Transcript Document

Special electronic structures of inverse spinels
LiMVO4(M=Ni and Cu): a first-principles study
S.Li (李晟) and Z.Q.Yang (杨中芹)
Department of Physics, Fudan University, Shanghai 200433, China
Motivation
Calculation Methods and Models
Recently, LiMVO4(M=Ni and Cu) are typical inverse spinels which have
attracted considerable attention due to their abundant and fantastic properties,
such as long-range orbital ordering and multiferroicity. LiNiVO4 and
LiCuVO4 have completely different magnetic behaviors due to random
distribution of Li and Ni in LiNiVO4 and ordered Li and Cu in LiCuVO4. But
there was no investigation on the mechanisms of the disorder/order of those
cations in the compounds yet. We would like to study the electronic structures
of them systematically from first-principles calculations.
Fig.1Crystal structure of LiMVO4(the left is LiNiVO4 and the other is LiCuVO4
Calculation method: The density functional theory calculations using VASP code. Valence
electrons were described by a plane wave basis set with the energy cutoff of 460 eV and
valence-core electron interactions were treated with projector augmented wave(PAW)
method at the level of generalized-gradient approxiamtion (GGA).16x16x16 k-point grids
were used for the calculations with primitive unit cell. To improve the converge in the
eigenstates at the Fermi level(EF), a Gaussian smearing of sigma=0.01 eV was applied.
The on-site Coulomb interactions were considered for Ni and Cu 3d states with the
parameters of U=6.0eV and J=0.8eV. Only commensurate(collinear or noncollinear)
magnetic ordering was considered in our calculations.
Fig.2 Total energy per unit cell versus lattice constant for LiNiVO4 and LiCuVO4
Results and Discussion
Energy(eV)
LiNiVO4
LiCuVO4
Space group
Li/M
M/Li
Fd3m
Imma
Fd3m
Imma
-97.720
-94.556
-93.644
-93.974
-97.720
-96.441
-93.644
-92.763
Since Li 2S electron is almost
lost in both LiNiVO4 and
LiCuVO4, it mainly occupies
above EF. It was not shown in
the DOS figure. When U is not
considered, there generates a
gap of 0.4eV near the EF, which
is formed mainly by the energy
interval of the occupied and
unoccupied Ni 3d states near the
EF.
Table-1 energy difference between two structures of LiMVO4
The total energies of LiMVO4(M=Ni and Cu) with different geometric structures.
“Li/M” →“M/Li” means Li ions exchange positions with M in the lattice. The bold
space group in Table I gives the ground state of the compound.
It is the stable cubic structure of LiNiVO4 that gives rise to random distribution of
Li/Ni ions in the lattice. It is expected that Li/Cu may also occupy randomly when
LiCuVO4 takes cubic structure at certain conditions.
LiCuVO4
GGA
NM
246.4
FM
72.4
AFM
70.4
A
Fig.3 Calculated total and partial densities of
states for AFM LiNiVO4 within GGA and
GGA+U methods, respectively.
NCM
B C D E
15.4 13.7 0 0 0
Table-2 Total energies(in meV) for different magnetic structures for LiCuVO4
based on GGA calculations without U or SOC considered.
Fig.4 Different types of NCM structures considered for LiCuVO4. For each
case, the left Cu line corresponds to line(I) marked in the right panel in
Fig.1,while the right Cu line,line(II)
After U is added into the calculations, the gap is widened to
2.3eV since the occupied and
unoccupied Ni 3d states both
move away from the EF with U
considered.
LiCuVO4(NCM)
A
B
C
D
E
GGA+SOC
GGA+U
37.3
26.3
0.6
19.2
0.0
17.2
0.0
17.2
0.7
17.2
GGA+U+SOC
9.2
2.0
2.6
2.5
0.0
Table-3 Total energies(in meV) for the five NCM structures shown in Fig.4 for
LiCuVO4.The lowest energies in the cases with U and without U are set as zero.
Fig.5 Calculated total and partial
densities of states for LiCuVO4
corresponds to the ground state of
NCM-(e) within GGA and GGA+U
methods, respectively.
Our calculation show that when the
on-site Coulomb interaction and the
SOC, which play a very important
role in the electronic structures of
such kind of inverse spinels. are
considered together, the correct NCM(e) ground state can be produced in
calculations.
Conclusion
• We studied the electronic structure of LiMVO4(M=Ni and Cu)from first-principles calculations.
• We find the cubic ground-state structure causes the Li and Ni ions to randomly distribute in LiNiVO4.Antiferromagnetic structure was found to be ground state of
magnetism for LiNiVO4 at low temperature.
• Semiconductor bands with a band gap of 1.5eV are obtained for LiCuVO4 under this noncollinear magnetic structure. To obtain this noncollinear ground state,
both the on-site Coulomb interaction and the spin-orbit coupling need to be employed in the calculations. The most stable xy orbit can rationalize why the spin of
Cu2+ ions rotates only along xy plane after the spin-obit coupling is considered.
•References
[1] B.J.Gibson, R.K.Kremer et al. Incommensurate antiferromagnetic order in the S=1/2 quantum chain compound LiCuVO4 Physica B 350(2004)e253-256
[2] C.Gonzalez,M.Gaitan et al. Structure and magnetic properties of LiMVO4(M=Co,Ni,Cu) spinels Journal of Materials Science 29 (1994) 3458-3460
[3] R.S.Liu, Y.C.Cheng, et al. Crystal and electronic structures of inverse spinel-type LiNiVO4 Materials Research Bulletin 36(2001)1479-1486
[4] H.J.Xiang and M.-H. Whangbo Density-Functional Characterization of the Multiferroicity in Spin Spiral Chain Cuprates PhysRevLett.99.257203