LEAD T I T A N A T E : CRYST.4L STRUCTURE, T E M P E R A T U R E OF FORAIATION, A N D S P E C I F I C GRAT’ITY DATA SAKDFORD S. COLE
ASD
H. ESPENSCHIED
Research Laboratory, T i t a n i u m Diuision. N a t i o n a l L e a d C o m p a n y , Sayreuille, Wew Jersey
Received A u g u s t 25, 1936 IXTRODUCTION
The study of the kinetics and theory of reactions in the solid state has been carried on principally by Jander (B), Hedvall (4),Tamman (9), Goldschmidt (3), Taylor (lo), and their coworkers. Their investigations have dealt principally with reactions between the carbonates, or the halides and metal oxides. I n general, the reactions investigated yielded a readily soluble component or evolution of a gas, which affords a ready means of determining the rate of reaction. I n some instances, heating curves obtained b y means of differential thermocouples have been employed to observe the reaction temperatures. TThen studying the reactions between oxides, the amounts of resulting componentq or unreacted components are determined in order to measure the extent or the temperature of the reaction. If these oxides are extremely small particlec, as in the caSe of litharge and precipitated titanium dioxide, the differential thermocouple procedure will give high values for temperature of reaction, owing to low thermal conductivity of the powder and because the deflection in the heating curve is only pronounced when the reaction is rapid and when an appreciable quantity of heat is released or consumed. The formation of alkaline earth metatitanates b y reaction of alkaline earth oxides or carbonates with titanium dioxide in the solid state at elevated temperatures was first disclosed in t h e patent literature (3). The crystal habit, the possibility of other compounds in the system PbO-TiOz, and other physical data on lead titanate have not been published in the literature. Since this compound is now being produced as a commercial pigment, such data are of interest. A discussion of its properties in paints has appeared (8). Other divalent metal metatitanates have been investigated b y Goldschmidt (3), Barth (I), Posnjak ( 7 ) , Zachariasen (12), Hoffman ( 5 ) , and Taylor (10). -Iceording to their researches the MTiOa compounds crystallize in two types, i.e., ilmenite and perovskite. Wyckoff (11) has questioned the designation of the perovskite type as cubic, and has indicated that orthorhombic might 445
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be the proper qystem for certain titanates now assigned a perovskite structure. Goldschmidt has used the radius ratio of the ionic radii to predict the probable structure of a compound. The limits of the perovskite structure are between 0.99 and 0.77, according to the formula
RA
+ RB
=
W g (RB
+ Ex)
The tolerance factor ( t ) for lead titanate using Pauling's values for the ionic radii is calculated to be 0.985 and using Goldschmidt's values 0.95. Accordingly lead titanate should have a perorskite structure but, as will be shown by the powder diffraction pattern, it does not have such a structure. The ionic radius of Pb++ may not be correct, or the radius ratio as postulated by Goldschmidt does not apply in all cases. P R E P A R A T I O S O F LEAD T I T A K A T E
Hydrous titanium oxide (99.8 per cent TiOs) precipitated from an ilmenite solution and freed from sulfates was intimately mixed with fume litharge (99.9 per cent PbO). A second means of compounding the lead titanate was t o grind calcined titanium dioxide (commercially known as Titanox-A) with fume litharge in water. The mixtures were filtered, dried, and calcined at various temperatures. The mole ratios were varied from 3 P b 0 . T i 0 2 to Pb0.5TiOz, in order to determine if PbTiOs waq the only compound in the system PbO-TiOz. Lead titanate crystallized from 2 P b 0 . lTiOz at 9OO"C., a t which temperature the composition was a mixture of liquid and crystals. Crystalline lead titanate was also prepared by adding the calcined lead titanate to molten sodium tungstate a t 850°C. Alicroscopic examination of crystals obtained from this melt after three hours heating showed sufficient crystal growth to permit classification. Since the products of the lead titanate composition showed less than 2 per cent lead monoxide soluble in 5 per cent acetic acid, the reactions were over 98 per cent completed under proper calcination conditions. Long periods of heating a t several temperatures were employed in order to ascertain if dimorphism might exist, as in the case of cadmium titanate. RESULTS OBTAIKED
The x-ray data revealed only one compound in the system PbO-Ti02 between 89 per cent and 25 per cent PbO. Owing to the highly corrosive characteristics of lead monoxide, no attempt was made to obtain the liquidus-solidus curve of the system PbO-TiOz. The formation of lead titanate, as determined b y heating curves, has been reported by Tamman (9) to begin a t 470°C. The x-ray data showed that the reaction starts a t 360°C. and is complete in forty hours a t 375"C., or in four hours at 400°C.
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b y Dunii ( 2 ) for fiiic ponder- aiid pigment;. Thr. T aluo of T 52 waobvrved, nhile T 65 ha- hrcn calculated from thr. x-ray data. Tlii- iz
