Zr stabilized Ti5Te4-type hafnium telluride

Article abstract of DOI:10.1515/znb-2005-0308

A new Ti₅Te₄-type compound, Zr stabilized hafnium telluride was discovered and characterized with electron crystallography. Microanalyses and preparative work show that Zr stabilizes Ti₅Te ₄-type hafnium telluride and destabilizes the competing phase, Hf₃Te₂.

Full text of DOI:10.1515/znb-2005-0308

Zr Stabilized Ti5Te4-Type Hafnium Telluride
Meng He, Arndt Simon, and Viola Duppel
Max-Planck-Institut fu¨r Festko¨rperforschung, Heisenbergstraße 1, D-70569 Stuttgart, Germany
Reprint requests to Prof. Dr. Arndt Simon. Fax: +49 711 689 1642. E-mail: A.Simon@fkf.mpg.de
Z. Naturforsch. 60b, 284 – 288 (2005); received December 14, 2004
A new Ti5Te4-type compound, Zr stabilized hafnium telluride was discovered and characterized
with electron crystallography. Microanalyses and preparative work show that Zr stabilizes Ti5Te4-
type hafnium telluride and destabilizes the competing phase, Hf3Te2.
Key words: Hafnium, Zirconium, Ti5Te4-Type Structure, Electron Crystallography
Introduction
for zirconium tellurides. Ti5Te4-type [11] Zr5Te4 was
identified a long time ago [12] and recently structurally
characterized [13, 14]. Hf5Te4 adopting the same struc-
ture type was mentioned only [15, 16] by citing unpub-
lished work.
Hafnium and zirconium are commonly thought
to be very similar in chemistry. However, they
are surprisingly dissimilar in the chemistry of
metal-rich chalcogenides. For instance, Hf2S [1]
and Hf2Se [2] are comprised of hexagonal close-
packed double metal atom layers spaced by sulfur
or selenium while both Zr2S [3] and Zr2Se [4]
crystallize in a Ta2P-type structure [5]. Zr2Te [6]
and Hf2Te [7] adopt Sc2Te- [8] and Nb2Se-type
structures [9], respectively. Hf3Te2 [10] has a lay-
ered structure consisting of bcc metal slabs sand-
wiched by Te atoms, but no counterpart is reported
In the Ti5Te4-type structure, M6X8 clusters (M =
early transition metal, X = metalloid or post transition
metal) [17] are trans-condensed into one-dimensional
bcc metal columns which are further connected with
each other via metal-metal bonding (see Fig. 1). More
than 10 Ti5Te4-type compounds have been reported up
to now. It was found that this structure type is adopted
only when the valence electron count (VEC) [18] and
atomic sizes [19] meet certain criteria. We predicted
Ti5Te4-type Hf5Te4 to exist [19] because the above-
mentioned criteria are met in this case. However, it
turns out that Ti5Te4-type hafnium telluride exists only
with the stabilization by zirconium. Here we report the
synthesis and characterization of this phase.
Experimental Section
Synthesis: Hf powder (99.6%, including 2 — 3.5% Zr,
325 mesh, Johnson Matthey), Zr rod (99.2+%, including
≤ 4.5% Hf, 12.7 mm in diameter, Johnson Matthey) and Te
(99.997+%, Preussag) were used as starting materials. Zr was
lathed into small pieces before being used. HfTe2 was first
prepared in evacuated silica ampoules and then further re-
duced with Hf and Zr in sealed tantalum tubes which were
protected by a silica tube by adding some silica wool. When
Zr pieces were used, a short tantalum rod was added and the
ampoules were repeatedly shaken during the heat treatment
to crush Zr pieces and to homogenize the sample. The am-
Fig. 1. A projection of the Ti5Te4-type structure along the
tetragonal c axis is shown at the left side. The part marked
with a dashed circle in the projection represents a single
1∞[M5X4] column (M = early transition metal, X = metalloid
or post transition metal), which is shown in perspective view
at the right. Filled small circles depict early transition metal
atoms, open ones metalloid or post transition metal atoms.
Two crystallographic sites for early transition metal atoms
are denoted as M1 and M2, respectively.
◦
poules were heated at 1000 C for about 40 days and then
quenched in cool water.
c
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M. He et al. · Zr Stabilized Ti5Te4-Type Hafnium Telluride
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TEM and EDXS: TEM observations and EDXS measure-
ments were performed with a Philips CM30/ST electron mi-
croscope which was operated at 300 kV and equipped with
an energy dispersive X-ray spectroscopy system (Noran, Sili-
con detector). High resolution images and selected area elec-
tron diffraction (SAED) patterns were recorded with a Slow
Scan CCD Camera (Gatan, 1024 × 1024 pixels). The pro-
gram Crisp [20] was used to process high resolution images.
X-ray powder diffraction: X-ray measurements were
made on a Stoe StadiP system equipped with an incident
beam curved germanium monochromator. Samples were
contained in glass capillaries of 0.2 mm diameter. Data were
collected in Debye-Scherrer geometry with a linear position-
sensitive detector (2θ range 4◦).
Results and Discussion
A sample prepared with stoichiometric quantities of
starting materials (Hf:Te = 5:4) shows strong peaks be-
sides reflections of Hf3Te2 on the powder diffraction
pattern. According to the equation proposed in our pre-
vious publication [19], the unit cell volume of the hy-
3
˚
pothetical Hf5Te4 phase is predicted to be 428.3 A .
Supposing the ratio of a/c is the same as that of
Zr5Te4, the lattice constants of the hypothetical Hf5Te4
˚
˚
should be a = 10.63 A and c = 3.79 A. Most of the ad-
ditional peaks observed on the powder pattern can be
assigned to this unit cell. The lattice constants refined
˚
from powder diffraction data are a = 10.674(3) A and
c = 3.754(1) A, in good agreement with the predicted
˚
values.
The sample has been further examined with TEM
and EDXS. Thin Hf3Te2 platelets can be easily identi-
fied based on the diffraction patterns. EDXS measure-
ments on 14 Hf3Te2 crystals show an average compo-
sition of 58.0% Hf, 1.7% Zr and 40.4% Te, the high-
est Zr content measured being 2.5%. Ti5Te4-type crys-
tals have also been identified by SAED. These crys-
tals are chunky blocks with irregular shape. In most
cases, it is difficult to find a thin enough area for high
resolution observations, and even SAED patterns can
be of poor quality. A significant enrichment of Zr in
these crystals has been observed. Microanalyses made
on seven Ti5Te4-type crystals gave an average compo-
sition of 48.6% Hf, 7.9% Zr and 43.5% Te, the low-
Fig. 2. Selected area electron diffraction pattern (a) and cor-
responding high resolution image (b) of Ti5Te4-type hafnium
telluride with electron beam parallel to [001]. (c) Recon-
structed image after lattice averaging, compensating for the
contrast transfer function (CTF) and imposing the crystallo-
graphic symmetry in projection P4. A projection of the struc-
ture was superimposed on (c) to highlight the features of the
reconstructed image.
est Zr content being 4.4%. A SAED pattern along the symmetry in projection P4, the reconstructed image
[001] direction and the corresponding high resolution (Fig. 2c) can be interpreted directly as a projection of
image are shown in Figs 2a and b, respectively. After the Ti5Te4-type structure along [001]. The coordinates
lattice averaging, compensating for the contrast trans- of the metal atoms in the ab plane are measured from
fer function (CTF) and imposing the crystallographic this image by searching the local maxima of the po-
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M. He et al. · Zr Stabilized Ti5Te4-Type Hafnium Telluride
Table 1. Fractional coordinates of Zr stabilized Ti5Te4-type
hafnium telluride.
Atoms
Hf1
Hf2
Te
Site
2a
8h
x
0
y
0
z
0
0
0
0.297
0.062
0.375
0.260
8h
Note: The SAED pattern and high resolution image were taken from
a crystal of composition 42.0% Hf, 13.7% Zr and 44.3% Te, as de-
termined by EDXS.
˚
Table 2. Interatomic distances (A) in Zr stabilized Ti5Te4-
type hafnium telluride.
Hf 1
−2 Hf 1
−8 Hf 2
−4 Te
3.75
3.16
2.85
Te
−Hf 1
−Hf 2
−2 Hf 2
−2 Hf 2
−2 Te
−2 Te
−4 Te
−2 Te
2.85
2.79
2.80
2.81
3.75
4.03
4.19
4.44
Hf 2
−2 Hf 1
−2 Hf 2
−2 Hf 2
−2 Hf 2
− Te
3.16
3.41
3.60
3.75
2.79
2.80
2.81
−2 Te
−2 Te
tential. The potential distribution of Te is smeared out
and its coordinates cannot be determined reliably in the
same way. Inspection of other structures of the same
type reveals that nonmetal atoms are always in con-
tact with five M2 atoms at nearly the same distances
and one M1 atom at a slightly longer distance. The co-
ordinates of Te in the ab plane have been determined
with this geometrical restriction. The shortest distance
between Te and Hf2 in the projection is only about
˚
2 A, approaching the point resolution of the micro-
˚
scope, 1.9 A. The coordinates along the c axis were
1
fixed at 0 or /2 from the geometrical considerations.
Attempts were not made to refine the structure on basis
of the electron diffraction data. A detailed description
of processing high resolution images with the program
Crisp can be found in a previous publication [21]. The
atomic coordinates are listed in Table 1. Supposing the
lattice constants of the crystal are the same as those
refined from X-ray powder diffraction data, the inter-
atomic distances were calculated (Table 2).
Fig. 3. Selected area electron diffraction patterns of Hf2Te3.
The indexing is based on a HfTe2-related unit cell.
with a unit cell comparable with that of HfTe2. The
positions of other reflections show that the structure
of this phase is quite different from Hf2Se3 [22] and
Zr2Te3 (Zr1.3Te2) [23]. Besides sharp Bragg reflections
diffuse scattering along c∗ is observed which, however,
is not investigated further.
Another phase was identified by TEM. The compo-
sition averaged over 8 EDXS measurements is 36.9%
Hf, 3.0% Zr and 60.1% Te. The metal to nonmetal ratio
in this phase suggests a compound Hf2Te3. It is worth
noting that the Zr content of this phase is in between
The significant enrichment of Zr in Ti5Te4-type
that of Hf3Te2 and the Ti5Te4-type phase. SAED pat- crystals raises the suspicion that it is a Zr stabilized
terns shown in Fig. 3 indicate that this phase is closely phase, and introducing Zr intentionally into the starting
related to HfTe2. The strong reflections can be indexed materials may improve the yield of this phase. A sam-
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M. He et al. · Zr Stabilized Ti5Te4-Type Hafnium Telluride
287
which consists of one-dimensional bcc metal columns,
and destabilizes the Hf3Te2-type structure built up
from two-dimensional bcc metal slabs.
It has been found that Hf atoms prefer sites allowing
for more metal-metal bonding in the quasi-binary sys-
tem Hf-Zr-P [24]. Comparing the structures of Hf2Te
and Zr2Te, Harbrecht and coworkers [6] found that
metal atoms in the former have enhanced metal-metal
bonding compared to the latter. The preference of Hf
to form more metal-metal bonding was explained by
the greater expansion of the 5d orbitals compared to
the 4d orbitals of Zr [25]. The Hf3Te2-type struc-
ture allows for more metal-metal contacts than the
Ti5Te4-type structure does, so it is preferred by Hf, and
the Ti5Te4-type structure is adopted only when suf-
ficient Zr is present to stabilize it. Obviously, only a
small amount of Zr is enough to stabilize the Ti5Te4-
type structure or destabilize the Hf3Te2-type struc-
ture: The highest Zr content in Hf3Te2 and the low-
est Zr content in the Ti5Te4-type structure observed
by EDXS are 2.5% and 4.4%, respectively. The find-
ing that the Ti5Te4-type structure collects Zr from
the starting materials may be exploited to purify Hf
metal and, hence, may help to solve a long-standing
problem.
Fig. 4. Experimental powder diffraction patterns of samples
with nominal composition Hf4.5Zr0.5Te4 (a) and Hf5Te4 (c)
(Cu-Kα1, 1 min/0.1◦ in 2θ). Simulated patterns of Ti5Te4-
type hafnium telluride (b) and Hf3Te2 (d) are also given for
comparison. Peaks marked with “+” and “*” result from im-
purities of Hf2Te3 and monoclinic HfO2, respectively.
ple with nominal composition Hf: Zr: Te = 4.5: 0.5: 4
was therefore prepared. The X-ray powder diffraction
pattern of this sample is shown in Fig. 4 together with
that of the stoichiometric one. It can be seen that Zr
intentionally added to the starting materials improves
the yield of the Ti5Te4-type phase greatly. On the other
hand, the amount of Hf3Te2 in the product is negli-
gible. Clearly, Zr stabilizes the Ti5Te4-type structure
Acknowledgement
M. He thanks Max Planck Society for the financial
support.
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