Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels
The structure and morphology of low-temperature quench condensed binary alloys of hydrogen with argon and krypton were studied by the powder x-ray diffraction. The nominal hydrogen fraction c in both systems was varied from 0 to 50%; the condensation was performed at 5-6 K; both as-prepared and anne...
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irk-123456789-1288672018-01-15T03:03:46Z Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels Strzhemechny, M.A. Galtsov, N.N. Prokhvatilov, A.I. 3-й Международный семинар по физике низких температур в условиях микрогравитации The structure and morphology of low-temperature quench condensed binary alloys of hydrogen with argon and krypton were studied by the powder x-ray diffraction. The nominal hydrogen fraction c in both systems was varied from 0 to 50%; the condensation was performed at 5-6 K; both as-prepared and annealed samples were examined by the x-ray diffraction. Few, often only one reflection can be unambiguously detected for the as-grown alloy samples. In the Kr-H₂ condensates with c < 10%, the x-ray patterns show fine-grain krypton-rich crystallites with rather high actual hydrogen contents as estimated from Vegard`s law. At high nominal hydrogen fractions( %) c ≥ 10 , no the reflections attributable to the krypton lattice were recorded and the incoherent background showed no characteristic swelling around the position of reflection (111) from pure Kr but, instead, the reflections from a hydrogen-rich hcp phase were distinct. As the temperature was steadily raised, first the hydrogen reflections disappeared and then, at a certain temperature, the samples underwent an abrupt transformation, releasing heat and making the krypton component forms larger, x-ray detectable textured crystallites. In the as-grown Ar-H₂ samples, only (111) reflections from the argon-rich phase were recorded. Warmup led to the same consequences, viz., effusion of hydrogen and then recrystallization. In both systems, the recrystallization onset temperature depends substantially on the nominal hydrogen fraction in the gas. The shift of the lattice parameter in the as-grown argon-based phases suggests a strong suppression of the quantum nature of hydrogen in argon lattice environment. The entire set of the experimental findings can be treated as evidence that the quench-condensed hydrogen-containing alloys morphologically resemble helium-impurity solids (gels) whose structure and morphology are currently studied at Cornell University. 2003 Article Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels / M.A. Strzhemechny, N.N. Galtsov, A.I. Prokhvatilov // Физика низких температур. — 2003. — Т. 29, № 6. — С. 699-705. — Бібліогр.: 18 назв. — англ. 0132-6414 PACS: 61.10.-i, 61.43.Dq, 61.66 -f http://dspace.nbuv.gov.ua/handle/123456789/128867 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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3-й Международный семинар по физике низких температур в условиях микрогравитации 3-й Международный семинар по физике низких температур в условиях микрогравитации |
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3-й Международный семинар по физике низких температур в условиях микрогравитации 3-й Международный семинар по физике низких температур в условиях микрогравитации Strzhemechny, M.A. Galtsov, N.N. Prokhvatilov, A.I. Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels Физика низких температур |
| description |
The structure and morphology of low-temperature quench condensed binary alloys of hydrogen with argon and krypton were studied by the powder x-ray diffraction. The nominal hydrogen fraction c in both systems was varied from 0 to 50%; the condensation was performed at 5-6 K; both as-prepared and annealed samples were examined by the x-ray diffraction. Few, often only one reflection can be unambiguously detected for the as-grown alloy samples. In the Kr-H₂ condensates with c < 10%, the x-ray patterns show fine-grain krypton-rich crystallites with rather high actual hydrogen contents as estimated from Vegard`s law. At high nominal hydrogen fractions( %) c ≥ 10 , no the reflections attributable to the krypton lattice were recorded and the incoherent background showed no characteristic swelling around the position of reflection (111) from pure Kr but, instead, the reflections from a hydrogen-rich hcp phase were distinct. As the temperature was steadily raised, first the hydrogen reflections disappeared and then, at a certain temperature, the samples underwent an abrupt transformation, releasing heat and making the krypton component forms larger, x-ray detectable textured crystallites. In the as-grown Ar-H₂ samples, only (111) reflections from the argon-rich phase were recorded. Warmup led to the same consequences, viz., effusion of hydrogen and then recrystallization. In both systems, the recrystallization onset temperature depends substantially on the nominal hydrogen fraction in the gas. The shift of the lattice parameter in the as-grown argon-based phases suggests a strong suppression of the quantum nature of hydrogen in argon lattice environment. The entire set of the experimental findings can be treated as evidence that the quench-condensed hydrogen-containing alloys morphologically resemble helium-impurity solids (gels) whose structure and morphology are currently studied at Cornell University. |
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Strzhemechny, M.A. Galtsov, N.N. Prokhvatilov, A.I. |
| author_facet |
Strzhemechny, M.A. Galtsov, N.N. Prokhvatilov, A.I. |
| author_sort |
Strzhemechny, M.A. |
| title |
Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels |
| title_short |
Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels |
| title_full |
Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels |
| title_fullStr |
Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels |
| title_full_unstemmed |
Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels |
| title_sort |
quench deposited kr-h₂ and ar-h₂ mixtures: in quest of impurity-hydrogen gels |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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| topic_facet |
3-й Международный семинар по физике низких температур в условиях микрогравитации |
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http://dspace.nbuv.gov.ua/handle/123456789/128867 |
| citation_txt |
Quench deposited Kr-H₂ and Ar-H₂ mixtures: in quest of impurity-hydrogen gels / M.A. Strzhemechny, N.N. Galtsov, A.I. Prokhvatilov // Физика низких температур. — 2003. — Т. 29, № 6. — С. 699-705. — Бібліогр.: 18 назв. — англ. |
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| fulltext |
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6, p. 699–705
Quench deposited Kr–H2 and Ar–H2 mixtures:
in quest of impurity-hydrogen gels
M.A. Strzhemechny, N.N. Galtsov, and A.I. Prokhvatilov
B. Verkin Institute for Low Temperature Physics and Engineering
of the National Academy of Sciences of Ukraine, 47 Lenin Ave., Kharkov 61103, Ukraine
E-mail: galtsov@ilt.kharkov.ua
Received December 19, 2002
The structure and morphology of low-temperature quench condensed binary alloys of hydrogen
with argon and krypton were studied by the powder x-ray diffraction. The nominal hydrogen frac-
tion c in both systems was varied from 0 to 50%; the condensation was performed at 5–6 K; both
as-prepared and annealed samples were examined by the x-ray diffraction. Few, often only one re-
flection can be unambiguously detected for the as-grown alloy samples. In the Kr–H2 condensates
with c < 10%, the x-ray patterns show fine-grain krypton-rich crystallites with rather high actual
hydrogen contents as estimated from Vegard’s law. At high nominal hydrogen fractions ( %)c � 10 ,
no the reflections attributable to the krypton lattice were recorded and the incoherent background
showed no characteristic swelling around the position of reflection (111) from pure Kr but, in-
stead, the reflections from a hydrogen-rich hcp phase were distinct. As the temperature was
steadily raised, first the hydrogen reflections disappeared and then, at a certain temperature, the
samples underwent an abrupt transformation, releasing heat and making the krypton component
forms larger, x-ray detectable textured crystallites. In the as-grown Ar–H2 samples, only (111) re-
flections from the argon-rich phase were recorded. Warmup led to the same consequences, viz., ef-
fusion of hydrogen and then recrystallization. In both systems, the recrystallization onset tempera-
ture depends substantially on the nominal hydrogen fraction in the gas. The shift of the lattice
parameter in the as-grown argon-based phases suggests a strong suppression of the quantum nature
of hydrogen in argon lattice environment. The entire set of the experimental findings can be
treated as evidence that the quench-condensed hydrogen-containing alloys morphologically resem-
ble helium–impurity solids (gels) whose structure and morphology are currently studied at Cor-
nell University.
PACS: 61.10.–i, 61.43.Dq, 61.66.–f
1. Introduction
Binary systems like mixtures of hydrogen and rare
gas (RG) species are interesting for solid state physics
because the relative simplicity of the basic interac-
tions involved enable an unambiguous interpretation
of the phenomena observed. Because of the large mis-
match in the molecular parameters � and � of hydrogen
and the heavier rare gas atoms, one could hardly ex-
pect at low pressures an appreciable equilibrium solu-
bility in the solid of these mixtures at both ends of the
fraction range. Only application of a considerably
pressure makes some of these mixtures form stoichio-
metric compounds like Ar(H2)2 [1] or a series of such
compounds in hydrogen–methane mixtures [2]. There
are other high-pressure hydrogen-based compounds
(cf. Schouten’s review [3]). The low mutual miscibi-
lity of hydrogen with the heavier rare gas species
might be helpful in producing hydrogen-based self-
sustained condensed media. This idea with 4He as the
main binding material was first suggested [4] and sub-
sequently implemented [5] in Chernogolovka, Russia.
The preparation technique consists in blowing a jet of
the species chosen into superfluid 4He. The pivotal
factor is the extremely high thermal conductivity of
superfluid helium, which enables momentary dissipa-
tion of the energy coming with the particles in the jet.
Later these helium–impurity solids with different im-
purities were extensively studied by Gordon’s group
(for references see Ref. 6). Later studies at Cornell
[7–9] led to the conclusion that these materials are
highly porous amorphous media made up of particles
or small clusters surrounded by layers of helium,
© M.A. Strzhemechny, N.N. Galtsov, and A.I. Prokhvatilov, 2003
which could be considered solidified due to the external
forces of the impurity–helium interactions. Since hy-
drogen cannot guarantee a heat removal like in super-
fluid 4He, in order to get a hydrogen–impurity solid it
seems worthwhile to try quench condensation of the
corresponding mixture at high deposition rates. So, one
of the basic motivations was to look for possible highly
amorphized states in both systems. Proceeding from the
known mismatch of the molecular parameters of the
heavier rare gas atoms and hydrogen we expected to ob-
tain condensates that would have properties resembling
those of impurity–helium solids (gels).
Apart from this principal aim, our motivation was
also to elucidate the structural consequences of admix-
ing heavy RG species to hydrogen and quench condens-
ing the mix. It is important for a few reasons. It was
mostly interesting to try to determine the equilibrium
miscibility edge at the RG side, which is important for
the purposes of low temperature thermodynamic inves-
tigations on dilutions of various molecular species in
solid hydrogen (cf. an overview in the book by Man-
zhelii et al. [10]).
In this paper we report results of powder x-ray stud-
ies on the Ar–H2 and Kr–H2 binary systems in the con-
centration range of mixtures rich in the rare gas ele-
ments. Special attention was paid to the structure and
phase state of the samples and the effect of deposition
rates of the gas mixes. These results are analyzed in the
light of possible hydrogen–impurity solids (gels).
2. Experimental
These structure studies were performed on a x-ray
diffractometer DRON-3M equipped with a liquid-he-
lium cryostat. Radiation Fe K � was used. Diffracto-
meter control, data collection and processing were
accomplished from PC. Samples were produced by con-
densing gas mixtures of known compositions directly
into the solid phase onto a flat copper substrate at a
temperature of 5 or 6 K. This procedure allowed us to
suppress the natural tendency of hydrogen to grow in
large-size single crystals and, instead, to obtain 0.1 mm
thick polycrystal samples with coherent scattering area
sizes of order 10–4–10–5 cm. The purity of the gases for
H2, Ar, and Kr was 99.99%. The fraction of normal hy-
drogen in the gaseous Ar–H2 and Kr–H2 mixes was var-
ied from 1 to 50%. The relative error in the composition
of the binary gas mixtures did not exceed 5% with re-
spect to the amount of hydrogen introduced.
The sample preparation procedures employed in this
investigation was based on our experience of working
with solid hydrogen [11] as well as hydrogen doped with
neon [12] and some other simple molecules [13]. Growth
of condensates was performed using two deposition
regimes, which differed in the amount of material in a
single gas deposition burst. In one of the regimes,
every deposition step resulted in a 3 to 4 Torr drop of
the pressure inside the gas mixing chamber; during
the other regime the pressure drop was typically twice
as large. In order to avoid selective freeze-out of the
less volatile rare gas component from the gas mix on
its way from chamber to substrate, the temperature of
the filling duct was maintained at a sufficiently high
level, close to room temperature. To this end, the
thin-walled stainless steel filling capillary was con-
nected to the cooling chamber through a Teflon
capillary.
The samples were x-ray diffraction examined at
once after deposition and subsequently on warmup
in a step-like manner (every 5 or 10 K) up to 50 K.
At every working point the temperature was stabi-
lized to within �0.05 K. Because of the small number
of recordable reflections and overlap of separate re-
flections from different phases, the error in the lat-
tice parameter determination was noticeably larger
than typical of x-ray studies of pure cryocrystals but
did not exceed �0.05%.
3. Results and discussions
One of the main goals of this work was to deter-
mine the structure state of as-grown samples and,
when possible, to evaluate the limiting solubility of
hydrogen in the two rare gas solids under the con-
densation conditions specified above. The structure
of freshly grown samples differed substantially for
different nominal hydrogen contents c in the gas
mixture.
Krypton
In experiments with elevated (c = 10 to 40%) hy-
drogen contents we obtained patterns with rather
unexpected properties. First, all as-grown samples
deposited on the 5 K substrate did not produce re-
flections characteristic of the krypton lattice. Sec-
ond, incoherent background did not contain smeared
maxima (diffuse halo), typical of the systems with
well formed short-range order. Third, in spite of the
lower fraction of hydrogen in the mix, we saw dis-
tinct reflections from a crystal phase that could be
attributed to the hydrogen crystal, possibly with a
certain nonzero krypton content (Fig. 1). The essen-
tial qualitative changes in diffraction patterns be-
tween 4 and 10% gave us ground to infer that the
samples with nominal hydrogen content of 10% and
higher have an unusual morphology. By all appear-
ances, the krypton-rich phase in those as-grown sam-
ples was in an amorphous or/and highly finely dis-
perse state, while part of the hydrogen fraction with
a relatively low krypton content was crystallized
700 Fizika Nizkikh Temperatur, 2003, v. 29, No. 6
M.A. Strzhemechny, N.N. Galtsov, and A.I. Prokhvatilov
into a fine-grained textured phase, which was record-
able by x-rays. The hydrogen-rich phase observed
within the range of nominal concentrations 10–40%
was nonstrained and structurally perfect; reflection
(002) had a small width of 0.1–0.15 degrees. Varia-
tions of the deposition rate did not affect the sample’s
parameters. By applying the straightforward Vegard
law, i.e., making use of the cube root of the molar vol-
umes of pure krypton and hydrogen, we estimated the
actual krypton concentration xKr in hcp H2 as being
around 4%. It should be added here that the angular
position of mixed crystals depends not only on the im-
purity concentration but also on the density of stack-
ing faults �, amount of which might be rather high,
especially in freshly grown samples. It is quite safe to
evaluate the impurity concentration using the (002)
reflection of a hcp structure, because the position of
precisely this reflection is not affected by stacking
faults [14]. As to the fcc reflection (111), which we
have at our disposal for the Kr-rich phases, it shifts to
higher angles owing to stacking faults [14]. The re-
flection (111) from pure quench condensed krypton in
Fig. 1 is appreciably shifted to larger angles. Taking
this shift as being due to stacking faults we find [14]
that � corresponds to an average number of planes be-
tween faults equal to 11 � 3, which is a very high den-
sity but fairly consistent with the reflection width and
intensity. The strong effect of stacking faults on the
position of reflection (111) makes the estimation of
the actual hydrogen concentration uncertain with a
tendency for underestimation because both factors
(impurities and faults) shift the (111) line to higher
angles.
There is also another consideration that should be
taken into account, when using Vegard’s law in esti-
mations of the actual H2 content in Kr. As it will be
proven below, when H2 is dissolved in the argon lat-
tice, the quantum nature of hydrogen molecules is
considerably suppressed. Although we do not have di-
rect unambiguous evidence for Kr-based mixtures, it
seems quite reasonable to assume that this consider-
ation is applicable for the Kr matrix as well. If the
zero-point motion of the hydrogen molecules is igno-
red, then the lattice parameter appearing in Vegard’s
law should be calculated not from the actual molar of
H2 but [15] rather from the value �3, where � =
= 2.96 Å is the van der Waals radius of the hydrogen
molecule.
When the samples with high nominal hydrogen
content were warmed up to 14 K or higher, the reflec-
tion from the hydrogen phase disappeared (compare
curves a and b in Fig. 2). When the temperature was
increased to 30 K, a very broad and weak reflection,
attributable as (111) of a krypton-rich phase, ap-
peared (curve c). As the temperature was further
raised, the reflection grew in intensity and narrowed
(curve d). It can be surmised that within this temper-
ature range, effusion of hydrogen from krypton star-
ted. At a certain temperature Ts an abrupt annealing
and (re)crystallization occurred, which manifested it-
self in much higher intensities and smaller widths of
the reflection.
Traces b, c, and d in Fig. 2 can be utilized to eval-
uate the actual concentration xH in the Kr-rich phase
at higher temperatures. Here we must take into ac-
count the numerous stacking faults inevitably pres-
ent in the sample. If we assume their density to be the
same � ; 1/11 as found in the quench condensed
sample of Fig. 1, we evaluate xH to be (3.8 � 0.4)%
Quench deposited Kr–H2 and Ar–H2 mixtures: in quest of impurity-hydrogen gels
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6 701
30 32 34 36 38
pure Kr
(111) Kr
a
2�, deg
Kr-20% H2 (002) H2bIn
te
n
si
ty
Fig. 1. Fragments of powder x-ray patterns from pure
krypton (a) and a Kr–20%H2 alloy (b), both obtained by
quench deposition from the gas on a flat copper substrate
at T = 5 K. The arrows indicate the reflections from the
respective crystal planes. Note absence of Kr reflections
from the hydrogen-doped as-grown sample.
32 33 34 35 36 37 38 39 40
0
200
400 2Kr–H
a
b
c
d
2�, deg
(0
0
2
)�
H
2
(1
1
1
)
K
r
(0
0
2
)�
H
2
In
te
n
si
ty
Fig. 2. Effect of warmup on the structural state of a
Kr–30%H2 alloy, quench deposited at T = 5 K. All pat-
terns were recorded without filter to see more clearly the
evolution of the sample. Pattern (a) is from the sample
freshly quench deposited on 5 K; only the (002) reflection
from the basal plane of the hexagonal hydrogen-rich lat-
tice can be seen; a K� reflection from the hydrogen is seen
at smaller angles. Patterns (b) through (d) correspond to
annealing temperatures, respectively, 35, 43, and 48 K.
at 35 K and (3.5 � 0.5)% at 43 K. (We made use of the
lattice parameters measured for pure Kr at different
temperatures by Losee and Simmons [16].) After the
quick recrystallization is over (trace d), one can expect
the sample to be virtually free of hydrogen. Indeed, the
position of line (111) coincides within the aggregate er-
ror with the value for pure Kr at the same temperature,
which implies that the stacking-fault and impurity con-
centrations are close to zero.
At lower nominal hydrogen fractions (c < 10%) we
saw only reflections that could be attributed to line
(111) from a cubic krypton-rich lattice with a some-
what smaller lattice parameter than in pure krypton.
Within a rather large uncertainty the respective lattice
parameter shifts for c = 1, 2, and 5% may be considered
constant and corresponding to the absolute lattice insta-
bility edge. If uncorrected Vegard’s law is applied, we
obtain an appreciable hydrogen concentration x (H2 in
Kr) = (1.4 � 0.3) %, the presence of hydrogen manifest-
ing itself in a slight lattice constriction. However, it
should be born in mind that this estimate does not ac-
count for stacking faults. The reflections observed were
the more intensive the lower the nominal hydrogen frac-
tion. Irrespective of the condensation rate the reflec-
tions were broadened, which suggests small grain sizes
and a high concentration of lattice defects.
Effect of the gas deposition rate mainly manifested
itself in variations of the temperatures Ts where recry-
stallization and annealing of amorphous specimens oc-
curred. A general trend was that the crystallization tem-
perature somewhat increased at higher condensation
rates. Thus, for the low-rate condensation regime, crys-
tallization of the apparently amorphous Kr–20%H2 con-
densate was observed at a temperature of 35 K. Under
the twice as fast deposition rate, crystallization began
at around 40 K.
A more pronounced dependence of the crystallization
starting temperature for krypton-based mixtures was
observed with varying composition of the source mixes
(Fig. 3). Samples with 10 to 25% H2 began to recrystallize
at 35–40 K. The crystal phase in recrystallized samples was
strained and imperfect, which was inferred from broad and
low-intensity reflections. Keeping the samples for 5
hours even at the crystallization onset temperature
resulted in a complete recrystallization, the intensity
and width of the Kr reflections approaching the val-
ues observed previously for pure krypton. A slight
temperature increase resulted in quickening the pro-
cess considerably. Samples with the nominal hyd-
rogen content ranging from 30 to 40% underwent
recrystallization at higher temperatures in the range
45–55 K (Fig. 3). One can see that the curve for the
Kr–H2 system has a kink at lower c values, which
might reflect the basic differences in structure and
morphology between samples grown from hydro-
gen-dilute and hydrogen-concentrated gas mixes.
The distinct shift of the crystallization onset tempe-
rature to higher values at higher nominal hydrogen
fractions indicates that hydrogen tends to stabilize
the unusual amorphous krypton phase in as-grown
samples.
The latest results of the Cornell group [9,18]
show that morphologically the so-called impu-
rity–helium solids are highly amorphous media in
which the impurities form a multi-connected net-
work of separate atom, n-mers, chains, nano-parti-
cles, etc., isolated from one another by layers of he-
lium, presumably in the solid state. Basically, the
impurity system can be treated as a non-self-sus-
tained gel, which is stabilized at low enough tempe-
ratures by the helium spacer layers. As the sample is
warmed up, the impurity fraction starts to coalesce
and the latent heat makes helium atoms effuse,
causing a burst-like reorganization of the matter. In
many aspects, the hydrogen-containing krypton-based
mixtures studied in this work resemble the impu-
rity–helium media. There are a few pieces of evidence
that confirm this assumption, namely, the highly
amorphous state of the krypton-rich phase; the
strong dependence of the crystallization onset tem-
perature Ts on the nominal and, presumably, actual
hydrogen concentrations; the very existence of Ts at
which a sample undergoes a cardinal and fast recon-
struction. What can be said for sure is that hydrogen
is a factor that stabilizes those amorphous states.
In Fig. 4 we plot the lattice parameters a of the
samples after annealing as a function of the nominal
hydrogen fraction c in the gas mix. All a values were
determined at 5 K. The «final» a is virtually indepen-
dent of c. Assuming no stacking faults and using cor-
rected Vegard’s law (dotted line), we evaluate the ac-
tual hydrogen concentration xH to be close to 1% (use
of uncorrected Vegard’s law yields 4%). This figure
should be treated as the ultimate estimate for the equi-
librium solubility of H2 in solid crystalline krypton.
702 Fizika Nizkikh Temperatur, 2003, v. 29, No. 6
M.A. Strzhemechny, N.N. Galtsov, and A.I. Prokhvatilov
0 10 20 30 40
20
30
40
50
Kr-H2
Ar-H2
T S
,
K
c, mol.% H 2
Fig. 3. The annealing onset temperatures of Kr–H2 (�) and
Ar–H2 (�) alloys as a function of the nominal hydrogen
fraction in the source gas mix.
Argon
The structural characteristics of as-grown Ar–H2
condensates differ considerably from those observed
on Kr–H2 condensates. There was an important pecu-
liarity, namely, the brightest reflections (111) from
the Ar cubic phase and (002) from the hexagonal H2
phase overlap, which makes reflection identification
difficult. Use of other reflections was impossible be-
cause they were, as in the Kr–H2 system, absent,
which might be indicative of a texture even in very
fine-grained samples. Therefore, some conclusions for
the Ar–H2 system are less reliable compared to the
Kr–H2 system.
The following findings should be noted for the
Ar–H2 mixtures. First, irrespective of the growth re-
gime, within the entire range of nominal hydrogen
concentrations up to c = 50% the mixed samples gave
reflections that were broader and less bright as com-
pared to that from pure solid argon prepared under
similar deposition conditions (Fig. 5). Pure argon was
easily annealed at a comparatively low temperature
around 16 K. The annealing onset temperature increa-
sed almost linearly with increasing c, remaining how-
ever always lower than for the Kr–H2 system (Fig. 3).
Second, although the assignment of the only record-
able reflection was dubious (Gaussian deconvolution
always gave only one line), judging from its position
and the general behavior with temperature, we attrib-
uted this reflection as the (111) line from the ar-
gon-rich fraction. Over the entire nominal H2 concen-
tration range we did not identify reflections from the
hydrogen-rich phase, either upon condensation or af-
ter annealing during 1 hour at 10–12 K.
The above findings allow us to conclude the fol-
lowing. Since it is unlikely that hydrogen formed a
superfine-grain phase, we surmise that H2 entered
completely into the argon lattice as well as the hydro-
gen-argon gel, existence of which cannot be ruled out.
The following facts favor this assumption: 1) ref-
lections from as-grown condensates are weaker and
broader compared to those from pure as-grown sam-
ples; 2) reflections from the argon-rich phase are sub-
stantially shifted to larger angles compared to pure ar-
gon; 3) a considerable increase of the recrystallization
onset temperature with increasing nominal hydrogen
fraction was observed. At the same time, annealing of
the samples resulted not only in larger intensities and
smaller widths of the reflection but also in substantial
shift to smaller angles, i.e., in larger lattice parame-
ters of the argon lattice closer to the values typical of
pure crystalline argon. Effect of annealing on the in-
tensities and line widths of the argon matrix is clearly
demonstrated in Fig. 6. Annealing at T > 20 K re-
sulted in brighter and narrower reflections from plane
(111) of the argon matrix. A shift of this line to
smaller angles is distinctly seen, which again means an
increase in lattice parameter. Figure 7 shows effect of
hydrogen impurities on the cubic lattice parameter a
of the Ar matrix in as-grown and annealed samples.
The unusual finding is that a in as-grown argon-based
phase, where a finite concentration of hydrogen is ex-
pected, is smaller than in annealed samples, which are
presumably devoid of hydrogen impurities, opposite to
what can be expected from a straightforward Vegard
Quench deposited Kr–H2 and Ar–H2 mixtures: in quest of impurity-hydrogen gels
Fizika Nizkikh Temperatur, 2003, v. 29, No. 6 703
0 10 20 30
5,55
5,58
5,61
5,64
c, mol.% H 2
a
, Å
Kr-H2
Fig. 4. The cubic lattice parameter of the krypton-rich
phases in Kr–H2 mixtures versus the nominal hydrogen
fraction in the source gas (empty and solid circles are re-
spectively for as-grown and completely annealed samples).
The long-dash curve shows uncorrected Vegard’s law for
this system. The dotted curve is corrected Vegard’s law
(for explanation see the argon subsection in Sec.3).
34 35 36 37 38 39 40
0
100
200
300
c
b
a
2�, deg
Ar-H2
In
te
n
si
ty
Fig. 5. Effect of hydrogen impurities on the angular posi-
tion and intensity of the (111) reflection from quench
grown Ar–H2 samples: pure argon (a); condensates with
nominal hydrogen fractions of 10% (b) and 30% (c).
34 36 38 40 42
0
500
1000
b
a
2�, deg
In
te
n
si
ty
1
1
1
Ar-H2
Fig. 6. Effect of annealing on the intensity and half-width
of reflection (111) in the Ar–H2 system. Trace (a) is from
an as-grown Ar–20%H2 condensate; trace (b) is from the
same sample upon annealing.
line, which must have a positive, however small, slope.
This fact suggests the following reasoning. The inter-
molecular spacing in pure hydrogen is governed by an
effective potential, renormalized considerably to ac-
count for large zero-point vibrations. If a hydrogen mo-
lecule is placed in the argon lattice, one can expect that
due to much stronger interactions with the argon envi-
ronment the quantum nature of the H2 molecule will be
substantially suppressed. If this suppression is ultimate,
then the intermolecular distance used in Vegard law es-
timates should be close to the hydrogen van der Waals
diameter �, as is the case with any «classical» heavy
particle. That is, the effective hydrogen molar volume
to be used in Vegard law calculations at the argon side
of the (a, x) phase diagram should be [15] �3. The
Vegard line corrected in that way is shown in Fig. 7 as
the (relatively) steep line with negative slope. Using
now the corrected Vegard law, we estimate that the hy-
drogen concentration in as-grown argon-based phase to
be 3.6%. Account of stacking faults inevitably present
in as-grown samples must reduce this estimate at least
twice. The corrected actual hydrogen concentration (1
or 2 %) in as-grown samples and the still smaller value
in annealed samples (see Fig. 7) compares well with the
upper estimate for the solubility of hydrogen in solid ar-
gon of 0.5 mol% found by Loubeyre et al. [1].
4. Conclusions
1. When room-temperature Kr–H2 and Ar–H2 gas
mixtures are quench condensed on a substrate at 5 K,
the condensates tend to grow as highly amorphized
structures. This is in particularly true for Kr-based
samples with nominal hydrogen fractions above
10%, when as-grown samples did not give at all
reflections that could be attributed to a krypton-rich
phase.
2. When quench condensed samples were warmed
up, the recrystallization onset temperature depended
appreciably on the nominal hydrogen fraction, which
is indicative of certain differences in morphology and
structure of as-grown samples, including the actual
hydrogen content in the phases rich in both rare
gases. (Re)crystallization occurred rather quickly, re-
leasing an appreciable amounts of latent heat.
3. Proceeding from the above experimental find-
ings, we think that the states formed in as-grown
samples were in many aspects analogs of the
helium–impurity media (gels). These mixtures,
which comprise two substances differing strongly in
molecular parameters and are self-sustained at low
temperatures, have a highly amorphous structure
and undergo an explosion-like separation at a certain
temperature, the value of which depends on the
impurity employed.
4. As-grown Ar–H2 condensates exhibited a lesser
tendency to form hydrogen–impurity media as com-
pared to Kr-based samples. This finding can also be
an indirect confirmation of the assumption made,
since the difference in Kr–H2 system is larger than in
the Ar–H2 one, by virtue of which the latter system
is expected to be less reluctant to form the standard
substitutional solid mixtures.
Acknowledgments
This work was supported by the CRDF (grant
UP2-2445-KH-02). The authors thank V. V. Khme-
lenko for communicating results prior to publica-
tion and A. A. Solodovnik for valuable discussions.
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5,30
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5,32
Ar-H2
c, mol.% H
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