Mar.,1956
CRYSTAL STRUCTURE OF TRICESIUM MONOXIDE
345
THE CRYSTAL STRUCTURE OF TRICESIUM MONOXIDE' BY KHI-RUEYTSAI,P. M. HARRIS AND E. N. LASSETTRE Contribulion from the Departmat of Chemistry, The Ohio Stale University, Columbus,Ohio Received Auaueli8, 1066
Tricesium monox ide, CssO, has been prepared and found to pomess ma.ny metallic properties, a D&C; cm structure with two molecules per unit cell. The observed interatomic distances indicate that the bond between cesium and oxygen is ionic as in the crystal of cesium monoxide, while the bond between cesium and cesium is metallic.
Introduction The existence of four suboxides of cesium, Cs70, CsrO, Cs702and Cs30, was first discovered by Rengade,2 through thermal analysis of the cesium-oxygen system. Part of the phase diagram (from Cs t o C S O ~ .has ~ ) been substantiated recently by Brauer3 by meanx of X-ray powder diagrams and measurement of the resistivity-temperature coefficients of the samples. Brauer observed, however, that CQO gave an abnormal X-ray powder pattern consisting of only two lines. No structure work on any of these suboxides has been recorded in the literature, despite the fact that they are of great interest from the point of view of valency and structural chemistry. Experimental (a) Preparation of Tricesium Monoxide, CsrO.-The suhoxide was prepared by direct combination of pure cesium with the calculated amount of pure oxygen admixed with a small amount of argon, which served as an inert gas to prevent excessive volatilization of the alkali metal. Toward the later stage of the oxidation; the reaction temperature was raised to 170" to decompose any higher oxides of cesium. The molten reaction product was allowed to solidify and cool to room temperature. It was pulverized in dried argon purified by passing through copper turnings at 350O.5 Crystalline samples of tricesium monoxide were also prepared by distilling small amounts of a lower suboxide, CsrOz, in thin-walled Pyrex capillaries (sealed in vacuo) at 120130" I n the distillation process, however, yellowish films of cesium monoxide were also formed above the dark greenish crystals of tricesium monoxide. The lower suboxide, CspOz, was prepared by direct combination of pure cesium and oxygen in the presence of a small amount of argon. (b) Study of the Crystal Properties.-The tricesium monoxide prepared by direct combination of the elements was obtained as a dark greenish, translucent solid, with a metallic luster, soft and malleable, difficult to pulverize. Analyzed by decomposition with water, the sample gave 0.337 equivalent of hydrogen for each equivalent of total alkali; hence the composition was Cs00.w. The method of analysis is similar to that recently described by Libowitz' for determining the excess of metallic barium in barium oxide crystals. The following physical properties were observed : (1) Dark greenish, translucent solid; m.p. w. 165', as observed in Pyrex capillary tube. (2) Density: 2.73 f 0.03 g./cc. at 30.2",determined by displacement of dried, oxygen-free toluene in a pyknometer. (3) Magnetic susceptibility at 30" xm = 61 X 10c.g.s./unit per mole, as compared with x,,, = (29 - 2 X 35 - 10) X 10- = -51 X 10- c.g.5. unit calculated from Wiedeman's law for CssO Cs. and with x,,, = 29 X 10c.g.s. unit for metallic cesium. The magnetic measurement was done by the standard Gouy method.6 (4) Electrical resistivity at 30': 7.21 X 10-6 ohm-cm. (as compared with 2.08 X 10-6 ohm-cm. for metallic cesium a t 18"); resistivity-temperature coefficient: a =
+
(1) This work was supported both by the University Committee for Allocation of Reaeamh Foundation Grants and by the U. 9. Army Engineer Corps under Contract DA 44-009-eng-405. ( 2 ) E. Rengade. Bull. doc. chin., I , 994 (1909). (3) V. 0. Brauer, 2. onora. Chem., SSS, 101 (1947). (4) C. 0. Libowitz. J . Am. Chsm. Soc., 'IS, 1501 (1953). (5) L. G. G o w . Compt. r d . , 109,935 (1889).
0.0025 er degree. The measurement was done potentiometricaey by determining the voltage-drop across a column of solidified suboxide in a conductivity ipet standardized with mercury. These observations delnitely show that CsaO possesses the physical properties characteristic of an alkali metal. (c) X-Ray Diffraction Experiments.-Debye-Scherrer diagrams of tricesium monoxide were obtained with Cu K a radiation and with Mo KCYradiation in an 11.4-cm. camera a t room temperature. The pattern was readily indexed graphically by the simple hexagonal system with a c/a ratio of 0.86. A highly imperfect crystal of tricesium monoxide was obtained by melting a small sample in a thin-walled Pyr;x capillary tube and allowing it to cool very slowly to 150 . The crystal was of irregular shape with one (hexagonal) aaxis approximately parallel to the length of the capillary. A rotation photograph taken with Cu K a radiations and with an a-axis as the rotation axis confirmed the hexagonal symmetry with a glide extinction of the (h0.l)- and (Ok.2)type with odd 1-indices. The cell constants are a = 8.78 f 0.01 A. c = 7.52 f 0.01 A. The calculated density for two molecules (Cs,O, formula wt. = 414.73) per unit cell 18 Aal0.= 2.7: g./cc., as compared with dObs. = 2.73 0.03 g./ cc.at30.2 .
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Determination of the Structure.-The presence of strong hhl-reflections with off E-indices and the systematic absence of (h0.Z)-and (Ok.1)-reflections with odd indices show the presence of a (1i.o)glide, rather than a (11.0)-glide (equivalent to a (10.0)-glide). Hence the possible space group symmetriess are Did - Di,, C:, - C 6cm, Dih CG/mcm. The shortness of the c-axis and the strong (10.0)-reflection eliminate the possibility of putting the six cesium atoms a t the combined two and four equivalent positions possible with these space-groups. Thus the cesium atoms must lie on six equivalent positions. This means that the six cesium atoms in the hexagonal unit cell of the suboxide are crystallographically alike. The two space groups, Did and Dih, give the same set of six equivalent positions: uu0; OUO; 800; iiUi/2; Ou1l2; uO1l2. The six equivalent positions possible with the space group C:, differ from this set only in the choice of the origin along the c-axis. The relative intensities of the (10.0)and (11.O)-powder lines are approximately in the ratio of 5 : 1. This fixes u at about 0.24. With this approximate parameter for the positions of the cesium atoms, and assuming that the observed extinctions are true, the only reasonable positions for the two oxygen atoms are OOi/4 and 003/4, corresponding to a Dih structure. Comparison of the observed and calculated intensities of the powder lines (Table I) gives u = 0.250 A 0.001. Based upon this structure and with u = l/d for the parameter of the cesium atoms, the calculated relative intensities of the rotation spots were also found to (6) "International Tabellen zur Bestimmung von Kriatallatrukturen." Bd. 1. Gebruder Borntraeger. Berlin, 1935.
KHI-RUEYTmr, P. M. HARRIS AND E. N. LASSETTRE
346
Vol. 60
TABLE I OBSERVED AND CALCULATED INTENSITIES OF X-%Y POWDER LINESOF TRICESIUM MONOXIDE Hexagonal indices, hk.1
10.0 00.1 10.1 11.0 20.0 11.1 00.2 20.1 10.2 21 .o 11.2 21.1 20.2 30.0 00.3 30.1 10.3 21.2 22.0 11.3 31.0 22.1 30.2 20.3 31.1 40.0 22.2 21.3 00.4 40.1 31.2 10.4 32.0 11.4 32.1 40.2 20.4
Planar spscin d Calod. %d.
7.60 7.52 5.35 4.39 3.801 3.793 3.760 3.371 2.857 2.673 2.535 2.507 2.402 2.380 2.283 2.195 2.176 2.109 2.108 2.101 2.092 2.031 1.901 1.896 1.889 1.880 1.844 1.826 1.728 1.699 1.697 1.685
Relative intensities
7.62
60
4.39
10
3.80 (broad)
100
u
62.6 0 0 8.5 6:; 24.3 0 10.2
3.37
10
2.87
15
2.68
50
2.54
2
2.28
5
2.175
5
9.2 3.0 0 0 0 12.0 1.5 15.3
2.103
5
0
1
10.5
1.9
J
15
100
1.833
2
1.74 (?)
1
1.696
10
3.0 1.4 13.3 14.4 1.4
1
100
U
+ '/Id
('/4
52.9 0 0 9.6 100
0 9.2
0 8.2
17.4
17.2
17.0
68.5
65.0
61.6
3.8 0 0 0 11.2 1.5 15.5
4.4 0 0 0 10.2 1.4 15.7
7.0
7.8
0 0
0 0
38.8
37.0
0
0
6.4
6.4
6.3
2.8
2.9
3.0
29.1
29.0
28.8
6.0
40.1
21 .o 3.7 0
'/c
57.6 0 0 9.1
0 0 1.891 (broad)
-
Relative intensities calcd., Icalcd.
1ob.d.
be in good qualitative agreement with the ob- (10.0)-, (10.1)-, (11.0)-, and (10.2)- powder-line served values obtained by visual estimation with intensities for these two structures with the obthe triple-film technique. However, in some served intensities ((10.1)-reflection absent) shows cases, such as the (11.0)-, (11.1)-. and (11.3)-reflec- that these two structures must be ruled out in favor tions on the Cu K a rotation photograph, the ab- of the - C6/mcm structure. sorption ( p = 894 cm.-l) appeared to be quite apThe X-ray powder and single-crystal data are preciable. Unfortunately, it was not possible to given in Tables I and 11. The powder data indicorrect for absorption because of the irregular cate an abnormally high temperature factor, BT,of shape of the crystal. the order of 10 x cm.2, probably due to latSince the diffracting power of the oxygen atoms tice defects. The absorption correction for the is very small compared with that of the cesium powder sample appeared to be small (from comatoms, the possibilities of placing the two oxygen parison of the Cu K a and Mo Ka photographs), atoms at positions other than those required by the since the sample was spread out in a thin film of observed extinctions must be examined. Thus with small crystallites on the wall by rapid melting and u = l/4 for the parameter of the cesium atoms, we cooling in the capillary tube. still have to consider the possibilities of putting the The observed interatomic distances are shown in two oxygen atoms a t one of the following two sets of Fig. 1. The Cs-0 distance in tricesium monoxide, 2/3, 1/4; 2/3, l/3, two equivalent positions: (1) CsaO, is very close to the observed Cs+-0' distance 3/4, for a DS - C632 structure, in which the Cs-0 in the cesium monoxide (CsnO) layer crystal, indi2/3) 0; 2 / ~ , distance would be 3.84 8.; or (2) cating that the Cs-0 bond in tricesium monoxide is l/3) I/:; for a C&-C63/m structure, with Cs-0 = ionic; while the Cs-Cs distance is about 8% higher 3.35 A. However, comparison of the calculated than the interatomic distances in metallic cesium
CRYSTAL STRUCTURE OF TRICESIUM MONOXIDE
Mar., 1956
TABLEI1 OBSERVED AND CALCULATED INTENSITIESOF BRAGG SPOTS ON ROTATION PHOTOGRAPHS OF TRICESIUM MONOXIDE CRYSTAL (Cu Ka radiation, a-axis rotation.) Hexagonal I ndices, hk.1
01.0
r
Reciprocal lattice ooordinates € (Ito a )
(I1 to a)
0
2.0
0
02.0 1 00.2 01.2 02.2 03.0 03.2 04.0 00.4 02.4 10.0 11.0 11.1 11.2 12.1 12.2 11.3 13.0 12.3 10.4 2i.o
20.0 2i.1
1
2i.2 20.2 21.2
E} 21.3 20.4 3i.o 3i.1 30.0
Intensities. I Calcd. (u = ‘ / r )
Obsd. I
50
37
3
0
4.1
100
100
0 0 0 0 0 0
0
4.6 5.7 6.1 7.3 8.2 8.3 8.4
10 14 2 1 5 4 1
14 16 7 6 31 31 7
0
9.1
6
46
1.76 1.76 1.76 1.76
1.0 3.0 3.6 4.2
75 4 63 17
76 14 100 15
1.76
5.1
21
26
1.76 1.76 1.76 1.76 1.76
5.5 6.5 6.8 7.3 7.9
28 6 8 3 9
53 12 35 5 26
1.76
8.2
3
12
(3.52)
i (out of range)
..
..
3.52
2.0
80
116
3.52
4.1
8
20
3.52
4.6
54
86
3.52
5.7
5
14
3.52
6.2
9
24
3.52
7.4
6
38
3.52
8.4
7
26
(5.27)
i (out of range)
..
..
5.27 5.27
2.3 3.1
77 5
171 16
347
3i.2
5.27
4.2
9
21
3 30.2 1’0}
5.27
5.1
3
17
3i.3 a 31.2 32.1
5.27 5.27 5.27
6.3 6.6 7.4
7 2 6
36 6 26
(Cs-Cs = 5.36 8.,a t room temperature), probably due to the polarizing effect of the oxide ions. In view of the observed metallic properties of the suboxide, the observed Cs-Cs distance suggests that the Cs-Cs bonds in CsaO crystals have metallic character. The structure can be regarded as consisting of hexagonal columns of CsfO (formed by piling up the hypothetical pyramidal tricesiumoxonium ions, CslO, according to the symmetry of a 6a screw axis), the columns being bonded together by “metallic” electrons.
Fig. 1.-Diagram showing the positions of the cesium and the oxygen atoms (denoted, respectively, b the closed and the open circles) in the hexagonal unit celrof CssO crystal at room temperature. The observed cell constants and 0.01 A., c = 7.52 f internuclear distances are: a = 8.78 -i 0.01 A., u = 0.250 0.001.; ao = 2.89 0.02 A. (cf. Cs+-O- = 2.86 0.01 A. in CszO crystal)’; 2 = 4.34 f 0.03 k.(cf. Cs+-Cs+ = 4.19 i0.02A. in CsgO crystal)’; = 5.80 i 0.04.A.,W = 5.78 0.05 A. (cf.CS-CS = 5.36 A. in metallic cesium).
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Brauera has observed that the lower suboxides of cesium (CsrO, and other suboxides lower in oxygen content) are metallic (electrical) conductors. Probably these also possess partial metallic structures. Silver subfluoride, Ag2F, has also been found by Terry and Diamond’ to possess metallic properties and partial metallic structure (anti-CdIz structure) in which the Ag-F distance is about the same as that in silver fluoride (AgF) crystals and the Ag-Ag distance the same as that in metallic silver. Thus the metal suboxides and subhalides probably belong to the same class as far as structure chemistry is concerned. Further study of this class compounds appears to be desirable. Our thanks are due Dr. D. Tuomi for his valuable help in connection with this work. (7) H. Terry and H. Diamond, J . Cham. Soc., 2820 (1928).
