NOTES
July, 1960
941
THE GROWTH OF BARIUM TITANATE (4)
The bonding strengt,h of CH3Br:AlBr3 is then g l 9 kcal. Other aluminum bromide solutions may be interpreted in a similar manner. Kespita13 by the use of dipole measurements
SINGLE CRYSTALS FROM MOLTEN BARIUM FLUORIDE BY R. C. LINARES Bell Telephone Laboratones, Inc., Murray Hall, New Jersey Recetved January 86,1960
Single crystals of barium titanate have been of interest for ferroelectric studies. Most of these found that an addition molecule, - -A1Br3, crystals have been grown from molten salts, and at moderate temperatures (1000-1250°), because Baexisted in small concentration in benzene solutions TiOa undergoes a change from the cubic to hexaof aluminum bromide. U l r i ~ h , by ~ cryoscopic gonal form a t 1460°.1 Some of the solvents sucmeasurements in very dilute solutions of aluminum cessfully used include: BaC12,2KF,3PbF2,4PbO4 bromide in benzene, obtained results consistent and Na2C03.h Of these, the KF process has been with the conclusions of Nespital. Both of these the most widely used ; however, potassium from studies are consistent with the equilibrium the solvent and platinum from the crucible enter the crystals as impurities. The purpose of this paper is to report the solubility of BaTiOI in BaFL and point out that BaTiOs crystals can be grown from BaF2 solutions. Although this method has for which K = 700 @ 20'. This, combined with some drawbacks, which will be discussed, crystals an estimated (Sackur-Tetrode equation) A S = can be grown free of potassium and platinum. +17.9 e.u., yields an estimated bond strength of Solubility Curve -An approximate phase diagram for tho
Q
the addition compound
0
- - -AlBrs of 14.6 kcal.
The vapor pressure measurements of Fairbrother and Field" on solutions of aluminum bromide in cis-pent-2-ene a t 0' also can be interpreted as due to the existence of an addition molecule between an olefin and A1Br3 in equilibrium with AIzBrg and free olefin. I n Fig. 2 are plotted the apparent molecular weights of aluminum bromide in pentene solution as were experimentally determined by Fairbrother and Field. The dotted line represents calculated values of this same molecular weight function assuming that equilibrium (6) exists in the liquid phase Olefin---A1Br3
AllBraf olefin
(6)
and has a value of 3.0 a t 0'. Again combining an estimated A S = f17.5 kcal. with this value for the Kequilibriiim, a bond strength of 17.5 kcal. is estimated for the pentene-AlBr3 molecule. Experimental Apparatus.--..lll of the experiments were carried out with high vacuum apparatus and techniques in which the materials came in contact only with glass and mercury. Procedure.-A weighed amount of synthesized aluminum bromide was resublimed several times into a glass vessel under dry nitrogen. The vessel was attached to the vacuum system and evacuated. A measured amount of methyl bromide was condensed on the aluminum bromide and the vessel was agitated until solution was complete. Temperature control was by the use of appropriate slush baths. Pressure was read from a mercury manometer. The composition was varied by allowing fractional portions of the vapor to transfer to the vacuum system. Compositions reported as "CH~B~/AlBr3ratio" were corrected for the presence of CHaBr in the vapor space above the system. The vapor pressure of the methyl bromide recovered after a given experiment was identical t o that recorded before the experiment. The volume of gas recovered also was very nearly identical to that charged. (3) W. Nerrpital, Z . physak. Chsm., B16, 153 (1932).
(4) H. Ulrich. ibid., B18, 427 (1931). ( 5 ) F. Fairbrother and K. Field, J . Chem. SOC.,2614 (1956).
system BaFrBaTiOj was determined by a simple quenched melt technique. A charge of barium titanate and barium fluoride was heated in an electric furnace for four hours at a temperature 20' above the desired temperature. .4t the end of the time the temperature was dropped 20' and held for another two hours. Yext the temperature close to the crucible was determined with a Pt-Pt 10% Rh thermocouple probe, the crucible was removed from the furnace, and the melt was poured off and quenched in a platinum crucible Only those runs in which there was an excess of barium titanate in the crucible a t the time of pouring were used for the solubility determination The eutectic mixture was made by heating a mixtiire of 30 g. of BaTiOl and 60 g. of BaFZ to 1350' and cooling slowlv to 1200". The BaTi03 crystals were removed merhanicallv leaving the eutectic mixture and the melting point of this mixture was determined in a platinum wound micro furnace. The average of seven readings was taken as the melting point and was found to be 1260" with the maximum deviation being 14'. The composition of the melts was determined by analveiP for barium and titanium by an X-ray fluorescence techniqiie 6 The results are considered to be corrert to within 1% The molar composition was calculated on the assumption that barium and titanium were present onlv as RaF2 and RnTiOI Although X-ray pictures show only these two phases prcsent in low temperature runs, long runs (24 hr ) at high temperatures (1420') show a small amount of an iinidmtified third phase. Also if the weight per cent. of hariiim titanate and barium fluoride in these samples is ralculitcd, these total shghtlv more than 100% indicating the possible prewnrc of another phase. I t is possible that this nhmr is 't b?rium platinum oxide formed by slow decomposition of BnF2 to BaO which would attack the platinum rrucihle The proposed phase discrram for the svstem RaFn-RnTiO, is given in Fig. 1. This Rhows the solubility of RaTiOl to range from 16 5 mole yc a t t h e euteciir point of 1260' to 49 2 mole % at 1.393". Thp acciii-acv of thp ciirvc' has been confirmed bv observations made during crvstal growth bv pulling When using a melt of a givrn rompohition, crystallization began very near the temperature predicted by the solubility curve as drawn. The vapor pressure of BaF2 is quite low even a t elevatcti
(1) Phase Dirtgiarns for Ceraniists. 1956, p . 42. (2) Blattner, Matthias. Mera and Schemer, Helu. Clqrn. &!a, 20, 225 (1947) ; Ezperientia, 3, 4 (1947). (3) J. P. Remeika, J . A n . Chem. SOC.,76, 940 (1954). (4) J. W. Nielsen, personal communication. ( 5 ) Shoao Sawada, Choichiro Nomura and Shin'ichi Fugii, IZept. I n s t . Sci. Technol., Uniu. Tokyo, 6, 7 (19513. (6) T. C. Loomis, unpublished work.
NOTES
942
and rooling was begun of 2.5 degrees per hour; a t 12751300" the grown crystal was withdrawn from the melt, and allowed to cool slowly to room temperature. Crystals up to 7.5 g. have been grown in this manner. Both butterfly twins and cubes were used as seed material in these runs. No appreciable difference could be found in the growth behavior between the two type seeds; however, the orientation of the seed was very important. The crystal has a great tendency to grow as a cube or a rectangular parallelopiped; thus the (111) and (110) faces cap out rapidly leaving the crystal bounded by (100) faces. A crystal grown from the (110) face capped out quickly in that direction and then grew in steps as a pile of cubes. Crystals grown from the (100) face were more regular, and although they grew slightly stepped on the side they were sound internally. Despite the slow growth rate there was a great tendency for the crystals to twin. For this reason the crystal could not actually be pulled during growth; the slowest practical pulling rate was faster than the rate of deposition a t the desirable cooling rate.
140C
1380 0 (0
n
136C
u
W
n
g 1340 w
a 2
5
Vol. 64
1320
lx W
R
I
;i' 1300
Conclusions Barium titanate butterfly twins and cubes can be grown from molten BaF2. BaTiOs single crystals can be grown on a rotating seed by slowly cooling moderately concentrated solutions of BaTiOl in 1260 10 20 30 40 50 60 BaF2. This method of growing a crystal allows Ba F2 M O L PER CENT B a T i O , the crystal to be removed from the melt when the Fig. 1. growth is completed and allows one large crystal rather than many small ones to be grown from a temperatures which makes it desirable as a solvent. The melt. This procedure should also prove useful in attack on the platinum crucible is only slight7 but any contamination of the BaFz-BaTiOl melt with silicates causes other systems where the solvent is difficult to remove from the crystals. The solubility of the rapid attack on the crucible and eventual failure. Growth of Crystals. Butterfly Twins.--A typical crystal material need not be high provided that the slope growth run was made by heat'ing a mixture of 50 g. of BaTi- of the solubility curve is such that a reasonable 0 3 and 50 g. of BaFp a t 1350' for 4 hours in a Pt crucible. .4t t.he end of this time, a crucible containing 5 g. of BaTiOa amount of material can be deposited during the was placed in the furnace and alloved to reach furnace cooling cycle and that the transport can be accutemperature (about 10 minutes). Then t.he molten solution rately controlled. was poured onto the BaTiOt powder and the growth cycle Acknowledgments.-The author wishes to thank was begun. The growth cycle consisted of holding the temperature constant from 0 to 2.5 hours, followed by cool- J. W. Nielsen for his helpful discussions and suging a t rates from 1 to 25' per hour. At the end of the cool- gestions on this work. Thanks are also due to W. ing cycle (1270 to 1300") the crucible was removed from the Hartmaim and T. C. Loomis for their excellent furnace and the melt was poured off. In the bottom of the crucible were the dark blue barium titanate butterfly tv-ins chemical analysis and to Miss A. D. Mills for X-ray identification of melt phases. and cubes. 1280
\
.'.\J '\
~
~
Discussion of Twin Growth.-The barium titanate rrystallized from barium fluoride melts is dark blue to black in color. This is due to an oxygen deficiency in the crystal like that of BaTiOI grown from KF in a nitrogen atmosphere.8 Attempts were made to grow clear twins from barium fluoride by growing in an osygen atmosphere. The crystals irppeared no light,er in color, indicating that the oxygen pressure of the transparent crystals a t t'hese temperatures may be greater than one :ttmosphere. These crystals and others were heated in an oxygen atmosphere for 4 days at' 1000° with no indication of losing their dark color. However, by raising the temperahre to 1050-1100° some lightening in color did occur after a week of oxidation. Dark blue crystals grown from KF oxidize more rapidly under those same conditions. This would indicate that t8heoxygen defect concentration in the barium fluoride grown crystals is considerably greater than the defect concentration in potassium fluoride grown crystals. Growth on Seeds.-While barium fluoride is an excellent solvent for barium titanate, i t is very difficult to remove from the crystals and from the crucible. For this reason it was decided to explore the possibility of pulling the BaTiO, from a BaF2 melt while slowly cooling the melt. It was also t'hought, that the method itself should be explored to determine its usefulness in other systems. A crystal was grown by first preparing a saturated BaTiOBBaFp solution at about 1360" in a platinum crucible. Then a seed as lowered to, and rotated on t,he surface of the melt, .
-.
Analysis of t h r w s ~ i i a , a t rbami,le+ oi crjstalb s l t o ~ c dIehs than U.0057cplatinum. ( 8 ) J. W. Xielsen. to be published. ii)
T H E F1,UORESCESCE OF ACETA4LDEHPDE
YAPOR' BY E. M V R A D ~ D e p , z r l i ~ i ~ i iot j ( ' I < iiimt,.]~, r x i r e r s i t y o,f Kochesrcr, XochFster. .V.
1..
IIereivrd January $7, 1960
I'revious ~ o r kon the fluorescence of acet,aldehyde has been cursory and limited to visual observation of the fluorescence.3 f 4 This visible emission has been attributed by hlatheson and Zaborj to t)he sensitized emission of biacet'yl, which is a product of the phot'ochemical decomposition of acet'aldehyde vapor. (1) This work was supported in part bg t h e United States Air Force through t h e .4ir Force Office of Scientific Research of the Air Research a n d Development Command, under Contract No. A F lE(600) 1528. Reproduction in whole or in p a r t is perniitted for a n y purpose by the United States Government. (2) Department of Chemistry, Cnirersity of Wisconsin, Madison 6, Wie. (3) P. A . Leigllton and F. E. Rlacet, J . . i n Chem. Soc.. 65, 1766 (1933).
(4) G. li. Rdlefson and r). C. Grabamp, J . Chem. r h u s . , 7 , 775 (1939). ( 5 ) .?i.Y. .?fatheson a n d J. W. Zabor, ibid., 7 . '536 (183U).
