ond J. D. R. Th Welsh College of Advonced Technology Cardiff, United Kingdom
I
Alkali Metal Nitrides
I
The interaction of metals with gaseous nitrogen, a process sometimes described as nitridation, produces either ionic, covalent or interstitial nitrides. Only the highly electropositive elements give the ionic type of compound. These contain the unusual N-a anion, display characteristics of crystalline conduction, and on electrolysis in the fused state give nitrogen at the anode ( 1 ) : N-I
+ 3e
-
-
Element
1.i
AHcslr.0
-76
-t,:
It might be expected that the remaining Group I elements in virtue of their greater reactivity would also form corresponding nitrides. Theoretical calculations based on the Kapustinskii equation (6) and the BornHaber cycle, -U
T
Table 1. Lattice Energies, Free Energies and Entholpies of Formation (kcal mole-') of Group I and Group II Nitrides
'/%N2
Of the alkali metals, it is frequently claimed that only lithium will yield the nitride. Depending on experimental conditions, Group I metals form either nitride, or azide, or a mixture of both. Lithium nitride was first prepared in high yield from the elements at red beat (8), although considerable reaction is possible even a t room temperature (5). Because molten lithium attacks most ceramics, contamination of lithium nitride samples is unavoidable. However, the use of fused lithium fluoride-coated zirconium oxide crucibles, which are completely resistant to liquid lithium up to 800°C, has permitted preparation of very pure nitride as ruby red material (4), compared with the previously reported dark grey substance. Unlike other alkali metals, small quantities of nitride, but no azide, result when lithium is exposed to electrically activated nitrogen (5).
MaNAs)
indicate the definite existence of lithium nitride. (Table 1).
3M+Yg7)
+
xN-'(8) 7
K
Na
-36
20
Rb
Cs
Re
Me
Ca
Sr
43
75
-85
-121
-83
-75
Ba
0
Calculated from the Kapu~tititikiiequation (6). b Caloulhted from the above Born-Haber cycle. a
Except for lithium nitride, no experimental AH values are available for the Group I nitrides. However, the free-energy change (AG) is the fundamental property in predicting the ability of a reaction to proceed. Inspection of Table I shows that the entropy contribution to AH giving AG is approximately 15 kcal mole-' for the Group I1 nitrides. Allowing similar entropy trends throughout Group I, it is possible that sodium, like lithium, might he capable of forming a nitride. Sodium Nitride
Although sodium greatly accelerates nitridation of both calcium and strontium (7), Guernsey and Sherman have established that neither Na3N nor NaNp are formed from the elements a t 800°C even in the presence of iron catalyst (8). Indeed, this fact has been exploited to obtain lustrous sodium and for blanket in^ the liquid metal during- manipulation pocedurei. Under different experimental circumstances the nitride can be nrenared. Passage of electrical dis. charges through sodium under low nitrogen pressures for periods of less than five minutes gives solely nitride (9). More prolonged discharging causes both nitride
.
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and azidc to be formed (9). A black substance, formulated S a r N , collects in a vessel fitted with a sodium cathode after nitrogcu discharge (10). Disrharge between sodiuni electrodes immersed in liquid argon-nitrogen mixture (90: lo), is said to produce the nitride, but. it is contaminated with an inseparably fine dust (11). Moldenhauer aud Xottig consider that this dust is a mixture of sodium metal and azide, and that nitride is not formed under such conditions (5). During evaporation of liquid ammonia containing sodium azide and sodium, up to 50% of the original azide is converted to nitride, probably according to the equation (9)
-
3NaNa
NaaN
+ 4N2
Alliali nletal azides when heated in vacuo are reported to yield 2074 potassium, 40% rubidium, and 10% caesiuni nitride, respectively (18). Grou]) I1 nitrides can he obtained by heating the appropriatc amide: 3\1(NTI&
-
+ 4NHa
MzN2
Similar experiments witahalkali metal aniides have produced conflicting results. Thus, sodamide aud potas.saniide givc the respective nitride as black and green compounds (IS) (14). Titherly was unable to confirm either preparation, the sole products being the metal, hydrogen and nitrogen (15). It is evident that while the thermal method will not realise sodium nitride as wit,h lithium, it can be made in various states of purity by alternative meaus (9, 10, 11). Chemically it hehaves like the lithiuni analogue as illustrated in the following equations. (hl = Li or S a ) . \LN 1lsN 313N
+ 31190
+ 317% + 3EtOH
--
3\tt 3RIII
+ 3 0 W + NNx + NIT, + NHI
3Et,011
Sodium nitride has bee11employed (16) as target material for CI4 production in high density neutron beams: A514
+ no'
-
Ce'+
Htl
Nitrides of Potassium, Rubidium and Caesium
Sitrogen in its usual form does not react, with potassium, rubidium, or caesium, and presun~ablyfrancium would behave sinlilarly. Again as with sodium, this knowledge has been uncd to purify potassium, and for safe handling of the metal. The nitrogen absorptiou following discharge of nitrogen betweeu potassium coated electrodes, or Po a-partirle bombardment of nitrogen-potassium mixtures, is found on chemical analysis to be due to potassium nitride (17). Electrically excited nitrogen will react with potassiun~,rubidium, and caesium, hut the time lapse prior to azide formation is considerably shorter. Thus, with potassium, t,he azide is d o tectable after only 30 sec (9). According to Moldenhauer and AIottig (6) this method apparently produces t,he nitrides indirectly, due to decon~positionof the
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Journal of Chemical Education
first formed azide, and even then only in small quantities: 3MNa
-
MaN
+ 4N?
However, in a recent review of inorganic azides (18) there is no suggestion that the well-linown thermal degradation of ionic azides proceeds via an intermediate nitridc, as proposed by Fischer and Schroter (11). Furthermore, nitrides have considerably greater lattice Table 2. Lattice Energies and Enthalpies of Formation (kcal mole-') of Group I and Group II Arides ( 1 8 )
Element Li
U
Na
194 175 2.6 5.1
AH
K
Rb
157 -0.3
152 -0.1
Cs
Ca
Sr
lia
469 146 517 494 -2.4 11 1 . 7 -5.3
energies than their azide counter~larts. (Tables 1 and 2 . ) Fischer and Schroter (11) maintain that both potassium and rubidium nitrides can be synthesized by the same discharge method employed for sodium under liquid argon-nitrogen mixture. The compounds so formed are contaminated with fine dust, and are black. The fact that these materials int,eract with acids to give ammonia, and not hydrazoic acid, N,-a
+ HCI
-t
N,H
+ C1-
supports the existence of potassium and rubidium nitrides. nIoissan has reported the forn~ation of rubidium and caesium nitrides by pasqing nitrogen over the corresponding hydrides (19).
Literature Cited
(1) (2) (3) (4) . . (5) (6) (7) (8) (9) (10) (11) (12)
MARDUPUY, E., Ann. Chfm.,2, 527 (1057). OWRARD, L., Compl. rend., 114, 120 (1892). DESLANDRES, H., Compt. rend., 121,886 (1895). ZINTL,E., A N D WOLTERSDORF. . G.,. Z. electrochem.,. 41,. 876 (1985). MOLDENHAUER, W., AND MOTTIG, H., Ber., 62,1954 (1929). KAPUS~'INSKII, A. F., &a71. Rev., lo, 283 (1956). See also MOODY, G. J., AND THOMAS, J. D. R., THIS JOURNAL 42,204 (1965). ANTROPOFF, A. V., AND K R ~ G E R K., H., Z . P h g ~ Ch€?n., . A167, 49 (1933). GUERNSEY, E. W., AND SHEWAN, M.S., J . A m w . Chem. Sac., 47, 1933 (1825). WATTENBERG, H., Be?., 63, 1667 (1930). ZEHNDER, L., Wied. Ann., 52, 56 (1804). FISCHER, F., AND S C A R O ~FR .,,Ber., 43,1465 (1910). SUHRMANN, It., AND C ~ u s m s ,K., 2.anorg. chem., 152, 52
-- .
114?fil , -, A
(13) GAY-Luss~c,J. L., AND THSNARD, L., Reserehes phvsicoehemie, Paris, 1, 337 (1811). (14) DAVY,H., Phil. Trans., 99, 40, 450 (1809). A. W., J . C h m . Soc., 65, 504 (1894). (15) TITHERLY, (16) FRIES.B. A,. U . S . P a t a t . 2.532.490. December 1050. i17j N E W ~ A F.'H., N, Trans. Amkr. ~ l e e t . ' ~ o e44,77 ., (1023) (18) GRAY,P., Quart. Rev., 17, 441 (1863). H., Compt. mmd., 136, 587 (1903). (19) MOISSAN,
