Mat. Res. Bu l l . , V~ . 19, pp . 849-855, 1984. Pr in ted in the USA. 0025-5408/84 $3.00 + .00 Copyr igh t (c) 1984 Pergamon Press Ltd.

COPPER DOPED ZINC OXIDE Y2BaZnO 5 : ERS AND OPTICAL INVESTIGATION

Claude Michel and Bernard Raveau Laboratoire de Cristallographie, Chimie et Physique des Solides, L.A. 251

ISMRA-Universit~, 14032 Caen Cedex, France

(Received March 15, 1984; Refereed)

ABSTRACT In copper doped Y2BaZnO5 oxides, copper exhibits a distorted square py- ramidal coordination which is consistant with the values of g and A ten- sors obtained from 0 band ERS spectrum for a sample containing about

= - -_ = - _ = ~ . %'-.

The electronic spectrum shows three bands at I|700, 145OO and 20500 cm -l which can be assigned to the transitions A I + BI, B 2 + B I and E ÷ B I respectively. The orbital reduction parameters are calculated and the bondin~ covalency is discussed.

INTRODUCTION

In a recent paper (1) a series of zinc oxides Ln2BaZnO 5 were synthesized in which Zn 2+ ions exhibit a distorted square pyramidal coordination. This unusual coordination of Zn 2+, was confirmed by the infra-red and electron spin resonance (ESR) study of the solid solution Y2BaZnl_xCUxO5 . The ESR X band spectra of these oxides were characterized by a hyperfine structure typical of individual Cu 2+ ions in axial symmetry ; the experimental values of g and A tensors are consistent with those observed for square pyramidal complex~ (2-6). However, the interpretation of the perpendicular part of the spectrum was dif- ficult in spite of the existence of a hyperfine structure. It was not possible to decide between one unique gl value and two very close glvalues. The presence of extra peaks resulting from the large anisotropy of the Spin-Hamiltonian tensors (7-10) or of forbidden transitions induced by important nuclear quadrupole interaction (If-12) could not be ruled out. Q band can be a useful tool for the accurate determination of g~ because extra singularities and second order peak-shifts are expected to be less important at Q band frequencies. In addi- tion, the different components are better separated in Q band spectra than in X band spectra and it is easier to detect a deviation from tetragonal symmetry. This paper deals with the study of the O band spectra of copper doped V2BaZnO 5 samples in order to interprete the behavior of copper in an approximate, square pyramidal coordination. In order to determine the orbital reduction parameters the d-d transitions were studied from the diffuse reflectance spectra.

EXPERIMENTAL

Compositions in the range | %-10 % Cu have been synthesized by heating

849

850 C. MICHEL, et a l . Vol. 19, No. 7

mixtures of Y203 , ZnO, CuO and BaCO 3 in platinum crucibles, in air, fir6t at 9OO°C for 12 hours, then at ]lOOOC for 24 hours and finally by quenching to room temperature. The pureness of powders has been inspected by means of X- ray diffraction patterns.

Q band ESR spectra were recorded with a Varian spectrometer V45OO (f = 34.8 GHz) for room temperature. The Varian '!strong pitch" was used as a standard and was found to give a signal at H = 12420 Gauss. A Beckman Spec- trometer 5240, supplied with a sphere for diffuse reflectance, was used to ob- tain the NIR-VIS spectra. The compound V2BaZnO5 was used as a standard.

RESULTS AND DISCUSSION

ESR : The ESR spectrum of a powder sample containing about ] % Cu was recorded zn Fig.)]a, and the only perpendicular region with an extended field scale in Fig. lb. The parallel and perpendicular signals are well separated and each

I i ! 11000 ~,

/ / I J

t

b

a2,gl

t

t

, , > 12000

H ( G a u s s )

a ; !

12200H(Gauss)

i !

I I i

FIG. 1

g=: 2,0028

a) Q band ESR spectrum of 1% Cu doped Y2BaZnO 5 at room temperature, with~mpli- fication of the parallel signal showin~ the satellites due to the uJCu isotope (arrows). b) Perpendicular region with extended field scale : solid lines experimental spectrum; dotted line : calculated spectrum (half width at half height : 6C).

Vol. 19, No. 7 ZINC OXIDES 851

signal is composed of four peaks due to hyperfine interactions. The gff and IAffl values can be easily calculated ; they are equal to those previously found from the X band spectrum (1) : gff = 2.275 and ~HI = 147.5 IO -4 cm -I. The peaks due to the 65Cu isotope can be also easily observed on the magnification of the parallel signal. Contrary to the X band spectrum (Fig. 2), the Q band

H (GBuss)., 3 410() I 28100 l 3010 0 l 32100

s

L

q3

i I

Ii £

U

/

I

i i II II

FIG. 2

X band spectrum of 1% Cu doped Y2BaZnO5 a t room tempera ture ( s o i i d e I i n e ) and calcula~e_d spectrum wi th the r e s u l t s of Q band (do t t ed l i n e ) , The c o n t r i b u t i o n of the v~Cu i s o t o p e was not in t roduced in the c a l c u l a t i o n s .

one shows a perpendicular line which is only composed of a set of four peaks leading to one g_L value or two very close g_l values as it was previously assu- medo Computer calculations of theoritical spectra give the best fit for two gL and A±values with the following values :

= I -l gl = 2"0495' g2 2"O515' I AII = 13 In -4 cm- , I A21 = 10 1~ -4 cm

Theoretical X band spectrum, calculated with these values but omitting second order terms and nuclear quadrupole interaction is shown in Fig. 2. The most important feature is that two peaks are missing in the theoritical spectrum with regard the experimental one. This explains that no good fit could be obtained in our previous study (1). Since g] = g2, it results from ESR that the pyramidal CuO 5 polyhedron is probably sllghtly less distorted than it ap- pears from the X-ray study and its geometry'is probably not very far from a

852 C. MICHEL, et a l . Vol . 19, No. 7

a

E

'B 1 Cd x2.y2 )

,A 1 (.d z2)

~>ry ..B2C.dxy )

~xz,yz

b -E (dxz,dyz)

FIG. 3

a) Square pyramidal geometry with notations used to d -d tions.

b) Energy levels in crystalline field of C4v symmetry.

transition calcula-

square pyramid (C4v symmetry). If oxygen atoms lie near the x, y, z axes (Fig. 3a), the splitting of the 3d copper orbitals by the crystal field is illustrated in Fig. 3b and the expressions of wave function of "d" levels and of g and A tensors are given in Table I. The unpaired electron lies into a dx2_y 2 , antibonding orbital.

TABLE 1

In these expressions, contribution of p orbitals and ligands are neglected. ~, a I, B, 81 are the molecular orbital coefficients, X0the spin-orbit coupling constant for the free Cu(ll) ion (X 0 = - 828 cm-l), A denotes the energy sepa- ration between the ground state and the level given as subscript. P is the dipolar hyperfine splitting parameter for 3d orbital (P = 360 IO -4 cm -l for free Cu(ll) ion) and K is the Fermi contact parameter (K 0 = 0.43 in free Cu(ll) ion).

Wave functions of molecular antibonding orbitals

~B 1 = a]x 2 _ y2>

~AI = ~llZ2>

~B2 = 8 Ixy >

~E = 611xz>

g and A tensors in C4v symmetry 8 X 0 c~ 2 B 2

gz = g / / = 2 - Axy 2 t O a 2, 821

gx = gy = gi = 2 - Axz,yz

A// -- P[- a2(K + 4/7) + 3/7 ~g± + 6g//]

A i -- Pie2(2/7 - K) + ll/14 ~gi]

Vol. 19, No. 7 ZINC OXIDES 853

This assumption is not unreasonable: under symmetry lower than C4v , such as C2v (pyramid with orthorhombic distortion), the d. 2 .2 and dz2 levels are mixed and the ground state wave function can be written:

~GS = =(cos 8) Ix 2 _ y2> + (sin B) [z 2> (1)

where sin ~ is the admixture of the d z orbital in the ground state. Under these conditions, the g shifts can be expressed by the followin~ equations :

2 %0k2x(cos ~ + /3 sin ~)2 (2) ~gx = - - Ayz

- 2 ~ok2y(cos B - /~ sin B) 2 (3) ~gy = Axz

- 8 %0k2z cos 2 B (4) 6gz = Axy

Since gl = g2' it can be assumed that k x ~ k. and Ayz ~ Axz Using equations y 2) and 3), cos B can be calculated, it is found to be equal to 0.99998 = 1. So the admixture of the dz2 orbital in the ground state is negligible and the C4v symmetry can be assumed for the Cu(ll) site.

d-d transitions:diffuse reflectance study. The diffuse reflectance spectra Of 1% and 5 % Cu doped Y2BaZnO 5 samples are plotted in Fig. 4 : in the range 5000-35000 cm -I four broad bands are observed at about 11700 cm -I (shoulder),

R %

50

1 0 0 0 0 2 0 0 0 0 3 0 0 0 0 cm-' I I I

FIG. 4

Diffuse reflectance spectra of I ~ and 5 Z Cu doped V2BaZnO5 at room tempera- ture.

854 C. MICHEL, et al. Vol. 19, No. 7

14500 cm -I, 20500 cm -I and 30000 cm -I, increasing the copper amount does not change the feature of the spectra. It must be noted that the copper d-d transitions are generally observed at energies lower than 25000 cm -I, Axy ran- ging from 13000 to 16000 cm -I for five coordinated copper complexes (2, 3, 13- 15). Taking into account these latter results and the scheme of Fig. 3b, the three first ban~ can be assigned to the transitions AI ~ B 1 (Az2) , B 2 ÷ B1 (Axy) and E ÷ B 1 (Axz = gy z) respectively whereas the band at 30000 cm -I cor- responds to a charge transfer band. If the assignment of the band at 14500 cm -I is supported by several papers, the assignment of the band at 20500 cm -I is open to criticism. Nevertheless a theoritical value can be cal- culated from the expressions of Wasson et al. (16) for 3d energy levels for a 3d I or 3d 9 ion in a C4V symmetry with non equivalent ligands. Assuming a square pyramidal configuration with four equivalent oxygen atoms (noted B)in the square basal plane and one non equivalent oxygen atom (noted A) lying along the axis of the pyramid, and if e is the angle between the ~O A and M-O B directions (Fig. 3a) the d-d transitions can be written :

Axy = 5/3 e~ Sin 40 (5)

4 [2 ~ (3 cos 20 - I) + ~2 A] + ~/6 Sin 40 - 5/14(~ Cos 40 Az2 = - V

- IO COS 20 + l) 1 - 5/21 al (6)

Axz = - ~ - I) + ~2] + ~ /6 Sin + 5/14( Cos

- 10 Cos me + I)] + 5/21 ~i (7)

In these expressions, ~2 and ~4 are the orbital splitting parameters for the basal (~B) and axial (~A) ligands : =2 = 7Ds = q e2 <r2>/R3 ; ~ = 6Dq = q e 2 <r4>/R 5, where qe is the ligand charge, R the metal ligand ~istance and <r n> the expected value of the nth power of theAd electron distance from the metal nucleus. ~From these expressions, ~ and ~o can be easilv evaluated as a function of ~ and ~B respectively • ~ ~

2 ""

A R B 5 B A RB 3

~4 = (~A) a4' a2 = (~A) =~

SO, if e, Axy, Az2 are known, ~ and ~ can be calculated from equations (5) and (6) and then introduced in equation (7) ~o_ calculate~ ~z- In Y~ BaCuO~_ (I~, RA, R B and e are found to be R A = 2.29 A, R B - 2.09 ~, e = 94 . If we assume that the CuO 5 polyhedron is ~he same in diluted and undiluted copper oxides, we find Axz = 20320 cm -I. The same calculation made with the assump- tion that the CuO~ polyhedron in diluted copper oxides is like the ZnO 5 poly- hedron (R A = 1.98~ ; R B = 2.02 ~ ;8 = 1OO ° ) leads to Axz = 21250 cm -I. These two value are very close to the exptected one and the band at 20500 cm -I can be assigned to the E ÷ B 1 transition.

Bonding parameters : including these d-d transition values in g shifts expres- sions (Table I), it comes for the reduction orbital parameters :

~2B2 = k2~ = ~2B~ = k21 = 0.60 (8)

These values of k2~ and k2l are not'unreasonnable since they lie within the limits generally observed :--i.e. 0.4 - 0.9. e2 can be calculated from the hy- perfine coupling constant expressions given in table I, then B and ~I from equation (8). Assuming that A~ and A| are negative (A l = I/2(A 1 +A2)), the following values are obtained : --

2 ~2 "o = 0.74 = B~ = 0.81 K = 0.38

Vol. 19, No. 7 ZINC OXIDES 855

2 Note that the same value for ~ can be obtained using the expression (18)

2 A//+~ :--~- 0gH + 3/7 ~g~ + O.OA

which is frequently used when Ai cannot be measured from the ESR spectra. In these calculations the effects of ligands are omitted. In order to increase the accuracy of the orbital reduction parameters we have attempted to calcula- te them, using the expressions of g shifts and hyperfine coupling factors esta- blished by Kivelson and Neiman (18) although they were written for a D4h point symmetry. In this theory the overlap between the copper and ligand orbitals is considered. In first approximation, we have used for the overlap integral S and T(n) term the same values as those calculated by these authors for oxy- gen ligands in copper complexes ; we obtain : ~2 = 0.74 ; B 2 = B~ = 0.88. It

• . 9 ~ • 1

appears that ~2 remains unchanged whlle B ~ and ~I Increase of about IO %.

In a square pyramidal configuration, the dx2 -2 and dz2 orbitals of . . . . -y .

the 3d metal partlclpate to o bondlngs whlle the dxy , dxz , dy z orbltals par- ticipate to ~ bondings. Since the bonding is ~onic when the reduction orbital parameters are equal to unity ; it appears from these results that the bonding is appreciably covalent in this oxide and that covalency is mainly due to g interactions.

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