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Dielectric constant and EPR spectraof Fe doped TlInS2 crystal near thestructural phase transitionsS. Kazan a , M. Açikgöz b , F.A. Mikailov a c , T. Mammadov c & B.Aktaş aa Department of Physics, Gebze Institute of Technology, Gebze,Kocaeli, Turkeyb Faculty of Art and Sciences, Bahcesehir University, Beşiktaş,Istanbul, Turkeyc Institute of Physics, National Academy of Sciences, AZ-143,Baku, Azerbaijan

Available online: 05 Jun 2008

To cite this article: S. Kazan, M. Açikgöz, F.A. Mikailov, T. Mammadov & B. Aktaş (2008): Dielectricconstant and EPR spectra of Fe doped TlInS2 crystal near the structural phase transitions, PhaseTransitions, 81:6, 581-589

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Phase TransitionsVol. 81, No. 6, June 2008, 581–589

Dielectric constant and EPR spectra of Fe doped TlInS2 crystal near

the structural phase transitions

S. Kazana, M. Acikgozb, F.A. Mikailovac*, T. Mammadovc and B. Aktas� a

aDepartment of Physics, Gebze Institute of Technology, Gebze, Kocaeli, Turkey; bFaculty of Artand Sciences, Bahcesehir University, Bes� iktas� , Istanbul, Turkey; cInstitute of Physics,

National Academy of Sciences, AZ-143, Baku, Azerbaijan

(Received 14 January 2008; final version received 27 February 2008)

Dielectric constant and electron paramagnetic resonance (EPR) spectra ofFe-doped TlInS2 crystal has been investigated in the temperature range of5–300K. The influence of Fe impurities on dielectric properties of TlInS2 crystalhas been considered in comparison with earlier observed results from pure TlInS2compounds. Considerable decrease of the dielectric constant, as well as thechange of the shape of its temperature dependence in a result of influence ofdoped iron atoms, has been observed. It has been established that EPR linesexhibit remarkable splitting and appearance of additional resonance lines at thetemperatures lower than 200K, which is attributed earlier as the temperature ofthe ferroelectric phase transition. The simulation for low-temperature EPRspectra has been performed taking into account the appearance of four additionalcenters in addition to four equivalent centers observed from EPR spectra at roomtemperature. Such transformations are considered as result of nonequivalentdisplacements of different groups of Tl atoms during the structural phasetransitions.

Keywords: electron paramagnetic resonance; ferroelectrics; phase transitions;crystal fields

1. Introduction

The layered chalcogenide TlInS2 crystal crystallizes in monoclinic system and belongs toa space symmetry group of C6

2h at room temperature [1]. The crystal structure of TlInS2is characterized by metal-chalcogen layers composed of In4S10 polyhedron complexesrepresenting a combination of four elementary InS4 tetrahedra linked by commonchalcogene atoms at the corners (Figure 1). The unit cell has two layers containingsuccessive rows of the tetrahedrons. The layers are turned away from each other by 90�

and shifted a quarter of a unit along [1, 1, 0] and 1, �1, 0� �

directions. Monovalent Tl ionsare located in the trigonal cavities between metal–chalcogen layers.

The presence of ferroelectric phase transition in TlInS2 was first reported in [2]. Sincethat time the structural phase transitions in this compound were investigated using a great

*Corresponding author. Email: faik@gyte.edu.tr

ISSN 0141–1594 print/ISSN 1029–0338 online

� 2008 Taylor & Francis

DOI: 10.1080/01411590802017476

http://www.informaworld.com

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number of experimental methods. It has been established that, on cooling, TlInS2 exhibitssuccessive phase transitions to an incommensurate (IC) and commensurate (C) phases.According to neutron [3] and X-ray scattering investigations [4], the transition to IC phase,which takes place at Ti1� 216K, is associated with condensation of a soft mode at a pointin the Brillouin zone characterized by qi(�, �, 0.25), where � is the incommensurationparameter (�¼ 0.012). On subsequent cooling, TlInS2 exhibits IC–C phase transition at thetemperature of Tc1� 200K with condensation of the soft mode at qc¼ (0, 0, 0.25), which isaccompanied by the quadrupling of the unit cell volume along the direction perpendicularto the layers. However, the presence of the ferroelectric soft mode with Curie temperatureat about Tc� 200K and with the Curie constant �103 was discovered by submillimeterspectra and dielectric constant measurements [5]. It was revealed that the spontaneouspolarization vector of the ferroelectric phase lies in the plane of layers.

In addition to the successive phase transitions described above, a phase transition to aweak ferroelectric phase at Ttr¼ 79K was observed in [6]. The authors reported that thiseffect can be explained using the model of two non-equivalent sublattices, proposed byDvorak and Ishibashi [7]. So, according to this model, the phase transition is accompaniedwith the appearance and increasing of additional polarization with direction oppositeto that of the ferroelectric spontaneous polarization.

The first EPR investigations of TlInS2 crystal were performed by some authors of therecent paper [8,9]. The room temperature EPR spectra of Fe doped TlInS2 wereinterpreted in [8] to correspond to the transitions between spin multiplets (S¼ 5/2, L¼ 0)of the Fe3þ ion, located at the center of the InS4 tetrahedra formed by S atoms. Theorthorhombic symmetry of the crystal field (CF) at the Fe3þ site was determined.According to the results, the resonance lines originate from the contributions of fourstructurally equivalent Fe3þ centers localized at different tetrahedron arrays of the

Figure 1. Crystal structure of TlInS2 composed of InSe4 tetrahedrons. – Tl ions; – In ions;– S ions.

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crystal structure. It was established that the symmetry axis of the axial component in the

CF is making an angle of about 48� with the plane of layers of TlInS2 crystal.

Experimental results indicate that Fe ions substitute into the In site at the center of the

InS4 tetrahedrons, and the rhombic distortion of the CF is caused by Tl ions located in the

trigonal cavities between the tetrahedral complexes.Besides, low-temperature EPR investigations of Fe doped TlInS2 single crystal [9]

showed that the resonance lines exhibit a slight shift and remarkable splitting on

decreasing the temperature to lower than 200K, which was attributed to the effect of

the ferroelectric phase transition. It has been established that the splitting of EPR lines

results from the doubling of the number of the resonating centers at low temperatures.

The simulation of EPR spectra can be performed using the same fitting parameters as

used before [8] for room temperature spectra. However, a small rotation of the

principal axis system of the fine structure terms in the spin Hamiltonian should be

taken into account on performing the simulation. It has been revealed that the

low-temperature transformations of EPR lines are related to the rearrangement of Tl

atoms in the plane of layers, which is accompanied by nonequivalent displacements of

two different groups of Tl atoms. As mentioned in [9], detailed analysis of the

low–temperature data, which is very complicated due to large number of parameters

involved, would be published later.This article presents detailed results of low-temperature EPR investigations of

Fe-doped TlInS2 crystal in the context with macroscopic measurements of the temperature

dependence of dielectric constant in the temperature interval of well-known phase

transitions in this compound.

2. Experimental technique

TlInS2 single crystals were grown in evacuated quartz tubes using the modified Bridgman

method. For EPR experiments, Fe was added to the growth mixture in amounts

corresponding to a molar ratio Fe/In of about 0.7%. The crystals were cleaved easily into

plane parallel plates since the morphology of the crystal permits performing 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�3 mm2 with an inter-electrode distance of 1mm.Measurements of the dielectric constant were performed with a HP 4194A Impedance

Gain/Phase Analyser at a 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 0.2Kmin�1 and to stabilize the

temperature with accuracy better than 0.05K. The temperature was measured by GaAlAsdiode sensor with an accuracy of 0.01K. The measurements were performed in the

temperature range of 80–300K.The EPR spectra were recorded using Bruker EMX model X-band spectrometer

(9.480GHz). The static magnetic field was varied in the range of 0–16000G. 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 15–300K using continuous helium gas flow cryostat made by Oxford

Instruments.

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3. Results and discussion

The temperature dependences of dielectric constant of pure and iron doped TlInS2 crystalsmeasured during heating cycle are shown in Figure 2. As it is seen from the figure, thedielectric constant values of the doped TlInS2 crystal are considerably lower than thoseof the pure crystal in a wide temperature interval including previously known phasetransitions. The temperature behaviour of the real part of the dielectric susceptibility of thedoped crystal exhibits a single wide anomaly at the temperatures 190–210K instead of thepresence of three different anomalies at the temperatures 196, 199, and 215K for purecrystal. Additionally it should be pointed out here that the measurements of the dielectricconstants performed in heating and cooling regimes revealed a more remarkabletemperature hysteresis for doped crystals.

This result can be interpreted taking into account strong interaction of the domain-likestructure with lattice defects and impurities near the commensurate phase transitions in thecrystals with incommensurate phases. As it is mentioned in [6], annealing of the purecrystal at the certain temperatures inside the incommensurate phase results in remarkableshifts of the dielectric constant anomalies of pure crystal and scanning the temperatureunder certain regimes gives rise to transformation of the temperature behaviour of thedielectric constant, which was also interpreted as a result of the influence of defects.The same mechanism was considered as a main reason for the observed changes of thedielectric constant values, which take place as a result of strong influence of the defectstructure on the mobility of the domain walls.

According to the Landau theory of incommensurate systems [10,11], their dielectricsusceptibility consists of normal and ‘‘anomalous’’ parts. ‘‘Anomalous’’ part of thedielectric constant arises due to movements of discommensurations (domain walls) in

Figure 2. Temperature dependences of the dielectric constant of pure and iron doped TlInS2crystals.

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response to applied electric field and this part determines mainly a magnitude of thedielectric susceptibility in the temperature range of incommensurate–commensurate phasetransitions. So, the influence of defects, impurities and other imperfections on the dielectricconstant is very remarkable and observable phenomena on investigating the crystalspossessing successive IC and C phase transitions and is therefore of particular interest.

Thus, in this context, the observed changes of the dielectric constant for doped TlInS2crystal can be interpreted as a result of the influence of Fe impurities on thediscommensurations of IC state, domain structure of the polar state, as well as on theactive atomic groups responsible for the ferroelectric polarization.

Electron Parmagnetic Resonance spectra of Fe3þ doped TlInS2 crystals measured inmicrowave frequency of 9.5GHz (X band) at various temperatures between 5K and 300Kon applying the static magnetic field in (100) and (010) planes of the plate-like sample(so-called ‘‘out of plane’’ geometries) are presented in Figure 3. The temperatures inthe figure are selected in accordance with the significant temperature changes of EPRspectra. For the measuring of the temperature dependence the best orientation of the staticmagnetic field for the simplest EPR spectra is selected. The temperature dependences ofthe positions of the observed resonance field lines between 20K and 300K are shown inFigures 4(a) and (b) for the same geometries as in Figures 3(a) and (b), respectively. Thesedependences obviously show the processes of splitting and multiplication of the resonancelines on lowering the temperature and passing through the phase transition temperaturepoint at about 200K.

The angular variations of the EPR spectra, which have been obtained on rotating thesample 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 simulatedrotational patterns of the resonance fields are presented in Figures 5(a) and (b),respectively. Both measurements reveal the doubling of the spectral lines at low

Figure 3. EPR Spectra of Fe ions in TlInS2 measured at various temperatures on applying the staticmagnetic field in (100) and (010) planes (the directions perpendicular to the plane of layers).

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Figure 5. (a) EPR spectra of Fe3þ ions in TlInS2 obtained at T¼ 20K at various orientations ofstatic magnetic field applied on the plane perpendicular to the layers and rotated around the rotationaxis which lied on the layer plane; (b) rotational patterns of the resonance fields obtained from EPRspectra (full curves present the results of simulation using the spin-Hamiltonian parameters given inTable 1).

Figure 4. Temperature dependences of the positions of the observed resonance lines of EPR spectraof Fe3þ doped TlInS2 crystals for the static magnetic field applied in (100) and (010) planes(the directions perpendicular to the plane of layers).

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temperatures, which is attributed to the structural phase transformations at thetemperatures near 200K.

Low-temperature EPR spectra of Fe doped TlInS2 can be analyzed with an effectivespin Hamiltonian consisting of Zeeman electronic (Z) terms and ZFS terms expressed in

extended Stevens (ES) operator notation [12]:

H ¼ Hz þHZFS ¼ �BBgSþX

BqkO

qk ð1Þ

where �B is Bohr magneton, B is the applied magnetic field, g is the spectroscopic splittingtensor (g-factor), S is the effective spin operator and B

qk are the ZFS parameters associated

with extended Stevens operators Oqk [12,13]. At this stage one can neglect the cubic terms in

the spin Hamiltonian, because as shown in [8], they are very small compared to verystrong second-order terms in this crystal. So, we have used the same form of Spin

Hamiltonian, which takes into account the tetragonal and orthorhombic symmetry for theligand field [14,15]:

HZFS ¼ B02O

02 þ B2

2O22: ð2Þ

The evaluated parameters, which were used to simulate the rotational patterns in

Figure 5, are listed in Table 1. Here it should be pointed out that the g-value of Fe3þ ionsin many systems such as [14,16,17] is close to the free spin value of 2.0023. TheLS-coupling introduced a small admixture of exited state 4P5/2 in to the ground state 6S5/2.

For this reason, that is a good approximation the g-value would be as 2.0.Hence, we obtained in total eight Fe3þ centers having two different fitting parameters

and different ligand field distortion angles at low temperatures. The analysis and results of

computer simulations of low temperature EPR spectra of Fe3þ centers in TlInS2 crystalusing the spin Hamiltonian form in (1) revealed that the splitting of the EPR spectra attemperatures lower than 200K is due to doubling in the number of the resonating Fe3þ

centers. The rotation patterns in Figure 5(b) showed two magnetically nonequivalent setsof Fe spin multiplets, which are substituted into chemically equivalent In sites of the

crystal structure.As it was established in [8,9], the symmetry axis of the axial component in the CF is

making an angle of about 48� with the plane of layers of TlInS2 crystal at roomtemperature. The principal axes of fine structure splitting were determined in [9]. The

detailed investigations of low-temperature EPR spectra and study of rotational patternspresented in Figure 5 brought to determination of the principal axes of fine structure

splitting (and/or changes of directions of CF distortions due to Tl displacements) of eightcenters at low temperatures as follows:

x1,2 ¼ ½sinð44�Þ, 0, �cosð44�Þ�, y1,2 ¼ ½0, 1, 0�, z1,2 ¼ ½sinð44

�Þ, 0, �cosð44�Þ�,

x3,4 ¼ ½1, 0, 0�, y3,4 ¼ ½0, sinð45�Þ, �cosð45�Þ�, z3,4 ¼ ½0, sinð45

�Þ, �cosð45�Þ�,

x5,6 ¼ ½sinð46�Þ, 0, �cosð46�Þ�, y5,6 ¼ ½0, 1, 0�, z5,6 ¼ ½sinð46

�Þ, 0, �cosð46�Þ�,

x7,8 ¼ ½1, 0, 0�, y7,8 ¼ ½0, sinð47�Þ, �cosð47�Þ�, z7,8 ¼ ½0, sinð47

�Þ, �cosð47�Þ�:

Table 1. Fitting parameters for the rotational patterns observed at 15K.

Centers g-factor B02 (Oe) B2

2 (Oe)

1, 2, 3, 4 2.0 2450 17005, 6, 7, 8 2.0 2600 1645

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It is worth mentioning that the analysis of the structure of the low-temperaturephase using EPR spectra, along with the determination of the direction and amountof atomic displacements, is also of great interest. In our opinion, the observedlow-temperature doubling in the number of resonating centers is related to the peculiarrearrangement of Tl atoms, which is accompanied by non-equivalent displacements of twodifferent groups of Tl atoms under different directions in the crystal structure of TlGaSe2.In this matter, it is worth mentioning the suggestion made in [6] about the presence of two-order parameters and two-polar sublattices in TlInS2. At room temperature Tl, atoms liealong the plane of layers having the structurally equivalent positions. Analysis of thelow-temperature rotational patterns revealed that on lowering the temperature thearrangement of Tl atoms can take zigzag form, which might lead to the observed twinningof EPR patterns and the above mentioned multiplication of centers.

Thus, lowering the temperature brings to multiplication of centers (doubling of thenumber of resonating centers) of EPR spectra of four Fe centers observed at roomtemperature, and as a result, the presence of four couple of structurally equivalent butmagnetically nonequivalent Fe centers. We think that such a situation can be reached onlyin the case of rearrangement of Tl atoms, which is accompanied by nonequivalentdisplacements of two different group of Tl atoms arranged along different rows aroundIn4S10 tetrahedral complexes.

4. Conclusion

Thus, the temperature dependence of the dielectric constant exhibited a single and wideanomaly at the temperatures between 190 and 210K and considerable decreases in thevalues of the dielectric constant in a whole temperature interval as a result of influence ofFe impurities. It has been established that resonance lines of EPR spectra exhibit shiftingand remarkable splitting on lowering the temperature. The rotational patterns show thatthe EPR lines, which originate from Fe centers located at the centers of InS4 tetrahedrons,exhibit a multiplication of number of the centers at low temperatures. The simulation canbe performed using the same fitting parameters as it has been used for room temperaturespectra, taking into account the appearance of four additional centers in addition to fourequivalent centers observed from EPR spectra at room temperature. We think that such asituation can be reached only in the case of rearrangement of Tl atoms, which isaccompanied by nonequivalent displacements of two different group of Tl atoms followingby their zigzag rearrangement, which, in our point of view, is caused by sticking of crystallayers during structural phase transitions at this temperature interval.

Acknowledgements

F.A. Mikailor is indebted to Research Projects Commission of Gebze Institute of Technology forsupporting this work by the Grant No. 2007-A-14 and to The Scientific & Technological ResearchCouncil of Turkey (TUB_ITAK) for supporting by Project. No. 106M540.

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