Pis’ma v ZhETF, vol. 116, iss. 9, pp. 614 - 615

Theoretical modeling of high spin to low spin transition and structural

stability under pressure in CaFeO₃

A. O. Shorikov¹⁾

M. N. Mikheev Institute of Metal Physics of Ural Branch of the Russian Academy of Sciences, 620108 Yekaterinburg, Russia

Department of Theoretical Physics and Applied Mathematics, Ural Federal University, 620002 Yekaterinburg, Russia

Submitted 7 September 2022

Resubmitted 22 September 2022

Accepted 22

September 2022

DOI: 10.31857/S123456782221008X, EDN: lhjogt

Compounds with a perovskite structure have been

tum ESPRESSO package [8]. We use the exchange-

intensively investigated during the last decades due to

correlation potential in the form proposed by Perdew,

the variety of electronic and magnetic properties and

Burke, and Ernzerhof [9]. A similar approach was previ-

the great interest in their practical applications [1-7].

ously successfully applied for modeling structural tran-

CaFeO₃ is of a spetial interest due to unusual valence of

sition and for description of the evolution of magnetic

iron ions Fe⁴⁺ which is in a high spin (HS) state at am-

properties in correlated materials under pressure [10].

bient pressure with electronic configuration t^32ge^1g. Since

To take into account the correlation effects, U = 4.5 eV

the double degenerate e_g subshell is occupied by one

and J = 0.95 eV were applied to the Fe d-shell.

electron, the instability known as the Jahn-Teller effect

We consider three magnetic orders, namely AFM-A,

could arise. It is energetically favorable to lift the degen-

AFM-C, and AFM-G for both structures: P nma and

eracy by distorting the octahedron around the transition

P2₁/n. For each magnetic order full structure optimiza-

metal. Another way for lifting the degeneracy and mini-

tion was performed. The AFM-A ordering has the lowest

mizing the total energy is charge ordering or charge dis-

total energy for both structures, which agrees with ex-

proportionation (CD) which is observed in the CaFeO₃

perimental data for the monoclinic structure. One can

in the low temperature phase.

see that the AFM-A ordering is persistent against the

CaFeO₃ has a perovskite-like structure consisting of

structural transition and its traces could be observed in

corners-shared octahedrons FeO₆ and Ca ions in the

the P nma structure at high pressure. Optimization of

spaces of this 3D structure. The structure of CaFeO₃

the crystal structures in the framework of the GGA + U

is distorted by the tilt of the octahedrons, leading

method shows that both structures are stable at ambi-

to orthorhombic or monoclinic symmetry. Takeda et

ent pressure. The monoclinic structure P 2₁/n has a to-

al. showed that at room temperature CaFeO₃ has

tal energy 0.029 eV lower than the orthorhombic P nma

Pnma space group with the average Fe-O bond length

structure in agreement with the experimental phase di-

1.91870(6)Å. With decreasing temperature the CD

agram.

state occurs at TCD ≈ 290 K, that leads to a phase

A series of calculations with full relaxation of the

transition to the monoclinic structure with P 2₁/n space

crystal structure was performed for varios external pres-

group and with two inequivalent FeO₆ octaherons [1].

sures (cell volumes). The unit cell parameters obtained

The average Fe-O bond length for the small octahedron

in the GGA + U calculation are in a good agreement

is 1.87244(6)Å, and 1.97317(6)Åfor the larger one.

with the experimental data [1]. If the shape of the unit

At ambient pressure and temperature above 290 K

cell was not preserved during relaxation of the crystal

CaFeO₃ is paramagnetic without CD and all iron ions

structure, the orthorhombic phase keeps the symme-

are equivalent and have Fe⁴⁺ valence. With decreasing

try below 30 GPa only. At higher pressures FeO₆ oc-

temperature the CD state with Fe³⁺ and Fe⁵⁺ ions is

tahedrons become non-equivalent indicating that the

observed below 290 K, but CaFeO₃ remains paramag-

monoclinic structure might be favorable. The length

netic until the CD AFM phase occurs at T_N = 115 K.

of the Fe-O bonds at varios pressures obtained in

The GGA + U calculations were performed using

GGA + U is shown in Fig. 1. One can see that the

the pseudopotential method implemented in the Quan-

bond length is tending to the values obtained for mon-

oclinic phase, which gives two metastable structures

¹⁾e-mail: shorikov@imp.uran.ru

with non-equivalent octahedrons. However, high pres-

614

Письма в ЖЭТФ том 116 вып. 9 - 10

2022

Theoretical modeling of high spin to low spin transition and structural. . .

615

sure XRD shows that orthorhombic structure is stable

leading to the HS-to-LS transition at ≈ 35 GPa. How-

above 30 GPa at room temperature [2]. This result can

ever, the structural phase transition cannot be repro-

indicate that the transition is entropy driven and proper

duced in the framework of the GGA + U method. The

accounting of phonon entropy and temperature is neces-

explanation of this could be the following: the ground

sary. Similar results were obtained for transition under

state energy calculated in DFT + U assumes T = 0 K,

pressure in FeS [11]. It was shown recently, that tran-

whereas the impact of temperature and vibrational en-

sition from P 2₁/n to P nma structure under pressure

tropy [11] could be important for a proper description

could be reproduced in the framework of DFT + DMFT

of the phase transition under pressure in CaFeO₃.

method [12] at finite temperature without full relax-

Calculations were carried out within the state

ation of atomic positions but with accurate accounting

assignment of the Ministry of Science and Higher

for many body correlation effects, which underscores the

Education of the Russian Federation (theme “Electron”

importance of the temperature effects.

#122021000039-4) and supported by the Russian

Foundation for Basic Research (RFBR): grant

#20-42-660027.

This is an excerpt of the article “Theoretical mod-

eling of high spin to low spin transition and struc-

tural stability under pressure in CaFeO₃”. Full text

of the paper is published in JETP Letters journal.

DOI: 10.1134/S0021364022601993

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2022