The density of states, band structure, tolerance factor and Jahn–Teller splitting energy were calcu-lated.. CaMnO3bulk structure Dependence of the total energy on the lattice parameter o
Trang 1Influence of doped rare earth elements on electronic properties
of the R 0.25 Ca 0.75 MnO 3 systems
Nguyen Hoang Linha,*, Nguyen Thuy Tranga, Nguyen Tien Cuonga, Pham Huong Thaob, Bach Thanh Conga
a
Faculty of Physics, Hanoi University of Science, Vietnam National University – Hanoi, Vietnam
b
Faculty of Physics, Hue University of Education, Hue University, Vietnam
a r t i c l e i n f o
Article history:
Received 29 October 2009
Received in revised form 25 February 2010
Accepted 1 March 2010
Available online 1 April 2010
Keywords:
Calcium manganese
Density functional theory
Doping compound
Doped rare – earth elements
a b s t r a c t
The influence of doped rare earth elements on the some electronic properties of perovskite systems
R0.25Ca0.75MnO3(R = La, Nd, Eu, Tb, Ho, Y) is investigated using the density functional theory with Dmol3 code The density of states, band structure, tolerance factor and Jahn–Teller splitting energy were calcu-lated By doping the different rare earth elements, the systems show different changing in the crystal structure, hopping amplitude, and electrical resistivity Among these doping compounds, the
Eu0.25Ca0.75MnO3 exhibits the strongest structural change corresponding to the largest Jahn–Teller splitting
Ó 2010 Elsevier B.V All rights reserved
1 Introduction
During recent decade, the rare earth-doped calcium manganese
perovskites have attracted much attention due to their exotic
behaviors, such as the colossal magnetoresistance (CMR), the large
magnetocaloric effects, as well as their potential for applications in
electronics industry[1]
Like other classic perovskites, the pure CaMnO3 crystal has a
perfect perovskite structure with cubic symmetry belonged to
Pnma space group The lattice parameter of the cubic CaMnO3,
a = 3.75 Å, was determined by Wollan and Koehler[2] Moreover,
CaMnO3 exhibits a G-type antiferromagnetic insulator with the
band structure as a Mott insulator[1] Doping with rare earth
ele-ment makes the ion concentration ratio between Mn3+/Mn4+
alter-ing from 0 to 1 This induces a big change in the conductivity of the
doped systems For example, Sousa et al found that by substituting
Holmium for Calcium, Ho1xCaxMnO3exhibit a significant decrease
in the electrical conductivity and the metal–insulator transition
temperature increases with increasing of the holmium
concentra-tion[3] In[4], the melting of the charge ordering state by
Ruthe-nium doping for manganese in calcium-praseodymium
perovskite was also observed
By using the density functional theory (DFT), this paper
concen-trated to study on electronic properties of the typical rare
earth-doped calcium manganese oxides with the general chemical
for-mula, R0.25Ca0.75MnO3 (R = La, Nd, Eu, Tb, Ho, Y) The calculations were effectively carried out using the Dmol3 package
The pure CaMnO3unit cell has been constructed with the sym-metric standard coordinates of atoms as following: Ca (0, 0, 0); Mn (1/2, 1/2, 1/2); O1 (0, 1/2, 1/2); O2 (1/2, 0, 1/2); O3 (1/2, 1/2, 0) Then, the modeling bulk crystals of the R0.25Ca0.75MnO3 systems were built by replacing ion Ca2+ with ion R3+ at the symmetric coordinates as showed in the supercell with the size 2 2 1 gi-ven inFig 1
In the present study, we chose Perdew–Wang correlation func-tional (PW91) of generalized gradient approximation (GGA) A spin restricted calculation (treating separately for spin up and spin down electrons) for a system having odd number of electrons is also applied K-point was chosen with 7 7 7 for pure CaMnO3
model and 3 3 7 for doping model Fine orbital cutoff quality was set as default of DMol3 code: 5.5 Å for pure CaMnO3 model and 5.8 Å for doping model The accuracy of these calculations is improved by all electron relativistic treatment for cores
2 Results and discussion 2.1 CaMnO3bulk structure Dependence of the total energy on the lattice parameter of the cubic CaMnO3 bulk crystal was calculated in Hatree unit (1 Ha = 27.221 eV) and plotted inFig 2 From this figure the opti-mized lattice constant, a = 3.75 Å, was founded This value is in 0927-0256/$ - see front matter Ó 2010 Elsevier B.V All rights reserved.
* Corresponding author Tel.: +84 912489852.
E-mail addresses: linknh@gmail.com (N.H Linh), congbt@vnu.edu.vn (B.T Cong).
Trang 2agreement with previous experimental and theoretical studies
[2,3]
The model of Goodenough, which is known as semi-covalent
exchange interaction model, shows clearly antiferromagnetic state
of CaMnO3[5] In this compound, the antiferromagnetic coupling
between magnetic manganese ions results from their indirect
interaction via intermediate oxygen ion with fully occupied
p-orbi-tal So, the spin coupling of two nearest neighboring manganese ions was indirectly oriented by two anti-parallel spins in the full orbital of the neighboring oxygen ion.Fig 3shows the density of spin states of ion Mn4+in the bulk CaMnO3at the lattice parameter
a = 3.75 Å With spin-up and spin-down states of ion Mn4+ distrib-uted equally at the energy levels, it is clearly seen that the system
is antiferromagnetic one This also agreed with the density of spin states of total systems The insulating properties show clearly in the band structure with the band gap estimated 0.038 Ha (1.034 eV)
2.2 Rare earth-doped calcium manganese perovskite compounds The lattice constants of doping compounds are optimized by plotting total energy as a function of lattice parameter In Fig 4, the dependences of total energy on lattice parameter of La-doped and Ne-doped compounds are showed as typical examples The Fig 2 The total energy as function of lattice parameter of the cubic CaMnO 3
Fig 1 The modeling supercell of doped R 0.25 Ca 0.75 MnO 3 perovskite compounds
(R = La, Nd, Eu, Tb, Ho, and Y).
Fig 3 Density of spin states of ion Mn 4+
in the cubic CaMnO 3 with Fermi levels was shifted to zero (dashed line) Where, the blue and red lines stand for the spin-up and the spin-down states of the system CaMnO 3 , respectively Black and green lines stand for the spin up and spin down of d-level of ion Manganese (For interpretation
of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Trang 3similar calculations for other compounds were carried out by the
same way
The influence of rare earth elements on the structure change is
examined through the tolerance factor: f ¼pffiffi2<R AO >
<R MnO>, where hRAOi,
hRMnOi correspond to the mean distances from an ion A (Ca2+or
R3+ions) to an ion O2and from an ion Mn (Mn3+or Mn4+ions)
to an ion O2, respectively In the tight-binding model, hopping parameter t betweend3z2 r2anddx2 y2(denoted as e1
g and e2 g
Fig 5 Dependence of the tolerance factor f (a), hoping parameter t (b) and Jahn–Teller energy (c) on the rare earth ions radii for the doping R 0.25 Ca 0.75 MnO 3 compounds (R = La, Nd, Eu, Tb, Ho, Y).
Fig 6 The electron spin up (blue line) and down (red line) energy band structure for the R 0.25 Ca 0.75 MnO 3 systems (R = La, Nd, Eu, Tb, Ho, Y) at the symmetric G (0, 0, 0), F (0, 1/2, 0), Q (0, 1/2, 1/2), Z (0, 0, 1/2) points e 1 , e 2 correspond to d 3z 2 y 2 and d x 3 y 3 orbitals.
Trang 4of doping rare earth ion radii were given in Fig 5a, b and c,
respectively
As seen inFig 5a, the strongest deviation from the ideal
perov-skite structure (f = 1) is observed in the europium doping
com-pound As a consequence, the largest Jahn–Teller splitting is also
seen in Eu0.25Ca0.75MnO3(Fig 5c) The hoping parameter t seems
to reduce with an increasing of the substituted rare earth ions radii
(seeFig 5b) We see that not only the size of doping rare earth ions
but also its electronic structures cause non-monotonic change of
these parameters
Fig 6illustrates the changes of electronic band structures of the
doping crystals One can see fromFig 6that the Jahn–Teller effect
dominates over spin splitting of egband for the case of europium
doping In contrast, in the case of La doping, the spin splitting of
degenerated e2
g band is essentially larger than magnitude of the
Jahn–Teller effect In the other case (with R = Nd, Tb, Ho, Y), the
spin splitting occurs mainly on e1
gband; so that the magnitude of Jahn–Teller effect is not as large as the spin splitting
According to Zener’s double exchange (DE) model [1,6], the
hoping parameter t plays an important role to understand the
conductivity of manganese oxides where mixed valence states
of manganese ions (Mn3+/Mn4+) occur A larger hoping
parame-ter t corresponds to a higher hoping probability of electrons from
the occupied egorbital of Mn3+to the empty egorbital of Mn4+via
oxygen ion It means the higher conductivity For example,
shows largest change This change directly influences on the mag-nitude of Jahn–Teller effect, the electron hoping parameter t and the spin splitting of the degenerated egband The hoping parame-ter t has decreasing tendency with an increasing of the doped rare earth ion radius The Jahn–Teller splitting of the degenerated eg le-vel has biggest magnitude also in the case of the Europium substitution
Acknowledgments
We would like to thank the Project QGTD 09-05 (VNUH) for re-search support We also would like to thank the TRIG A project to financial publishing support
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[2] E.O Wollan, W.C Koehler, Phys Rev 100 (1955) 545.
[3] D Sousa, M.R Nunes, C Silveira, I Matos, A.B Lopes, M.E Melo Jorge, Mater Chem Phys 109 (2008) 311.
[4] P.Q Thanh, C.T.A Xuan, N.H Luong, B.T Cong, J Mag Mag Mater 310 (2007) e720.
[5] J.B Goodenough, Phys Rev 100 (1955) 564.
[6] C Zener, Phys Rev 81 (1951) 440.