LOTHARH . BRIXNER
3214
tion.) Since the steady state established in diffusion or conductance is the result of a complicated interplay between competing forces, the resultant concentration dependence of these properties can be very different from the pattern typical of small strong electrolytes. To treat the conductance it would be necessary to compute the retarding force due to ion atmosphere lag and relaxation, an effect which does not alter the diffusion coefficient because all ions move with the same mean velocity. I t is anticipated that the asymmetry of the ion atmosphere plays a relatively larger role in relaxation phenomena, which themselves result from atmosphere asymmetry, than in electrophoretic phenomena which result from the bulk motion of [CONrRIUUTION FROX
E.I. DU
VOl.
so
the entire atmosphere. The corresponding problems for polyelectrolytes are still more complex. It would be of great interest to construct well defined models of polyions by synthesizing electrolytes with small numbers of charges distributed in some known manner on the molecular skeleton. Triply charged molecules with dimensions of the order of magnitude of those 01 the bolaform electrolytes considered herein would be a further step in bridging the gap between small electrolytes and polyelectrolytes. The study of such model compounds would undoubtedly contribute to the construction of a more definitive theory of polyelectrolyte solutions. CHICAGO 37, ILLINOIS P0N.I' D E
NEMOURS &
CO., INC.]
Preparation and Structure of the Strontium and Barium Tantalates Sr3Ta05., and Ba3Ta05., BY LOTHAR H. BRIXNER RECEIVEDNOVEMBER 23, 1957 SrsTa06.sand BasTaOs.5 have bcen prepared by the reaction of TanOjwith equivalent amounts of the corresponding carbonates a t 1100" in oxygen. The compounds crystallize in the face centered cubic stcucture of (NH4)aFeFe with, the space group, Oh5. Four molecules constitute the unit cell with a lattice constant of 8.34 A. for SraTa05.j and 8.69 A. for BaaTa05.&. The pycnometric densities of 5.935 g. cc.-I and 6.798 g. cc.-I are in good agreement with the X-ray densities of 6.088 g. cc.-I and 6.906 g. cc.-I, respectively. The dielectric constant of Sr3TaOj.s is 14.3, and that of Ba3Ta05.5 is 13.18.
Introduction Pauling' was the first to determine the structure of ("4)3FeF~, which is common to a series of compounds of the &B&-type. Between the straight complex fluorides, such as (h"4)3FeF6, and the ternary oxides such as A3BO6, (NH4)&1003F3 constitutes an intermediate link, which also was investigated by Pauling earlier.' In 1951, Steward and Rooksby2 found a series of tungstates, such as di-barium mono-calcium hexa-oxo-tungstate, which also belong to this class; and Ruedorff3 characterized Ba3UO6 as another member of this series. -4 fully-coijrdinated tantalate of the same type can only be expected when one of the cationic alkaline earth ions is replaced by a trivalent rare earth ion, such as trivalent lanthanum. Theref p e , the compounds SrzLaTaO6 with an a0 = 8.57 A. and Ba2LaTa06with an a. = 8.62 A. were first prepared. However, when TazOs was treated with BaCO3 or SrCOj alone in a mole ratio 1:3, the X-ray pattern of the resulting compound could also be completely indexed according to the (NH4)3FeFcstructure. This behavior can only be interpreted if one assumes an anion deficient structure for Sr3Ta05.5 and Ba3Ta05.5, similar to SrTi02.5,which Ward4 recently found to crystallize in the perovskite structure of SrTiOs. Experimental The new anion deficient strontium arid barium tantalates are obtained readily by treating strontium or barium carbon(1) L. Pauling, THISJ O U R N A L , 46, 2738 (1924). ( 2 ) Rooksby and Steward, A c t a Cvysl., 4 , 503 (1951). ( 3 ) IV. Ruedorff, 2. A'iiluij'orsciz., 9b, Heft 8, 568 (1954).
(4) A f . Kestigian, 1. G. Dickinsm and i 3 9 8 (A'J27).
R. Ward,
THIS JOURNAL,
79,
ate with Tal06 according t o the equation 6MeC03 Taz05 = 2MeaTaOs.s 6COz(Me = Sr, Ba) rhis mixture is presintered a t 800" for one hour in air and then ball-milled in a mechanical agate hall mill to ensure a homogeneous reaction mixture. The second firing takes place at 1100" in oxygen, using a platinum boat as a container. If amounts of 10-20 g. are processed, the evolution of CO, usually is complete after two hours, as is evident from the results in Table I.
+
+
Con EVOLUTION
DURISG A
TABLE I %FIR. FIRISCP E R I O D
AT
1l ( ) o oI N
0 2
Compound
COXcalcd., g .
CO, given off, f i .
SraTaOj.6 3.582 3.590 BarTaOj.; 2.985 2.970 The Con-free compounds were again hall milled, pressed into pellets under 40,000 p.s.i. and finally fired a t 1200" for 1-2 hours. The strontium compound is white, while the barium compound is slightly yellow t o pink. Chemical Analysis.-The separation of strontium and tantalum was first attempted by dissolving the compouiid it1 molten K2Sz0,, precipitating SrSOc b y diluting the melt in water, while tantalum was kept in solution with ammonium oxalate. The results are erratic because varying quantities of tantalum were precipitated with the strontium. Successful separation was effected by chlorinating the compounds with phosgene a t 600" and leaching out the tantalum chloride with absolute alcohol. The alkaline earths were determined as sulfates; the tantalum, by precipitating in freshly filtered ammoniacal solution and igniting in air t o the pentoxide. The analytical results were as follows: SraTaOs.j, found, 34.97% T a , 48.49yc Sr, Sr:Ta = 1:2.97; calcd., 34.01% Ta, 49.44y0 Sr, Sr:Ta = 1:3.00. For BasTaOs.,, found, 26.49YG T a , 60.63% Ba, F3a:Ta = 1:3.01; calcd., 26.565; T a , 6 0 , 5 1 % B a , B a : T a = 1:3.00. DielectrG Constant.-For the tueasureineiit o f the dielectric constant a pellet, 1.25 cnl. in diameter, 2 iiiiii. thick, was pressed at 40,000 p.s.i. and silver-coated on both sides. The measurement was carried out at 200 kilocycles. The dielectric constant of SraTaOs.5 was found to be 14.3, and that of BaaTaOs.s, 13.18.
ORGANICDISULFIDES AS INITIATORS OF POLYMERIZATION
July 5 , 1958
TABLE I1 hkl
do
do
IO for
BalCaWOsb
do
loa
4.815 m+ 111 4.82 4.170 m200 4.17 2.949 s+ 220 2.948 2.514 m 311 2.516 222 2.403 2.408 w 2.085 s400 2.082 1.913 m331 1.913 1.865 m 420 1.864 422 1.702 1.702 s 1.635 m 333 1.604 1.474 m+ 440 1.474 1.410 m531 1.411 1.390 w 442 1.391 1.319 m+ 620 1.320 1.272 w533 1.272 1.257 w622 1.258 1.294 w 444 1.204 1.168 \v 711 1.168 1.157 w-640 1.157 1.115 in+ 642 1.116 a s = strong, in = moderate, rv = from A.S.T.M. card 6-0400.
4.84 4.20 2.97 2.53 2.42 2.10 1 ,925
22 1 100 9 4 30 4
.. 1.713 1.615 1.483 1.418
37 4 15 4 , .
1.327 1.279 1 ,265 1 211 1.175
15 1 1 6 3
.. 1.121 weak.
22 D a t a taken
X-Ray Analysis.-The compounds were investigated in powder form in hcrmetically sealed Lindemann glass capillaries in order to avoid any hydrolysis. Copper radiation (A = 1.5418) was used, filtering the @-radiationwith a nickel filter. In Table I1 are summarized the observed and calculated interplanar spacings as well as the observed2intensities of BazCa\;VOa for ocomparison. The lattice constant for SraTaOs.b of 8.34 A. is very close to that for BalCaWO6 of 8.39 A. Steward and Rooksby' found that this compound belongs to the cubic (NH&FeF~-type which crystallizes in the space group Oh5. The observed diffraction lines have either all odd or all even indices, indicating a face-centered cubic structure. Since the patterns of Sr3Ta06.Sand Ba~CawO6are essentially identical and since the concordance of the intensities is good, it can be stated that SraTa0s.s crystallizes in the same structure, that is, in the (NH4)3FeF~-type. In Table I11 are given the observed and calculated interplanar spacings for the com[ COXTRIBUTION FROM
THE
3215
TABLE I11 OBSERVED A N D CALCULATED IKTERPLANAR SPACINGS OF hkl 111 200 220 311 222 ,100 331 4 "0 422 333 410 531 442 620 j33 822 444 711 640 642
BaaTaOa.. do do 5.01 4.34 3.07 2.618
IC
5.017 m 4.345 w 3.Oi2 s+ 2.620 w 2,509 2,170 2.172 s 1.992 1.994 m 1.943 1.772 1.774 s 1.671 1.672 m 1.534 1.536 m + 1.468 1.469 w 1.448 1.373 1.374 m + 1.323 1.325 w 1.310 1.253 1.254 w 1.217 1.217 w1.205 1.160 m
BazLaTaOe do d, 4.98 4.32 3.06 2.60 2.50 2.16 1.97 1.93 1.76 1.66 1.53 1.45
4.97 4.31 3.045 2.599 2.485 2.155 1.977 1.927 1.760 1.658 1.522 1.457 1.436 1 . 3 6 1.361 1.313 1.300 1 . 2 5 1.243 1,207 1.195 1 . 1 5 1.151
Ii
SrpLaTaOe de do
IO
4.77 4.77 m 4.13 4.13 m S+ 2.92 2.921 s + w2.49 2.492 m w2.40 2.385 w s2.06 2.062 S w1.90 1.896 w w - 1.86 1.849 w s 1 . 6 9 1.689 s w1.59 1.590 m m + 1 . 4 6 1.461 m + w1.39 1.398 w 1 . 3 8 1.378 w m i 1.31 1.306 w 1 . 2 6 1.261 w 1.247 w 1.19 1.192 w 1.158 1.136 m+ 1.104
w w
poun{s BazLaTa06 with a0 = 8.62 A., SrzLaTao~with an = 8.27 A . and BasTaOs.swith a0 = 8.69 A.
Discussion The reaction between stoichiometric amounts of Taz05and SrC03 or BaC08 to form Me3Ta05.6 as a pure phase, indicates that, in the case of the (NH4)sFeFe structure, there might be a similar range of anion deficient structures as has been observed for the perovskites. I n the case of tungsten and uranium, compounds like Ba3WO5.5 and BaaU05.5 can be postulated with the transition elements in the 5-valent oxidation state. Since there are four molecules per cell, a total of two oxygens are missing in the unit cell of the compound SraTaO5.6. The question as t o whether these vacancies are randomly distributed or located a t definite lattice sites can only be answered by a more precise X-ray investigation. WILMINGTON, DELAWARE
FRICK CHEMICAL LABORATORY, P R I N C E T O N UNIVERSITY]
Organic Disulfides as Initiators of Polymerization. Part 11' BY T. FERINGTON~ AND A. V. TOBOLSKY RECEIVED JULY 31, 1957 The study of the compound tetramethylthiuram disulfide (TMTD) as an initiator of the polymerization of methyl methacrylate has been extended t o 60 and 80". The experimental points are fit by the equation R'i/(cat.) = 4.80 X loL4exp(-31.6 kcal./RT). This equation falls in the same range as the similar expression for, e.g., benzoyl peroxide, although the S-S bond is much stronger than the 0-0 bond. T M T D also has a retarding effect which was studied a t 30' where the initiating effect does not appear. Retardation is due to radical attack on the T M T D molecule to produce an inhibiting T M T D radical residue. Evidence on the photoactivity of T M T D also was obtained. Various polar reagents were studied as to their effect on the rate of generation of radicals by T M T D but no pronounced change was found. A comparison with vulcanization data for T M T D indicates similar primary steps in vulcanization and polymerization by this compound. Tetramethylthiuram monosulfide ( T M T M ) was studied in the same way as T M T D a t 70, 80, 90". No retardation was found. The experimental points for initiation were fit by the equation (R'ilcat.) = 9.0 X 10l1 exp( -29.4 kcal./RT). The energy of activation is very similar to that for T M T D . Experimental observations on the photoactivity of T M T M are given. Dipentamethylenethiuram tetrasulfide was found to have a purely retarding behavior with methyl methacrylate. Diphenyl disulfide, while much less active than T M T D , shows a very similar combination of initiating and retarding ability at 100'. A mechanism is presented which fits the above observations, Retardation is due to polysulfide radical species, RS,. ( x > 1)and initiation through the intermediate radical RS.. In the case of thiuram compounds this latter is unstable and further decomposes to CS2 (CH,)N.. This mechanism is consistent with most of t h e observed d a t a on thiuram vulcanization.
+
Introduction I n the previous publication the decomposition of the compound tetramethylthimam disulfide (1) Part I is T . Ferington and A. V. Tobolsky, THIS JOURNAL, 7 7 , 4510 (1956).
(TMTD) was studied at 70" in the presence of the common vinyl monomers styrene and methyl (2) This article is based upon a dissertation submitted by Thomas Ferington in partial fulEliment of the requirements for the degree of Doctor of Philosophy a t Princeton University.
