PHYSICAL REVIEW B VOLUME 49, NUMBER 17

Optical phonons in NdzaaMO5 (M =Zn, Cn)

1 MAY 1994-I

M. V. Abrashev, G. A. Zlateva, M. N. Iliev, and M. GyulmezovFaculty ofPhysics, Softa Uniuersity, BG 112-6Sofia, Bulgaria

(Received 12 July 1993)

We report the polarized Raman spectra of Nd2BaZn05 and the nonpolarized infrared-absorptionspectra of Nd2BaM05 (M=Zn, Cu). Thirteen out of a total of fifteen Raman-allowed modes(3A &~+28&g+4B&g+6Eg ) were observed. The optical-phonon assignment was made using molecular-site group analysis, taking account of the fact that the Zn04 (Cu04) groups are relatively isolated. Inspite of the different shapes of Zn04 (tetrahedron) and Cu04 (square) units resulting in different spacegroups of Nd2BaZn05 (l4/mcm) and Nd2BaCu05 (P4/mbm), a comparison between the modes in thetwo structures is done. We argue that the high-frequency bands in ir spectra involve mainly vibrationsof Nd-plane oxygen atoms.

INTRODUCTION

Compounds with chemical formula L1BaMOs (L rareearth, M=Cu, Zn} belong to three different structuraltypes. One was reported by Michel and Raveau. ' It isorthorhombic [space group Pbnm, Z=4 (four formulaunits in the unit cell}], contains isolated MOs square-pyramidal units, and exists for I.=Sm-Lu and Y. Laterit became known as the "green phase" in studies of Y-Ba-Cu-0 superconductivity ceramics. The other twostructures exist for L =La,Nd. They are tetragonal andhave isolated MO4 units. For M =Cu, the MO4 units aresquares and the space group of the structure isP4 jmbm, Z=2 [see Fig. 1(a)]. For M=Zn, the MO4units are tetrahedra, the c axis of the lattice is double,and the space group of the structure is l4imcm(Z =4}[see Fig. 1(b}]. In both structures, the shape and symme-try of the L-O(1) layers between the M[O(2)4]-Ba slabsare the same. The similarity between these two struc-tures justifies their parallel study.

Nonpolarized Raman spectra of Nd2BaCu05 were re-ported by Loo, Burns, and Xidis without discussion.Both nonpolarized Raman and ir-transmission spectra ofthe same compound were presented by Baran et al. and

tentative assignment of the vibrations of the Cu04 unitswas given on the basis of the irreducible representationsof a square-planar D4& molecule. In a previous paper'we presented the polarized Raman spectra of this com-pound and discussed the mode assignment in the frame-work of molecular-site group analysis. In this work weuse the same approach to analyze the polarized Ramanand ir-transmittance spectra of Nd1BaMOs (M=Zn, Cu}.The model used predicts the existence of Davydov's pairsof modes, both of which are Raman active forNd2BaCu05 and one of which is Raman active and one iractive for Nd2BaZn05. The analysis allows us to deter-mine approximately the frequencies of some of the ir-active modes of Nd1BaMOs (M =Zn, Cu). The observa-tion of the corresponding bands in the ir spectra of thesetwo compounds can be consistently explained by phononmodes involving motion of the identical Nd-O(1) layers inboth structures.

EXPERIMENTAL

The Nd1BaZn05 samples were prepared by mixing thestarting components (Nd101, BaCO1, and ZnO) in the ap-propriate ratios and calcining in air at 900'C for 17 h.

~() n

o~~~o0—Koo 0

C'o

o~ o oo)II-Ik (II

~l Q

O—~ Ba,cu(sn)

()4—0(2) -+

Nd, o(1)m O O())

o )() FIG. 1. Structure of (a) Nd2BaCu05 and (b)

Nd2BaZn05. For better comparison two adja-cent cells along c for Nd2BaCu05 are given.

. a

(b)

0163-1829/94/49(17)/11783(6)/$06. 00 49 11 783 1994 The American Physical Society

11 784 ABRASHEV, ZLATEVA, ILIEV, AND GYULMEZOV 49

The resulting materials were ground and pressed into 1-gpellets. To obtain ceramics with grains of sufBcient sizefor micro-Raman measurements (about 5 pm), the pelletswere annealed at temperatures from 1000 to 1150'C forseveral days. The synthesis of Nd28aCu05 was similar,as described elsewhere. '

The purity of the samples was tested with a PhilipsSEM 515 scanning electron microscope, equipped withan EDAX 9100/72 energy-dispersive x-ray spectrometer.Measurements of the chemical content of the investigatedmicrocrystals confirmed that the ratio Nd:Ba:Zn(Cu) is2:1:1.

The Raman spectra were measured at room tempera-ture using a triple multichannel spectrometer Microdil 28(DILOR) equipped with an optical microscope. A 100Xobjective was used to focus the incident laser beam in aspot of about 1 pm diameter on the surface of the micro-crystals and to collect the backward-scattered light. The488.0-nm Ar+ laser line was used for excitation. The irtransmission spectra from CsI pellets containing 1% byweight NdzBaMO5 (M=Zn, Cu) powder were obtainedusing a Fourier transform infrared spectrometer (BomemDA3).

RESULTS

First we present the classification of the I -point modesbased on factor-group analysis. NdzBaZn05 crystallizesin the tetragonal l4lmcm (D4i, ) (Z=4) structure, "whereas Nd2BaCuO& crystallizes in the P4lmbm (D4& )

(Z=2) structure. The site symmetry of the atoms, thenumber and symmetry of their I -point modes, and thesymmetry-allowed directions of vibrations for both struc-tures are given in Table I. It is seen that Nd, O(1), andO(2) atoms in both structures have identical site symme-try and types of modes. The Ba and Cu atoms in

NdzBaCu05 are located at the center of symmetry andcontribute only to ir-active modes, whereas the Ba andZn atoms in Nd28aZnO~ can give rise to ir- as mell asRaman-active modes. In order to identify the symmetryof the Rarnan lines, we measured polarized Raman spec-tra from three difFerently oriented surfaces of the micro-crystals, namely, zx, zx', and xy(x'y') (z, x, y, x', andy'refer to the [001], [100], [010], [110],and [110]crystallo-graphic directions, respectively). The A, modes shouldappear in parallel zz, xx (yy ), and x'x' (y'y') scatteringconfigurations, E in crossed zx and zx', B, in parallelxx (yy) and crossed x'y', and 82g in parallel x'x'(y'y')and crossed xy geometry. Note that even if one cannotdistinguish the [100] from the [110] direction, the B,sand B2g modes can be separated into two groups, withoutspecifying which group contains B&g and B2g modes. Inour case, the number of B, and Bzg modes, however, isdifferent (28~g and 482') and thus the assignment be-

comes unambiguous.The polarized Raman spectra of Nd2BaZn05 as ob-

tained from the zx, zx', and xy surfaces are shown inFigs. 2 and 3. %e observed all three A& and six Emodes, but only one B&g and three B2g modes. In an at-tempt to assign the Raman lines to definite atomic vibra-tions we begin with those of Ba and Nd. The Ba atomsparticipate in only one Raman-active E mode (vibrationin the xy plane). As the Ba atoms have the lowestcharge-mass ratio in the unit cell, we tentatively assignthe lowest of the Raman lines (at 79 cm ') to their vibra-tions. For identification of the Nd modes one can takeinto account the similarity between the Nd environmentsin Nd2BaZn05 and Nd2BaCuO&. For R 28aCu05(R =La,Nd) all four modes of La (Nd) atoms have beenobserved, as follows: A, mode at 154 (152), E at 173,

B&g at 196, and B2g at 231 cm '. ' The same four Nd

TABLE I. Normal modes in Nd28aMO5 (M =Cu, Zn).

BaNd

2b

4gC4A

C2„

Cu0(1)0(2)

2c2a8k

C4c,"

VA'ckofF SiteAtom notation symmetry Normal modes

Nd28aCuOg, P4/mbm(D4A ),A I„+A2u+2E„A ]g + A2g + A2 +B]g

+B,„+B2g+Eg +2E„A2u+BI +2EA, u+ A2„+2Eu2A, g+ A iu+ A2g+2A2u+B&g

+2Blu+2B2g+B2u +3Eg+3E„

B)g

X,y X ,y X ,y

xy planexy plane

zx' plane x',y' zx' plane all

z xy planez xy planezx' plane all

Symmetry-allowed directions of Raman andir-active vibrations

Alg B2g A2u

a=6.7015 A, c=5.8211 A, Z=2'

BaNd

Zn0(1)0(2)

4a8h

4b4c161

D4C2,

D2C4ACd

xy planexy plane

xy plane zz zX,y

xy plane z xy planez xy planezx' plane allzx' plane x',y' zx' plane all

Nd2BaZn05, I4/mcm(D4&), a=6.747 A, c=11.537 A, z=4A2g + A2u +Eg +Eu

x',y' x',y'+Blu +B2g +Eg +2Eu

zA lu+ A2u+2Eu2A Ig+ A )„+A2 +2A2„+BI

+2BI +2B2g+B2u+3Eg+3Eu

'Reference 5.Reference 7.

49 OPTICAL PHONONS IN Nd, BaMO5 (M =Zn, Cu) 11 785

F. R+glLER F 8 g g QA

I

200I

400I

600

Raman shift (cm-')

FIG. 2. Polarized Raman spectra obtained from an x'z (110)surface (the top three spectra) and from an xz (010) surface (thebottom spectrum) of Nd2BaZnO& microcrystals. The symmetryof the lines is indicated.

300 500 700

Raman shift (cm-~)

FIG. 3. Polarized Raman spectra obtained from an xy (001)surface of Nd2BaZn05 microcrystals. The positions of the Eglines forbidden for these polarizations are indicated by asterisks.

modes are allowed for NdzBaZn05, too. Thus, it is natu-ral to assign the 149- and 164-cm ' lines in theNd2BaZn05 spectra to Nd A,s and Eg modes [the weakshift towards lower frequencies can be explained by theslight increase of lattice parameter I in the zinc com-pound compared to a in the copper compound (see Table

I)]. We could not observe, however, the 8, and Bzmodes of Nd in the NdzBaZn05 spectrum near their ex-

pected frequencies. Thus, we suppose that all remainingmodes observed in these spectra originate from Zn04 vi-

brations.The assignment of the O(2) and Zn modes meets with

definite difficulties due to their large number, possiblemixing, and weaker limitations for the shape of thesemodes from group-theory considerations [in particularfor the three E modes of O(2)]. In order to avoid thesedifficulties we used molecular-site group analysis, i.e., anapproximation that regards the normal modes of O(2)and Zn in a Nd2BaZn05 crystal as normal modes in a freeZn04 molecule. Such an approach has been used byBaran et al. for assignment of the optical phonons inNd2BaCu05. In the latter study, however, there was alack of information on the symmetry of the observed linesand on the correlation between the modes in a free Cu04molecule and modes of Cu and O(2) atoms in the crystal.This approximation has also been used by Popovicet al. ' to analyze the phonons in the Y2BaCu05 "greenphase, " considering the Y205 unit as an isolated mole-cule. In a recent study of vibrational properties ofBi2Cu04, ' which also contains isolated Cu04 squareslike Nd2BaCu05, the results for the shapes and frequen-cies of the normal modes were obtained using lattice-dynamical calculations (on the basis of a rigid-ion model).The results for the shapes of Cu and 0 modes in

Bi2Cu04, however, many excellently be interpreted usinga molecular-site group analysis alone.

The shapes of the normal vibrations of a free Zn04 unitwith Td symmetry and of a Cu04 unit with D4& symme-

try are given in Fig. 4. The correlations between the vi-brations of the free M04 molecule and the normal modesoriginating from M and O(2) atoms in both structures, asobtained using the techniques developed in Refs. 14, 15,are given in Table II. Placed at a crystal site of lowersymmetry, the degenerate vibrations of the free M04molecule split. This splitting (Bethe splitting) may belarge, depending on the anisotropy and intensity of thecrystal field at the site. One has to take into account alsothe directions of vibrations. As the primitive cell con-tains two M04 units, to each mode of the M04 moleculecorrespond two MO4 modes in the crystal (Davydovpartners). These modes, which are identical for eachM04 unit if considered separately, difFer in the phase ofvibration in two adjacent units. The splitting of thesemodes is very small. It depends on the correlation be-tween two adjacent units, placed on equivalent sites andis known as the correlation field or Davydov splitting.In the case of Nd2BaCu05 the Davydov partners havebeen found within experimental error to have the samefrequencies. '

The line with the highest frequency in the Raman spec-trum of Nd2BaZn05 is at 639 cm ' and is of A

&gsymme-

try. We assign it to the Zn-O(2) fully symmetric stretch-ing vibration (A& breathing mode) of ZnO~ units. InNd2BaCu05 the corresponding line was observed at 634cm '. In Bi2Cu04 the same vibration was found at 584cm ', ' i.e., the frequency of this mode is the highest

11 786 ABRASHEV, ZLATEVA, ILIEV, AND GYULMEZOV

Qi

Q/

B2g

E„

E„

0

0

00

0

B1

02 02

FIG. 4. Normal modes of free ZnO& (Td symmetry) and freeCu04 (D41, symmetry) molecules. All distances (A) of MO4(M=Zn, Cu) units in Nd2BaMO& crystals are given from Refs.7,11.

among the internal vibrations of the MO4 unit.In the range 291—607 cm ' one observes six 1ines

(1A &g, 1B&s, 2Bzs, and 2Es ). According to Table II, theremaining three internal modes of the Zn04 moleculeyield three Raman-active pairs: A

&g +B, originatingfrom the doubly degenerate E bending molecular mode,and 2(Bzs+E ) from the two triply degenerate Fz

modes, one of them mainly antistretching and the othermainly bending (see Fig. 4). Using these correlations, weassign the 607-cm ' (Bz ) and 493-cm ' (E~) modes tothe internal I'2 antistretching vibration, the 361-cm(Bzz) and 291-cm ' (Ez) to internal Fz bending vibra-tion, and the 368-cm '

( A&

) and 247-cm ' (B&g ) modes

to the internal E scissorslike vibration of the Zu04 units(the same scissorslike vibration in NdzBaCu05 was foundat 410 cm '). For the two (Bzg+Eg) pairs, the Bzgmodes (involving vibrations of Zn along z ) have higherfrequency than the E counterparts (involving vibrationsof Zn in the xy plane, see Table I). This can be explainedby the fact that the Zn04 tetrahedra are elongated in thez directions (see Fig. 4). Similarly, the A

&gmode is hard-

er than the B,g mode (in the former mode the varying an-gle 0-Zn-0 is smaller than in the latter mode).

Modes originating from translational and rotationalmotions of Zn04 molecule are expected at lower frequen-cies. We assign the lines at 179 cm '

(Bzg) and 101cm (Ez), to the two Raman-active modes originatingfrom translations of Fz symmetry, one Bzg (along z ) andone Eg (in the xy plane). Again, as for the internal vibra-tions, the frequency of Zn04 vibrations along z is harderthan of the vibration in the xy plane (the stronger atomicbonding along the z direction correlates with the fact thatthe crystals grow in a needlelike shape along the c axis).We assign the remaining Raman line at 219 cm (Eg ) tothe librational mode of Zn04 units (arising from the rota-tional F, mode of Zn04 around an axis lying in the xyplane).

For analysis of the ir-transmittance spectra ofNdzBaMO& (M =Zn, Cu) we note that in both structuresthe atoms of the Nd-O(1) layers have identical site sym-metry and types of modes. Six modes (2Az„and 4E„)correspond mainly to O(1) and Nd motions. Thus, oneexpects that in the ir spectra of both compounds there

TABLE II. Correlation table connecting the modes of free Zn04 (CuO)4) units and modes of Zn (Cu) and O(2) atoms in the crys-tals (int. , rot. , and tr. indicate internal, rotational, and translational modes, respectively).

Symmetry offree unit

Td

Sitesymmetry

D2d

Nd, BaznO,Crystal

symmetryD4

Observed lineRaman ir(cm ')

Symmetry offree unit

D4~

Sitesymmetry

D2h

Nd, BaCu05Crystal

symmetry

D4~

Oberved lineRaman ir

(cm ')

int Al

int F2

int E

int F2

rot Fl

tr F2

int Al

int 82int Eint Alint Blint 82int E

rot A2rot E

tr 82tr E

Aq„+82g+E

A lg +Bl„A l„+BlA q„+82gEg+E„

607493368247361291

=610

=375?

A2g +82„Eg+E„ 219 223

A2„+82gEg +E„

179101

A &g +B&u 639 int Algint B,gint E„int B,g

int E„int A2„int 8,

„

rot Eg

rot A2gtr E„tr A2„

int Agint 82gint 83„int Bz„int Ag

int 83„int 82„int Bl„int A„rot 83grot 82grot Blgtr 83„tr 82„tr Bl„

A lg+82gEgA,„+8,

„

A, g +82

A2„+81 „

A l„+82„A2g+BlgEg

A,„+8,„

634572

410

??436

=564=514

=350=350230

?167?

OPTICAL PHONONS IN Nd2BaMO, (M =Zn, Cu) 11 787

will exist six corresponding bands. Comparing the spec-tra obtained (see Fig. 5), we suggest that these corre-sponding bands are at 581, 404, 327, 269, 192, and 146cm ' for Nd2BaZu05 and at 594, 425, 352, 287, 211, and157 cm ' for NdzBaCuO&.

For identification of the ir-active modes arising fromthe M04 motions we will make use of two assumptions.First, as all ir-active modes of the Zn04 units areDavydov partners of Raman-active modes (see Table II),it is natural to expect that their frequencies are close.Second, although the Cu04 and Zn04 molecules are ofdifferent shape, they have two similar (antistretching andbending) internal modes (see Fig. 4}: two E„for theCu04 and two Fz for Zn04. Thus, the ir-active modesarising from these vibrations must be close in frequency.

The arrows in Fig. 5(a) shows the positions expectedfor the ir-active modes of Zn04 units (provided they havethe same frequencies as their Raman counterparts). Onlythe 223-cm ' band can definitely be assigned to the E„li-brational mode. The weak bands near the 361- and 607-cm ' Raman frequencies may tentatively be assigned tothe Az„modes (bending and antistretching) of Zn04. Asa whole, following this scheme of assignment one cancome to the conclusion that all the bands of internalZn04 ir-active modes are very weak.

In the case of Na2BaCu05 [Fig. 5(b)] the splitting be-tween A2„and E„modes, arising from the internalmolecular vibrations, should be weaker, as the deforma-tion of the Cu04 square is less strong than the deforma-tion of the Zn04 tetrahedron (see Fig. 4). The two weakbands above 500 cm ' could be assigned to the E„andA z„antistretching modes, whereas the E„andA z„bend-ing Zn04 modes probably lie around the 352-cm Nd-O(1}band, resulting in its broadening. The two clearly pro-nounced bands at 230 and 167 cm ' can tentatively beassigned to the E„(internal A2„)and E„(translationalE„}modes, which do not have analogues in theNd2BaZn05 structure.

It should be noted that our assignment of the ir-activemodes (in particular for these at 594, 425, and 287 cmfor Nd2BaCu05 and the corresponding ones at 581, 404,and 269 cm ' for NdzBaZnOs) differs from that proposedin earlier reports. ' In those papers a comparison be-tween the ir-spectra of Nd2BaCu05 and NdzCu04 (Ref.17) was used and the modes were assigned to internal vi-brations of the Cu04 units. The structures of these com-pounds however, are rather different from each other(isolated Cu04 units and Nd-0 single layers in

Iv

~ W

100 3001

500 700

Wave number (cm-')

Nd2BaCu05, and infinite Cu02 layers and Nd-02-NdCaF2-type layers in Nd2Cu04} and in our opinion such acomparison is inappropriate.

In summary, the polarized Raman spectra ofNd2BaZn05 and ir-absorption spectra of Nd2BaMO,(M=Zn, Cu) were measured and the symmetry ofRaman-active phonons was determined. Based onmolecular-site group analysis and using the similarity be-tween the two structures, the origin of all the observedoptical phonons was analyzed.

ACKNOWLEDGMENTS

We thank E. Dinolova for EDAX measurements andG. Bogachev and L. Georgieva for their technical help.We are grateful to V. G. Hadjiev for valuable discussions.This work was supported in part by Grant No. F-1/1991(2001 NIS) of the Bulgarian National Foundation for Sci-ence.

FIG. 5. ir-transmission spectra of (a) Nd2BaZn05 and (b)Nd&BaCuO&. The bands assumed to correspond to the modes ofO(1) and Nd in the two structures are connected with lines. Thearrows indicate the frequencies of the Raman-active modeswhich have ir-active Davidov partners.

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