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On the crystal structure of TaTe4
SHORT COMMUNICATIONS
On the crystal structure
231
of TaTe,
As a link in studiesIof the properties of the polychalcogenide compounds of the transition metals of subgroups IV, V and VI, the crystal structure of TaTed has been investigated. The TaTed phase was identified by UKRAINSKII et a1.4, but the composition TaTeh was first established by the present authors”. TaTe4 cq-stallizes in the tetragonal class with a = 2 s (7.514A, c = 3 x 6.809’1, I:,/&= 3;‘~ x 1.045. The py~nometric density, 7.901 g cm-3 (at 25Y), corresponds to 12 4 z (Xc = 12 Y 1.99) TaTeJ-groups per unit cell. Systematic extinctions in the X-ray
photographs
are ME for 1 = zn _t I and okl! for 1 == 212 + I, the Laue symmetry
is qjmmm and the possible space groups are P4cc (Ci,) and Pqlmcc (@A). Although oscillation, Weissenberg and precession photographs clearly showed the doubling of the a-axis and the tripling of the c-axis the additional reflections are faint and the full cell may be regarded as a superstructure of the smaller cell (subcell). Both the full cell and the subcell have the same extinctions and therefore the same possible
space groups. The present
paper concerns
the determination
of the crystal
structure of the TaTe* subcell. The subcell contains two tantalum and eight tellurium atoms. The two tantalum atoms must thus be located in a twofold position and as no other extinctions
have been observed
the eight tellurium
atoms must be in an eight-
fold position. It was decided to start with a Patterson projection along [OOI], where both space groups have symmetry p4.n~. Intensity measurements of the lzko reflections on W’eissenherg photographs were carried the combined Lorentz and polarization
out microphotometrically. factor and for absorption
Corrections were made.
for For
calculation of Fc-values the atomic scattering factors were taken from FORSYIYH A?JD \VELI*\ sj. Tentative atomic positions were easily deduced from the Patterson projection. Based on these approximate
atomic parameters
a set of signs and a Fourier projection
were evaluated. The latter clearly showed all atoms resolved. The atomic parameters were refined by means of one Fourier and two difference syntheses. The final Fourier map is shown in the lower part of Fig. I. The best atomic sponded
to an R-value
2Ta in o,o 8Te in x = 0.1440,
(X =
Ci[Fo(
y = 0.3280;
-
IFcll / ClFol)
parameters,
of 0.107,
which corre-
are:
etc.
with B = 0.5 for both kinds of atoms. In terms of the space group of highest
symmetry
P4lmcc
(D&) the atoms
would
have fixed z-coordinates. With 8 Te atoms in (m) and z Ta atoms in (a) (z Ta atoms in (h) would lead to unreasonable Ta-Te distances) a comparison of observed and calculated structure factors for the ho1 reflections was carried out. Intensity measurements of the hoi reflections on precession photographs were carried out microphotometrically. absorption
Corrections for the combined Lorentz and polarization factor but not for were made. Atomic scattering factors were taken from International
Tables”. Although the Fourier map (the upper part of Fig. I) appears as expected, the R-value was as high as 0.212. A new calculation was therefore carried out for the same atomic arrangement described in terms of the space group P4cc (Cz,). One of the two kinds of atoms may be given arbitrary z-coordinates. In the following calculation
232
SHORT
COMMUNICATIONS
the 8 Te atoms in (d) were given a fixed z-coordinate (zre = 0). A set of calculated structure factors was then prepared for values of .zTabetween o and 2 and the corresponding reliability index R calculated. (In these calculations the x and y parameters of the tellurium atoms were assumed to be the same as those obtained from the projection along [OOI]. x and y values in close agreement with those obtained in the
Fig. 1. Electron density projections of the TaTe4 subcell. In the lower part of the diagram along [OOI] ad in the upper part along [OIO]. Contours are at intervals of 12.5 e kZ. The zero contours are broken.
oar-projection were also obtained from the Fourier projection along [oIo].) R is shown in Fig. 2 as a function of zTs. The curve has an absolute minimum and three secondary minima. The absolute ~~rnurn is located at ZTa = 0.2245 (R = 0.166), whereas at zTa = t there is a slight maximum. The best z-parameter is taken as the z-value at the absolute minimum. The deduced atomic arrangement is thus described in terms of the space group P4cc (Ci,) as follows ; zTain(a) o,o,z;o,o,~+z with z = 0.2245 J. Less-Common
Melds,
7 (1964)
231-234
SHORT
COMMUNICATIONS
233
8 Te in (d) x, y, z; 1, 7, z; X, y, 3 + z; x, 9, + + z; jj, x, 2; y, x, 2; y, x, + + 2; 7, x, + + 2 with x = 0.1440, y = 0.3280, z = 0. It should be pointed out that it is difficult to distinguish
01 Fig.
L. The reliability
from the effect
factor
’
1
1
0.00
0.05
0.10
H as a function
of strongly
1
0.15
0.20
shifts in the z%parameter
I
0.25
of ZT~. The curve is symmetric ZT?. = :_.
anharmonic
vibrations
of the atoms
around
zn
=
o and
in the
c-direction
(compare Fig. I). For this reason, and also because the present structure determination only concerns the substructure of TaTe4, the difference AZ = -0.0255 fromz = 1 will not be discussed in detail at present. Three-dimensional X-ray data are now icing collected in order to relate the superstructure to the substructure. In the substructure each tantalum atom is surrounded by eight tellurium atoms in a slightly distorted square antiprism. Each tellurium atom is coordinated by two tantalum atoms and eleven tellurium atoms. Five of the tellurium atoms form a planar, irregular pentagon and six a trigonal prism. The interatomic distances between nearest neighbours are listed in Table effect of the deviation from + for ,zra is only noticed in the Ta-Te distances, give two different sets (4 + 4) of distances instead of eight equal distances for z ~~ = i. With reference to these distances, bonds are considered
I. The which (2.889x) to exist
between each tantalum atom and all the eight surrounding tellurium atoms and between each tellurium atom and two tantalum atoms and one of the other tellurium atoms. The existence of Te-Te bonds is of considerable interest in relation to the predictions of the general (8-N) rule discussed in our previous communication”. The rule seems in the present case to give an almost correct prediction for the number of Te-Te bonds. Further comments will, however, have to await the interpretation of the superstructure. J. Less-Common
Metals,
7 (1964) 231~~34
SHORTCOMMUNICATIONS
234
Although there is a slight difference in z-parameters, TaTe4 should be regarded as isostructural with NbTe.8. It is also interesting to compare these structures with the CuAlz type structure (cf. PEARSON~).The only difference (reflected by the difference TABLE I INTERATOMIC
Ta - z Ta -
4 Te
-4Te
: 3.405 : 2.790 : 3.0”
DISTANCES IN TaTed (A) _--. ~~~ --Te -- I Ta : 2.790 - I Ta : ~.CJI~ ._ I Te : 2.923 - 2 Te : 3.300 - 2 Te : 3.642 - 2 Te : 3.804 - 2 Te : 3.888 - 2 Te : 4.076
in composition) is that the Cu atoms located at 4, 4, b and $, 4, 2 in the CuAlz structure are absent in the structures of TaTe4 and NbTe4. The reasons for this behaviour cannot evidently be explained from geometrical considerations as the absent atom would have slightly more space than the one present in these structures. The authors wish to thank Professor H. HARALDSENfor his interest in this study and for making laboratory facilities available. E. B JERKELUND A. K JEKSHUS
Kjemisk Xnstitutt A, University of Oslo, Blindern (Norway) H. HARALDSEN,
A. KJEKSHUS, E. KBST AND A. STEFFENSEN, ActaChenz. &and., r7 (1963) 283. E. BJERKELUND AND A. KJEKSHUS, 2. Anorg. Allgem. Chem., 328 (1964) 235. K. SELTE AND A. KJEKSHUS, Acta Ckem. Scmd., x8 (1964)960. Yu. M. UKRAINSKII, A. V. NOVOSELOVA AND Yu. P. SIMANOV, Russian J. Imrg. Chem., (English Transl.), 4 (1959) 60. J. B. FORSYTH AND M. WELLS, Acta Cry&., 12 (1959) 412. Internation& Tables for X-ray Crystallography, III, The Kynoch Press, Birmingham, 1962. W. B. PEARSON, A Handbook of Lattice Spacings and Structwes of Metals and Alloys, Pergamon Press, London, 1958.
Received April gth, 1964 J.
_hS-COWWcl?a
Metals, 7 (1964)
231-234
On the crystal structure
231
of TaTe,
As a link in studiesIof the properties of the polychalcogenide compounds of the transition metals of subgroups IV, V and VI, the crystal structure of TaTed has been investigated. The TaTed phase was identified by UKRAINSKII et a1.4, but the composition TaTeh was first established by the present authors”. TaTe4 cq-stallizes in the tetragonal class with a = 2 s (7.514A, c = 3 x 6.809’1, I:,/&= 3;‘~ x 1.045. The py~nometric density, 7.901 g cm-3 (at 25Y), corresponds to 12 4 z (Xc = 12 Y 1.99) TaTeJ-groups per unit cell. Systematic extinctions in the X-ray
photographs
are ME for 1 = zn _t I and okl! for 1 == 212 + I, the Laue symmetry
is qjmmm and the possible space groups are P4cc (Ci,) and Pqlmcc (@A). Although oscillation, Weissenberg and precession photographs clearly showed the doubling of the a-axis and the tripling of the c-axis the additional reflections are faint and the full cell may be regarded as a superstructure of the smaller cell (subcell). Both the full cell and the subcell have the same extinctions and therefore the same possible
space groups. The present
paper concerns
the determination
of the crystal
structure of the TaTe* subcell. The subcell contains two tantalum and eight tellurium atoms. The two tantalum atoms must thus be located in a twofold position and as no other extinctions
have been observed
the eight tellurium
atoms must be in an eight-
fold position. It was decided to start with a Patterson projection along [OOI], where both space groups have symmetry p4.n~. Intensity measurements of the lzko reflections on W’eissenherg photographs were carried the combined Lorentz and polarization
out microphotometrically. factor and for absorption
Corrections were made.
for For
calculation of Fc-values the atomic scattering factors were taken from FORSYIYH A?JD \VELI*\ sj. Tentative atomic positions were easily deduced from the Patterson projection. Based on these approximate
atomic parameters
a set of signs and a Fourier projection
were evaluated. The latter clearly showed all atoms resolved. The atomic parameters were refined by means of one Fourier and two difference syntheses. The final Fourier map is shown in the lower part of Fig. I. The best atomic sponded
to an R-value
2Ta in o,o 8Te in x = 0.1440,
(X =
Ci[Fo(
y = 0.3280;
-
IFcll / ClFol)
parameters,
of 0.107,
which corre-
are:
etc.
with B = 0.5 for both kinds of atoms. In terms of the space group of highest
symmetry
P4lmcc
(D&) the atoms
would
have fixed z-coordinates. With 8 Te atoms in (m) and z Ta atoms in (a) (z Ta atoms in (h) would lead to unreasonable Ta-Te distances) a comparison of observed and calculated structure factors for the ho1 reflections was carried out. Intensity measurements of the hoi reflections on precession photographs were carried out microphotometrically. absorption
Corrections for the combined Lorentz and polarization factor but not for were made. Atomic scattering factors were taken from International
Tables”. Although the Fourier map (the upper part of Fig. I) appears as expected, the R-value was as high as 0.212. A new calculation was therefore carried out for the same atomic arrangement described in terms of the space group P4cc (Cz,). One of the two kinds of atoms may be given arbitrary z-coordinates. In the following calculation
232
SHORT
COMMUNICATIONS
the 8 Te atoms in (d) were given a fixed z-coordinate (zre = 0). A set of calculated structure factors was then prepared for values of .zTabetween o and 2 and the corresponding reliability index R calculated. (In these calculations the x and y parameters of the tellurium atoms were assumed to be the same as those obtained from the projection along [OOI]. x and y values in close agreement with those obtained in the
Fig. 1. Electron density projections of the TaTe4 subcell. In the lower part of the diagram along [OOI] ad in the upper part along [OIO]. Contours are at intervals of 12.5 e kZ. The zero contours are broken.
oar-projection were also obtained from the Fourier projection along [oIo].) R is shown in Fig. 2 as a function of zTs. The curve has an absolute minimum and three secondary minima. The absolute ~~rnurn is located at ZTa = 0.2245 (R = 0.166), whereas at zTa = t there is a slight maximum. The best z-parameter is taken as the z-value at the absolute minimum. The deduced atomic arrangement is thus described in terms of the space group P4cc (Ci,) as follows ; zTain(a) o,o,z;o,o,~+z with z = 0.2245 J. Less-Common
Melds,
7 (1964)
231-234
SHORT
COMMUNICATIONS
233
8 Te in (d) x, y, z; 1, 7, z; X, y, 3 + z; x, 9, + + z; jj, x, 2; y, x, 2; y, x, + + 2; 7, x, + + 2 with x = 0.1440, y = 0.3280, z = 0. It should be pointed out that it is difficult to distinguish
01 Fig.
L. The reliability
from the effect
factor
’
1
1
0.00
0.05
0.10
H as a function
of strongly
1
0.15
0.20
shifts in the z%parameter
I
0.25
of ZT~. The curve is symmetric ZT?. = :_.
anharmonic
vibrations
of the atoms
around
zn
=
o and
in the
c-direction
(compare Fig. I). For this reason, and also because the present structure determination only concerns the substructure of TaTe4, the difference AZ = -0.0255 fromz = 1 will not be discussed in detail at present. Three-dimensional X-ray data are now icing collected in order to relate the superstructure to the substructure. In the substructure each tantalum atom is surrounded by eight tellurium atoms in a slightly distorted square antiprism. Each tellurium atom is coordinated by two tantalum atoms and eleven tellurium atoms. Five of the tellurium atoms form a planar, irregular pentagon and six a trigonal prism. The interatomic distances between nearest neighbours are listed in Table effect of the deviation from + for ,zra is only noticed in the Ta-Te distances, give two different sets (4 + 4) of distances instead of eight equal distances for z ~~ = i. With reference to these distances, bonds are considered
I. The which (2.889x) to exist
between each tantalum atom and all the eight surrounding tellurium atoms and between each tellurium atom and two tantalum atoms and one of the other tellurium atoms. The existence of Te-Te bonds is of considerable interest in relation to the predictions of the general (8-N) rule discussed in our previous communication”. The rule seems in the present case to give an almost correct prediction for the number of Te-Te bonds. Further comments will, however, have to await the interpretation of the superstructure. J. Less-Common
Metals,
7 (1964) 231~~34
SHORTCOMMUNICATIONS
234
Although there is a slight difference in z-parameters, TaTe4 should be regarded as isostructural with NbTe.8. It is also interesting to compare these structures with the CuAlz type structure (cf. PEARSON~).The only difference (reflected by the difference TABLE I INTERATOMIC
Ta - z Ta -
4 Te
-4Te
: 3.405 : 2.790 : 3.0”
DISTANCES IN TaTed (A) _--. ~~~ --Te -- I Ta : 2.790 - I Ta : ~.CJI~ ._ I Te : 2.923 - 2 Te : 3.300 - 2 Te : 3.642 - 2 Te : 3.804 - 2 Te : 3.888 - 2 Te : 4.076
in composition) is that the Cu atoms located at 4, 4, b and $, 4, 2 in the CuAlz structure are absent in the structures of TaTe4 and NbTe4. The reasons for this behaviour cannot evidently be explained from geometrical considerations as the absent atom would have slightly more space than the one present in these structures. The authors wish to thank Professor H. HARALDSENfor his interest in this study and for making laboratory facilities available. E. B JERKELUND A. K JEKSHUS
Kjemisk Xnstitutt A, University of Oslo, Blindern (Norway) H. HARALDSEN,
A. KJEKSHUS, E. KBST AND A. STEFFENSEN, ActaChenz. &and., r7 (1963) 283. E. BJERKELUND AND A. KJEKSHUS, 2. Anorg. Allgem. Chem., 328 (1964) 235. K. SELTE AND A. KJEKSHUS, Acta Ckem. Scmd., x8 (1964)960. Yu. M. UKRAINSKII, A. V. NOVOSELOVA AND Yu. P. SIMANOV, Russian J. Imrg. Chem., (English Transl.), 4 (1959) 60. J. B. FORSYTH AND M. WELLS, Acta Cry&., 12 (1959) 412. Internation& Tables for X-ray Crystallography, III, The Kynoch Press, Birmingham, 1962. W. B. PEARSON, A Handbook of Lattice Spacings and Structwes of Metals and Alloys, Pergamon Press, London, 1958.
Received April gth, 1964 J.
_hS-COWWcl?a
Metals, 7 (1964)
231-234