DSpace at VNU: Origin of the forbidden phonons in Raman scattering spectra of uranium-doped Ca2CuO3, a spin 1 2 chain system

Research Article Received: 10 April 2008 Accepted: 22 July 2008 Published online in Wiley Interscience: 19 September 2008 (www.interscience.wiley.com) DOI 10.1002/jrs.2100 Origin of the forbidden phonons in Raman scattering spectra of uranium-doped Ca2CuO3, a spin 1/2 chain system Nam Nhat Hoang,∗ Thuy Trang Nguyen, Hong Van Bui and Duc Tho Nguyen The Raman scattering spectra of uranium-doped Ca2 CuO3 were investigated The small doping of uranium (≤5%) in this one-dimensional spin 1/2 chain system induced three new first-order scattering bands and two new multiphonon bands in the structure of forbidden phonons The first-order bands were found to agree well with the existing theoretical results from the ab initio and tight-binding calculation Among them, the 470 and 665 cm−1 bands appeared as the basic wavenumbers of which the multiphonon overtones were composed The grain size effect in this strongly anisotropic system was proposed not to originate from the classical phonon confinement but rather as a result of the segmentation of one-dimensional spin chains due to doping, which in turn allowed the new vibrational modes and implied the appearance of higher overtones in the scattering spectra Copyright c 2008 John Wiley & Sons, Ltd Keywords: uranium; phonon; Raman; spin chain Introduction 170 It has been shown that interesting phenomena occur in Cu–Obased low-dimensional systems – the one-dimensional (1D) spin 1/2 chain compounds A2 CuO3 (A = Ca, Sr) These systems exhibit the covalent insulation besides the spin-charge separation – a property that is a key factor for their application in quantum devices.[1] Ca2 CuO3 has a structure that can be derived from the K2 NiF4 -type structure of its superconducting sister compound La2 CuO4 by removing one oxygen atom from the (ab) basal plane The presence of this oxygen creates a two-dimensional (2D) Cu–O network in La2 CuO4 , whereas its absence reduces the dimension of this network to 1D in Ca2 CuO3 (Fig 1(a)) It is known that there is a systematic dependence of the optical charge-transfer gaps between the bottom of the conduction band and the top of the valence band on the dimension of Cu–O network.[2] This dimension influences the number and the valence of the apical ions, which consequently decide the p–d hybridization between the Cu2+ 3dx −y and O2− 2pσ orbitals Both La2 CuO4 and Ca2 CuO3 show clear evidences for a strong electron–phonon coupling, as it was demonstrated that some forbidden phonons are associated with the optical charge-transfer process.[3,4] Owing to its 1D spin exchange, antiferromagnetic Cu–O chain Ca2 CuO3 is expected to provide a model platform for spin transportation Several doping studies have been presented, e.g in the Sr-doped[3] and the U238 doped Ca2 CuO3 [5] Besides the effect of doping on other properties of material, i.e on its dielectric constant, conductivity, life-time of optical decays, and optical transition, these studies also provide an opportunity to look into the modification of the phonon structure, which might in turn shed light on the spin manipulation In this paper, the Raman active optical phonons are investigated for the family of compounds Ca2 CuO3 : Ux (x = 0.0–0.05) Both pure U238 and Ca2 CuO3 are extensively used as the precursors in the preparation of highly homogeneous copper oxide–based superconductors.[6,7] The doping of uranium in Ca2 CuO3 itself is aimed at achieving the specific optical property, besides the J Raman Spectrosc 2009, 40, 170–175 fact that the resistivity of bulk samples can easily be controlled within a specified range while preserving their covalent insulating character.[5] The effect of this doping is essential for direct application of Ca2 CuO3 in devices as a novel material with high dielectric constant The Raman scattering spectra of the uraniumdoped samples differ from the known cases by the presence of several new scattering lines that are intrinsic to 1D chain scattering and not obviously originate from the vibration of a heavy atom such as uranium Their clarification is important for understanding what happens in the spin chain system when chain length varies Experimental Ca2 CuO3 : Ux (x = 0.0, 0.005, 0.01, 0.025, 0.05) polycrystalline samples of size 0.8 cm × 0.7 cm × 0.1 cm were prepared by a modified sol–gel route using citric acid as reagent The efficiency of this method in the preparation of highly homogeneous Ca2 CuO3 powder was demonstrated in Ref and a detailed discussion is given in Ref Here we summarize the main aspects of this technique First, a mixture of the metal nitrates Ca(NO3 )2 , Cu(NO3 )2 , and ammonium diuranate ((NH4 )2 U3 O7 ) was prepared in the required molar ratio Ca : Cu : U = : : x, and then citric acid (C6 H8 O7 ) as reagent was added The solution was heated up to 80 ◦ C until the appearance of a blue gel After drying, the gel was calcinated at 550 ◦ C and a so-called xerogel was obtained, which was then pressed into cylinders and consequently sintered at 870 ◦ C for 24 h to yield the final Ca2 CuO3 : Ux products The ∗ Correspondence to: Nam Nhat Hoang, Center for Materials Science, College of Sciences, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam E-mail: namnhat@gmail.com Center for Materials Science, College of Sciences, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam Copyright c 2008 John Wiley & Sons, Ltd Raman scattering spectra of uranium-doped Ca2 CuO3 (a) (b) Figure The 1D and 2D Cu–O networks in the crystal structures of Ca2 CuO3 and La2 CuO4 These structures may be considered as being composed of the Cu–O layers stacking between the double layers of Ca–O or La–O (a) The calculated grain (nanoparticle) and monocrystallite size according to SEM and Fourier fitting of X-ray diffraction profiles (b) This figure is available in colour online at www.interscience.wiley.com/journal/jrs J Raman Spectrosc 2009, 40, 170–175 in Ref On the other hand, the visible greater rigidity of the b-axis (i.e the Cu–O(2) bonding along 1D chain) was considered as a support argument for the strong electron–phonon coupling along this direction As the forbidden phonons appeared only along the 1D Cu–O chain, it would reasonably suggest that all structural aspects of this chain, including its rigidity, finite length, segmentation, and defects, would have significant impact on the structure of those forbidden phonons Hence, the doping would provide a valuable base for studying the phonons if it would only affect the spin chains while retaining the other aspects of structure This seems to be the case of uranium doping, as the only influence on the structure was a visible reduction in the particle size Both monocrystallite and grain size were sufficiently reduced when the uranium content increased While a reduction in grain size was visible in the scanning electron microscope (SEM) images of surfaces, a refinement of monocrystallite size was shown by the Fourier fitting of peak shape for the strongest peaks in the corresponding X-ray diffraction profiles (Fig 1(b)).[5] Therefore, a systematic shortening of the spin chains due to Copyright c 2008 John Wiley & Sons, Ltd www.interscience.wiley.com/journal/jrs 171 samples were subjected to X-ray diffraction measurement and Rietveld analysis of phases to ensure their high homogeneity.[5] The Raman spectra were recorded using three different excitation lasers: a He–Ne laser (632.8 nm), a Nd : YAG laser (1064 nm) and an argon ion laser (514 nm) The He–Ne laser source was provided with the Dilor-Jobin micro-Raman system LABRAM 1B, the Nd : YAG with the Nicolet 6700 NRX FT-Raman system, and the argon ion laser with the Renishaw Invia Raman Microscope The excitation density of these sources could be varied from 105 W/cm2 down to 10 W/cm2 by using neutral density filters The spot radius was around µm From the structural analysis,[5] all doped compounds crystallized in the Immm space group (no 71) with atoms lying at the following Wyckoff sites: Cu at 2d, O(2) at 2a, Ca and O(1) at 4f While the lattice structure of the samples showed no significant changes upon doping, there was an observable flux of the Ca–O(1) bond distance parallel to c-axis The easier movement of Ca atoms along this axis was also observed in the undoped Ca2 CuO3 [8] and was attributed to the Ag mode phonon observed at 306 cm−1 N N Hoang et al doping is expected This observation also led to the presumption that the doped uranium atoms rather occur at the interstitial areas between the monocrystallite interfaces than substituting for Ca in the lattice.[5] Owing to the valence mismatch between Ca2+ and U6+ , the substitution of uranium in the lattice is not probable This issue is important for understanding the forbidden modes in the Raman scattering spectra of the doped samples: these modes not include the contribution from uranium Results and Discussion 172 The obtained Raman scattering spectra for the uranium-doped Ca2 CuO3 are shown in Fig Some scattering bands appeared only under the argon ion laser excitation [890, 943, and 1390 cm−1 (Fig 2(b))] or under the He–Ne laser excitation [e.g 1337 cm−1 (Fig 2(c))] The bands marked by arrows, which are better visible in the doped samples, correspond to 235, 435, 1003, and 1390 cm−1 These four bands may be practically considered as absent for x = (the first two of them were also not seen for x = in the Sr-doped Ca2 CuO3 [10] ) For the purpose of comparison, the Raman scattering spectra for the Sr-doped Ca2 CuO3 , as obtained from Refs and 10, are given in the inset in Fig Two studies have been previously reported for Sr-doped Ca2 CuO3 by Yoshida et al.[3] and by Zlateva et al.[10] As they used single crystals, the Raman-allowed phonons could be identified According to group theory, the six Raman active modes ( -point phonons, k = 0) in the space group Immm (D25 2h ) are 2Ag +2B1g +2B2g (the other nine modes, 3B1u +3B2u +3B3u , are the IR active) The Raman active modes are associated with the ) vibration of Ca and O(1) (Wyckoff site 4f and site symmetry C2v along the c-axis (Ag ) and a, b-axes (B1g , B2g ) Hence, the Ag mode phonons could be observed in the c(a, a)c and a(c, c)a scattering configurations Indeed, they were the only observed features here, and therefore could unambiguously be associated with the Ca movement (307 cm−1 ) and O(1) movement (530 cm−1 )[3,10] along the c-axis For reasons unknown, no structure due to the B1g and B2g modes was observed The rest of the bands, which appeared only when the light polarization was parallel to b-axis, could not be explained by group theory and were considered as the forbidden modes They include 13 bands: 200, 235, 280, 435, 470, 665, 890, 943, 1003, 1142, 1217, 1337, and 1390 cm−1 Among them, only nine bands were visible for the undoped samples.[3,5,10,11] Considerable effort has been made to identify the origin of these forbidden modes in the undoped Ca2 CuO3 There were two theoretical studies from Drechsler et al.[4] and Hoang et al.:[9] the first one based on the tight-binding approach and the second on the ab initio cluster modeling According to these studies, the forbidden modes are associated with a charge-transfer process and should be regarded as the zone boundary phonons originating from the local distortion in 1D Cu–O chain This assignment well interprets the observed fact that the wavenumbers of the forbidden modes showed only a small variation among the doped and undoped samples, the crystalline and polycrystalline samples, despite differences in chemical content and structure.[3,5,10,11] The coincidence in wavenumbers is apparent especially for a group of higher resonances at 890, 943, 1140, 1215, and 1337 cm−1 (Fig 2), which correspond to 880, 940, 1140, 1200, and 1330 cm−1 in Sr-doped Ca2 CuO3 (the inset, Fig 2).[3] This group could be well interpreted as the overtones: i.e 880 is 440 + 440; 940 is 470 + 470; 1140 is 440+690; 1200 is 500+690 and 1330 is 440+440+440 The similarity continues for the rest of the bands In Sr-doped Ca CuO3 , as Sr was substituted for Ca, an Ag mode shift at around 204 cm−1 www.interscience.wiley.com/journal/jrs should be observed in addition to a 306 cm−1 shift because of the mass effect: υ ∝ (mSr /mCa )1/2 ≈ 1.5 Indeed, a peak was seen around 200 cm−1 besides four other bands at 235, 440, 500, and 690 cm−1 Weak shoulders also appeared at 470, 640, 1000, and 1390 cm−1 All these modes, except at 500 and 640 cm−1 , correspond well to their counterparts in the scattering spectra of uranium-doped samples, i.e 200, 235, 435, 470, 665, 1003, and 1390 cm−1 (Fig 2) Two wavenumbers can easily be recognized as the combination of two phonons, i.e 1000 is 500 + 500 and 1390 is 690 + 690 Yoshida et al.[3] did not consider the three modes 200, 470, and 640 cm−1 and gave no interpretation to them The core group of four bands at 235, 440, 500, and 690 cm−1 (of which the overtones are composed) were ascribed to the in-chain vibration of Cu (235) and the T-point phonons (kx = ky = 0.5, kz = 0) involving O(2) (440, 500, 690).[3] For the uranium-doped samples, forbidden modes among 13 could undoubtedly be considered as the first-order phonons They are 200, 235, 280, 435, and 470 cm−1 , which correspond closely to 211, 231, 288, 440, and 461 cm−1 obtained from the ab initio calculation (Fig 3(b)).[9] According to this result, the first three bands, 200, 235, and 280 cm−1 , follow from the movement of O(2) along the a-axis and some movement of Ca in (b, c) plane Recall that the inclusion of Ca here also explains the widening of 211 cm−1 band toward 204 cm−1 when Sr was substituted for Ca in the Sr-doped Ca2 CuO3 No such widening was seen for the uranium-doped cases The next two bands, 435 and 470 cm−1 , originate from a combined vibration of O(1) and O(2): that is, O(1) along c-axis plus a symmetric movement of O(2) along b To extend this theoretical interpretation for the purpose of identifying the phonon modes in the chains of different lengths, we studied the optical excitation in the ideal chains (Cu–O)n of the singlet spin state for the length up to nm using density functional theory (DFT) with B3LYP (Becke, three-parameter, Lee-Yang-Parr) hybrid functionals The 6-31G basis sets were used For evaluation of the basis set effect, the LANL2DZ (Los Alamos double zeta) and the DGDZVP (DGauss double zeta valence plus polarization) basis sets were also used The calculation was performed using the Gaussian 03 software package[12] without optimization of geometry (Cu–O distance was fixed at 0.189 nm) The results include the following (Fig 3(a)): (1) The Raman activity of a symmetric oscillation of oxygen along the chain direction (420–440, 470–485 cm−1 ) was dominating and showed an exponential growth according to the chain length up to 4.3 nm; (2) A lower activity was seen for the asymmetric movement of oxygen along the chain (504–507, 540–550, and 640 cm−1 ); (3) The stretching movement of a boundary oxygen along the chain caused the appearance of a band at 355–370 or 400–410 cm−1 ; (4) A band relating to the symmetric vibration of Cu along the chain in the static host lattice of oxygen was seen at 240–250 cm−1 ; (5) A small Raman activity induced by a movement of oxygen perpendicular to the chain direction was observed at 200–240 and 280–285 cm−1 ; (6) The other modes corresponded to the bending or stretching of the chain, to the movements of atoms perpendicular to the chain, and to the combined symmetric/asymmetric movements of atoms within the limited chain segments In general, the number of modes increased with the chain length The appearance of the new modes is apparently associated with the local asymmetries of atomic positions in the 1D chain of finite length with respect to electrostatic interaction along this chain (In an infinite chain, the Coulomb field is always equal for all positions, but it is just not so in a finite chain.) Another important factor that may contribute to local asymmetries of long chains is the limited range of magnetic Copyright c 2008 John Wiley & Sons, Ltd J Raman Spectrosc 2009, 40, 170–175 Raman scattering spectra of uranium-doped Ca2 CuO3 Figure The Raman scattering spectra of the uranium-doped Ca2 CuO3 The spectra are offset for clarity The features that are developed according to the uranium content are denoted by arrows The inset shows the spectra for Sr-doped system as reported in Refs and 10 J Raman Spectrosc 2009, 40, 170–175 partly the 470 cm−1 also), were better visible at higher doping concentration This also explains the lower activity of the Ag modes obtained in a cluster Ca18 Cu8 O28 used in Ref 9, which is of dimensions 2a×2b×c/2, in comparison with that of the first-order forbidden modes Following the theoretical results, two first-order bands at 505–512 and 630–640 cm−1 were missing in the uranium-doped Ca2 CuO3 Both these modes were observed in the Sr-doped cases and attributed mainly to the asymmetric movement of O(2), either along the b-axis or in the (a, b) plane, with a combined vibration of O(1) along the a-axis.[9] Instead, the uranium-doped samples showed a band at 665 cm−1 , so it deserves more attention It is worthwhile to mention that this band was not reported for Ca1.8 Sr0.2 CuO3 [3] but for Ca1−x Srx CuO3 (x = 0.0–0.4)[10] (in both cases, single crystals were used) It coincides well with a mode at 669 cm−1 calculated from the tight-binding approach using the Cu–O chain with 3/4 filled bands (Fig 3(b)).[4] Within a frame given in Ref 4, this mode was explained as composing of the symmetric movement of O(2) in a breathing lattice along the c-axis As for Copyright c 2008 John Wiley & Sons, Ltd www.interscience.wiley.com/journal/jrs 173 interaction and the possible formation of 1D magnetic domains Therefore, a mode at lower n may split into several modes at higher n, each of which occurs over a limited chain segment However, for ≤ n ≤ 12 (2.8–4.3 nm), the activity arising from the position asymmetries appeared to be negligible in comparison with that arising from symmetry of the middle positions, so the peaks (5) and (6) were not observed in the spectra The final appearance of the rest of the peaks in the Raman scattering spectra of doped Ca2 CuO3 depends on the ratio of their activity to the activity of the Ag modes As observed from experimental data, the contribution from first-order modes was rather small at the chain length above 22 nm But, as the activity of the Ag modes (scattering along c-axis) depends on the number of unit cells that are available in the monocrystallites along the c-axis, which is about three times longer than the b-axis in Ca2 CuO3 , the activity of these modes is expected to fall faster than that of the first-order forbidden modes when the average monocrystallite size reduces Below 4.3 nm, the forbidden modes should dominate This might be a reason why several forbidden modes, e.g the 235 and 435 cm−1 (and N N Hoang et al (a) (b) Figure The calculated Raman scattering spectra at various chain lengths for a chain model (Cu–O)n (a) and the movements of oxygen in some forbidden modes (b) The illustration shows the modes at 231 and 461 cm−1 according to Ref 9, the 440 (symmetric mode) and 640 cm−1 (asymmetric mode) as obtained from a chain (Cu–O)12 , and the 669 cm−1 according to Ref This figure is available in colour online at www.interscience.wiley.com/journal/jrs 174 Ca1.8 Sr0.2 CuO3 [3] the 690 cm−1 band was observed instead of 665 cm−1 , Drechsler et al.[4] have misstated that the calculated 669 cm−1 mode corresponded to this 690 cm−1 band According to the analysis given in Ref 8, there was a considerable amount of impure phases, CaO, CuO, and CaCu2 O3 , in the Ca2 CuO3 samples prepared by the ceramic and oxalate precipitation technique (∼5%) Therefore, the 690 cm−1 nd might have its origin in these impurities The appearance of the 665 cm−1 mode as a first-order mode is a very specific feature of the uranium-doped samples and, as we discuss later, it contributes largely to the www.interscience.wiley.com/journal/jrs observed multiphonon bands Unlike the Sr-doped cases,[3] the multiphonon bands here cannot be composed of the two modes 500 and 690 cm−1 as they are absent; therefore, the assignment will differ from the known cases First, the lines 235, 470, 943, and 1390 cm−1 seem to imply the series of wavenumbers (1f, 2f, 4f, 6f) or, in a more accurate match, 943 and 1390 cm−1 appear as the second- and third-order phonons of 470 cm−1 The 1337 cm−1 line also corresponds well with a second-order phonon of 665 cm−1 , and the 890 cm−1 line can be considered as both 435 + 435 or 435 + 470 Complication Copyright c 2008 John Wiley & Sons, Ltd J Raman Spectrosc 2009, 40, 170–175 Raman scattering spectra of uranium-doped Ca2 CuO3 Conclusions The effect of uranium doping on the structure of forbidden phonons in the quantum spin chain system Ca2 CuO3 is believed to be associated with the smaller segmentation of the 1D spin chain due to doping The grain size effect in this strongly anisotropic system originates rather from the intrinsic vibrational modes of the spin chain than from the classical phonon confinement Theoretically, the shortening of the chain may introduce many new modes, but among them only the modes associated with the symmetric movement of oxygen in the direction perpendicular to the chain (235 cm−1 ) and along the chain (435, 470, and 665 cm−1 ) were observed The modes relating to asymmetric movement of oxygen, which might be seen at 504–507, 540–550, or 640 cm−1 , were absent The reason for this might lie in their low activity in the uranium-doped samples In the other known cases, e.g in the Sr-doped samples, some modes were really observed Acknowledgements Figure The Raman scattering spectrum for a binary xerogel (CaO)2 –CuO in the early sintering state The dots show the phonons originating from the Ca2 CuO3 phase The inset shows the X-ray diffraction profile with Rietveld refinement of phases for the same sample after sintering according to Ref J Raman Spectrosc 2009, 40, 170–175 References [1] R Neudert, M Knupfer, M S Golden, J Pink, W Stephan, K Penc, N Motoyama, H Eisaki, S Uchida, Phys Rev Lett 1998, 81, 657 [2] Y Tokura, S Koshihara, T Arima, H Takagi, S Ishibashi, T Ido, S Uchida, Phys Rev B 1990, 41, 11657 [3] M Yoshida, S Tajima, N Koshizuka, S Tanaka, S Uchida, S Ishibashi, Phys Rev B 1991, 44, 11997 [4] S L Drechsler, J Malek, M Yu Lavrentiev, H Koppel, Phys Rev B 1994, 49, 233 [5] N N Hoang, D C Huynh, T T Nguyen, D T Nguyen, D T Ngo, M Finnie, C Nguyen, Appl Phys A 2008, 92, 715 [6] S X Dou, S J Guo, H K Liu, K E Easterling, Supercond Sci Technol 1989, 2, 308 [7] (a) R Weinstein, US Patent 6083 885, Method of forming textured high-temperature superconductors, 2000, 4; (b) R Weinstein, R P Sawh, Supercond Sci Technol 2002, 15, 1474 [8] D C Huynh, D T Ngo, N N Hoang, J Phys.: Condens Matter 2007, 19, 10625 [9] N N Hoang, T H Nguyen, C Nguyen, J Appl Phys 2008, 103, 093524 [10] G A Zlateva, V N Popov, M Gyulmezov, L N Bozukov, M N Iliev, J Phys.: Condens Matter 1992, 4, 8543 [11] Ya S Bobovich, V N Denisov, B N Mavrin, T I Chuvaeva, Opt Spectrosc 2000, 89, 407 [12] M J Frisch, G W Trucks, H B Schlegel, G E Scuseria, M A Robb, J R Cheeseman, J A Montgomery, Jr., T Vreven, K N Kudin, J C Burant, J M Millam, S S Iyengar, J Tomasi, V Barone, B Mennucci, M Cossi, G Scalmani, N Rega, G A Petersson, H Nakatsuji, M Hada, M Ehara, K Toyota, R Fukuda, J Hasegawa, M Ishida, T Nakajima, Y Honda, O Kitao, H Nakai, M Klene, X Li, J E Knox, H P Hratchian, J B Cross, C Adamo, J Jaramillo, R Gomperts, R E Stratmann, O Yazyev, A J Austin, R Cammi, C Pomelli, J W Ochterski, P Y Ayala, K Morokuma, G A Voth, P Salvador, J J Dannenberg, V G Zakrzewski, S Dapprich, A D Daniels, M C Strain, O Farkas, D K Malick, A D Rabuck, K Raghavachari, J B Foresman, J V Ortiz, Q Cui, A G Baboul, S Clifford, J Cioslowski, B B Stefanov, G Liu, A Liashenko, P Piskorz, I Komaromi, R L Martin, D J Fox, T Keith, M A AlLaham, C Y Peng, A Nanayakkara, M Challacombe, P M W Gill, B Johnson, W Chen, M W Wong, C Gonzalez, and J A Pople, Gaussian 03, Revision B.03, Gaussian: Pittsburgh, 2003 Copyright c 2008 John Wiley & Sons, Ltd 175 occurs only in the interpretation of the 1003 and 1217 cm−1 bands The simplest forms lead to 1003 = 280 + 280 + 435 and 1217 = 470 + 470 + 280 Hence, these two wavenumbers are not composed of the same wave vectors and imply the violation of the conservation of momentum This situation points back to the finite size of nanocrystals and shortening length of Cu–O chains in the doped polycrystalline Ca2 CuO3 As the chains shorten, there is more chance for the higher wavenumbers to appear in the spectra of the doped samples (the same as the case with acoustic devices when the shortening of a string makes higher tones) This explains why the 1003 and 1390 cm−1 bands are more visible at higher doping concentration So it provides a good example of the grain size effect on the Raman scattering spectra of Ca2 CuO3 , especially when the phonon confinement does not bring observable changes to half-widths and intensities of the major scattering bands The phonon confinement model is known to neglect the surface/boundary phonons and is valid only for uniform grain size and shapes, so is not suitable for cases where strong anisotropy takes place The supporting argument for this interpretation is provided in Fig 4, where the Raman scattering spectrum for the binary (CaO)2 –CuO xerogel in the early sintering state is shown Among the peaks that correspond to the nonreacting CaO, CuO, and middle products, all highorder phonons originating in the Ca2 CuO3 phase appear in the spectrum but, to our surprise, there was none for the first-order phonons except for the features at 470 and 665 cm−1 There is no clear evidence for the 235, 500, or 690 cm−1 bands, e.g for the generators in Ref It is possible that the 280 cm−1 line is also present in the spectrum but is overlapped by the peak at 290 cm−1 that corresponds to the nonreacting CuO phase Therefore, Fig clearly argues for the essence of the 470 and 665 cm−1 bands (and possibly the 280 cm−1 too) as the true basic wavenumbers of all observed overtones The authors would like to thank Vietnam National University, Hanoi, for financial support (Project No QG-07-02 (2008)) and help during the completion of this work www.interscience.wiley.com/journal/jrs ... observed in the spectra The final appearance of the rest of the peaks in the Raman scattering spectra of doped Ca2 CuO3 depends on the ratio of their activity to the activity of the Ag modes As observed... the appearance of a band at 355–370 or 400– 410 cm 1 ; (4) A band relating to the symmetric vibration of Cu along the chain in the static host lattice of oxygen was seen at 24 0 25 0 cm 1 ; (5) A. .. Raman Spectrosc 20 09, 40, 17 0 17 5 Raman scattering spectra of uranium-doped Ca2 CuO3 Conclusions The effect of uranium doping on the structure of forbidden phonons in the quantum spin chain system