The new binary intermetallic YbGe2.83

Article abstract of DOI:10.1016/j.jssc.2010.06.022

The new compound YbGe_2.83 was obtained from the reaction of Yb and Ge in liquid indium. The crystal structure of YbGe_2.83 adopts the trigonal, P3m1 space group with a=b=8.3657(12) and c=7.0469(14) . The structure of YbGe_2.83 is a variant of the CaAl₂Si₂ structure type with ordered vacancies. Germanium atoms form double layers of puckered hexagons creating slabs that sandwich the Yb atoms. YbGe_2.83 can be classified as a Zintl compound with the formula Yb ^(2x)(Ge_2.83)^(2x)-. The deficiencies at the Ge sites cause a mixed/intermediate valent state of ytterbium (Yb^2.35). Valence bond sum calculations suggest an average valence of Yb ions in YbGe _2.83 of 2.51 consistent with an intermediate valence compound.

Full text of DOI:10.1016/j.jssc.2010.06.022

Journal of Solid State Chemistry 183 (2010) 2077–2081
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
The new binary intermetallic YbGe2.83
n
C. Peter Sebastian, Mercouri G. Kanatzidis
Department of Chemistry, Northwestern University, 2145 N. Sheridan Road, Evanston, IL 60208-3113, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The new compound YbGe2.83 was obtained from the reaction of Yb and Ge in liquid indium. The crystal
Received 3 June 2010
Received in revised form
structure of YbGe2.83 adopts the trigonal, P 3¯ m1 space group with a¼b¼8.3657(12) A˚ and c¼7.0469(14)
˚
A. The structure of YbGe2.83 is a variant of the CaAl
2
Si
2
structure type with ordered vacancies.
2
8 June 2010
Germanium atoms form double layers of puckered hexagons creating slabs that sandwich the Yb atoms.
Accepted 28 June 2010
Available online 23 July 2010
(2+x)+
(2+x)ꢀ
YbGe2.83 can be classified as a Zintl compound with the formula Yb
(Ge2.83
.35+
) . The deficiencies
2
at the Ge sites cause a mixed/intermediate valent state of ytterbium (Yb
). Valence bond sum
Keywords:
calculations suggest an average valence of Yb ions in YbGe2.83 of 2.51 consistent with an intermediate
valence compound.
Yb–Ge system
Mixed valence
Superstructure
Zintl phase
&
2010 Published by Elsevier Inc.
Intermetallics
1
. Introduction
During our studies on Yb
in indium metal flux, we have observed many rod shaped crystals of
side products, such as Yb Ge and Yb Ge5ꢀx. In addition, we observed
triangular shaped single crystals of a new binary YbGe which it was
subsequently determined to have the composition of YbGe2.83. Here
we report the synthesis and crystal structure of YbGe2.83
x
T
y
Ge
z
(T¼transition metal) compounds
The first compound discovered in the Yb–Ge system was
5
3
3
Yb
following year Smith et al. [2] reported the crystal structure of
Yb Ge , a Th Pd type structure. In 1983 Eremenko et al. [3]
discovered Yb11Ge10, a Ho11Ge10 type structure and a carbon
stabilized Yb Ge , Mn Si type structure and they proposed the
phase diagram of Yb–Ge system for the first time. In 1990, all
those previous results were compiled by Massalski et al. [4] in the
book Binary Alloy Phase Diagrams. In 2003, Pani and Palenzona [5]
synthesized and elucidated the structure of Yb
Yb Ge (Mn Si type), Yb Ge (Sm Ge type) and Yb
structure type) to complete the Yb–Ge phase diagram.
There are few REGe
(RE¼Y, Gd–Er) binary compounds
crystallized in orthorhombic Cmcm space group with the DyGe
structure type [6–11]. Except for DyGe , a deficiency in the Ge is
not uncommon in these systems, as evidenced by the composi-
tions YGe2.69 [6], GdGe2.57 [7], TbGe2.7 [8], HoGe2.7 [10] and
ErGe2.83 [11]. From the perspective of Yb, 1:3 compounds are
known with terelides (Al, Ga, In, Tl) [12,13] and tetrelides (Sn and
2
Ge
3
, an AlB
2
type reported in 1964 by Gladyshevskii [1]. In the
3
.
3
5
3
5
3
5
5
3
2
. Experimental section
.1. Synthesis
The YbGe2.83 was obtained as a side product from the reactions
2
2
Ge (PbCl
2
type),
5
3
5
3
5
4
5
4
3
Ge (its own
8
by combining 3 mmol of the Yb metal, 2 mmol of transition metal
(Ag or Pd), 6 mmol of Ge and 45 mmol of In in an alumina crucible
under inert nitrogen atmosphere inside a glove-box. The crucible
was placed in a 13 mm fused silica tube, which was flame sealed
3
3
3
ꢀ
4
under vacuum of 10 Torr, to prevent oxidation during heating.
The reactants were then heated to 1000 1C over 10 h, maintained
at that temperature for 5 h to allow for proper homogenization,
followed by cooling to 850 1C in 2 h and held there for 48 h.
Finally, the system was allowed to cool to 50 1C in 48 h. The
reaction product was isolated from the excess indium flux by
heating at 350 1C and subsequent centrifugation through a coarse
frit. Any remaining flux was removed by immersion and
sonication in glacial acetic acid for 48 h. The final crystalline
product was rinsed with water and acetone. The ternary
compounds obtained from the Pd and Ag based reactions were
Pb) [14,15]. Except for YbGa
Cu
Au structure type (Pm 3¯ n space group) and feature fully
occupied Ge sites. YbGa is hexagonal and crystallizes in its own
structure type but with a deficiency in the Ga position (YbGa2.64
16]. Surprisingly, the binary YbSi and YbGe are missing
3
, all others crystallize in the cubic
3
3
)
[
3
3
compounds in this category.
YbPd
products obtained from the reaction were well formed crystals of
Yb Ge and Yb Ge compounds. In addition, we observed a few
2 2 4 8
Ge [17] and Yb AgGe [18], respectively. The binary side
n
Corresponding author. Fax: +1 847 491 5937.
E-mail address: m-kanatzidis@northwestern.edu (M.G. Kanatzidis).
5
3
3
5
0022-4596/$ - see front matter & 2010 Published by Elsevier Inc.
doi:10.1016/j.jssc.2010.06.022

2
078
C. Peter Sebastian, M.G. Kanatzidis / Journal of Solid State Chemistry 183 (2010) 2077–2081
Table 1
Crystal data and structure refinement for YbGe2.83 at 293(2) K.
Formula weight
Wavelength
378.29
.71073 A
Trigonal, P 3¯ m1
0
˚
Crystal system, space group
Unit cell dimensions
a¼8.3657(12) A,
b¼8.3657(12) A˚ ,
˚
a
b
¼90.001
¼90.001
c¼7.0469(14) A˚ ,
g
¼120.001
Volume
27.10(12) A
˚
3
4
3
Z, density (calculated)
Absorption coefficient
F(000)
4, 5.883 g/cm
ꢀ
1
41.177 mm
642
3
Crystal size
150 ꢂ 150 ꢂ 150
2.81–29.221
mm
Theta range for data collection
Index ranges
ꢀ11rhr11, ꢀ11rkr11, ꢀ9rlr9
Reflections collected
Independent reflections
8016
466 [Rint¼0.0790]
Completeness to
y
¼29.221
100%
2
Refinement method
Full-matrix least-squares on F
Data/restraints/parameters
466/36/28
1.295
2
Goodness-of-fit on F
Fig. 1. Image of the typical single crystal of YbGe2.83 and EDS data from the
selected spot (spectrum 1) and the area indicated by the blue square (spectrum 2)
of the crystal. The analytical data indicate absence of other elements. (For
interpretation of the references to colour in this figure legend, the reader is
referred to the web version of this article.)
Final R indices [42
Extinction coefficient
Largest diff. peak and hole
s
(I)]
R
obs¼0.0483, wRobs¼0.0969
0.0023(3)
2.438 and ꢀ2.633 e A
˚
ꢀ3
2
2
2
4
1/2
R¼
S
99F
o
2
9ꢀ9F
c
99/
S
9F
o
2
9, wR¼{
S
[w(9F
o
9 ꢀ9F
c
9 ) ]/
S
[w(9F
o
9 )]}Wy
and calc w¼
2
2
2
1
/[s
(F
o
)+(0.0245P) +15.6684P], where P¼(F
o
c
+2F )/3
3
small triangular shaped crystals which were identified as ‘‘YbGe ’’
from the energy dispersive X-ray spectroscopy (EDS) measure-
ments. A scanning electron microscopy (SEM) image of a typical
As a check for the correct composition, the occupancy parameters
were refined in a separate series of least-squares cycles. Initially,
all sites were kept completely occupied, however, the refinement
was largely unsatisfactory and marked with relatively high
residuals (R149%) and large thermal ellipsoids around the Ge2
3
single crystal of YbGe is shown in Fig. 1.
The particular synthesis conditions under which YbGe2.83 was
discovered appear to be essential for its stabilization. More direct
attempts made to synthesize the compound in bulk quantity for
physical property studies by varying the starting composition and
using various techniques (including Ge flux) such as arc-melting
and high frequency induction heating were unsuccessful. The
main product observed from these experiments was the more
˚
˚
atom (U11¼U22¼0.66 A and U33¼0.69 A). In order to avoid these,
the Ge2 position was kept unoccupied and observed a pronounced
˚
decrease in the residuals (R1¼2.1 A) and thermal ellipsoids
˚
˚
(U
11¼U22¼0.10 A and U33¼0.13 A). This reduces the Ge2 atom
occupancy to 42% in the 2d Wyckoff position. The initial crystal-
3 5
thermodynamically stable Yb Ge .
lographic data showed some additional reflection along [100] and
[010] directions suggesting the presence of a superstructure. In
2
.2. Elemental analysis
order to take those weak reflections into account, we repeated the
data collection with increased exposure time and refined the
structure successfully within same crystal system and space
group but with doubling of a and b axes. In order to check the
presence of In or Pd in the crystal structure, the germanium atoms
at 2d (Ge1 and Ge3) and 6i (Ge2 and Ge4) positions were mixed
with In and Pd in separate cycles and showed a slight rise in the
equivalent isotropic displacement parameter indicated the
absence of Ge/In and Ge/Pd mixing in YbGe2.83. The details of
the data collection and complete refinement are shown in Table 1.
The atomic coordinates, anisotropic displacement parameters and
interatomic distances are listed in Tables 2–4, respectively.
Quantitative microprobe analyses of the compounds were
performed with a Hitachi S-3400 scanning electron microscope
SEM) equipped with a PGT energy dispersive X-ray analyzer.
Data were acquired with an accelerating voltage of 20 kV and a
0 s accumulation time. The EDS analysis taken on visibly clean
(
6
surfaces of the single crystal used for the data collection gave an
atomic composition that is in good agreement with the results
derived from the single crystal X-ray diffraction refinement. The
EDS detector cannot detect elements lighter than Be.
1
Further information on the structure refinements is available.
2.3. X-ray crystallography
2.5. Bond valence sum (BVS) calculations
The X-ray intensity data were collected at room temperature
using a STOE IPDS 2T diffractometer with graphite-monochroma-
˚
Based on the Pauling’s concept [20] the BVS surrounding the
ytterbium atom is equal to the oxidation state zYb, as shown
below
tized MoK
a (l¼0.71073 A) radiation. The X-AREA (X-RED and
X-SHAPE within) package suite was used for the data extraction,
integration and analytical absorption correction. Triangular
shaped single crystals (Fig. 1) obtained from the reaction were
used for the data collection.
X
zYb
¼
sYbGe
ð1Þ
The valences of the individual bonds, SYbGe in Eq. (1), can be
calculated from the observed bond lengths using
2.4. Structural refinement
SYbGe ¼ exp½R
0
ꢀRYbGeꢁ=b
ð2Þ
Indexing of all the reflections lead to a trigonal cell with the
space group P 3¯ m1. The direct method was used and the structure
was refined using Shelxl-97 (full-matrix least-squares on F ) [19]
with anisotropic atomic displacement parameters for all atoms.
1
Details may be obtained from: Fachinformationszentrum Karlsruhe, Abt.
PROKA, D-76344 Eggenstein-Leopoldshafen, Germany, by quoting the Registry no.
CSD-421475.
2

C. Peter Sebastian, M.G. Kanatzidis / Journal of Solid State Chemistry 183 (2010) 2077–2081
2079
Table 2
Atomic coordinates ( ꢂ 10 ) and equivalent isotropic displacement parameters
high thermodynamically stable Yb–Ge binaries it is difficult to
synthesize the compound as a pure phase. Byproducts of these
4
2
3
(
A˚ ꢂ 10 ) for YbGe2.83 at 293(2) K with estimated standard deviations in
parentheses.
2 2 4 8 3 5 5 3
reactions include YbPd Ge , Yb AgGe , Yb Ge and Yb Ge and
recrystallized germanium. The compound YbGe2.83 crystallizes
from indium flux as triangular shaped single crystals (Fig. 1). The
crystals are stable in air for several months. At first we suspected
that the transition metal may be incorporated into the structure
to producing a ternary compound similar to the Yb–Si system
where the inclusion of silver stabilizes the new compound
YbAg0.28Si1.72 [29] Therefore, we looked carefully at the possibility
of transition metal or indium inclusion. Several EDS/SEM analyses
systematically carried out on selected single crystals with long
accumulation times of ꢃ6 min did not indicate any transition
metal present in the specimens (Fig. 1). Multiple measurements in
different regions of the selected single crystals were also
performed for better statistics (two of them are shown in Fig.1)
and clearly ruled out the presence of any detectable element other
than Yb and Ge. Although the material does not form without a
transition metal being present in the synthesis giving instead
a
Label
Wyckoff site
x
y
z
Occupancy
U
eq
Yb(1)
Yb(2)
Ge(1)
Ge(2)
Ge(3)
Ge(4)
1a
3e
2d
6i
0
0
0
1
4(1)
1(1)
2(1)
2(1)
4(1)
6(2)
0
5000
6667
8333(1)
3333
6669(6)
0
1
3333
1667(1)
6667
8334(3)
2580(3)
7423(2)
3551(7)
6275(5)
1
1
2d
6i
0.566(11)
0.361(8)
a
U
eq is defined as one third of the trace of the orthogonalized Uij tensor.
Table 3
Anisotropic displacement parameters ( A˚ 2 ꢂ 10 ) for YbGe2.83 at 293(2) K with
3
estimated standard deviations in parentheses.
Label
U
11
U
22
U
33
U
12
U
13
U
23
3 5
Yb Ge and the ternary phases mentioned above, we speculate
Yb(1)
Yb(2)
Ge(1)
Ge(2)
Ge(3)
Ge(4)
4(1)
1(1)
2(1)
2(1)
3(1)
6(2)
4(1)
1(1)
2(1)
2(1)
3(1)
6(2)
4(1)
1(1)
2(1)
2(1)
6(2)
5(2)
2(1)
1(1)
1(1)
1(1)
2(1)
3(1)
0
0
0(1)
0
0(1)
0
that the transition metals could act as a nucleating agents for the
growth of YbGe2.83 crystals. A similar such behavior was reported
0(1)
0
0(1)
0
5 2 6
in the case of Yb Al Sb [30].
The thermal stability and phase transformation of YbGe2.83
were investigated. Pani and Palenzona [5] synthesized and refined
0(1)
1(1)
The
anisotropic
displacement
factor
exponent
takes
the
form:
3 8
the crystal structure of Yb Ge (YbGe2.67), which was close in
2
2
*2
* *
ꢀ2p [h a U11+?+2hka b U12].
composition to our compound, triclinic, but it is triclinic P 1¯ space
group and crystallizes in its own structure type. They observed
the formation of the compound at 865 1C. In order to check for any
phase transition of YbGe2.83 we annealed the sample up to 800 1C.
We observed no change in the crystal structure of the compound
but at 850 1C the single crystal decomposed completely and Ge
melt formed.
Table 4
Bond lengths ( A˚ ) for YbGe2.83 at 293(2) K with estimated standard deviations in
parentheses.
Label
Distance
Label
Distance
Yb(1)–Ge(2) ꢂ 6
Yb(1)–Ge(4) ꢂ 6
Yb(2)–Ge(2) ꢂ 4
Yb(2)–Ge(1) ꢂ 2
Yb(2)–Ge(3) ꢂ 2
Yb(2)–Ge(4) ꢂ 4
Ge(1)–Ge(4) ꢂ 3
Ge(1)–Ge(3)
3.0219(13)
3.566(4)
Ge(2)–Ge(4) ꢂ 2
Ge(2)–Yb(2) ꢂ 2
Ge(2)–Yb(1)
2.546(2)
3.0217(10)
3.0219(13)
2.5103(19)
2.726(5)
3.477(3)
2.546(2)
2.548(4)
2.605(4)
3.566(4)
3.568(3)
Other synthetic techniques such as arc-melting, high
frequency induction heating, direct heating in resistive furnaces
also failed to produce the compound. Since YbGe2.83 is the most
germanium rich compound in the Yb–Ge system we also tried to
grow crystals using germanium flux but were not successful.
3.0217(10)
3.0230(14)
3.478(3)
Ge(3)–Ge(2) ꢂ 3
Ge(3)–Ge(1)
3.568(3)
Ge(3)–Yb(2) ꢂ 3
Ge(4)–Ge(2) ꢂ 2
Ge(4)–Ge(1)
2.548(4)
2.726(5)
Ge(1)–Yb(2) ꢂ 2
Ge(2)–Ge(3)
3.0230(14)
2.5103(19)
2.546(2)
Ge(4)–Ge(2)
3.2. Crystal structure
Ge(4)–Yb(1)
Ge(2)–Ge(4) ꢂ 2
Ge(2)–Ge(4)
Ge(4)–Yb(2) ꢂ 2
Crystal structure refinement shows that YbGe2.83 is a germa-
2.605(4)
2 2
nium deficient variant superstructure of the CaAl Si type [31]
structure. The details of the data collection, complete refinement,
the positional parameters and interatomic distances are listed in
Tables 1–4. The evolution of YbGe2.83 superstructure from
CaAl Si is shown in Fig. 2. In the substructure of YbGe2.83, the
0
where RYbGe is the observed bond length in YbGe2.83, and R is the
constant dependent on the nature of YbGe pair. The constant b
was determined to be 0.37 [21], which is a generally accepted
2
2
value [22–25]. The R
valence. The usual procedure was to assume an oxidation state
and to use a previously determined R value appropriate to the
bond being considered. The R value was calculated using the
software VALENCE [26] from the bond lengths and the oxidation
state of YbAl Ge [27]. The valences of Yb1 and Yb2 were
calculated using the program CIFTOOL [28].
0
value can be viewed as a bond length of unit
Yb atom occupies the Ca position, while the Ge1 and Ge2 atoms
occupy the Al and Si positions, respectively. The Yb and Ge1 sites
are fully occupied while the Ge2 site is only 42% occupied. In the
superstructure the a- and b-axis are doubled and every atom site
splits into two different sites. Both the Yb and Ge1/Ge2 sites are
fully occupied. The Ge3 and Ge4 sites are deficient but the degree
of deficiency is different with 57% and 36%, respectively. Also two
types of coordination environments are present for the Ge atoms:
a flipped tetrahedron or umbrella shaped coordination for the Ge1
and Ge2 and a tetrahedron coordination for Ge3 and Ge4 atoms.
Fig. 3 shows the slabs of double puckered layers of fused Ge
hexagons which stack along the c-axis to sandwich the Yb atom
layers. The closest distance between the anionic germanium slabs
0
0
2
2
3
. Results and discussion
3.1. Reaction chemistry
˚
The combination of Yb, Ge and Pd or Ag in excess indium
is 4.4028(9) A. YbGe2.83 can be classified as a Zintl compound and
(2+x)+
(2+x)ꢀ
resulted in the formation of the new binary compound YbGe2.83
.
can be written, more expressly, as Yb
separate the two different Ge sites in order to make a more direct
comparison with its parent compound, CaAl Si compound which
is regarded as a Zintl phase [32]. In CaAl Si the Al and Si atoms
(Ge
2
Ge2ꢀx
)
. We
We did not observe this new compound from similar stoichio-
metric reactions with other transition metals we studied such as
Cr, Mn, Fe, Co, Ni, Co, Ru and Au. Unfortunately, because of other
2
2
2
2

2
080
C. Peter Sebastian, M.G. Kanatzidis / Journal of Solid State Chemistry 183 (2010) 2077–2081
2 2
Fig. 2. Structural derivation of the superstructure of YbGe2.83 from CaAl Si through its substructure is shown. The unit cell is emphasized as blue lines. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web version of this article.)
value was estimated at 2.618 using the program VALENCE [26]
which for YbGe2.83 indicated an oxidation state of 2.48 for Yb1
and 2.53 for Yb2 with an overall average value of 2.51. This is
slightly higher than the postulated valence state using the Zintl
2
.35+
concept (Yb
). This small increase in the Yb valence probably
due to the defects at the germanium sites (Ge3 and Ge4) which
were not taken into account for the VBS analysis.
˚
The Yb–Ge distances of 3.0219(13) and 3.0217(10) A in
YbGe2.83 are consistent with the 4 +2 oxidation state of the Yb
atoms. By comparison, the shortest Yb–Ge distances in Yb Ge are
2
˚
˚
˚
3
.127(2) A, in Yb
5
˚
Ge
3
2.899(2) A, in Yb
5
Ge
˚
3 8
3.030(1) A, in Yb Ge 2.941(1) A.
4
3.061(1) A, in
˚
Yb11Ge10 2.839(1) A, in Yb
3
Ge
5
This diversity in the bond distances is a consequence of the
varying average oxidation state of Yb in these systems.
˚
The average Ge1–Ge2 bond distances (2.5871 A) in YbGe2.83
are comparable with the Ge–Ge distances in other mixed valent
˚
compounds, e.g. Yb
differences in CaAl Si
Zheng et al. [32]. In comparison to that our compound shows
3
Ge
5
(2.576(1) A) [5]. The bond length
2
2
type Zintl compounds were discussed by
2
ꢀ
different types of Ge–Ge distances in the (Ge
4
)
layers, the short
˚
˚
one with 2.5103(19) A and the large one with 2.726(5) A. The
nominal Ge zigzag chains of YbGe2.83 are very similar to those in
CaAl
resulting Ge–Ge–Ge zigzag angles average at 111.23(7)1 are pretty
close to the corresponding angles in CaAl Si , which are close to
2 2
Si in that the dihedral angles of the chains are 01. The
2
2
1
12.1041.
Fig. 3. Superstructure of YbGe2.83 is shown along the a-axis. The unit cell is
emphasized as blue lines. (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
4
. Concluding remarks
being
Ca (Al
4-connected
consistent
with
the
formulation
YbGe2.83 is a new binary compound in the Yb–Ge system
obtained from indium flux. The crystal structure of YbGe2.83 is a
vacant variant of the CaAl Si structure type and can be
2
+
2ꢀ
2
Si
2
)
. The two dimensional arrays of Yb ions are
(2+x)ꢀ
separated by tightly bound (Ge
expected that each atom in the Ge
surrounded by other Ge atoms. However, Ge3 and Ge4 may
exhibit 3-connected puckered nets as in the structure of -As
2
Ge2ꢀx
)
layers (Fig. 3). It is
2
2
4
layer is 4-connected and
rationalized on the basis of the Zintl concept. The deficiency at
the Ge sites and the Zintl phase description of YbGe2.83 suggest an
a
+2.35
overall mixed/intermediate valence for Yb (Yb
).
because of the deficiency at the Ge3 and Ge4 sites. The Zintl
description requires that the ytterbium atoms either mixed or
intermediate valent with an overall oxidation state of 2.35.
Acknowledgments
3 8
Interestingly, except for Yb Ge , all other Yb–Ge binaries were
interpreted as classic Zintl phases [5]. Pani and Palenzona [5]
indicated a probable mixed valence of Yb in those compounds
based on the Zintl concept. So far, the experimental evidence for
Research supported by the US Department of Energy, Office of
Basic Energy Sciences, Division of Materials Sciences and
Engineering under Award (Grant DE-FG02-07ER46356).
mixed valency is available only for Yb
magnetic susceptibility and electrical resistivity measurements
33].
Valence bond sum (VBS) calculations for YbGe2.83 were carried
out to assess the oxidation state of the Yb ions. We used the
isostructural compound YbAl Ge [27] as standard for the
estimation of R because the divalent nature of Yb has been
established from magnetic susceptibility measurements. The R
3 5
Ge coming from the
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[
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[
[
3] V.N. Eremenko, K.A. Meleshevich, Y.I. Buyanov, Dopov. Akad. Nauk Ukr. RSR
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