# Lattice Dynamics of Gd1−xYxMn2O5 Investigated by Infrared Spectroscopy

Authors: Javed Ahmad; Jawaria Mansoor; Mehr Khalid Rehmani; M. Tufiq Jamil; Syed Hamad Bukhari
Journal: Advances in Materials Science and Engineering
 (2017)

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## Abstract

We present infrared (IR) reflectivity ofGd1-xYxMn2O5 with x = 0, 0.2, 0.4, 0.6, 0.8, and 1 in the frequency range 30–1000 cm−1. A total of 18 IR active phonons were observed for GdMn2O5 (x=0) and three additional phonons have been observed with increasing x, marking a total of 21 phonons in YMn2O5 (x=1). A systematic investigation was performed to map out the structural distortion through the lattice vibration and discuss the consequences of frequency shifts in phonon modes. In addition, we have calculated the real part of optical conductivity (σ1(ω)) which reflects the semiconducting nature of Gd1-xYxMn2O5.


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## Body

## 1. Introduction

Multiferroic materials are promising for future technology due to the simultaneous existence of electric and magnetic orders [1, 2]. Multiferroic RMn2O5 received much attention due to strong coupling between magnetic and ferroelectric orders, puzzling magnetic structure, large ferroelectric polarization, and interesting physics [3–5]. At ambient condition, RMn2O5 has an orthorhombic structure with Pbam space group for a broad range of R ions as shown in Figure 1 [6, 7]. The MnO6 octahedra are linked in the form of an infinite chain parallel to c-axis. However, the chains of MnO6 are cross-linked with pyramidal MnO5 with an edge-sharing. The R atoms form an eightfold coordination to oxygen atoms. From high resolution diffraction method it has been found that lattice parameters of RMn2O5 system show no significant effect on field or temperature variation [8]. However, the low temperature (below TN~40–45 K) behavior is quite complex, originating from magnetic interaction between 4f and 3d magnetic moment of R and Mn3+/Mn4+ ions, respectively. Below TN, RMn2O5 shows various phase transitions upon temperature variation [5, 9].Figure 1
Crystal structure ofRMn2O5.In the family ofRMn2O5, GdMn2O5 is a unique multiferroic with largest polarization under strong magnetic field among the known multiferroics [3]. Very recently, we have observed a magnetodielectric effect in the paramagnetic phase of GdMn2O5 [10] as well as in DyMn2O5 [11]. This unusual magnetodielectric behavior in GdMn2O5 is consistent with the softening of Raman active phonons, indicating a distinct magnetic correlation slightly above TN [12]. Moreover, the softening of infrared active modes has also been observed in DyMn2O5 above TN [13]. In case of GdMn2O5, magnetoelectric effect and spin-lattice coupling are commonly acknowledged in several investigations [4, 10, 12, 14]; little is known about the correlations between structural distortion and lattice dynamics. Infrared spectroscopy demonstrates the local lattice distortion and provides the information about the flexible crystalline lattice. Several investigations on IR reflectivity of RMn2O5 (R=Tb [15, 16], Dy [13, 17], Ho [18], and Bi [19]) have been performed. In this regard, we are motivated to conduct a systematic investigation of Gd1-xYxMn2O5 and probe the structural distortion through IR phonons. This system is interesting due to many aspects: (i) large difference in ionic radius of Y (rY=1.019 Å) and Gd (rGd = 1.05 Å), (ii) large difference in ionic mass of Y (mY=89 amu) and Gd (mGd = 157 amu), and (iii) Gd having strongest magnetic moment (4f) and Y being nonmagnetic. It is noteworthy to mention that the intermediate members (x = 0.2, 0.4, 0.6, and 0.8) of Gd1-xYxMn2O5 are prepared for the first time, to the best of our knowledge, and are expected to have the same crystal structure as the end members. A detailed investigation of Raman and infrared measurements on Eu1-xYxMnO3 system has shown increase in structural distortion from orthorhombic to hexagonal with increasing x (x>0.5) [20]. As majority of vibrational modes are sensitive to structural distortion induced in both Mn polyhedra and with respect to R centers, thus a significant change in the ionic mass and radii at R site would be interesting.The question whether the continuous substitution of Y into Gd site may lead to any kind of disorder effect due to large difference in mass and ionic radius has not been addressed yet. Therefore, we present lattice dynamics study ofGd1-xYxMn2O5 using the IR reflectivity spectroscopy. The study of optical phonons and their correlation to distortion allows us to have closer look on structural evolution in Gd1-xYxMn2O5.


## 2. Experiment

G
d
1
-
x
Y
x
M
n
2
O
5 ceramic samples have been synthesized by using sol-gel method, similar to other RMn2O5 compounds [10, 11, 21]. Fourier transform infrared (FTIR) spectrometer (Vertex 80v) has been used to measure the IR reflectivity. Pellets of 13 mm diameter were made smooth prior to spectroscopic measurements. IR reflectivity has been measured in the frequency range of far (30–680 cm−1) and mid (550–7500 cm−1) infrared regions at room temperature under vacuum purge. Detail of measurements can be seen elsewhere [22].


## 3. Results and Discussion

Figure2 demonstrates X-ray diffraction (XRD) patterns for the composition Gd1-xYxMn2O5 which exhibit the quite similar phase sequences for all concentration x. The obtained XRD patterns have been analyzed by using the Rietveld refinement and the calculated lattice parameters are given in Table 1. A peak at 2θ~28 has been identified only for intermediate member of Gd1-xYxMn2O5 (x = 0.2, 0.4, 0.6, and 0.8) and is attributed to an additional peak as it was not fitted with the Rietveld refinement (Figure 2). In addition, the peaks at 2θ~32 were only found in x=0 and x=1 and well fitted with the Rietveld refinement which remain absent for the intermediate x (Figure 2). However, crystal structure of all members of Gd1-xYxMn2O5 was found to be orthorhombic and the obtained lattice parameters for end members (GdMn2O5 and YMn2O5) of the series Gd1-xYxMn2O5 are in excellent agreement with reported values [23]. As observed, the peak positions slightly shift towards low 2θ values with increasing x. Moreover, lattice parameters show a nonmonotonic dependence on x which may be caused by the substitution of small ionic radius of Y into Gd ions which leads to affecting the bond length and bond angle of Mn-O-Mn [9]. These facts are manifesting a structural distortion, which will be of particular focus through IR spectroscopic analysis.Table 1
Summary of lattice parameters and refinement data forGd1-xYxMn2O5.
X
0
0.2
0.4
0.6
0.8
1
a (Å)
7.35(6)
7.27(6)
7.28(2)
7.29(5)
7.33(5)
7.25(2)
b (Å)
8.54(6)
8.46(3)
8.48(6)
8.48(4)
8.21(2)
8.44(2)
c (Å)
5.69(2)
5.66(4)
5.66(3)
5.64(3)
5.62(5)
5.65(4)
V (Å3)
357.1
348.2
349.5
348.7
340.2
345.8
R
P
6.57
11.05
11.10
10.62
11.63
6.85
R
W
P
8.23
13.92
14.03
13.48
14.78
8.62
GOF
1.04
1.06
1.07
1.08
1.11
1.03Figure 2
XRD pattern ofGd1-xYxMn2O5 (x=0, 0.2, 0.4, 0.6, 0.8, and 1) at room temperature. ∗ shows the additional peaks not fitted with Rietveld refinement.Figure3 demonstrates the IR reflectivity spectra obtained for Gd1-xYxMn2O5 at room temperature. The experimental spectra were fitted by using Lorentz oscillator model that correlates the optical reflectivity (R) with the dielectric function by Fresnel’s formula as(1)Rω=ϵω-1ϵω+12.Figure 3
Infrared reflectivity ofGd1-xYxMn2O5 (x=0, 0.2, 0.4, 0.6, 0.8, and 1) at room temperature. The black open circle shows the experimental curve and the red line shows the fitted data. The spectra are vertically shifted for clarity.To quantify the infrared phonon contribution, the dielectric functionϵ(ω) is defined as(2)ϵω=ϵ∞+∑jωTO,j2SjωTO,j2-ω2-iωγj,where ϵ∞ is the high frequency dielectric constant indicating the contribution to the electronic polarization. ωTO(j), Sj, and γj are the optical phonon frequency, oscillator strength, and damping factor of jth phonon, respectively. Both (1) and (2) together can give the measured reflectivity spectra. The dispersion parameters (ωTO(j), Sj, and γj) obtained from the best fit to the experimental curve of Gd1-xYxMn2O5 are summarized in Tables 2 and 3. According to lattice dynamic calculations there are 36 IR active modes in RMn2O5 system; however we were able to observe maximum 21 phonons in YMn2O5. The origin of this discrepancy simply lies in the polycrystalline nature of our samples having mixed response of all crystallographic axes.Table 2
Phonon-fit values of transverse optical modeωTO (given in cm−1) of the IR reflectivity spectra of Gd1-xYxMn2O5 with x = 0, 0.2, 0.4, 0.6, 0.8, and 1 at room temperature.
ωTO
X
Assignment
0
0.2
0.4
0.6
0.8
1
ωTO1
151
165
163
156
161
166
Relative motion of Mn polyhedra and Gd(Y) ions
ωTO2
167
—
—
—
—
176
ωTO3
194
190
185
166
181
191
ωTO4
217
217
217
216
218
215
ωTO4′
—
228
—
—
—
229
Mn-O bending motions inMnO6 octahedra
ωTO5
251
255
256
252
255
255
ωTO6
260
267
266
261
265
267
ωTO7
291
286
282
274
278
289
ωTO8
327
328
329
325
329
323
Mn-O twisting motions in Mn polyhedra
ωTO9
371
370
368
368
370
370
ωTO9′
—
380
381
378
379
380
ωTO10
402
405
406
404
404
399
ωTO11
433
445
443
439
444
434
ωTO12
462
464
466
462
463
468
Mn-O bending motions within equatorial
ωTO12′
—
—
—
—
—
486
M
n
O
2 equatorial in MnO6 octahedra
ωTO13
484
494
488
486
485
498
ωTO14
511
519
516
514
514
522
ωTO15
534
540
536
535
533
545
Mn-O stretching motions inMnO6 octahedra
ωTO16
565
569
565
565
563
576
ωTO17
628
633
630
629
629
639
ωTO18
684
692
681
685
688
697
ε∞
2.76
2.95
3.45
2.87
2.92
1.98
ε∘
5.02
5.31
5.45
4.87
5.37
3.41Table 3
Phonon-fit values of oscillator strength (Sj) and damping factor (γj) of Gd1-xYxMn2O5 with x = 0, 0.2, 0.4, 0.6, 0.8, and 1 at room temperature.
S
j
X
γj
X
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
S
1
0.20
0.14
0.22
0.06
0.25
0.08
γ
1
9.29
7.60
11.22
5.13
14.52
7.69
S
2
0.28
—
—
—
—
0.05
γ
2
8.51
—
—
—
—
5.05
S
3
0.11
0.25
0.28
0.02
0.21
0.17
γ
3
15.14
11.63
16.44
4.14
18.15
11.22
S
4
0.02
0.06
0.04
0.06
0.13
0.02
γ
4
4.46
5.83
4.98
9.10
8.25
11.42
S
4
′
—
0.03
—
—
—
0.03
γ
4
′
—
6.28
—
—
—
8.48
S
5
0.05
0.05
0.11
0.06
0.13
0.02
γ
5
4.73
6.80
10.12
6.07
11.35
7.20
S
6
0.22
0.32
0.27
0.24
0.17
0.17
γ
6
18.26
18.35
14.56
17.75
13.43
23.19
S
7
0.01
0.05
0.05
0.02
0.03
0.04
γ
7
10.77
8.09
6.34
7.39
5.94
11.02
S
8
0.05
0.05
0.02
0.18
0.02
0.12
γ
8
9.73
11.12
4.37
43.19
4.06
34.58
S
9
0.27
0.13
0.04
0.16
0.09
0.02
γ
9
27.28
15.93
5.78
19.15
8.80
15.02
S
9
′
—
0.05
0.03
0.03
0.07
0.01
γ
9
′
—
9.02
3.83
7.45
7.49
5.11
S
10
0.10
0.09
0.16
0.08
0.14
0.01
γ
10
12.72
10.93
11.31
11.74
11.07
7.43
S
11
0.12
0.04
0.06
0.08
0.16
0.01
γ
11
27.72
11.29
9.01
25.40
21.08
4.16
S
12
0.10
0.24
0.28
0.20
0.46
0.10
γ
12
18.08
20.01
12.50
26.73
20.86
33.51
S
1
2
′
—
—
—
—
—
0.03
γ
1
2
′
—
—
—
—
—
12.86
S
13
0.20
0.12
0.15
0.22
0.16
0.09
γ
13
18.25
11.10
9.55
16.25
11.94
17.59
S
14
0.01
0.02
0.06
0.02
0.04
0.01
γ
14
7.15
7.97
10.57
7.50
9.61
7.12
S
15
0.16
0.23
0.34
0.20
0.25
0.09
γ
15
17.31
17.99
18.12
17.51
18.29
27.73
S
16
0.23
0.25
0.17
0.24
0.14
0.10
γ
16
36.58
44.36
29.60
38.71
31.79
46.84
S
17
0.08
0.10
0.13
0.10
0.08
0.04
γ
17
23.22
21.17
26.74
21.75
23.69
25.80
S
18
0.02
0.02
0.05
0.02
0.02
0.14
γ
18
32.57
30.59
51.16
32.10
25.94
28.78In order to elucidate the lattice dynamics of individual phonon, we have analyzed the ionic displacement within the crystal. Vibrational properties are better understood in the limits of harmonic oscillatorω=(k/μ)1/2, where k denotes force constant and μ is the reduced mass of the ions involved in the corresponding phonon mode. Thus it is natural to expect that low frequency phonons (≤200 cm−1) are attributed to the vibrations of Gd(Y) and Mn ions, while the intermediate phonon modes are due to the bending and twisting motion of Mn-O polyhedra. At higher frequencies (≥450 cm−1), stretching of the MnO6 octahedra (including the bending motion of equatorial planes) will contribute to the vibration of phonons [13]. Thus it is expected that substitution of Y into Gd sites always leads to increase in frequency of phonon modes at low frequency due to decrease in reduced mass.Figure4 shows a closer look of the low frequency phonon modes dynamics as indicated by arrows. Interestingly, there is a significant increase in frequency for the two most prominent phonon modes ωTO1 and ωTO3. Moreover, the mode ωTO2 was only observed for GdMn2O5 at 167 cm-1 and almost disappeared and reappeared for YMn2O5 at 176 cm-1. Although it shows an increase in the frequency as expected the absence of ωTO2 for the intermediate x is quite surprising. One possible explanation is some structural changes that totally damped these mode and is quite reasonable as we have observed some new peaks in XRD patterns at 2θ~28 that is only present for intermediate x and thus may be responsible for the absence of ωTO2. The substitution of Y at Gd site takes the system in combined effect of Y/Gd ions that may vibrate in opposite direction relative to Mn polyhedra results in damping the vibration as observed. Moreover, small ionic radius of Y directly affects the Mn-Mn interaction which leads to affecting the relative motion of Y/Gd ions with respect to Mn polyhedral. Thus, a consistent change in both rare-earth ionic radius and Mn-Mn bond distance may result in damped and discontinuous evolution of frequencies. Thus, we have observed strong lattice dynamics in low frequency range (<200 cm−1) as expected.Figure 4
Detailed view of the low frequency (<250 cm−1) for Gd1-xYxMn2O5 (x=0, 0.2, 0.4, 0.6, 0.8, and 1). The black open circle shows the experimental curve and the red line shows the fitted data. Arrows shows the mode evolution with x and the spectra are vertically shifted for clarity.With a further insight into lattice dynamics of intermediate frequency modes, one can see that the modeωTO4 shows no obvious frequency shift (Figure 4). Interestingly, a new phonon mode (ωTO4′) has been observed only for x=0.2 and x=1 (Figure 4). Moreover, the phonon modes from ωTO5 to ωTO12 that lies in the intermediate frequency range have shown no significant frequency shift as can be seen from Table 2. More interestingly, corresponding values of Sj and γj (j = 5–12) remain almost the same for the complete series Gd1-xYxMn2O5 (Table 3). This simply reflects that Y substitution into Gd sites does not disturb much at higher frequencies. The effect of Y substitution can also be observed in high frequency region as a result of two new phonons. One new phonon (ωTO9′) has been observed for x=0.2 at a frequency of 380 cm−1 and remains almost unchanged with increasing x. The second new phonon is ωTO12′ that has been observed only for YMn2O5 at frequency of about 486 cm−1. Thus the origin of three new phonon modes (ωTO4′, ωTO9′, and ωTO12′) after the substitution indicates the strong structural distortion produced, which is difficult to observe in XRD patterns.In contrast, the high frequency modesωTO13 to ωTO18 exhibit strong frequency hardening as shown in Figure 5. These modes represent the stretching and bending motion of MnO6 octahedra, as in this frequency range, oxygen atoms vibrate as per harmonic oscillator limitation due to its reduced mass as compared to Gd, Y, and Mn ions. While increasing Y content, the whole crystal reduces its volume due to the substitution of lighter element Y into heavy ion Gd. The phonon dynamics is also a function of volume that ultimately gives rise to the change in frequency of phonon modes [24]. Hence, the modes ωTO13 to ωTO18 harden with decrease in volume, giving rise to stretching of MnO6 octahedra to vibrate at higher frequency (Figure 5).Figure 5
Transverse optical frequency (ωTO) of the high frequency phonon mode ωTO13 to ωTO18 as a function of concentration x.We have also calculated the static dielectric constant, which is given by(3)ϵ∘=ϵ∞+∑jSj.ϵ
∘ represents the static dielectric response obtained from the sum of all the oscillator dielectric strength (Sj) and the electronic polarizability (ϵ∞). It is important to note that we have observed less number of phonons as compared to those theoretically predicted and thus the dielectric strength of the missing modes must be relatively weaker than those observed. The decrease in ϵ∘ with x, similar to the trend of ϵ∞, is clearly reflecting the small values of the ∑j‍Sj, which mainly decreases with x (see Figure 6). However, an overall decrease in the value of ϵ∞ means that energy band gap expands with increasing Y content [24].Figure 6
The high frequency dielectric constant,ϵ∞ (closed circle), and static dielectric constant, ϵ∘ (closed square), as a function of concentration x.We have calculated the real part of optical conductivity (σ1) through the measured reflectivity spectra by using the relation σ1(ω)=ωϵ2/4π. Here, ω and ϵ2 are wavenumber and complex part of the dielectric function obtained from measured reflectivity spectra [24]. As a representative, the obtained σ1 for GdMn2O5 is shown in Figure 7. It is noteworthy that the spectra have strong absorption peaks, indicating semiconducting behavior of GdMn2O5. In addition, spectra are structureless below 100 cm−1 and have shown no conduction mechanism reflecting the absence of free charge carriers, which is a typical semiconducting nature, in contrast to the Drude-type metallic behavior [25, 26]. Similar behavior has been observed for all samples.Figure 7
Real part of optical conductivity (σ1ω) for GdMn2O5.


## 4. Conclusion

We have performed a systematic investigation on structural distortion through lattice dynamics of the phonon modes inGd1-xYxMn2O5. By utilizing the discriminative sensitivity of IR reflectivity technique, we explicitly observed different types of phonon modes in Gd1-xYxMn2O5 depending upon their symmetry and participating ions. The substitution of Y ion in GdMn2O5 leads to hardening of phonons caused by the difference in mass and thus decrease in cell volume. Strong movements of Gd(Y) ions exhibit disorder induced effects through low frequency phonons shifts. The Mn sites remain almost unchanged upon increasing x, as reflected through almost constant frequency of the intermediate phonon modes. However, O-ions contribute at higher frequency modes inducing the stretching motion in MnO6 octahedra caused by change in cell volume. Moreover, optical conductivity indicates the semiconducting nature of the prepared compounds.



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