Lattice dynamics of FeMnP0.5Si0.5 compound from first principles calculation

doi:10.1016/j.jmst.2018.09.009

Abstract

Abstract:

Understanding the role of lattice vibrations on first-order magnetic transitions is essential for their fundamental description, as well as for the optimization of the related functional properties. Here, we present a first principles study on the lattice dynamics of the MnFeP_0.5Si_0.5 compound. The phonon spectra are obtained by Density Functional Theory (DFT) calculations in combination with frozen phonon method. DFT calculations reproduce most of the features observed in experiments including the lattice softening across the magnetic phase transition and the pronounced shift of phonon peak. The site projected phonon density of states (pDOS) shows that the local vibrations of Mn atoms have an essential contribution to the overall lattice softening. Moreover, the local lattice vibrations of Mn atoms are rather featureless in the paramagnetic state (PM) and thus the total pDOS evolution across the transition appears to be dominated by Fe. The lattice vibrations of both Fe and Mn in the PM state are very sensitive to the local environment, which shows that the magnetic order and the local chemical environment are strongly coupled in this compound.

Key words: Lattice dynamics, Magnetic phase transition, Phonon softening, First principles calculation

B. Wurentuya, Shuang Ma, B. Narsu, O. Tegus, Zhidong Zhang. Lattice dynamics of FeMnP_0.5Si_0.5 compound from first principles calculation[J]. J. Mater. Sci. Technol., 2019, 35(1): 127-133.

Figures/Tables 10

Fig. 1. Supercell structure of the FeMnP0.5Si0.5 compound.

Table 1 Lattice parameters, volume, bulk modulus, local magnetic moment, atomic position of Fe 3f (x1, 0, 0) and Mn 3?g(x2, 0, 0.5), Debye temperature.

		a (?)	c (?)	V (?³)	B (GPa)	μ_Fe (μ_B)	μ_Mn (μ_B)	x₁	x₂	θ_D (K)
FM	cal. (0?K)	6.1666	3.1993	105.36	188.5	1.499	2.854	0.2589	0.5980	593
FM	exp. (295?K)	6.1949	3.3040	109.81	-	-	-	-	-	405^a
PM	cal. (0?K)	5.8915	3.4957	105.08	171.2	0.795	2.723	0.2529	0.5832	564
PM	Exp. (450?K)	6.0736	3.4676	110.58	-	-	-	-	-	374^a

Fig. 2. Phonon dispersion and the density of state of the FeMnP0.5Si0.5 compound. (a) Phonon dispersion in the ferromagnetic state; (b) phonon dispersion in the paramagnetic state; (c) comparison of phonon density of states in different magnetic states.

Fig. 3. Calculated Fe specific (a) and Mn specific (b) phonon density of state (pDOS) in different magnetic states for FeMnP0.5Si0.5. The pDOS difference (pDOSPM- pDOSFM) is shown for Fe (c) and Mn (d) with the units of the horizontal axis converted to meV.

Fig. 4. Site projected phonon density of state (pDOS) of four nonequivalent Fe sites in the model FeMnP0.5Si0.5 compound: (a) Fe-1, (b) Fe-2, (c) Fe-3, (d) Fe-4. pDOS curves are compared for different magnetic states.

Fig. 5. Site projected phonon density of state (pDOS) of four nonequivalent Mn sites in the model FeMnP0.5Si0.5 compound: (a) Mn-1, (b) Mn-2, (c) Mn-3, (d) Mn-4. pDOS curves are compared for different magnetic states.

Fig. 6. Mn specific, Fe specific and total phonon density of states (pDOS) of FeMnP0.5Si0.5. (a) Mn pDOS constructed from Mn-1 and Mn-3; (b) Fe pDOS constructed from Fe-1 and Fe-3; (c) total pDOS just includes Fe-1, Fe-3, Mn-1 and Mn-3. The pDOS was normalized to unity.

Fig. 7. Element resolved electronic density of state in the PM and FM states. (a) Fe-1 and Fe-3 in the PM state, (b) Mn-1 and Mn-3 in the PM state, (c) Fe-1 and Fe-3 in the FM state, and (d) Mn-1 and Mn-3 in the FM state. The majority channel is denoted by positive values, the minority spin channel by negative values.

Table 2 Bader charge difference (CPM-CFM) across FM to PM phase transition.

j	Fe_j	Mn_j	Si_j	P_j
1	0.0365	-0.0304	0.0258 (1b)	-0.0331 (1b)
2	0.1626	0.0115	-0.1330 (2c)	0.0048 (2c)
3	0.0554	-0.0213	-0.1336 (2c)	0.0044 (2c)
4	0.0182	-0.0080	-	-

Fig. 8. Temperature dependence of (a) vibrational entropy difference ΔSvib = SPM - SFM and (b) heat capacity difference ΔCP=CPPM-CPFM and ΔCV=CVPM-CVFM.

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