THELANTHANUM-BORON SYSTEM
June, 1961
probability at room temperature. Then the invocation of dimers to account for room temperature fluorescence quenching would either require differently structured dimers a t high and low temperatures or a single form of dimer to which the ad hoc hypothesis of a strongly temperaturedependent collisional or internal conversional quenching is applied. The indications of absorption spectrum broadening a t room temperature,’O we believe, are due to scattering by suspended particles in saturated solutions. For these reasons we feel that the dimer stable a t low temperature is probably not the prevalent form of chlorophyll important for in vivo photosynthesis. We do not mean to exclude the role of reversible changes in the small fraction of chlorophyll molecules participating chemically in the photosynthetic process. The low stability of the chlorophyll dimer is not surprising. Practically all the authenticated cases of reversible room temperature-stable dye dimers have occurred with ionic dyes, such as thionin, acridine and crystal violet cations and fluorescein and eosin anions. Levinson, et al.,
909
attribute the stability of such dimers to the coulombic forces of ion pairing as influenced by charge delocalization. l3 Non-ionic dimers would then be restricted to those formed only in the excited states by electronic excitation delocalization, such as pyrene,14 or to those whose ground-state stabilization is of the much weaker van der Waals type. This latter category, including chlorophyll, would be expected only a t low temperatures. The relative inability of allomerized chlorophyll to dimerize a t low temperature may be due to steric barriers of the bulky alkoxy substituent a t carbon-10. The changes reported here differ by their concentration dependence from the temperatureand solvent-dependent changes reported by Freed and eo-workers and ascribed to reversible solvation. 15,16 Freed’s measurements were all made with dilute chlorophyll solutions. (13) G. 9. Levinson, W. T. Simpson and W. Curtis, J . Am. Chem. Soc.. 79,4314 (1957). (14) T. Forster, 2. Elektrochem., 69, 976 (1955). (15) S. Freed and K. M. Sancier, J . Am. Chem. Soc., 76, 198 (1064). (16) S. Freed, Science, 126, 1248 (1957).
THE LANTHANUM-BORON SYSTEM’ BY ROBERT W. JOHNSON AND -4. H. DAANE Institute f o r Atonic Research and Department of Chemistry, Iowa State University, Ames, Iowa Received August 11 1960 ~
From thermal, metallographic, X-ray and electrical resistance data a phase diagram is proposed for the lanthanum-boron system. Two compounds are forTed, LaB4 and La&. The former has a very narrow range of homogeneity and decomposes peritectically a t 1800 i 15 The crystal system is tetragonal, and the com ound is a metallic type conductor. LaB6exists in the range 85.8 to 887, boron, melts above 2500”) and has a simple cubic Tattice. The color of this compound changes with composition, going from purple to a bright blue with increasing boron content. The addition of boron t o lanthanum has no measurable effect on the melting point or transition points of the metal. The addition of lanthanum to boron appears t o have very little effoect on the melting point of boron. There is metallographic evidence for an allotropic transformation in boron above 2100 Evidence also is given for a new compound CaB4, which appears to be isomorphous with LaB,.
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Introduction The past decade has seen an increasing interest in compounds between transition metals and boron, carbon, nitrogen and silicon. This group of refractory compounds is under study not only because they possess useful properties, but also because knowledge about the nature of their bonding is expected to contribute to the understanding of metallic bonds. Work on metal-boron systems has been hampered until recent years because elemental boron was not available in sufficient purity to permit reliable experimental results to be obtained, and as a result the state of knowledge of borides is less developed than that of carbides, nitrides and silicides. There is additional incentive for study of borides rather than the other refractory compounds because “electron deficient” bonding found in the boron hydrides and their derivatives is present in the boron “frameworks” of some borides. It would be desirable to seek a common basis for iiriderstanding the boron bonding in both sets of compounds. (1) Contribution No. 908. Work was performed in the Anies Laboratory of the U. S. Atomic Energy Cornmission.
The employment of the lanthanides (and scandium and yttrium) in the study of a set of compounds such as borides can be very useful because the size of the metal atom can be varied while other factors are nearly constant, thus helping the investigator to distinguish between size effects and effects due to other factors. The study of the lanthanum-boron system was undertaken for the above reasons, and also as a part of a program of investigation of the effects of interstitial type elements on the properties of the rare-earth metals. Kiessling,2 Kieffer and Benesovsky, and Robins4 have reviewed borides and other refractory compounds, and references 5-17 include most of the recent work on rare-earth borides. (2) (a) R. Kiessling, Acta Chim. Scand., 4, 209 (1960); (b) Powder Metallurgy, No. 3 (1959). (3) Kieffer and Benesovsky, ibid., No. 1/2 (1958). (4) D. A. Robins, ibid., No. 1/2 (1958). ( 5 ) L. Brewer, D. L. Sawyer, D. H. Templeton and C. €1. Dauben, J . Am. Ceram. Soc., 34, 173 (1951). 16) J. M. Lafferty, J . Appl. Phys., 22, 299 (1951). (7) F. Bertrtut and P. Blum, Compl. rend., ‘234, 2621 (1852). (8) A. Zalkin and D. H. Templeton, Acta Cryst.. 6,269 (1963). (9) H. C. Longuet-Hipgins and M. De V. Robert.?, Proc. Roy. SOC. (London), A!d24, 336 (1954).
ROBERT W. JOHNSON AND A. H. DLWE
910 2500-
-
/
/
2400 2300 2200-
/ /
I
I
21002000-
I I I
1900-
I
800 700 -
B La + LOB4
600-
500400-
300: 2oo0
IO
Fig. 1.-Phase
a , t a f La BI 20 30 40 50 60 7 0 80 ATOMIC % BORON. diagram proposed for the lanthanum-boron system.
Experimental Materials.-The lanthanum was prepared a t this Laboratory by reduction of sintered LaF3 with distilled calcium metal that was vacuum melted just prior to the reduction. The reactants were contained in a tantalum crucible, and the reduction carried out under an atmosphere of purified argon. Spectrographic analysis of the lanthanum used for investigating the effect of boron on the melting point and the upper transition point revealed traces of AI, Ca, Cu, Fe, M g , Mn, Xi and Si. Most of these were barely detectable, meaning that the probable amount of each was less than 0.0170. The oxygen content was estimated by a spectrographic method to be less than 1450 p.p.m. Chemical analysis showed 52 p.p.m. carbon and 19 p.p.m. nitrogen. The lanthanum used for investigating the liquidus curve and for preparing LaB4 averaged 500 p.p.m. carbon, 200 p.p.m. nitrogen and greater than 2000 p.p.m. oxygen, the metallic impurities being about the same as in the other sample. Spectrographic analysis of the LaB4 that was prepared revealed barely detectable amounts of Al, Ca, Cr, Cu, Fe, Mg, Mn, Si and Ta. Chemical analysis did not detect the presence of carbon or nitrogen in the LaB4. KO carbon was detected in the La&, but the nitrogen content is unknown, due to analytical difficulties. (10) F. Bertaiit and P. Blum, Acta Crysl., I , 81 (1954). i l l ) F. Gaume-hfahn, Bull. soc. chim. Fyance, 1862 (1966). (12) B. Post, D. hioskoa,itr and F. W. Glaser, J . A m . Chem. Soc., 7 8 . 1800 (1956). (13) I. Binder, Powder M e t . Bull., 7, 74 (1956). (14) E. J. Felten, I. Binder and B. Post, J . Am. Chem. Soc., 80, 3479 (1958). (15) H. Eick and P. Gillis, Abst. of Papers of 133rd meeting of the ACS, 37Q, 1938. (16) H. Eick a n d P.Gillis, Ahst. of Papers of 134th nierting of t h e ACS, 1.4, 1958. 117) A. U. Bcyholt, Gen. Elec. Co. Researcli Report K O ,T,O-RL2180, 959.
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The boron was obtained from Fairmont Chemical Co., and a vacuum fusion analysis showed 2.09y0 oxygen before arc melting and 0.26Y0 oxygen after arc melting. Preparation of Samples.-The samples of composition between pure lanthanum and LaB, were prepared by heating weighed amounts of lanthanum and LaB, in a tantalum crucible under a vacuum to 1500" for 15 min..in an induction furnace. The LaB4 was made by arc melting pressed boron powder with a two to threefold excess of lanthanum metal. Impurities such as carbon, nitrogen and oxygen tended to stav in the lanthanum phase while pure LaB, crystallized in the melt on cooling. The lanthanum and impurities were dissolved in dilute hydrochloric acid, leaving black LaBr crystals having smooth shiny faces and sharp edges. Lanthanum hexaboride was prepared by heating the required amounts of LaBl and boron in uacuo a t 1600 for 15 min. Alloys of higher boron content than LaBs were prepared by arc melting the required amount of LaBs v-ith previously arc melted boron. Analysis-The composition of alloys between lanthanum and La& was knownfrom the exact weights of the lanthanum and LaB, used to make up the allor. hIelting of the coniponents in a tantalum crucible did not alter the coniposition since neither lanthanum nor the boron in it seriouslv attacked the crucible, unless the carbon content of the lanthanum was abnormally high. The composition of LaB4 and LaBGv-as obtained from the measured density and lattice constants as described below, the densities being measured by pycnometric methods. The composition of arc-melted high boron samples was known from the weights of the LaB6 and boron used to make up the sample. Thermal Analysis.-Heating and cooling curves of pure lanthanum and lanthanum-rich alloys were carried out in a vacuum resistance furnace with a tantalum wire heating element which was not in direct contact with ceramics. .4 pressure of 1 x 10-4 mm. or less mas maintained during runs made with this furnace. The e.m.f. of the Pt-Pt, 13% Rh thprmocouple was read on a portable precision potentiometer a t one minute intervals. The cooling rate \$-as about one degree per minute. cooling curve v a s first obtained for the pure metal before adding LaB, to it, so that any change due to the addition could be drtected to qithin about one degree. Since the liquidus curve (see Fig. I ) could not be found by the usual cooling curve method, a different method was used for its estimation. An alloy of knowm composition was heated sloivly until the LaB, crystals floating on the liquid surface just disappeared, and this temperature recorded. The temperature of reappearance of the crystals on cooling was also recorded, and the liqiiidus temperature chosen between these two temperatures for the allo: observed. The decomposition temperature of LaB, was apparent as a sudden change of resistance that occurred in the compound on heating. This method of measurpment had been calibrated a t the melting point of platinum, which \vas taken as 1773". The composition of the liquid formed by the peritectic reaction of LaBa was chosen bv making the assumption that the B6octahedral groups present in 99.%y0; xenon > 99.92%; and nitrogen > 99.996%. Several samples of oxygen were generated from KC101. The samples were irradiated in glass tubes held in dewars containing liquid X2, 0 2 or He. The ozone content after irradiation was determined dilatometrically or by titrating iodine released in passing the gases through unbuffered potassium iodide solutions. When the former method was used the data of the Armour
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