Hg-1223 with Re concentrations of 10, 20, and 25 %. Again, the set of high-frequency peaks appears only in the Resubstituted samples, while the spectrum of the pure Hg-1223 remains completely featureless above the apex phonon band. Note that all these peaks, with the exception of the peaks at 747 and 510 cm 1, are clearly modified and evolve upon increasing Re doping. We believe this is the strongest argument in support of the hypothesis that the high-frequency peaks are intrinsic of the Re-doped samples. We also note the great similarity of these spectra to that of Hg-1201 Fig. 1 b, with the exception of the relatively weak peak at 720 cm 1 of Hg This mode in Hg-1223 appears either to be shifted to 747 cm 1 and considerably enhanced in intensity or it is present only in the highest Re concentration as FIG. 3. Raman spectra of a pure and b d Re-doped Hg in the zz scattering geometry. a small shoulder to the dominant 747 cm 1. In the other members of the family the mode at 720 cm 1 does not appear see below and therefore its origin remains unclear. Besides, a weaker peak at 510 cm 1 appears in Figs. 2 b 2 d which is evidently Re induced since it is absent from the spectrum of the Re-free sample Fig. 2 a. In Fig. 3 we clearly see the different behavior of the 747 cm 1 peak, which is almost independent of the Re content, contrary to the intensity of all the other high-frequency peaks which exhibit a clear evolution as Re concentration increases. Concerning the apex phonon band, comparison between Figs. 3 a and 1 a shows a clear enhancement of the 570 cm 1 component relative to its 590 cm 1 counterpart as we pass from Hg-1201 to Hg1223, in accordance with previously reported works. 20,21 Such behavior has been interpreted on the basis of the tendency of the higher members of the Hg series to hold higher excess oxygen content see Introduction. Moreover, as in the case of Hg-1201 Fig. 1, an interchange in intensity between the 570 and 590 cm 1 components of the apex band takes place in the spectra of Hg-1223 upon increasing Re substitution. Besides, from the spectra of Fig. 3 the evolution of this interchange with Re content becomes clear confirming the above conclusion about the role of Re ions in increasing the oxygen content of the Hg superconducting compounds. Figure 4 exhibits the xx/yy spectra of the pure and Redoped Hg-1223 samples. In accordance with the corresponding spectra of Hg-1201 Fig. 2, the apex modes as well as

5 3248 N. POULAKIS et al. PRB 60 FIG. 4. Raman spectra of a pure and b d Re-doped Hg in the xx scattering geometry. the strong mode at 640 cm 1 have been completely suppressed. On the contrary, the high-frequency peaks at 510, 747, 803, and 858 cm 1 have considerable in-plane components, and the latter two having also been observed in the xx spectra of Hg-1201 with consistent relative intensities. The peak at 747 cm 1 clearly dominates the xx spectra of the Re-doped Hg-1223 while, as in the corresponding zz spectra, shows no dependence on Re concentration. Quite interestingly, this mode has been shown to exhibit more or less the same Raman intensity in both out-of-plane zz and in plane (xx/yy) polarizations. So, this 747 cm 1 peak seems to correspond to a roughly isotropic mode, in the sense that its Raman tensor consists of three almost equal components ( xx yy zz ). Such a situation is rather unusual in HTSC s in which the strong anisotropy along c axis and, not rarely, between a and b axes has as a consequence very different values between xx yy and zz, like, e.g., in the aforementioned case of the apex phonon peaks. For a phonon peak to appear with equal intensities in all polarizations, it should correspond either to the triply degenerate vibrations T g symmetry along x, y, and z of an isotropically surrounded ion or, to the completely symmetric breathing mode (A g symmetry of an ionic complex of the form AO 6 with octahedral or almost octahedral symmetry like the ReO 6 complex conjectured for the Re-doped Hg-HTSC s. The former case may surely be excluded because a T g mode has a nondiagonal Raman tensor which, of course, is not the case. On the other hand, the tightly bound ReO 6 octahedra, as has been implied from the neutron-scattering measurements, with their shortened Re-O bond distances and the reduced c-axis anisotropy, are very likely to behave like pseudomolecular complexes acting as pinning centers and giving rise to such a decoupled breathing isotropic mode of the six oxygen atoms. The strong Re-O bonds may account for the high frequency of this peak, while the shifting of the oxygen ions towards Re and the subsequent decoupling of the ReO 6 group from the surrounding lattice may very well account for the low width 7 cm 1 of this mode. Moreover, the small tetragonal distortion of the ReO 6 octahedra due to their apical elongation Re-O apex Å and Re-O in-plane 1.80 Å) 9 could be responsible for the small difference observed between the zz and the in-plane (xx/yy) Raman intensities ( xx yy zz ). As concerns the other high-frequency peaks, these at 803 and 858 cm 1 are seen with xx/yy polarizations, while the peaks at 640, 775, and 794 cm 1 have almost exclusively zz components like the apex band. Based on this latter correlation, it is reasonable to consider the zz peaks as being related to c-axis vibrations of the oxygen ions of the ReO 6 complex. The 640 cm 1 peak may be attributed to c-axis vibrations of the apex oxygen due to the proximity and similar behavior of this mode to the cm 1 band of the apex oxygen in the vicinity of Hg. The higher by 50 cm 1 frequency of the apex phonon of the ReO 6 octahedra, as compared to the apex phonon in the Hg vicinity, could be interpreted by the strongest Re-O apex bond. The 640 cm 1 mode owes its zz intensity to the same reasons as the cm 1 apex band, i.e., to the near resonant conditions for the interaction of the laser ev with the excitation between the Cu(3d z 2) and O apex (2p z ) bands and the strong deformation potential produced in these bands by this mode because of its stretching character the apex oxygen ion vibrates along its bond with the plane Cu. The higher zz intensity of the cm 1 band as compared to the 640 cm 1 peak in the Re-doped samples could not be interpreted by simple stoichiometric arguments. While the concentration ratio of Hg to Re in the samples of Fig. 5 left is 3:1, the ratio between the integrated intensities a measure of the abundance of the various ions of the cm 1 to the 640 cm 1 band is apparently much higher. This may be attributed to an extra polarization that the apex phonon gains due to its bond with the Hg ion. In a way completely analogous to that described above for the case of the Cu plane -O apex bond, the apex phonon may partly gain its intensity from the deformation of its bond with the Hg(d z 2) orbital provided the d z 2 and d x 2 y2 orbitals of Hg lie around the Fermi level. Such an explanation for the higher apex phonon intensity in the Hg domains agrees well with evidence, from other experiments, for metallization of the HgO layers. 9 On the other hand, the other Re-induced peaks with the pure zz polarizations, namely that at 775 and 794 cm 1, may be related to the in-phase and out-of-phase vibrations along the c axis of the four oxygen ions of the ReO 6 octahedron lying on the HgO plane. The high frequency of this phonon mode is likely to be related to the quite short Re-O in-plane bond 1.80 Å, as quoted above. Such a phonon mode is more likely to gain its Raman polarizability because of the bonding of the plane oxygens to the Ba ions, which are

6 PRB 60 Re-INDUCED RAMAN ACTIVE MODES IN FIG. 5. Comparison of the Raman spectra in the zz left and xx right scattering configurations of Re-doped Hg compounds with a n 3, b n 4, and c n 5. lying below them. The main excitation which is expected to be modulated in the course of the c-axis vibrations of the inplane oxygen ions is between the O(2p z ) and Ba(6s) orbitals provided the former orbital lies near the Fermi level the valence of the Ba ion is 2 with its Ba(6s) orbital lying above the Fermi level. Note that the vibrations of the inplane oxygen ions are normally not Raman-active because they are located in a center of inversion of the unit shell. However, as mentioned earlier, these oxygen ions do not occupy the ideal interstitial positions in the center of the HgO squares, but are shifted towards the Re ion and strongly bound to it. In this way, the local symmetry of the in-plane oxygens in the lattice is destroyed and their mode appears in the Raman spectra. Based on the above exhibited justification for the polarizability of the 794 cm 1 mode, it is reasonable to expect that the c-axis vibrations of the Ba ion should also gain intensity upon Re doping since they modulate the same bond in a way completely analogous to that of the in-plane oxygens. This is actually the case with the lowfrequency phonon peaks cm 1 as one may infer from the comparison between the spectra of the pure and Re-doped samples see Figs. 1 and 3. Comments on this phonon mode are given below. As regards the high-frequency peaks, which appear with xx/yy polarizations, namely the 803 and 858 cm 1, they may be tentatively attributed to vibrations of the in-plane ions of the ReO 6 octahedra along x and y axes. Such an assignment is in accordance with the high frequencies of these modes been affected by the very strong Re-O in-plane bond. These modes are likely to gain their Raman polarizability through the modulation of this bond which is mainly between O(p x,p y ) and Re d x 2 y 2. HIGHER MEMBERS In Fig. 5, representative zz left and xx/yy right spectra of the Re-doped Hg-1223, Hg-1234, and Hg-1245 samples are depicted in common for the sake of comparison. It is important to note that the spectra of Hg-1223 and Hg-1234 are almost identical at least for the apex zone and the highfrequency Re-induced region including the 510 cm 1 peak. The only remarkable difference between the spectra of the three members is the gradual decrease in intensity of the 590 cm 1 apex component with increasing number n of the CuO 2 layers. This resulted from the analysis of the apex band with two peaks but may also easily be discerned by simple inspection of the spectra as the apex phonon band of the Hg-1223 shows a hump at 590 cm 1 while in the Hg-1245 the band becomes more symmetric. The similarity between the spectra of Fig. 5 is further evidence that the apex and the high-frequency peaks are related to vibrations of ions quite apart from the system of the CuO 2 layers. It should be advisable, at this point, to stress the great difference between these spectra and the corresponding spec-

7 3250 N. POULAKIS et al. PRB 60 tra of the Re-free samples with the same number of CuO 2 layers reported by Zhou et al. 20,21 The latter showed two strong and very broad bands around 540 and 470 cm 1 which dominated the apex phonon band for the highest members of the series (n 4,5). Based on laser heating experiments, these bands were attributed to the excess oxygen vibrations parallel and perpendicular to the HgO planes. 20,21 In our spectra there is not even a trace of such bands. As long as a considerable content of excess oxygen exists also in our samples, the question arises as to whether the origin of these bands lies in the presence of the excess oxygen and the disordered way in which these ions are distributed in the HgO planes, as Zhou et al. have proposed. 20,21 Contrary to the rich structure of the Hg compounds especially the Re-doped ones in the high-frequency part of the Raman spectra, in the low-frequency region the only feature observed is the narrow 4 7 cm 1 peak at 163, 99, 86, and 75 cm 1 for n 1, 3, 4, and 5, respectively. The most likely assignment for this peak is the Ba vibration mode along the c axis. The strongest argument supporting this assignment is that this mode is the only one allowed in the case of Hg-1201 as predicted by group-theoretical analysis of the site symmetry. Actually this peak is observed at 163 cm 1, as may be seen in Fig. 1. However, it is striking that this peak considerably softens to 99 cm 1 in the Hg-1223 Fig. 3. Because of this softening, an objection could arise as to whether the low-frequency peak is related to the Ba phonon or should be attributed to the Ca c-axis vibrations which become Raman active in the case of Hg-1223 and higher members. Of course, no definite answer can be given to this question unless one carries out isotopic substitution of either Ba or Ca atoms. We can only intuitively assert the assignment of the low-frequency peak to the Ba mode since otherwise it would seem unexplained why the Ba mode is suppressed so much in passing from the phase with one to the phase with three or more CuO 2 layers. Besides, as one can see from Fig. 5 left, this peak suffers a further considerable softening in Hg-1234 and Hg-1245, a fact which points towards the existence of a mechanism imposing a strong dependence of the frequency of this mode on the number of CuO 2 layers. Indirect evidence supporting the assignment of this peak to the Ba z-polarized phonon has been presented above with respect to the great polarizability this peak gains in the Redoped samples Figs. 1 and 3. Based on that analysis, it is hard to understand how the intensity of the vibrations of the Ca atom which lie between the CuO 2 layers can be affected by the Re ions located in the intermediary HgO layers which are more than 6 Å apart. However, beyond any uncertainty about the identity of this low-frequency peak, it should be stressed that based on its considerable dependence on the number of CuO 2 layers it may very well serve as a criterion to discriminate between different phases of the Hg series. The observation of this peak could provide a probe of the coexistence of different phases in a microcrystal. CONCLUSIONS In conclusion, among the changes induced in the Raman spectra of the Hg compounds by Re substitution the most striking is the appearance of a number of high-frequency phonon peaks attributed to the oxygen vibrations of the ReO 6 octahedra. Due to the high frequency, small width, and the special Raman polarizability characteristics of these modes it is evident that they resemble very much the internal oxygen modes of strongly bound ReO 6 octahedra almost decoupled from the surrounding lattice. A characteristic Re-induced redistribution of spectral weight between the 570 and 590 cm 1 apex phonon bands of the Hg site shows that Re doping favors the increase of the excess oxygen content in the Hg compounds. Finally, the strong frequency dependence of the low-frequency mode on the number of CuO 2 layers provides a simple way of identifying the existence of microdomains of multiple phases in the Hg-based high-t c compounds. 1 E. V. Antipov, J. J. Capponi, C. Chaillout, O. 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