A 57Fe MÖSSBAUER STUDY OF THE HIGH PRESSURE PHASE TRANSITION IN THE SYSTEM V1-xFexO2 (x = 0.024)

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A 57Fe MÖSSBAUER STUDY OF THE HIGH

PRESSURE PHASE TRANSITION IN THE SYSTEM

V1-xFexO2 (x = 0.024)

G. Amthauer, S. Hafner, J. Pebler, K. Schmidt

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Dbcembre 1976, page C6-599

A 57Fe MOSSBAUER STUDY

OF

THE HIGH

PRESSURE

PHASE

TRANSITION IN THE SYSTEM

V,

-

,Fe,O,

(x

=

0.024)

G . AMTHAUER, S. S. HAFNER, J. PEBLER and K. SCHMIDT Department of Geosciences and Department of Chemistry, University of Marburg, 3550 Marburg, Germany (F. R. G.)

RbumB. - La transition M2, M3 de V0.976Feo.02402 a kt6 ktudike jusqu'h 60 kbar en utilsant l'effet Mossbauer dus7Fe. On observe une variation brutale de l'effet quadrupolaire et une variation continue du deplacement isomkrique avec inversion des valeurs relatives au moment de la transition M2, M3.

Abstract.

-

The M2, M3 transition of V0.9~6Fe0.02402 was studied up to 60 kbar using the Mossbauer effect of 57Fe. A discontinuous change of the quadrupole splittings and a continuous change of the isomer shift with a crossing over at the M2, M3 phase transition is found.

In the systems Vl-,Fex02 and V,-,CrxO,

-

T (0 < x

<

0.04), four different phases are found which K 1 may be related to the rutile type [I-51. At room tem- LOO.

perature, pure VO, has a monoclinic structure called R

MI, with the space group P2,/c and four formula units

Tt(xJ --. XA-'

in the unit cell. Vanadium and two sets of oxygen *I\ .. .. atoms occupy general positions. At T, = 340 K,

there is a phase transition to the ideal rutile type R " a M2

associated with an increase of the electrical conducti- 3 0 0 - I, TY(x)

vity. If vanadium is substituted by chromium or iron

b

two additional, monoclinic phases, M2 and M3, appear : M2 is the high-temperature and M3 is the low-temperature phase. X-ray diffraction studies of Vo~,7,Cro~,,,0, single crystals showed [5] that M2

200' MI

M3

and M3 have the same spacegroup C2/m with eight formula units in the unit cell, but there is a significant discontinuity of the cell volume at the M2, M3 phase transition. The vanadium atoms are in the special positions 4g (symmetry 2) and 4i (symmetry m) ;

oxygen is located at a general position and two positions 4i.

X 01 .o 2 0 3 0 L

The systems V1-,Fe,O, and 57Co activated V, -,CrXO, were studied in the region 0

<

x

<

0.04 by use of the Mossbauer effect of 57Fe [6-81. The phase diagrams of the two systems are principally the same [6-81. Figure 1 shows the V,-,Fex02 system using the results of [6-71. Typical Mossbauer spectra are illustrated in figure 2. The results and conclusions may be summarized as follows :

1. In the R phase, the iron atoms occupy only one crystallographic position as expected. In the M2 and M3 phases two distinct quadrupole splittings are observed indicating a nearly disordered distribution of iron over the two positions 4g and 4i.

2. At the M2, R transitions, the isomer shifts and

quadrupole splittings show a discontinuity. At the M2,

FIG. 1. - Phase diagram of the system V1-xFex02 in the region

0 < x

<

0.04.

M3 transition (T,. = 275 K for

x

= 0.024), the change of the isomer shifts is continuous (Fig. 3), whereas the quadrupole splittings show an abrupt decrease from higher to lower temperatures (Fig. 4).

3. At the M2, M3 transition, crossing over of the isomer shifts is observed (Fig. 3), which coincides always with the transition, independent of

x

[6, 71.

4. The discontinuity of the quadrupole splittings at the M2, M3 transition indicates a crystallographic change, e. g. a first order phase transition, which generally requires a change of volume and isomer shift as observed for the M2, R transition. The conti-

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G. AMTHAUER, S. S. HAFNER, J. PEBLER AND K. SCHMIDT

FIG. 2.

-

57Fe spectra of V0.976Fe0.02402 at different tempe-

ratures. Upper spectrum : R phase (absorber at 373 K). Middle spectrum : M2 phase (absorber at 314 K). Lower spectrum :

M3 phase (absorber at 275 K).

FIG. 3. -Plot of the Fe3+ isomer shift 6 at the two cation positions in V0.976Fe0.02402 against temperature.

1.25 Fell I ' I I Fel21 0.25 % 1 1 6 I , Y -

@I

Tf Tt

iw

T - 100 200 3 00 LOO

-

K FIG. 4. -Plot of the Fe3+ quadrupole splittings AEQ at the two cation positions in V0.976Fe0.02402 against the temperature.

High pressure experiments were performed at room temperature up to 60 kbar using a cell similar to that described by [8, 91. Here, the y-rays pass through anvils consisting of B,C. They are parallel to the direc- tion of the applied force. The pressure was calibrated with the known changes of electrical conductivity of Bi, Yb, and Ba. At room temperature, the M2 phase transforms to the M3 phase with rising pressure. The fairly sharp transition could be observed at 17.5 (f 1.0) kbar. As illustrated in figure 5-7 (cf.

nuous behavior of the isomer shifts points to a first

order transition with AV = 0, High pressure Moss-

u2L-.----

-3.60 -1.80 o 1.80 3.60 ~ / m n s - l bauer yield jnformation On the

_FIG.

_5.

_-

_{57Fe spectra of V0.976Fe0,02402}_{at different pres-}

nature of the ~ 2 , M3 transition, since a decrease of sures. LOW, spectrum : M2 phase (absorber at 10 kbars).

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57Fe MOSSBAUER STUDY O F THE HIGH PRESSURE PHASE TRANSITION I N THE SYSTEM V1-,FeX02 C6-601

FIG. 6. -Plot of the Fe3+ isomer shift 6 at the two cation positions in V0.976Fe0.02402 against the pressure.

FIG. 7. -Plot of the Fe3+ quadrupole splittings AEQ at the cation positions in VO. 97 6Fe0. 0 2 4 0 2 against the pressure.

also Table I), the transition was accompanied by a discontinuous behavior of the quadrupole splittings and a continuous change of the isomer shifts again with

Nuclear hyperfine parameters of 57Fe

in V0.976Fe0.02402 at dzjierent pressures ( T = 295 K )

Pressure P &bar) - 10 25 40 50

Isomer shift (a)

6

(mmls) Phase Fel Fe2

- - -

M2 0.198(8) 0.192(8) M3 0.185(8) 0.190(8) M3 0.168(8) 0.187(8) M3 0.157(8) 0.186(8) (a) Referred to 57Co-Pd source at 295 K.

Quadrupole splitting ( 0 ) A& (mmls) Fel Fe2 -

-

1.30(1) 0.28(1) l.ll(1) 0.23(1) 1.09(1) 0.21(1) 1.12(1) 0.20(1)

a crossing over. The nature of the pressure dependence of the two shifts suggests that the volume of the Fe2 coordination octahedron remains constant while the volume of the Fel coordination octahedron is reduced with rising pressure. The volumes of the vanadium octahedra were calculated on the basis of the structure data at different temperatures given by [ 5 ] . The volume of octahedron 1 in M2 is 9.62 A3 ( T = 295 K )

and 9.45 A3 (T = 180K) in M3 (AV = 0.17 A3), whereas the volume of octahedron 2 in M2 is 9.70 A3 (T = 295 K) and 9.71 A3 ( T = 180K) in M3

(AV = 0.01 A3). A similar temperature and pressure dependence of the iron octahedra is suggested by the Mossbauer spectra. The significant change of volume and isomer shift at one of the two octahedra seems to be indicative of a distinct change of chemical bonding. The bulk volume of the unit cell diminishes conti- nuously at the phase transition. A martensitic type of the M2, M3 transformation is thus quite likely.

References

[I] GOODENOUGH, J. B., J. Solid State Chem. 3 (1971) 490. [6] PEBLER, J. and SCHMIDT, K., Proc. Int. Conf. Moss. Spec., [2] VILLENEUVE, G., BORDET, A., CASALOT, A. and HAGEN- Cracow (1975) to be published.

MULLER, P., Mat. Res. Bull. 6 (1971) 119.

[31 ~ H I I ,

s., KOSUGE,

K. and o K I N A ~ ~ , H., J .

state

171 PEBLER, J., SCHMIDT, K. and WASINSKI, H., Z. Naturforsch.

Chem. 6 (1973) 258. (1976) in press.

[4] WERTHEIM, G. K., BUCHANAN, D. N. E. and GUGGENHEIM, H. _{[8] KLEIN,}_U._{F., Dissertation, Miinchen, 1976.}

J., Bull. Am. Phys. Soc. 12 (1967) 23.

[5] MAREZIO, M., MCWHAN, D. B., REMEIKA, J. P. and DER- 191 SCHILLING, J. S., KLEIN, U. F. and HOLZAPFEL, W. Be, Rev.