Influence of hydrogen absorption on structural and electrical transport properties of
alloys
G. SrinivasV. SankaranarayananS. Ramaprabhu
Citation: Journal of Applied Physics 102, 063706 (2007); doi: 10.1063/1.2757004
View online: http://dx.doi.org/10.1063/1.2757004
View Table of Contents: http://aip.scitation.org/toc/jap/102/6
Published by the American Institute of Physics
Influence of hydrogen absorption on structural and electrical transport
properties of Ho
1−x
Mm
x
Co
2
alloys
G. Srinivas
Low Temperature Laboratory, Department of Physics, Indian Institute of Technology, Madras,
Chennai 600 036, India and Alternative Energy Technology Laboratory, Department of Physics,
Indian Institute of Technology, Madras, Chennai 600 036, India
V. Sankaranarayanan
Low Temperature Laboratory, Department of Physics, Indian Institute of Technology, Madras,
Chennai 600 036, India
S. Ramaprabhu
a兲
Alternative Energy Technology Laboratory, Department of Physics, Indian Institute of Technology, Madras,
Chennai 600 036, India
共Received 14 April 2007; accepted 2 June 2007; published online 19 September 2007兲
The structural and electrical transport properties of Ho
1−x
Mm
x
Co
2
共x=0, 0.1, 0.2, 0.3, and 0.4 and
Mm=mischmetal兲 alloys and their hydrides in the hydrogen concentration 共y兲 range of 0艋 y
艋3.6 have been determined through the powder x-ray diffraction 共XRD兲 and temperature
dependence of electrical resistivity 关
共T兲兴 measurements. At room temperature, these compounds
crystallize in MgCu
2
-type 共C15兲 structure. The crystalline nature and lattice expansion of
hydrogenated alloys have been studied using the hydrogen concentration dependence of XRD peak
analysis indicating the coexistence of two hydride phases at intermediate hydrogen concentrations.
The temperature dependence of the electrical resistivity of alloys has been discussed based on the
conduction electron scattering and spin fluctuation scattering mechanisms. The changes in magnetic
ordering and transition temperature upon Mm substitution and at different concentrations of
hydrogen loadings have been discussed. Furthermore, the transformation from metalliclike
conductivity to thermally activated conduction mechanism and different conduction regimes of
temperature dependent resistivity upon increasing H concentration have been well described by
Kondo-type and electron-electron scattering effects. © 2007 American Institute of Physics.
关DOI: 10.1063/1.2757004兴
I. INTRODUCTION
Most of the Laves phase AB
2
compounds formed among
rare earths 共R 兲 and elements of the d-transition series 共M兲
form stable hydrides and, very often, hydrogen absorption
leads to strong changes of the macroscopic structural, elec-
tronic, and magnetic properties of these materials due to the
bonding between interstitial H atoms.
1–9
Laves phase RCo
2
and LR
1−x
HR
x
Co
2
共LR=light rare earth and HR=heavy rare
earth兲 compounds show unusual varieties of physical prop-
erties and the results indicate that the physical properties are
strongly dependent on the type of rare earth element and the
lattice constant.
10–15
HoCo
2
belongs to the RCo
2
family, crys-
tallizes in MgCu
2
-type structure, and has enhanced paramag-
netic state at room temperature. It readily absorbs large
amount of hydrogen, about 3.5–4 H/f.u. and forms stable
hydrides. The hydrogen absorption in HoCo
2
leads to the
weakening of magnetic interactions.
8,16,17
At higher hydro-
gen concentrations, the system goes from metallic state to
semiconducting- or insulating-type thermally activated
conduction.
8
These hydrogen induced metal-semiconductor
transitions greatly depend on the type of the rare earth and
the substituting elements.
8,9
It is reported that Ce-based com-
pounds lead to stronger thermally activated conduction pro-
cess upon hydrogenation than HoCo
2
. However, in the Zr
substituted HoCo
2
compounds, the hydrogen induced ther-
mally activated conduction process drastically gets reduced
with increasing Zr content.
8
Therefore, it is of interest to investigate the structural
and electrical transport properties of Laves phase rare earth
and Co compounds and their hydrides with a mixture of LR
and HR. In order to obtain a better understanding of the role
played by hydrogen into driving the modifications of the
structural, electrical, and magnetic behavior of these Laves
phase compounds, here we present a detailed x-ray diffrac-
tion and electrical resistivity studies in the Ho
1−x
Mm
x
Co
2
共x=0, 0.1, 0.2, 0.3, and 0.4 and Mm=mischmetal兲 alloys and
their hydrides. Mischmetal is a cheap natural ore and con-
tains the light rare earth metals: 50% Ce, 35% La, 8% Pr, 5%
Nd, 1.5% other rare earth elements and 0.5% Fe. Recently,
studies on the hydrogen absorption and desorption properties
of Ho
1−x
Mm
x
Co
2
alloys
18
using pressure-composition 共P-C兲
isotherms indicate the formation of two hydride phases, i.e.,
one intermetallic phase dissolving hydrogen 共solid solution
of the hydrogen in the metal,
␣
phase兲 and another hydride
phase 共

phase兲, separated by an 共
␣
+

兲 two-phase region.
Further, thermogravimetric 共TG兲 analysis shows that the hy-
drides are in very stable nature at room temperature. In this
a兲
Author to whom correspondence should be addressed. Electronic mail:
ramp@iitm.ac.in; Tel.: ⫹91-44-22574862. FAX: ⫹91-44-22570509.
JOURNAL OF APPLIED PHYSICS 102, 063706 共2007兲
0021-8979/2007/102共6兲/063706/8/$23.00 © 2007 American Institute of Physics102, 063706-1
paper we are concerned with crystalline hydrides of
Ho
1−x
Mm
x
Co
2
–H
y
with 0艋 y 艋 3.6. The coexistence of two
hydride phases at lower hydrogen concentrations and the lat-
tice expansion at higher H contents have been reported. The
changes in magnetic transition and ordering temperature
upon Mm concentration and hydrogen loadings have been
discussed. Further, the effect of hydrogen interstitials on the
transition from metallic state to thermally activated conduct-
ing state in these LR and HR mixture compounds has been
discussed.
II. EXPERIMENTAL DETAILS
Ho
1−x
Mm
x
Co
2
共x =0,0.1,0.2,0.3, and 0.4兲 alloys were
prepared by arc melting of stoichiometric amounts of con-
stituent elements Ho, Mm, and Co with a purity better than
99.9% under a protective argon atmosphere. 6 wt % excess
of Ho and Mm were taken in order to prevent the formation
of Co-rich phases. The ingots were melted several times to
ensure homogeneity. Samples thus obtained were sealed in
an evacuated quartz tube and homogenized at 1123 K for
4 days. Hydrogen absorption experiments were performed
using the Sieverts-type apparatus. The ingot of annealed
samples were preheated under high vacuum at 700 K for 2 h
and then exposed to a controlled amount of hydrogen pres-
sure, 0.001–1 bar at 400 K. The amount of H absorbed was
determined volumetrically by monitoring the pressure
changes in a calibrated volume. In order to get a homoge-
neous hydrogen distribution, the samples were annealed in a
hydrogen atmosphere at 400 K. The hydrogen desorption has
been carried out by continuously evacuating the system and
increasing the temperature up to 1000 K. The structural and
phase identification of alloys and their hydride samples have
been carried out using powder x-ray diffraction 共XRD兲. The
powder XRD measurements were performed at room tem-
perature using X’pert Pro, PANalytical diffractometer with
2
values between 15° and 90°, in steps of 0.05° using
Cu K
␣
radiation. The unit cell parameters were determined
by a least-squares refinement method using silicon 共5N兲 as
an internal standard. The electrical resistivity measurements
have been carried out using four-point contact technique on
the circular disk shaped bulk Ho
1−x
Mm
x
Co
2
alloys, buttons
with about 8 mm diameter and 1–2 mm thickness, obtained
by cutting the arc melted alloy using low speed diamond
wheel saw and on samples made of compressed powders of
its hydrides having the same dimensions. The densities of the
bulk alloys and compressed hydride powders are listed in
Table I. The voltage and current leads were attached to the
samples using silver paint. The samples were mounted on an
electrically insulating support and kept near to the cold head
of a closed cycle refrigerator. The temperature was measured
using a platinum resistance thermometer 共PT100兲 in the
range of 20–300 K. The resistivity measurements down to
5 K have been carried out using a dip-stick helium cryostat,
and the temperature has been measured using cernox sensor
in the range of 5–30 K. The dc current through the sample
was set in the range of 1⫻ 10
−1
–1⫻10
−3
A and the current
was first applied in one direction and then reversed, in order
to minimize the thermo-emf effects. Measurements at tem-
peratures higher than 300 K could not be made due to the
desorption of hydrogen from the hydride samples.
III. RESULTS AND DISCUSSION
A. Crystal structure and lattice constant of alloys
The XRD of homogenized Ho
1−x
Mm
x
Co
2
共x=0, 0.1, 0.2,
0.3, and 0.4兲 alloys with Rietveld analysis shows
19
that all
the alloys crystallize in cubic Laves phase with the
MgCu
2
-type structure 共space group Fd3
¯
m兲. The lattice pa-
rameters 共a兲 at room temperature and the cell volume expan-
sion upon Mm substitution are summarized in Table I. The
lattice parameters increase linearly with increasing Mm con-
centration, which is ascribed to the larger radius of the Mm
than that of Ho, since the mischmetal is an ore of lighter rare
earths, which have larger radii than the heavier rare earth Ho.
B. Hydrogen absorption and structural behavior of
hydrides
In order to understand the formation of different hydride
phases and the contribution of each hydride phase to the
structural property of Ho
1−x
Mm
x
Co
2
alloys, the XRD of hy-
drogenated Ho
1−x
Mm
x
Co
2
alloys with various hydrogen con-
centrations are investigated. For example, Fig. 1 shows the
XRD patterns of the parent, the dehydrogenated 共hydrogen-
ated and then dehydrogenated兲, and hydrogenated
Ho
0.8
Mm
0.2
Co
2
alloys with different hydrogen concentra-
tions in the range of 0–3.6 H/f.u. at room temperature. The
XRD pattern of the dehydrogenated sample resembles that of
the parent alloy, without a shift of peak positions or appear-
TABLE I. The lattice parameters, unit cell volume expansion, and densities of alloys and their hydrides.
Alloy hydride
composition
Alloys Hydrides
Lattice
Parameter
共Å兲
Density
共g/cc兲
Volume
expansion,
⌬V/V
共Mm=0兲
%
Two phase Hydride phase
Volume expansion,
⌬V/V
共y=0兲
%
Lattice
parameter 共Å兲
Density
共g/cc兲
Lattice
parameter 共Å兲
Density
共g/cc兲 Two phase
Hydride
phase
HoCo
2
–H
y
7.1781共2兲 10.12 ¯ 7.60 共y =1.7兲 6.72 7.70 共y =3.8兲 5.77 18.69 23.44
Ho
0.9
Mm
0.1
Co
2
–H
y
7.1839共2兲 9.88 0.243 7.57 共y =1.9兲 6.00 7.72 共y =3.5兲 5.62 16.82 23.91
Ho
0.8
Mm
0.2
Co
2
–H
y
7.1900共1兲 9.46 0.498 7.61 共y =1.9兲 5.89 7.73 共y =3.5兲 5.47 18.43 24.43
Ho
0.7
Mm
0.3
Co
2
–H
y
7.1966共2兲 9.31 0.775 7.62 共y =2.1兲 5.63 7.77 共y =3.7兲 5.12 18.75 25.95
Ho
0.6
Mm
0.4
Co
2
–H
y
7.2040共1兲 9.11 1.086 7.61 共y =1.9兲 6.46 7.72 共y =3.6兲 5.10 17.64 23.16
063706-2 Srinivas, Sankaranarayanan, and Ramaprabhu J. Appl. Phys. 102, 063706 共2007兲
ance of the new peaks, except for a small decrease in the
intensity. These two facts indicate that there is a slight de-
crease in grain size due to the plastic deformation. The large
cell volume expansion upon hydrogenation and brittle nature
of these alloys lead to fine hydride powders. The hydrides of
Ho
1−x
Mm
x
Co
2
, with different concentrations maximum up to
y =3.6 H/f.u., were prepared by controlling the sample tem-
perature and pressure in the range of 0.001–1 bar. A single-
phase C15 MgCu
2
-type diffraction pattern is observed for y
艋0.5 and y 艌 2. In the concentration range 0.5艋 y 艋 2, all
the diffraction lines split into two sets of identical lines, rep-
resenting the coexistence of the two hydride phases. In ac-
cordance with the well-defined plateau in the P-C isotherms
of Ho
1−x
Mm
x
Co
2
–H
y
,
18
XRD, perturbed angular correlation
共PAC兲 and Mossbauer measurements of isostructural Laves
phase RM
2
–H
y
共Refs. 20–22兲 suggest that this region exhib-
its coexistence of two hydride phases. One phase 共i.e.,
␣
phase兲 has a lattice parameter close to that of the unhydro-
genated Ho
1−x
Mm
x
Co
2
alloys and the other phase 共

phase兲
corresponds to the hydride phase of Ho
1−x
Mm
x
Co
2
. The ap-
pearance of new set of diffraction lines with large shift to-
wards the lower angle side represents large lattice expansion,
due to the growth of the

phase at the expense of the
␣
phase. However, within the two-phase region, the relative
change of amounts of the two phases changes, i.e., the ratio
of amount of

phase to
␣
phase increases as observed by the
change in the relative intensities of two identical sets of
XRD lines 共Fig. 1兲. Whereas in the

-phase region 共y 艌 2兲,
the diffraction lines systematically shift further towards the
smaller angles with increasing y, indicating increase in the
lattice constant. The concentration dependence of the expan-
sion in lattice constant and the unit cell volume of
Ho
0.8
Mm
0.2
Co
2
–H
y
are shown in Fig. 2. In the two-phase
region, the absorbed hydrogen causes a negligible lattice ex-
pansion. In addition, the hydrogen concentration dependence
of the lattice constant and the corresponding unit cell volume
expansion of Ho
1−x
Mm
x
Co
2
–H
y
in the two-phase and hy-
dride phase region are summarized along with the lattice
parameters of parent alloys in Table I. The lattice expansion
in the

phase is consistent with the typical volume increase
of 2.9 Å
3
per hydrogen atom observed for different metal-
hydrogen systems.
23
The present work shows that the forma-
tion of hydrides in Ho
1−x
Mm
x
Co
2
does not lead to amor-
phization and all the hydrides retain their cubic C15 host
lattice structure accompanied by substantial increase of the
lattice constant.
C. Temperature dependent electrical resistivity of
Ho
1−x
Mm
x
Co
2
alloys
The temperature dependence of the electrical resistivity
共T兲 of Ho
1−x
Mm
x
Co
2
alloys for x =0, 0.1, 0.2, 0.3, and 0.4
is shown in Fig. 3. The steplike behavior in the
共T兲 plots
indicates the magnetic transition temperature from paramag-
netic state to magnetically ordered state.
13,14
The magnetic
ordering transition temperature T
C
has been determined from
the maximum in d
/dT vs T plots. The observed T
C
of the
parent HoCo
2
is in agreement with earlier published
data.
8,10,13,16
The
共T兲 curves of the Ho
1−x
Mm
x
Co
2
com-
pounds are characterized by nearly the same temperature de-
pendence in the paramagnetic region. The total resistivity of
Ho
1−x
Mm
x
Co
2
includes several contributions and can be ob-
tained from the Matthiessen’s rule given by
共T兲 =
0
+
ph
共T兲 +
mag
共T兲, where
0
is the residual resistivity,
ph
is
the phonon contribution,
mag
关=
sf
共T兲 +
spd
共T兲兴 is the mag-
netic contribution containing two spin-dependent parts,
where
sf
is the spin fluctuation resistivity due to the scatter-
ing of the conduction electrons by spin fluctuations within
the Co 3d band and
spd
is the spin disorder contribution
arising from the scattering of R 4f moments.
spd
is indepen-
dent of temperature above T
C
, but it shows a T
2
dependence
below T
C
.
24
The phonon contribution to resistivity
ph
at low
temperatures varies as
ph
=bT
5
.
14,24
The resistivity plots of
Ho
1−x
Mm
x
Co
2
below T
C
could be fitted well to the expres-
sion
共T兲 =
0
+AT
2
+BT
5
. The fit indicates that the T
2
term is
FIG. 1. Powder XRD patterns of Ho
0.8
Mm
0.2
Co
2
–H
y
, with hydrogen con-
centration range of 0 –3.6 H/f.u.
FIG. 2. Variation of lattice constant and corresponding unit cell volume of
Ho
0.8
Mm
0.2
Co
2
–H
y
upon increase in hydrogen concentration.
063706-3 Srinivas, Sankaranarayanan, and Ramaprabhu J. Appl. Phys. 102, 063706 共2007兲
dominating. Thus inset 1 of Fig. 3 shows the T
2
dependence
of resistivity well below T
C
, which is attributed to the
spd
.
24
A tendency for strong saturation in
共T兲, observed above T
C
in all the cases, is a peculiar behavior pertaining to the RCo
2
compounds. Such behavior is not seen in the isostructural
RTM
2
compounds with TM=Al, Fe, or Ni and is attributed
to the presence of spin fluctuations.
14
The sharp drop in
共T兲
plots at T
C
indicates the sudden appearance of the molecular
field due to the spontaneous alignment of the R moments.
The molecular field induces magnetic moments on the Co
sites and thus suppresses the spin fluctuations below T
C
.
13,14
The slight increase in
vs T is observed when approaching
T
C
from the paramagnetic temperature range. This can be
ascribed to short-range order effects existing in both local-
ized 4f and itinerant 3d sublattices, which enhances the spin-
density fluctuations in the Co sublattice due to the f-d ex-
change coupling.
As can be seen, upon substituting Mm for Ho, the re-
sidual resistivity
0
increases drastically and modifies the
vs T curves significantly in the low temperature region. Un-
like the dominant T
2
dependence of the low temperature re-
sistivity, pronounced minimum appears in the
共T兲 curve for
x=0.4. Usually, minima in the temperature dependence of
共T兲 are associated with freezing of the magnetic moments
of ferro/ferrimagnetic clusters in random directions.
25–27
An
increasing extent of spin fluctuations and a probable instabil-
ity of the R moments are also evidenced by low temperature
specific heat measurements in the dilute R concentrations.
28
The variation of T
C
upon Mm substitution is shown in inset
2 of Fig. 3. The strong nonlinear decrease of T
C
is attributed
to the decreasing f-d exchange interactions with increasing
Mm content.
11
The effect of dilution of the magnetic R ions
and influence on the resistivity have been investigated for
several pseudobinary 共R ,Y兲Co
2
and LR
1−x
HR
x
Co
2
compounds.
8,13,14,29–32
These studies suggest the weakening
of magnetic interactions due to the dilution of the R moment
by a non- or less magnetic element, thereby decreasing the
molecular field. Therefore, the observed decrease in the T
C
is
attributed to the weak magnetic interactions due to the Mm
substitution.
D. Temperature dependence of electrical resistivity of
Ho
1−x
Mm
x
Co
2
hydrides
Figures 4共a兲–4共e兲 display the temperature dependence of
the reduced representation of electrical resistivity
共T兲 for
the Ho
1−x
Mm
x
Co
2
–H
y
samples in the temperature range of
20–300 K. Each figure contains parent alloy, dehydroge-
nated 共hydrogenated and then dehydrogenated兲 and a series
of hydrogenated Ho
1−x
Mm
x
Co
2
samples at different hydro-
gen concentrations in the range of 0艋 y 艋 3.6. The resistivity
of dehydrogenated samples shows a smooth change in mag-
netic transitions and a large increase in the reduced residual
resistivity, unlike the sudden changes observed in the parent
alloys. It is reported that the dehydrogenated samples of
PrCo
2
and NdCo
2
fail to show the sudden change in resistiv-
ity as observed in parent PrCo
2
and NdCo
2
.
1
Therefore, the
共T兲 plots indicate that the dehydrogenated Ho
1−x
Mm
x
Co
2
samples exhibit weak magnetic interactions.
At lower hydrogen concentrations up to about y 艋2in
these alloy hydrides, the magnetic transition temperature
gradually disappears with increasing hydrogen concentra-
tions, as observed by the smoothening of
共T兲 plots from the
steplike behavior of unhydrogenated alloys. There is a sub-
stantial increase in lattice parameters confirmed from the
XRD studies of hydrogenated Ho
1−x
Mm
x
Co
2
samples. In ad-
dition, the previously reported XRD, Mossbauer, magnetiza-
tion, ac susceptibility, and electrical resistivity studies on the
R–H
y
and Laves phase RCo
2
–H
y
type systems
1,2,5,8,16,17,33–36
conclude that with increasing hydrogen concentration, there
is a large increase in lattice parameters, substantial reduction
of conduction electron density, lack of saturation in magne-
tization, a decrease in the Co moment and fanning of the rare
earth moments, and gradual reduction or disappearance of
magnetic transition temperature. Therefore the large lattice
expansion and decrease in the number of conduction elec-
trons due to charge transfer between hydrogen and the 3d
band in hydrogenated Ho
1−x
Mm
x
Co
2
lead to a decrease of
the strength of various exchange interactions of the local
moments mediated by the conduction electrons. This de-
crease modifies the magnitudes of the 3d moments and
weakening of R –Co interactions, which will affect the mag-
netic scattering contribution to the electrical resistivity
significantly.
8,16
Consequently, the type of magnetic order
changes with increasing hydrogen concentrations and the
broadening of transition at T
C
has been observed in the hy-
drides. As y increases, the magnitude of the
/
共300 K兲
values
at 20 K increases considerably due to the decrease in the
conduction electron density.
However,
vs T plots show different behaviors at inter-
mediate H concentrations and they exhibit several general
characteristic features as a function of T. As a typical ex-
ample, we consider the results for the Ho
0.9
Mm
0.1
Co
2
–H
y
sample with y =2 关Fig. 5共a兲兴. With respect to the sign of the
temperature derivative of
共i.e., d
/dT兲, three conduction
FIG. 3. Temperature dependent electrical resistivity of Ho
1−x
Mm
x
Co
2
alloys
for x =0, 0.1, 0.2, 0.3, and 0.4. Inset 1: T
2
dependence of electrical resistivity
below the magnetic transition temperature. Inset 2: variation of Curie tem-
perature with Mm concentration, defined by the maximum in the d
/dT vs
T.
063706-4 Srinivas, Sankaranarayanan, and Ramaprabhu J. Appl. Phys. 102, 063706 共2007兲
