Electric field gradients of 111Cd in the copper oxides CuO and Cu2O

Volume 130, number 3

PHYSICS LETTERS A

4 July 1988

ELECTRIC FIELD GRADIENTS OF ‘“Cd IN THE COPPER OXIDES CuO AND Cu20 A. BARTOS, W. BOLSE, K.P. LIEB and M. UHRMACHER II. Phvsikalisches Institus der Universität and Sonderforschungsbereich 126, Gottingen/Clausthal, D-3400 GOttingen. FRG Received 28 March 1988; accepted for publication 5 May 1988 Communicated by D. Bloch

Perturbed angular correlation measurements were performed after “In implantation into CuO and Cu20 powder samples and 1 urn thick Cu20 surface layers. The quadrupole hyperfine interaction of” ‘Cd was studied in isochronal annealing cycles at 370— 1170K covering the CuO—.Cu20 phase transition. The electric field gradients obtained for ‘‘‘Cd on substitutional Cu lattice sites were associated with the repective oxygen coordinations. Annealing of Cu20 surface layers on copper foils resulted in a texture with the efg pointing preferentially out of the surface plane.

Since the pioneeringwork ofPasquevich et al. [1] and Wodniecki and Wodniecka [21, the perturbed angular correlation (PAC) technique with radioactive “In has proven to be a powerful microscopic tool to study the oxidation ofmetals. In this manner, oxidation of many fcc metals has been investigated in detail by Boise and co-workers [3,41.A typical feature of these experiments is the observation of broadly distributed electric field gradients (efg) in contrast to more favourable cases in which unique efg’s can be attributed to defined lattice sites of the probing atoms. If “In is implanted into a metal or a metal oxide, broad efg distributions may be caused by the correlated radiation damage as observed e.g. in Ag20 [4]. On the other hand, the implantation of “In into an 1n203 film [4] resulted in a large fraction on undisturbed lattice sites. The knowledge of the hyperfine parameters for substitutional “In in the defect-free oxide surrounding offers the possibility to study the efg of the In—O bond in terms of the crystallographic structure of the host material. First hints of a systematic behaviour were recently found [4,5]. Previous experiments with “In diffused into the copper oxides only yielded broad efg distributions [6,7,]. Oxidation of “In implanted Cu resulted in efg distributions too; here the drastic shiftofthe mean quadrupole frequency with temperature was attributed to a changing oxygen stoichiometry [3]. The

lattice rearrangement taking place during such a phase transition may help to anneal out radiation damage and to incorporate the probe atoms on substitutional sites. Therefore we have now performed PAC experiments with the copper oxides considering in detail the transformation Cu0—~’Cu2O.An additional motivation for these measurements was the advent of the new high T~superconductors where CuO planes seem to play an important role [8]. The experiments were performed either with pressed CuO and Cu20 powder pellets, or with Cu20 surface layers produced by oxidation of 25 ~tmthick copper foils of SN purity at T0~= 523 K for 15 mm in 200 mbar 02. The thickness (typically> 1 ~tm) and stoichiometry of the oxide layers were checked by Rutherford backscattering with 1 MeY a-particles. About 10,2 “Ink ions were implanted with an energy of 400 keY, using the Göttingen ion implanter IONAS [9]. The calculated projected range in Cu20 for such ions is about 80 nm. In the course of the PAC measurements the samples were isochronally annealed for 1 h in vacuo (partial) oxygen pressure P02 10_6 mbar) in the temperature range between 300 and 1200 K. All PAC spectra were taken at room temperature. As the PAC method has previously been explained in detail [101, we here only mention a few details: The spectra were taken with a set-up offour 2” x 2” Nal (Tl) detectors arranged in 90°geometry [4]. In

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177

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PHYSICS LETTERS A

order to identify the higher harmonics, some coincidence spectra were also taken with four BaF2 de-

oi -~t

tectors having a time resolution of 1.0 ns FWHM for the 17 1—245 keY ‘y—y cascade in the “Cd daughter. From the eight N(90°, 1) and four N(180°, I) co incidence spectra, the ratio

0.1

2N(l 5, t) =A N(l80°,t)—N(90°,t) 800 t)+2N(90 22 >~JG2(t),

0

R(t)=

5

~ -_________________

__________

©

773 K

s

5

__

~ f~=l ,

01

G2(t)=

0

~S2fl(~)

_

cii

was formed. The R(t) function contains up to five fractionsf with different hyperfine interactions, each characterized by its quadrupole coupling constant v,-~ and asymmetry parameter j. The perturbation function G2(t) of each fraction is given by cos[gfl(~)vQt]

n=

2/a)}. xexp{[—g~(~)öQt]’ In all PAC oxidation studies broad quadrupole frequency distributions were encountered which can be described by either a gaussian (a = 2) or lorentzian (a= 1) distribution with width ÔQ. This parametrization, however, fails to determine i~correctly [4]. In the Fourier transform of R ( t), the maximum of the broad frequency distribution was found to ~cincide with the lowest Fourier component w, =g, (z~)VQ of the corresponding unique efg. Consequently, the broad distribution is interpreted to indicate a similar surrounding ofthe “In probe, which is affected by more distant lattice imperfections. Fig. 1 a shows the annealing sequence of “In implanted into Cu20 powder after 60 mm annealings at Ta = 300—1070 K. Both the Cu2O powder and the Cu20 layers on Cu show two fractionsf, and.!2 after implantation which vanish after the first annealing step at Ta = 423 K. We therefore attribute them to radiation damage. Annealing between 470 and 820 K forms a broad quadrupole distribution around = 111(12) MHz; f~ reaches 100% and then decreases rapidly at higher annealing temperatures. Kaufmann et al. [11] have observed the thermal behaviour of Cu vacancies with photoluminescence under nearly the same annealing conditions. They found that Cu vacancies disappear between 720 and 178

4 July 1988

1100

0

K

,~

0 0

-~--~-~“~

40 4

20

~ ___________

0.1

)

~.

~/~/~

0

0 40

_________

J~

20 .~

0 0

200 TIME Ins]

400

0

500

1000

FREQUENCY 1MHz]

Fig. I. (a) Development ofthe R (1) functions and Fourier transforms in the annealing cycle of “In implanted into a Cu 20 powder sample. (b) Same as in (a) for an Cu20 surface layer annealed up to 1070 K and with the foil placed perpendicular respectively parallel to the detector plane.

870 K which is in good agreement with the thermal behaviour offractionf3. We tentatively attribute this fraction to the formation and decay of In—V—O cornplexes. The two fractions f4 and f5 are formed in the decay off3: fraction f4 which has a sharp quadrupole frequency of VQ4= 123(1) MHz and 7~4=0increases up to 30% at Ta= 1170 K. This fraction and the broadly distributed fraction f5 with 94(10) MHz and ~~=0.6(l) have the same centre frequencies w, and are therefore attributed to very similar matrix environments. Additional X-ray respectively RBS analyses after these measurements confirmed the Cu20 structure of the sample. Implantation of “In into Cu20 films in general shows the same annealing behaviour. In some samples we succeeded in locating all “In probes into site 4 after the 1070 K

Volume 130, number 3

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4 July 1988

annealing. For these samples, a monocrystalline structure can be deduced as the amplitude ratio of the Fourier spectra depends on the orientation ofthe lattice relative to the detector plane as shown in fig.

damage. The fraction f~ reaches a maximum of 50% at Ta 670 K and vanishes at Ta~820 K. According to the phase diagram of the copper—oxygen system, the Cu0—~Cu2Ophase transition [13] takes place at

lb. This pattern arises if the efg preferentiallypoints out ofthe surface plane. It is known [12] that during annealings at elevated temperatures a recrystallizalion takes place leading to a texture of the Cu20 surface film. The perturbation functions and Fourier spectra displayed in fig. 2 show the annealing behavior of “‘In implanted into cupric oxide (CuO). The fitted hyperfine parameters are given in table lb. Three fractionsf6, f7 and j~are observed immediately after implantation. The fractionsf6 andf7 exhibit the same w ,-value: f6 has a sharp frequency, whereas f., shows a broad distribution. Annealing at Ta473 K destroys f6 and reduces f7, as expected for radiation

TPh 750 K, at partial oxygen pressure of P02 = 106 mbar. The long thermal stability of fraction f~ up to TPh suggests that f8 can be assigned to “In in undisturbed CuO lattice sites. (The asymmetry pararneter 773=0.42 was determined with a PAC measurement in which BaF2 detectors were employed.) Around Ta = 770 K a new broadly distributed efg is observed. Its fraction f~reaches a maximum of 100% around Ta = 900 K. It then transforms into two fractions the hyperfine parameters of which agree with that of complexes 4 and 5 observed in Cu20. In fact, an X-ray analysis of the sample after this treatment shows that CuO had fully converted to Cu20. The occurrence of so many electric field gradients summarized in table 1 pictures the complex thermal behaviour of the copper oxides. However, we will here discuss only those fractions which we attribute

0.1 -

10

RI t)

as

r-~r-

implontad

~ o 0

~

0

___________________

0.1

to ‘‘‘Inareoncomplex substitutional sitesand in the Cu sublattices. These 4 in Cu2O complex 8 in CuO. This interpretation is supported by the phase diagram and calculations with the point charge model. In the case of Cu20, fractions4 (and 5) are detected after the reduction CuO—~Cu2Oand after the oxidation Cu—÷Cu20.The asymmetry parameter cal-

773 K

riF’~r’1~

©~

0 1

t

ir~k~. .

873 K

0

1~fl©

~

01 1073 K

/ 0

®

0

©

1123 K

A V

0

A

A

A

‘~,

j~

5

\~4

o

200 TIME InsI

400

0

500

1000

FREOIJENC’Y (MHZI

Fig. 2. Annealing cycle after ‘‘‘In implantation into a CuO powder sample.

culated with the point charge model is zero, which is in agreement with 14 = 0. It relfects the colinear coordination of the two nearest oxygen ions. In CuO the thermal stability (both in isochronal annealings and in thermal equilibrium) and the disappearance ports the assignment off~ “Cd sites off8 according to the p (toT)substitutional stability diagram supin CuO. Here “Cd is surrounded by four oxygen ions in a rhombic planar coordination. The calculated asymmetry parameter value 77=0.3 is close to the measured valued 778=0.42(1). Very recently the La Plata group reported on PAC results for ‘‘‘Cd in the copperoxides [6,7], focusing on semiconductor aspects of the system. In these experiments, “In was introduced into Cu by melting and the CuIn alloys were then fully oxidized. These PAC spectra are rather different from ours, both concerning the absence of well defined efg and the values of the quadrupole frequencies. We attribute 179

Volume 130, number 3

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4 July 1988

Table I PAC-parameters observed after ‘‘‘In implantation into the copper oxides.

(%)

w, (MHz)

Interpretation

~ 0.95 0.1(1)

3 ~30

359(9) 249(9)

radiation damage

111(12)

0.7(1)

40

152(12)

In—O—V(?)

4 5

124(1) 84(10)

0 0.65(9)

3 45

116(1) 109(13)

‘‘‘CdCu2O

~470 ~670

6 7

238(2) 220(8)

0.62(5) 0.7(1)

3 15—35

303(3) 300(10)

radiation damage

470—820

8

420(1)

0.42(1)

3

468(1)

‘‘‘CdCuO

770—970

9

147(6)

0.84(6)

37

219(10)

oxygen-vacancyinCuO(?)

~970

4 5

123(l) 94(5)

0 0.60(5)

3 45

116(1) 117(10)

‘‘‘CdCu2O

i

v,~(MHz)

i~

~ 420

1 2

216(1) 261(9)

470—820

3

~870

TA (K)

(a) Cu

20 samples

(b) CuO samples

the discrepancies to the different sample preparation in which the formation of In—O complexes or small In203 clusters cannot be excluded, and to the very limited statistics. In particular, the broad frequency distribution found in refs. [6,7] centers at in our Cu20 experiments, Theoretical predictions of electrin field gradients are difficult to make. In ionic crystals the sample point charge model often often reproduces the asymmetry parameter ~ well, but usually underestimates the quadrupole coupling /3=[14]) v~/ 11pcm~ which ranges from constantby /3=1 (“Cda factor in ZnO uptofl~80(“CdinAl [15]). “Inwas introduced into Cu by2O3 melting andPlacing the CuIn alloys were then fully oxidized. These PAC spectra are rather different from ours, both concerning the absence of well defined efg and the values of the quadrupole frequencies. attribute theindiscrepancies to the different sampleWepreparation which the formation of In—O complexes or small 1n 203 clusters cannot be exluded, and to the very limited statistics. in particular, the broad frequency distribution found in [6,7] centers at in our Cu2O experiments. Theoretical predictions of electric field gradients are difficult to make. In ioniccrystals the simple point charge model often reproduces the asymmetry parameter ~ well, but usually underestimates the quadrupole coupling constant by a factor /3= v~,,.,,/v,,~,,,, which ranges from /3= 1 (“Cd in ZnO [14]) up to fla~80(‘‘‘Cd in Al203 [15]). Placing ‘‘‘Cd on the 180

ö

substitutional metal sites in different oxides allows one to study the role of the oxygen coordination on the efg. We compared the present results for the copper oxides to those previously found in the silver oxides [4], since the structures of Cu2O and Ag20 are identical and those of CuO and AgO are very similar [16]. The measured efg parameters are given in table 2. Surprisingly, the values of Vq and ~ for Ag20 and Cu20 are in perfect agreement, whereas the point charge model predicts a 30% higher efg for “Cd in Cu20 due the smaller lattice constant. Orgel for attempted toto explain the formation of linear bonds 2~in ionic oxides d’°-cationslike and Cd by assuming an Cu~,Ag~ sd~ 2-hybridisation[17]. His calculations predict the same type of binding mechanism for Cu20 and Ag20. Recently, Nagel performed MSXa-cluster calculations to determine the2-clusefg on the cation in Cu20, Ag20approximates and a (Cd02)the situter [18,19].siteThe latter case ation with “Cd occupying a cation site in the oxide. In all three cases the main contribution to the efg resulted from 4d,5p or 3d,4p orbitals. The calculated efg turned out to be nearly independent of the distance between the cation and the oxygen ion, in perfeet agreement with our observations. No similar calculations for CuO and AgO are known to us. Although the crystallographic structures are slightly different, the planar coordination of the four oxygen atoms dominates. The higher oxygen content of the lattice results in a more ionic

Volume 130, number 3

PHYSICS LETTERS A

4 July 1988

Table 2 Comparison of efg parameters for ‘‘‘Cd on cation sites in the silver [4] and copper oxides. v~(MHz)

p~

2)

/3°’

V. (l0~’V/rn

exp.

point charge model

‘‘‘CdCu 2O ‘‘‘CdCuO

124(1) 420(1)

‘‘‘Cd~g~~ 129(1) ‘‘‘Cd,~g2 323(5) a)

/1= V~xp/Vca~cwhere

~

0 0.42

4(1) 19(3)

6.2(1.0) 20.9(3.3)

1.573 1.076

0 0

5(1)

20(3)

6.4(1.0) 16.1(2.5)

1.164 0.773

is calculated with the point charge model.

character of these oxides. In fact, the efg for CuO and AgO are different, but give the same antishielding factor (/3=20) within the point charge model (see table 2), i.e. the efg scales with the lattice constant. More important than the difference in the efg-scaling behaviour of mono- and di-oxides is the similarity of the efg parameters within each crystal class. Although the cation—oxygen distance in the copper and silver oxides is very similar, the efg for ‘‘‘Cd in the mono-oxides is about three times higher than in the di-oxides. This underlines the importance of the number and spatial distribution ofoxygen ions in the neighbourhood of the ‘‘‘Cd probe. Most metals form oxides and therefore our hypothesis can, in principle, be checked in many more cases. A recent complitation of hyperfine interaction parameters in chemical compounds clearly shows a lack of data for ‘‘‘Cd in oxides [20].

References [1] A.F. Pasquevich, A.G. Bibiloni, C.P. Massolo, F.H. Sanchez and A. Lopez-Garc,a, Phys. Lett. A 82 (1981) 34. [2] P. Wodniecki and B. Wodniecka, Hyp. mt. 12 (1982) 95.

[3]W. Boise, M. Uhrmacher and K.P. Lieb, Mater. Sci. Eng. 69 (1985) 375. [4] W. Boise, M. Uhrrnacher and K.P. Lieb, Phys. Rev. B 36 (1987) 1818.

[5]W. Boise, M. Uhrmacher and J. Kesten, Hyp. mt. 35

(1987) 931. [6] J. Desimoni, A.G. Bibiloni, L.A. Mendoza-Zélis, L.C. Damonte, F.H. Sanchez and A. Lopez-Garcia, Hyp. mt. 34 (1987) 271. M. Renteria, J. Desirnoni and A.G. Bibiloni, [7] C.P. Massolo, to be published. [8] P.W. Anderson, Science 235 (1987) 196. [9] M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Purschke and K.P. Lieb, Nuci. Intrum. Methods B 9 (1985) 234. [10] W.D. Hamilton, ed., The electromagnetic interaction in nuclear spectroscopy (North-Holland, Amsterdam, 1975). [11]R.G. Kaufmann and R.T. Hawkins, J. Electrochem. Soc. 133 (1986) 2652. [121G. Blankenburg and K. Kassel, Ann. Phys. (Leipzig) 10 (1952) 14. [131 J.P. Neumann, T. Zhong and Y.A. Chang, Bull. Alloy Phase Diagrams 5 (1984). [141W. Witthuhn, Hyp. mt. 24—26 (1985) 547. [15 1 J. Kesten, private communication. [16] S. Asbrink and L.J. Norrby, Acta Crystallogr. B 26 (1970) 8. [17]L.E.Orgel,J.Chem.Soc.58 (1958)4186. [18] 5. Nagel, J. Phys. Chem. Solids 46 (1985) 743. [19] S. Nagel, private communication. [20] A. Lerfand T. Butz, Hyp. Int. 36 (1987) 275.

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