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Twinned EPR spectra of Fe3+ centers in ternary layered TlGaS2 crystal
Solid State Communications 138 (2006) 239–241 www.elsevier.com/locate/ssc
Twinned EPR spectra of Fe3C centers in ternary layered TlGaS2 crystal F.A. Mikailov a,b,*, S. Kazan a, B.Z. Rameev a,c, A.M. Kulibekov d, E. Kerimova b, B. Aktas¸ a a
Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey b Institute of Physics, Azerbaijan Academy of Sciences, 370143 Baku, Azerbaijan c Kazan Physical-Technical Institute, 420029 Kazan, Russian Federation d Department of Physics, Mugla University, 48000 Mugla, Turkey Received 23 December 2005; accepted 4 March 2006 by E.L. Ivchenko Available online 23 March 2006
Abstract TlGaS2 single crystal doped by paramagnetic Fe3C ions has been studied by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe3C ions was observed. The spectra reveal a nearly orthorhombic symmetry of the crystal field (CF) on the Fe3C ions. Two groups each consisting of four equivalent Fe3C centers were observed in the EPR spectra. The local symmetry of the crystal field on the Fe3C centers and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of the GaS4 tetrahedrons. The rhombic distortion of the sulfur ligand CF is attributed to the effect of Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines indicates a presence of two non-equivalent positions of Tl ions that confirms their zigzag alignment in the TlGaS2 crystal structure. q 2006 Elsevier Ltd. All rights reserved. PACS: 75.10.Dg; 76.30.Da; 76.30.Kv; 71.70.Ch Keywords: A. Ferroelectrics; B. Crystal field; E. Electron paramagnetic resonance; E. Fine structure splitting
1. Introduction TlGaS2 belongs to a group of ternary layered semiconductors 6 . According to X-ray diffraction with space group symmetry C2h measurements [1], the crystal structure of TlGaS2 consists of alternating two-dimensional metal–chalcogen layers arranged parallel to (001) plane. Each successive layer is turned through a right angle relative to the preceding one. The fundamental structural unit of a layer is the Ga4S10 polyhedron representing a combination of four elementary GaS4 tetrahedrons, which are linked together by common chalcogen atoms at the corners (Fig. 1). Monovalent Tl atoms are in trigonal prismatic cavities resulting from the combination of the In4S10 tetrahedra into a layer. The Tl atoms form nearly planar chains along the [110] directions. As one can see from the figure, each and ½110 successive lower layer is shifted along the [010] direction by the length of the edge of the small GaS4 tetrahedron with respect to * Corresponding author. Address: Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey. Tel.: C90 262 6051311; fax: C90 262 6538490. E-mail address: [email protected] (F.A. Mikailov).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2006.03.005
the upper layer. As a result, a deviation from the tetragonal symmetry appears. The angle between the monoclinic c angle and the layer plane is about 1008. In this paper, we report on the results of the first investigations of EPR spectra of the TlGaS2 compound doped by paramagnetic Fe3C ions, which substituted Ga sites as local probes of ligand crystal field.
2. Experimental details TlGaS2 single crystals were grown in evacuated quartz tubes by using the modified Bridgman method. For EPR experiments, iron was added to the growth mixture in amounts corresponding to a molar ratio Fe3C/Ga3C of about 2%. The crystals were cleaved easily into plane parallel plates. The morphology of the crystal permits to perform this operation along the a–b plane, which is parallel to the layers. The EPR spectra were recorded by using Bruker EMX model X-band spectrometer (9.8 GHz). The static magnetic field was varied in the range 0–16,000 G. The field derivative of microwave power absorption (dP/dH) was registered as a function of the static magnetic field (H). The angular dependences of EPR spectra were obtained for different orientations of the static magnetic field with respect to the
240
F.A. Mikailov et al. / Solid State Communications 138 (2006) 239–241
Fig. 1. Crystal structure of TlGaS2 composed of GaS4 tetrahedrons: ( ) Tl ions; ( ) In ions; ( ) S ions.
crystalline axes. The temperature dependence of EPR spectra was studied in the range of 5–300 K using continuous helium gas flow cryostat made by Oxford instruments. The temperature stability was no more than 0.5 K.
3. Results and discussion The EPR measurements of Fe-doped TlGaS2 single crystal revealed that the EPR spectra were strongly anisotropic with the resonance fields up to 16 kG. Angular dependences of the resonance fields of various lines in EPR spectra of the Fedoped TlGaS2 crystal are shown in Figs. 2 and 3. Fig. 2 shows
Fig. 2. The observed and fitted rotation patterns of the resonant field values for different orientations of the static magnetic field (H) in (1,0,0) plane (in-plane orientation). Full curves are calculated using the spin-Hamiltonian parameters listed in the table.
the angular dependence of the resonance fields for the static magnetic field rotated in the plane of layers (‘in-plane’ orientation), while Fig. 3 presents the angular dependence on rotation the sample in the (010) plane, which is perpendicular to the layers (‘out-of-plane’ orientation). Strong anisotropy of the EPR lines as well as the mere fact of their registration at room temperature indicates that the observed EPR spectra are due to the trivalent paramagnetic Fe ions in the TlGaS2 crystal host. As evident from the rotation patterns presented in Figs. 2 and 3, the EPR spectra originate from the groups of equivalent Fe3C centers. The rotational pattern observed on applying the static magnetic field in the plane of layers (Fig. 2) revealed a presence of two twinned
Fig. 3. The observed and fitted rotation patterns of the resonant field values for different orientations of the static magnetic field (H) in (0,1,0) plane (out-ofplane orientation). Full curves are calculated using the spin-Hamiltonian parameters listed in the table.
F.A. Mikailov et al. / Solid State Communications 138 (2006) 239–241
241
Table 1 Fitting parameters for the observed rotational patterns g-Factor
B02 (G)
B22 ðGÞ
B4
B04
B24
2.0 (G0.02) 2.0 (G0.02)
2100 2000
1050 1010
Negligible small Negligible small
Negligible small Negligible small
Negligible small Negligible small
groups of equivalent centers with symmetry axes almost perpendicular to each other. The rotational patterns of two out-of-plane rotation planes (Fig. 3) allowed to make a conclusion about the presence of eight centers. Each group consists of (1) two centers with symmetry axes lying in the (100) plane and (2) two other centers demonstrating the same symmetry in the (010) plane. The difference between these two groups of centers is in the angle between the symmetry directions of the centers and the layer plane: it is about 40 and 468 for different twinning groups. To obtain parameters of CF at the Fe3C ions the special computer program has been developed to simulate digitally the angular dependences of the resonance fields at arbitrary rotation plane. According to CF theory [2,3] the crystalline electric field at a positive ion site has a dominant component arising from an array of surrounding negative charges (ligands). The degeneracy of spin multiplet of the S-state Fe3C ion (SZ5/2, LZ0) is removed due to the crystal field. Using the equivalent Stevens operators, the energy level splittings of Fe3C ion can be described by the spin-Hamiltonian in most general form as following 2 4 X X m m H Z HZ C HCF Z bBð gððSð C Bm Bm (1) 2 O2 C 4 O4 mZ0
orthorhombic symmetry of the crystal field can be caused by the influence of Tl atoms, which are located in the trigonal cavities between the tetrahedral complexes. As seen from Fig. 1, the Tl ions can cause distortions of the S atom positions in the two neighbour GaS4 tetrahedra as well as contribute in the CF as next-neighbour ligand in the directions both along the layer plane (the rhombic term in CF) under the angles mentioned above (the axial component in CF). As it is mentioned above, the unit cell contains two layers containing successive rows of the tetrahedrons, which turned away from each other by 908. Thus, the symmetry of ligand field around the centers located in different layers is also turned away by the same angle. In this way one obtains a group consisting of four structurally equivalent Fe3C centers in the TlGaS2 structure. Besides, it is seen from the structure of TlGaS2 (Fig. 1) that there are two possible structural positions of Tl ions due to zigzag alignment of their chains in a vertical plane between arrays of GaS4 tetrahedrons. Therefore, two groups of the equivalent Fe3C centers appear, which are slightly different by the angle between the symmetry axis of axial component and the layer plane as well as by the spin-Hamiltonian parameters. 4. Conclusion
mZ0
where SZ5/2 is the electronic spin and b is Bohr magneton. The first term HZ accounts for the Zeeman interaction, the second term HCF is the crystal field Hamiltonian. The Stevens operators Om 4 are defined according to Abragam and Bleaney [2]. Computer simulations of the EPR spectra of Fe3C centres in TlGaS2 crystal shows that the observed rotational patterns may be adequately described by following spin Hamiltonian: pffiffiffi 2 HCF ZK B4 ðO04 C 20 2O44 Þ C B02 O02 C B04 O04 C B24 O24 (2) 3 3C Here, the local cubic symmetry at Fe site was found to be orthorhombically distorted with z-axis (axial term) directed nearly along the trigonal axis of sulfur tetrahedron and the rhombic component lying in the layer plane. It was established that there is a very strong rhombic component in the crystal field at Fe ion positions in the TlGaS2 structure. As it is mentioned above, two groups of structurally equivalent Fe3C ions were assumed and the rather good agreement between experimental and modelled rotational patterns of the resonant field values was obtained (Figs. 2 and 3). The evaluated parameters for these two groups of centers, which were used to fit the observed rotation patterns, are listed in the Table 1. These results can be understood on the base of the assumption that the paramagnetic probes Fe3C substituted for Ga3C at the central positions of GaS4 tetrahedra. It is known that perfect GaS4 tetrahedron has cubic symmetry at the Ga site due to the effect of nearest ligands (S). Therefore, the
The fine structure of EPR spectra of paramagnetic Fe3C ions was observed and the spin-Hamiltonian parameters were extracted. The spectra were interpreted to correspond to the transitions among spin multiplet of the Fe3C ions located at the center of the GaS4 tetrahedra formed by the S atoms. Two groups of four equivalent Fe3C centers were observed in EPR spectra and the local orthorhombic symmetry of the crystal field and the CF parameters were determined. The orthorhombic CF on Fe3C sites was attributed to the effect of the Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines can be attributed to a presence of two possible structural positions of the zigzag-aligned Tl ions in the TlGaS2 crystal structure. Acknowledgements The authors are indebted to Research Projects Commission of Gebze Institute of Technology for supporting this work by the Grant no. 2005-A-09. References [1] D. Muller, H. Hang, Z. Anorg. Allg. Chem. 438 (1978) 258. [2] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, 1970. [3] C.P. Poole, H.A. Farach, Theory of Magnetic Resonance, Wiley, New York, 1987.
Twinned EPR spectra of Fe3C centers in ternary layered TlGaS2 crystal F.A. Mikailov a,b,*, S. Kazan a, B.Z. Rameev a,c, A.M. Kulibekov d, E. Kerimova b, B. Aktas¸ a a
Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey b Institute of Physics, Azerbaijan Academy of Sciences, 370143 Baku, Azerbaijan c Kazan Physical-Technical Institute, 420029 Kazan, Russian Federation d Department of Physics, Mugla University, 48000 Mugla, Turkey Received 23 December 2005; accepted 4 March 2006 by E.L. Ivchenko Available online 23 March 2006
Abstract TlGaS2 single crystal doped by paramagnetic Fe3C ions has been studied by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe3C ions was observed. The spectra reveal a nearly orthorhombic symmetry of the crystal field (CF) on the Fe3C ions. Two groups each consisting of four equivalent Fe3C centers were observed in the EPR spectra. The local symmetry of the crystal field on the Fe3C centers and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of the GaS4 tetrahedrons. The rhombic distortion of the sulfur ligand CF is attributed to the effect of Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines indicates a presence of two non-equivalent positions of Tl ions that confirms their zigzag alignment in the TlGaS2 crystal structure. q 2006 Elsevier Ltd. All rights reserved. PACS: 75.10.Dg; 76.30.Da; 76.30.Kv; 71.70.Ch Keywords: A. Ferroelectrics; B. Crystal field; E. Electron paramagnetic resonance; E. Fine structure splitting
1. Introduction TlGaS2 belongs to a group of ternary layered semiconductors 6 . According to X-ray diffraction with space group symmetry C2h measurements [1], the crystal structure of TlGaS2 consists of alternating two-dimensional metal–chalcogen layers arranged parallel to (001) plane. Each successive layer is turned through a right angle relative to the preceding one. The fundamental structural unit of a layer is the Ga4S10 polyhedron representing a combination of four elementary GaS4 tetrahedrons, which are linked together by common chalcogen atoms at the corners (Fig. 1). Monovalent Tl atoms are in trigonal prismatic cavities resulting from the combination of the In4S10 tetrahedra into a layer. The Tl atoms form nearly planar chains along the [110] directions. As one can see from the figure, each and ½110 successive lower layer is shifted along the [010] direction by the length of the edge of the small GaS4 tetrahedron with respect to * Corresponding author. Address: Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey. Tel.: C90 262 6051311; fax: C90 262 6538490. E-mail address: [email protected] (F.A. Mikailov).
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2006.03.005
the upper layer. As a result, a deviation from the tetragonal symmetry appears. The angle between the monoclinic c angle and the layer plane is about 1008. In this paper, we report on the results of the first investigations of EPR spectra of the TlGaS2 compound doped by paramagnetic Fe3C ions, which substituted Ga sites as local probes of ligand crystal field.
2. Experimental details TlGaS2 single crystals were grown in evacuated quartz tubes by using the modified Bridgman method. For EPR experiments, iron was added to the growth mixture in amounts corresponding to a molar ratio Fe3C/Ga3C of about 2%. The crystals were cleaved easily into plane parallel plates. The morphology of the crystal permits to perform this operation along the a–b plane, which is parallel to the layers. The EPR spectra were recorded by using Bruker EMX model X-band spectrometer (9.8 GHz). The static magnetic field was varied in the range 0–16,000 G. The field derivative of microwave power absorption (dP/dH) was registered as a function of the static magnetic field (H). The angular dependences of EPR spectra were obtained for different orientations of the static magnetic field with respect to the
240
F.A. Mikailov et al. / Solid State Communications 138 (2006) 239–241
Fig. 1. Crystal structure of TlGaS2 composed of GaS4 tetrahedrons: ( ) Tl ions; ( ) In ions; ( ) S ions.
crystalline axes. The temperature dependence of EPR spectra was studied in the range of 5–300 K using continuous helium gas flow cryostat made by Oxford instruments. The temperature stability was no more than 0.5 K.
3. Results and discussion The EPR measurements of Fe-doped TlGaS2 single crystal revealed that the EPR spectra were strongly anisotropic with the resonance fields up to 16 kG. Angular dependences of the resonance fields of various lines in EPR spectra of the Fedoped TlGaS2 crystal are shown in Figs. 2 and 3. Fig. 2 shows
Fig. 2. The observed and fitted rotation patterns of the resonant field values for different orientations of the static magnetic field (H) in (1,0,0) plane (in-plane orientation). Full curves are calculated using the spin-Hamiltonian parameters listed in the table.
the angular dependence of the resonance fields for the static magnetic field rotated in the plane of layers (‘in-plane’ orientation), while Fig. 3 presents the angular dependence on rotation the sample in the (010) plane, which is perpendicular to the layers (‘out-of-plane’ orientation). Strong anisotropy of the EPR lines as well as the mere fact of their registration at room temperature indicates that the observed EPR spectra are due to the trivalent paramagnetic Fe ions in the TlGaS2 crystal host. As evident from the rotation patterns presented in Figs. 2 and 3, the EPR spectra originate from the groups of equivalent Fe3C centers. The rotational pattern observed on applying the static magnetic field in the plane of layers (Fig. 2) revealed a presence of two twinned
Fig. 3. The observed and fitted rotation patterns of the resonant field values for different orientations of the static magnetic field (H) in (0,1,0) plane (out-ofplane orientation). Full curves are calculated using the spin-Hamiltonian parameters listed in the table.
F.A. Mikailov et al. / Solid State Communications 138 (2006) 239–241
241
Table 1 Fitting parameters for the observed rotational patterns g-Factor
B02 (G)
B22 ðGÞ
B4
B04
B24
2.0 (G0.02) 2.0 (G0.02)
2100 2000
1050 1010
Negligible small Negligible small
Negligible small Negligible small
Negligible small Negligible small
groups of equivalent centers with symmetry axes almost perpendicular to each other. The rotational patterns of two out-of-plane rotation planes (Fig. 3) allowed to make a conclusion about the presence of eight centers. Each group consists of (1) two centers with symmetry axes lying in the (100) plane and (2) two other centers demonstrating the same symmetry in the (010) plane. The difference between these two groups of centers is in the angle between the symmetry directions of the centers and the layer plane: it is about 40 and 468 for different twinning groups. To obtain parameters of CF at the Fe3C ions the special computer program has been developed to simulate digitally the angular dependences of the resonance fields at arbitrary rotation plane. According to CF theory [2,3] the crystalline electric field at a positive ion site has a dominant component arising from an array of surrounding negative charges (ligands). The degeneracy of spin multiplet of the S-state Fe3C ion (SZ5/2, LZ0) is removed due to the crystal field. Using the equivalent Stevens operators, the energy level splittings of Fe3C ion can be described by the spin-Hamiltonian in most general form as following 2 4 X X m m H Z HZ C HCF Z bBð gððSð C Bm Bm (1) 2 O2 C 4 O4 mZ0
orthorhombic symmetry of the crystal field can be caused by the influence of Tl atoms, which are located in the trigonal cavities between the tetrahedral complexes. As seen from Fig. 1, the Tl ions can cause distortions of the S atom positions in the two neighbour GaS4 tetrahedra as well as contribute in the CF as next-neighbour ligand in the directions both along the layer plane (the rhombic term in CF) under the angles mentioned above (the axial component in CF). As it is mentioned above, the unit cell contains two layers containing successive rows of the tetrahedrons, which turned away from each other by 908. Thus, the symmetry of ligand field around the centers located in different layers is also turned away by the same angle. In this way one obtains a group consisting of four structurally equivalent Fe3C centers in the TlGaS2 structure. Besides, it is seen from the structure of TlGaS2 (Fig. 1) that there are two possible structural positions of Tl ions due to zigzag alignment of their chains in a vertical plane between arrays of GaS4 tetrahedrons. Therefore, two groups of the equivalent Fe3C centers appear, which are slightly different by the angle between the symmetry axis of axial component and the layer plane as well as by the spin-Hamiltonian parameters. 4. Conclusion
mZ0
where SZ5/2 is the electronic spin and b is Bohr magneton. The first term HZ accounts for the Zeeman interaction, the second term HCF is the crystal field Hamiltonian. The Stevens operators Om 4 are defined according to Abragam and Bleaney [2]. Computer simulations of the EPR spectra of Fe3C centres in TlGaS2 crystal shows that the observed rotational patterns may be adequately described by following spin Hamiltonian: pffiffiffi 2 HCF ZK B4 ðO04 C 20 2O44 Þ C B02 O02 C B04 O04 C B24 O24 (2) 3 3C Here, the local cubic symmetry at Fe site was found to be orthorhombically distorted with z-axis (axial term) directed nearly along the trigonal axis of sulfur tetrahedron and the rhombic component lying in the layer plane. It was established that there is a very strong rhombic component in the crystal field at Fe ion positions in the TlGaS2 structure. As it is mentioned above, two groups of structurally equivalent Fe3C ions were assumed and the rather good agreement between experimental and modelled rotational patterns of the resonant field values was obtained (Figs. 2 and 3). The evaluated parameters for these two groups of centers, which were used to fit the observed rotation patterns, are listed in the Table 1. These results can be understood on the base of the assumption that the paramagnetic probes Fe3C substituted for Ga3C at the central positions of GaS4 tetrahedra. It is known that perfect GaS4 tetrahedron has cubic symmetry at the Ga site due to the effect of nearest ligands (S). Therefore, the
The fine structure of EPR spectra of paramagnetic Fe3C ions was observed and the spin-Hamiltonian parameters were extracted. The spectra were interpreted to correspond to the transitions among spin multiplet of the Fe3C ions located at the center of the GaS4 tetrahedra formed by the S atoms. Two groups of four equivalent Fe3C centers were observed in EPR spectra and the local orthorhombic symmetry of the crystal field and the CF parameters were determined. The orthorhombic CF on Fe3C sites was attributed to the effect of the Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines can be attributed to a presence of two possible structural positions of the zigzag-aligned Tl ions in the TlGaS2 crystal structure. Acknowledgements The authors are indebted to Research Projects Commission of Gebze Institute of Technology for supporting this work by the Grant no. 2005-A-09. References [1] D. Muller, H. Hang, Z. Anorg. Allg. Chem. 438 (1978) 258. [2] A. Abragam, B. Bleaney, Electron Paramagnetic Resonance of Transition Ions, Clarendon Press, Oxford, 1970. [3] C.P. Poole, H.A. Farach, Theory of Magnetic Resonance, Wiley, New York, 1987.