Copper-doping effects in electronic structure and spectral properties of SmNi₅
The electronic structure and optical properties of the SmNi₅₋Cux (x = 0, 1, 2) compounds are studied. The band spectra of the studied intermetallics were calculated with LDA+U+SO method supplementing the local density approximation with a correction for strong electron interaction on the shell of...
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irk-123456789-1282922018-01-08T03:03:16Z Copper-doping effects in electronic structure and spectral properties of SmNi₅ Knyazev, Yu.V. Lukoyanov, A.V. Kuz’min, Yu.I. Kuchin, A.G. Низкотемпературная оптическая спектроскопия The electronic structure and optical properties of the SmNi₅₋Cux (x = 0, 1, 2) compounds are studied. The band spectra of the studied intermetallics were calculated with LDA+U+SO method supplementing the local density approximation with a correction for strong electron interaction on the shell of the rare-earth element. Optical properties were studied by ellipsometry method in the wide wavelength range. It was found that the substitution of copper for nickel leads to local changes in the optical conductivity spectra. Both the spectroscopic measurements and theoretical calculations demonstrate the presence of a broad absorption band around 4 eV associated with the Cu 3d → Ni 3d electron transitions and increasing with the grown of copper content. The experimental dispersion curves of optical conductivity in the interband absorption region were interpreted using the results of the calculations. 2015 Article Copper-doping effects in electronic structure and spectral properties of SmNi₅ / Yu.V. Knyazev, A.V. Lukoyanov, Yu.I. Kuz’min, A.G. Kuchin // Физика низких температур. — 2015. — Т. 41, № 12. — С. 1313–1317. — Бібліогр.: 15 назв. — англ. 0132-6414 PACS: 71.20.–b, 71.20.Eh, 78.30.–j, 78.40.–q http://dspace.nbuv.gov.ua/handle/123456789/128292 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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Низкотемпературная оптическая спектроскопия Низкотемпературная оптическая спектроскопия |
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Низкотемпературная оптическая спектроскопия Низкотемпературная оптическая спектроскопия Knyazev, Yu.V. Lukoyanov, A.V. Kuz’min, Yu.I. Kuchin, A.G. Copper-doping effects in electronic structure and spectral properties of SmNi₅ Физика низких температур |
| description |
The electronic structure and optical properties of the SmNi₅₋Cux (x = 0, 1, 2) compounds are studied. The
band spectra of the studied intermetallics were calculated with LDA+U+SO method supplementing the local
density approximation with a correction for strong electron interaction on the shell of the rare-earth element. Optical
properties were studied by ellipsometry method in the wide wavelength range. It was found that the substitution
of copper for nickel leads to local changes in the optical conductivity spectra. Both the spectroscopic
measurements and theoretical calculations demonstrate the presence of a broad absorption band around 4 eV associated
with the Cu 3d → Ni 3d electron transitions and increasing with the grown of copper content. The experimental
dispersion curves of optical conductivity in the interband absorption region were interpreted using the
results of the calculations. |
| format |
Article |
| author |
Knyazev, Yu.V. Lukoyanov, A.V. Kuz’min, Yu.I. Kuchin, A.G. |
| author_facet |
Knyazev, Yu.V. Lukoyanov, A.V. Kuz’min, Yu.I. Kuchin, A.G. |
| author_sort |
Knyazev, Yu.V. |
| title |
Copper-doping effects in electronic structure and spectral properties of SmNi₅ |
| title_short |
Copper-doping effects in electronic structure and spectral properties of SmNi₅ |
| title_full |
Copper-doping effects in electronic structure and spectral properties of SmNi₅ |
| title_fullStr |
Copper-doping effects in electronic structure and spectral properties of SmNi₅ |
| title_full_unstemmed |
Copper-doping effects in electronic structure and spectral properties of SmNi₅ |
| title_sort |
copper-doping effects in electronic structure and spectral properties of smni₅ |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
| publishDate |
2015 |
| topic_facet |
Низкотемпературная оптическая спектроскопия |
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http://dspace.nbuv.gov.ua/handle/123456789/128292 |
| citation_txt |
Copper-doping effects in electronic structure and spectral properties of SmNi₅ / Yu.V. Knyazev, A.V. Lukoyanov, Yu.I. Kuz’min, A.G. Kuchin // Физика низких температур. — 2015. — Т. 41, № 12. — С. 1313–1317. — Бібліогр.: 15 назв. — англ. |
| series |
Физика низких температур |
| work_keys_str_mv |
AT knyazevyuv copperdopingeffectsinelectronicstructureandspectralpropertiesofsmni5 AT lukoyanovav copperdopingeffectsinelectronicstructureandspectralpropertiesofsmni5 AT kuzminyui copperdopingeffectsinelectronicstructureandspectralpropertiesofsmni5 AT kuchinag copperdopingeffectsinelectronicstructureandspectralpropertiesofsmni5 |
| first_indexed |
2025-07-09T08:48:26Z |
| last_indexed |
2025-07-09T08:48:26Z |
| _version_ |
1837158536954511360 |
| fulltext |
Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 12, pp. 1313–1317
Copper-doping effects in electronic structure
and spectral properties of SmNi5
Yu.V. Knyazev1, A.V. Lukoyanov1,2, Yu.I. Kuz’min1, and A.G. Kuchin1
1Institute of Metal Physic UD of RAS, S. Kovalevskoy, 18, Ekaterinburg 620990, Russia
E-mail: knyazev@imp.uran.ru
2Ural Federal University, Mira, 19, Ekaterinburg 620002, Russia
Received May 14, 2015, published online October 23, 2015
The electronic structure and optical properties of the SmNi5–xCux (x = 0, 1, 2) compounds are studied. The
band spectra of the studied intermetallics were calculated with LDA+U+SO method supplementing the local
density approximation with a correction for strong electron interaction on the shell of the rare-earth element. Op-
tical properties were studied by ellipsometry method in the wide wavelength range. It was found that the substi-
tution of copper for nickel leads to local changes in the optical conductivity spectra. Both the spectroscopic
measurements and theoretical calculations demonstrate the presence of a broad absorption band around 4 eV as-
sociated with the Cu 3d → Ni 3d electron transitions and increasing with the grown of copper content. The ex-
perimental dispersion curves of optical conductivity in the interband absorption region were interpreted using the
results of the calculations.
PACS: 71.20.–b Electron density of states and band structure of crystalline solids;
71.20.Eh Rare-earth metals and alloys;
78.30.–j Infrared and Raman spectra;
78.40.–q Absorption and reflection spectra: visible and ultraviolet.
Keywords: rare-earth compounds; intermetallics, optical properties; electronic structure.
Introduction
The RNi5 group of intermetallic compounds (where R
is a rare earth) and their substitutional derivatives have
been extensively studied because of a variety of interesting
properties promising for practical applications, such as
magnetocaloric effect, hydrogen storage capacity, magnet-
ic anisotropy and high coercivity [1–6]. Their diverse
magnetic and electronic characteristics are associated with
both localized moments of R atoms and itinerant electrons
of Ni atoms in hexagonal CaCu5-type structure. Besides
studying the properties of binary compounds, studying the
influence of partial substitution of the rare earth or the
nickel on the physical properties and the electronic struc-
ture also attracted considerable attention. For example,
substitution of Ni by some of p- or d-elements in RNi5 can
significantly affect some properties owing to changes in
the electronic structure, crystal field effects and exchange
interaction. In particular, various pseudobinary RNi5–xMx
compounds with M = Al, Ga, Si and Cu, as shown in nu-
merous investigations, exhibit substantial concentration
dependences of the crystalline, electronic, magnetic and
thermodynamic properties comparing to the parent com-
pounds. Study of these materials is of special interest due
to their ability to absorb and store atomic hydrogen. It was
found that substitution of nickel in binary intermetallics by
certain metals can drastically influence the hydrogen sorp-
tion characteristics of the prototype compound.
Significant modifications of some properties owing to the
doping effect were found also in ferromagnetic SmNi5–xCux
system (the Curie temperature TC for binary SmNi5 is 30 K
[2]). The substitution of Cu for Ni is accompanied by de-
creasing of spontaneous magnetic moment and increasing of
coercive force, which indicates the presence of crystal elec-
tric field effect [2]. Also it was shown that TC displays
nonmonotonic concentration dependence with the maximum
at x ~ 1. In ternary SmNi4Cu compound the x-ray photoemis-
sion spectroscopy of the valence band region revealed pecu-
liarities related to the Cu impurity [7]. To explain the exper-
imental data, one needs more detailed investigations on the
electronic structure of the SmNi5–xCux series for different x.
In this paper we report the results of band structure cal-
culations of intermetallics of the SmNi5–xCux (x = 0, 1, 2)
© Yu.V. Knyazev, A.V. Lukoyanov, Yu.I. Kuz’min, and A.G. Kuchin, 2015
Yu.V. Knyazev, A.V. Lukoyanov, Yu.I. Kuz’min, and A.G. Kuchin
system with the aim to obtain a deeper insight into their
electronic structures. In addition to electronic structure
analysis, the optical measurements were performed. Opti-
cal spectroscopy is a suitable technique to study the energy
and the intensity of the electronic excitations as well as the
changes in the density and the mobility of the carriers be-
cause it allows the determination of the plasma and relaxa-
tion frequencies. The experimental spectral data were ana-
lyzed in accordance with the computed band structures.
Experimental and calculation details
The polycrystalline samples of SmNi5–xCux were syn-
thesized by a standard induction melting procedure in the
argon protective atmosphere using induction furnace, with
stoichiometric quantities of the reactant elements of at least
99.9% purity. The resulting ingots were inverted and melt-
ed several times to insure a better homogeneity. To obtain
a single-phase state, the annealing at ~ 1100 ºC was per-
formed for 10 h. The prepared samples were checked for
phase purity using standard powder x-ray diffraction meth-
od. The hexagonal CaCu5-type crystal structure of space
group P6/mmm was confirmed from diffraction spectrum.
Rare-earth atoms occupy the 1a site (0,0,0), two Ni2 atoms
are in the 2c sites (1/3,2/3,0) and three Ni2 atoms are in 3g
sites (1/2,0,1/2). The obtained lattice parameters were used
in our theoretical calculations.
The studies of the optical properties were performed
at room temperature in the wavelength range of λ =
= 0.22–16 µm (photon energies E = 5.64–0.078 eV). The
optical constants, i.e., refractive index n(λ) and absorp-
tion coefficient k(λ) were derived from the ellipsometry
measurements using the Beattie technique. Spectroscopic
ellipsometry is based on the fact that the state of polari-
zation of incident light is changed on reflection. This
change is directly related to the dielectric function of
reflecting material. Mirror samples surfaces were ob-
tained by means of mechanical polishing with diamond
pastes. The measured values n and k enable to determine
a number of spectral functions that characterize the opti-
cal response of the medium, including the permittivity
ε = ε1 – iε2, reflectivity R and the most sensitive parame-
ter, namely the optical conductivity σ(ω) = ε2ω/4π (ω is
the frequency of light wave).
The electronic structure of SmNi5–xCux compounds for
x = 0, 1, 2 was calculated with the LDA+U+SO method [8]
in the framework of TB-LMTO-ASA (tight binding, linear
muffin-tin orbital, atomic sphere approximation). The
LDA+U+SO supplements the local density approximation
with the Hubbard U correction for strong electronic corre-
lations and spin-orbit coupling in the 4f shell of samarium.
The values of direct Coulomb U = 6.3 eV and exchange
Hund J = 0.6 eV parameters were calculated in additional
constrained LDA calculations [9]. In all calculations we
used a k-mesh of 512 = 8×8×8 points and muffin-tin radii
r(Sm) = 3.6 a.u., r(Ni,Cu) = 2.7 a.u. In order to account for
Cu in different positions, self-consistent total and partial
densities of electronic states (DOS) were averaged over all
possible configurations of Cu atoms substitutions for Ni in
the unit cell.
In the calculations we obtained significant magnetic
moments of samarium and almost negligible moments of
nickel with the maximum orbital moment of 0.05 µB on the
Ni ions similar to previous experimental and theoretical da-
ta, e.g., [10]. Since spin-orbit coupling in the 4f shell of sa-
marium was explicitly taken into account in the
LDA+U+SO method, total magnetic moment can be calcu-
lated including the orbital component [11]. The following
configuration of the Sm ions was found for all Cu concentra-
tions: 2S = 4.8, L = 4.7, J = 2.3, g = 0.27, gJ = 0.63. These
values are rather close to Sm3+: 2S = 5.1, L = 5.0, J = 2.45,
g = 0.286, gJ = 0.7.
Total and partial (for Sm 4f and Cu 3d electrons) den-
sities of states for SmNi5–xCux (x = 0, 1, 2) compounds
are shown in Fig. 1. The DOS up to energy of 4 eV below
the Fermi level EF are primarily associated with the Ni 3d
electrons. The sharp dark peaks belong to the partial den-
sities of Sm empty and filled 4f states above and below
the EF, respectively. The grey regions in Figs. 1(b),(c)
correspond to the filled Cu 3d states whose DOS are larg-
est in the range –2…–4 eV. The intensity and extension of
this structure become more significant when substituting Ni
Fig. 1. Total (solid curve) and partial for Cu 3d (grey regions)
and Sm 4f (dark regions) densities of states calculated for
SmNi5 (a), SmNi4Cu (b) and SmNi3Cu2 (c) compounds in the
framework of LDA+U+SO method. The Fermi level corre-
sponds to zero on the energy scale.
–8 –6 –4 –2 0 2 4 6 8
0
10
20
E, eV
(c)
0
10
20 (b)
0
10
20
(a)
D
O
S,
st
at
es
/e
V
D
O
S,
st
at
es
/e
V
D
O
S,
st
at
es
/e
V
1314 Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 12
Copper-doping effects in electronic structure and spectral properties of SmNi5
by Cu atoms increases. The calculated energy maximum of
the localization of the impurity Cu 3d electron band centered
near 3.5 eV is close to the previously obtained values for
other compounds of this type, where the Ni atoms are substi-
tuted by Cu [12–14]. The evolution of the electronic struc-
ture when replacing Ni by Cu is illustrated in Figs. 1(b),(c).
An increase in the concentration of Cu atoms leads to a
modification of the spectral profile of the total DOS mostly
below EF. These changes manifest themselves in the fact
that the broad minimum in the range of –1…–2 eV between
two groups of peaks observed in SmNi5 is less pronounced
in the ternary compounds. Besides that, the doping of the Cu
atoms leads to essential enhancement of the total DOS in the
energy region below ~ –3 eV.
It should be noted that the calculated density of states
for ternary SmNi4Cu is in good agreement with the exper-
imental x-ray photoemission spectrum of this compound
[7]. The localization and width of the main structural fea-
tures revealed in this spectrum due to the Ni, Cu 3d states
and Sm 4f states below the Fermi level are close to those
obtained in our calculation. The decrease of DOS values at
EF going from x = 0 to x = 2 follows the same trend as the
low-temperature magnetic susceptibility and heat capacity
of these type compounds.
Result and discussion
Figure 2 shows the experimental dependences n(λ) and
k(λ) for the SmNi5–xCux (x = 0, 1, 2) compounds. Over most
of the wavelength range, except for the interval λ < 1.5 µm,
the values of these parameters increase monotonically. Be-
sides that, in the whole spectral region k > n, such a relation
is inherent for a media with metallic conductivity. Typical
metal-like behavior was observed also in the ε1(E) and R(E)
dependences (see Fig. 3): the ε1 values are negative in the
entire energy range, while R tends to unity in the low-photon
energy interval. The low-frequency growth of R(E), as well
as large and negative values of ε1, are stipulated by the
intraband (Drude-type) light absorption. The intraband ab-
sorption is determined by the kinetic parameters of the con-
duction electrons — the relaxation γ and plasma ωp frequen-
cies. The relaxation frequency 2 1/γ = ε ω ε additively takes
into account all types of electron scattering upon excitation
by the electromagnetic field, and in the limit ω → 0 it is de-
termined by the static electrical resistivity. The squared
plasma frequency 2 2 2 2
1 2 1( )/Pω = ω ε + ε ε is proportional to
the Fermi velocity of electrons and their concentration. It is
known [15] that in the single-electron approximation for
an arbitrary dispersion relation E(k) the 2
Pω is propor-
tional to the density of states at EF. In the long-
wavelength region λ > 10 µm, the γ and 2
Pω parameters
are frequency independent being stabilized at the values:
γ = 1.4·1014 s–1, 2
Pω = 34.4·1030 s–2 (SmNi5), γ =
= 1.8·1014 s–1, 2
Pω = 32.6·1030 s–2 (SmNi4Cu) and γ =
= 2.3·1014 s–1, 2
Pω = 31.3·1030 s–2 (SmNi3Cu2). As one
can see, the dependence γ(x) for the studied system tends
to increase, pointing out a direct effect of the Cu-doping
on this parameter. The dependence 2 ( )P xω indicated that
DOS for the studied compounds shows the tendency of re-
duction which is in a qualitative agreement with the band
calculations. The values 2
Pω were used to estimate the con-
centration of conduction electrons as 2 2/4PN m e= ω π
(m and e are the mass and the charge of free electron, respec-
tively), which gave us: N = 1.05·1022 cm–3 (SmNi5),
N = 0.99·1022 cm–3 (SmNi4Cu), N = 0.95·1022 cm–3
(SmNi3Cu2).
In Fig. 4 the experimental optical conductivity spectra
σ(E) of these compounds are given (note that the curves
Fig. 2. Dependences of the refractive index n and absorption
coefficient k on the wavelength of the incident light for SmNi5,
SmNi4Cu and SmNi3Cu2 compounds. The inset shows the short-
wavelength range.
1 2
4
8
2 4 6 8 10 12 14
10
20
30
0
n
kk
nn
k,
0n
k,
λ µ, m
λ µ, m
SmNi5
SmNi Cu4
SmNi Cu3 2
Fig. 3. Energy dependences of the real part of the permittivity
and reflectivity (inset) of the SmNi5, SmNi4Cu and SmNi3Cu2
compounds.
0 1 2 3 4 5 6
0.4
0.6
0.8
1 0.
0 1 2 3 4 5 6
–100
–80
–60
–40
–20
R
E, eV
E, eV
ε1
0
SmNi5
SmNi Cu4
SmNi Cu3 2
Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 12 1315
Yu.V. Knyazev, A.V. Lukoyanov, Yu.I. Kuz’min, and A.G. Kuchin
are shifted with respect to each other along the vertical axis
by 10 units). In the low-energy range a monotonic growth
of σ(E) is related to the Drude-type of electron excitation,
2 2 2/4 ( ).D Pσ = ω γ π ω + γ Above ~ 0.5 eV the shape of
σ(E) dependence indicates the dominant role of interband
absorption. The spectra of the optical conductivity for all
alloys in this region are characterized by the broad asym-
metrical absorption band with the abrupt low-energy edge
and some maximum, which intensity and position depend
on the compound composition. It can be seen that the ab-
sorption band has almost the same width for the all studied
compounds, whereas its structure is substantially trans-
formed with variation in the amount of the impurities. In
particular, the peak located at the energy ~ 3 eV in the
spectrums σ(E) of SmNi5 and SmNi4Cu compounds dis-
appeared in the corresponding dependence for SmNi3Cu2.
The maxima near 4 eV, in turn, are conspicuous only in the
σ(E) curves of the ternary alloys. The formation of high-
energy maxima is connected with the substantial changes
in the electronic energy structure of compounds upon sub-
stitution Cu for Ni atoms. The localization of these features
in the experimental σ(E) spectra and the enhancement of
their intensities with the increase of Сu content are corre-
spondent to the theoretical DOS in Fig. 1. The similar
maxima centered at ~ 4 eV were also found in optical con-
ductivities of some RNi5–xCux alloys [12–14] and identi-
fied as Cu 3d → Ni 3d transitions. According to the calcu-
lated DOS, the formation of intense absorption structures
in the range E < 3.5 eV can be related mainly with the Ni
3d → Ni 3d, Sm 4f electron transitions.
The obtained DOS of SmNi5–xCux (x = 0, 1, 2) com-
pounds were used to interpret the experimental data. The
interband optical conductivities were calculated directly from
the electronic structure through the convolution of the total
DOS both below and above the EF. The calculations were
performed in approximation that the direct and indirect tran-
sitions are equally probable. The results of such calculations
are given in Fig. 5 (shown in arbitrary units) together with
the experimental interband contributions to the optical con-
ductivity σib = σ(E) – σD(E). The calculations for all com-
pounds predict the existence of a strong absorption region up
to 6 eV, which is formed by electronic transitions between
the states characterized by larger values of the total DOS.
Note that calculations showed the presence of the broad max-
ima near 4 eV for SmNi4Cu and SmNi3Cu2 compounds.
There are also shown the partial contributions to the
interband optical conductivity from quantum transitions in-
volving electrons of the Sm 4f (dashed lines) and Cu 3d
bands (dash-dotted lines). Dotted lines in Fig. 5 identify the
Drude contribution. For all the alloys the fine structure in
σib(E) spectra obtained from the total DOS is qualitatively
similar to the corresponding dispersion curves determined by
the partial contributions involving Sm 4f electrons (Ni 3d →
→ Sm 4f and Cu 3d → Sm 4f transitions). This suggests a
significant role played by Sm 4f electrons in the interband
absorption in SmNi5–xCux compounds.
Fig. 4. Energy dependences of the optical conductivity of
SmNi5–xCux (x = 0, 1, 2) compounds. The curves are shifted up-
ward along the ordinate axis relative to one another by 10 units.
0 1 2 3 4 5 6
20
40
60
σ,
1
0
s
–1
4
–1
E, eV
SmNi5
SmNi Cu4
SmNi Cu3 2
Fig. 5. Interband optical conductivity spectra of SmNi5 (a),
SmNi4Cu (b) and SmNi3Cu2 (c). Circles refer to experiment and
the solid curve corresponds to calculations in arbitrary units.
Dashed and dash-dotted curves represent the partial contributions
from the transitions involving the Sm 4f electron bands and Cu 3d
bands, respectively. Dotted lines show the Drude contribution.
0
10
20
30
40
0
10
20
30
40
1 2 3 4 5 6
10
20
30
40 (c)
0
(b)
(a)
E, eV
σ,
1
0
s
–1
4
–1
σ,
1
0
s
– 1
4
– 1
σ ,
1
0
s
–1
4
–1
1316 Low Temperature Physics/Fizika Nizkikh Temperatur, 2015, v. 41, No. 12
Copper-doping effects in electronic structure and spectral properties of SmNi5
A comparison of the experimental dependences of
interband optical conductivity with the theoretical ones
shows both their certain similarity (the position of some fea-
tures and the same energy range of quantum absorption) and
distinctions, which manifest themselves mainly in the curves
for the SmNi4Cu. On the whole, we can conclude that the
energy dispersion of σ(E) of SmNi5–xCux compounds with
x = 0, 1, 2 within the fundamental absorption band is ade-
quately described by the calculated band structure.
Conclusions
The evolution of the electronic structure and optical
properties of SmNi5–xCux (x = 0, 1, 2) compounds by the
substitution of copper for nickel atoms has been investigat-
ed. The energy dependences of the total and partial elec-
tronic densities of states have been calculated by
LDA+U+SO method taking into account strong electron–
electron interactions in the Sm 4f shell. The nature of elec-
tronic states involved in formation of interband optical
absorption spectra in the energy range EF ± 6 eV has been
determined. Optical properties were studied ellipsome-
trically in a broad spectral range. It has been shown that
the frequency dependences of the optical conductivities in
the quantum light absorption region are satisfactory ex-
plained in terms of the calculated densities of electronic
states. The spectral data in the infrared region were used to
obtain the relaxation and plasma frequencies of conduction
electrons.
The research was carried out within the state assignment
of FASO of Russia (theme “Electron” No. 01201463326),
supported in part by RFBR (projects 13-02-00256 and
13-02-00050), Program of UrB RAS (project 15-8-2-4)
and the Dynasty Foundation. Some results reported in
this work were obtained using ”Uran” supercomputer of
IMM UrB RAS.
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Introduction
Experimental and calculation details
Result and discussion
Conclusions
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