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Review article

Superconductivity in U-T alloys (T

¼ Mo, Pt, Pd, Nb, Zr) stabilized in

the cubic

g-U structure by splat-cooling technique

N.-T.H. Kim-Ngan

a,*

, L. Havela

b

a_{Institute of Physics, Pedagogical University, Podchorazych 2, 30 084 Krakow, Poland}

b_{Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 12116 Prague, Czech Republic}

a r t i c l e i n f o

Article history:

Received 15 April 2016
Received in revised form
25 April 2016

Accepted 25 April 2016
Available online 15 May 2016
Keywords:

Superconductivity
Electrical resistivity
Crystal structure

g-U phase
U-based alloys

a b s t r a c t

We succeed to retain the high-temperature (cubic)g-U phase down to low temperatures in U-T alloys
with less required T alloying concentration (T¼ Mo, Pt, Pd, Nb, Zr) by means of splat-cooling technique
with a cooling rate better than 106_{K/s. All splat-cooled U-T alloys become superconducting with the}

critical temperature Tcin the range of 0.61 Ke2.11 K. U-15 at.% Mo splat consisting of theg-U phase with

an ideal bcc A2 structure is a BCS superconductor having the highest critical temperature (2.11 K).
© 2016 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.

This is an open access article under the CC BY license ( />

1. Introduction

The large interest in stabilization of U-based alloys with a cubic

g

-U structure has camefirst from the viewpoint of metallurgy. In
the late 1970s massive research programs were launched in USA to
develop the low enriched uranium (LEU,< 20%235_{U) fuels}_[1,2]_.

The research showed that the U-Mo alloys with

g

-U phase were
the most promising candidates for LEU fuels, e.g. they have a
higher stability under irradiation and are more resistant to
swelling (than

a

-U alloys)[3e5]. Indeed, U-10Mo (U-10 wt%Mo
(uranium alloying with 10% weight percent of molybdenum)) has
been selected for the U.S. reactors, while many European reactors
have used the U-7Mo[2]. This concentration (7e10 wt% Mo) in
uranium is sufficient to reach the

g

-U phase stability. In Vietnam,
the high enriched uranium (HEU,_{> 90%}235_{U) rods of the nuclear}

reactor in the Central Highlands of Da Lat City have been

exchanged by LEU ones since 2011.

From the fundamental research viewpoint, the 5f electronic
states in many uranium-based compounds are generally close to
the verge of localization, which brings up fascinating many-body
physics. However, the fundamental physical properties of

elemental uranium have been investigated thoroughly for the
orthorhombic

a

-U phase (space group Cmcm)[6,7], since only this
phase is stable at and below room temperature. The
supercon-ductivity of natural uranium wasfirst discovered at Tc¼ 1.3 K in

1942[8]. Most recent reports gave Tc¼ 0.78 K[9,10]. However, no

signature of the superconductivity was found down to 0.02 K at
ambient pressure in good-quality single crystals of uranium,
although the charge-density-wave (CDW) states[10]were found
to be developed fully at low temperatures in those crystalline
uranium specimens[11].

We remind here that pure uranium metal exhibits three
allo-tropic phases. The

a

-U with an orthorhombic structure
(mentioned above) exists below 940 K down to ambient
temper-ature. Between 940 K and 1045 K the

b

-U phase with a tetragonal
structure exists (space group P42/mmm), while the

g

-U phase with

a body-centered-cubic A2-type structure is stable only between
1049 K and 1408 K (space group Im3m)[6,7]. The cubic

g

-U phase
can be retained to the room temperature by alloying with Zr, Nb,
Mo, Pd, Pt, etc.[12]. Mo has a large solubility in U (z35 at.%) and

thus is considered as a good candidate to stabilize

g

-U. For instance
the single-phase

g

-U alloy has been reported for U-8 wt% Mo (y
U-16.5 at.% Mo (equivalently uranium alloying with 16.5% atomic
percent of molybdenum)) under normal furnace cooling
condi-tions[13].

* Corresponding author. Tel.: ỵ48 12 6627801; fax: ỵ48 12 6358858.
E-mail address:(N.-T.H. Kim-Ngan).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

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The basic thermodynamic properties of

g

-U phase alloys have
been much less investigated and remained practically unknown.
Except for old reports from 1960s on the superconductivity of the

g

-U phase around 2 K in water-quenched U-Mo and U-Nb alloys

[14,15], there are no more detailed data on fundamental
low-temperature properties of the

g

-U alloys.

We have been interested in stabilization of

g

-U alloys and
characterization of their fundamental electronic properties,
espe-cially their superconductivity. It was shown earlier that the rapid

quenching (with a cooling rate of about 105K/s) of certain alloys
from the melting point could lead to a formation of new
meta-stable phases and/or amorphous solid phases [16]. Indeed, the
splat-cooling technique has been used for searching novel
micro-structure or amorphous uranium [17]. Recently, using ultrafast
cooling from the melt to room temperature, we were able to retain
the cubic

g

-U phase in U-T alloys (T¼ Mo, Pt, Pd, Nb, Zr). In our
equipment, the molten metal drops between two colliding
massive copper anvils, yielding a cooling rate better than 106K/s.
We can then proceed with characterization of low-temperature
properties. Starting with Mo alloying, we succeeded to suppress
the

a

-U phase with about 11 at.% Mo[18e20]. We have extended
our investigations on other U-T alloys (T¼ Pt, Pd, Nb, Zr), focusing
in particular on their superconductivity. This work is a review of
our results obtained up to date.

2. Experimental

U-T alloys (T¼ Mo, Pt, Nb, Zr) with low T concentrations (up to
30 at.%) were prepared using natural U (2N8 purity or better) and T
element (3N8 or better) by arc-melting on a copper plate in argon
atmosphere. The sample ingots were turned over 3 times to ensure
the homogeneity. Up to 4 samples could be obtained in one
arc-melting cycle without breaking a vacuum, thanks to a special
construction of copper crucible and the chamber. The splat-cooled
sample was prepared from the alloy-ingot by splat-cooling
tech-nique (using the HV splat cooler from Vakuum Praha) and had a
shape of irregular disc with a diameter of approx. 20 mm and a
thickness of 100e200

m

m, as shown in Fig. 1. More details of
preparation of the splats have been reported earlier [18e20].

Throughout our work, the T-content is given in the atomic percent
(at.%).

The crystal structure of the splat-cooled alloys (splats) was
investigated by X-ray diffraction (XRD) using the Bruker D8
Advance diffractometer with Cu-Karadiation. The resistivity and

specific heat measurements were carried out in the temperature
range 0.4e300 K by means of standard techniques using e.g.
Closed Cycle Refrigerator system (CCR) and Quantum Design
Physical Properties Measurement System (PPMS) described earlier

[18]. For investigations around the superconducting transitions,
we performed those measurements in applied magneticfields up
to 7 T. Additional phase purity analysis was performed by scanning
electron microscope (SEM) equipped with an energy dispersive
X-ray (EDX) analyzer. The splats show in most cases a homogeneous
distribution of the alloying elements with concentrations
corre-sponding the nominal ones. Electron backscattering diffraction
(EBSD) analysis has been employed to study the microstructure
and texture of several splats.

3. Results and discussion
3.1. Crystal structure of U-T splats

The crystal structure of U-Mo splats (with Mo concentration of
0 (pure U splat), 1, 2, 4, 6, 10, 11, 12, 13, 15 and 17 at.%) has been
thoroughly investigated in order to determine precisely the
min-imal Mo concentration necessary for obtaining the pure cubic

g

-U
phase. Details of our investigations of crystal structure and phase

stability in U-Mo system have been reported earlier[18,19]. For a
necessity of a comparison with other U-T splats, we summarize
briefly the main outcome obtained on U-Mo splats: 1) the
(orthorhombic)

a

-U phase has disappeared and the (cubic)

g

-U
phase or its tetragonally distorted variant (

g

0-U phase) has
developed fully in the alloys with Mo larger than 11 at.%. A pure
cubic

g

-U phase without any distortion is revealed only for
U-15 at.% Mo (Fig. 2a) and U-17 at.% Mo splat, and 2) the stable

g

-U
alloys were obtained in the as-formed state without any additional
sample treatment. Thus, the effect of the splat cooling can be seen
in a better capability in retaining the bcc-type of structure for
lower (by several at.%) Mo concentrations.

No aging or phase transformation/decomposition was observed
for all splat-cooled alloys when exposed to air. They show even a
very good resistance against any hydrogen absorption in the
hydrogen atmosphere with the pressure below 2.5 bar[21,22].

As small amount of orthorhombic

a

-U phase is dif_{ficult to}
recognize by XRD if it coexists with the cubic

g

-U phase, EBSD
analysis has been performed on several U-Mo splats. Earlier
pub-lished EBSD results for pureeU and U-15 at.% Mo splats[18,20]

corroborated the XRD data. For instance, the EBSD maps for
U-15 at.% Mo splat have revealed a pure cubic

g

-U phase with an
equigranular grain structure without twinning and preferred
crystallographic texture. For as low as 12 at.% Mo, the EBSD maps
exhibited a full crystallinity with grain size of several micrometers
and no evidence for

a

- or

a

-U related phases[23].

Recently, we have extended our studies to the splat-cooled
U-based alloys with other T metals (T¼ Pt, Pd, Zr, Nb). Some of the
results were included in our recent publications[23,24]. We
pre-sent here a comparison of selected results.

The XRD patterns of U-Pt splats in the as-formed state are
shown inFig. 2b. For an easier comparison, we display normalized
intensities. Increasing the Pt concentration leads to merging of
several reflections around 36_{, suppression of the low-index}

_a



-reflections, vanishing of the high-index

a

-reflections and a
development of

g

-reflections. The situation is very similar to U-Mo
alloys, showing a coexistence of both

a

- and

g

-U phase for splats
with less than 10 at.% alloying level. The XRD pattern of U-15 at.%
Pt revealed four characteristic reflections of the

g

-type structure
(

g

(110),

g

(200),

g

(211) and

g

(220) respectively at 36.8, 53.0,
65.3and 78.2), indicating a stabilization of the cubic

g

-U phase.
However, unlike U-15 at.% Mo with very narrow

g

-reflections

Fig. 1. Photograph of splat-cooled disc (right) produced by HV splat cooler from the
bulk sample ingot with a mass of approx. 300 mg (top, left) prepared by the
arc-furnace.

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indicating the fundamental cubic A2 structure, there is a certain
broadening for all the

g

-reflections in U-15 at.% Pt, similar to those
observed in the U-13 at.% Mo splat. It is interesting to compare our
findings with respective binary phase diagrams. The maximum
reported solubility in

g

-U of Pt or Pd does not exceed 5 at.%

[12,25e27]. Our results reveal that using the splat cooling we not

only retain the bcc phase to low temperatures, but also extend its
occurrence for much higher concentrations of alloying Pt/Pd
metals. However, SEM analysis indicated that a small amount of
the binary phase UPt occurring at the grain boundaries, which is
accompanied by the U-Pt alloy depleted in Pt, so the splat cannot
be taken as single phase.

The normalized XRD patterns of the splat-cooled U-Nb alloys in
the as-formed state are shown inFig. 3a. In general, the increase of
the Nb concentration leads to the suppression of

a

-U reflections
and the development of

g

-U reflections. It causes the overlap of
low-index reflections around 36 _{and then the combined re}

flec-tion becomes narrower for 10 at.% Nb. For the U-15 at.% Nb alloy,
the splitting of the

g

-reflections into doublets was observed for all
four prominent

g

-reflections. For instance, the

g

(110) reflection of
U-15 at.% Nb splits into doublet located around 36.3(

g

0(110)) and
37.0 (

g

0(101)). The situation is similar to that of alloying with
11e12 at.% Mo which stabilizes the

g

0-U phase. (The

g

0-U phase
has a body-centered tetragonal structure with the c/a
ratioz 0.98e0.99. It is considered as a cubic structure with a small
tetragonal distortion). In general, our results show a similarity
between the U-Nb and U-Mo systems. Moreover, we expect that
using ultrafast cooling could reduce the necessary Nb
concentra-tion. Indeed, it turned out that the

g

0_{-U phase is found to be}

sta-bilized by 15 at.% Nb alloying, i.e. lower than the minimal content
for stabilization of such a phase in water-quenched (16.8 at.% Nb)

[28]or in argon quenched ones (16.2 at.% Nb)[29]. Using a

com-bined arc-melting, hot-rolling, annealing and water-quenching,
the

g

-U phase was stabilized in U-7 wt% Nb (i.e. U-15 at.% Nb)
alloy[30].

In the case of Zr system, the situation is similar to that of
U-Nb, i.e. the complete miscibility in the high-temperature bcc phase.
The normalized XRD patterns of the splat-cooled U-Zr alloys in the
as-formed state are shown inFig. 3b. The results illustrate the phase
transformation from the

a

-phase to

g

with increasing Zr
concen-tration. Unlike other T alloying, the

a

(110) and

a

(111) reflections
still persist for U-11 at.% Zr and U-15 at.% Zr. They become very
broad for U-20 at.% Zr and then vanish for U-30 at.% Zr. Existing
reports indicate that the single-phase

g

-alloys were obtained for Zr
concentrations between 25 at.% and 80 at.%[31]. In our case the
single

g

-U phase can be considered only for U-30 at.% Zr splat.
Moreover, most of

g

-reflections (including the main peak

g

(111) at
35.9) are broadened. We attribute such the broadening to an
additional disorder (microstrain) by randomly distributed Zr atoms
especially in alloying with high Zr concentrations. In all splats,
UC(111) and UO2(111) impurity reflections were observed in the

low-angle part of the XRD patterns attributed to surface
segrega-tion. Additionally for U-Zr system, ZrC presence is revealed by most
intense reflections ZrC(111) and ZrC(200) at 33.4 _{and 38.7}_,

respectively. Traces of carbon are ubiquitous in uranium metal.
However, it seems that it couples preferentially only with Zr
(among all investigated T alloying) and has a high surface
segre-gation tendency.

The lattice parameters estimated for the

g

-U phase alloys are
given inTable 1. The atomic radii of Nb (1.47Å), Pd (1.37 Å) and Pt
(1.39_{Å) are equal or close to that of Mo (1.40 Å), all which are lower}
than the nominal atomic radius of U (1.56Å), while the Zr atomic
radius (1.60 Å) is larger[32]. The lattice parameters of the alloys can
be compared with that of

g

-U at 1050 K (3.52 Å) and the value
extrapolated to room temperature considering the thermal
expansion (3.48 Å). It is evident that the largest lattice parameters
for the Zr alloying are related to the Zr atomic diameter. A
remarkable fact is the large tetragonal distortion for the Nb
alloy-ing, which apparently exhibits c> a, i.e. opposite than for the

g

0-U
phase at U-Mo alloys.

Fig. 2. X-ray diffraction (XRD) patterns of the as-formed splat-cooled U-Mo alloys (a) and U-Pt alloys (b). Each curve was normalized to the maximal intensity of the most intense
peak at 2q¼ 36o

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3.2. The electrical resistivity of the cubic

g

-U phase

For a brief summary of the change of the temperature coefficient
in splat-cooled U-T alloys with increasing T content in the normal
state in the temperature range 3e300 K, we show inFig. 4a the
temperature dependence of the (normalized) electrical resistivity
of U-Mo splats. (We show the data of all investigated U-Mo splats in
one Figure here, while they were already reported separately
earlier [18,33,34]). We concentrate on the two limit cases which
reveal a striking difference, i.e. the pure-U splat (consisting of

a

-U
phase) and the U-15 at.% Mo splat (consisting of the

g

-U phase). The
pure-U splat exhibits a quadratic temperature dependence below
50 K and then an almost linear dependence up to 300 K, i.e. with a
positive temperature coefficient (d

r

/dT> 0). Unlike such a common

metallic behavior, for U-15 at.% Mo, the resistivity weakly decreases
with increasing temperature in the normal state in the whole
temperature range, i.e. with a negative temperature coefficient (d

r

/
dT< 0). The temperature dependence of the resistivity of other
U-Mo splats lies between such the two limits. The U-U-Mo alloys
con-sisted of both

a

- and

g

-U phase (with<10% Mo alloying) have still
positive d

r

/dT, all U-Mo alloys with (cubic)

g

-U phase (with11%
Mo alloying) have the negative d

r

/dT. As such a change of the

temperature dependence appears in conjunction with increase of
the absolute resistivity value, we can deduce that a large disorder
effect plays an important role in the splat-cooled alloys, similar to a
strong disorder observed e.g. in some (superconducting)
amor-phous systems or disordered alloys and compound[35e37]. The
reason for the negative slope can be seen in the weak localization,
i.e. a quantum interference effect (e.g. the anomalous dispersion of
the conduction electrons) occurring in strongly disordered systems

[38]. In our case, there is certainly still some extra contribution to
the disorder produced by ultrafast cooling, affecting the grain size.
It is interesting to review the resistivity behavior of all
splat-cooled U-T alloys (T¼ Mo, Pt, Pd, Nb, Zr) formed in the (cubic)

g

-U structure. The temperature dependence of the resistivity of these
alloys in zero-field and in the temperature range of 3e300 K is
shown inFig. 4b. The resistivity values at 300 K and 4 K are given in

Table 1. The

r

(T) curves of U-15 at.% Mo and U-15 at.% Nb splat are
quite similar. Besides, the residual resistivity

r

0(

r

4K) and the

re-sistivity at room temperature (

r

300K) are also similar. For the

U-15 at.% Pt splat, although the resistivity values are twice higher, the
relative change of the resistivity in U-15 at.% Pt (the

r

/

r

300K (T)

curve) is very similar to that of U-15 at.% Mo (as well as U-15 at.%
Nb). Namely, from room temperature down to temperature just

Fig. 3. (Normalized) X-ray diffraction (XRD) patterns of the as-formed splat-cooled and U-Nb alloys (a) and U-Zr alloys (b). The same notation of the color vertical ticks are used as
those inFig. 2.

Table 1

Summary of low-temperature properties of U-T splat alloys havinggeU structure: resistivity values at 300 K and at 4 K (r300K,r4K), superconducting transition temperatures

(Tc) determined from ther(T) jump and/or from the specific heat C(T), the width of the superconducting transition in the resistivity (DTr), the Sommerfeld coefficient of
electronic specific heat (ge) and Debye temperature (QD). The structure types (the orthorhombica-U, the cubicg-U and the tetragonalg0-U (or the cubic with a small tetragonal

distortion)) and lattice parameters (a,c) are given as well.
T Content

(at.%)

Type a,c

(Å)

r300K

(mUcm)

r4 K

(mUcm)

Tc(K)

(r(T))

DT_r
(K)

Tc(K)

(C(T))

ge

(mJ/K2_mol) Q_(K)D

Pure U a 53 14 1.24 0.20 0.65 11.0 179

15% Mo g 3.441 89 95 2.11 0.02 2.11 16.0 139

15% Pt g 3.469 164 166 0.95/0.61 0.08/0.04 19.5 145

15% Nb g0 _{3.435 (a)}

3.565 (c)

83 86 1.90 0.15 1.90 13.7 153

30% Zr g 3.543 75 73 0.81 0.08 0.60 11.8 165

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above the superconducting transition, the electrical resistivity
ex-hibits a negative temperature coefficient (d

r

/dT< 0). For U-30 at.%
Zr containing the

g

-U phase, the negative slope does not develop
yet. Instead, we found a very small (close to zero) but still positive
slope of the temperature dependence in this splat. It should be
mentioned that a negative temperature coef_{ficient (d}

r

/dT_{< 0) was}
indeed reported for U-Zr system, but for sample with 70 at.% Zr

[38]. We assume that the negative slope can be also observed for
higher Zr concentrations than 30 at.%.

3.3. Superconductivity in U-T splats with

g

-U phase

All investigated U-Mo splats become superconducting at low
temperatures below 2.2 K. The superconducting transitions
revealed by abrupt resistivity drops in zero magnetic field are
shown inFig. 5. We focus_{first on the two cases: the pure U splat}
and the U-15 at.% Mo splat (Fig. 5a). The transition is manifested by
a single drop at Tc¼ 1.24 K and 2.11 K, respectively[19,33]. We

remind here a very small width of the transition

D

T_r ¼ 0.02 K
observed for U-15 at.% Mo, while a wider transition

D

T_r¼ 0.2 K was
found for pure U splat. However, unlike a

l

-type anomaly for
U-15 at.% Mo, the superconducting transition in the pure U splat was
revealed only as a small feature around 0.65 K in the specific heat

[19]which is a clear evidence against the bulk nature of

super-conductivity. We assume that only a small fraction of the sample

becomes superconducting. As the impurity phase has to form a 3D
network to reach a zero-resistance state, it must be related to the
grain boundaries. For other

g

-U alloys, such as 11 at.% Mo and
U-12 at.% Mo, the superconducting transition also appears as a single
resistivity drop, although broader than that in U-15 at.% Mo. We pay
particularly attention to the superconducting transition in the
U-6 at.% Mo splat [23], i.e. the intermediate range of Mo alloying
consisted of both

a

- and

g

-U phases. The phase coexistence is
re-flected by a flat but still a metallic-type overall temperature
dependence (d

r

/dT> 0). In the low-T range, the resistivity starts to
decrease rapidly below 1.6 K. This decrease ends in an abrupt drop
into the zero resistance state at Tc¼ 0.78 K. The obtained results

suggest that there are two different superconducting phases in the
U-6 at.% Mo splat (we have to assume the coexisting

a

and

g

-U
phase), each of them exhibiting its own superconductivity. The
lower Tc may be associated to the

g

-U phase, as it revealed by a

sizeable anomaly in the specific heat[23].

The low-temperature

r

(T) dependence of U-15at.% T (T¼ Nd, Pt)
splats measured in zero_{field is shown in}Fig. 5b. We add in the
samefigure the data for U-30 at.% Zr splat consisting of

g

-U phase.
In all cases, a very sharp resistivity drop was observed at Tc. The

estimated values for Tcand

D

Trare given inTable 1. U-15 at.% Nb

becomes superconducting at similar critical temperature

(Tc¼ 1.90 K with

D

Tr¼ 0.15 K) as for other splat alloys consisting of

g

0-U structure (with 11e12 at.% Mo). U-30 at.% Zr exhibits a
superconducting transition revealed by a single drop at Tc¼ 0.81 K

(with

D

T_r¼ 0.08 K)[24]. The superconductivity in U-15 at.% Pt is
characterized by a sharp drop at Tc¼ 0.61 K (with

D

Tr¼ 0.04 K).
Fig. 4. Temperature dependence of electrical resistivity in zero-field in the normal

state of all investigated U-Mo splats (a) and of splat-cooled U-T splats having theg-U
phase (b). For an easier comparison the curves were normalized to respective
re-sistivity values at T¼ 300 K. All U-Mo splats with theg-U phase (11 at.% Mo alloying)
have a negative temperature coefficient (dr/dT< 0). Other alloys with 15 at.% T alloying
(T¼ Nb, Pt) have a negative dr/dT, while U-30 at.% Zr having a positive one but close to
zero.

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Despite of a similarity in the crystal structure (

g

-U) and lattice
parameter between U-15 at.% Mo and U-15 at.% Pt (resulted from
alloying with elements with a similar atomic radii), U-15 at.% Pt
becomes superconducting at much lower temperature. In addition,
a second small drop was observed at Tc(h) ¼ 0.95 K (with

D

T_r¼ 0.08 K). As a complicated phase situation was detected for the
U-15 at.% Pt splat at the grain boundaries (a small amount of
ferromagnetic UPt phase plus U-Pt matrix depleted in Pt), we
cannot be conclusive about intrinsic behavior of U-Pt alloys. More
detailed investigations of superconducting phase transition in
U-15 at.% Pt are in progress in order to understand the two transitions
below Tcand Tc(h). We note here that, even if for the U-5 at.% Pt

splat consisted of a mixed

a

-U and

g

-U phase, the superconducting
phase transition is revealed by only a single drop in the resistivity at
0.7 K[23]. One can also see a certain parallel to recently observed
two transitions in the skutterudite-related La3Rh4Sn13and La3

R-u4Sn13[39].

Applying external magneticfields, the superconducting
transi-tions shift towards lower temperatures, as expected. The estimated
values for critical magneticfields at zero temperature (

m

0Hc) and

the critical slopes at Tcof the Hc2vs T curves (

m

0(dHc2/dT)Tc) for

selected U-Mo splats were reported earlier[18,19]. InTable 1we
listed only the values for pure U and U-15 at.% Mo splat, for a
comparison with other T-alloying splats. The estimated values for
(

m

0Hc) and for (

m

0(dHc2/dT)Tc) are respectively in the range of

2e7 T and 2e4 T/K. These values are close to that found for the
strongly interacting Fermi liquid superconductor U6Fe (

m

0(dHc2/

dT)Tc ¼ 3.42 T/K) [40]and Chevrel-phase superconductors (2 T/

K (

m

0(dHc2/dT)Tc) 8 T/K)[41]. One difference is that for those

splat-cooled

g

-U alloys, the Tcvalues are lower than 2.2 K, while

Chevrel-phase superconductors have much higher Tc(>10 K).

The temperature dependence of specific heat, Cp(T), has been

studied for selected splats over the whole temperature range,
including both the low-T and high-T parts for characterizing the
superconducting behavior as well as the electronic and phonon
contribution. The estimated values for Sommerfeld coefficient of
electronic specific heat (

g

e) and the Debye temperature (

Q

D) are

given inTable 1. A clear evidence of an increase of density of states
at the Fermi level for

g

-U is observed only for U-15 at.% Mo, as
shown by an enhancement of the

g

e value by Mo alloying

(

g

e¼ 16 mJ/K2mol (¼18.8 mJ/K2mol U for U-15 at.%Mo, in a

com-parison with that for pure U

g

e¼ 11 mJ/K2mol U)). It is ascribed to

the increasing atomic volume and higher UeU spacing. The
enhancement of the

g

evalue is found to be larger for Pt alloying,

while it was smaller for Nb and Zr alloying (seeTable 1).

The temperature dependence of the specific heat and its field
variations have been performed down to 0.3 K for selected
splat-cooled UeT alloys. The jump in the specific heat at Tcwithin the

BCS theory in the weak coupling approximation is:

D

C¼ 1:43

g

eTc

We estimated the height of the experimentally observed
specific-heat jump (

D

C) and then compared to the estimated BCS

values by using the

g

eand Tcvalues determined from our

experi-ments. InFig. 6, we shown the C-T curves in zero_{field for selected}
investigated U-T splats. Only a very small feature related to the
superconducting transition was revealed at 0.65 K in the specific
heat for the pure-U splat (Fig. 6a). The results suggest that only a
small fraction of the sample is really superconducting. For U-15 at.%
Mo splat (consisting of single

g

-U phase with ideal bcc A2
struc-ture), a pronounced

l

-type specific-heat anomaly was observed.
The height of the experimentally observed specific-heat jump (

D

C)
is in a good agreement with that estimated from BCS theory. For
other U-Mo splats with lower Mo contents (<15 at.%), a broader

peak with a smaller (but non-negligible) specific-heat jump was
observed close to the superconducting transition temperature Tc

de_{fined from the resistivity measurements. The experimentally}
estimated jump for instance for U-6 at.% Mo splat amounts to only
about 55% of the BCS value[23]. The specific heat of other U-T splats
containing the

g

-U phase measured down to 0.4 K in zero magnetic
field is shown inFig. 6b. Only a weak and broad bump with a small
height was observed in the C(T) curve of U-15 at.% Nb[24]. The
crystal structure, the resistivity jump and the Tcvalue of this splat

are similar to that of U-12 at.% Mo splat, but a much larger peak was
observed for U-12 at.% Mo in the C(T) curve. The specific heat peak
related to the superconducting transition in U-30 at.% Zr splat is
visible at Tcdetermined from the resistivity jump, proving that the

superconductivity in this splat is a real bulk effect.

4. Conclusions

We have stabilized the

g

-U phase in the U-T alloys by a
combi-nation of ultrafast cooling and alloying with 15 at.% T content
(T¼ Mo, Pt, Nb) and 30 at.% Zr content. An ideal bcc A2 structure
was found only in the U-15 at.% Mo splat. It is crucial that using
ultrafast cooling we are able to reduce the necessary concentration
of the T elements (T¼ Mo, Nb, Zr), i.e. the

g

-U phase can be
sta-bilized by a lower concentration of alloying elements. Moreover,
ultrafast cooling could also extend the solubility of Pt metal (up to
at least 15 at.%) and thus we are able to stabilize also

g

-U phase in
U-15 at.% Pt splat. We emphasize again that all splat-cooled alloys

Fig. 6. Specific-heat anomalies related the superconducting phase transition for U-Mo
splats (a) and of selected U-T splats (b). A pronouncedl-type specific-heat anomaly
was observed only for U-15 at.% Mo splat consisting of singleg-U phase with ideal bcc
A2 structure. The bars show the estimated jumps from BCS theory.

</div>
<span class='text_page_counter'>(7)</span><div class='page_container' data-page=7>

were obtained without any additional treatment and that they are
very stable when exposing to ambient conditions.

All the U-T splats become superconducting with the lowest and
highest Tcof 0.61 K and 2.11 K respectively for 15 at.% Pt and

U-15 at.% Mo. The prediction of BCS superconductivity for the specific
heat jump at Tcwas found to be entirely fulfilled in the U-15 at.% Mo

among all investigated splats.

Our investigations have provided new data to the data-base for

low-temperature properties of the U-T system with low-T content
(< 30 at.%, T ¼ Mo, Pt, Pd, Zr, Nb).

Acknowledgments

We express our thanks to all colleagues, in particular our four
Ph.D. students from Prague and Krakow (Ilya Tkach, Mykhaylo
Paukov, Magdalena Krupska, Sylwia Sowa), who have performed
the experiments in the scope of our‘splat-cooling’ project.

This review paper is a tribute to Peter Brommer.
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