Scaling of the electric field gradient of 111Cd impurities in the bixbyite oxides of Y, Sc, Dy and Yb

Volume 157, number 8,9

PHYSICS LETTERS A

12 August 1991

Scaling of the electric field gradient of “Cd impurities in the bixbyite oxides of Y, Sc, Dy and Yb * A. Bartos, K.P. Lieb, A.F. Pasquevich

‘,

M. Uhrmacher and ISOLDE Collaboration

II. Physikalisches Instilut, Universität Gottingen, W-3400 Gottingen, Germany Received 25 February 1991; revised manuscript received 17 May 1991; accepted for publication 12 June 1991 Communicated by J. Flouquet

The quadrupole hyperfine interaction of” ‘Cd in the cubic C-form sesquioxides ofSc, Y, Dy and Yb has been investigated over the temperature range 295—8 50 K with perturbed angular correlation (PAC) spectroscopy. The “Cd probe nuclei were either produced in the EC-decay of “In or in the y-decay of the “Cd isomeric state and introduced via ion implantation into the oxide matrices. A simple scaling ofthe components of the electric field gradient with the lattice constant has been observed on both substitutional octahedral sites.

The perturbed angular correlation (PAC) method is a powerful tool for investigating lattice defects, solid state reactions and phase transitions on a microscopic scale [1]. PAC measurements require a radioactive probe with a sensitive nuclear state the

reported [71.All rare earth metal oxides can be obtamed in the cubic C-form (bixbyite) which belongs to the 1a3(T~)space group. A unit cell contains 8 cations at position 8b having D3d point group symmetry, 24 cations at position 24d with C2 point group

electric quadrupole moment Q ofwhich interacts with the electric field gradient(s) (efg) generatedby ionic and electronic contributions of the nearest neighborhood. In the past, we have extensively used this method to study hyperfine interactionsbetween “Cd probe nuclei and oxygen during oxidation of fcc metals [2] as well as structural, antiferromagnetic and oxidation phase transitions in many oxides [3—6]. Recently, Kesten et al. [3] discovered a pronounced, but so far unexplained correlation of the efg for substitutional “Cd with the crystal structure and the nearest-neighbor oxygen coordination. The subject of the present PAC study is to investigate the scaling of the efg for substitutional “Cd atoms in several rare earth oxides of bixbyite structure. Under suitable conditions, all rare earth elements formand a sesquioxide buttypes polymorphism is common up to five RE203, crystalline have been

symmetry and 48 oxygen ions in position 48e [8]. Consequently, there exist two possible substitutional sites referred to as site “C” (asymmetric site) and “D” (regular or symmetric site), to be occupied by a probe atom with relative fractions in the ratio 3:1. Besides the rare earth elements, also 1n203, Sc203 and Y203 share the cubic C-form. A comparison of the efg in Sc203 (lattice constant a=0.9857 nm), Y203 (1.0621 nm), 1n203 (1.0116 nm), Yb203 (1.0433 nm) and Dy203 (1.0665 nm) thus offers the possibility to compare the 4f rare earth oxides and the d-element oxides of In [9], Y and Sc and to study the variation of the efg over quite a large range of the lattice constant. The oxide powder targets (Johnson Matthey; Y203: ultrapure, Sc203, Yb203, Dy203: 99.999%) 11n + ions were rolled onto Ag backings. About 1013 “ implanted at 400 keY using the Gottingen ion implanter IONAS [10]. After implantation, the samples were annealed5 for 1A= 1 h at TA= 1075 K under vacuum (p~10— mbar) to remove radiation damage. About 10” ions of hhlmCd+ were implanted with an energy of 60 keY at ISOLDE/CERN, followed by

*

Supported by Deutsche Forschungsgemeinschaft (L,325/2-l) and Stiftung Volkswagenwerk. Permanent address: Facvltad de Ciencias, Universidad Nacional de La Plata, La Plata, Argentina.

0375-9601/91/S 03.50 © 1991

—

Elsevier Science Publishers B.V. (North-Holland)

513

Volume 157, number 8,9

PHYSICS LETTERS A

o

is

°.020

o io

Ta

1075 K,

Tm=

295

K

0 015

0.05 0.

12 August 1991

0 010

~

0 .005

-0.05

0 000

0 10~

Tm=

605

K

:.: : 0.005

0.10

:.:J~ ~

Tm= 860 K

0.015

—

0.003

—.

Tm= 950 K

0 10

0.015

:.::

—0.05

~

1 100

—

:.: :

—0.05

0

•-

200 (ns)

300

0.0

0.5

Frequency

1.0

~

(GHz)

Fig. 1. Development ofthe perturbation functions R ( t) and Fourier transforms for “In in Y

203 asfunction of the measuring tempera-

ture Tm. 975 K in air, thermal for ta= were 1 h atperformed TA= The PACannealing measurements with the isomeric 5/2k state of “Cd, which was populated either by the EC decay of “In (T,, tmCd (T 2=2.8 d) or by the 151 keY 7-decay of “ 112=49 mm). The PAC-sensitive state is characterized by the mean life r= 110 ns, the quadrupole moment Q=0.85 b and the magnetic momentu= —0.7656(25) ~N [1]. The perturbation of the angular correlation of the 171— 245 KeY 7-cascade of “In ( “Cd) was measured with a conventional slow—fast set-up of four 2” x2” Na!(Ti) detectors, while the PAC data of the 151— 245 keY 7-cascade of “Cd were taken at room temperature with a fast—fast set-up of four BaF2 detectors arranged in 90°geometry. From the eight N(90°,t) and four N(180°,1) co—~

514

incidence spectra, the perturbation function R(t), R(t)=2 N(180°,t)—N(90°,t) N(180°,t)+2N(90°, t) 5

A22

•~,JG22(t)

(1)

was

formed after background subtraction = 1). For polycrystalline samples, a static electrical quadrupole interaction can be described by the perturbation factor G2(t),

(>

~.,

3

G2(t)= ~ S2~(~) cos(w~t)G~,(ô)

(2)

G~1(ô)=exp{—[g~(~)ôt]°/a},

(3)

,

Volume 157, number 8,9

PHYSICS LETTERS A

12 August 1991

Table I Electric quadrupole interaction parameters of ‘‘‘Cd in the bixbyite oxides Y 20,, Sc203, Yb203, Dy203 and 1n203 [9,12]. The ‘‘‘In and uImcd data are labelled by I and II, respectively. Lattice

Fraction D PQ

Fraction C ‘7

(MHz) “203 Sc203 Yb203 Dy203 In203

I~> Ib)

II 1 II 1 I II

150(2) 166(2) 164(1) 154(2) 150(1) 149(2) 154(1) 154(1)

0 0 0 0 0 0 0 0

Fraction 3 ‘7

(MHz)

(MHz)

<1.5 <2 3.5(5) 1.5(7) 1.1(5) <2

89(2) 132(1) 134(2) 97(1) 97(1) 82(2) 118(1) 118(1)

2.0(5),

(5 (MHz)

0.77(2) 0.71(1) 0.71(2) 0.75(2) 0.77(1) 0.81(5) 0.71(2) 0.71(1)

‘1

(5 (MHz)

166(5) 193(5) 216(10) 107(5)

0.32(1) 0.73(1) 0.76 0.0

15 3—13 17(5) 2—10

179(7)

0.35(5)

16(2)

(MHz)

<2 3—13 4.4(5) 3(1) 17(2) 1.2(8) 6(1)

~ Two additional fractions with VQ4= 75(5) MHz, 174=0.8(1), (54=20(5) MHz; VQ5=O, ö~=20(5) MHz were required to fit the perturbation function. b) Two additional fractions with v~=68(5) MHz, 174=0.7(1), (54= 15(3) MHz; 0Q5=O, J~=20—50 MHz were found in the fit.

and with the precession frequencies w~=g~(~)vQ. The electrical hyperfine interaction of the “Cd nucleus is characterized by the coupling constant VQ x: V~ and the asymmetry parameter = ( V.~—V~,) / v~.,assuming ~ ~ ~ Foreign atoms, inhomogeneities in structure or stoichiometry may be the reason for a distribution of field gradients at the probe nuclei that can be described by a Lorentzian (a = 1) or a Gaussian (a = 2) frequency distribution with width ö around VQ. Y203: Fig. 1 displays PAC spectra in yttria taken at different measuring temperatures Tm after “In implantation. These data require some detailed discussion. The spectra taken at Tm~600 K show that most an probe atoms occupysite, twolabelled well-defined sites: axially symmetric site “D”,lattice with the coupling constant VQD = 150(2) MHz, and a second site “C” without axial symmetry, ‘lc = 0.77(2), and VQC=89(2) MHz. Note the very small ô values of these two fractions which lead to nearly undamped perturbation functions. Above 700 K a third fractionf 3 with VQ3= 166(5) MHz and 113=0.32(1) is observed which is correlated with a corresponding decrease of f~.The ratio of the fractions (J~+f3)/ fD 3 corresponds to ~the one expected for the two substitutional sites in cubic C-form sesquioxides. The coupling constants VQD and ~QC show a very weak dependence on the measuring temperature increasing by about 3 MHz for site D and about 8 MHz for .

site C between 300 and 750 K. All “In PAC parameters are listed in table 1. Compared to the spectra taken at Tm = 750 K, the room temperature PAC spectrum is strongly damped. However, this damping at the two sites C and D can neither be described by a Lorentzian nor a Gaussian static frequency distribution. Furthermore, the experimental hard-core value S20 is smaller than the value expected for polycrystalline quadrupole interaction. This points to a dynamic hyperfine interaction as previously found in PAC studies on various oxides (a-Fe203, Zr02, In203 [11, 12]). Here we follow refs. [12,13] to account for the damping of PAC spectra for “In in 1n203: A+~

2R+AG)t]. (4) exp[—( The damping of the perturbation function is assumed to be due to an efg fluctuating with the time Gdyfl=~+A

constant ~ 2R is the Abragam—Pound relaxation constant [14]. When dynamic perfine interaction, theintroducing perturbationthis factor G,, (ö)hyin eq. (1) was replaced by

~

)~G,)~R;

ö)

=fdYflGdYfl(~c,AR)+(l—fdYfl)GSt(d) .

(5)

The fraction fdyfl takes into account that part of the probe nuclei which are affected by the dynamic interaction which is characterized in yttria by 515

Volume 157, number 8,9

PHYSICS LETTERS A 0.15~

0020

Sc

203

0 10

iE

12 August 1991

Tm°650

.

1

K 0

~

015

~~ __________________ Yb203

Tm730

K

001O

~

-0.05~

0 000 Y203

Tm=750

K

0.010

-0.05~

1 Dy~O3

Tm750

0.10

0 015

~ ~0.O5E 0

0.000

K

~

100

200

.~OO

ins)

::;:~ 0000L ~ 0.0

0

Frequency

5

1.0 u

(GHz)

Fig. 2. High temperature perturbation functions and Fourier transforms of “In in the different sesquioxides discussed here.

3O(lO)MHz and )~R=2OO(5O)MHz. )~G= Sc 203: the PAC measurements of “InSc2O3 revealed similar results as in yttria. At 295~Tm~600 K, the perturbation functions were fitted with four fractions (see table 1): two well defined fractionsfD and f~and two fractions with frequency distributions around VQS=O and v04~75 MHz. Above Tmi~i550 K another fractionf3 with VQ3= 193 MHz and 113 = 0.72 appeared. Again, (fD +f3 ) /f~ 3 is valid for Tm> 550 K. The dynamic fraction can be characterized by ‘~~G = 20(10) MHz and 2 R = 200(100) MHz. Dy203, Yb203: For these rare earth oxides, nearly identical results have been obtained. The main features of the predominant fractions fD, fc and f3 remain as can be seen from table 1. Again, these frac516

tions are in the ratio (J~+f3)/fD=3.In contrast to our results in Y203 and Sc203, fraction f3 disappears in Dy203 above Tm> 350 K and in Yb203 above Tm> 650 K. No additional fractions (J~,.f~)were necessary to fit the spectra at room temperature. Contrary to Y203 and Sc203, only a very small amount of probe atoms in both oxides is affected by dynamical hyperfine interaction at room temperature. Fig. 2 illustrates the similarity of the high ternperature perturbation functions for “In in the four matrices and their Fourier transforms. All PAC parameters derived are listed in table 1, together with the results of the hhlmCd measurements at ISOLDE! CERN. Evidently the agreement between these two sets of efg parameters for sites C and D is excellent

Volume 157, number 8,9

PHYSICS LETTERS A

symmetry is found (V~~=~ in agreement with the symmetric oxygen coordination at this site. The efg component V~at the irregular site C varies nearly linearly with a—3, while the different slopes of V~, and Vi,,, reveal that the asymmetry slightly increases for increasing a. This points to a small structural change of the atomic positions in these sesquioxides as also found in the X-ray and neutron diffraction data [8]. For 1n 203, we have therefore included the prediction of the efg components based on the crystallographic coordinates determined in X-ray diffraction [17], and the commonly adopted antishielding factor of Cd, flwl_y=Vzz/V~M=32.9 [18]. The distinct variation of ~ and V,,,, with increasing lattice constant and the axial symmetry found at site D for all oxides investigated appears to exclude distortions of the oxygen octahedra by the

8. V E >

4

site D

0

Sm

~

Dy Y Yb

In

Sc

Vxx Vyy

s,te C

~

4 Vu

>

2

0

Vxx

•

~

8

9 4/a3

(A3)

10

11

10 Fig. 3. Deduced efg components V~,V,~,and V~for sites C and D plotted versus a ~. The open circles indicate the values predicted from the point charge model using the coordinates from X-ray diffraction 1171 and the Cd antishielding factor fl= 32.9

[181.

and suggests that in both experiments the probe ions are located at substitutional and (most probably) defect free cation sites. On the other hand, the fractions of nuclei which experience dynamical hyperfine interactions are distinctly different after “In~ and UlmCd+ implantations. These “after-effects” will be discussed in a separate paper [15]. All investigated sesquioxides share the same crystal structure bixbyite, with the lattice constant a varying by 8.3%. The bonding in cubic sesquioxides is supposed to be predominantly ionic [7]. In a purely ionic (point charge type) model, a linear 3 isscalexing of the at substitutional sites a— pected. Theefgprecisely determined efgwith parameters VQ and 11 allow us to deduce the diagonal components V~, V~,and ~ of the efg tensor for “Cd on both crystallographic sites. They are plotted in fig. 3 as function of a including the previous results for In 2O3 [12] and recent work on Sm2O3 by Shitu [16]. For both sites a steady variation of the efg components with a ~ is observed: For the regular site D the variation is very small and no deviation from axial ~,

12 August 1991

Cd probe atoms. In conclusion, the present high precision PAC study on “Cd impurities in a number of sesquioxides with bixbyite structure has revealed a simple scaling behavior ofthe electric field gradient components with the lattice constant a, in agreement with the predominant ionic bonding. This is the first systematic PAC study in which the same hyperfine probe was used in a series of isostructural compounds covering a large range in the lattice constant. The simple scaling observed as well as the previously found correlation of the antishielding factor with the crystallographic structure [3] provide very sensitive tests for theoretical interpretations of the efg in compounds. The authors are indebted to D. Purschke for his efficient “In implantations, to J. Shitu for the permission to quote from unpublished work and to K. Schemmerling for his help in setting up the fast—fast BaF 2 detector system.

References

[1] G. Schatz and A. Weidinger, Nukleare Festkorperphysik (Teubner, Stuttgart, 1985). [2] W. Boise, M. Uhrmacher and K.P. Lieb, Phys. Rev. B 36 (1987) 1818 [3) J. Kesten, W. BoIse, K.P. Lieb and M. Uhrmacher, Hyp. Int. 60 (1990) 683.

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[4]W. Boise et al., Bet. Bunsenges. Phys. Chem. 93 (1989) 1285; D. Wegner, Z. Inglot and K.P. Lieb, Bet. Bunsenges. Phys. Chem.94(l990) 1. [51A. Bartos, M. Uhrmacher, K.P. Lieb and W. Boise, Hyp. mt. 50(1989) 619; Phys. Lett.A130(l988) 177. [61M. Uhrmacher, A. Barbs and W. Boise, Mater. Sci. Eng. A 116(1989)129. [7] L. Eyring, in: Handbook on the physics and chemistry of the rare earths, eds. K.A. Schneider and L. Eyring (NorthHolland, Amsterdam, 1979) p. 339. [81 G. Bauer, in: Progress in the sciences and technology ofthe rare earths, ed. L. Eyring (Pergamon, Oxford, 1966) p. 312.

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[91D. Taylor, Br. Ceram. Trans. J. 83 (1984) 92. [10)M. Uhrmacher et a!., Nuci. Instrum. Methods B 9 (1985) 234. [1l]K.Asaieta!.,Phys.Rev.B41 (1990) 6124; H.T. Su et al., submitted to J. Am. Ceram. Soc. [12]A.G.Bibilonietal.,Phys.Rev.B29(1984) llO9;32(1985) 2393. [!3]U.Bäverstametal.,Nuci.Phys.A186(1972)500. [14] A. Abragam and R.V. Pound, Phys. Rev. 92 (1953) 943. [151 A. Bartos et a!., in preparation. [16] J. Shitu, private communication. [171 M. Marezio, Acta Crystallogr. 20 (1966) 723. [181 N.C. Mahapatra et al., Phys. Rev. B 16 (1977) 39.