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Variation of the local properties in TmxSe (0.8 < x ≤ 1.0) : 169Tm Mössbauer and X-ray measurements
J.A. Hodges, G. Jéhanno, D. Debray, F. Holtzberg, M. Loewenhaupt
To cite this version:
J.A. Hodges, G. Jéhanno, D. Debray, F. Holtzberg, M. Loewenhaupt. Variation of the local properties in TmxSe (0.8 < x≤ 1.0) : 169Tm Mössbauer and X-ray measurements. Journal de Physique, 1982, 43 (6), pp.961-971. �10.1051/jphys:01982004306096100�. �jpa-00209475�
Variation of the local properties in TmxSe (0.8 x ~ 1.0) :
169Tm Mössbauer and X-ray measurements
J. A. Hodges (1), G. Jéhanno (1), D. Debray (2), F. Holtzberg (3) and M. Loewenhaupt (4)
(1) DPh. SRM, CEN Saclay, 91191 Gif sur Yvette, France
(2) Laboratoire Léon-Brillouin, CEN Saclay, 91191 Gif sur Yvette Cedex, France (3) I.B.M. Research Center, Yorktown Heights, NY 10498, USA
(4) Institut für Festkörperforschung, KFA, 5170 Jülich, W. Germany (Reçu le 8 octobre 1981, révisé le 1er février 1982, accepté le 17 février 1982)
Résumé. - Nous rapportons des mesures Mössbauer, effectuées sur 169Tm entre 1,4 K et 295 K, relatives à cinq
échantillons de TmxSe couvrant toute l’étendue de composition où la symétrie globale reste cubique. Pour les
échantillons de plus grand paramètre de réseau (a = 5,712 Å, a = 5,711 Å) il n’y a pas d’évidence d’un écart à la symétrie locale cubique. Nous observons une distribution de champs hyperfins dans la région ordonnée. Pour les échantillons de paramètres intermédiaires (a = 5,685 Å, a = 5,672 Å) on observe une distribution de dédouble- ments quadrupolaires et donc d’environnements locaux. Pour l’échantillon le plus éloigné de la stoechiométrie
(a = 5,626 A), les observations Mössbauer révèlent, curieusement, l’existence d’un seul site non cubique. Une
étude radiocristallographique de cet échantillon, à la température ambiante, montre la présence de fines réflexions
de surstructure résultant d’une ségrégation de lacunes cationiques dans chaque second plan { 111 } à l’intérieur d’une structure en domaines. Les formes de raies Mössbauer dans la région paramagnétique, à basse température, témoignent d’effets de relaxation magnétique, dans le cas des trois échantillons de plus petits paramètres. Les spectres ont été convenablement ajustés en utilisant un modèle de champ hyperfin fluctuant.
Abstract - 169Tm Mössbauer absorption measurements have been made on the system TmxSe in order to investi-
gate the variation of the local properties with stoichiometry. Five samples spanning the whole range of composition
where the overall symmetry remains cubic were examined from 1.4 to 295 K. For the samples with the highest
lattice constants, a = 5.712 and 5.711 Å, there is no evidence of any significant departure from local cubic symmetry in the paramagnetic region, and in the magnetically saturated region we observe a distribution of hyperfine fields.
For each of the samples with intermediate lattice constants (a = 5.685 and 5.672 Å) a distribution of quadrupole splitting is observed in the paramagnetic region probably related to a distribution of noncubic environments.
Surprisingly, for the most nonstoichiometric sample (a = 5.626 Å) this distribution vanishes and there is only
one dominant noncubic site present. A room temperature X-ray examination of this sample shows the presence of sharp superstructure reflections indicating a cationic vacancy segregation in every second {111} plane within
a multi-domain structure. For the three smallest lattice parameters samples, in the paramagnetic region at low temperatures, the Mössbauer line shapes are influenced by magnetic relaxation. Adequate fits are obtained using
a fluctuating hyperfine field model.
Classification
Physics Abstracts
76. 80 - 61.708
1. Introduction. - The intermediate valence system Tm.,Se has been extensively studied by a wide variety
of techniques (see fM’Bexample reference [1] and further references therein). As for other NaCI type compounds
a range of compositions exists such that the overall symmetry remains cubic. However, the role of stoi-
chiometry is particularly important in Tm,,Se as it is closely related to the mean thulium valence state which in turn has an important influence on the pro-
perties of this system [2-5]. At the present time there is
no general agreement as to the precise correspondence
which exists between the chemical formula, the mean
Tm valency and the lattice r, As to the rela- tion between the lattice parameter and the mean Tm valence, for the largest lattice parameter (a - 5.71 Å)
for which the overall structure remains cubic, different
values have been proposed for the precise average
valency depending on the approach used. By extra- polating room temperature Curie constant data, a
value near 2.50 has been proposed [3] whereas from a
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01982004306096100
Vegard law extrapolation a value near 2.70 has been
deduced [4]. A microscopic measurement using the
L X-ray absorption edge [6] made for a sample with
a = 5.711 A established that the average valency was
2.58 in agreement with results obtained from X-ray photo-emission studies [7]. This mean valency is
closer to that obtained from the magnetic result than to that derived from lattice constant data. For the smallest lattice parameter (a - 5.63 A) for which the overall structure remains cubic, Tm is usually taken
to be present uniquely in the 3 ’ state.
Differences also exist concerning the proposed cor- respondence between the lattice parameter and the chemical composition. The value a - 5.71 A has often been taken to correspond to stoichiometric TmSe. But it has been suggested that this same lattice parameter corresponds to Tml.osSe and that
a - 5.69 A corresponds to stoichiometric TmSe [8].
It is to be noted that, in these last formulae the value for
x refers to the Tm/Se ratio and not to the occupancy number as there can be something like 1 % of Schottky
defects in all the Tm.,Se compounds including stoi-
chiometric TmSe [9]. At the other end of the range for
a - 5.63 A, it has been suggested that the Tm defi- ciency is as low as to correspond to the formula Tmo.79Se [4] whereas it has also been suggested
that nearly the same lattice parameter corresponds
to a limiting formula Tmo.87Se [2].
As the lattice parameter varies considerably over
the range where the overall symmetry remains cubic, it is a convenient parameter for initially character- izing the various samples studied here. The lattice parameters were measured, using copper radiation
(AKal = 1.540 56 Á), on powders resulting from crushing single crystals. The room temperature lat- tice parameters for the five samples examined were
5.712 3 (3) A, 5.710 9 (3) A, 5.684 6 (3) A, 5.671 6 (3) A,
5.626 0 (3) A and thus cover the complete range for Tm,,Se which retains overall cubic symmetry.
The last two samples originate from the same bat-
ches that gave samples 4 and 5 of reference [3], where
the lattice parameters are given as 5.665 A and 5.625 A respectively. At the high lattice parameter limit, two samples made in separate laboratories, having very
close, but not identical lattice parameters, were examined in order to study any possible sensitive dependence of the microscopic properties on the
lattice parameter in this region. The present study
concerns Mossbauer measurements on 169Tm in all the various samples at temperatures in the range 1.4 K to 295 K together with a room temperature X-ray analysis of the most non-stoichiometric sample (a = 5.626 A) which shows evidence of vacancy
segregation. No X-ray evidence for segregation was
found in the other samples. The particular interest
of a Mossbauer study is related to the fact that in contrast to most techniques used so far to study Tm,,Se, it is a microscopic method. It provides the
local hyperfine parameters so giving information on
the local environment and also on any possible spatial distribution of the local properties.
2. Results and discussion. - The results are group- ed into three sections. The first relates to the two
samples with large lattice constants, both situated towards the stoichiometric end of the compositional
range. The second relates to the two samples with
intermediate lattice constants and the third relates to the sample with a small lattice constant situated towards the most nonstoichiometric end of the com-
positional range.
2.1 SAMPLES WITH a = 5.712 A AND a = 5.711 A.
- In the paramagnetic region both samples show single lorentzian Mossbauer lines. The full width for both samples increases progressively, as the tempera-
ture is lowered, from 17.5 mm/s (120 MHz) at 295 K
to 25.5 mm/s (170 MHz) at 4.2 K. The present line widths are narrower than those reported by Triplett
et al. [10, 11] on a nominally stoichiometric sample
with an unspecified lattice constant for which, at
4.2 K, a full line width of 37.0 mm/s (250 MHz) was reported.
The temperature dependent line width could most
easily be associated with a small quadrupole splitting
of the nuclear levels induced by an electric field
gradient arising from the presence of small distortions from local cubic symmetry. If this association were
to be made, it would still not be possible to use it
to obtain any quantitative assessment of the depar-
ture from local cubic symmetry as the nature of the Tm electronic states are not known. In fact the existence of discrete crystal field levels in Tm,,Se with
lattice parameters near 5.71 A is still an open ques- tion [12].
Alternatively, the broadening could be associated with paramagnetic relaxation (intermediate relaxation region). It is possible also to associate the broadening
with the combined presence of both small nuclear
quadrupole splittings and paramagnetic relaxation.
The simultaneous presence of both these processes is, in fact, more clearly evident for the three samples of
the next two section. Whatever the origin of the tempe-
rature dependent broadening, it seems reasonable to say that there is no clear cut evidence to show that the local symmetry for Tm,,Se with a lattice parameter
near 5.71 A is other than cubic.
The absorption spectrum at 1.4 K, in the ordered
region (type I structure) [13, 14], for the sample with
a = 5.711 A is shown in figure 1. The spectrum for the sample with a = 5.712 A is essentially the same. The absorption spectrum is typical of an Ig = 1/2, Ie = 3/2
level system showing a dominant magnetic hyperfine
interaction and a smaller quadrupole interaction.
The two central lines are fairly sharp (full line width
15.5 mm/s (105 MHz)) and are well resolved. The other lines are however asymmetrically shaped with sharp
outside and shallower inside edges. This is especially
Fig. 1. - Mossbauer spectrum at T = 1.4 K for TmxSe
with a = 5.711 A. The solid line was obtained using a dis- tribution of hyperfine parameters (see text).
evident for the two outermost components. Previous data on a nominally stoichiometric sample also show-
ed the same asymmetry both at 1.4 K and at much lower temperatures [10, 11] suggesting that it repre- sents the low temperature limiting behaviour. The line widths in the ordered region were again much larger
than for those of the two samples reported here and no analysis other than that giving the mean hyperfine
field was reported.
The two most evident possibilities for explaining
the line broadening are magnetic relaxation and/or a
distribution of the hyperfme parameters. We did not obtain adequate fits using a single set of hyperfine parameters and a simple relaxation model involving a fluctuating hyperfine field. This approach was inade- quate as it generally gives a line shape where the
individual absorption lines are symmetrically broaden-
ed and not asymmetrically broadened as observed experimentally. We did however obtain adequate fits using an effective hyperfine field approach by intro- ducing distributions in the hyperfine field and in the nuclear quadrupole interaction. For these fits the line width was blocked at the value found for the two central lines (15.5 mm/s (105 MHz)), and the isomer shift was blocked at zero.
As the hyperfine field is the dominant interaction,
its distribution can be most readily found. For both
samples, the best fits to the total absorption spectrum
were obtained by assuming a distribution of hyperfine
fields of a roughly triangular form, the largest hyper-
fine field being present in the highest proportions.
As the outside edges of the lines are sharp, the maxi-
mum hyperfine field present is well defined and is 2 100 ± 25 k0e for the sample with a = 5.711 A
and 2 130 ± 25 k0e for the sample a = 5.712 A. The
lowest hyperfine field present is near 1 600 k0e in the
two cases. The particular fit shown in figure 1 was
obtained by assuming six discrete spectra, the asso-
ciated hyperfine fields being distributed evenly over the
mentioned field range and weighted’ following a triangular distribution.
The weighted mean value of the distribution of
hyperfine fields was near 1 950 kOe and agrees with the average value (1 930 kOe) obtained previously on a nominally stoichiometric sample [10, 11]. The hyper-
fine field range 1 600-2 100 k0e corresponds to 4f
electron magnetic moments in the range 1.62 Jlø to
2.12 YB if the conversion factors for Tml ’ are used
and 1.51 Jlø to 1.98 Jlø if the conversion factors for Tm 2+ are used [15]. A neutron diffraction study on a sample with a = 5.71 A gave an average moment of 1.7 ± 0.2 ,uB [ 13]. The present Mossbauer data extends all the previous data, in that, in addition to giving the
average value, they clearly show the presence of a
significant distribution in the hyperfine fields (and
in the associated 4f electronic moments).
The quadrupole splitting is small and lies roughly in
the range 3 mm/s to 11 mm/s (20 to 75 MHz). These
values which were obtained by assuming that the principal axis of the electric field gradient, at least partially induced by the exchange field, is collinear
with the hyperfine field. They encompass the previous- ly proposed mean value of about 4 mm/s (27 MHz) [10, 11]. The percentage variation is thus much bigger for
the quadrupole interaction than for the hyperfine field.
There is some indication, but no clear proof, that the quadrupole interaction also follows a roughly trian- gular distribution with its largest value associated
with the largest value of the hyperfine field.
As the line widths both above and well below TN,
for the two samples discussed here, are narrower than those of the sample of references [10] and [11], this suggests that both of the present samples are cha-
racterized by relatively narrower distributions in the
hyperfine parameters. It can be asked if, by going to
another sample with a slightly different lattice para- meter, whether this distribution would disappear leaving only one sharply defined set of hyperfine parameters for all the Tm centres in the sample. As
the two samples studied here, which are both very
near the high lattice parameter end of the TmxSe system, gave the same distribution we can conjecture
that such a distribution is in fact intrinsic.
Distributions in the saturated hyperfine parameters
obtained from Mossbauer measurements have often been encountered in the case of amorphous sys- tems [16] where they are related to the presence of
differing local environments. We can again conjec-
ture that here the observed distribution also stems from inhomogeneities which are linked in this case to the Schottky defect [9]. The relatively large distribu-
tion in hyperfine fields in proportion to the percen- tage of defects would be linked to the strong depen-
dence of the Tm properties on variations in the local environment.
One last possible explanation of the line broaden-
ing should be mentioned For the 169Tm Mossbauer measurements the sample necessarily has to be in the approximate form of a monolayer of small (- 10 ym) grains. This raises the possibility that strains may be responsible for the presence of the observed distribu- tion in the hyperfine parameters. This seems unlikely
as in the present case this distribution is most clearly
visible in the saturated ordered region whereas usually in this region the influence of small random strains is wiped out by the saturating magnetic
order [ 17].
2.2 SAMPLES WITH a = 5.685 A AND a = 5.672 A.
- These two samples fall towards the middle of the range of lattice parameters studied. They are grouped together here as they have comparable characteristics
especially when compared to the samples of
sections 2 .1 and 2 . 3 which are situated at the two ends of the lattice parameter range.
An important difference between the two present samples and those of the previous section is that sizeable splittings are now clearly visible in the para-
magnetic region especially at lower temperatures (Figs. 2a and 2b). This is direct evidence that the
majority of the thulium nuclei are situated in envi- ronments where the local symmetry is lower than cubic. Furthermore, even in the paramagnetic region,
each of the samples has absorption line shapes which
show the simultaneous presence of differing hyperfine
parameters probably related to the simultaneous presence of differing thulium environments.
At room temperature the absorption in both samples can be well fitted with a single Lorentzian
line shape with a full line width near 22 mm/s (150 MHz) (Figs. 2a, 2b). This is roughly 25 % higher
than the room temperature line width of the more
stoichiometric samples of section 2.1. At lower tem-
peratures the data fits were made by blocking the
individual line widths at this room temperature value.
To obtain the fits shown in figures 2a and 2b we have adopted the approach of introducing, at each tempe- rature, the lowest number of subspectra necessary to obtain an adequate fit. Moreover, in order to
compare the two samples with each other, at a given temperature we assumed the same number of sub- spectra for the two samples. At 77 K two quadrupole subspectra were used, the weighted quadrupole split- ting being near 14 mm/s (95 MHz) for both samples.
At 30 K three quadrupole spectra were needed for the fits shown with weighted mean 20.5 mm/s (140 MHz)
for the sample with a = 5.685 A and 28 mm/s (190 MHz) for the sample with a = 5.672 A. Thus at
this temperature the sample having the lower lattice parameter, that is the sample which is the more remo-
ved from the stoichiometric composition, shows a higher mean quadrupole splitting.
For both samples then, as the temperature decreases the mean quadrupole splitting increases. This is similar behaviour to that generally observed in inte-
Fig. 2. - Mossbauer spectra of Tm,,Se at various tempe-
ratures. The solid lines were obtained as an agregate of subspectra as described in the text. a) Sample with
a = 5.685 A ; b) Sample with a = 5.672 A.
