R 368
Philips Res. Repts 14, 143-150, 1959
CRYSTAL STRUCTURE OF SODIUM-POTASSIUM
.ANTIMONIDE (NaiKSb)
by J. J. SCHEER and P. ZALM
548.736.4
Summary
X-ray analysis of Na2KSb - a photoemissive material discovered by
Som mer - has led to the determination of its crystal structure. The
unit cell is described by the space group Fm3m-01l5 with four antimony
atoms at (0,0,0; O,t,!; t,O,t; t,t,O) + 0,0,0; four potassium atoms at
(
) + t,t,! and eight sodium atoms at (
) + t,t,t and t,t,t.
The crystal structure of Na2KSb has a great resemblance to that of
CS3Sb and differs strongly from that of Na3Sb and K3Sb which both
crystallize in the Na3As type.
Résumé
L'analyse par rayons X d'une rnatière photoémissive - Na2KSb _
découverte par Sommer, à conduit à la détermination de sa structure
cristalline. La cellule unitaire est décrite par le groupe spatial Fm3m-Oh5
avec quatre atomes d'antimoine à: (0,0,0; O,t,t; t,O,t; t,t,O) + 0,0,0,
quatre atomes de potassium à: (
) + t,t,t et huit atomes de sodium
à: (
) + t,t,t et t,t,t -,La structure cristalline de Na2KSb ressemble beaucoup à celle de CS3Sb et diffère fortement de celle de Na3Sb
et K3Sb qui cristallisent tous deux suivant le mode Na3As.
Zusammenfassung
Die Röntgenanalyse von Na2KSb - ein von Sommer, entdecktes photoemittierendes Material-ermöglichte
die Bestimmung der Kristallstruktur
dieses Stoffes. Das Raumgitter wird beschrieben durch die Raumgruppe
Fm3m - OhS mit vier Antimon-Atomen bei (0,0,0; O,t,t; t,O,t; t,t,O)
+ 0,0,0, vier Kalium-Atomen bei (
) + t,t,t und acht NatriumAtomen bei (
) + {,t,t und t,t,t. Die Kristallstruktur
von
Na2KSb hat groûe Ähnlichkeit mit der von CS3Sb und weicht stark ab
von der Struktur von NaaSb und K3Sb, die beide nach dem Na aAs-Typ
kristallisieren .:
1. Introduetion
The properties of A-B compounds in which A represents an alkali metal and B
an element of the Vth group ofthe periodic systemhave been studied for various
reasons. Zintl and coworkers have investigated these compounds to clarify their
crystal chemistry, and in particular to determine the boundary ("Zintl Grenze")
at which the transition of the type of bonding of alloys from metallic to ionic
takes place. For this purpose the crystal structures of various A3B compounds
(LisP, Li3As, Li3Sb, Na3P, Na3AS, Na3Sb, K3As, K3Sb and K3Bi) have
been investigated by Brauer and Zintl+). It appears that all these compounds
crystallize in the Na3As type (hexagonal) although in the case of Li3Sb only
the high-temperature modification is hexagonal, its low-temperature modification being cubic (probably CU3AItype).
144
J. J. SCHEER and P. ZALM
Recently, Klemm and co-workers have continued the study of I - V compounds. Gnutzmann 2) has investigated the· binary systems Rb-Bi, Cs-Bi and
K-Bi by means ofthermal analysis. He has also determined the crystal structure
of the following AaB compounds: RbaSb (NaaAs type) and Rb-Bi, CSaSb,
CsaBi (CuaAl type).
The discovery by Görlich a) of the excellent photo-emissivity of CSaSband
CsaBi is another reason why the properties .of I - V compounds have been
studied. The composition of the cesium-antimony compound used as photocathode material was found by Sommer 4), Lukjanov 5) and Chlebnikov 6) to
be CSaSb. In order to explain the photoelectric properties of CSaSb Jack and
Wachtel 7), being unacquainted with the investigations of Klemm e.a., have
I
I
I
I
I
I
.....
...«:
..........
""r
I
I"
I
I
I
I
_._
I ..........
..........
_r:r----
Unit cellof
.:CSasSbas
O:Cs
CS3Sb
96630
Fig. 1. Unit cell of CS3Sb according to Jack and Wachtel.
investigated once more its crystal structure. Surprisingly their result differs
from that of Gnutzmann. According to the latter author the unit cell of CsaSb
shows a complete ordering of Cs and Sb, whilst Jack and Wachtel report a
random distribution of Cs and Sb over equivalent positions to a certain extent.
The unit cell of CSaSb as determined by Jack and Wachtel is shown in fig. 1,
that according to Gnutzmann in fig. 2 and the unit cell ofNaaAs in fig. 3.
Some years ago Sommer 8) found that the combination of two or more alkali
metals with antimony leads in some cases to photocathodes with a sensitivity
higher - especially at long wavelengths - than that of CSaSb or CsaBi.
CRYSTAL STRUCTURE
145
OF Na,KSb
.:Sb
O:Cs
96631
Fig. 2. Unit cell of CSaSb according to Gnutzmann.
The composition of the most promising compound is reported to be probably
Na2KSb with some Cs to lower the electron affinity of this material.
Unit cell of Na3As
£:As
96632
O:Na
Fig. 3. Unit cell of NaaAs.
146
J. J. SCHEER and P. ZALM
It was thought therefore worth while to investigate whether an ordered
compound exists with the composition Na2KSb.
2. Experimental
For the preparation of samples of sodium-potassium antimonide suitable
for X-ray analysis we have adopted the method used by Jack and Wachtel 7)
for CS3Sb: antimony powder was heated in an atmosphere of potassium and
sodium vapours at 230°C during 20 hours.
The alkali metals were obtained by radio-frequency heating of pellets containing sodium chromate and potassium chromate, respectively, together with
ferrotitanium as a reducing agent. By preliminary investigations the amount
of sodium and of potassium that could be obtained from such pellets was determined. It turned out that within a few per cent the yield of alkali metals from
the pellets was a constant fraction of the total amount of alkali metal present
as chromate.
Powder diagrams of the samples were made at room temperature with CuKa
radiatiçn in a 9-cm Debye-Scherrer camera. After X-ray analysis the ratio
Na/K in the reaction product was determined flame-photometrically.
Most attention was paid to compounds with the composition Na2KSb and
we will restrict ourselves to a discussion of the structure of this 'material.
3. Structure determination of Na2KSb
The X-ray powder photographs of Na2KSb show - in contrast with that
of CS3Sb- only little background. The pattern could be indexed on the basis
of a cubic unit cell with a lattice constant a = 7·74 ± 0·01 Á.
From the systematic absence of reflections with mixed odd and even indices
it must be concluded that this compound possesses a face-centred structure.
If Na2KSb would have a fully ordered structure, thus with Na occupying
eight-fold sites, and both antimony and potassium four-fold sites, then the
choice of a space group is limited to the three groups Fm3-Th3 (International
tables nr 202), F432 - 03 (nr 209) and Fm3m - Oh5 (nr 225) which, moreover,
appear to be identical with respect to thè occupation of the afore-mentioned
sites. In this case the Na2KSb structure is described by the following positions:
four antimony atoms at (0,0,0; O,t,t; t,O,t;
four potassium atoms at (
) + t,t,t;
eight sodium atoms at (
)
t,1,t and
+
t,t,O) + 0,0,0;
i,i,i.
The structure amplitudes are
h,k and I even, h
h,k and I even, h
h,k and lodd
+ k +" I = 4n
+ k + I ~ 4n + 2
Fe -8fNa
Fe = -8fNa
Fs = -4ji<:
+ 4ji{ + 4fSb
+ 4ji{ + 4fsb
+ 4fSb
CRYSTAL STRUCTURE
147
OF Na,KSb
TABLE I
Powder diagram of Na2KSb with CuKa radiation
sin2 8
obs.
sin2 8
calc.
0·0295
0·0425
,0·0788
0·1103
0·1192
0·1590
0·1908
0·2019
0·2410
0·2714
0·3224
0·3488
0·3588
0·4012
0·4270
0·4391
0·4791
0·5070
0·5147
0·5592
0·5868
0·6428
0·6644
0·6743
0·7145
0·7445
0·7545
0·7939
0·8227
0·8333
0·8714
0·9025
0·9509
0·9802
0·0297
0·0396
0·0792
0·1089
0·1188
0·1584
0·1882
0·1981
0·2377
0·2674
0·3169
0·3466
0·3566
0·3981
0·4258
0·4357
0·4753
0·5051
0·5150
0·5546
0·5843
0·6340
0·6635
0·6734
0·7130
0·7427
0·7526
0·7922
0·8219
0·8319
0·8715
0·9012
0·9507
0·9803
,
..
I
h,k,l
111
200
220
"311.
222
400
331
420
422
511, 333
440
531
600, 442
620
533
622
444
711, 551
640
642
731, 553
800
733
820, 644
822, 660
751, 555
662
840
911, 753
842
664
931
844
933, 771, 755
Int. *)
calc.
Int.
obs.
62
80
384
85
51
108
58
100
293
58
110
77
82
184
32
58
56
60
58
320
91
43
34
134
289
97
86
248
162
219
325
162
581
616
m
m
vs
m
w
ms
wm
ms
s
w
ms
m
m
s
vw
wm
wm
wm
wm
vs
m
w
vw
ms
ms
m
m
ms
ms
ms
ms
m
s
s
*) The intensities are corrected for absorption and Lorentz-polarization factors, and not for
temperature factor.
148
J. J. SCHEER and P. ZALM
The calculated intensities of the reflections for this structure (see fig. 4) are
in good agreement with the observed values (see table 1).
In the Na2KSb structure each atom has eight ligands equidistant at 3·34 Á.
Each sodium atom is surrounded by 4 potassium and 4 antimony atoms as
nearest neighbours, whilst the potassium and antimony atoms each have 8
sodium atoms as neighbours. The sum of the ionic radii of Na and Sb is 3·40Á
Unit celI of N02 K Sb
.:Sb
O:K
fj.: No
96633
Fig. 4. Unit cell of Na2KSb.
and that of the metallic radii 3·31.À 9). The interatomie distance in Na2KSb is
just between these two values. For the Na-K distance we find 3·34 Á in comparison with the sum of the ionic radii: 2·28 Á and with that of the metallic radii:
4·08Á.
The main conclusion from the foregoing is that an ordered compound exists
with the composition Na2KSb.
.
4. Structure of CS3Sb
To trace whether the, different conditions of preparation are responsible for
the differences in the crystal structure of cesium-antimony as found by Gnutzmann and by Jack and Wachtel, we again made some CS3Sb according to the
method of the latter authors. For obtaining X-ray patterns with sufficiently
sharp lines it was necessary to heat the reaction tubes at a temperature of
240°C for 16 hours. The background intensity on the photographs, however"
CRYSTAL STRUCTURE
149
OF Na,KSb
+ +
was still high and we were only able to find the reflections for which h k
I=
4n with h, k, I all even. So we have no reasons to conclude that CS3Sbhas any
other structure than that of the CU3AItype as found by Gnutzmann, although
it is conceivable that our product has still the structure as described by Jack
and Wachtel and that the lack of observation of other reflections is due to the
high background. Another explanation, however, is that CS3Sb can exist in
both structures. In this case we expect the CU3AI-typestructure to be the stable
modification of CS3Sband the structure as proposed by Jack and Wachtel to be
an unstable modification.'
,
At any rate one may conclude from the investigations of Jack and Wachtel
that the crystal structure of CS3Sb in the actual photocathodes has cesium and
antimony distributed over equivalent sites.
5. Discussion
The type of bonding in Na2KSb is perhaps best described as a' normal
valency intermetallic compound, as was done by Jack and Wachtel 7) when discussing the type of interatomie bonding of cesium-antimonide. The considerations given by these authors for CS3Sbapply equally to this material.
Concerning CS3Sbit is of interest to note that, if the hypothesis is right that
both a stable and a metastable structure can occur, this would account for the
observations that a prolonged heating ofphotocathodes results in an irreversible
decrease in photosensitivity 10);,From the crystal structure of CS3Sb as determined by Jack and Wachtel it is clear that an excess of Cs in the crystallattice
would lead to a p-type semiconductor. Chemical analysis of sensitive photocathodes of CS3Sbinvariably indicated the presence of such an excess of Cs.
The experience that photo-emitters exhibiting a high quantum efficiency are
p-type semiconductors can be understood *) by noting that the amount of
absorbed Cs at the surface of a given semiconductor as Cs+ ions is the greater
the stronger the material is p-type 11). The greater the amount of Cs+ at the
surface the stronger the bending of the energy bands will be, and this facilitates
the emission of the electrons which are excited to the conduction band. The
apparent electron affinity of the photo-emitter can thus be made equal to or
smaller than zero and this would account for the observation that light absorption and photo-emission have identical thresholds (see also Wright 12».
Acknowledgement
The authors are indebted to Dr A. J. van Bommel and Dr P. B. Braun for
their constructive criticism.
Eindhoven, December 1958
*) The authors are indebted to Dr H. A. Klasens for this suggestion.
150
J. J. SCHEER and P. ZALM
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1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
11)
12)
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S. J. Lukjanov, J. tech. Phys. Moscow 13,3-12, 1943.
N. S. Chlebnikov,
J. tech. Phys. Moscow, 17, 333-340, 1947.
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