Superconducting La2CuO4+x prepared by oxygenation at high pressure: a Raman-scattering study

Superconducting crystals of La2CuO4+ prepared by high-pressure oxygenation have been analyzed by Ramán spectroscopy. A direct comparison of the role of excess oxygen was made by examining the same crystals with and without excess oxygen. La2CuO4+, like non-superconducting La2CuO4.O, is found to have a soft phonon that drives an orthorhombic to tetragonal phase transition. In both its tetragonal and orthorhombic forms, La2CuO4+ has a phonon peak at 630 cm1 that is absent in La2CuO4.O. The frequency of this peak is suggestive of a peroxide-like species in La2CuO4+. Surprisingly, the Ag phonons of La2CuO4.O and La2CuO4+ occur at essentially the same frequency. While La2CuO4.O has a well-defined peak from double-magnon scattering, no welldefined double-magnon scattering is observed in La2CuO4+, even in its phase-separated form.


I. INTRODUCTION
High-temperature superconductivity was first achieved by doping nonsuperconducting (NSC) La2CuO~o with Ba or Sr to give Laz, R CuO& 0 (R =Ba or Sr).' Illustrated in Fig. 1(a) is the structure of the parent compound La2Cu04 o.It is possible, however, to convert La2Cu04 0 into a superconductor in another manner, namely, by putting excess oxygen into the structure.'  When sufhcient excess oxygen is introduced by heat treatment in high-pressure oxygen, a stable, bulk superconductor is obtained.
The resulting materials, La2Cu04+, have average oxygen stoichiometries of about 4.03 -4.04, superconducting transition temperatures, T"of up to 39 K, and superconducting volume fractions of over 50%.
From the viewpoint of charge balance, introducing excess anionic oxygen achieves the same effect as does substitut-  ing Ba or Sr (formal valence +2) for La (formal valence + 3).
While it is accepted that Ba and Sr substitute for La on the La site, ' the structural defect and the configuration formed by the excess oxygen of superconducting (SC) LaCu04+ is not well established.Similar uncertainty is associated with the valence and local bonding of the ex- cess oxygen.Early measurements of oxygen content by gravimetric analysis and chemical titration suggested that the excess oxygen was in the form of superoxide, 02 This view was supported by analysis using photoelectron spectroscopy.
Zhou, Sinha, and Cxoodenough argued that disparities between gravimetric and titration mea- surements could be explained by surface oxygen, a result that is consistent with more recent single-crystal stud- ies.' Other research has suggested that the excess oxy- gen is in the form of peroxide, 02 .For example, neutron-diffraction measurements ' of a single crystal of average composition La2Cu04 o3$ showed as one interpre- tation that the excess oxygen resulted in a short 0 -0 bond of 1.59 A, consistent with bond lengths of peroxides.Fig. 1(b) illustrates the structural position of the ex- cess oxygen as suggested by the neutron-diffraction study.
Like NSC LaCu04o, SC LaCu04+ has a tetragonal-to- orthorhombic (T-0) phase transition upon cooling.With further cooling, SC LaCu04+ undergoes phase separa- tion into two distinct orthorhombic phases." One of the two coexisting phases is known to contain the excess oxy- gen and to be superconducting.
While the other phase is known to be essentially stoichiometric La2Cu04 o, '" lit- tle is known about its electrical or magnetic properties.
Here, Raman scattering has been used to examine sin- gle crystals of La2Cu04+".A direct comparison was made between the same samples with excess oxygen (i.e. , superconducting La2Cu04+") and after removing the ex- cess oxygen (i.e. , nonsuperconducting LazCu04 o).If the excess oxygen of SC LaCu04+ is indeed in the form of some dioxygen complex such as Oz or 02 ', then these species should have local vibrational modes associated with them.Since the vibrational frequency of dioxygen complexes is highly sensitive to the valence of the com- plex, then vibrational analysis of La2Cu04+ should help clarify the nature of the excess oxygen.In addition, this study has investigated the phase transition and phase sep- aration of SC LaCuO4+ using Raman spectroscopy.A soft phonon mode is found to be associated with the T-0 phase transition in SC LaCu04+ .Finally, a comparison is made between light scattering from magnetic spin Auc- tuations (i.e. , magnon scattering) from SC LaCu04+     NSC LaCu040.At room temperature, SC LaCu04+" has no well-defined peak from two-magnon scattering.However, two-magnon scattering is observed from the La2Cu04 0 phase of phase-separated SC LaCu04+ II.EXPERIMENTAL light are polarized perpendicular to the c axis.In (zz) polarization, the incident and scattered light are polarized parallel to the c axis.

OF SUPERCONDUCTING La2Cu04+ "
Superconducting crystals of La2Cu04+ were prepared by heating nonsuperconducting single crystals of La2Cu040 (NSC LaCu040) in either 3 kbar of Oz at 575'C for 12 h or in 3 kbar of O2 at 550'C for 60 h.'   Crystals prepared by the former conditions, referred to as SC LaCu04+ No. 1, exhibited the onset of superconductivity at 39 K. Crystals prepared in the latter manner, re- ferred to as SC LaCuO4+"No.2, exhibited the onset of superconductivity at 38 K.A direct comparison of the effect of the excess oxygen was made by examining the SC LaCu04+ No. 2 crystals by Raman spectroscopy be- fore and after removing the excess oxygen by heating at 700 C in one atmosphere of Oz.Raman measurements were made in a 180' backscattering geometry using either 488-or 514.5-nm excitation.All the results shown here are for 488-nm excitation.The majority of Raman mea- surements were made on the interior surfaces of the crys- tals exposed by mechanical fracture.All spectra of SC LaCu04+ shown here are from fracture surfaces, while the NSC LaCu04 0 spectra are from surfaces prepared by mechanical polishing.Polarization geometries are refer- enced to the axes of the tetragonal structure illustrated in Fig. 1.In (yy) polarization, the incident light propagates perpendicular to the c axis and the incident and scattered Like La2Cu04 0, SC LaCu04+ undergoes a tetragonal-to-orthorhombic (T-0) transition upon cooling (space group l4/mmm to Cmca).The T-0 transition temperature is strongly dependent on the exact oxygen content of the sample.For example, the T-0 transition occurs at about 530 K for SC La2Cu04+ .' In contrast, the SC LaCu04+ samples of this study are tetragonal at room temperature (295 K) and the T-0 transition occurs at about 280 K. ' The Raman-active phonons allowed in the I4/mmm structure are two of A & symmetry and two of E symmetry (see Table I).In the upper Raman spectrum of Fig. 2 I).The lower spectrum (77 K) of Fig. 2 shows the five A phonons of orthorhombic SC LaCu04+".The frequencies of the five A phonons of orthorhombic SC LaCuO4+ are listed in Table II.
SC LaCu04+ has an additional feature not present in NSC LaCu04 0, namely SC LaCuO~+ separates into two distinct orthorhombic phases upon cooling." The phase-separation temperature is strongly dependent upon the exact amount of excess oxygen.For example, the ma- terial used in the study of  'Included for completeness since Fmmm has been suggested as one of the two phases in phase- separated SC LaCu04+" (Ref. 11). the I4lmmm to Cmca transition at about 430 K and phase separation into two orthorhombic phases occurs at about 320 K.In contrast, the samples of the present study contain a greater amount of excess oxygen and the phase separation occurs at a lower temperature, about 260 K. ' While originally one phase of the phase- separated material was thought to be Fmmm, " both structures are now thought to be Cmca: Neutron-difFraction analysis ' of a crystal prepared in a similar manner as those of the present study has estab- lished that one of the two coexisting phases is essentially stoichiometric LaCu04 o.The other coexisting phase contains the excess oxygen and has approximate composition La2Cu040~.Interestingly, the existence of these two coexisting phases is not apparent in the Raman data, even at liquid helium temperatures.Specifically, no split- ting of the five Ag phonons is observed in Fig. 2, as might be expected from two di6'erent phases with slightly di6'erent lattice parameters and vibrational force con- stants.
The orthorhombic Cmca phases di8er from the tetrag- onal l4/mmm phase by essentially a nearly rigid rotation of the CuQ6 octahedra about the original [110]tetragonal axis.This rotation is a Raman-active vibration' and ex- hibits soft-mode behavior in SC LaCu04+ as in NSC La- CuQ4 o (see Sec. IV).As shown in Fig. 2, the A phonon at 131 cm ' at 77 K has softened to 112 cm ' at 250 K.This phonon could only be traced to about 250 K; above this temperature the phonon was not observable.Figure 3 shows the temperature dependence of the soft phonons of SC LaCu04+ and NSC LaCu040.Since the soft Ag mode is allowed only in orthorhombic SC LaCu04+"but not in the tetragonal phase, the disappearance of the soft Raman Shift (cm') While NSC LaCu04o  II.
The frequencies reported here agree extremely well (to within 3 cm ') with the results of Sugai.' This agree- ment is convincing evidence that these are truly the five phonons of orthorhombic LaCu04 o. Figure 3 shows the temperature dependence of the soft 3 phonon asso- ciated with the T-0 transition.This soft phonon has been extensively studied by both Raman spectroscopy ' ' and inelastic neutron scattering.' Several comments comparing SC LaCu04+ and NSC LaCu04 o are in order.First, the temperature depen- dence of the soft 3 mode is nearly identical for the two materials below about 2SO K.However, the soft mode of SC LaCu04+"essentially disappears above about 250 K, roughly the temperature at which phase separation and the T-O transition occurs.Second, the 3 phonons of or- thorhombic SC LaCu04+ and orthorhombic NSC La- Cu04 o occur at essentially the same frequency (see Table II).This insensitivity to the excess oxygen is surprising in view of the rather large structural distortion [see Fig.

1(b)
] that the majority [ -70% (Ref.5)] of the sample has undergone at low temperature.However, difFerences do exist in the relative intensities of the 3 phonons.Figure 4 shows a comparison between the orthorhombic forms of NSC LaCu04 o (upper spectrum), SC LaCu04+ No. 1 (middle spectrum), and SC LaCu04+ No. 2 (bottom spectrum).Relative to the axial-oxygen vibration [O(l) in Fig. 1(a)] at 427 cm ', the peak height of the soft phonon ( -131 cm ' at 77 K) decreases from NSC LaCuO~o to SC LaCu04+ No. 1 to SC LaCu04+ No. 2. The La vi- bration (228 cm ) displays the opposite trend, i.e. , it in- creases in relative intensity.Since SC LaCu04+"has a higher oxygen content than NSC LaCu04O, it is clear that the relative intensity of the soft phonon decreases with increasing oxygen content.Further, as supported in the next section, it is believed that the soft phonon of SC LaCu04+ No. 2 is weaker than that of SC LaCuO4+ No. 1 due to the higher overall oxygen content of the former material.It is interesting that the same trends ob- served here in relative phonon intensities as La2Cu04 o is doped with excess oxygen are also observed as LazCu04 o is doped with strontium.
V. MAGNETIC SCATTERING Raman spectroscopy has been extensively used to study spin fluctuations in the Cu-0 based materials.In the parent compound La~Cu04 o, two-magnon scattering is found as a broad but well-defined peak at about 3000 cm ' with B, s (tetragonal) symmetry. [See Fig. 5 Refs.15 and 2D].Since the two-magnon peak is dominat- ed by short-range magnetic order, it is little affected by temperature and persists above T&.' As shown in Figs.
5(b) and 5(c) at 295 K neither SC LaCu04+"No. 1 or SC LaCu04+"No. 2 exhibit a well-defined two-magnon peak.This is not surprising since Lyons and Fleury ' have shown that the two-magnon scattering of YBa2Cu306+ "shifts to lower frequency and greatly broadens upon doping the parent nonsuperconducting compound YBa2Cu306 with oxygen.For the supercon- 2. Spectra were obtained from the same region of each crystal as the corresponding spectra of Fig. 4. The 77-K spectra are off'set from the 295-K spectra for clarity.The short horizontal lines in (b) and (c) represent zero intensity for the 77-K spectra.The notation x (yy)x refers to polarization of both the incident and scattered light along the b =y axis with the incident laser beam and the scattered light propagating along the a =x axis.
ductor YBazCu306 9 (T, =9D K), there is no well-defined peak from magnon scattering.Similar results are ob- served in the Tl-based system.That is, the antiferromag- netic insulator T1YBa2Cuz07 has a well-defined two- magnon peak but the doped superconductor T1CaBa2Cu207 does not.SC LaCu04+" is more complicated since it separates into two coexisting phases below about 260 K. Neutron- diffraction analysis has shown that from a structural standpoint, one of these two phases is essentially La2Cu04 o. '" The other phase contains the excess oxy- gen and is known to be superconducting." Less is known about the La2Cu04 0 phase in phase-separated SC LaCu04+", although it has been suggested to be nonsu- perconducting.' Figures 5(b) and 5(c) illustrate the magnetic scattering observed from SC LaCuO4+ No. 1 and SC LaCu04+ No. 2 at 77 K, well below the temperature of phase separation.SC LaCu04+ No. 1 [Fig.5(b)] has a well-defined two-magnon peak at about 3000 cm '.c)] has an extremely weak two-magnon peak at about 3000 cm '.Since the phase with the excess oxygen is known to be superconducting, it will not exhibit a well-defined two-magnon peak, as discussed above.Therefore, the two-magnon peak observed in SC LaCu04+"at 77 K must result from the La2Cu04 o phase and this phase must be antiferro- magnetic and nonsuperconducting.
At 77 K, the two-magnon peak of SC LaCu04+"No. 1 is intense, while the two-magnon peak of SC LaCu04+ No. 2 is barely discernable.
This implies that SC LaCuO4+ No. 2 has less of the antiferromagnetic phase La2CuO4O than does SC LaCu04+"No. 1.Since the amount of the La2Cu04 o phase that forms is reduced as the average oxygen content of the material increases, " it is concluded that the SC LaCu04+ No. 2 material has a higher oxygen content than does SC LaCu04+ No. 1.This conclusion is consistent with the observation of Sec.IV, where the intensity of the soft A phonon was found to be related to the amount of excess oxygen.Specifically, in Fig. 4, the soft phonon of SC LaCu04+ No. 2 (bottom spectrum) was weaker than that of SC LaCu04+ No. 1 (middle spectrum).
As seen in Fig. 5, both SC LaCu04+ and NSC La- Cu04 o have numerous Raman features between about 700 and 1500 cm '.For NSC LaCu040, these high- frequency peaks have been interpreted as either second- order phonons enhanced by some resonance process' '  or magnetic scattering from a spin-density wave in the antiferromagnetic structure.
These peaks are largely absent in tetragonal La2CuO40, and can be largely el- iminated by heat treatment in low pressures of oxygen.
For SC LaCu04+, this high-frequency structure is large- ly absent at 295 K [Figs. 5(b), 5(c), and 6] but is present at lower temperatures.
The intensity of the structure be- tween 700 and 1500 cm ' varies greatly from sample to sample.Compare, for example, Figs. 5(b) (SC LaCuO4+ No. 1) and 5(c) (SC LaCu04+"No. 2).Even within a sample, the intensity of this structure was variable, for example, the spectra of Figs. 6 and 5(c), taken from different spots on the same crystal.', labeled with an arrow, is present in some but not all regions of SC LaCu04+ No. 2 crystals.This feature was not observed in samples SC LaCu04+"No. 1 and NSC La- Cu04 z.The spectra are offset for clarity and the short horizontal line represents zero intensity for the 77-K spectrum.
VI. THE 630-CM ' PEAK OF SC LaCuO4+"NO.2: OBSERVATION AND DISCUSSION A substantial difference exists between SC LaCu04+" No. 2 and NSC LaCu04 ~when examined in a (yy) polar- ization geometry.As shown in Fig. 6, SC LaCu04+ No. 2 has a scattering peak at 630 cm ' at both 295 and 77 K, that is, in both the tetragonal and orthorhombic (phase-separated) forms.This peak is totally absent in (zz) polarization spectra The 6. 30-cm ' feature has been observed in spectra from "as-received" surfaces of two different crystals prepared by heating in 3 kbar of 02 at 550'C for 60 h (i.e. , the conditions denoted by the nota- tion SC LaCu04+ No. 2).Interestingly, this feature was observed from some but not all fracture surfaces of SC LaCu04+, No. 2. In contrast, this peak was absent from NSC LaCu04 o and SC LaCu04+ No. 1 at all locations and temperatures examined.Further, this peak has not been observed in La2 Sr CuO4.' The presence of the 630-cm ' peak on fracture surfaces and its strong polar- ization dependence greatly diminish the possibility that the peak is impurity related.Instead, it can be argued that the 630-cm peak is observed in regions of high average oxygen content.%'hen the 630-cm ' peak was observed using (yy) polarization, examination of the same spot using (zz) polarization revealed that the soft 2 phonon was weaker than in any of the spectra of Fig. 4.This includes the bottom spectra of Fig. 4, obtained from a re- gion of SC LaCu04+ No. 2 where the 630-cm ' peak   was not observed.Given the relationship between the in- tensity of the soft phonon and oxygen content, we con- clude that the 630-cm peak is associated with regions having the highest amounts of excess oxygen.The absence of the 630-cm ' peak within some regions of the SC LaCuO~+ No. 2 crystals indicates that the crystals are not uniformly loaded with oxygen.This is not surprising, given the size of the crystals (about 1 -2 mm in thickness).
There are two sources of Raman-scattering peaks in SC LaCu04+ .phonon scattering and magnetic scattering.
(An additional source of intensity is electronic scattering from charge carriers.However, this scattering, if present, should be in the form of very broad features, i.e. , a continuum).It is unlikely that the peak at 630 cm ' re- sults from magnetic scattering.Below its Neel tempera- ture (T~) of -300 II, ' NSC LaCu04 ~has 3D antiferro- magnetic ordering of the Cu + spins.Above T&, the spins are still ordered in two dimensions.As discussed in the last section, two-magnon (spin-pair) scattering occurs as a well-defined peak at about 3000 cm ' in nonsuper- conducting La2Cu04 o.
The frequency of one-magnon scattering in LazCu04 o is essentially zero near the zone center (i.e. , k =0), as probed by light scattering.Clearly, the 630-cm ' peak is not associated with two-magnon scattering or one-magnon scattering near the zone center.
It is possible that defects such as vacancies could result in observable scattering from a localized, single spin ftip.Indeed, a single spin Aip next to a vacancy has been sug- gested as a possible origin of the -1430-cm ' peak of La2Cu04~.'   However, a similar explanation is not possible for the 630-cm ' peak.If one takes the ex- change parameter J=1000 -1300 cm ' (Refs.26 and 29)   and spin= -, ', then 630 cm ' requires more than two va- cancies around a copper atom, ' an unphysical prospect.
Since magnetic scattering can be ruled out, it is concluded that the 630-cm ' peak of SC LaCu04+ is phonon scattering.
One interpretation is that the peak at 630 cm ' is a lo- cal phonon mode associated with the excess oxygen in the structure.Following this possibility and literature sug- gestions that the excess oxygen forms a dioxygen com- plex in the structure, it is interesting to compare the fre- quency of 02 vibration as a function of 0 -0 bond dis- While this interpretation cannot be rigorously defended (see below), the relationship would suggest that if the 630-cm peak is indeed from dioxy- gen vibration, then the dioxygen complex is more like peroxide (02 ) than like superoxide (02 ').The ob- served polarization dependence of the 630-cm ' peak is qualitatively consistent with the structural model proposed by Chailout and co-workers.' As illustrated in Fig. 1, the short O(3) -O(4) bond of 1.59 A lies largely within the x-y plane.Therefore, vibration of O(3) -O(4) along this bond should have a large effect on the polarizability measured by (yy) polarization and a smaller influence on the polarizability measured by (zz) polarization.The experimental observations are consistent with this intuitionthe 630-cm peak is strong for (yy) po- larization (Fig. 6) but is absent for (zz) polarization.However, arguments based upon strong scattering for (yy) polarization are weakened by the fact that anoma- lous scattering peaks are observed with (yy) polarization for La2Cu04 0 (see Sec. V).
Despite the reasoning given above, the 630-cm ' peak cannot be definitely assigned to vibration of a dioxygen complex.To begin with, 630 cm is within the frequen- cy range expected for vibrational modes of oxygen in the stoichiometric La2Cu04 p structure.For example, an X- point mode of tetragonal La2CuO4p has been calculated from first principles to occur at 731 cm '.This mode is mainly the planar-breathing vibration of O(2) with some contribution from the axial vibration of O(1).In or- thorhombic La2Cu04 p, this mode is folded back into the zone center and is Raman allowed.Indeed, Weber et al. have assigned a peak observed at 710 cm ' in or- thorhombic La2Cu04 p to the planar-breathing mode of O( 2).There exist several mechanisms that might allow phonons that are formally Raman inactive to gain Ra- man intensity even in tetragonal SC LaCu04+ .The ex- cess oxygen can be viewed as a defect that destroys the long-range order and local symmetry of the lattice and thus relaxes selection rules associated with momentum conservation and phonon symmetry.This could allow vi- brational modes that are not at the zone center or are normally only ir-active to gain Raman intensity.In addi- tion, the SC LaCu04+ structure may have a larger unit cell than even the underlying orthorhombic structure of La2Cu04p.This large unit cell could have additional Raman-allowed phonons not directly related to the speculated dioxygen species and the 630-cm ' peak could be such a phonon.Finally, the 630-cm ' peak could be a second-order phonon.
Despite the complications discussed above, several conclusions can be drawn.The 630-cm ' peak of SC LaCu04+ No. 2 is of phonon origin.Since it is not present in NSC LaCuO4p or La2 "Sr Cu04, ' the 630- cm ' peak results from the introduction of excess oxygen into the structure.Further, the 630-cm ' peak is not re- lated to either the orthorhombic distortion or phase sepa- ration since it occurs in tetragonal SC LaCuO4+

VII. SUMMARY
In certain aspects, the excess oxygen of SC LaCu04+ has little inhuence on the Raman-allowed phonons.For example, at low temperature, five A phonons are found at essentially the same frequency in both SC LaCu04+ and NSC LaCu04 p. Further, like NSC LaCuO4p, SC LaCu04+ has a soft-mode phonon associated with the orthorhombic-to-tetragonal transition and the temperature dependence of this mode below about 250 K is near- ly identical in the two materials.However, key differences do exist between SC LaCu04+ and NSC La- CuO4p.The soft A phonon of SC LaCu04+ is only observed below about 250 K, approximately the tempera- ture of phase transition and phase separation.' ' In contrast, the T-0 transition in NSC LaCu04 p occurs at about 530 K.At room temperature, SC LaCu04+"does not have a well-defined peak from two-magnon scattering, in contrast to NSC LaCuO4o.At 77 K (i.e. , in its phase-separated form), SC LaCu04+"exhibits a well- defined peak from two-magnon scattering.This estab- lishes that the La2Cu04 p present in phase separated SC LaCu04+ is antiferromagnetic and not superconduct- ing.In both the tetragonal and orthorhombic forms, certain regimes of SC LaCu04+"have a phonon peak at 630 cm, unlike NSC LaCu04p.This peak is believed to occur in samples containing the most excess oxygen.If this peak results from a local vibrational mode of a dioxy- gen species in the structure, then the observed frequency is consistent with peroxide but not superoxide.However, a definitive assignment of the 630-cm ' phonon is not possible at this time.
mode above about 250 K is consistent with the T-0 transition observed by other techniques near 280 K.' FIG.3.Temperature dependence of the soft Ag phonon of SC LaCu04+ and NSC LaCuO4 o.Open squares are from crys- tal SC LaCuO4+"No. 1 and solid squares are from crystal SC LaCu04+"No.2.

TABLE I .
Symmetries of Raman-allowed phonons in diferent structures of La2Cu04+ .

TABLE II .
Comparison of frequencies of A~phonons of SC LaCu04+ and NSC LaCu04 0.
has been extensively studied by Ramans spectroscopy, the literature is inconsistent.(See review articles by Ferraro and Maroni' and Feile' for a summary of the literature results.) The five allowed A~F IG. 2. Raman spectra of SC LaCu04+ No. 1 in (zz) polar- ization geometry permitting A&g (tetragonal I4/mmm) and Ag symmetries (orthorhornbic Cmca).The 131-cm ' phonon (77 K) exhibits soft-mode behavior associated with the tetragonal- to-orthorhombic phase transition.Spectra are o8'set for clarity.

TABLE III .
Vibrational frequency of dioxygen complexes of coordination compounds as a function of valence.Data from Nakamoto (Ref.31)..As seen in TableIII, the O-O vibration becomes weaker with increasing 0 -0 bond distance.Table III also lists the 630-cm ' frequency and the short O -O tance