Nonstoichiometric Compounds

17 Investigations of Nonstoichiometric Sulfides I. Titanium Sulfides, TiS and Ti S 2

2

3

JACQUES BENARD and YVES J E A N N I Ν Ecole Nationale

Supérieure de Chimie,

Paris,

France

The nonstoichiometric phases TiS and Ti S Downloaded by UNIV OF ROCHESTER on August 24, 2013 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch017

2

2

3

have

been studied in sulfides prepared by direct synthesis at 1000° and 800° C.

Lattice constants

and densities have been measured as functions of composition.

Characteristics

of

the

transition

from the TiS crystal structure to the Ti S struc2

2

3

ture have been investigated.

c o m p o u n d s with layer lattices constitute a family characterized b y special crystallochemical properties which have been the object of comparative studies (10). It is customary to relate the chemical behavior of these compounds to their crystallographic structure composed of more or less complex layers, or sheets. A n example of a layer structure is offered b y cadmium iodide. It is an important example, since many sulfides, selenides, a n d tellurides of the transition metals are of this same crystal type. T h e crystal lattice is composed of planar sheets, ( C d l ) , consisting of C d l octahedra joined at their edges (Figure 1) (7) ; the different sheets are held together b y V a n der Waals forces. It is possible to insert metal atoms between these sheets; thus, cases are known i n w h i c h a continuous transition has been observed between M X and M X — t h a t is, between the C d l structure and that of N i A s . T h e plane situated between two ( M X ) sheets is completely filled w i t h metal atoms, becoming identical to the M plane of one of the ( M X ) sheets. A classic example is furnished b y the system C o T e - C o T e (8). T h e sulfides of titanium are, i n this connection, of particular interest. Inasmuch as T i S crystallizes w i t h the N i A s structure and T i S w i t h the C d l structure, the existence of an intermediate compound of the sesquisulfide type has long been the object of speculation (1, 3), until the last few years when its existence has been definitely established (4, 5, 9). F o r our part, we have concentrated our attention on titanium disulfide, titanium sesquisulfide, and the manner of transition from one to the other. 2

w

6

2

2

2

2

W

2

2

Experimental

n

2

Methods

Preparation. T h e sulfides were prepared b y mixing weighed amounts of titanium a n d sulfur i n the required proportions; the S / T i ratio was known w i t h a precision of 0.2%. T h e titanium, i n spongy form, was made available through the courtesy of Ε. I. d u Pont de Nemours & C o . T h e impurity content is: 191 In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.

ADVANCES IN CHEMISTRY SERIES

Downloaded by UNIV OF ROCHESTER on August 24, 2013 | http://pubs.acs.org Publication Date: January 1, 1963 | doi: 10.1021/ba-1964-0039.ch017

192

Figure 1.

0 Fe G 2

0.043% 0.045% 0.025%

The structure of titanium bisulfide, ing nature of L·yers N Gr Si 2

0.004% 0.005% 0.007%

H

2

show

0.014%

The titanium-sulfur mixture was placed i n a transparent silica tube w h i c h was evacuated, then sealed under a vacuum estimated at 1 0 m m . of H g . A l though silica can, according to theory, be reduced by titanium, our experience has shown such attack to be exceptional, at least i n the case of the particular sulfur-rich sulfides that we have studied. The reaction tube and its contents were then brought to the desired temperature. T h e heat treatment was extended over 2 to 3 weeks. I n certain cases, particularly when single crystals were grown, an intermediate grinding was carried out. T h e silica tube was cooled by d i p p i n g it i n water. The homogeneity of the sulfide obtained was verified b y examination w i t h an optical microscope and w i t h x-rays (sharpness of the bacjo-reflections). Analysis. T o confirm the value of the S/Τίratio, the sulfides were chemically analyzed b y roasting i n air at 900° C . T h e weighings, made w i t h a balance sensitive to 0.01 mg., gave the ratio w i t h a precision of 0.2% for a quantity of oxidized sulfide on the order of 0.1 gram. In the great majority of cases, experi ment showed the discrepancy between the values of the S / T i ratio determined i n the preparation and i n the analysis to be less than 0.5%. X - R a y Methods. A camera w i t h a curved quartz crystal monochromator served to identify the crystal type of the sulfide under study a n d to verify that it belonged to a single-phase system. T o reduce the effect of the titanium fluores cence excited by the copper radiation used, an aluminum filter 0.03 m m . thick was placed over the photographic film. Under these conditions, well-focused weak lines stood out clearly against a continuous background reduced as m u c h as possible. - 4

In Nonstoichiometric Compounds; Ward, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1963.


17. BENARD AND

JE ANN IN

Titanium Sulfides

193

The lattice constants were measured b y a back-reflection method w i t h an internal standard of sodium chloride. The sulfides studied possess hexagonal symmetry. Thus two diffraction lines are needed for a determination of the two constants. The two lines were chosen i n such a way as not to increase unduly the experimental error, w h i c h was of the order of 0.15%. O n l y diffraction lines whose Bragg angles exceeded 73° were photographed. Therefore it was not possible to perform an extrapolation that w o u l d have per mitted reducing the error due to absorption. However, since the mean Bragg angle of the lines used was 78°, the residual systematic error should be relatively slight. X - r a y intensities were measured photometrically. T o reduce the effect of absorption as much as possible, the sulfide was diluted w i t h magnesium oxide. The value of the coefficient μΆ was experimentally determined. Moreover, to eliminate the effect of preferred orientation, w h i c h arises when the sample assumes the form of a cylindrical rod, we adopted a spherical sample shape of 0.3-mm. diameter. Density Measurements. T h e measurements were made according to the method of Archimedes. As the amount of sulfide was limited to 0.2 or 0.3 gram, the resulting loss of precision was offset by choosing a l i q u i d whose density was as close as possible to those of the sulfides studied. T h e density of 1,1,2,2-tetrabromoethane, the l i q u i d chosen, was determined i n advance as a function of tem perature. T h e sulfides, placed i n a special weighing boat, were immersed under vacuum i n the apparatus shown i n Figure 2. U n d e r these conditions, the densities of the sulfides were determined w i t h a precision of 0.2%.

Figure

Nonstoichiometric

2.

Apparatus for measurement of density by mersion of sulfide under vacuum

im

Phase, TiS

2

Crystals of titanium disulfide occur i n the form of golden hexagonal plates w i t h an extremely marked metallic luster. If the reaction tube does not con tain enough sulfur to give a S / T i ratio of 2, a powdery compound w i l l be formed w h i c h has the same unit cell as the stoichiometric sulfide, but w h i c h maintains a S / T i ratio of less than 2. T w o series of sulfides obtained i n this manner were prepared; they are dif ferentiated only by the temperatures of preparation, 1000° and 800° C . T h e experimental results are collected i n Table I. Lattice constants and density i n crease linearly w i t h a decreasing S / T i ratio (Figures 3 and 4 ) . Calculating the mass of the unit cell and studying its variation as a function of composition indicate the nature of the lattice defects responsible for the variations from stoichiometry:


194

ADVANCES IN CHEMISTRY SERIES

They correspond to the insertion of titanium between layers (Figures 5 and 6) Table I.

(6).

TiS Phase 2

S/Ti Preparation

Analysis

c, A.

a, A.

G./Cc.

p,

Series P r e p a r e d at 1000 ° G . 1.667


T13S5 1..804 1,.821 1..812 1,.814 1..820 1,.865 1..850 1..874 1,.900 1,.952 1,.900 1,.905 1,.915 TiS

O u t s i d e of phase 1 .823 1 .816 1 .819 1 .816 1 .825 1 .841 1 .856 1 .869 1 .875 1 .903 1 .904 1 .919 1 .919

2

2

5.7170 5.7176 5.7166 5.6166 5.7153 5.7138 5.7119 5.7089 5.7077 5.7040 5.7060 5.7019 5.7028

3.4138 3.4131 3.4123 3.4134 3.4132 3.4125 3.4123 3.4109 3.4109 3.4095 3.4089 3.4091 3.4085

3.355 3.358 3.345 3.344 3.357 3.328 3.326 3.313 3.296 3.298 3.306 3.293 3.294

C a n n o t be isolated at this temperature Series P r e p a r e d at 8 0 0 ° G .

T13S5 1 .817 1 .821 1 .818 1 .824 1 .851 1 .873 1 .883 1 .917 1 .904 1 .920 1 .943 1 .982 TiS

O u t s i d e of phase

1.667 1 .818 1 .818 1 .819 1 .825 1 .854 1 .870 1 .897 1 .898 1 .908 1 .922 1 .928 1 .943 2

2

3.4127 3.4127 3.4125 3.4124 3.4108 3.4098 3.4090 3.4092 3.4094 3.4079 3.4075 3.4075

5.7142 5.7140 5.7139 5.7129 5.7090 5.7053 5.7025 5.7027 5.7013 5.7002 5.6977 5.6976

3.362 3.352 3.345 3.354 3.311 3.327 3.291 3.309 3.315 3.289 3.285 3.266

C a n n o t be isolated at this temperature

Stoichiometric T i S cannot be isolated at 1000° or at 800° C . Indeed, at these temperatures it decomposes to give sulfur vapor and a nonstoichiometric sulfide with the T i S structure: 2

2

Sulfide ( S / T i = 2) - > sulfide ( S / T i = 2 -

a ) + sulfur v a p o r

The value of a is less i n proportion as the temperature is lower. O n e may expect an excess of sulfur vapor to affect the value of a. This is the reason for finding the sulfur-rich limit i n analyzing the solid sulfide from a reaction tube containing just a slight excess of sulfur. W a s h i n g the sulfide w i t h carbon disulfide assures that this excess does not lead to a false analytical result. The lattice constants of the stoichiometric disulfide may be obtained b y extrapolating the curves of Figures 3 and 4. T h e values found agree within experimental error. W e accept the following values for the lattice constants of the stoichiometric disulfide: a =

3.4048 ±

0.0004 A .

c = 5.6904 ±

0.0007 A .

O n the low-sulfur side the phase limit is less clearly defined. Indeed, if this limit was deduced from lattice constants variation curves, it should be located at


17. BENARD AND JEANNIN

Titanium Sulfides

195

5.715

5.710

5.705


5.700

5.695

Nonstoichiometric Compounds

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