Pis’ma v ZhETF, vol. 109, iss. 4, pp. 256 - 257

February 25

DFT and Mössbauer spectroscopy study of FeTe_0.5Se_0.5 single crystal

A. G. Kiiamov⁺¹⁾, D. A. Tayurskii⁺, F. G. Vagizov⁺, D. Croitori^∗, V. Tsurkan^∗×, H.-A. Krug von Nidda^×,

L.R.Tagirov^+◦∇

⁺Institute of Physics, Kazan Federal University, 420008 Kazan, Russia

^∗Institute of Applied Physics, MD-2028 Chisinau, R. Moldova

^×Experimental Physics V, Center for Electronic Correlations and Magnetism, University of Augsburg, 86135 Augsburg, Germany

^◦Zavoisky Physical-Technical Institute, Federal Research Center Kazan Scientific Center Russian Academy of Sciences,

420029 Kazan, Russia

^∇Institute of Applied Research, Tatarstan Academy of Sciences, 420111 Kazan, Russia

Submitted 11 December 2018

Resubmitted 11 December 2018

Accepted 12

December 2018

DOI: 10.1134/S0370274X19040118

Iron chalcogenides attract significant attention of the

surements were carried out with a continuous flow cryo-

scientific community after the discovery of superconduc-

stat (model CFICEV from ICE Oxford, UK).

tivity in iron based compounds [1]. Iron telluride is the

The ab initio calculations were based on density

parent compound for a substitutional series FeTe_1-xSe_x

functional theory (DFT) [7]. Exchange and correla-

of iron based superconductors. FeTe orders antiferro-

tion effects were accounted for by the generalized

magnetically [2], whereas the substitution of Se at the

gradient approximation (GGA) as parametrised by

Te site in FeTe introduces superconductivity and simul-

Perdew, Burke, and Ernzerhof (PBE-sol) functional [8].

taneously suppress the antiferromagnetic ordering [3, 4].

The Kohn-Sham equations were solved with projector-

The highest superconducting temperature for this Fe

augmented wave (PAW) potentials and wave func-

chalcogenide series is achieved at approximately 15 K

tions [9] as implemented in the Vienna Ab-Initio Simula-

for FeSe_0.5Te_0.5 at ambient conditions [3, 4]. It should

tion Package (VASP) [10], which is a part of the MedeA

be noted, that iron telluride has a strong tendency

software of Materials Design [11].

to off-stoichiometry of iron, and such off-stoichiometry

The obtained from ab initio calculations hyperfine

is clearly detected by Mössbauer spectroscopy experi-

parameters for FeTe_0.5Se_0.5 with nine percent occu-

ments [5]. In the FeTe iron atoms can occupy two dif-

pancy factor for Fe2 centers are similar to those of iron

ferent crystallographic positions. Iron atoms in the first

atoms presented for Fe_1.05Te in [5]. Three groups of iron

position (2a) form in-plane Fe-Te layers with tellurium

atoms with different hyperfine parameters can be identi-

atoms while in the second position (2c) interstitial iron

fied. One of them (Fe2 group) is formed by only Fe2 in-

atoms are located in the interlayer space between the

terstitial iron atoms, whereas the other two are the first

Fe-Te layers. It was shown that even a small amount of

(Fe1/1 group) and the second (Fe1/2 group) coordina-

excess iron atoms leads to a modification of the elec-

tion rings around the Fe2 consisting of four and eight

tronic and magnetic properties of Fe_1+y Te [5]. An iron

Fe1 atoms, respectively.

off-stoichiometry is also typical for Fe_1+yTe_0.5Se_0.5 com-

The room-temperature Mössbauer spectrum is pre-

pounds [6]. In the present study we investigate the influ-

sented in Fig.1. In accordance with the ab inito re-

ence of iron off-stoichiometry on the magnetic state of

sults, it is reasonable to model the Mössbauer spectrum

FeTe_0.5Se_0.5 compound combining ab initio calculations

with three doublets. Each of the doublets is character-

and Mössbauer spectroscopy experiments. The refined

ized by isomer shift (IS), quadrupole splitting value,

stoichiometry for iron were 0.907(3) and 0.093(3) for

angle θ, and partial area (A). Under the assumption

Fe1 and Fe2 ions [6].

that the Lamb-Mössbauer factor is the same for all iron

Mössbauer effect measurements were carried out

atoms in the compound, the distribution of partial ar-

at temperatures of 5 and 295 K using a conventional

eas should represent the ratio between number of iron

constant-acceleration spectrometer (WissEl, Germany)

nuclei in different groups (one, four, and eight nuclei in

with⁵⁷Co as γ-radiation source. Low-temperature mea-

Fe2, Fe1/1, and Fe1/2 groups, respectively). Initial val-

ues of quadrupole splitting (QS) and θ for the fitting

procedure were taken from our ab initio calculations,

¹⁾e-mail: AiratPhD@gmail.com

whereas the initial value for the isomer shift was taken

256

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2019

DFT and Mössbauer spectroscopy study of FeTe_0.5Se_0.5 single crystal

257

groups, and the excess iron atoms Fe2 affect the charge-

density distribution up to a third coordination ring as

it was observed in Fe_1.05Te [5]. The low-temperature

magnetic state could be characterized by a distribution

of hyperfine fields on⁵⁷Fe nuclei, which may indicate a

distribution of magnetic moment values of iron atoms

of different groups. The hyperfine field and magnetic

moment distributions may also indicate an incommen-

surate spin-density waves phase, which coexists with su-

perconductivity as it was observed in other iron-based

superconductors [15].

Mössbauer spectroscopy experiments and ab initio

calculations were funded by Russian Foundation for Ba-

sic Research according to the research project # 18-32-

00342.

Full text of the paper is published in JETP Letters

journal. DOI: 10.1134/S0021364019040027

Fig. 1. (Color online) Room-temperature Mössbauer spec-

trum of single-crystalline FeTe0.5Se0.5 with nine percent

Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono,

occupancy factor of Fe2 centers (black symbols) and sub-

J. Am. Chem. Soc. 130, 3296 (2008).

spectra (colored) of the fitting (red line) corresponding to

F. Ma, W. Ji, J. Hu, Z.-Y. Lu, and T. Xiang, Phys. Rev.

various groups of iron atoms

Lett. 102, 177003 (2009).

M. H. Fang, H. M. Pham, B. Qian, T. J. Liu, E. K. Vehst-

from [5]. During the fitting procedure the IS, QS and θ-

edt, Y. Liu, L. Spinu, and Z. Q. Mao, Phys. Rev. B 78,

angle values were changed to reproduce the experimen-

224503 (2008).

A. V. Fedorchenko, G. E. Grechnev, V. A. Desnenko,

tal spectrum by the proposed model. It could be seen

A. S. Panfilov, S. L. Gnatchenko, V.V. Tsurkan,

from Fig. 1 that the model, based on ab initio calcula-

J. Deisenhofer, H.-A. Krug von Nidda, A. Loidl,

tions, describes well the experimental Mössbauer spec-

D. A. Chareev, O. S. Volkova, and A.N. Vasiliev, Low

trum of FeTe_0.5Se_0.5 with nine percent occupancy factor

Temp. Phys. 37, 83 (2011).

of Fe2. Therefore we can conclude that the iron atoms

A. G. Kiiamov, Y.V. Lysogorskiy, F. G. Vagizov,

in the this compound are divided into three groups.

L. R. Tagirov, D. A. Tayurskii, D. Croitori, V. Tsurkan,

The low-temperature Mössbauer spectrum of

and A. Loidl, Annalen der Physik 529, 1600241 (2017).

FeTe_0.5Se_0.5 with nine percent occupancy factor of Fe2

V. Tsurkan, J. Deisenhofer, A. Günther, Ch. Kant,

centers exhibits a complex shape. The combination

M. Klemm, H.-A. Krug von Nidda, F. Schrettle, and

of three magnetic sextets, an approach based on the

A. Loidl, Eur. Phys. J. B 79, 289 (2011).

presence of three different iron centers, was not able

P. Hohenberg and W. Kohn, Phys. Rev. 136, B864

(1964).

to reproduce the experimental spectrum. The shape of

J. P. Perdew, K. Burke, and M. Ernzerhof, Phys Rev.

the spectrum is similar for that obtained for the spin

Lett. 77, 3865 (1996).

density waves phases (SDW) [12].

P. E. Blöchl, Phys. Rev. B 50, 17953 (1994).

The short-range ferromagnetic/antiferromagnetic

10.

G. Kresse and J. Furthmüller, Phys. Rev. B 54, 169

correlations between nearest-neighbour spins in

(1996).

Fe_1+yTe_1-xSe_x compounds were reported in [13]. It

11.

MedeA, Materials Design, Inc., Angel Fire, NM, USA,

was proposed, that it could imply the coexistence and

2015.

competition between SDW order and superconductivity

12.

A. Blachowski, K. Ruebenbauer, P. Zajdel, E. E. Ro-

in this system [13]. Because of the significant fraction

driguez, and M. A. Green, J. Phys. Condens. Matter

of the excess interstitial iron atoms, it is reasonable to

24(38), 386006 (2012).

use a distribution of magnetic sextets with different

13.

J. Wen, G. Xu, Zh. Xu, Z. W. Lin, Q. Li, W. Ratcliff,

values of hyperfine magnetic field on⁵⁷Fe nuclei. The

G. Gu, and J. M. Tranquada, Phys. Rev. B 80, 104506

(2009).

average value of the hyperfine field, corresponding to

14.

M. Kurokuzu, S. Kitao, Y. Kobayashi, M. Saito, R. Ma-

the distribution, was 110 kOe. This result is in a good

suda, T. Mitsui, Y. Yoda, and M. Seto, Hyperfine Inter-

agreement with the average hyperfine field value, 〈H〉,

actions 239, 9 (2018).

reported for F_1.1Te compound in [14].

15.

M. H. Fang, H. M. Pham, B. Qian, T. J. Liu, E. K. Vehst-

In conclusion, our results demonstrate that the iron

edt, Y. Liu, L. Spinu, and Z. Q. Mao, Phys. Rev. B 78,

atoms in FeTe_0.5Se_0.5 with nine percent occupancy fac-

224503 (2008).

tor for interstitial Fe2 centers are divided into three

Письма в ЖЭТФ том 109 вып. 3 - 4

2019