pss basic solid state physics b status solidi www.pss-b.com physica REPRINT Structural and optical high-pressure study of spinel-type MnIn 2 S 4 F. J. Manjón 1 , A. Segura 2 , M. Amboage 3 , J. Pellicer-Porres 2 , J. F. Sánchez-Royo 2 , J. P. Itié 4 , A. M. Flank 4 , P. Lagarde 4 , A. Polian 5 , V.V. Ursaki 6 , and I. M. Tiginyanu 6 1 Dpto. de Física Aplicada, Univ. Politècnica de València, Cno. de Vera s/n, 46022 València, Spain 2 Dpto. de Física Aplicada-ICMUV, Univ. de València, c/Dr. Moliner 50, 46100 Burjassot, Spain 3 European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France 4 Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin – BP 48 91192 Gif-sur-Yvette Cedex, France 5 Physique des Milieux Denses, IMPMC, Université P. et M. Curie, 140 Bvd de Lourmel, 75015 Paris, France 6 Institute of Applied Physics, Academy of Sciences of Moldova, 2028 Chisinau, Moldova Received 10 July 2006, accepted 9 August 2006 Published online 27 November 2006 PACS 61.10.Ht, 61.10.Nz, 62.50.+p, 78.40.Fy We report a combined study of the structural and electronic properties of the spinel-type semiconductor MnIn 2 S 4 under high pressures by means of X-ray diffraction (ADXRD), X-ray absorption (XAS), and op- tical absorption measurements. The three techniques evidence a reversible structural phase transition near 7 GPa, that according to ADXRD measurements is to a double-NaCl structure. XAS measurements evi- dence predominant tetrahedral coordination for Mn in the spinel phase that does not noticeably change with increasing pressure up to the phase transition. XAS measurements indicate that the static disorder in- creases considerably when the sample reverts from the double-NaCl phase to the spinel phase. Optical ab- sorption measurements show that the direct gap of MnIn 2 S 4 exhibits a nonlinear behaviour with a positive pressure coefficient at pressures below 2.5 GPa and a negative pressure coefficient between 2.5 and 7 GPa. The pressure behavior of the bandgap seems to be affected by the defect concentration. The dou- ble-NaCl phase also exhibits a bandgap with a negative pressure coefficient. phys. stat. sol. (b) 244, No. 1, 229 – 233 (2007) / DOI 10.1002/pssb.200672522
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REPRINT
Structural and optical high-pressure study
of spinel-type MnIn2S4
F. J. Manjón1
, A. Segura2
, M. Amboage3
, J. Pellicer-Porres2
, J. F. Sánchez-Royo2
,
J. P. Itié4
, A. M. Flank4
, P. Lagarde4
, A. Polian5
, V.V. Ursaki6
, and I. M. Tiginyanu6
1
Dpto. de Física Aplicada, Univ. Politècnica de València, Cno. de Vera s/n, 46022 València, Spain
2
Dpto. de Física Aplicada-ICMUV, Univ. de València, c/Dr. Moliner 50, 46100 Burjassot, Spain
3
European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France
4
Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin – BP 48 91192 Gif-sur-Yvette Cedex, France
5
Physique des Milieux Denses, IMPMC, Université P. et M. Curie, 140 Bvd de Lourmel, 75015 Paris,
France
6
Institute of Applied Physics, Academy of Sciences of Moldova, 2028 Chisinau, Moldova
Received 10 July 2006, accepted 9 August 2006
Published online 27 November 2006
PACS 61.10.Ht, 61.10.Nz, 62.50.+p, 78.40.Fy
We report a combined study of the structural and electronic properties of the spinel-type semiconductor
MnIn2S
4under high pressures by means of X-ray diffraction (ADXRD), X-ray absorption (XAS), and op-
tical absorption measurements. The three techniques evidence a reversible structural phase transition near
7 GPa, that according to ADXRD measurements is to a double-NaCl structure. XAS measurements evi-
dence predominant tetrahedral coordination for Mn in the spinel phase that does not noticeably change
with increasing pressure up to the phase transition. XAS measurements indicate that the static disorder in-
creases considerably when the sample reverts from the double-NaCl phase to the spinel phase. Optical ab-
sorption measurements show that the direct gap of MnIn2S
4exhibits a nonlinear behaviour with a positive
pressure coefficient at pressures below 2.5 GPa and a negative pressure coefficient between 2.5 and
7 GPa. The pressure behavior of the bandgap seems to be affected by the defect concentration. The dou-
ble-NaCl phase also exhibits a bandgap with a negative pressure coefficient.
F. J. Manjón*, 1, A. Segura2, M. Amboage3, J. Pellicer-Porres2, J. F. Sánchez-Royo2,
J. P. Itié4, A. M. Flank4, P. Lagarde4, A. Polian5, V.V. Ursaki6, and I. M. Tiginyanu6
1 Dpto. de Física Aplicada, Univ. Politècnica de València, Cno. de Vera s/n, 46022 València, Spain 2 Dpto. de Física Aplicada-ICMUV, Univ. de València, c/Dr. Moliner 50, 46100 Burjassot, Spain 3 European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France 4 Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin – BP 48 91192 Gif-sur-Yvette Cedex, France 5 Physique des Milieux Denses, IMPMC, Université P. et M. Curie, 140 Bvd de Lourmel, 75015 Paris,
France 6 Institute of Applied Physics, Academy of Sciences of Moldova, 2028 Chisinau, Moldova
Received 10 July 2006, accepted 9 August 2006
Published online 27 November 2006
PACS 61.10.Ht, 61.10.Nz, 62.50.+p, 78.40.Fy
We report a combined study of the structural and electronic properties of the spinel-type semiconductor
MnIn2S4 under high pressures by means of X-ray diffraction (ADXRD), X-ray absorption (XAS), and op-
tical absorption measurements. The three techniques evidence a reversible structural phase transition near
7 GPa, that according to ADXRD measurements is to a double-NaCl structure. XAS measurements evi-
dence predominant tetrahedral coordination for Mn in the spinel phase that does not noticeably change
with increasing pressure up to the phase transition. XAS measurements indicate that the static disorder in-
creases considerably when the sample reverts from the double-NaCl phase to the spinel phase. Optical ab-
sorption measurements show that the direct gap of MnIn2S4 exhibits a nonlinear behaviour with a positive
pressure coefficient at pressures below 2.5 GPa and a negative pressure coefficient between 2.5 and
7 GPa. The pressure behavior of the bandgap seems to be affected by the defect concentration. The dou-
ble-NaCl phase also exhibits a bandgap with a negative pressure coefficient.
MnIn2S4 is an n-type semiconductor of the AIIBIII2C
VI4 family that crystallizes in the cubic spinel structure
(space group Fd3m) with Mn occupying tetrahedral positions in the lattice and In occupying octahedral positions. This family of materials is interesting for optoelectronic applications because they have shown nonlinear optical properties, like nonlinear optical susceptibility combined with natural birefringence [1]. In particular, MnIn2S4 is a very promising candidate for optoelectronic applications such as solar cell windows because it has an optical direct energy gap of 1.95 eV [2] that can be modified after suitable doping or by changing the cation concentration [3]. An important aspect of this semiconductor is that both cations Mn and In have nearly the same ionic radii for the same coordination [4]. This usually leads to a high degree of inversion; i.e., a number of Mn and In cations can interchange their positions in the lattice. Therefore, MnIn2S4 is a defect semiconductor with a relatively high concentration of antisite defects that can be tailored for different applications.
This semiconductor still lacks a systematic characterization of their structural and optical properties so in this work, we have used pressure as an excellent tool to give an insight of the ambient pressure proper-ties and to obtain information about the structural and optical modifications introduced by compression.
2 Experimental
Spinel-type MnIn2S4 samples used in this study were obtained from a single crystal grown by chemical vapour transport using iodine as a transport agent [5]. Angle-dispersive X-ray diffraction measurements (ADXRD) at room temperature (RT) inside a diamond anvil cell (DAC) were performed up to 15 GPa in the ID09 beamline at the ESRF with a wavelength λ = 0.40896 Å in powder samples obtained from a bulk single-crystal ingot and with nitrogen as pressure medium. X-ray absorption (XAS) measurements at RT in powder samples were performed at the Mn K-edge (6.539 keV) at the LUCIA beamline of SLS (PSI, Villingen, Switzerland), in the transmission mode with a spot focused to 10 × 20 µm2. For XAS measurements the powder sample was inserted in a 80 µm hole drilled in a stainless steel gasket mounted on a DAC with partially hollow diamonds (200 µm in diameter) to reduce the absorption of diamonds at the low-energy Mn K-edge. For optical absorption measurements at RT in the UV-VIS-NIR range under pressure, bulk single crystals (15–30 µm thick) with parallel faces were loaded in a DAC with methanol-ethanol-water (16:3:1) as pressure transmitting medium. The optical set-up consists of a halogen lamp, fused silica lenses, microscopic objectives, a UV-VIS-NIR spectrometer, and a silicon or germanium photodiode. In all the experiments the pressure was determined by the ruby fluorescence technique [6].
3 ADXRD measurements
Figure 1 shows the RT ADXRD patterns of MnIn2S4 at increasing pressures up to 15 GPa. The ambient pressure pattern can be clearly attributed to the cubic spinel phase. A Rietveld refinement of the structure at 1 atm allows us to obtain a volume of 1233.5(5) Å3 and an inversion parameter of i = 0.25 in agree-ment with previous Raman measurements under pressure [7]. The inversion parameter does not change appreciably with pressure. The spinel phase is stable up to around 7 GPa, where a phase transition is observed. The X-ray patterns above 7 GPa can be reasonably fitted with an ordered double-NaCl struc-ture of the LiTiO2 type. At the phase transition pressure (7 GPa) the volume contracts by about 4%. The
space-group remains Fd3m, but Mn moves from the tetrahedral 8a position to the octahedral 16c posi-tion, with a 0.5 occupancy, while In remains in the 16d position and S in the 32e position. The phase transition is reversible and the spinel phase is recovered when releasing pressure, as shown by the last pattern of Fig. 1. The inversion parameter at 3.9 GPa during downstroke is i = 0.12, that is, half of the value during upstroke. This result is consistent with the decrease of the inversion parameter in MnIn2S4 under compression observed from Raman measurements under pressure [7], and suggests that with increasing pressure In is more compressible than Mn so they tend to their normal positions in the lattice. The unit cell volume as a function of pressure is well described by the Murnaghan equation of state [8] which yields the bulk modulus and its pressure derivative at ambient pressure: B0 = 73(2) GPa and B′ = 2.8(6) for the spinel phase, and B0 = 40(3) GPa and B′ = 5.1(5) for the high pressure LiTiO2-type phase. It is interesting to note that the bulk modulus of the spinel phase of MnIn2S4 is half the value of spinel oxides (around 200 GPa) and similar to that of Se spinels, like CdCr2Se4 (101 GPa) [9]. Since the volume fraction occupied by the anion in spinels is near 75% and the bulk moduli of spinels are re-lated to the dominant atom in the unit cell [10], our result suggests that the compressibility of S is similar to that of Se in the spinel structure.
4 XAS measurements
Figure 2 shows the X-ray absorption spectra of MnIn2S4 at the Mn K-edge at different pressures up to 12 GPa. The general spectral features remain essentially unchanged up to 7 GPa, where noticeable modi-fications occur. In particular, the pre-edge feature, associated to transitions to Mn d empty levels, disap-pears. At low pressures, the pre-edge feature suggests the absence of inversion symmetry at the Mn site, as it is the case in tetrahedral coordinated Mn. The absence of significant changes in the XAS spectra before the phase transition suggests that the predominant tetrahedral coordination for Mn is maintained up to the phase transition, in agreement with XRD and Raman results [7]. The lack of the pre-edge fea-ture above 7 GPa is coherent with the octahedral coordination of Mn in the LiTiO2 phase obtained in XRD measurements.
Fig. 2 Room-temperature XAS spectra of MnIn2S4 at the Mn K-edge. Numbers next to the spectra indi-
cate pressure in GPa. Spectra labelled d are taken in the downstroke. The inset zooms the pre-edge feature.
232 F. J. Manjón et al.: Structural and optical high-pressure study of MnIn2S4
Extended X-ray absorption fine structure (EXAFS) analysis performed with ab-initio phases and am-plitudes (FEFF code [11]), results in a Mn–S distance of 2.45 ± 0.02 Å at ambient pressure, and a Mn–S bond length compressibility given by χ = (4.5 ± 0.6) 10–3 GPa–1. Although it has not been possible to deduce the inversion parameter from EXAFS analysis, the rather high value of the pseudo Debye–Waller factor (~0.009 Å2) reveal the presence of significant static disorder, which does not vary appre-ciably under high pressure, in agreement with XRD and Raman results [7]. On decreasing pressure the sample reverts to its original spinel phase. The absorption spectra in the spinel phase recorded on down-stroke reveal a higher static disorder than that on upstroke, showing a slightly larger pseudo Debye–Waller factor (~0.012 Å2). This higher static disorder is likely caused by the defects introduced by the pressure-induced phase transition.
5 Optical absorption measurements
Figure 3 shows the absorption spectra of MnIn2S4 single crystal at different pressures up to 20 GPa. The bandgap energy of MnIn2S4 shows a nonlinear behaviour under pressure. It exhibits a positive slope at pressures below 2.5 GPa, reaching the maximum value of the bandgap energy around this pressure. Above 2.5 GPa the bandgap energy exhibits a negative pressure coefficient up to the phase transition observed near 7 GPa. This nonlinear behaviour of the bandgap in MnIn2S4 under pressure contrast with the linear dependence of the indirect bandgap in CdIn2S4 that shows a positive pressure coefficient up to 6 GPa [12]. On decreasing pressure from 20 GPa the spinel phase is recovered below 6.5 GPa, however, the band-gap energy did not revert to its original value at 1 atm. Furthermore, on downstroke the sample did not gain the maximum bandgap energy at 2.5 GPa, but at 1 GPa, thus showing a dependence of the bandgap on either the defect concentration in the sample or on the different inversion parameter. A different bandgap is observed in spinels with same stoichiometry but different inversion parameters [13]. The dependence of the bandgap on the inversion parameter is related to the different amount of hybridization of the cation electron wavefunctions to give the topmost valence and lowest conduction bands. There-fore, the different bandgap energy measured at 1 atm (and the different pressure for the maximum of the bandgap) before and after the pressure run seems to be related to the different inversion parameter of the spinel phase during upstroke and downstroke. Since the bandgap usually decreases with the increase of the inversion parameter between 0 and 0.25 [13], the larger bandgap found at 1 atm on downstroke with respect to upstroke is coherent with the decrease of the inversion parameter observed by XRD after the pressure run. Finally, we must note that the double-rocksalt phase exhibits a linear dependence of the bandgap between 7 and 20 GPa with a negative pressure coefficient (see Fig. 3).
Energy (eV)
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4
α(cm
-1)
0
1000
2000
3000
4000
0.2GPa
19.5
GPa
2.5GPa
5.2GPa
7.1GPa
8.7GPa
11.7
GPa
13.1GPa
14.8
GPa
12.7
GPa
7.4GPa
1 atm1 GPa
Fig. 3 Room temperature optical absorption
spectra of MnIn2S
4 at different pressures up to
20 GPa. Solid lines represent the absorption coef-
ficient during upstroke and dotted lines represent
We have studied the pressure dependence of the atomic and electronic structure of spinel MnIn2S4. ADXRD measurements up to 15 GPa show that the spinel structure transforms into a double NaCl struc-ture of the LiTiO2 type above 7 GPa and reverts to the spinel phase on downstroke. XAS measurements up to 12 GPa show predominant tetrahedral Mn coordination in the spinel phase, with significant disor-der which is not considerably affected with increasing pressure, but which however increases after the reversal of the sample to the spinel phase on downstroke. Finally, optical absorption measurements up to 20 GPa show a highly nonlinear behaviour of the optical gap of the spinel phase under pressure. The bandgap increases initially with increasing pressure up to 2.5 GPa and decreases above that pressure. After the reversal of the sample to the spinel phase, the bandgap exhibits again its nonlinear behaviour but the bandgap did not recover its initial value after releasing pressure. Furthermore, the pressure for the maximum energy of the bandgap was lower than during upstroke, thus showing a certain memory effect and a clear dependence of the bandgap energy on the inversion parameter.
Acknowledgements This study was made possible through financial support from the Spanish MCYT under
grants Nos. MAT2002-04539-CO2-01/-02. We thank the ESRF for the provision of synchrotron radiation facilities.
F.J.M. acknowledges the financial support from the “Programa de Incentivo a la Investigación” of the Universidad
Politécnica de Valencia. This research project has been supported by the European Commission under the 6th
Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastruc-
tures, RII3-CT-2004-506008.
References
[1] A. N. Georgobiani, S. I. Radautsan, and I. M. Tiginyanu, Sov. Phys.-Semicond. 19, 121 (1985).
[2] N. N. Niftiev, Solid State Commun. 92, 781 (1994).
[3] R. K. Sharma, A. C. Rastogi, S. Kohli, T. W. Kang, and G. Singh, Physica B 351, 45 (2004).
[4] R. D. Shannon, Acta Crystallogr. A 32, 751 (1976).
[5] R. Nitsche, J. Cryst. Growth 9, 238 (1971).
[6] H. K. Mao, J. Xu, and P. M. Bell, J. Geophys. Res. 91, 4673 (1986).
[7] V. V. Ursaki, F. J. Manjón, I. M. Tiginyanu, and V. E. Tezlevan, J. Phys.: Condens. Matter 14, 6801 (2002).
[8] F. D. Murnaghan, Am. J. Math. 49, 235 (1937).
[9] L. Gerward, J. Z. Jiang, J. Staun Olsen, J. M. Recio, and A. Waskowska, J. Alloys Compd. 401, 11 (2005).
[10] A. M. Pendás, A. Costales, M. A. Blanco, J. M. Recio, and V. Luaña, Phys. Rev. B 62, 13970 (2000).
[11] J. Ankudinov, B. Ravel, and S. Conradson, Phys. Rev. B 58, 7565 (1998).
[12] H. Nakanishi and T. Irie, phys. stat. sol. (b) 126, K145 (1984).
[13] S. D. Mo and W. Y. Ching, Phys. Rev. B 54, 16555 (1996).
