Investigation of the local geometry of Ni2+ ions in an Al2O3 crystal from EPR data

Investigation of the local geometry crystal from EPR data Zheng

Wen-Chen

The electron paramagnetic resonance (EPR) parameters D. g,, and Q (=g,, - g i ) for Ni” in corundum (Al-O,) were measured by Marshcll and coworkers [ I.21 several decades ago. but until now no satisfactory theoretical explanation related to the structural data has been made for these parameters. Marshell et al. [I] thought that a satisfactory explanation for these parameters must be based on a reasonable consideration of the local lattice distortion produced by the substitution of Ni’& for an Al” ion. However. the details of this distortion were not shown. In this paper, we will study the distortion by fitting the theoretical values of EPR parameters to the observed values. The reasonableness of this distortion is also discussed. The theoretical calculation rests on high-order perturbation formulas based on the ‘quasi-intermediate-field’ approach [3]. Thus, for a Ni‘ ’ ion in trigonal symmetry [3]. we have

’ Addrex~

of Ni’ ’ ions in an Al@,

i1XV3Bk&’ + w,w,w,

@

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6Bkf Amwtw.

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where W, (i = I-S) are detined in ret. l_Jj. 3, 2.0023. u and U’ arc the trigonal field paramc-tcrs. From the point-charge model. wc ha\c

for correyx~ndence

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Zheng Wen-Chen

u’ =

z

3vT2 ~eq(‘*)(3cos20i

XL

I Displacement of Ni”

- 1)/R?

I=1

5v2 - 168 eq ( r”) (35 cos”@ - 30 cos’f3, + 3) /R;S -&eq(r4)sin’OjcosOjcos3~i/R~

1,

(5)

where q is the effective charge of ligand. R,, 0, and & are the structural parameters of the studied crystal. For the Al’+ site of the A&O, crystal [4], R, = 1.966 A, R, = 1.857 A, 19,= 47.7”, 0, = 63.1”, 4, = 0 and & = 4.13”. Utilizing the empirical d-orbit of the Ni2+ ion [5] and introducing a parameter N (k = N*) to denote the average covalency reduction effect, we have B = 1208N4 cm-’

,

C ==4459N4cm-’

.$ = 636N’cm-’

,

(r*)

= 1.8904N’

a.u. ,

(r’)

= 13.4043N’

a.u.

,

(6)

The parameter N is determined from the optical absorption spectra of the studied crystal. According to the optical spectra of the Al,O, : Ni’+ obtain N -0.964, Dq -‘I crystal [6], we The comparison of optical spectra 1000 cm-‘. between calculation and experiment is shown in table 1. Thus, the adjustable parameter q can be obtained from the parameter Dq by using the formula 3ti 2 Dq = - 16 eq (Ir4) c sin”tl; cos 0, cos 3&lRj . (7)

1=I

Table 1 Cubic theoretical values and experimental data for the optical absorption spectra of an AlzO, :Ni” crystal. Theory Energy

‘A,(‘F) ‘T2(‘F)

0 10 000

‘E(D) ‘T,(‘F)

15 467 16560

“‘Ref.

[6]

(cm-‘)

Level

Energy

‘A,(‘A>) ‘A,(‘Tz) ‘E, ‘T>) ‘E( ‘E) ‘AZ(3T,) ‘E( ‘T, )

0 10 500 9800 15 800 16 590 16 500

(cm-‘)

323

Substituting the parameters N, q and the structural data related to the exact Al”+ site in Al,O, into the above formulas, we can calculate the EPR parameters D, g,, and Ag. They are compared with the observed values in table 2. It can be seen that the calculated EPR parameters (particularly the zero-field splitting D and the parameter Ag) based on the structural data of the host crystal are indeed in disagreement with the observed values. So, the local lattice distortion produced by the substitution of Ni2+ for the Al”+ ion should be taken into account. The EPR experiments [1,2] indicate that the Ni’+ center in Al,O, still has a trigonal symmetry; we therefore assume that the Ni*+ ion is displaced along the C, axis from the exact site of the replaced Al” ion (note: for simplicity, the positions of oxygen ligands are assumed to be unchanged). Considering that the calculated values of 1DI and Ag are greater than the observed values, to reach a good fit between theory and experiment, the Ni’+ ion should be shifted towards the central site of the oxygen octahedron, which results in a smaller trigonal distortion and hence the smaller calculated values of IDI and Ag. The quantitative calculation shows that the displacement of the Ni’+ ion is 0.05 A. The comparison between calculation and experiment is shown in table 2. Noteworthily, the above displacement direction is reasonable in physics. As is known, the Al,O, lattice can be visualized as a stacking of equidistant oxygen planes perpendicular to the C, axis. The oxygens are arranged in a triangular lattice and form an almost hexagonal closepacked structure. Along the C, axis, the oxygens Table 2 EPR parameters

of an AlzO,

: Ni’ ’ crystal.

Calculation

Experiment”’

Level

in Al,O,

D (cm-‘) & 4

Experiment

1J,

IIh’

I”

-2.3972 2.2245 0.0181

-1.3108 2.2160 0.0099

-

IId’

1.3760 2.1957 0.0098

- 1.3287 2.196 0.009

“‘Calculated from the structural data of the host crystals. “‘Calculated by taking into account the impurity displacement AZ = 0.05 A. “Ref. [ 11. “‘Ref. [2].

constitute

a sequence

of

distorted

octahedra.

successively containing Al” , Al” ions vacant site. The electrostatic repulsive

ions displaces these cat-

between the two Al’ ions from the midway layers

and leads

schematic cations

view

the oxygen

to the positions closer vacancies.

of

the

oxygen

is shown between

cation i\ replaced hy

an

plancs

in fig. these

electrostatic

the

upon

between

to A

neighbouring

included

the separation

plane

them

distinctive

tk

and a force

cations

repulsive impurity

the

and

1. Obviously, force.

&pen& If

the

having different

charge. it can be expected that the impurity ion dots not occupy the exact site of the replaced Ali’

ion, but is displaced along the c’; axis by

amount the ion.

AZ hccause

impurity In

differs

general.

the repulsive from when

force acting

an

charge

than

the

replaced

ion.

host

the

C~C‘C‘II-o-

static repulsive force acting on the impurity i4 smaller than that on the host ion and the imput-ity should

bc shifted

towards

the

plant

midway

bctwecn oxq’gcn layera. whereas. fog- the impurity carrying tion

with

extra

rehpcct

bccaLlac

of

force.

the

OhviousIy.

charge.

the displacement

larger for

electrostatic the

direc-

plane ih opposite

to the midway

repukivc

Ni”

AI,O,:

crystal.

the ~~bovc clisplacemcnt tiircction of the Ni’ is consistent with what WC‘ expect and can regarded that

physically

as

by studying

the

reasonable. EPK

infol-mation about the lc~al impurity ion cm be obtained.

data.

It

bc

appears

some

gcometr!

ion

useful

of

the

on

that on the host Al’ the

impurity

has

Icss

Acknowledgement ‘lliia

proJect

Foundation tee of (‘hina I<‘MP!92iO.3).

References

LV;I\ \upportcd

of the National

134’ the Education

and the Foundation

Science (‘ommit-

of ICMP

(no.