Chemical pressure effect on the transport and electronic band structure of Fe
2
V
1-x
Nb
x
Al
C. S. Lue, R. F. Liu, and M. Y. Song
Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan
K. K. Wu and Y. K. Kuo*
Department of Physics, National Dong Hwa University, Hualien 97401, Taiwan
Received 11 June 2008; revised manuscript received 2 September 2008; published 17 October 2008
We report the effects of partial substitution of Nb onto the V sites of Fe
2
VAl by measuring the electrical
resistivity, Seebeck coefficient, and thermal conductivity as a function of temperature. It is found that the Nb
substitution effectively produces a negative chemical pressure in the system. As a result, the Nb-substituted
materials show enhanced semiconductinglike behavior in their electrical resistivity. In addition, the Seebeck
coefficient changes sign from positive to negative while replacing V with Nb. These phenomena have been
associated with the change of the band features, mainly due to the decrease in the number of the hole carriers.
To identify this scenario, we performed ab initio calculations to investigate the electronic band structures of
Fe
2
V
1-x
Nb
x
Al, focusing on the band variation around the Fermi level. Theoretical results indicate a significant
reduction in the hole pockets through Nb substitution, which is consistent with experimental observations.
DOI: 10.1103/PhysRevB.78.165117 PACS numbers: 72.15.Eb, 72.15.Jf, 71.20.Be
I. INTRODUCTION
Heusler-type intermetallics with a general formula X
2
YZ
where X and Y are transition metals and Z is often an ele-
ment from columns III through VI in the periodic table have
attracted considerable attention because of their various
transport and magnetic features. Semiconductors, semimet-
als, normal Pauli metals, weak ferromagnets, antiferromag-
nets, as well as half-metallic ferromagnets exist in this class
of materials. Fe
2
VAl, a material of this prototype, has been
characterized as a nonmagnetic semimetal from intense ex-
perimental and theoretical researches.
1–12
The semimetallic
nature of Fe
2
VAl has been attributed to the presence of a
pseudogap around the Fermi level, arising from a slightly
indirect overlap between the electron and hole pockets.
7–11
The exotic behavior observed in the electrical resistivity, the
nuclear-magnetic-resonance NMR Knight shift, and the
spin-lattice relaxation rate have been associated with the
thermally excited quasiparticles across the pseudogap.
1,2
An
optical-conductivity study on Fe
2
VAl has further confirmed
the existence of a pseudogap in the vicinity of the Fermi
level.
3
Theoretical calculations have indicated that the pseudogap
in Fe
2
VAl will open to a real gap if the volume is expanded
by about 6%.
10
Fe
2
NbAl, an artificial Heusler compound
with a larger lattice constant, was thus predicted to be a
semiconductor with a gap of about 0.2 eV.
10
While a real
sample of Fe
2
NbAl does not exist, the partial substitution of
Nb atoms onto the V sites of Fe
2
VAl yielding a series of
Fe
2
V
1-x
Nb
x
Al compounds may be achieved. Since the nio-
bium atom is isoelectronic to the vanadium and has a larger
atomic radius, the substitution would cause a lattice expan-
sion, which is an effectively negative chemical pressure in
Fe
2
VAl. With this regard, it allows us to study the evolution
of electronic band structures by the chemical pressure effect.
In this work, we performed a detailed transport investiga-
tion by means of the electrical resistivity, the Seebeck coef-
ficient, as well as the thermal-conductivity measurements on
Fe
2
V
1-x
Nb
x
Al with x ranging from 0 to 0.1. It is known that
Seebeck coefficient is very sensitive to the electronic band
features near the Fermi surfaces, and the results can be used
to interpret the change of the band structures through Nb
substitution. In a parallel study, ab initio calculations were
also employed to investigate the electronic band structures of
Fe
2
V
1-x
Nb
x
Al, focusing on the features around the Fermi
level. Theoretical results indicate that the Nb substitution has
an effect that reduces the hole pockets in the vicinity of the
Fermi level, which is consistent with experimental observa-
tions.
II. EXPERIMENTAL DETAILS
Polycrystalline Fe
2
V
1-x
Nb
x
Al x =0, 0.03, 0.06, and 0.1
samples were prepared by an ordinary arc-melting technique.
Briefly, a mixture of appropriate amounts of high-purity el-
emental metals was placed in a water-cooled copper crucible
and then melted several times in an argon flow arc melter.
The weight loss during melting is less than 0.5% for each
compound. To promote homogeneity, these ingots were an-
nealed in a vacuum-sealed quartz tube at 800 ° C for two
days and followed by furnace cooling. This is a typical pro-
cess that forms in a single-phase L2
1
Heusler-type
structure.
13–17
Room-temperature x-ray diffraction taken with Cu K ra-
diation on the powder Fe
2
V
1-x
Nb
x
Al specimens is identified
to the expected L2
1
structure—a more detailed analysis of
the x-ray data, in which the Heusler-type structure was re-
fined with the Rietveld method. We thus obtained the lattice
constant, a, for each composition with the variation as a
function of x illustrated in Fig. 1. It clearly demonstrates that
the lattice constant consistently increases as Fe
2
VAl deviates
from its stoichiometry, indicating that the V sites are success-
fully replaced by Nb atoms according to Vegard’s law. It
should be mentioned that a sample with the nominal compo-
sition of x = 0.2 was also synthesized under the same prepa-
ration condition. The x-ray powder-diffraction pattern exhib-
its no expected L2
1
phase, suggesting that this substitution
level is beyond the solubility limit for Nb in Fe
2
VAl.
Electrical resistivity for the Fe
2
V
1-x
Nb
x
Al alloys was ob-
tained by a standard dc four-terminal method. Seebeck coef-
PHYSICAL REVIEW B 78, 165117 2008
1098-0121/2008/7816/1651175 ©2008 The American Physical Society 165117-1