1760
VOl. 64
NOTES
with the trichloroacetate ion the potential +I bands centered a t about 2000, 2150, 2300, 2400 and 2550 pi., with wave length uncertainties effect is prevented from taking place. If decanoic acid and 1-hexanol do not associate which range from 1 2 5 t o A40 A. His observed through hydrogen bonding the difference of 1.8 wave lengths are listed in Table I and have been kcal. in the AH* of the trichloroacetate ion re- claimed to be independent of cation. Audubert action in these two solvents (lines 3 and 6 of Table suggested in most of the papers cited that the bands 11) may be attributed to the additional electron could be explained in terms of the known energy withdrawal due to a -E effect on the carbonyl levels of Nz, and in one paperI3 specifically comcarbon atom of decanoic acid evoked by the nega- pares four of his band wave lengths with those of tive charge on the attacking ion. The decrease of one band each of four different Nz band systems. 7 e.u. in the AS* of the reaction on going from 1It is the purpose of this note to suggest that the hexanol to decanoic acid is commensurate with the broad features probably arise from groups of bands difference in the number of carbon atoms between of the Nz Vegard-Kaplan (A% -+ X1&) system the monomers of the two compounds plus the which originate from the v’ = 0, 1, 2, 3 levels of the steric effect (screening effect) of the carbonyl A32: state. oxygen atom in close proximity to the nucleophilic The band-head vacuum wave lengths from hydroxyl group. 1900-2600 k. of the gas-phase Vegard-Kaplan The results of this investigation strengthen bands are also given in Table I together with their the hypothesis that the electron repelling influence Franck-Condon factors Q ~ which ~ ~ give J ~ an of the negative charge on the trichloroacetate ion indication of the relative band intensities for equal tends to evoke unfavorable inductive and electro- populations in the upper levels. It will be noticed meric effects, and that the electron attracting in- in Table I that strong Vegard-Kaplan bands fluence of the electrophilic, carbonyl carbon atom ( p v ~ v ~ ~0.1 or greater) occur in the region of each of the un-ionized malonic acid tends to evoke of the bands reported by Audubert. Positive favorable inductive and electromeric effects. identifications are, of course, difficult with such Further work on this problem is contemplated. large uncertainties on wave lengths but the followAcknowledgments.-(1) The support of this re- ing paints lend strong support t o identification of search by the National Science Foundation, Wash- the Vegard-Kaplan system as the most plausible ington, D. C., is gratefully acknowledged. (2) The emitter. (A) It is physically more reasonable potassium trichloroacetate used in this research to attribute all of the bands to one system than to was supplied by Dr. James D. Solomon, Blackburn rely on individual wave length coincidences between Laboratories, St. Elizabeth Hospital, Washington, the bands and those of difference Nz systems. D. C. (B) In considering the mechanisms of dissociation of simple molecules, Henri16 compared lengths (re) and characteristic frequencies (we) of bonds ON T H E NATURE OF ULTRAVIOLET LIGHT within the molecules with those of the electronic WHICH ACCOMPANIES THE states of diatomic species into which the molecule may decompose. In particular he quotes the DECOMPOSITION OF SOME AZIDES’ values re = 1.13 k., we = 2060 cm.-l for the BY R. W. NICHOLLS* terminal NN bond in NaN3 t o be compared with Department of Phyaice, Univsrsit of Western Ontario, London, Onlar%o, the values re = 1.144 A.; we = 2046 cm.-l for 8ando free Nz in the C311ustate. From the close agreeReceived April $Q8 1960 ment between these data he went on to propose Some years ago Audubert and his colleagues that Nz is formed in the CSII, state in the slow reported that a very feeble luminosity accompanied thermal decompositio? of NaN3. The more recent the slow thermal decomposition of NaN3, 3,4.6.6 values of re = 1.148 A.; we = 2035.1 cm.-l for the ICN3,3AgNa,s*7 P ~ ( N s )C~B, ~ ( N ~ )Ba(N3)z,3 ~,~ T1N8.8 C3IIU state,17 and the fact that the characteristic It also occurs near the anode during electrolysis frequency we = 2141 cm.-l is ncw associated with of NaN3 and The spectrum of the lumi- the antisymmetric stretching of the Na group as a nosity was studied over the wave length range 1800- whole18 while re for the terminal NN bond is 2600 k. a t very low dispersion using a quartz 1.128 k.l9 does not detract from the usefulness of monochromator and a copper iodide photon Henri’s suggestion which is really based (insofar counter,l0-lZand was found to be limited to broad as rough equality of re is concerned) upon the Franck-Condon principle. (1) This work ha8 been supported in part by contracts with the Air Force Office of Scientific Research, The Office of Naval .Research. The If it therefore be supposed that Nz is formed Air Force Cambridge Research Center and the Department of Defence during slow thermal decomposition of NaNa in the Production of Canada. 0’ = 0 level of C311u,three radiative transitions in (2) Temporarily on leave of absence at the National Bureau of Standards, Washington, D. C. cascade will be.expected to occur, viz., C311u -+
-
~
(3) R. .4udubert and H. Muraow, Compt. rend., 204, 431 (1937). (4) R . Audubert, ibdd., 204, 1192 (1937). ( 5 ) R. Audubert, ibid., 206, 748 (1938). (6) R. Audubert and J. Mattler, {bid., 206, 1639 (1938).
(7) R. Audubert, ibid., 206, 133 (1937). (8) R. Audubert. and C. R a w , ibid., 208, 1810 (1939). (9) R. Audubert,, ibid., 208, 1984 (1939). (IO) R. Audubert, J . Phys. et Rod.. 6, 452 (1935). (11) R. Audubert, Compt. rend., 200, 389 (1935). (12) R. Audubert, ibid., BOO, 918 (1935).
(13) R. Audubert. Trans. Faraday Soc., 86, 197 (1939). (14) R. W. Nicholls, Ann. Geophys.. 14, 208 (1957). (15) W. R. Jarmain, P. .4. Fraser and R. W. Nicholls, Astrophy. J . , 118, 228 (1953). (16) V. Henri, Compl. rsnd., 203, 67 (1936). (17) R. S. Mulliken, “The Threshold of Space,” Pergamon Preaa, Inc., New York. N. Y., 1957, p. 169. (18) E. H. Eyster and R. H. Gillette. J . Chem. Phye., 8, 369 (1940). (19) E. H. Eyster, i b d . , 8, 135 (1940).
Nov., 1960
1761
NOTES
TABLE I BAND-HEAD VACUUM WAVELENGTHS AND FRANCK-CONDON FACTORS FOR VEQARD-KAPLAN BANDS IN THE SPECTRAL REGION 1900-2600 A., AND AUDUBERT’S BANDSARISINQFROM TEIERMAL DECOMPOSITION AND ELECTROLYSIS OF SOME AZIDES Y V e g a r d - K a p l a n bandsA
(h.)
qv’d’b
1901 1936 1954 1990 2010 2027 2046
0.009 .07 ,003 .043
2085 2108 2125 2148 2189
.096 .004 ,044 .06 .lo2
2258 2302 2334 2347
.111 .Oil .053 ,044
2379 2425 2463 2474
.124 000 .lo6 .07
NaNa (Thermal)
Audubert’s features ‘ I 1N: (Thermal)
AgNi (Thermal)
(H.)
NaN;
(Elect)
-
HNa (Elect)
1950 f 25
1975 f 25 1990 f 52
.001
.099 .019
.
2040 f 30
2140 f 30 2150 f 25
2150 f 25
2300 f 35
2300 & 35
2400 f 40
2400 f 40
2140 f 25 2280 f 35
2511 .076 2561 .041 2605 ,158 2614 .019 * The bands predicted as occurring strongly are italicized. bert cites this band as doubtful.*
2130 f 25
2270 f 35 2300 f 35
2450 f 35
2390 f 40 2425 f 35
2500 f 40 2550 f 40
q,‘,”
2600 i 40 (2650)” is the Franck-Condon factor of the band.“ Audu-
B311, (Second Positive Bands), BaIIg - A3Z: (First Positive Bands), AaZ: + X’Z’, (VegardKaplan Bands). Tables of Franck-Condon Factors which are available for these transitions16J0 may then be used to predict which bands will be most strongly radiated. They are
The intrinsic weakness of the intercombination Vegard-Kaplan system may account in some part for the very feeble intensities observed by Audubert. The system nevertheless has been excited in the gas phase by electrical discharges of various kinds by Kaplan,21 Wulf and Melvin,22 Janin2a and Herman and Herman,24 by electron beams by Nz Second positive bands. v’ = 0 u” = 0, 1, 2 Bernard,26and in solid N2by VegardZ6and by Peyron N 1First positive bands. v‘ = 0 u” 0, 1, 2 v’ = 1 Y” 0, 2, 3, 4 and B r ~ i d a . ~ ~ v’ = 2 v“ = 0, 1, 2, 5, 6 The bands observed during electrolysis of azides NzVegard-Kaplan bands: v’ = 0 v” = 4, 5, 6, 7, 8 v’ = 1 v” = 3, 4, 5, 9, 10, 11 probably are excited in electrical discharges inside (From 1900-2600 A.) v’ = 2 v” = 2, 3, 6, 7, 8, 11, the small bubbles of N P liberated a t the anode. 12, 13 Ozonizer-like electrical discharges caused by surface v‘ = 3 v” = 1, 2, 3, 5, 6 , 7, changes in such bubbles are known to give rise to 13, 14, 15, 10 light2* and as Wulf and Melvin point outz2 such The strong Vegard-Kaplan bands predicted by discharges are favorable for the excit,ation of the this scheme, which lie in the wave length range of Vegard-Kaplan system. Table I have been italicized there and it is noted One test for the operation of the cascade mechathat such strong bands appear in each of the five nism discussed in B above is to observe the pregroups into which the table is divided. Additional dicted strong bands of the N2 second positive and Vegard-Kaplan bands can be predicted in a similar first positive bands (which lay outside Audubert’s fashion if it be assumed that the v’ = 1, 2, etc., range of observation and which are also outside the levels of C311, are also populated by thermal dis- visible spectrum). Experiments to search for these sociation of azides. (C) It is possible that N, bands are in preparation. The absence of them is formed during the decomposition of the azides will not weaken the case for identification of directly into the AaZ: state from which the (21) J. Kaplan, Phys. Rev., 46, 675 (1934). Vegard-Kaplan system is radiated directly. The (22) 0. R. Wulf and E. H. Melvin, Phys. Rev., 86, 687 (1939). primary Franck-Condon parabola for the system (23) J. Janin, Ann. Phye., [1211, 538 (1946). passes (in the usual v’, V” array) in the vicinity of (24) R. Herman and L. Herman, J . Phya. et Rad., 7, 203 (1946). the (3, 2), (2, 3), (1, 3), (1, 4) and (0,5 ) bands one (25) R. Bernard, Compt. rend., 300, 2074 (1935). (26) L. Vegard. Skriftar Norske Videnekaps. Akad. Oslo, No. 9 of each of which is associated in Table I with each (1938). of Audubert’s spectral features. (27) M. Peyron and H. P. Broida, J. Chem. Phys.. 30, 139 (1950). (20) W. Jarmain and R. W. Niaholls. Con. J . Phys., 31,201 (1954).
(28) P. Jarman, Proc. Phys. Sac., 73, 628 (1959).
Vol. 64
1762
Audubert's features with Vegard-Kaplan bands, but will make the cascade mechanism less likely.
THE GROWTH OF LARGE SINGLE CRYSTALS OF ZINC OXIDE' BY J. IT. NIELSEX~ AND E. F. DEARBORN Bell Telephone Laboratories, Inc., Murray H i l l , New Jersey Received M a y 3 , 1050
Single crystals of hexagonal zinc oxide can be grown from the vapor phase by the method of Scharowsky. 3 Lander4 has improved upon Scharowsky's method and recently Laudise and Ballmanj have shown that zinc oxide can be grown hydrothermally. Both of these methods produce crystals which have a needle-like habit with the needle axis and the crystallographic c-axis coinciding. Growth perpendicular to the c-direction is usually quite slow and seldom exceeds a millimeter. Growth of single crystals from molten salt solution was attempted in the hope crystals large in the a-dimension could be obtained which could be used as seeds for the hydrdhermal method. As is usually t'he case with molten salt systems, insufficient phase equilibrium dat,a were available t'o choose a solvent for ZnO, and the selection had to he made by trial and error with the help of previous experience. It was quickly discovered that LiCl arid KF were very poor solvents for ZnO. PbF2, however, dissolved appreciable quantities of ZnO, and this solvent was selected. PbFz previously had been used by Jona, Shirane and Pepinsky6 to grow single crystals of lead zirconate, and it had been used by the authors? to grow cryst.als of Mn203 and Lao.,Pb~.3Mn03. ZnO crystals grown from molten PbFz between 1050 and 1150" exhibited the desired platey habit, the plane of the plate coinciding with the (001) crystallographic plane. Experimental All crystal growth experiments were done in resistance furnaces which have been described previously.* The samples were contained in 100 ml. platinum crucibles fitted with lids. They were prepared from reagent grade chemicals or materials prepared from reagent grade salts. Two hundred g. of PbF2 and 22 g. of ZnO were convenient amounts to use. Temperatures were measured with Pt10% Rh-Pt thermocouples. It was found that 1150" was a convenient temperature from which to begin crystal growth. At this temperature the volatility of PbF2, though high, was not excessive. The samples usually were held a t 1150" for from 2 to 4 hours and then cooled a t a constant rate of from 1 to IO" per hour. The system reaches a steady state quickly and it is possible a holding time of less than two hours would bc satisfactory. The samples usually were withdrawn near 800" and allowed t,o cool in air. Both air and oxygen at____(1) Presented at, the 135th Meeting of the American Chemical Society, Boston, Massachusetts, April 1959. (2) Now at Airtron, Division of Litton Industries, ;LIorris Plains, N. J. (3) E. Scharowsky, Z. physik., 156, 318 (1953). (4) J. J. Lander (to be published). ( 5 ) R. A. Laudise and A. A. Ballman (to be published). (6) F. Jona, G. Shirane and R. Pepinsky, Phys. Rev., 97, 1584 (1955). (7)J. W. Nielsen and E. F. Dearborn (unpublished work). ( 8 ) J. n . Nielsen and E . F . Dearborn, Phys. Chem. of Solids,6, 202 (1958).
PbF,
5
13 15 20 25 M O L E P E R CENT ZflD.
Fig. 1.-Solubility
33
35
curve of ZnO in PbF2.
mospheres were used in the growing furnace and no change in growth characteristics resulted. Since no method of chemically separating PbF? from ZnO has been found, the crystals must be broken away from the melt. It is still possible, however, to separate crystals which are 1 to 5 cm. in their largest dimension (the adimension) by tapping the bottom of the crucible containing the sample. It is found that the PbFz does not cling very tenaciously to the ZnO crystals. Solubility Data.-A rather crude solubility curve of ZnO in molten PbF2 was determined. The system has only a simple eutectic between 0 and 35 mole % ZnO. The curve, which is shown in Fig. 1, was obtained by preparing melts with a large excess of ZnO present, holding them for a t least two hours a t the desired temperature and quenching them in water as quickly as possible. The excess ZnO which had floated on the surface of the melt was analyzed for zinc and lead. The percentage zinc oxide present was calculated assuming all the zinc was present as ZnO and the lead as PbF2. The eutectic composition m-as determined from a crystal run which was allowed to cool to solidification. The eutectic was found to be a t 8.8 mole % ZnO and 733". Two of the points near 850" were determined by approaching the equilibrium from the other side, that is, slowly cooling the melts to that temperature from 1150°, then quenching them. Since these points fell on the curve nicely, i t was assumed that two hours is ample time to allow for a steady state condition to be reached by samples held a t constant temperature. The average error in the ZnO ronrentration determined by this method is 5273 except a t the eutectic where the ZnO concentration is known more accurately. I t should be pointed out that the quench technique is not as reliable as one might wish when melts are highly fluid, but the great volatility of PbFz makes any other technique extremely difficult. Another possible source of error lies in side reactions. X Ray powder photographs of quenched melts of ZnO and PbFz revealed that only ZnO and P-PbF2 with a small amount of a-PbF2, were present. Thus side reactions, if they occur, do not yield products in amounts detectablr by X-ray diffraction . Spectrochemical analyses of the crystals indicated :t total impurity rontcnt of less than O.lTo.
Discussion Although the habit of the ZnO crystals 011 the melt surface was platey, it was discovered that crystals nucleated a t temperatures below 1050" tended to be more drum shaped. Thus the ratio of the rate in the a-direction to the rate in the cdirection varied from about 50 to 1150" to near unity a t 1050". It was also observed that crystals
