RÉSONANCESTHE INVESTIGATION OF THE FERROELECTRIC PHASE CHANGE IN BaTiO3 USING ELECTRON SPIN RESONANCE AND THE MÖSSBAUER EFFECT

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RÉSONANCESTHE INVESTIGATION OF THE FERROELECTRIC PHASE CHANGE IN BaTiO3 USING ELECTRON SPIN RESONANCE AND THE

MÖSSBAUER EFFECT

H. Maguire, L. Rees

To cite this version:

H. Maguire, L. Rees. RÉSONANCESTHE INVESTIGATION OF THE FERROELECTRIC PHASE CHANGE IN BaTiO3 USING ELECTRON SPIN RESONANCE AND THE MÖSSBAUER EFFECT.

Journal de Physique Colloques, 1972, 33 (C2), pp.C2-173-C2-176. �10.1051/jphyscol:1972259�. �jpa-

00214995�

RÉSONANCES

THE INVESTIGATION OF THE FERROELECTRIC PHASE CHANGE IN BaTi0

³

USING ELECTRON SPIN RESONANCE

AND THE MÔSSBAUER EFFECT

H. G. MAGUIRE

(Allen Clark Research Centre, The Plessey Co., Caswell, Towcester, Northants, England) and

L. V. C. REES

(Department of Physical Chemistry, Imperial College of Science and Technology, London S. W. 7)

Résumé. — Les mesures des paramètres de l'Hamiltonien de spin de F

^{+ 3}

sont en accord avec des déterminations précédentes. On présente des preuves de l'existence d'une lacune associée aux sites et possédant un caractère rhombique. Les mesures d'effet Mossbauer de dédoublement quadripolaire et de déplacement isomérique ne coïncident pas avec les conclusions antérieures.

Abstract. — Measurements of the spin Hamiltonian parameters of Fe

3+

is in agreement with earlier determinations. Evidence is presented for vacancy associated sites and those possessing rhombic character. Mossbauer measurements of quadropole splitting and isomer shift do not concur with previous findings.

Introduction. — Butterfly twin platelets were grown in a KF flux and all measurements performed on selected crystals from the same batch. These were etched in orthophosphoric acid at 180 °C to remove surface flux residues. Good square hystereses loops were obtained with these platelets at room temperature which narrowed with increasing temperature, finally collapsing to indicate a Curie point at 122 + 1 °C.

EPR measurements were performed on c-domain platelets with a Varian V4502-15 spectrometer ope- rating at X-band (9.3 GHz). Mossbauer measurements were performed with an Elron spectrometer using a constant acceleration output and 400 channel analyser (Intertechnique). Co

57

was diffused into five crystals as described elsewhere [5] varying in activity from 0.7-0.3 mC. Specimen temperatures were controllable to within + 0.5 °C.

Results and discussion EPR. — Fe

^{3 +}

in the

⁶

S

^{5 / 2}

state substitutes for Ti

4 +

in BaTi0

3

which is tetra- gonal at room temperature and the effect of the crys- talline field and external magnetic field are given by a spin-Hamiltonian of the form

36 = j?H.g.S + DS, + aSt + b(Sj + S

⁴

) where z is parallel to the tetragonal axis (c-domain).

D and a are related to the coefficients A2 and A

4

the axial and cubic components of the crystal field potential [1], [2], [3],

F

crys

= A

2 0

r

2

Y

2 0

+A°

4

r

4

*

x [Y

⁴

°+(5/14)*(Y

^{4 4}

+ Y

⁴

"

⁴

)] + - D vanishes in a cubic field above the Curie point, when a = b. In agreement with previous determina- tion [4] the main five line room temperature spectrum is shown in figure 1 with the « c » axis parallel to the d. c. magnetic field. Setting a — b evaluation of the spin-Hamiltonian parameters at room tempera ture gives | D [ = 0.092 + 0.002 c m

- 1

, g = 2.003, I a I = 0.016 ± 0.0002 c m

- 1

. This value for D is greater than that in [6] but about half that most recently quoted [7], which is surprising in view of the reasonable agreement for a.

The temperature variation through the Curie point follows closely that previously observed [4] except for the collapse of the crystal field which occurs here at 129 °C (Fig. 2). This variation was reproducible for the several crystals chosen, and shows quite clearly the strong temperature dependence of D. Of note here are the three sharp lines of low intensity from fig. 1 labelled a, £, y at fields of 670 (g = 10), 1100 (g = 6) and 1770 (g = 3.7) G respectively of which only a has been previously observed [4].

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972259

C2-174

H.

G . MAGUIRE AND L. V. C. REES

FIG. 1.

-

The room temperature spectrum of Fe3+ in BaTiO3 with the c >> axis parallel to the d. c. magnetic field showing clearly the three sharp lines a ^(g = lo),

P

(g = 6) and y (g = 3.7).

The S4 terms in the Hamiltonian mix I + ³¹² >

with I ^f 512 > states, and the matrix elements transitions between the + ^{312 and}

⁾

512 states is possible though of low probability and hence low intensity which depend on a and not on D and is thus temperature independent. Such mixing of the states would give transitions occurring at g

=

10

( w

660 G) and g = 6

(-

1100 G) and would appear at first to be consistent with the lines a and p. However, we see from Fig. 2 that a and y, though temperature independent are not phase independent since they disappear at the Curie point, while P does not. This suggests a common origin for a and

y,

different from that for

/3.

The normal five line spectrum is associated with Fe3+ substituted for Ti4+ in a perfect environment ; however, even at these low impurity concentrations

(--

0.001 %) charge considerations necessitate the inclusion of oxygen vacancies which as possible nearest neighbours to some of the substituted Fe3+

gives rise to a strong axial distortion perpendicular to the

⁽⁽ c c)

axis, which would persist into the cubic phase.

This is consistent with a large D

(%

gPH) which for H

⁼

H,,, splits the lowest lying doublet at g

=

6. In addition for a site symmetry lower than tetragonal, introduction of an E term

($

gPH) in the Hamiltonian would produce splittings at g

=

10 and g

=

4.3, the former being strongly anisotropic in 8. This was confirmed over a limited

8

range only owing to reduc- tion in intensity as the magnetic field was rotated from H /!

c

to H 1 a. Departure from g

=

4.3 of

y(g =

3.7) may be due to a small D term. However, positive identification must for the present remain uncertain.

Application of an electric field to force the crystal into the ferroelectric phase was possible up to 125 OC only

;

no double hysteresis loops were observed above this temperature, and it is difficult to see why P, and D go to zero at different temperatures. It is possible that the d-electron orbitals and hence D would be more sensitive to the effect of the changing crystal field than the ion and hence P,.

Isomer shift and quadrupole splitting. - Good agreement was obtained in the measured values for the quadrupole splitting and isomer shift between the various crystals

;

these were 0.46 and 0.50 f 0.02 mm.

s-' respectively w.r.t. stainless steel (corrected), which is in agreement with previous determinations [5].

Figure 3 shows the room temperature spectrum obtain- ed. In some of the spectra weak additional peaks occur- red which could be attributed to ferrous ion and iron atoms which may have entered the lattice interstitially.

However, there is no doubt that the main absorption peak is due to substituted ferric ion as its isomer shift corresponds to that of a 3d5 configuration [8], [9], and that its ionicity is about 60% in agreement with previous determinations [S], [LO], [l 11.

In contrast to [5] our quadrupole splitting does not go to zero at the Curie point and the temperature variation of the isomer shift is perhaps indicative of a change in the s-electron density at the iron nuclei at the phase transition (Fig. 4). This temperature dependence does not concur with previous findings [5]

but is similar to that found in other perovskite sys- tems [12]. Also, no anomaly is detectable in the area of the absorption peak through the Curie point which would be indicative of a temperature dependent T. 0.

mode [13]. In this connection it is worth pointing out the difference found in the sign of the anomaly in the recoilles fraction5 near the Curie point in potassium ferrocyanide trihydrate [14], [IS] together with no detectable anomaly [16].

The quadrupole splitting AEQ which is a measure of the field gradient is directly related to the coefficient A; of V,, the axial component of the crystal field.

Using values of AEQ from [5] and our D values and

THE INVESTIGATION OF THE FERROELECTRIC PHASE CHANGE C2-175

Degrees Centigrade

FIG. 2. - The temperature dependence of the normal five line spectrum resulting in a collapse of the crystal field at 129 OC in contrast to the behaviour of the lines a,

8,

y. The shaded area is that range over which the

temperature was varied in 1 OC intervals.

.'I .

, .

... . . . . . . .

_{. . . .}

. , .. .,

.

'- *::,.

...

:.

.'

y..

...

...

. . .

^0.9

...

D

. . . . ... ...;

^:. ...

. .:. . *."

^"'

'.

.'. . . . . . . . .

::.:.:.:.

.

^." .:

.. ....*:

.- .

..: _. .

_,.:.

.:

_'

. . ^\,...;:

^{, *}

.:...:.. .

^:. : ^:.:, .;< . ^;.:.::.

.

c i ' 0.8

"

. : . t

. _{. . . .}

_.:... .. _{. . . .}

_.

0 ' 7 -

.:

. ;.

0.6

' C

.

^0.5

. . . ,

. ... .

_:>:.

. ." _,.

^{0 . 4}

, r : : _{-1 -2 -3}

0.3-

FIG. 3. -The room temperature spectrum obtained with 0.5 mC of C057 showing clearly the quadrupole splitting. O"-

0.1

c v o

I

2 0 40 6 0 80 100 120 140 160 ' c

and that to a first approximation D is linear in V,,.

Previous attempts to verify such a relationship using

FIG. 4. - The temperature dependence of the quadrupole

splitting and the isomer shift through the Curie point. Note the

Fe3+ were possibly because different

non vanishing of AEQ above the Curie point.

crystals were used for the Mossbauer measurements 121.

- -0.46

A E m

-

.

- 0-39

- -

-

-

21

^{0: o!, d.4}

d.5

0!6 d.7 0!s 0!9 I:o

plotting these against reduced temperatures shows that they lie on a smooth curve (Fig. 5). This indicates

'/TC

9.5 FIG. 5. -A comparison of D and AEQ (E. F. G.) against reduced temperature showing their similar behaviour over the same range

of distortion.

,I I e r r o r bar

that D and the E. F. G. vary in a like manner between

room temperature and the Curie point at the Ti site,

H. G. MAGUIRE AND L. V. C. REES

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(Kyoto),

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