NEUTRON DIFFRACTION, MÖSSBAUER AND MAGNETIC INVESTIGATION OF RUTILE-TYPE TANTALATES MTaO4 (M = Ti, V, Cr, Fe)

(1)

HAL Id: jpa-00213988

https://hal.archives-ouvertes.fr/jpa-00213988

Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

NEUTRON DIFFRACTION, MÖSSBAUER AND MAGNETIC INVESTIGATION OF RUTILE-TYPE

TANTALATES MTaO4 (M = Ti, V, Cr, Fe)

N. Fadeeva, R. Ozerov, V. Smirnov, V. Makarov, E. Makarov, V. Povitsky, N.

Krjukova, R. Zorin

To cite this version:

N. Fadeeva, R. Ozerov, V. Smirnov, V. Makarov, E. Makarov, et al.. NEUTRON DIFFRAC- TION, MÖSSBAUER AND MAGNETIC INVESTIGATION OF RUTILE-TYPE TANTALATES MTaO4 (M = Ti, V, Cr, Fe). Journal de Physique Colloques, 1971, 32 (C1), pp.C1-503-C1-505.

�10.1051/jphyscol:19711167�. �jpa-00213988�

(2)

JOURNAL

DE PHYSIQUE

Colloque C 1, supplément au n° 2-3, Tome 32, Février-Mars 1971, page C 1 - 503

NEUTRON DIFFRACTION, MOSSBAUER AND MAGNETIC INVESTIGATION

OF RUTILE-TYPE TANTALATES MTa0

⁴

(M = Ti, V, Cr, Fe) N. A. FADEEVA, R. P. OZEROV, V. P. SMIRNOV

Karpov Institute of Physical Chemistry, Moscow, U. S. S. R.

V. A. MAKAROV, E. F. MAKAROV, V. A. POVITSKY Institute of Chemical Physics, Moscow, U. S. S. R.

N. A. KRJUKOVA, R. B. ZORIN

Institute of Physico-Technical and Radio Measurements, Moscow, U. S. S. R.

Résumé. — Nous avons effectué par diffraction des neutrons la recherche de l'ordre des atomes et des paramètres de l'oxygène dans les tantalates du type rutile MTa(>4 (M = Ti, V, Cr, Fe). Des mesures magnétostatiques, par effet Môssbauer et par diffraction neutronique révèlent trois phases magnétiques dans FeTaÛ4 ; l'une d'elles est paramagné- tique et les deux autres sont ordonnées au point de vue magnétique.

Abstract. — Neutron diffraction investigation of atomic ordering and oxygen parameters in rutil-type tantalates MTa04 (M = Ti, V, Cr, Fe) has been carried out. Magnetic, Mossbauer and neutron diffraction measurements reveal three magnetic phases in FETaC>4, one of them being paramagnetic and two-magnetically ordered.

3 d-transition metal tantalates are of interest because of their magnetic, electric and other properties. Neu- tron diffraction, Mossbauer and magnetic investigation of the tantalates M T a 0

4

(M = Ti, V, Cr, Fe) have hence been undertaken.

The tantalate powder samples were synthesized by Rozhdeststvensky [1]. He found by X-ray analysis that all these compounds are rutile-type structures () with the metal atoms perfectly ordered ; the proposed ordering is in disagreement with the result of the X-ray investigation of Keller [2] who found complete disorder* in the same compounds. Some optic, magnetic and electric properties were investigated in [3, 4, 5, 6].

We have reinvestigated the samples prepared in [1].

Neutron diffraction patterns were recorded with a neutron powder spectrometer at three different tempe- ratures 300, 77 and 4.2 °K. X-ray analysis of the samples had been made with a multipurpose diffrac- tometer at room temperature. Static magnetic pro- perties were mesured by the Faraday method in magnetic fields up to 6 kOe, in the temperature range 300-4.2 °K. Mossbauer spectra were measured at fixed sample temperatures of 300, 85 and 4.2 °K with Co

5 7

in a chromium matrix as the source.

The large difference in neutron scattering amplitudes of Ti and V (b = - 0.38 and - 0.05 respectively) and Ta (b = 0.70 x 1 0

^{- 1 2}

cm) greatly increases the sensi- tivity in determining the metal atom ordering as compared with the use of X-rays. The ordering in Cr and Fe compounds were also studied (fe

Fe

= 0.96, Z»

^Cr

= 0.352 x 10"

^{1 2}

cm). The discrepancy factor R for two models-ordered and disordered metal arrange- ment-are given in Table I. The temperature factor was neglected but its introduction in the case of V T a 0

4

improved the R value (see Table I). The results are consistent with a complete disorder model: Ta and M atoms occupy position 2a of the rutile structure in a disordered manner.

TABLE I

Discrepancy factor Rfor ordered and disordered models

Ti V Fe

ft -"•disordered

0.083 0.080 (0.055) (*) 0.080

-^ordered

0.72 0.62 0.17 () With temperature factor B = 0.4 A.*

The large contribution of oxygen to the neutron scattering permits us to refine the only structure para- meter x

0

. The minimum R-value corresponds to JC

0

= 0.295 + 0.005 in all substances.

The F e T a 0

⁴

sample has been investigated in greater detail. Figure 1 represents the results of magnetic measurements, figure 2, the neutron diffraction pat- terns, figure 3, the Mossbauer spectrum. The present

Msp(x10", 0e/g)

F'G. 1. — Results of magnetic measurements. The upper curve (scale et left) represents the reciprocal susceptibility versus temperature ; the lower curve represents the specific magneti-

zation versus temperature at H = 950 Oe.

() The rutile structure belongs to the tetragonal space group T>™*

4b

— P 4

2

/mnm, Z = 2, a = b = 4,651, c = 3.031 A (for FeTaCU, according to [1]) ; M atoms are in 2 (a) positions, oxygen atoms are in 4 (f) with one parameter xo & 0.33.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711167

(3)

C l - 5 0 4

N.

A. FADEEVA ET COLL.

0 :-

M 20 90 40 50.

2

8 FIG. 2. -Neutron diffraction patterns of FeTaO* at 300 OK.

77 OK and 4.2 OK. The additional unindexed peaks are due to scattering from the cryostat and to impurities. More parti- cularly, the peak at 100 does not change significantly when heating to 600 O C and was considered to be caused by an

impurity.

FIG. 3.

-

Mossbauer spectrum of FeTa04 at a) 300 OK and 85

OK

b) 4.2 OK.

The zero point is taken as the center of the C057 (Cr) line.

interpretation of our experimental results are now given.

There are three phases in FeTa04.

I. The paramagnetic high temperature phase (Fig. 4a), is stable at room temperature. There is aqua- drupole splitting in the Mossbauer spectrum (Fig. 3a).

The value of this splitting is A

=

(0.53 +_ 0.03) mm/s, and the isomer shift is

6 =

(0.68

$-

0.03) mm/s (scaled with respect to sodium ferrocyanide). They are characteristic of Fe3+ in an octahedral environ- ment

[7].

The doublet lines are sharp, 2 r =,0.3 mm/s, perhaps due to the small influence of disor- dered Ta5+ atoms on the electric field gradient in the ~e~ " positions. The paramagnetic moment of Fe3' is p

=

5.3 p,, the Curie-Weiss constant

C,

=

1.117

x

and 0,

=

- 456OK (Fig. 1).

FIG. 4.

-

Atomic and magnetic structure of FeTa04 a) paramagnetic,

b) antiferromagnetic, c) ferrimagnetic phase,

The finely hatched circles have

t

⁼0.5 (see text), the openhat- ched circles have 1 = 0.75 ; the crossed circles havekA-= 0.25.

11. The intermediate phase 2 exhibits the same

Mossbauer spectrum as phase 1. The neutron diffrac-

tion pattern at 77 OK, however, has an additional

magnetic peak at the 100 position. Qualitative peak

intensity analysis leads us to the conclusion that in

phase 2, antiferromagnetic ordering associated with

atomic disorder takes place (Fig. 4b). The magnetic

moments in the corners of the unit cell are directed

along the c-axis and opposite to the moments in the

body-centered position. The magnetic structure belongs

to the Shubnikov group

^0::69-

P4;/mnm1 (accord-

ing to the nomenclature of [8]) with metal atoms in

(4)

NEUTRON DIFFRACTION, MOSSBAUER AND MAGNETIC INVESTIGATION C 1

-

⁵⁰⁵

2a positions 000 and 4 + + (for notation, see

[ 9 ] )

and oxygen atoms in 4f positions with

x, =

0.295. The effective magnetic moment of Fe3+ is 2.96 pB.

The combined nuclear and magnetic discrepancy factor R is 4,3 %.

Thus there is disagreement between the Mossbauer and neutron diffraction data. This discrepancy can be resolved by proposing a dynamical ordering effect for the paramagnetic spins of the intermediate phase 2.

The mean life-time of these ordered spins must be

z

with >

z

> 1 0 - l ~ s. The macroscopic magnetic properties are consistent with this proposal (see Fig. I).

111. The low temperature phase 3 differs from phases

1

and 2. The Mossbauer spectrum consists of six broa- dened lines (Fig. 3b) (with 2 r ^-- 1.5 mm/s) in the magnetic hyperfine structure. The effective magnetic field on the Fe57 nuclei is He,,

=

(486 f 5) kOe. If we suppose that He,, is proportional t o the magnetic moment pFe, and equal to 550 kOe [lo] for Fe3+ in an octahedral environment, then in our case at 4.2 OK, p ~ e

=

4.4

fit%.

The neutron diffraction pattern at 4.2

OK

has some additional peaks, for example, 200, see figure 2. The presence of both 100 and 200 force us t o accept a

ferrimagnetic model with different moments in 000 and 3 3 3 positions (fig.

4c).

This differene can be associated with partial atomic ordering or with a distortion of the oxygen octahedra. In both cases it is necessary to choose a new unit cell with A

⁼

B

=

J2a, c

=

c, z

⁼

4, space group D~~

19

- Cmmm, atomic positions

:

2 Fe (if completely ordered) -in2a,2Ta-in2~,4O-i114g with x x 0.30, 4 0 in 4 j with

y R!

0.20. The best agreement between experimental and calculated intensities is found with 1

=

0.25 (1 is the proportion of Ta atoms in 2a posi- tion), with an effective magnetic moment for the atoms in the 2a-position pa

=

3.28 p,, and with pc

=

1.12 p ~ .

The broadening of the Mossbauer lines at low temperature (see Fig. 3b) is consistent with the proposal that two different sublattices coexist

:

each line consists of two unresolved lines causing the broadening.

Attempts to get a good fit of calculated and obser- ved data on any realistic values of moments are still unsuccessful.

Acknowledgements. - The authors wish to thank F. Rozhdeststvensky for giving us the samples, and D. Astrov and B. Al'shin for valuable discussions.

References

[I] ROZHDESTSTVENSKY

(F.

A.), Thesis, Sverdlovsk, 1969. [61 KRYLOV (E. I.), ROZHDESTVENSKY (F. A), PILI-

[2]

KELLER (G.),

2.

anorg. nllg. Chem., 1962,

318,

89. PENKo (G. I.),SOLODOV(V. P.), ibid. 1968, 4,477.

[3]

ROZHDESTSTVENSKY (F. A.), KASIMOV (G. G.), KRY- 171 BELJAEV (L. M.), LUBUTIN (I. S.) , MILL (B. V.).

LOV

(E. I.), FiS. Tverdogo Tela, 1969,

11,

1689. POSITSKY (V. A.), Fis. Tve~dogs Tela, 1969,