Optical phonons, crystal-field transitions, and europium luminescence-excitation processes in Eu2BaCoO5: Experiment and theory

PHYSICAL REVIEWER BVOLUME 50, NUMBER 13 1OCTOBER 1994-I Optical phonons, crystal-field transitions, and europium luminescence-excitation processes in Eu2BaCoO&. Experiment and theory S. Taboada and A. de Andres Instituto de Ciencia de Materiales de Madrid, Consej oSuperior de Investigaciones Cientijicas, Facultad de Ciencias C-g, Cantoblanco E88-0)g, Madrid, Spain J.E.Munoz Santiuste Escuela Politecnica Superior, Unieersidad Carlos III de Madrid, Aeenida del Mediterraneo 20, Leganes E-M928, Madrid, Spain C. Prieto and J.L. Martinez Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientificas, Facultad de Ciencias C-g, Cantoblanco EMO)g, -Madrid, Spain A. Criado Instituto de Ciencia de Materiales de Seeilla, Departamento de Fcsica de la Materia Condensada, Universidad de Sevilla, Box 1065 E)1080, S-pain (Received 16 November 1993;revised manuscript received 13 May 1994) The europium compound Eu&BaCo05 has been studied by means of Raman and x-ray absorption spectroscopies. The eigenfunctions and frequencies ofthe optical normal modes have been calculated from an adequate potential showing good accordance with the observed phonons. In addition to Raman allowed normal modes, several infrared and luminescence bands are observed between 10 K and 300 K. The temperature dependence ofthese processes has allowed us to determine the complex interrelation between these two kinds of elementary excitations. Abroad luminescence band (at 2.3 eV) is tentatively attributed to electronic transitions between Co +3d crystal field levels in the gap of the material. The mixing of these quasiatomic levels with apical oxygen orbitals, along the very short Co-O(2) bonds in the chains, can be the reason for the simultaneous enhancement of the intensities of the luminescence band and of the apical oxygen infrared modes through aresonant electron-phonon coupling. From the dependence of the phonon frequencies and the analysis of the extended x-ray-absorption fine structure spectra with the temperature it can be concluded that on decreasing the temperature the a-axis becomes shorter, while the other two axes remain nearly unchanged. The emission spectrum in the visible range has been observed and interpreted in the frame of the crystal-field theory. We have studied the dependence of the intensity of the electronic transitions between 4f levels of the europium ions with the temperature and the energy of the exciting light. Prom the behavior of the Eu +luminescence peaks it has been possible to determine the processes of excitation and emission, which are shown to involve lattice phonons. The crystalfield parameters of the Eu +ions have been calculated &om the energies of the lower terms of the ion. I.INTRODUCTION The compounds with R2BaM05 formula, where R=rare earth, and M= transition metal, that crystalize in the Immm structure are attracting vast interest because of the peculiarities of this structure composed of isolated chains of M06 octahedra. Its simplicity makes the study of the related physical properties fascinating. Magnetic characterization of these nearly perfect onedimensional (1D) Heisenberg antiferromagnets is being actively studied. Nickel oxides, with R=Nd to Yb, have been recently structurally described as belonging to the Immm space group. i2This structure is also adopted by some Co oxides. The Immm structure consists of one-dimensional chains of Co-0 octahedra. These octahedra are only connected by the apical oxygens called O(2) along the aaxis, but the chains are not directly connected and the Co-Co distances are quite long so that the system can really be considered as 1D &om the magnetic point of view. The octahedra present astrong tetrahedral deformation with very short Co-O(2) bonds (1.89 A) and quite larger Co-in plane-0 bonds (2.22 A.).In the case of the Ni compounds, the en'ect of the orbitals on the deformation of the Ni-0 octahedra has been evaluated from asemiempirical model. 0163-1829/94/50(13)/9157(12)/$06. 00 50 9157 1994 The American Physical Society 9158 S.TABOADA et al. 50 Optical Raman phonons have been studied in compounds of the same family with different structures and the force constants were evaluated. 'The compounds with the Immm structure present Raman spectra that agree with the group theory predictions, but aweak forbidden Raman peak, the IR-active stretching mode ofthe apical oxygen, is observed probably due to the presence of some defects, like small quantities of different rare-earth lons. The study of the rare-earth crystal-field transitions by means of avery high resolution spectroscopic technique can give information on the magnetic order in the sample as has been done as afunction of the temperature in nickelatess and cuprates7 with R2BaMOs composition. The coupling between alow-energy crystal-field transition and aphonon of similar energy has been observed by Raman spectroscopy and explained theoretically in the case of NdBa2Cu307 The electronic structure, in related compounds, of the localized states and bands, has attracted enormous interest and several experimental approaches have been used. For example, in cuprates and nickelates, through optical conductivity and reflectivity and infrared absorption, 'peaks around 1.7— 2eU have been assigned to transitions between different kind of electronic levels [charge transfer (CT) gap, transitions from Cu localized states to band states, oxygen 2p intraband]. A recent study has also been done in insulating Co compounds (Bi2MsCoqOs+~), where abroad infrared absorption is observed as in the conducting 3d transition metal oxides; the CT and/or d— dexcitations of Co are found to lie above 4eV. Some features of the electronic structure, in this kind of compound, have been explained but the problem is far Rom being resolved. Still, the theoretical calculations, often, can explain not even the gap, so that much more information is still necessary even in these relatively simple oxides. In this work we study the complex behavior of the Eu2BaCo05 compound as afunction of the temperature by Raman scattering and x-ray absorption in combination with different theoretical models. The Raman technique allows to observe phonons and some electronic transitions, as luminescence peaks or bands, too weak to be detected with another luminescence setup. The scheme of the results presented in this work is as follows: Section III is dedicated to the optical phonons. After the description of the model used to calculate the &equencies and the eigenvectors, we discuss the assignment of the observed one-phonon peaks to normal modes at the different temperatures. Afterwards, the processes which produce the appearance of multiphonons and luminescence bands are discussed. In Sec. V, the combination of Raman and extended x-ray-absorption fine structure (EXAFS) spectroscopies have made possible the determination of the variation of some particular interatomic distances with the temperature. In Sec. VI we present the observed emission spectra of the europium ion, in the visible range, as afunction of the temperature (between 10 and 300 K) and of the excitation energy of the incident laser beam. Because the state &om which we observe most of the emission lines is asinglet, the spectra are not very complicated and a clear picture of the fundamental states can be obtained. Atheoretical approach has been done in order to describe the crystal Geld acting on the Eu +ion. In a Grst step, we have calculated the crystal-field parameters based on asingle point charge electrostatic model using the x-ray structural data of Eu2BaCo05. The energy level fitting is done as asecond step, by diagonalizing the interaction matrix of the 4f configuration taking into account the observed splitting of the I' levels. II. EXPERIMENTAL DETAILS The Eu28aCo05 compounds were prepared as polycrystalline samples mixing stoichiometric amounts of the high purity oxides Eu20s (99.999%),CoO (99.99%), and BaCOs (99.999%%up). The homogenized mixture was heated at 950'C for 12 h; then it was regrounded and reheated at 1050'C for another 12 h. The heating treatment was done in argon flow due to the high instability of Co + ions in air at the high temperatures necessary to prepare these compounds. Raman-scattering experiments have been performed with an X— YDilor multichannel spectrometer using a Spectra Physics Ar+ laser as excitation source with less than 10 mW on the samples, which were always in helium atmosphere, inside the chamber of an Oxford cryostat, in order to avoid heating damage. The luminescence of the samples was excited with the 476.5, 488, and 514.5nm lines ofthe Ar+ laser. The spectra were recorded between 300 Kand 9Kin the available experimental spectral range (21000— 13000 cm ~). The apparatus response is strongly dependent on the &equency of the detected light in asuch wide energy range, so that we have corrected all the spectra by the experimental set-up sensitivity. The x-ray-diffraction data show that the samples are single phase and present the Immm structure. The structure has been checked only at room temperature (RT) with x-ray diffraction. Nevertheless, the Immm structure presented by these compounds is very stable and does not admit oxygen defects, and so no structural changes are expected when the temperature is varied. The EXAFS experiments were carried out on the XAS3beam line at the DCI storage ring (Orsay, France) with an electron beam energy of 1.85 GeV and an average current of 250 mA. Data were collected with afixed exit monochromator using two Si(311) crystals in transmission mode. Detection was made by using two ion chambers filled with air. The energy resolution was estimated to be better than 2eV based on the Cu foil 3d near-edge feature. The energy calibration was monitored using the Cu foil sample, and was set as 8991 eV at the first maximum above the edge. Samples were prepared by grinding, sieving, and then selecting particles less than 10 pm in size by floating the powder. The particles were then spread on atape. Several layers of tape were used to fabricate samples with an adequate absorption jump (0.1 Apx (1.0) and amaximum for the total absorption of less than 1.5(px &1.5). 50 OPTICAL PHONONS, CRYSTAL-FIELD TRANSITIONS, AND. ..9159 III. OPTICAL PHONONS The Immm space group is orthorhombic with the D2i, point group (Z =1in the primitive cell). The factor group analysis of the Immm structure4 gives that 9even modes are Raman active (SAs+1Bis+2B2s+3Bss) and 14 odd modes are infrared active (5Bi„+5B2„+4B3„). In this highly symmetric structure only Eu and O(1) can contribute to Raman modes. The apical oxygens O(2), which catenate the successive octahedra, can contribute, in principle, only to infrared modes. Among the Raman modes, two of them (those with the lowest energies) correspond to Eu motions, five to the oxygens O(1), which form the basal planes of the Co-0 octahedra, and the two modes left involve the two kinds of ions. These last two modes are not observed in our Raman spectra. The infrared (IR) modes involve always several kinds of ions, but it is possible to say that the higher-frequency ones (over 400 cm i) are basically due to motions of oxygen [O(1) and O(2)] and cobalt ious. A. Lattice dynamics calculation Crystal lattice modes have been calculated using the Born— von Karman formalism, is in which the dynamical matrix is set up in terms of Cartesian force constants where the force constant relating atomic displacements along nand P(u, P=x, y, z) of atoms kand k' belonging to unit cells land l' can be obtained as the second derivative of the crystal energy with respect to these displacements. The dynamical matrix can be obtained from these force constants as D(q]kk')(mI, m&) /)4'(Ok, lk') xexp{iq[r(l'k') — r(0k)]), where qis the wave vector and r(lk) and mg are the position vector and mass of atom (lk), respectively. When Coulombic long-range forces are present the snm over all unit cells l' converges very slowly and it is necessary to accelerate the convergence using the Ewald method. In order to be able to build up the dynamical matrix we need amodel for the crystal lattice energy. In the case of ionic crystals, the equilibrium structure can be viewed as acompromise between the Coulombic attractive forces and the short-range repulsive forces arising from the overlapping of atomic orbitals. We have modeled the Coulombic forces considering for each ion its formal charge in the empirical formula whereas for the short-range forces, several parametrizations are available in the literature. We have chosen that proposed by Gilbert, ~where the repulsion between two ions iand j can be expressed as V~(rep) =fp(B; +B~)exp[(A;+ A~ — r;~)/(B; +B~)] . B. One-phonon processes Figure 1shows the Raman spectra obtained at different temperatures between 9Kand 300 K. The room TABLE I. Static crystal energy parameters V(r,~) =Z;Z,.e/r;~ +fp(B; +B~)exp[(A;+ A,.— r;~)/(B, +B~)] with fo =1kcal mol A. .Afactor of 0.51 was applied to this function in order to calculate the lattice frequencies. A(A.)B(A) 1.733 0.0959 1.953 0.101 1.285 0.066 1.853 0.168 Z +3 +2 +2 -2 Here, Aand Bare characteristic constants of the chemical kind of the ions involved and r;z is the distance between them. The values of the Aand Bconstants for barium, cobalt, and oxygen atoms have been taken from the literature. For europium, no available data could be found and we proceeded to adetermination of the A and Bparameters. Aleast-squares refinement of these parameters was carried out in order to get an energyminimized crystal condguration as close as possible to the experimental one. The minimization was performed with respect to the lattice parameters and the atomic positions maintaining the constraints of the Immm crystal symmetry. For the optimized Aand Bparameters we found adiscrepancy of 1.5%, 0.1% and 0.7% between minimized and experimental a, band clattice parameters, whereas the maximum atomic shift was 0.07 A during the minimization process starting Rom the experimental structure. In Table Iwe show the final values of the parameter adopted in this work. Using this potential model derived from static considerations, we have calculated the lattice dynamics at q=0 by diagonalization of the dynamical matrix for q=0. Our approach is within the rigid-ion model which deals the ions as rigid units, which precludes atomic polarization effects. The inclusion of such efFects would require much more sophisticated approaches such as shell models. An alternative approach which retains the simplicity of the rigid-ion model considers effective Coulombic charges which account for polarization effects. The effective charge values are lower than the formal charge ones, modelling in this way the Coulombic screening because of polarization. The use of effective charges has been successfully applied to ionic materials such as KNbOs Ref. 19 and YBa2CusOr. In our case, the calculation of the Raman frequencies using the parameter values of Table Igave too large frequencies and we had to scale the free-ion charge values with afactor of 0.71 in order to get effective charges showing the best agreement between experimental and calculated frequencies. In order to keep the balance between Coulombic and repulsive forces, the V;~(rep) terms were scaled accordingly with afactor of 0.51. Table II collects the phonons observed by Raman spectroscopy at room and low temperatures together with the calculated frequencies. S.TABOADA et al. TABLE II. Calculated and observed optical phonon frequencies (in cm ); upper part, even modes, and lower part, odd modes. The phonons have been observed by Raman scattering at temperatures between 300 Kand 10 K. Species B2g Ag B3g B3g Bgg B~g B3g Ag Ag Bg B3„ Bg„ Bg„ B3„ B2„ Bg„ A„ B2 Bg„ B3tt Bg„ Bg„ Bg„ B3%l Kind of ion involved in the motion Eu Eu+O(l) Eu+O(1) O(1) O(1) O(1) O(1) O(1) Calculated normal modes 118 171 174 181 306 356 410 445 550 54 61 86 114 161 184 189 221 288 304 315 436 436 602 709 Observed phonons 300 K 118 168 508 708 Observed phonons 10 K 121 172 302 342 411 508 (silent) 270 446 446 724 E~BaCo05 488 nm a9K bVOK c150K dSOOK ~i+I r} 50 160 260 860 460 560 $60 760 Raman Shift (cm ) FIG. l. One-phonon region of the Eu2BaCo05 Raman spectra at different temperatures between 300 Kand 9K with the 488 nm excitation laser line. The 300 Kspectrum is equivalent to the one of Ref. 5. temperature spectra of isomorphous nickelates have been studied in previous works. 'The very difterent atomic masses of the yttrium and the rare-earth ions (Ho, Er, and Tm) have already allowed us to observe, very clearly, the correct dependence of the frequency with the lanthanide mass in the first two observed peaks showing that they correspond exclusively to movements of Rions. The remaining observed peaks are only related to oxygen motions. These two conclusions are now checked by the calculations of the normal mode eigenvectors and frequencies. The peak at 708 cm, which is observed around 740 cm as aweak shoulder in other studied nickelates, is the most important peak in the spectrum of Eu2BaCoOs [Fig. 1(d)j. We assign this peak to the Bs„ infrared M-O(2) stretching mode. The Co-O(2) distance is very short (around 1.89 A), and so the frequency corresponding to its stretching is expected to be the highest of the one-phonon peaks. When the temperature decreases, several changes are observed in the one-phonon region (below 750 cm ~): (i) Some new peaks appear (their frequencies are compiled in Table II).(ii) The intensity ofthe infrared mode at 708 cm shows astrong increase and the relative intensity of the remaining peaks also changes. (iii) The frequencies of the observed peaks vary in two diferent ways: The R ion modes and the 708 cm 1peak increase around 2%, while the O(l) peaks remain unchanged or even decrease slightly. The calculation of the optical phonons has been done to obtain the eigenvectors and frequencies of the Ramanand in&ared-active modes in order to compare them with the observed phonons. Table II collects the observed phonons by Raman spectroscopy at room and low (10 K) temperatures together with the calculated frequencies. The so large intensity of the Raman-forbidden 708 cm peak in the Eu compound can be produced by some kind of disorder in the structure. For example, the presence of different rare-earth ions as an impurity would break the inversion symmetry of the apical oxygens and transform this mode into aRaman-allowed one. Another possibility is that the structure is not correctly described by the Immm group but by some less symmetric group or that some kind of local distortion is present. Nevertheless, we have rescued these possibilities because (1) the x-ray diKraction spectra at room temperature are clearly indexed in the Immm group, (2) the structure is very stable, and (8) the oxides employed in the sample preparation are extremly pure. Another argument is that, with the only exception of the 708 cm ~peak, the number and the frequencies of the Raman peaks, at room temperature, correspond to those expected &om group theory. It is known that cobalt oxide compounds are unstable when heated in air. We have performed Raman spectra with the sample in air, instead of the usual helium atmosphere, and have obtained that, actually, the heating produced by the 10 mW laser is enough to locally damage the sample. Nevertheless, the obtained spectrum (Fig. 2) contains none of the previously reported peaks (Fig. 1). Another possibility, which we think is the correct one, is that the incident or scattered energies can be in resonance with some electronic transition which induces, 50 OPTICAL PHONONS, CRYSTAL-FIELD TRANSITIONS, AND. .. 9161 EuNEaCoOI Damaged. 50 160 860 360 460 560 860 760 Raman Shift (cm ) FIG. 2. Raman spectrum of an EuqBaCo05 sample damaged by the reaction of the hot spot zone with air. through electron-phonon Frohlich interaction, the Raman activity for the LO odd phoaons. This is supported by the presence of the second, third, and even fourth order of this phonon marked in Fig. 3. Note that the intensity is clearly vanishing with the increasing order of the phonon. On the other hand, the peaks observed in Fig. 1when the temperature decreases can have several origins. The possibility of astructural phase transition which would induce the observation of new Raman-active phonons is disregarded because ofthe previously mentioned stability of the structure. Moreover, neutron-diffraction studies between RT and 1.5Kon the isostructural nickelates show that no structural phase transition happens in this system. We rather propose that these aew peaks have these two origins: Two peaks (at 302 and 342 cm ') are Raman modes not observed at RT whose intensities increase when the temperature is low enough, and the other ones (at 270 and 446 cm i) are infrared modes (as the 708 cm ione), forbidden but observed by Raman scattering. The two first mentioned peaks are detected in nickelates with the same structure at room temperature, probably because the scattering efBciency for all modes is higher. The 446 cm ipeak is wider than the other phonons, probably because it comes from two infrared modes (1Bi„and1B2„, see Table II) whose calculated frequencies are identical, but could be slightly different, producing the observed broadening. These two infrared modes (270 and 446 cm )involve basically CoO(2) bond bending together with Co-O(1) bond beading and/or stretching, respectively. The highest-eaergy Bs„ mode is principally due to the Co-O(2) bond stretching, and so it is clear that on lowering the temperature the infrared phonons that involve principally the Co-O(2) bonds are those that appear in the Raman spectra. Eu&BaCo0& a9K b70K c',)150K d) 3DDK coo ' ' bio ''who II II 0I..)W. ~) I 8kg~~ b) I 0I I II II ~d) T 500 940 1300 1700 Ri00 2500 8900 Raman Shift (cm— 1) FIG. 3. Raman spectra of EuqBaCoOS showing the evolution of the intensity and frequency of the multiphonons with temperature. The excitation wavelength is 488 nm. The inset shows the enhancement ofthe B3„phononas the temperature decreases. Labels I— 4indicate the order of the H3„phonon. IV. INTERACTION BETWEEN PHONONS AND LUMINESCENCE PROCESSES Figure 3shows the spectra recorded at temperatures betweea 300 Kand 9Kusing the 488 nm laser liae as excitation in the range between 500 and 3000 cm where the multiphonons are observed. The frequencies of the peaks in the lowest-temperature spectrum are 724, 1448, 2170, and around 2910 cm i, which correspond to the first, second, third, and fourth order of the Bs„phonon, respectively. The shift in the energy of the firstand second-order peaks can be followed in all the temperature range from 708 to 724 cm iand from 1417to 1448 cm, respectively, showing aconsistent variation. This figure clearly shows how the intensity ofthe Bs„peakand its multiples increases when the temperature decreases. Note that the "background" also changes considerably. The inset of Fig. 3presents the evolution of the intensity and the &equency of the 708 cm mode. The spectra at low temperature for two incident wavelengths (476.5and 488 nm) are shown in Fig. 4. The broadband corresponds to aluminescence process. Note that the phonon peaks are observed at the same position in the "Raman shift, "while the luminescence peaks and band (marked with an arrow), where the absolute position in energy must be preserved, are observed with ashift equal to the separation between the two incident beam energies (b,ufo= 494 cm i, in the present case). This broadband, observed in the 476.5nm (20986 cm ), 9162 S.TABOADA et al. 50 enhancement of the intensity of the B3„phonon related to Co and apical O(2) movements. 2.3eV. TEMPERATURE DEPENDENCE OF THE STRUCTURE 500 i50G 2500 $500 4500 5500 Raman Shift (cm— 1) FIG. 4. Spectra of EuqBaCo05 at 20 Kfor the 488 and 476.5nm excitation wavelengths showing the broad luminescence-type band, which corresponds to an absolute energy of 2.3eV. spectrum around aRaman shift of 2400 cm, corresponds to an absolute energy of (20986— 2400 cm i=) 18586 cm i(= 2.32 eV). The sharp luminescence peaks correspond to crystalfield transitions between the 4f levels localized in the Eu ions. These peaks are very sharp as it corresponds to emissions between these kinds of electronic levels. The assignment of the observed peaks and the calculation from two different models of the crystal-field parameters at the Eu site are presented in Sec. VI. The intensity of the multiphonons of the Bs„modeincreases extraordinarily (about 10 times; see the inset of Fig. 3) when the temperature decreases: The spectra of Figs. 1, 3, and 4have been recorded exactly at the same conditions and with the same incident beam power. Only some kind of resonant process can explain such behavior. The observation of the luminescence wide band at 2.3eV (Fig. 4) with aparallel behavior leads one to think that these two processes should be related. As we shall see later, on lowering the temperature the Co-O(2) distance decreases about 2%%uo. This effect could induce asufficient change in the crystal field on the Co +ions to make partially eKcient transitions between the quasiatomic 3d cobalt electronic levels which leads to the observed luminescence band. The band gap of this material, whose conduction and valence bands are mainly due to oxygen 2p and Co 3d levels, respectively, is expected to be larger than the nickelates one, which is around 4eV. 'For that reason no interband transition is expected under our experimental conditions. We are inclined to think that the transitions are to and from (first absorption and later emission) these quasiatomic cobalt crystal-field 3d levels, which lay in the gap and which have very probably some O(2) orbitals mixture because of the very short Co-O(2) bonds parallel to the aaxis of the chains. In that way, through the mixing between the Co 3d crystal-field levels and the 0orbitals, we can understand the selective It has been already pointed out that only the frequencies of phonons related to O(2) and Eu motions increase about 2'%%up when the temperatures decreases from 300 to 80 K, while the O(1) phonons remain invariable. From a structural point of view, this fact can be explained by a contraction ofthe aaxis when the temperature decreases, where as the 6and caxes are nearly unchanged. In order to check this point, EXAFS spectra at the Co Kedge and Eu L~gy edge were carried out. The available energy range for the Co Kedge is unfortunately very short (about 340 eV), because of the presence of the LI Eu edge. The energy range of the spectrum is not good enough to obtain reliable fits ofthe several Co-O(l), O(2), Eu, and Ba distances. The Eu ion has seven oxygens as nearest neighbors at three difFerent distances: Four of these are in the XY plane and the other three are in the YZ plane (Fig. 5). Adecrease in the Xaxis means that only the four oxygens in the XY plane may change their Eu-0 distance on cooling. The analysis of the EXAFS signal, in order to get the variation with the temperature of the distances of the neighbors around the absorbing atoms, has been done using the well-known expression: g(k) =)„' exp( — 2k o;)exp ~ kR (k) xf,(k) sin[2kR, +Cz(k)j . This equation describes the EXAFS oscillations for a Gaussian distribution of N~ atoms at mean distances B~ around the absorbing atom considering the single scattering and plane-wave approximation. kis the photoelectron wave vector, related to the electron mass (m, ) and with the threshold energy (Eo) by k=[2m, /5 (E— NL, . =Z o(2) eEu ~co FIG. 5. Eu +environment in EuqBaCo05 with the Immm structure with seven oxygen first neighbors and three difI'erent distances. 50 OPTICAL PHONONS, CRYSTAL-FIELD TRANSITIONS, AND. ..9163 EU-0 '"' ~I RY ----- LNT 0.01.02.08.04.iIi 5.0S.O Distaxice (/&) FIG. 6. Fourier transform of the EXAFS spectra at the Eu L«& edge at room temperature (RT) and at liquid nitrogen temperature (LNT). Eo)j ~; N~ is the average coordination number for the Gaussian distribution of distances centered at the R~ value, 0& is the Debye-Wailer contribution, and 4z(k) = 2h(k) +p~(k) is the phase shift, b(k) and p~(k) being the central and backscattering atom phase shifts, respectively. f~(k) is the magnitude of the backscattering amplitude of the jth-neighbor atoin, and k/I'~ is the mean &ee path of the photoelectron. Figure 6shows the Fourier transform of the EXAFS spectra at the Eu Ly~~-edge at RT and at the liquid nitrogen temperature (LNT). These functions are related to the radial distribution function around the absorbing atom, and the main peaks are related to the different coordination spheres of the Eu ions. The positions of the peaks must be corrected by the phase functions C'~(k) to obtain the true distances of the difFerent coordination spheres in real space. We have taken the amplitude and phase functions reported by McKale et al. The peak located at about 2Ais related to the first oxygen coordination sphere and peaks centered at 3.0Aand 3.6Aare related to the Co, Eu, and Ba neighbors. The positions of these two peaks are compatible with the difFraction data, but no fine analysis can be performed because of the very similar distances (3.03, 3.66, and 3.94 A.,respectively) and the complexity of their backscattering amplitude functions. Table III shows the EXAFS parameters that better fit the oxygen-related peaks. The RT values of the distances and coordination numbers have been taken as the ones given by x-ray-diffraction experiments. 24 The Debye-Wailer, the mean free factor, and the energy shift have been fitted. The LNT parameters were obtained by fixing the shift and the I' factor equal to the RT ones. In that way, the analysis gives that the distance to the four oxygens in the XY plane decreases while the other two Eu-0 distances remain constant. This is consistent with the shortening only of the aaxis at low temperature inferred from the frequency decrease observed in the Raman spectra. TABLE III. EXAFS parameters of the best Bt for the oxygen-related peaks at room temperature (RT) and liquid nitrogen temperature (LNT). Temperature Pair EU-O(1) RT Eu-O(2) Eu-O(1) Eu-O(1) Eu-O(2) Eu-O(1) LNT 2.31 2.37 2.48 2.31 2.37 2.44 No. (A) 20.174 10.053 40.177 20.174 10.053 40.177 r(A )aE (eV) 0.47 -14.0 0.47 0.47 0.47 -14.0 0.47 0.47 VI. Eus+ CRYSTAL FIELD TRANSITIONS The environment ofthe Eu ion is shown in Fig. 5, its local symmetry being C2„with the twofold axis parallel to the Eu-O(2) bond (2.372 A). One mirror plane contains this axis and two oxygen ions O(l) at 2.313 A; the other plane is parallel to the paper. The four O(1), which are not on any symmetry element, are at the same distance from the Eu ion, at 2.481 A..The different distances are indicated in Fig. 5. The Eu +&ee-ion levels consist of aFground-state multiplet well separated from the excited states. The levels of the first excited sD multiplet are also far from any other level so that the mixing with other terms is not very important. Therefore, the assignment of the observed lines can be done keeping Jas agood quantum number. The first excited level being Do, the absorption to, or emission from, this singlet is sufficient to obtain the levels of the ground-state multiplet. These levels are the ones used to calculate the crystal-field parameters that describe the Eus+ environment. Because of the C2„local symmetry of the Eu ious, the degeneracy of the free-ion levels must be completely lifted so that (2J +1) levels are expected for each Jterm. In Fig. 7we present the emission spectra detected at 10 Kand 300 K, in the spectral region between 13000 cm and 18000 cm i(770 nm and 555.5nm) when the excitation is achieved with three different lines of an Ar+ laser (476.5, 488, and 514.5nm). No emission peaks are detected between 18000 cm iand 20 900 cm i. At room temperature, the peaks of Fig. 7correspond to crystalfield transitions between 4f levels localized in the Eus+ ious and are quite sharp as it corresponds to emissions between these kind of electronic levels (the width introduced by the experimental setup is around 5cm i). The most intense peaks, around 16000 cm, are characteristic of the Eu +ion and correspond to the Do — +I'2 transitions. The assignment of the observed luminescence peaks has been done taking into account those done for Eu +in Y203 Ref. 26 and in LiNb03, and based on the linewidths and the relative intensities of the observed peaks. Table IV summarizes the energies of the levels that can be deduced from the assignment of the observed lines. Figure 8shows the emissions &om Do to Fp Fy, and E2, in detail at room temperature. In this figure the characteristic increase of the linewidth associated with the electron-phonon interaction is clearly observed. 9164 S.TABOADA et al. 50 ~1+ cA V C II Eu BaCo 2 476.5nm 10K 488 nm 10K 514.5nm RT =-=- D-F 5D -7F lq 4 D-F 514.5nm 10 K D-F 5D 7F 01 D-F than the ground-state multiplet Fand the first excited multiplet D. This limited set of data is rarely sufhcient to determine the large number of parameters (free ion and crystal field) necessary for acomplete description of the Eu +ion inside acrystal. Nevertheless, because 4f electrons are only weakly perturbed by the crystal field, "&ee-ion" parameters introduced by the classical theories must be very similar to their free-ion values. So we can consider that the barycenters of FJ and DJ multiplets are only slightly deviated from their free-ion positions and we can use the spin-orbit and crystal-field terms as the more important ones in the Hamiltonian of Eu +inside acrystal structure. Moreover, it is well known that the levels of the first excited Dmultiplet are very far from the ground-state multiplet (the rFsDo gap is usually larger than 10000 cm i) and so the spin-orbit mixing between Fand Dlevels is not very important. We can estimate that the Fwave functions have apredominantly pure (95%) rF character, as in the ft.ee ion. This fact allows afirst attempt of crystalfield parametrization using, as restricted basis set, the 488 nm RT II TABLE IV. Experimental and calculated values of the lowest terms of Eu +in EuqBaCo05. 16000 18000 Energy (cm ) FIG. 7. Luminescence spectra ofEu2BaCo05 at 300 and 10 Kfor three excitation wavelengths 476.5, 488, and 514.5nm. Term (number of levels) 5D (3) 5D Level position (cm ) Experimental Calculated 18700 (18930-54) 17263 It is well known that the electronic transition from or to (corresponding to absorption or emission) the lowest crystal-field level of one term is the sharpest one and that the linewidth of the transitions to the other crystalfield levels of this term increases with the energy of the levels. 2This is due to the possibility of relaxation to lower levels via spontaneous emission of phonons leading to ashorter lifetime of the higher-energy levels and a consequent broadening in energy. In Table Vwe present the parameters of the Lorentzian curves used to fit some experimental lines that show how the linewidth depends on the possibility ofrelaxation by phonons: The sharpest line corresponds to the transition Do ~Fo where no relaxation of this kind is possible. The linewidth of transitions &om the Do to the three levels of Fq increases with the energy of these Fz levels. The same behavior is observed in the transitions to the "F2 levels (only two peaks have been fitted, the other two being too weak). Therefore we have assigned the 15886 cm peak to a Dq ~F4 transition rather than to aDo — +F2 one because its linewidth (40 cm )is much smaller than the expected one (more than 64 cm ). F3 (5) 3168 3113 3078 2806 2691 2620 2106 2021 1920 1872 1240 1170 951 881 3155 3125 3079 2817 2740 2683 2617 2447 2351 2097 2018 2009 1926 1874 1812 1738 1238 1165 966 872 823 A. Eu~+ crystal-field parameters Although the complete scheme ofthe 4f configuration ofEu +has 3003 ~nSLJM) energy levels, the experimentally obtained set of energy levels seldom extends further ?Q '+o 526 364 221 542 363 208 50 OPTICAL PHONONS, CRYSTAL-FIELD TRANSITIONS, AND. ..9165 Eu28aCo05 RT point charge effects (that is the covalent, multipole, exchange, etc.)are neglected. Because the Eus+ ion is a constitutional ion (not an impurity), this static calculation appears as aquite good first approximation because no local distortion of the europium site is expected. From the expression of the potential energy for the electrostatic interaction between the various atoms ofthe crystal lattice, the crystal-6eld parameters have the form Lorentzian Fit Experinmntal 15800 16300 16800 Energy (cm i) 17300 FIG. 8. Luminescence spectrum ofthe Do to Fo, F~, and Fq transitions at room temperature. The excitation wavelength is 488 nm. JI =B'C' '+B'[C' '+C' ]+B'C' +B [C( )+C( )] +B[C( )+C( )] +BC( ) +B2[C(s) +C(s)] +B4[C(s) +C(s)] +Bs[C(s) +C(s)] in which C~( )are the renormalized spherical harmonic tensors and B& ~are the crystal-field parameters. Initial estimates for the values of B&~ parameters were obtained by direct calculation using the single point charge electrostatic model (PCEM), where all but the TABLE V. Parameters of the Lorentzian curves used to St some of the observed transitions between Eu +4f levels. pure LS-type eigenfunctions for the states of the I' multiplet. For Cz„symmetry, the one-electron crystal-field Hamiltonian, can be written in the Wybourne form:I where gi are the electric charges (in electron charge units), zi =cos8~, and pi, Hi, P~ are the polar coordi nates ofthe atoms surrounding arare earth atom selected as the origin. Instead of the Hartree-Fock (r")HF mean radius values, we use the semiempirically corrected values (r") = (r")HF („" ), which include linear screening and scaling factors for rare-eart¹ion wave functions into asolid. For PCEM calculation reference axes must be chosen so that all the B& ~are real; we have selected astandardized axis set with the zaxis along the twofold axis and z and yaxes as shown in Fig. 5. Convergent values of B& obtained for PCEM calculations are shown in Table VI. Using these apriori parameters, the experimental crystal-field parameters were obtained by diagonalizing the crystal-field Hamiltonian in the restricted LS basis and parameter re6ning with aleast-squares method. As pointed out previously, azero-order approximation is carried out where LS-type eigenfunctions for the states ofthe ~Fg (J=0, 1,...,6) multiplet are considered for the fitting. Since each ~Fg state is (2J+1)-fold degenerate, this approximation ends up with a49 x49 matrix, including full Jmixing. In this truncated representation, the matrix elements of the Cq tensors can be easily calculated from the Wigner-Eckart theorem using the tabulated values of fractional parentage coefficients and Racah Bj and 6j symbols. s4 Only the components ofthe Fg levels with J=0, ...,4 are experimentally found, due to the weakness of the transitions to the I"5 and F6. Out of the 49 I"Jcomponents only 17 can be unambiguously derived &om the Transition Dp -+ Fp 5D TF SD TF Energy (cm ') 17236 17016 16871 16710 16257 16286 15886 FTHM (cm ') 10 20 48 60 32 64 40 Integrated intensity (arb. units) 325 930 1330 1000 14400 6240 1800 Bo B~ B4 Bo +6 ~4 B6 PCEM -2383 -561 -1153 -2536 1838 -417 -554 -523 -670 Fitted values -1266 +32 -458 +11 -1464 +37 -2062 +27 1482 +38 -782 +19 -519 +13 -433 +ll -591 +15 TAHOE Vl. Crystal-field parameters (in cm }of Eu +in Eu~BaCo05. RxH width at half maximum (FWHM).