Gregory J. McCarthy
Materiols Research Laboratory The Pennsylvania State University Unwersgty Park, Pennsylvan~o 16802
An Experiment in High Temperature Solid State Chemistry
A
surprisingly large proportion of chemistry graduates at all levels take jobs in industries specializing in phosphors, lasers, and other ceramic, electonic, nuclear, magnetic, and optical materials. Indeed, today more and more chemists are accepting jobs in these applied areas. Many of these materials are based on oxide systems and must be prepared at high temperatures and characterized non-destructively in the solid state. Although the chemistry departments of a few universities have exemplary training in hightemperature and solid state chemistry at the graduate level, most have none a t either the undergraduate or graduate level. Faculty inexperience and cost of elaborate equipment are often given as the primary reasons. We describe here a simple illustrative experiment in high-temperature preparation and solid state characterization which can be performed with relatively inexpensive equipment and little previous experience. It fits readily into four ACS recommended undergraduate courses. Background
The experiment calls for the preparation and characterization of the nonstoichiometric compound1
TiOl ,. Titanium monoxide is a rocksalt structure oxide possessing an unusually wide range of stoichiometry. It is stable in this structure only at high temperatures. At low temperatures TiOl.o has a monoclinic structure due to ordering of vacancies ( 1 ) . The figure is the phase diagram of the Ti-Ti02 system after Porter (8). A somewhat more recent but very similar phase diagram has been presented by Wahlbeck and Gilles (3). The extent of nonstoichiometry in Ti01 * is shown in the upper center of the diagram. At 1000°C for example, it has the approximate stoichiometry range T i 0 0 . ~ to~ TiOl.2i. The diagram also shows the compositions and stability fields of the metal-oxygen solid solutions ru, P, y, 6, and r , Ti203, TirOs and the homologous series of "Magneli phases" (TinO~n-~). Titanium monoxide has the distinction of possessing one of the largest concentrations of vacancy defects reported for any compound. A comparison of calculated (from the X-ray cell parameter) and measured densities indicates that the stoichiometric oxide TiOl.0 can have as much as 15% Schottky defects. Since heating any reduced titanium oxide in air will cause it to be oxidized to Tion, the preparation must take ik closed system. The existence of this unique stoichiometric oxide TiOz allows us to memure the gain in weight on oxidation and thereby determine the oxygen content of any reduced oxide. Sealing the sample in glass tubes under vacuum is one of the easiest methods of attaining a closed system. The reaction goes to completion in a reasonable time period only a t 90O0C or higher, well above the softening point of Pyrex. Thus, either Vycar or silica glass tubing must be used in the preparation. Silica is preferred because of its superior shock resistance during quenching into water. Water quenching is necessary to prevent formation of the low-temperature monoclinic modification of ti(02 tanium monoxide. X-ray powder diffraction is
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a
(I
Phore diagram of the system T i - 0 ahsr Porter (2).
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the simplest and quickest method identifying most solid compounds. The X-ray diffraction pattern of TiOl *. is the classical cubic rocksalt (f.c.c.) pattern with a cell parameter of approximately 4.2 1. The cell parameter varies systematically as a function of oxygen content and temperature of preparation. This thermal dependence of cell parameter requires that a preparation be quenched in a reproducible manner from a known temperature if results consistent from one composition to another are to be obtained. Experimental Procedure The starting materials are Ti01 and Ti metal. Appropriate weights of ench are ground in a mortar until homogeneous and tightly packed in a 6 8 in. d i c a tube. Two sample tubes, each sealed a t one end, can he made by slowly softening and pulling apart a. 12-16in. length of silica tubing in an oxyhydrogen flame. The tube is attached via vacuum hose to a standard laborrttorv
by dropping directly into a, rnntainer of water. The resulting powder has a metdlic luster and is gold for oxygen-rich stoichiometries and silver for oxygen-poor ones. It is ground to a fine powder for X-ray examination by the DebyeSchemer technique or on a diffractometer. The X-ray powder diffraction pattern for TiO,.o is listed in Table 1. Other oxygen stoichiometries will give reflections systematically shifted away from these values. is determined by heating in air The oxygen content of TiO, (700-1000°C) 8, carefully weighed amount of sample. The sample will increase in weight by a percentage of its original weight. Far anv titanium oxide, the general equation of oxidetion is (1 - 2/2)0, TiO, TiO. Tahle 2 lists percentage weight increases for the monoxide stoichiometry range derived from this equation.
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Additional Experiments The hesic experiment can be modified or added to, depending on the type of course in which i t is included. Quanlilatirm Analysis. The weight gain analysis for oxygen content can be performed on TiO. unknowns prepared by the instructor. Physical Chonisly. First, a more elaborate discussion of crystd symmetry can he coupled to the X-ray studies (for oxample, why a face-eentered cubic structure has so few reflections). Secondly, the cell parameter of 8. particular TiO, *, composition can he calculated from the positions of the reflec' tions (Tahle 1 ) by the cuhic formula aa2 = dyh= "k 2%) aa2 = cell parameter where (h.k.1) is the Miller index of the reflection beine measured
"X-ray") demrity for the particular TiO, the formula. MZ p = NaoB
..
can he derived from
where M is the formul&weight of the TiO, ..under consideration; Z , the number of formulaunits in theunit cell (Z = 4fora rocksslt structure); N , the Avagadro's number; and an, the cuhic cell parameter in cm. The cl~lculateddensity can then be compared to the pycnometrically measured density to determine the percentage of vacmcy defects. Advanced Inorganic Chmistry. As part of the study of transition metal oxides, a. whole series of titanium oxides e m be prepared by the same techniques described above. The homologous series of Ti,O*.-, oxides (Ti,O,, TijOn, etc.) shown in the figure and related oxides are discnved a t length in the ACS publica, tion "Nonstoichiametric Compounds'' (4) The Ti-0 systenl is typical of the W-0, Mo-0, and V-0 systems. Each bas a large number of compounds, including a homologous series of stmcture-related oxides. Phase identification is performed by X-my powder diffraction. Patterns of each of the titanium oxides can be found in the widelv rwailahle Powder Diffraction File (51. Insfrun rnrol . 4 n n l q s r ~ . X-IHJ.dii(racti~mrat, be studied ns a n annlylirnl rod fur a h l irystulline vrmlm~ncl,1,s h h ~ x a n , i n a r i ~ n of the many tirmim. ~uxideswloich C B U be prrp~redIn. rhr procedures outlined above. (for a After the variation of aa with oxygen content of TiO, particular preparation temperature) is known, the value of X-ray diffraction for semiquantitative analysis of solid solutions can be illustrated. A calibration plot of a. versus composition can be used to estimate the compositions of unknown Ti01 prepared a t the same temperature by the instructor. If the cell parameter is determined toahout one part in four thousand (for example, 4.180 f 0.001 A) the preoision of z in TiO,.. will be about *0.02 (2).
..
..
A p p a r a t u s a n d Supplies
Most of the apparatus and supplies mentioned are commonly available in chemistry departments or can be readily obtained from laboratory supply houses at a relatively small initial expense. An adequate vacuum pump is available for about $150 and a "crucible furnace" and Variac control capable of operating at 1000°C for about $250.2 Thermocouples for use in this temperature range are inexpensive. Silica tubing Table 1.
X-Ray Powder Diffraction Patterna of Ti0l.o'
+
..
' TiO, will he used to denote the monoxide; TiO. for any titanium oxide. % Asmall furnece, suitable for continuous overation to 1200PC may he constructed'in alarge (No. 10) can as?ollows. 1. On a. lathe wind Kanthal A-l resistance wire (The Kanthal Corp., Stamford, Conn.) a t 8-10 turns/in. over 4-5 in. of a I/,-in. mullite ceramic tube (MV-30, MeDaniel Refractories, Beaver Falls, Pa.). Use double the amount of wire for leads. 2. Cover the wire portion of the tube with refractory cement (Alandum, Norton Co., Worcester, Mass.) and let dry. 3. Place heating element in the center of the can and pack with MgO powder or, for Lower-temperature applicrttions, vermioulite. 4. Connect to a 10-amp variable auto transformer. Tempereture will vary over 10-25T as s. function of variations in line voltage. The cost of an individual unit is about $50, but the minimum order requirements of the manufacturers would require building perhaps 5 units in order to show substrtntid ~+savings. A useful bulletin describing do-it-yourself furnace building is twai1.ileble from the Norton Co., Worcester, Mass. (form 458). 210 / Journal of Chemicd Educafion
a
From B-117 of ref. (6).
hao = 4.177,&, Space Group Fm3m.
" Relative intensity. Table 2.
TiO,
Titanium Monoxide Percentage Weiaht " Increases
.
Percentage lncre~se
can be obtained from leading glass manufacturers. If a chemistry department does not own an X-ray diffraction unit, many physics and most geology departments do. Suitable interdepartmental arrangements for use of this equipment exist on many campuses. Summary
We have presented an experiment in high-temperaature preparation and solid state characterization which would fit into any one of four ACS recommended chemistry courses. The experiment requires little
previous experience of the instructor and can be performed with apparatus commonly available in college chemistry laboratories or those of its sister sciences. Lilerature Cited (1) WATANAB~. D.. C*em.es, J. R., Jos~oxs.A,, A N D MALIN,A. 8.. Act. Cry8lallogt.o. 23, 307 (1967). (2) PORTER. V., Ph.D. Dissertation. The Pennsylvania state university, University Park, Pa., 1965. (3) W A H L ~ E ~P.KG . . . A N D G l r ~ e s P. , W., J . Amsr. Csvoni. Soe.. 49, 180
(1986).
(4) W A ~ R. . (Editm), "Nonstoiohiometric Compounds,"
Advances i n Chemietrv Series No. 38. American Chemical Soeietv. .. Washincton. - . D. C.. 1923. (5) Powder Diffraotion File of the Joint Committee on Powder Diffraation
Standards. Philadelphia, Pa.
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