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Structural phase transitions in Fe3+ -doped ferroelectric TlGaSe2 crystal
Solid State Communications 145 (2008) 539–544 www.elsevier.com/locate/ssc
Structural phase transitions in Fe3+-doped ferroelectric TlGaSe2 crystal M. Ac¸ıkg¨oz a , S. Kazan b , F.A. Mikailov b,c,∗ , T.G. Mammadov c , B. Aktas¸ b a Bahcesehir University, Faculty of Art and Sciences, Bes¸iktas¸, 34100, Istanbul, Turkey b Department of Physics, Gebze Institute of Technology, Gebze, 41400, Kocaeli, Turkey c Institute of Physics, Azerbaijan Academy of Sciences, AZ-143, Baku, Azerbaijan
Received 13 August 2007; received in revised form 25 December 2007; accepted 5 January 2008 by E.V. Sampathkumaran Available online 12 January 2008
Abstract This paper presents the results of dielectric constant and Electron Paramagnetic Resonance (EPR) investigations of Fe3+ -doped TlGaSe2 single crystals in the temperature range of 15–300 K. The influence of Fe impurities on dielectric properties and phase transitions of TlGaSe2 crystal has been studied. The results were considered in comparison with earlier observed results from pure TlGaSe2 compounds. We observed the considerable decrease of the dielectric constant as well as the change of the shape of the temperature dependence of the dielectric constant in doped crystals. Some certain significant changes of EPR spectra, which are associated with a strong splitting and appearance of additional resonance lines, were observed at the temperatures below 110 K. Such transformations are considered as the result of non-equivalent displacements of different groups of Tl atoms during the structural phase transitions. c 2008 Elsevier Ltd. All rights reserved.
PACS: 75.10.Dg; 76.30.Da; 76.30.-v; 71.70.Ch Keywords: D. Crystal fields; D. Dielectric properties; D. Electron paramagnetic resonance (EPR); D. Phase transitions
1. Introduction TlGaSe2 is a ternary-layered chalcogenide crystal, which crystallizes in monoclinic system and belongs to a space 6 at room temperature [1]. According to symmetry group of C2h X-ray diffraction measurements [2,3], the crystal structure of TlGaSe2 is characterized by metal–chalcogen layers composed of Ga4 Se10 polyhedron complexes representing a combination of four elementary GaSe4 tetrahedra linked by common chalcogen atoms at the corners (Fig. 1). The elementary unit cell contains two layers containing successive rows of the tetrahedron complexes, which are turned away from each other by 90◦ and each of them is shifted by the length of the edge of the small GaSe4 tetrahedron with respect to another layer ¯ 0] directions. Monovalent Tl atoms are along [1, 1, 0] and [1, 1, located in trigonal prismatic cavities between metal–chalcogen layers. As a result, a deviation from the tetragonal symmetry ∗ Corresponding author at: Department of Physics, Gebze Institute of Technology, Gebze, 41400, Kocaeli, Turkey. Tel.: +90 262 6051311; fax: +90 262 6538490. E-mail address: [email protected] (F.A. Mikailov).
c 2008 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2008.01.009
appears. The angle between the monoclinic c axis and the layer plane is about 100◦ . As mentioned in [4], the origin of the disorder in the crystal structure of TlGaSe2 is due to the fact that owing to the stacking variants and the pseudo-tetragonal symmetry, the crystal frequently contains four layer type twins, where the monoclinic axis are exchanged or the tilting is in the opposite direction. The first publication about the presence of structural phase transitions and ferroelectricity in ternary-layered chalcogenide crystal TlGaSe2 was published about two decades ago [5]. Many researchers investigated the promising structural phase transitions of this compound by using a great number of experimental methods since that time. It has been concluded that, on cooling, TlGaSe2 exhibits a sequence of structural phase transitions, including transitions to an incommensurate (IC) and commensurate (C) phases. According to neutron [6] and X-ray scattering investigations [3], the transition to IC phase, which takes place at Ti1 ∼ 120 K, is associated with condensation of a soft mode at a point in the Brillouin zone characterized by qi = (δ, δ, 0.25), where δ is the
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Fig. 1. Crystal structure of TlGaSe2 composed of GaSe4 tetrahedrons: Tl ions; — Ga ions; — Se ions.
—
incommensuration parameter (δ = 0.02). On subsequent cooling, TlGaSe2 exhibits IC–C phase transition at the temperature of Tc1 ∼ 107 K with condensation of the soft mode at qc = (0, 0, 0.25), which accompanied by the quadrupling of the unit cell volume along the direction perpendicular to the layers. However, the presence of the ferroelectric soft mode with Curie temperature at about Tc ∼ 107 K and with the Curie constant ∼103 was discovered by submillimeter spectra and dielectric constant measurements [7]. The spontaneous polarization vector of the ferroelectric phase lies in the plane of layers. In addition to the successive phase transitions described above, a phase transition to weak ferroelectric phase was observed at the temperatures about 65 K by authors of [8]. This transition could be explained by using the model of two non-equivalent sublattices proposed by [9]. A very interesting result has been observed by Allakhverdiev et al. [10], after measuring the temperature dependencies of the dielectric constant of TlGaSe2 under bias electric field: the electrical field dependence of the commensurate phase transition temperature exhibited a behavior peculiar to antiferroelectric crystals. The detailed investigations of various physical properties of TlGaSe2 at low temperatures, such as optical absorption [11], heat capacity [12] and acoustic emission [13] experiments, revealed additional phase transitions at temperatures about 101–103 K and 246–253 K. As a result of detailed dielectric constant measurements [14–17], two conclusions have been drawn recently: the presence of the incommensurate phase at the temperature interval between 115 K and 242 K and the
coexistence of two strong-interacting polar sublattices in the low-temperature phase of TlGaSe2 . As it is seen, in spite of a number of experimental results, there was no information about active atomic groups, possible atomic displacements causing the dipole ordering, and the local symmetry changes during the low-temperature phase transformations in this compound. As mentioned above, neutron scattering [5] and X-ray studies [6] have shown that the low-temperature ferroelectric phase has a fourfold-commensurate structure. Additionally, the satellite reflections for the incommensurate phase are observed at qc = (0, 0, 0.25). Still, any identification of the active group responsible for the mentioned succession of the phase transitions in TlGaSe2 could not been done using these experimental methods. It is known that the EPR is a well-established method of the investigation for many problems in condensed matter. One of them is the direct identification of the active group for a structural transformation in crystals. EPR experiments utilize paramagnetic probes incorporated into crystal lattice to obtain information about local structural changes in their surroundings. In this article we report the results of the first investigations of the temperature dependence of EPR spectra of TlGaSe2 compound doped by paramagnetic Fe3+ ions, which substituted Ga sites as local probes. It is also known that the influence of impurity atoms on the phase transitions in ferroelectric crystals is of great interest. The present paper reports the results of measurements of the dielectric constant of Fe-doped TlGaSe2 crystal in the temperature region of successive phase transitions in comparison with earlier observed results from pure TlGaSe2 compounds as well. 2. Experimental techniques TlGaSe2 single crystals were grown in evacuated quartz tubes using the modified Bridgman method. The Fe was added to the growth mixture in amounts corresponding to a molar ratio Fe/Ga of about 2%. The crystals were cleaved easily into plane parallel plates since the morphology of the crystal permits to perform this operation along the a-b plane, which is parallel to the layers. The plates were gently polished, cleaned and covered with silver paste for dielectric constant measurements. The dimensions of the electrodes were 5 × 5 mm2 with an interelectrode distance of 1 mm. Measurements of the real and imaginary parts of the dielectric susceptibility were performed with a HP 4194A Impedance Gain/Phase Analyser at frequency of 5 kHz. A closed-cycle helium cryostat system and Lakeshore 340 model temperature controller were used in the measurements, which allowed to scan the temperature with a rate of about 0.2 K/min and to stabilize the temperature with accuracy better than 0.05 K. The temperature was measured by GaAlAs diode sensor with an accuracy of 0.01 K. The measurements were performed in the temperature range of 80–300 K. The crystallographic axes of the samples for EPR measurements were obtained by X-ray diffraction. The
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Fig. 2. EPR spectra of Fe3+ ions in TlGaSe2 measured at various temperatures for the static magnetic field applied along (a) and perpendicular to the plane of layers (b).
diffraction patterns have shown a presence of Fe ions, located in layer planes of the crystal structure. This result allowed us to make the preliminary assumption about the substitution of trivalent Ga atoms by Fe impurities. The EPR spectra were recorded using Bruker EMX model X-band spectrometer (9.480 GHz). The static magnetic field was varied in the range of 0–16000 G. The field derivative of microwave power absorption (dW/dH1 ) was registered as a function of the applied magnetic field H1 . The static magnetic field (H ) direction was oriented along (100) and (010) planes (directions parallel and perpendicular to the layers respectively). The temperature dependence of EPR spectra was studied in the range of 15–300 K using continuous helium gas flow cryostat made by Oxford Instruments. The temperature stability was better than 0.5 K. 3. Results and discussion In the previous work [18], the fine structure of EPR spectra caused by paramagnetic Fe3+ ions was observed in the Fe-doped TlGaSe2 at room temperature. The spectra were interpreted to correspond to the transitions between the spin multiplet (S = 5/2, L = 0) of the Fe3+ ion, which is split in the local crystal field (CF). Four equivalent Fe3+ centers have been observed in the room temperature EPR spectra. Local symmetry of CF at the Fe3+ site and the CF parameters were determined. It was established that the symmetry axis of the axial component in the CF is making an angle of about 43 degree with the plane of layers of TlGaSe2 crystal. These experimental results indicate that Fe ions substitute Ga ions at the center of the GaSe4 tetrahedrons, and the low-symmetry distortion of the CF is caused by Tl ions located in the trigonal cavities between the tetrahedral complexes. For the present study, the EPR spectra of paramagnetic Fe3+ -doped TlGaSe2 crystals measured in microwave
frequency of 9.5 GHz (X band) at various temperatures between 15 K and 300 K for different orientations. The transformation of EPR spectra on lowering the temperature is presented in Fig. 3. The spectra presented in Fig. 3a have been obtained by the application of the static magnetic field in the direction along the crystal layers (in-plane geometry). On the other hand, Fig. 3b shows the spectra in out-of-plane geometry — on applying the static magnetic field perpendicular to the plane of the crystal layers. The presented temperatures were selected in accordance with the significant temperature changes. The spectra exhibit two profound changes at the temperatures between 100 K and 120 K. As it is seen from the Fig. 2, these changes are accompanied with both splitting and multiplication of resonance lines. The temperature dependence of the positions of the observed resonance lines between 25 K and 300 K has been shown in Fig. 3a and b for in-plane and out-of-plane geometries respectively. These dependences obviously show the processes of splitting and multiplication of the resonance lines on lowering the temperature and passing through the phase transition temperature point at 110 K. Moreover, the temperature behavior of the resonance lines in the vicinity of the phase transition temperature reminds us a well-known temperature behavior of the spontaneous polarization during the ferroelectric phase transition with characteristic critical constants. Thus, considering the temperature dependence of EPR spectra of Fe3+ ions, we can make a conclusion about the active groups responsible for the phase transitions in TlGaSe2 crystal. As mentioned in [18], the low-symmetry distortion of the ligand crystal field on Fe3+ site is caused by Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed transformations of the resonance lines, which are clearly attributed to the mentioned distortion, obviously indicates that the ferroelectric ordering in TlGaSe2 is associated
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Fig. 3. Temperature dependences of the positions of the observed resonance lines of EPR spectra of Fe3+ -doped TlGaSe2 crystals for the static magnetic field applied along (a) and perpendicular to the plane of layers (b).
Fig. 4. (a) EPR spectra of Fe3+ ions in TlGaSe2 obtained at T = 25 K at various orientations of static magnetic field applied on the plane perpendicular to the layers and rotated around the rotation axis which applied on the layer plane; (b) rotational patterns of the resonance fields obtained from EPR spectra (full curves present the results of simulation using the spin-Hamiltonian parameters given in the text).
with the displacements and ordering of Tl ions located at trigonal cavities. This ordering results in the appearing of anomalies in the dielectric properties as well as the considerable changes in EPR spectra from Fe3+ sites. The angular variations of the EPR spectra, which have been obtained by rotating the sample in the “out-of-plane” geometry (the static magnetic field is rotated in (100) plane, which is perpendicular to the layer plane) as well as the experimental
and simulated rotational patterns of the resonance fields are presented in Fig. 4a and b, respectively. Almost the same result is obtained in the second out-of-plane geometry, which is perpendicular to the first one (the static magnetic field is rotated in (010) plane). Both measurements reveal the doubling of the spectral lines at low temperatures, which is attributed to the structural phase transformations at the temperatures near 110 K.
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M. Ac¸ıkg¨oz et al. / Solid State Communications 145 (2008) 539–544 Table 1 Fitting parameters for the rotational patterns observed at 25 K
Fig. 5. Possible rearrangements of Tl atoms around Fe3+ centers located in Ga4 Se10 tetrahedron complexes during the structural phase transition.
Low-temperature EPR spectra of Fe-doped TlGaSe2 can be analyzed with an effective spin Hamiltonian consisting of Zeeman electronic (Z) terms and ZFS terms expressed in extended Stevens (ES) operator notation [19]: X q q H = Hz + HZFS = µ B BgS + Bk Ok , (1) where µ B is Bohr magneton, B is the applied magnetic field, g is the spectroscopic splitting tensor (g-factor), S is the effective q spin operator, and Bk are the ZFS parameters associated with q extended Stevens operators Ok [19,20]. At this stage one can neglect the cubic terms in the spin Hamiltonian, because as shown in [18], they are very small compared to very strong second-order terms in this crystal. Thus, we have used the same form of Spin Hamiltonian, which takes into account the tetragonal and orthorhombic symmetry for the ligand field [21, 22]: H Z F S = D(S Z2 −
1 S(S + 1)) + E(Sx2 − S y2 ) 3
= B20 O20 + B22 O22 .
(2)
The relations between the orthorhombic ZFS parameters in Eq. (2) are then given by 3D Z Z = 3B20 (E S) 2 (D X X − DY Y ) E= = B22 (E S). 2 The evaluated parameters, which were used to simulate the rotational patterns in Fig. 5, are listed in the Table 1. Thus, the analysis and preliminary results of computer simulations of the EPR spectra of Fe3+ centers in TlGaSe2 crystal with the spin Hamiltonian in the above-presented form reveals that the splitting of the EPR spectra at temperatures lower than 110 K is due to doubling in the number of the resonating Fe centers. Obviously, the rotation patterns have shown that two non-equivalent sets of triplets having chemically equivalent Ga sites in the unit cell, which are D=
Site number
g-factor
B20 , Oe
B22 , Oe
1 2 3 4 5 6 7 8
2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00
2900 2900 2650 2650 2650 2650 2350 2350
1950 1950 1950 1950 1200 1250 1200 1250
substituted by Fe3+ ions. Based on this assumption, the simulations of the EPR spectra from all orientations resulted in a well fitting. Thus, there are totally eight Fe3+ centers with different ligand field distortion angles and two different fitting parameters at low temperatures. It is worth mentioning that the analysis of the structure of the low-temperature phase using EPR spectra, along with the determination of the direction and amount of atomic displacements, is additionally of great interest. In our opinion the observed low-temperature doubling in the number of resonating centers is related to the peculiar rearrangement of Tl atoms, which is accompanied by non-equivalent displacements of two different groups of Tl atoms under different directions in the crystal structure of TlGaSe2 . In this matter, it is worth mentioning the suggestions made recently by authors [15–17] about the presence of two order parameters and two polar sublattices in TlGaSe2 . At room temperature Tl atoms lie along the plane of layers having the structurally equivalent positions. Analysis of the low-temperature rotational patterns revealed that on lowering the temperature the arrangement of Tl atoms can take zigzag form, which might lead to the observed twinning of EPR patterns and the above-mentioned multiplication of centers. Possible displacements of Tl atoms arranged along different rows around Ga4 Se10 tetrahedral complexes are presented in Fig. 5. In our point of view, such displacements of Tl atoms are accompanied by changes of inter-layer distances, which also result in peculiar anomalies of thermal expansion coefficient along the direction perpendicular to the layers, observed earlier by authors [23,24]. The temperature dependences of the real and imaginary parts of the dielectric susceptibility of pure and Fe-doped TlGaSe2 crystals measured during heating cycle are shown in Fig. 6. As it is seen from the figure, the dielectric constant values of the doped TlGaSe2 crystal are considerably lower than those of the pure crystal in a wide temperature interval including previously known phase transitions. The temperature behavior of the real part of the dielectric susceptibility of the doped crystal exhibits a single wide peak at the temperature about 110 K instead of the presence of two different anomalies at the temperatures 107 K and 115 K for pure crystal. The measurements of the dielectric constants performed in heating and cooling regimes revealed a more remarkable temperature hysteresis for the doped crystals. The imaginary part of the complex dielectric susceptibility also decreases and exhibits a weak temperature dependence,
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spectra of Fe ion have been observed at the temperatures lower than 110 K, which was attributed to a strong splitting and appearance of multiplication. In the result, one can conclude that the observed phase transition at about 110 K occurs due to the non-equivalent displacements of different groups of Tl ions and accompanied by their zigzag rearrangement, which is possibly caused by sticking of crystal layers and changes of inter-layer distances at these temperatures. Acknowledgments FAM is indebted to Research Projects Commission of Gebze Institute of Technology for supporting this work by the Grant No. 2007-A-14 and to The Scientific & Technological Research ¨ ´ITAK) for supporting by the Grant No. Council of Turkey (TUB 106M540. Fig. 6. Temperature dependences of the real and imaginary parts of the dielectric susceptibility of pure and Fe-doped TlGaSe2 crystals.
which appears as a result of small dielectric losses due to lower domain mobility in the doped samples. These results can be interpreted by taking into account strong interaction of the domain-like structure with lattice defects and impurities near the commensurate phase transitions in the crystals with incommensurate phases. As mentioned in [16,17] annealing of the pure crystal at certain temperatures inside the incommensurate phase results in remarkable shifts of the anomalies at 107 K and 115 K and scanning the temperature under certain regimes gives rise to transformation of the two-peak structure to the one-peak anomaly, which was also interpreted as a result of the influence of defects. The same mechanism was considered as the main reason for the observed decreases of the dielectric constant values, which take place as a result of strong influence of the defect structure on the mobility of the domain walls. Therefore in this context, the observed changes of the dielectric constant for doped TlGaSe2 crystals can be interpreted as a result of the influence of Fe impurities on the domain structure of the polar state, as well as on the active atomic groups responsible for the ferroelectric polarization. 4. Conclusion Thus, the results of low-temperature EPR investigations and dielectric susceptibility measurements of Fe-doped TlGaSe2 crystal led us to understand the phase transitions in this compound. The temperature dependence of the dielectric constant exhibited a single and wide peak at about 110 K and considerable decreases of values of the dielectric constant in a whole temperature interval as a result of the influence of Fe impurities. The EPR study of TlGaSe2 crystal doped by paramagnetic Fe ions shown that the resonance lines originate from the contributions of at least two types of magnetically non-equivalent Fe3+ centers localized at different tetrahedron arrays of the crystal structure. Considerable changes of EPR
References [1] W. Henkel, H.D. Hochheimer, C. Carlone, A. Werner, S. Ves, H.G.v. Schnering, Phys. Rev. B 26 (1982) 3211. [2] D. M¨uller, H. Hahn, Z. Anorg. Allg. Chem. 438 (1978) 258. [3] D.F. McMorrow, R.A. Cowley, P.D. Hatton, J. Banys, J. Phys.: Condens. Matter 2 (1990) 3699. [4] Y. S Kashida,Kobayashi, J. Kor. Phys. Society 32 (1998) S40. [5] R.A. Aliev, K.R. Allakhverdiev, A.I. Baranov, N.R. Ivanov, R.M. Sardarly, Soviet Phys. Solid State 26 (1984) 775. [6] A.V. Vakhrushev, B.E. Kvyatkovskii, N.M. Okuneva, K.R. Allakhverdiev, R.M. Sardarly, Preprint No. 81, A.F. Ioffe Physico-Technical Institute, Leningrad 1984 (in Russian). [7] A.A. Volkov, Yu.G. Goncharov, G.V. Kozlov, S.P. Lebedev, A.M. Prokhorov, R.A. Aliev, K.R. Allakhverdiev, JETP Lett. 37 (1984) 517. [8] K.R. Allakhverdiev, F.M. Salaev, F.A. Mikailov, T.S. Mamedov, JETP Lett. 56 (1992) 153. [9] V. Dvorak, Y. Ishibashi, J. Phys. Soc. Japan 41 (1976) 548. [10] K.R. Allakhverdiev, N.D. Akhmed-zade, T.G. Mamedov, T.S. Mamedov, M.Y. Seidov, Low Temp. Phys. 26 (2000) 56. [11] K.R. Allakhverdiev, M.A. Aldzanov, T.G. Mamedov, E.Yu. Salaev, Solid State Commun. 58 (1986) 295. [12] M.A. Aldzanov, N.G. Guseinov, Z.N. Mamedov, Phys. Status Solidi A 100 (1987) K145. [13] Y.P. Gololobov, V.M. Perga, I.N. Salivonov, E.E. Shchigol, Sov. Phys. Solid State 34 (1992) 59. [14] F.A. Mikailov, E. Bas¸aran, E. S¸ent¨urk, L. T¨umbek, T.G. Mammadov, V.P. Aliev, Phase Transitions 76 (2003) 1057. [15] F.A. Mikailov, E. Bas¸aran, E. S¸ent¨urk, L. T¨umbek, T.G. Mammadov, V.P. Aliev, Solid State Commun. 129 (2004) 761. [16] F.A. Mikailov, E. Bas¸aran, E. S¸ent¨urk, L. T¨umbek, T.G. Mammadov, T.S. Mammadov, Phase Transitions 78 (2005) 413. [17] F.A. Mikailov, E. Bas¸aran, L. T¨umbek, E. S¸ent¨urk, T.G. Mammadov, J. Non-Cryst. Solids 351 (2005) 2809. [18] F.A. Mikailov, B.Z. Rameev, S. Kazan, F. Yildiz, T.G. Mammadov, B. Aktas, Solid State Commun. 133 (2005) 389. [19] S.K. Misra, C. Rudowicz, Phys. Status Solidi A 147 (1988) 677. [20] D.G. McGavin, J. Magn. Reson. 74 (1987) 19. [21] A. Abragam, B. Bleaney, Electron paramagnetic Resonance of Transition Ions, Oxford University Press, Oxford, 1970 (Chapters 3, 7). [22] C. Rudowicz, Magn. Reson. Rev. 13 (1987) 1. [23] N.A. Abdullaev, T.G. Mamedov, R.A. Suleimanov, Low Temp. Phys. 27 (2001) 676. [24] N.A. Abdullaev, T.G. Mamedov, R.A. Suleimanov, Phys. Status Solidi A 242 (2005) 983.
Structural phase transitions in Fe3+-doped ferroelectric TlGaSe2 crystal M. Ac¸ıkg¨oz a , S. Kazan b , F.A. Mikailov b,c,∗ , T.G. Mammadov c , B. Aktas¸ b a Bahcesehir University, Faculty of Art and Sciences, Bes¸iktas¸, 34100, Istanbul, Turkey b Department of Physics, Gebze Institute of Technology, Gebze, 41400, Kocaeli, Turkey c Institute of Physics, Azerbaijan Academy of Sciences, AZ-143, Baku, Azerbaijan
Received 13 August 2007; received in revised form 25 December 2007; accepted 5 January 2008 by E.V. Sampathkumaran Available online 12 January 2008
Abstract This paper presents the results of dielectric constant and Electron Paramagnetic Resonance (EPR) investigations of Fe3+ -doped TlGaSe2 single crystals in the temperature range of 15–300 K. The influence of Fe impurities on dielectric properties and phase transitions of TlGaSe2 crystal has been studied. The results were considered in comparison with earlier observed results from pure TlGaSe2 compounds. We observed the considerable decrease of the dielectric constant as well as the change of the shape of the temperature dependence of the dielectric constant in doped crystals. Some certain significant changes of EPR spectra, which are associated with a strong splitting and appearance of additional resonance lines, were observed at the temperatures below 110 K. Such transformations are considered as the result of non-equivalent displacements of different groups of Tl atoms during the structural phase transitions. c 2008 Elsevier Ltd. All rights reserved.
PACS: 75.10.Dg; 76.30.Da; 76.30.-v; 71.70.Ch Keywords: D. Crystal fields; D. Dielectric properties; D. Electron paramagnetic resonance (EPR); D. Phase transitions
1. Introduction TlGaSe2 is a ternary-layered chalcogenide crystal, which crystallizes in monoclinic system and belongs to a space 6 at room temperature [1]. According to symmetry group of C2h X-ray diffraction measurements [2,3], the crystal structure of TlGaSe2 is characterized by metal–chalcogen layers composed of Ga4 Se10 polyhedron complexes representing a combination of four elementary GaSe4 tetrahedra linked by common chalcogen atoms at the corners (Fig. 1). The elementary unit cell contains two layers containing successive rows of the tetrahedron complexes, which are turned away from each other by 90◦ and each of them is shifted by the length of the edge of the small GaSe4 tetrahedron with respect to another layer ¯ 0] directions. Monovalent Tl atoms are along [1, 1, 0] and [1, 1, located in trigonal prismatic cavities between metal–chalcogen layers. As a result, a deviation from the tetragonal symmetry ∗ Corresponding author at: Department of Physics, Gebze Institute of Technology, Gebze, 41400, Kocaeli, Turkey. Tel.: +90 262 6051311; fax: +90 262 6538490. E-mail address: [email protected] (F.A. Mikailov).
c 2008 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2008.01.009
appears. The angle between the monoclinic c axis and the layer plane is about 100◦ . As mentioned in [4], the origin of the disorder in the crystal structure of TlGaSe2 is due to the fact that owing to the stacking variants and the pseudo-tetragonal symmetry, the crystal frequently contains four layer type twins, where the monoclinic axis are exchanged or the tilting is in the opposite direction. The first publication about the presence of structural phase transitions and ferroelectricity in ternary-layered chalcogenide crystal TlGaSe2 was published about two decades ago [5]. Many researchers investigated the promising structural phase transitions of this compound by using a great number of experimental methods since that time. It has been concluded that, on cooling, TlGaSe2 exhibits a sequence of structural phase transitions, including transitions to an incommensurate (IC) and commensurate (C) phases. According to neutron [6] and X-ray scattering investigations [3], the transition to IC phase, which takes place at Ti1 ∼ 120 K, is associated with condensation of a soft mode at a point in the Brillouin zone characterized by qi = (δ, δ, 0.25), where δ is the
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Fig. 1. Crystal structure of TlGaSe2 composed of GaSe4 tetrahedrons: Tl ions; — Ga ions; — Se ions.
—
incommensuration parameter (δ = 0.02). On subsequent cooling, TlGaSe2 exhibits IC–C phase transition at the temperature of Tc1 ∼ 107 K with condensation of the soft mode at qc = (0, 0, 0.25), which accompanied by the quadrupling of the unit cell volume along the direction perpendicular to the layers. However, the presence of the ferroelectric soft mode with Curie temperature at about Tc ∼ 107 K and with the Curie constant ∼103 was discovered by submillimeter spectra and dielectric constant measurements [7]. The spontaneous polarization vector of the ferroelectric phase lies in the plane of layers. In addition to the successive phase transitions described above, a phase transition to weak ferroelectric phase was observed at the temperatures about 65 K by authors of [8]. This transition could be explained by using the model of two non-equivalent sublattices proposed by [9]. A very interesting result has been observed by Allakhverdiev et al. [10], after measuring the temperature dependencies of the dielectric constant of TlGaSe2 under bias electric field: the electrical field dependence of the commensurate phase transition temperature exhibited a behavior peculiar to antiferroelectric crystals. The detailed investigations of various physical properties of TlGaSe2 at low temperatures, such as optical absorption [11], heat capacity [12] and acoustic emission [13] experiments, revealed additional phase transitions at temperatures about 101–103 K and 246–253 K. As a result of detailed dielectric constant measurements [14–17], two conclusions have been drawn recently: the presence of the incommensurate phase at the temperature interval between 115 K and 242 K and the
coexistence of two strong-interacting polar sublattices in the low-temperature phase of TlGaSe2 . As it is seen, in spite of a number of experimental results, there was no information about active atomic groups, possible atomic displacements causing the dipole ordering, and the local symmetry changes during the low-temperature phase transformations in this compound. As mentioned above, neutron scattering [5] and X-ray studies [6] have shown that the low-temperature ferroelectric phase has a fourfold-commensurate structure. Additionally, the satellite reflections for the incommensurate phase are observed at qc = (0, 0, 0.25). Still, any identification of the active group responsible for the mentioned succession of the phase transitions in TlGaSe2 could not been done using these experimental methods. It is known that the EPR is a well-established method of the investigation for many problems in condensed matter. One of them is the direct identification of the active group for a structural transformation in crystals. EPR experiments utilize paramagnetic probes incorporated into crystal lattice to obtain information about local structural changes in their surroundings. In this article we report the results of the first investigations of the temperature dependence of EPR spectra of TlGaSe2 compound doped by paramagnetic Fe3+ ions, which substituted Ga sites as local probes. It is also known that the influence of impurity atoms on the phase transitions in ferroelectric crystals is of great interest. The present paper reports the results of measurements of the dielectric constant of Fe-doped TlGaSe2 crystal in the temperature region of successive phase transitions in comparison with earlier observed results from pure TlGaSe2 compounds as well. 2. Experimental techniques TlGaSe2 single crystals were grown in evacuated quartz tubes using the modified Bridgman method. The Fe was added to the growth mixture in amounts corresponding to a molar ratio Fe/Ga of about 2%. The crystals were cleaved easily into plane parallel plates since the morphology of the crystal permits to perform this operation along the a-b plane, which is parallel to the layers. The plates were gently polished, cleaned and covered with silver paste for dielectric constant measurements. The dimensions of the electrodes were 5 × 5 mm2 with an interelectrode distance of 1 mm. Measurements of the real and imaginary parts of the dielectric susceptibility were performed with a HP 4194A Impedance Gain/Phase Analyser at frequency of 5 kHz. A closed-cycle helium cryostat system and Lakeshore 340 model temperature controller were used in the measurements, which allowed to scan the temperature with a rate of about 0.2 K/min and to stabilize the temperature with accuracy better than 0.05 K. The temperature was measured by GaAlAs diode sensor with an accuracy of 0.01 K. The measurements were performed in the temperature range of 80–300 K. The crystallographic axes of the samples for EPR measurements were obtained by X-ray diffraction. The
M. Ac¸ıkg¨oz et al. / Solid State Communications 145 (2008) 539–544
541
Fig. 2. EPR spectra of Fe3+ ions in TlGaSe2 measured at various temperatures for the static magnetic field applied along (a) and perpendicular to the plane of layers (b).
diffraction patterns have shown a presence of Fe ions, located in layer planes of the crystal structure. This result allowed us to make the preliminary assumption about the substitution of trivalent Ga atoms by Fe impurities. The EPR spectra were recorded using Bruker EMX model X-band spectrometer (9.480 GHz). The static magnetic field was varied in the range of 0–16000 G. The field derivative of microwave power absorption (dW/dH1 ) was registered as a function of the applied magnetic field H1 . The static magnetic field (H ) direction was oriented along (100) and (010) planes (directions parallel and perpendicular to the layers respectively). The temperature dependence of EPR spectra was studied in the range of 15–300 K using continuous helium gas flow cryostat made by Oxford Instruments. The temperature stability was better than 0.5 K. 3. Results and discussion In the previous work [18], the fine structure of EPR spectra caused by paramagnetic Fe3+ ions was observed in the Fe-doped TlGaSe2 at room temperature. The spectra were interpreted to correspond to the transitions between the spin multiplet (S = 5/2, L = 0) of the Fe3+ ion, which is split in the local crystal field (CF). Four equivalent Fe3+ centers have been observed in the room temperature EPR spectra. Local symmetry of CF at the Fe3+ site and the CF parameters were determined. It was established that the symmetry axis of the axial component in the CF is making an angle of about 43 degree with the plane of layers of TlGaSe2 crystal. These experimental results indicate that Fe ions substitute Ga ions at the center of the GaSe4 tetrahedrons, and the low-symmetry distortion of the CF is caused by Tl ions located in the trigonal cavities between the tetrahedral complexes. For the present study, the EPR spectra of paramagnetic Fe3+ -doped TlGaSe2 crystals measured in microwave
frequency of 9.5 GHz (X band) at various temperatures between 15 K and 300 K for different orientations. The transformation of EPR spectra on lowering the temperature is presented in Fig. 3. The spectra presented in Fig. 3a have been obtained by the application of the static magnetic field in the direction along the crystal layers (in-plane geometry). On the other hand, Fig. 3b shows the spectra in out-of-plane geometry — on applying the static magnetic field perpendicular to the plane of the crystal layers. The presented temperatures were selected in accordance with the significant temperature changes. The spectra exhibit two profound changes at the temperatures between 100 K and 120 K. As it is seen from the Fig. 2, these changes are accompanied with both splitting and multiplication of resonance lines. The temperature dependence of the positions of the observed resonance lines between 25 K and 300 K has been shown in Fig. 3a and b for in-plane and out-of-plane geometries respectively. These dependences obviously show the processes of splitting and multiplication of the resonance lines on lowering the temperature and passing through the phase transition temperature point at 110 K. Moreover, the temperature behavior of the resonance lines in the vicinity of the phase transition temperature reminds us a well-known temperature behavior of the spontaneous polarization during the ferroelectric phase transition with characteristic critical constants. Thus, considering the temperature dependence of EPR spectra of Fe3+ ions, we can make a conclusion about the active groups responsible for the phase transitions in TlGaSe2 crystal. As mentioned in [18], the low-symmetry distortion of the ligand crystal field on Fe3+ site is caused by Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed transformations of the resonance lines, which are clearly attributed to the mentioned distortion, obviously indicates that the ferroelectric ordering in TlGaSe2 is associated
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Fig. 3. Temperature dependences of the positions of the observed resonance lines of EPR spectra of Fe3+ -doped TlGaSe2 crystals for the static magnetic field applied along (a) and perpendicular to the plane of layers (b).
Fig. 4. (a) EPR spectra of Fe3+ ions in TlGaSe2 obtained at T = 25 K at various orientations of static magnetic field applied on the plane perpendicular to the layers and rotated around the rotation axis which applied on the layer plane; (b) rotational patterns of the resonance fields obtained from EPR spectra (full curves present the results of simulation using the spin-Hamiltonian parameters given in the text).
with the displacements and ordering of Tl ions located at trigonal cavities. This ordering results in the appearing of anomalies in the dielectric properties as well as the considerable changes in EPR spectra from Fe3+ sites. The angular variations of the EPR spectra, which have been obtained by rotating the sample in the “out-of-plane” geometry (the static magnetic field is rotated in (100) plane, which is perpendicular to the layer plane) as well as the experimental
and simulated rotational patterns of the resonance fields are presented in Fig. 4a and b, respectively. Almost the same result is obtained in the second out-of-plane geometry, which is perpendicular to the first one (the static magnetic field is rotated in (010) plane). Both measurements reveal the doubling of the spectral lines at low temperatures, which is attributed to the structural phase transformations at the temperatures near 110 K.
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Fig. 5. Possible rearrangements of Tl atoms around Fe3+ centers located in Ga4 Se10 tetrahedron complexes during the structural phase transition.
Low-temperature EPR spectra of Fe-doped TlGaSe2 can be analyzed with an effective spin Hamiltonian consisting of Zeeman electronic (Z) terms and ZFS terms expressed in extended Stevens (ES) operator notation [19]: X q q H = Hz + HZFS = µ B BgS + Bk Ok , (1) where µ B is Bohr magneton, B is the applied magnetic field, g is the spectroscopic splitting tensor (g-factor), S is the effective q spin operator, and Bk are the ZFS parameters associated with q extended Stevens operators Ok [19,20]. At this stage one can neglect the cubic terms in the spin Hamiltonian, because as shown in [18], they are very small compared to very strong second-order terms in this crystal. Thus, we have used the same form of Spin Hamiltonian, which takes into account the tetragonal and orthorhombic symmetry for the ligand field [21, 22]: H Z F S = D(S Z2 −
1 S(S + 1)) + E(Sx2 − S y2 ) 3
= B20 O20 + B22 O22 .
(2)
The relations between the orthorhombic ZFS parameters in Eq. (2) are then given by 3D Z Z = 3B20 (E S) 2 (D X X − DY Y ) E= = B22 (E S). 2 The evaluated parameters, which were used to simulate the rotational patterns in Fig. 5, are listed in the Table 1. Thus, the analysis and preliminary results of computer simulations of the EPR spectra of Fe3+ centers in TlGaSe2 crystal with the spin Hamiltonian in the above-presented form reveals that the splitting of the EPR spectra at temperatures lower than 110 K is due to doubling in the number of the resonating Fe centers. Obviously, the rotation patterns have shown that two non-equivalent sets of triplets having chemically equivalent Ga sites in the unit cell, which are D=
Site number
g-factor
B20 , Oe
B22 , Oe
1 2 3 4 5 6 7 8
2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00
2900 2900 2650 2650 2650 2650 2350 2350
1950 1950 1950 1950 1200 1250 1200 1250
substituted by Fe3+ ions. Based on this assumption, the simulations of the EPR spectra from all orientations resulted in a well fitting. Thus, there are totally eight Fe3+ centers with different ligand field distortion angles and two different fitting parameters at low temperatures. It is worth mentioning that the analysis of the structure of the low-temperature phase using EPR spectra, along with the determination of the direction and amount of atomic displacements, is additionally of great interest. In our opinion the observed low-temperature doubling in the number of resonating centers is related to the peculiar rearrangement of Tl atoms, which is accompanied by non-equivalent displacements of two different groups of Tl atoms under different directions in the crystal structure of TlGaSe2 . In this matter, it is worth mentioning the suggestions made recently by authors [15–17] about the presence of two order parameters and two polar sublattices in TlGaSe2 . At room temperature Tl atoms lie along the plane of layers having the structurally equivalent positions. Analysis of the low-temperature rotational patterns revealed that on lowering the temperature the arrangement of Tl atoms can take zigzag form, which might lead to the observed twinning of EPR patterns and the above-mentioned multiplication of centers. Possible displacements of Tl atoms arranged along different rows around Ga4 Se10 tetrahedral complexes are presented in Fig. 5. In our point of view, such displacements of Tl atoms are accompanied by changes of inter-layer distances, which also result in peculiar anomalies of thermal expansion coefficient along the direction perpendicular to the layers, observed earlier by authors [23,24]. The temperature dependences of the real and imaginary parts of the dielectric susceptibility of pure and Fe-doped TlGaSe2 crystals measured during heating cycle are shown in Fig. 6. As it is seen from the figure, the dielectric constant values of the doped TlGaSe2 crystal are considerably lower than those of the pure crystal in a wide temperature interval including previously known phase transitions. The temperature behavior of the real part of the dielectric susceptibility of the doped crystal exhibits a single wide peak at the temperature about 110 K instead of the presence of two different anomalies at the temperatures 107 K and 115 K for pure crystal. The measurements of the dielectric constants performed in heating and cooling regimes revealed a more remarkable temperature hysteresis for the doped crystals. The imaginary part of the complex dielectric susceptibility also decreases and exhibits a weak temperature dependence,
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spectra of Fe ion have been observed at the temperatures lower than 110 K, which was attributed to a strong splitting and appearance of multiplication. In the result, one can conclude that the observed phase transition at about 110 K occurs due to the non-equivalent displacements of different groups of Tl ions and accompanied by their zigzag rearrangement, which is possibly caused by sticking of crystal layers and changes of inter-layer distances at these temperatures. Acknowledgments FAM is indebted to Research Projects Commission of Gebze Institute of Technology for supporting this work by the Grant No. 2007-A-14 and to The Scientific & Technological Research ¨ ´ITAK) for supporting by the Grant No. Council of Turkey (TUB 106M540. Fig. 6. Temperature dependences of the real and imaginary parts of the dielectric susceptibility of pure and Fe-doped TlGaSe2 crystals.
which appears as a result of small dielectric losses due to lower domain mobility in the doped samples. These results can be interpreted by taking into account strong interaction of the domain-like structure with lattice defects and impurities near the commensurate phase transitions in the crystals with incommensurate phases. As mentioned in [16,17] annealing of the pure crystal at certain temperatures inside the incommensurate phase results in remarkable shifts of the anomalies at 107 K and 115 K and scanning the temperature under certain regimes gives rise to transformation of the two-peak structure to the one-peak anomaly, which was also interpreted as a result of the influence of defects. The same mechanism was considered as the main reason for the observed decreases of the dielectric constant values, which take place as a result of strong influence of the defect structure on the mobility of the domain walls. Therefore in this context, the observed changes of the dielectric constant for doped TlGaSe2 crystals can be interpreted as a result of the influence of Fe impurities on the domain structure of the polar state, as well as on the active atomic groups responsible for the ferroelectric polarization. 4. Conclusion Thus, the results of low-temperature EPR investigations and dielectric susceptibility measurements of Fe-doped TlGaSe2 crystal led us to understand the phase transitions in this compound. The temperature dependence of the dielectric constant exhibited a single and wide peak at about 110 K and considerable decreases of values of the dielectric constant in a whole temperature interval as a result of the influence of Fe impurities. The EPR study of TlGaSe2 crystal doped by paramagnetic Fe ions shown that the resonance lines originate from the contributions of at least two types of magnetically non-equivalent Fe3+ centers localized at different tetrahedron arrays of the crystal structure. Considerable changes of EPR
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