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MÖSSBAUER INVESTIGATION IN THE Ni0.6Fe2.4-I
BORACITE
D. Barb, S. Constantinescu, I. Zheludev, I. Iarmuhamedov
To cite this version:
JOURNAL DE PHYSIOUE Colloaue C6, suppldment au no 12, Tome 37, Dkcembre 1976, page C6-595
M~~SSBAUER
INVESTIGATION
IN THE Ni,.,Fe,.,-I BORACITE
D. BARB, S. CONSTANTINESCU Institute of Atomic Physics, Bucharest, Romania I. S. ZHELUDEV and I. N. IARMUHAMEDOV
Institute of Crystallography, Moscow, U. S. S. R.
R6sum6. - Nous Btudions par effet Mossbauer les interactions hyperfines dans le boracite Ni0,6Fe2,4-I dans le domaine de tempkatures allant de 4,2 K ?I 300 K. L'analyse des spectres Mossbauer montre que les champs hyperfins, varient d'une manikre coherente avec le modkle de l'environnement local qui tient compte des interactions avec les atomes de fer et du nickel situCs en position de premiers et seconds voisins mktalliques.
Abstract.
-
Using the Mossbauer effect we studied the hyperfine interactions in Nio. sFez.4-I boracite in the temperature range from 4.2 K up to 300 K. The analysis of the Mossbauer spectra reveals that the hyperfine fields on Fes7 vary according to the local environment model which takes into account the interactions with the iron and nickel atoms located in the first and second neighbour shell of the metallic ions.1. Introduction.,
-
The boracites are improper ferroelectric compounds of composition M3B,013X where M stands for a divalent metallic ion Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn or Cd, and X denotes one of the halogens C1, Br, I. Their crystal structure is nearly the same as that of the natural boracite Mg3B7OI3C1 [I]. In the ferroelectric phase the Mossbauer measure- ments on Fe,-X boracites have shown the existence of an orthorombic to trigonal transition121.
For 3d transition metals boracites at low temperatures a combined electric quadrupole and magnetic dipole interaction is also observed [3, 41.An increasing interest has lately been paid to the Ni3-I boracite due to its special place among the simi- lar compounds. It has a low cubic to orthorombic transition temperature (about T I
--
60 K). Below the Tl temperature it shows magnetic ordering [5, 6, 7, 81.The aim of the present measurements is to investi- gate the influence of the substitutional Ni-ions on the hyperfine fields in the policrystalline Ni0.,Fe2.,-I boracite.
2. Experimental results.
-
The Ni0.,Fe2.,-I was prepared by the chemical gas transport reaction method. The Mossbauer measurements were carried out with a constant acceleration ASA-type equipment and an Oxford Instruments He cryostat. A 5 7 ~ osource in a copper matrix was used. Source velocities of 4 6 mm/s and
+
8 mm/s were choosen to obtain the quadrupole spectra and the combined electric quadrupole and magnetic dipole spectrum, respecti- vely.In order to analyse the hyperfine parameters of the spectra a simulation program [9] was used. This computer program simulated the spectra using the Hamiltonian :
where
sin 0
x,
=-
g, H. [ I , cos 0+
-(I;" e+
15
e+i)] 2when the angle 0, p give the relative directions between the main axes of the field gradient tensor
and effective magnetic field H,,.
In figure 1 an example of a simulated quadrupole patterns with the best adjustement to the experimen- tal spectra is given. The Mossbauer parameters of quadrupole spectra are presented in table I, for 300 K, 77 K and 35 K.
The spectrum obtained at 4.2 K shown in figure 2 is a rather complex. Therefore only a rough simulation could be attempted. The values of the hyperfine para- meters are given in Table 11.
The sets of the hyperfine parameters Sgn V,,, q, 0, p were taken identical to those of the sublattices found for Fe3-I boracite [4].
The errors of the parameters listed in Tables I and
C6-596 D. BARB, S. CONSTANTINESCU, I. S. ZHELUDEV AND I. N. IARMUHAMEDOV
The Mossbauer parameters of the quadrupole spectra : quadrupole splitting, 2 E (mm/s) ; isomer shift with respect to 5 7 C o / C ~ source, 6 (mm/s) and the weight of the different sublattices, w
(%)
Temperature 300 K Sublattice w 6 2 E - --
A ( : 20 0.86 1.29 42 0.86 0.45( :
5.1 0.87 1.02 10.3 0.86 0.43c [ :
4.5 9.0 0.86 0.87 0.82 0.38 2.2 0.86 0.60 4.5 0.86 0.34'
0
...
...
.-.
.
.
-
_....
...
FIG- 2. - The cornplex hyperfine s p e c obtained at
m.
.
...
.
....
.
*
.-,
TABLE I1FIG. 1.
-
The Mossbauer experimental and simulated spectra Sublattice w 6 Aof Nio.6Fe2.4-I boracite at 300 K, 77 K and 35 K. -
-
A -r H n 8 q D A - - -14.0 1.06 -2.33 1 191 51 0 18.6 1.07
-
2.33 1 192 74 0 a90 3 14.0 1.06-
2.33 1 133 46 21 I1 are,+
1%
for the weight,+
0.05 mm/s for 6 and 4 14.0 1.06-
2.33 1 120 78 10 2 E ,5
10 kOe for H and+
2 degrees for 8 and q .1 5 6.4 1.02
-
2.0 1 151 51 0- T=35K
1
The hyperfne parameters of the Ni,.,Fe,,-I boraciteI I I I at 4.2 K. The efective magnetic$eld, H, is given in kOe
3. Discussion.
-
As one can see from table I the quadrupole spectra were simulated by superposition of four hyperfine subspectra A, B, C and D, with two doublets, for 300 K and 77 K and respectively only one doublet for 35 K.The spectra at 300 K and 35 K are characteristic for the orthorombic and trigonal phases respectively. Thus the 57Fe spectrum shows two quadrupole pairs for the orthorombic phase and only one for the trigonal one.
In the orthorombic phase the inner doublet is two
-
1 0 1 2rnm I S and A ( = 2
&/dl
+
y '13) in mm/sMOSSBAUER INVESTIGATION IN THE Ni0.~Fe2.4-I BORACITE C6- 597 The spectrum obtained at 77 K correspond to the
orthorombic-trigonal transition region.
The existence of more than one subspectra indicates the presence of the different environments, around Fe2+ ion with different Ni neighbours.
Supposing that in Ni,Fe3 -,-I boracite, the Fe2+ ions can be substituted by Ni2+, the probability to have n-Ni ions and (8
-
n) Fe-ions in the first metallic neighborshell is given by
where all eight sites are considered to be equally occupied.
According to (5) for x = 0.6 the probabilities to have 0, 1, 2 and 3 Ni2+ are 61.7
%,
15.4%,
13.4 % a n d respectively 6.7%.
The simulation weights of the four hyperfine sub- spectra A, B, C and D permit us to assign them to the Mossbauer patterns of the 57Fe-nucleus with 0, 1 , 2 and 3 Ni2+ in the first metallic neighbor shell.
The lower quadrupole splitting of the A subspec- trum at 35 K relative to the Fe,-I one, points out the influence of the second metallic shell. As the numbers of Ni2+ in the first metallic shell increaseas the value of the quadrupole splittings becomes smaller.
It seems that Ni2+ in the first and second metallic shells produces not only a smaller local distortion [2],
but also a decrease of the dipole contribution to the electric field gradient tensor.
The experimental Miissbauer spectra show that this boracite has a lower value of the orthorombic to trigonal transition temperature T2 (= 35 K) when compared to the Fe3-I boracite (T2 = 200 K). At the same time at TN = 30 K a broadening of the Moss- bauer spectra takes place due to the appearance of an internal magnetic field.
The helium temperature spectrum was simulated by a superposition of two hyperfine subspectra A and B, corresponding to n = 0, 1 and respectively 2 of Ni2+, each of them having four complex hyperfine sublattices. The smaller values of the magnetic hyperfine fields compared to those of the Fe,-I boracite, can be explained by a smaller orbital contribution compared to the negative Fermi contact field [4].
In this way the substitution of ~ eions by Ni2+ ~ + produces a decrease of the value of the magnetic hypedine field parameter. This is also caused by the change of the local environment seen by the Fe5' nucleus.
Despite of the crystalline and magnetic phase transi- tion no discontinuity was observed in the isomer shift. For all the sublattices for a given temperature we found nearly the same 6 values. The average isomer shift at 300 K shows a large covalency of the iron ion
with its neighbours however the binding is still ionic. The lower values of 6 obtained in Ni,.,Fe2.,-I bora- cite were explained previously [2].
We conclude that the changes in the hyperfine para- meters obtained from the experimental Mossbauer spectra, can be explained by the local environment model.
References
[I] ITO, I., MORIMOTO, N . and SADANAGA, S., Acta Cryst. 4 (1951) 310.
[2] TROOSTER, 3. M., Phys. Stat. Solids 32 (1969) 179.
[3] JAGANNATHAN, R., TROOSTER, J. M. and VIEGERS, M. P. A.,
Int. J. Magnetism 4 (1973) 363.
[4] LINK, R. and WURTINGER, W., J. Physique Colloq. 35
(1974) C 6-581.
[5] QUBZEL, G. and SCHMID, H., Solid State Commun. 6 (1968) 447.
[6] ZHELUDEV, I. S., PEREKALINA, T. M., SMIRNOVSKAIA, E. M., FONTON, S. S., IARMUHAMEDOV, I. N., Pisma J. E. T. F. 20 (1974) 289.
[7] WARTBURG, W. V., Phys. Stat. Solids 21 (1974) 557.
[8] BARB, D., TARINA, D., CONSTANTINESCU, S., HIMICH, T. A., ZHELUDEV, I. S., IARMUHAMEDOV, I. N., Rev. Roum. Phys., 21 (1976) 105.
[9] BARB, D., CONSTANTINESCU, S. and LAZAR, D., ZFA SR
