2
B.G. Ganga, C. Ganeshraj and P. N. Santhosh*
Low Temperature Physics Laboratory, Department of Physics, Indian Institute of Technology Madras, Chennai-
600036, Tamilnadu, India.
* santhosh@physics.iitm.ac.in, Tel.: +91-44-2257 4882; Fax: +91-44-2257 4852
A detailed analysis of the magnetic structure of marcasite FeSe
2
is performed using density functional theory. Calculations were performed with Local density Approximation and Generalized Gradient
Approximation functionals and the effect of Hubbard
U eff parameter is studied.
Keywords: DFT,Ttransition metal chalcogenides, FeSe
2
Transition metal dichalcogenides, TX
2
, (T =
Mn, Fe, Co, Ni, Cu, Zn and X = S, Se and Te) show remarkable electronic and magnetic properties, ranging from semiconductor to metal and from ferro (anitferro) with U eff
= 1 and 2. Even after including U eff
parameter,
FeSe
2
is unable to support an AFM arrangement. For
U eff
= 2, FM state calculations predict a spin polarized solution, but calculated a and c parameters show large deviations from experimental values. Density of states
(FM, U eff
= 2) depicts a metallic behaviour and Fe-Se bond length increases, accompanying a decrease in its covalent nature. Even though lattice parameters of DM state remain greatly unaffected with increase in U eff parameter values, an increase in band gap is predicted from 0.57 eV to 1.02 eV. magnetism to paramagnetism [1]. Iron dichalcogenides are known to be semiconductors 2 . According to
Hullinger et. al., Fe ion is in low spin 3d 6 (t
2g
6 , e g
0 ), predicting a nonmagnetic FeSe
2
[2]. But, some of the experimental results disagree with this concept, where weak ferromagnetic signal was observed for FeSe
2
[3].
Density Functional Theory (DFT) calculations were performed using CASTEP within Generalized
Gradient Approximation (GGA) and Local Density
Approximation (LDA) formalisms [4].
Three magnetic configurations have been chosen namely, nonmagnetic (NM), antiferromagnetic
(AFM) and ferromagnetic (FM), where NM state corresponds to non-spin polarized solution in which Fe ion is in low spin state and FM and AFM states correspond to spin-polarized Fe ions with their spins in parallel and antiparallel configurations, respectively.
GGA and LDA calculations reveal that FeSe
2
is unable to support either FM or AFM, as it finally collapses into a nonmagnetic state with zero Fe ion spin and has the same energy as NM structure. If the spin on each Fe ion is fixed to 4, calculations result in a ferromagnetic solution for FM, whereas AFM structure converges to a non-magnetic solution with a non-zero Fe spin. NM is more stable and the energy difference between FM and
NM is 1.9 eV and it is revealed that all the configurations tend to converge to NM, unless forced the spin to stay in a high spin state. Hubbard U eff parameter has been introduced to account for electronelectron correlation and GGA calculations were tested
Figure 1: calculated DOS of FeSe2 in NM and FM configurations with U eff
= 2
We have used DFT based methodologies and functionals to study the magnetic structure of FeSe
2
. We found that FeSe
2
is unable to support either FM or AFM and the stable structure is a nonmagnetic semiconductor.
CASTEP.
Authors thank DBT for research funding for
[1] V. H. McCann and J. B. Ward, “A 57 Fe Mössbauer study of four pyrite compounds: CoS
2
, NiS
2
, CoSe
2
and
NiSe2”, J. Phys. Chem. Solids , 38 (1977) 991
[2] F. Hulliger and E. Mooser, “Semiconductivity in pyrite, marcasite and arsenopyrite phases”, J. Phys.
Chem. Solids, 26 (1965) 429
[3] G. Fischer, “FeSes, A Semiconductor containing iron”, Can. J. Phys ., 36 (1958) 1435
[4] S. J. Clark, M. D. Segall, C. J. Pickard, P. J.
Hasnip, M. I. J. Probert, K. Refson, and M. C. Payne,
“First principles methods using CASTEP”, Z.
Kristallogr . 220 (2005) 567