338
KHI-RUEYTSAI,P. M. HARRIS AND E. N. LASSETTRE
Vol. 60
THE CRYSTAL STRUCTURE OF CESIUM MONOXIDE' BY KHI-RUEYTSAI,P. M. HARRIS AND E. N. LASSETTRE Contribution from the Department of Chemistry, The Ohio State University, Columbus, Ohio Received August 18, 1066
The anti-CdClz type layer structure2 (D,Sd-R8m)of dicesium monoxide fCs20) has been confirmed by X-ray sin le crystal work. The variable parameter for the positions of the cesium atoms is found t o be n = 0.256, instead of u = 17, ( owder worka), whicb fails to account for some of the weak powder lines.* The abnormally large cesium-cesium distance &s+ CS: = 4.19 A.) between layers and the slightly shortened cesium-oxygen distance (Cs+ 0' = 2.86 A.) indicate that the cesium ions are highly polarized in this layer crystal.
-
Introduction The monoxide of cesium, CszO, is believed to play an important role in Cs-0-Ag photocathode^.^ This oxide, orange yellow a t room temperature, is also known to exhibit color changes upon heating and ~ o o l i n g . ~It , ~is the only compound which has been assigned an anti-CdClz type layer structure.2 However, there has been some doubts about this assigned structure which is based upon X-ray powder data. A further study of the structure of this oxide by means of single crystal work thus appeared to be desirable. Preparation of Dicesium Monoxide and Analysis of the Samples.-This monoxide was pre ared by distilling a lower suboxide of cesium (Cs,Oz) in a byrex vessel at 180-190" until no more cesium appeared to condense on the aircooled trap. The suboxide (CS~OZ), in turn, was prepared by direct combination of pure cesium with the calculated amount of pure oxygen admixed with a small amount of argon, the procedure being the same as described for the preparation of tricesium monoxide, CSaO.' The monoxide thus obtained was in the form of polycrystalline, laminated plates, orange yellow at room temperature, cherry red above B O " , and lemon yellow at Dry ce temperature. It was readily pulverized by shaking with glass beads in a thoroughly degassed Pyrex tube. On account of the small weight percentage (5.7%) of oxygen in dicesium monoxide, the composition of the sample cannot be accurately determined by alkalimetric determination of the cesium content alone. In the present investigation, the alkalimetric determination was supplemented by determination of excess cesium, or excess oxygen. The amount of gas evolved upon decomposition of the sample in thoroughly degassed Pyrex vessels was measured by means of a Topler pump and a McLeod gage, and the resulting alkaline solution titrated. Any excess cesium in the sample would produce an equivalent amount of hydrogen, whereas the presence of peroxides would be indicated by the liberation of oxygen. A sample of dicesium monoxide (orange yellow, crystalline powder) thus analyzed gave 0.001 mole of gas for each mole of the monoxide, showing an almost stoichiometric compound. A separate preparation yielded a sample (also orange yellow, crystalline powder) which gave 0.014 mole of gas for each mole of the monoxide; the gas was not identified, but was assumed to be oxygen due to a small leakage of atmospheric oxygen into the sample tube. This latter sample showed five extra powder-lines (weak), which were also present in X-ray powder photographs of other CszO samples known to be partially oxidized due to inadequate protection against atmospheric oxygen. However, both the pure and the partially oxidized sample were found to be diamagnetic, xg = -0.20 x 10-6 c.g.s. unit per gram. (1) Thie work was supported by the U. S. Army Engineer Corpe under contract DA-44-009-eng-405 and by the University Committee for Allocation of Research Foundation Grants. (2) A. Helms and W. Klemm, 2. anorg. Chem., 442, 33 (1939). (3) V. G. Brauer, ibid., 255, 101 (1947). (4) V. K. Zworykin and E. G. Ramberg, "Photo-eleotricity and Ite Applications," John Wiley and Sons, Inc., New York, N . Y., 1949, p. 46. ( 5 ) E. Rengade, Ann. Chem. Phys., 11,348 (1907): Bull. UOC. e h h . phiis., 69, 667 (1907). (6) Recent. investigation in this Laboratory, to be reported in a separate article.
Re-examination of the Powder Pattern.-The X-ray powder pattern of dicesium monoxide was first re-examined, using Cu K a radiation and an 11.4-cm. camera, the finely pulverized sample being sealed in a thin-walled Pyrex capillary tube of about 0.2 mm. diameter. The higher resolution of the camera made i t possible to observe many weak powder lines besides those observed by Brauer.* However, the powder pattern could still be indexed by the rhombohedral system with a hexagonal c/a ratio of 4.46 instead of 2.30 as employed by Helms and Klemm.* (Helms and Klemm's reported c/a ratio is for a hexagonal pseudo cell containing 3Cs; those weak powder lines which cannot be indexed by em loying this c/a ratio have odd Z-indices (hexagonal).) '$his shows that the 2Cs cannot be in a bodycentered rhombohedral setting; in other words, the parameter, u = I/,, given by Helms and Klemm is not quite correct. When a freshly pulverized sample was used, the powder lines derived from lattice planes parallel, or nearly parallel to the c-axis (Le., those lattice planes with small Z-indices) became considerably weakened, indicating a shearing disorder in the directions parallel to the basal plane. If the sample was annealed by heating for about an hour at 150°, or simply was allowed to stand at room temperature for a few days, and then photographed, the intensity distribution of the powder lines became normal, indicating that the shearing disorder resulting from mechanical disturbance could be removed by annealing. This together with the fact that the monoxide tends to crystallize in laminated plates with more or less perfect basal cleavage leaves little doubt that a layer structure is correct. The present powder data give a = 4.256 f 0.004 A., c = 18.99 f 0.02 A., for a hexagonal unit cell containing three Car0 "molecules." The calculated density is 4.71 g./ cc. as com ared with 4.60 g./cc. observed by Helms and Klemm.2 sased upon an anti-CdClz type structure ( D k ; 2Cs+ a t uuu and tiati, 0- at OOO.), the relative intensities of the powder lines were calculated from the expression
no corrections for the absor tion and temperature factors being made. As shown in t a b l e I, with u = 0.256, the agreement between the observed and the calculated intensities is quite satisfactory. However, the (hk.0)-reflections (or (hk.Z)-reflections with small I ) appear to have a slightly higher temperature factor (BT)than the (OO.I)-reflection (or (hO. &reflections with small h and large Z), indicating that there might still be an appreciable shearing disorder in the powder sample. Helms and Klemm2 and Brauera reported the (10.2)-, (00.6)-, and (10.4)-powder line intensities as about equal; this indicates that they must have used freshly pulverized owder samples. I n interpreting the intensity data from t f e powder sample of a layer crystal, special attention should be paid to the mechanical treatment of the sample.
Single Crystal Work.-Single crystals of dicesium monoxide were obtained by distillationdecomposition of a suboxide (Cs702) in Pyrex capillaries at 170-180". The orange-yellow crystal used in the present investigation was a thin, almost rectangular plate with the dimensions and crystallographic geometry shown in Fig. 1. The following rotation photographs were taken : (a) Cu K a radiation with the hexagonal base diagonal [11.0] as the rotation axis; (h) Cu Ka ra-
CRYSTAL STI~UCTUHE OF CESIUM MONOXIDE
Mar., 1956
339
TABLE I X-RAYPOWDERDATAFOR CESIUM MONOXIDE h
Hexagonal indices k i
0 1 1 0 1 1 1 1 0 1 1
0 0
0 1
0
0
2 2
0 0
1
2
1 0
1
o
2 0
0 0
1
2 1 2 2 2 1 2 2 2 1 1 0
2 2 3 3 0 2 3 0 2 1 2
o
o
Rhqmbohedsl mdices
H
3 1
i
2 6 4
0 0
-i 1 1
1
2
0
0
1
2
7 0 9 3
2
S i
o
5
i
1 1 1 2 2 2 3 1 3 2 3
K
1 0 1 2 1 2 2 0 3 1 3
L
1 0 0 2 1 1 2 1 3 0 2
6.33 3.433 3.159 2.911 2.638 2.177 2.124 1.995
i
i
i
2 3 2
0 2 2
0 1 0
8 o 2 io 1 8 5
3 4 4 3 4 4 2 2 5 3 4 3
1 4 4 3 3 2 0 1 4 1 4 2
3 1} 4 3 1 2 2 1 1 4 0 2 0
O1 i 2i 4 12I
5
4
'}3
0
5 5 4 2
5 3 2 i
5 3 1 i
2
2
i
4
3
1
1.201 1,144
2 2 2
i 2 0
o i
2 6
4 101 51 12 Ti
2 2
7
0
2
1
8 3
8 1 2 13 4
0 1
1
o i 1
0 0
1 0 0 3 1 0
9
2 8
15 11 7
8
0
8 8
31 31
3
s
8
6)
o
Z
i3
3 5
3 5
0 3J
1
3
10
5
3
2
2
15
6
5
4
11
5
4
2
2 1
1
2
1
3
1
2 3
Planar spacings, d Obsd. Calcd.5
2
3
0
8
0 9
2 5
0 2
2 2
3 s
9
4 6
1 6 1 2 i
0
0
18
4 6
2
2
3
3
1
1 1
3 2
i
2 3
1 o
2
1.806 1.766 1.717
I.688 1,580 1.559 1.497 1.457 1.378 1.359 1.336 1.324 1.269
1.229
6.330 3.620 3.435 3.165 2.911 2.643 2.185 2.128 2.110 2.017 1.995 1.835 1.810 1.766 1.718 1.684 1.583 1.563 1.525 1.498 1.456 1.390 1.379 1.359 1.337 1.323 1.308 1.270 1.266. 1.260 1.239 1.229
Obsd.
5 100 25 100 1 3 25
..
1.093
1.015
u = 0.256
5.0 0.3 100 27 88 0.5 2.6 35 0.5
6.2 0.2 100 26 88 0.9 3.5 35 0.8 1.4 24 0 16 29 17
10 20 10 10 5 2 1 5
10 1 10
3
.. ..
.. 3
25 0.1 16 29 17 0.2 3.8 1.8 0.7 1.1 9.2 0 14 0.9 14 5.1 0.2
}
11 0.4 0.8 0.7 5.6
0.3 3.5 2.4
1.o
1.7 8.8 0
14 1.3 14 4.9 0.3 0.6 1.1 1.o
5.6
1.206
..
0.1
0.2
1,202 1.146
10
9.4
9.1
0.6
0.8
5
1.123 1.088 1.085
1.069
u = 0.255
1.1
20
1.145 1.125
Relative intensities Calcd.
1.064
2 1
.. 1
3.2 6.4
1
6.1
1.8
2.5
1.1
1.5
3.8
3.8 0.6
1.062
..
0.4
1.055 1.049 1.021 1.017
.. ..
0.7 0.2
..
0
0.2 0
6.0
6.0
0.9
1.3
2.7
2.4
1
0.6
..
1
3
13
6
4
3
0
2
I6
6
6
4
Based upon a
--
4.256 A.,
c =
18.90 A.
1.009 0.998
0.098
.. 2
KHI-RUEYTSAI,P. M. HARRIS AND E. N. LASSETTRE
340
Vol. 60
with much less labor than that required for a graphical computation. The crystal employed in the present experiment can he treated as a thin rectangular plate with wedge-like top and bottom sections. For the case of rotation about the (11.01 axis, the major portion of the crystal is one of constant cross-section perpendicular to the rotation axis. This cross-section may be divided by the projections of the incident and reflected X-ray beams into appropriate regions for integrations of the absorption integral. For fixed hk, the absorption factor A h k . l can be plotted as a function of the Z-indices. This is illustrated in Fig. 2. An abrupt change in the slope of the curve indicates a change in the type of reflection.
0080070060 05-
Fig. I.-Diagrarn
of single crystal employed.
diation with the hexagonal a-axis [10.0] as the rotation axis; and (e) Mo K a radiation with the hexagonal a-axis [10.0] as the rotation axis. The rotation spots were readily indexed, the hexagonal base diagonal being equivalent to the a-axis of a larger hexagonal unit cell (h' = 2h k , k' = k h, I' = I; a' = g 3 a ) . The relative intensities were estimated visually by comparison with a blackening scale and measurement of the areas of the rotation spots. The rotation photographs exhibit layer disorder similar t o that recently described by Brindley and Ogilvie' for brucite, a CdL-type layer crystal. On both the [10.0] rotation photograph and the [11.0] rotation photograph, the (hk.0)-reflections appear as sharp spots, while the (00.1)-reflections appear as extended arcs. According to Brindley's interpretation the resulting angular displacement of the c-axis is approximately 2". Laue photographs taken along the c-axis, consisting essentially of streaks because of the slight disorder, indicate a Dad diffraction symmetry. This confirms the Dgd-Rh~rhombohedral space group, there being only one CsnO "molecule" in tlle rhombohedral unit cell. Treatment of the Single Crystal Data. (A) The Absorption Factor.-For a crystal containing a high percentage of heavy atoms, the absorption correction becomes very important even though the crystal is very thin. Hendershots has described an analytical method for computing the absorption factor for a rotating crystal bounded by polygonal faces. The formulas apply to the zerolayer reflections only. The graphical method recently described by Kowells8is rather time-consuming. For a thin crystal plate with rectangular crosssection and high absorbing power, the estimation of the absorptioii factor can be done analytically
+
( 7 ) G . W. Brindley and 0. J. Ogilvie. Acta Crust., 5, 412 (1952). ( 8 ) 0. P. Hendershot. Rev. Sci. Inslr.. 8, 324 (1937). (9) R. G. Howella, Acta Cryst., 3, 366 (1950).
O 00 I
O
La
I
W
-+ --+.- * _I- - lk I-
Fig. 2.-Calculated absorption factors versus l-indices for the Ill.01 rotation photograph taken with CuK, radiation.
I n the case of Mo K a radiation ( p = 192 cm.-l) and rotation about the [10.O]-axis, the crystal can be treated as a thin rectangular plate, contribution from the wedge-like edges being negligible. The crystal is divided into one parallelopiped (central) section and two triangular prismatic sections since the rotation axis is now inclined 30" to the [11.0] edges. However, i n the case of internal reflection through both major faces of the crystal plate, good approximation can usually be obtained by merely integrating through the thickness of the crystal plate. (B) The Temperature Factor and the Scale Factor.-The anti-CdC1, structure (D.0.2) and Hartree scattering fac- on both sides of the cesium nucleus. Their signifitor for the oxide ion as given in the International cance will be discussed presently. (D) Interionic Distances,-With u = 0.256 f Tabellen. F , based upon u = 0.256 gave a slightly better agreement with the observed structure fac- 0.001, 5 = 4.256 i 0.004 A., and c = 18.99 f tors. A least-squares treatment of the values of 0.002 A., the observed interionic distances were loglo (+/Fc) versus corres onding values of sin20/A2 found to be Cs+-0- = 2.86 i 0.01 A. and Cs+-Cs+ gave K = 0.829 and T = 3.24 X cm.2. = 4.19 f 0.02 A., as compared with 3.09 and 3.38 A., 1601
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