Structure, dielectric, and magnetic properties of Sr2TiMnO6 ceramicsK. R. S. Preethi Meher,1 M. Savinov,2 S. Kamba,2 V. Goian,2 and K. B. R. Varma1,a�

1Materials Research Centre, Indian Institute of Science, Bangalore 560012, India2Institute of Physics, ASCR, Na Slovance 2, 18221 Prague 8, Czech Republic

�Received 9 June 2010; accepted 12 September 2010; published online 9 November 2010�

Sr2TiMnO6, a double perovskite associated with high degree of B-site cation disorder wasinvestigated in detail for its structural, magnetic, and dielectric properties. Though x-ray powderdiffraction analysis confirms its cubic structure, first order Raman scattering and infrared reflectivityspectra indicate a breaking of the local cubic symmetry. The magnetization study reveals ananomaly at 14 K owing to a ferrimagnetic/canted antiferromagneticlike ordering arising from localMn–O–Mn clusters. Saturated M-H hysteresis loops obtained at 5 K also reflect the weakferromagnetic exchange interactions present in the system and an approximate estimation ofMn3+ /Mn4+ was done using the magnetization data for the samples sintered at differenttemperatures. The conductivity and dielectric behavior of this system has been investigated in abroad temperature range of 10 to 300 K. Intrinsic permittivity was obtained only below 100 Kwhereas giant permittivity due to conductivity and Maxwell–Wagner polarization was observed athigher temperatures. X-ray photoemission studies further confirmed the presence of mixed oxidationstates of Mn and the valence band spectra analysis was carried out in detail. © 2010 AmericanInstitute of Physics. �doi:10.1063/1.3500369�

I. INTRODUCTION

Double perovskites have recently attracted considerableattention due to their unique electrical and magnetic proper-ties. They are generally defined by the formula A2B�B�O6,where the A site is occupied by alkaline earth metals such asCa, Ba, Sr, etc., or rare earth ions of larger ionic radii, B� andB� octahedral sites are occupied by the transition metal cat-ions or lanthanides with smaller ionic radii. We are espe-cially concerned about the subgroup of these compoundswhere B� is a transition metal cation with no free electrons�3d0 or 4d0 electronic configurations� and B� is a transitionmetal cation belonging to 3d or 4d electronic configurationwith a large spin value. The structure and the electronic prop-erties of these compounds depend on the ionic radii and thevalence states of the cations. The ordering of B-site cationsoccupying the octahedral sites in a rocksalt or a layered lat-tice depends on the difference between the ionic radii/valence states of B� and B� cations. Previous studies havedemonstrated that higher the difference, the tendency to or-der is high in such compounds. Double perovskites exhibitinteresting combination of conductivity, dielectric, and mag-netic properties depending on the degree of octahedral tiltingthat changes the extent of orbital overlap between the B-siteoctahedral networks. There was revival of interest in study-ing this family of compounds after Sr2FeMoO6, a half-metallic ferromagnetic oxide that was found to exhibit colos-sal magnetoresistance at room temperature.1,2 New doubleperovskite oxides such as LaBaZnTaO6, LaBaZnNbO6, andBa2Zn0.5Ti0.5TaO6 have been investigated for their relativelylow k dielectric properties coupled with very low lossvalues.3 The recent discovery of magnetodielectric effect ormagnetocapacitance near room temperature in ferromagnetic

semiconductors such as La2NiMnO6 and La2MnCoO6 hasgiven a fresh perspective in terms of the search for the co-existence of dielectric and the magnetic properties in similarcompounds.4–6 However, high electrical conductivity exhib-ited by these compounds at room temperature makes themunfit for any direct applications in appropriate devices.Therefore, there is a constant search for compounds that ex-hibit strong spin orbit lattice coupling and still possessesbetter resistive properties. Recently, compounds of genericformula Sr2MMnO6 �where M is a diamagnetic transitionmetal cation of 3d0 electronic configuration such as Sb5+,Ti4+, Nb5+, W6+, Mo6+, Ta5+, etc.� are under intensive studydue to their interesting dielectric and magnetic properties.7–10

Previous studies have shown that these 3d0 cations of smallerionic radii have higher polarizability in a lattice. From themagnetism point of view, these compounds exhibit a varietyof properties when these are cooled below their orderingtemperature. The indirect superexchange interaction betweenthe magnetic cations is usually mediated by the non magneticcation octahedra such as Mn–O–M–O–Mn. The magneticproperties like Curie temperature and remnant magnetizationdepend on the cation antisite disorder and oxygen nonsto-ichiometry. Sr2SbMnO6 �SSMO� has been found to exhibitextreme values of dielectric constant due to the extrinsicmechanism such as Maxwell–Wagner polarization at thegrain/grain boundary interface. SSMO exhibits complexmagnetic characteristics with weak ferromagnetic correla-tions occurring below 30 K and high cation disorder owingto the presence of Mn–O–Mn clusters. The structure andmagnetic characteristics of Sr2TiMnO6 �STMO� ceramicswas first reported by Roa-Rojas et al.11 According to thisreport, the structure of STMO was tetragonal with latticeparameters a=5.4702 Å and c=7.7361 Å and also a ferro-magnetic to a paramagnetic �PM� transition was found tooccur at 44.8 K for STMO. The high dielectric constant val-a�Electronic mail: kbrvarma@mrc.iisc.ernet.in.

JOURNAL OF APPLIED PHYSICS 108, 094108 �2010�

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ues exhibited by STMO ceramics �though studies were car-ried out only in a very narrow temperature range of 180 to300 K� was reported in our earlier paper and were attributedto the Maxwell–Wagner type relaxation behavior.12 Recentlythere is another report on dielectric and magnetic character-istics of STMO ceramics whose nanosized powders werefabricated by liquid mix route technique.13 However, the cru-cial parameters such as oxygen stoichiometry and the cationdisorder vary significantly with different synthesis methods.The dielectric property of STMO has been reported only in ahigh temperature range of 250 to 700 K where the conduc-tivity is significantly high giving rise to more extrinsic ef-fects. A review of all the recent work done on STMO em-phasizes the need for a structural, dielectric and magneticcharacterization of STMO over a wide temperature range. Inthis work, we investigate the structure of STMO, which wassynthesized by conventional solid state reaction route, usingx-ray diffraction �XRD�, Raman and infrared spectroscopyfollowed by its magnetic and dielectric characteristics downto liquid helium temperatures. Also, the electronic structureof STMO has been studied using x-ray photoemission spec-troscopy.

II. EXPERIMENTAL

The STMO polycrystalline powder samples were pre-pared by conventional solid-state synthesis route. Thesamples were prepared using SrCO3, MnO2, and TiO2 thatwere preheated at 1073 and 1273 K for several times. Thepreheated powders were pressed into pellets and sintered at1573 and 1623 K for 24 h in the ambient atmosphere toobtain maximum density. High resolution XRD pattern of thepowders obtained from the sintered samples were collectedusing Cu K� radiation in the 2� range of 20° to 110° with astep size of 0.016° at a scan speed of 0.05° per minute.Rietveld refinement of the diffraction pattern was carried outusing the Bruker AXS TOPAS �version 3� software. The x-rayphotoelectron �XPS� spectroscopy measurement was per-formed on the sintered pellets of STMO using ThermofisherScientific-ESCA x-ray photoelectron spectrometer equippedwith Al K� monochromatic source �h�=1486.7 eV� oper-ated at a base pressure of 10−7 Pa. Magnetization measure-ments were carried out in a superconducting quantum inter-ference device �SQUID� magnetometer �physical propertymeasurement system �PPMS� Quantum Design� and ac sus-ceptibility measurement unit. Raman spectra were recordedwith a HR800 �Jobin Yvon� micro Raman spectrometer. Asmall quantity of the sample was placed on a glass slide onthe microscopy stage. A 514 nm Ar ion laser was used forexcitation of the sample. Power incident on the sample sur-face was about 0.5 mW, with a laser spot size of about1.5 �m with 100� objective. Dispersion of the beam wasachieved through prism and 1800 gr/mm grating. Data acqui-sition was done in the range 2000−100 cm−1 from the laserline for 30 s. Infrared reflectivity spectra were obtained usinga Bruker IFS 113V spectrometer equipped with a helium-cooled Si bolometer operating at 1.6 K. An Optistat continu-ous flow cryostat from Oxford instruments was used forcooling the sample down to 10 K. The conductivity and com-

plex dielectric permittivity in the frequency �1 Hz to 1 MHz�and temperature �10–300 K� range was measured using theImpedance Analyzer Alpha AN �Novocontrol� under a weakfield of 0.68 V/mm in conjunction with a Leybold–Heraeuscryostat. The sintered pellets with silver paste electrodeswere used for measurements and the sample was cooled at arate of 2 K/min. Transmission electron microscopy �TEM��Technai T20 Electron microscope at an operating voltage of200 kV� was also employed to study the crystal structure ofSTMO at nanoscopic level.

III. RESULTS AND DISCUSSION

The room temperature x-ray powder diffraction patternobtained for STMO polycrystalline powders derived fromthe pellets sintered at 1623 K is shown in Fig. 1. XRD re-veals a single phase cubic structure with no trace of the su-perstructure reflections like �101� or �103� pertaining to theB-site cation ordering. The absence of superstructure reflec-tions indicates the presence of high degree of cation disorderthough it might not be completely 100%. According toAnderson et al.,14 the cation ordering depends on criticalfactors such as the ionic radii of the cations occupying theoctahedral interstices and their corresponding oxidationstates. Studies on various double perovskite systems revealthat the cation ordering occurs only when there is a signifi-cant difference between the ionic radii or the difference be-tween their oxidation states is greater than 2. The ionic radiiof Ti4+ and Mn4+, which are closer to each other, could bethe reason for high degree of antisite disorder in STMO withno direct indication of unit cell doubling as evidenced byXRD. Rietveld refinement of the structure was performedusing TOPAS 4.0 software. The peak profiles were fitted usinga pseudo-Voigt function and the background was fitted with atenth order polynomial function. An Rwp factor of 3.7% and

GOF of 2.41 were obtained for a cubic structure �Fm3̄m�with the lattice parameter a=7.712 Å. The details of the

FIG. 1. �Color online� Refined powder x-ray powder diffraction pattern ofSTMO at room temperature.

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equivalent positions of the atoms and the corresponding oc-cupancies and thermal factors are listed in Table I. Howeverinfrared �IR� and Raman studies on this system could effec-tively be used for confirming the actual crystal structure andthe extent of cation disorder in STMO. Moreover, we haveprovided supplementary information15 on the evidence forthe existence of certain degree of local cationic ordering inSr2TiMnO6 through TEM studies.

IV. INFRARED REFLECTIVITY AND RAMANSCATTERING STUDIES

Room-temperature Raman and IR spectra are shown inFigs. 2�a� and 2�b�, respectively. Four distinct Raman peaksare seen in the spectrum, but detail fits of the spectrum re-vealed their splitting leading to seven modes such as 138,200, 487, 533, 746, 788, and 1283 cm−1 after deconvolution.IR relectivity spectrum consists of six polar modes where thecorresponding frequencies such as 117, 141, 165, 189, 467,and 539 cm−1 are marked in Fig. 2�b�. A disordered double

perovskite with Pm3̄m structure allows theoretically only3T1u IR modes and none are Raman active.16,17 However, fora completely ordered double perovskite system possessing

Fm3̄m crystal structure, four IR active modes �of symmetryT1u� and four Raman active modes �A1g, Eg, and 2T2g� areallowed. One can observe that the number of observedmodes in both the spectra is much higher than the predictedwhich conveys that the local crystal symmetry of STMO islower than a perfect cubic structure. The difference betweenthe observed values of active modes in IR and Raman spec-tra indicates that the structure is still centrosymmetric.17 Inother words, STMO cannot be a ferroelectric and it remainsparaelectric at least down to 10 K which was further con-firmed by IR measurements where no significant change wasobserved in IR relectivity spectra on cooling down to 10 K.Here A1g is a symmetric mode where the oxygen ions movetoward or away from the oxygen octahedra and in general isof very high intensity compared to the other vibration modes.In the case of Eg mode, four coplanar oxygens move towardor outwards with two apical oxygens moving outwards ortoward the center of the octahedra. Usually the Eg modegives a very weak signal in the case of cubic double perovs-kites. The Raman spectra recorded for STMO shown in Fig.2�a� exhibits distinct first order lines indicative of a break inthe perfect cubic symmetry. The higher intensity mode at788 cm−1 obtained by deconvolution is the high symmetryA1g mode corresponding to the totally symmetric vibrationof TiO6�MnO6� octahedron �the peak split indicates clearpresence of more than one vibration mode—inset in Fig. 2�a�shows the deconvolution of the peaks�. In the case of Mgdoped SrTiO3, the mode which occurs closer to the highsymmetry A1g mode was identified to be an X2 mode whichbecomes Raman active owing to the inherent cation disorder-ness in the system.18 STMO is also a similarly cation disor-dered system giving rise to the possibility of the mode ob-served at 746 cm−1 to be an X2 mode. Further the two peaksat 487 and 533 cm−1 are attributed to the T2g modes. The T2g

mode in the first order Raman spectrum of SrTiO3 occurs ataround 475 cm−1 and splitting of this peak could be indica-tive of a break in the local symmetry as observed in the caseof the double perovskite Sr2TiFeO6 which belongs to the

Fm3̄m structure.19 Any distortion of oxygen octahedra in-duced by the presence of mixed Mn3+ /Mn4+ cations couldalso lead to the distinct splitting of T2g mode in the case ofSTMO. The low frequency modes at 200 cm−1 could be at-tributed to the T2g mode whereas the broad low intensitymode at 1283 cm−1 was identified as a combination mode�A1g+T2g�.

V. MAGNETIC CHARACTERIZATION

dc magnetization measurements were carried out on sin-tered pellets of STMO in the temperature range of 4.2–300 Kat an applied field of 100 Oe under both zero-field-cooled�ZFC� and field-cooled �FC� conditions. Samples of STMOsintered at different temperatures such as 1573 and 1623 Kwere also subjected to dc magnetization measurements inorder to ascertain the effect of synthesis temperature on themagnetic ordering. Therefore, the samples are being identi-

TABLE I. Details of the atoms, their refined equivalent positions, occu-pancy, and the thermal factors.

Atoms x y z Occupancy Beq

Sr 0.25 0.25 0.25 1 1.82�21�Mn 0.0 0.0 0.0 1 0.86�30�Ti 0.5 0.5 0.5 1 1.756�22�O 0.235 33�96� 0.0 0.0 1 1.839�52�

FIG. 2. �Color online� �a� Room-temperature Raman scattering spectrum �b�and infrared reflectivity spectra of STMO ceramics at 10 and 300 K. Theinset of �a� shows the deconvolution of Raman peaks and the peak positions.Frequencies of polar modes are marked with arrows in IR relectivity spectra.The low temperature infrared spectrum was taken only below 600 cm−1 dueto opacity of polyethylene windows in cryostat.

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fied as STMO1573 and STMO1623, respectively. The ZFCcurve of STMO1623 exhibits two anomalies around 44 Kand 14 K, respectively, as indicated in Fig. 3 and there wasno significant change in these temperatures with the synthe-sis conditions. The peak around 44 K observed in the ZFCcurve is accompanied by a saturation of the magnetizationvalue in the FC curve indicating a ferromagneticlike behav-ior. Before one is tempted to conclude that the anomaly at 44K is due to the long range magnetic ordering of STMO, it isworth noting that the antiferromagnetic ordering temperatureof Mn3O4 also lies around the same temperature and anytraces of Mn3O4 �not detected in the XRD� can clearly giverise to an anomaly in the magnetic measurements.20 How-ever more intriguing is the weak anomaly at 14 K that isclearly observed as a peak in the ZFC behavior. Such lowordering temperatures have been reported even in the case ofhigh antisite disordered Sr2SbMnO6 owing to the antiferro-magnetic interaction between the Mn–O–Mn clusters presentin the lattice.8 The PM region of 1 /� versus T was plotted inthe temperature range of 180 to 300 K as shown in Fig. 4.The linear behavior was fit according to Curie–Weiss lawgiven by 1 /�= �T−Tc� /C where C= �Ng2�B

2 s�s+1� /3�kB.Here, C is the Curie–Weiss constant, N is the number of PMatoms per unit volume, g is the gyromagnetic ratio, kB is theBoltzmann constant, TC is the Curie temperature, and s is thespin angular momentum values possessed by the magneticentity. C was determined from the slope of the linear fit and

the s�s+1� was calculated. The effective magnetic moment�eff per Mn cation is given by �eff= �4s�s+1��1/2�B. In thecase of systems with long range antiferromagnetic ordering,one defines the Neel’s temperature �N instead of TC, which iscalculated from the intercept of 1 /� versus T. The magnitudeof �N describes the strength of the antiferromagnetic interac-tion between the spins. The calculated values of the effectivemagnetic moment �eff /Mn cation and �N for STMO arelisted in Table II. The calculated effective magnetic momentsare higher than 3.8 �B expected only for Mn4+ cation indi-cating the presence of mixed oxidation states of Mn. TableIII lists the calculated effective spin magnetic moment forvarious fractions of Mn3+ and Mn4+ �where x=1 for pureMn4+� by assuming complete quenching of orbital angularmoment �l=0�. Comparison of the calculated and the ob-served effective magnetic moment per Mn cation reveals thepresence of comparatively higher fraction of Mn3+ inSTMO1623 samples than that of STMO1573 indicating mar-ginally higher oxygen nonstoichiometry. Very high negativevalue of �N for both the samples seems to support the exis-tence of very strong antiferromagnetic superexchange inter-actions in the lattice. Magnetic hysteresis loops �M versus Hbehavior� were recorded at three different temperaturesnamely 10, 40, and 100 K that covers the transition tempera-ture range as shown in Fig. 5. M versus H hysteresis loopstudy at 100 K reveals a linear behavior indicating PM na-ture of these compounds. A thin and unsaturated M-H loop

TABLE II. The magnetization parameters observed in the M versus H behavior, the effective magnetic mo-ments obtained for STMO1573 and STMO1623 samples.

�N

�K�

�eff /Mn ioncalculatedfrom PM

region�100–300 K�

��B�

Saturation magneticmoment at 10 K

��B�Temperature

�K�MSpont

��B�Msat �5 T�

��B�Coercive field

�kOe�

STMO1573 �356 4.21�2� 0.12 10 0.0158 0.119 0.31840 0.0076 0.089 0.051

STMO1623 �293 4.52�2� 0.16 10 0.0228 0.160 0.31140 0.0107 0.117 0.048

FIG. 3. �Color online� ZFC and FC magnetization curves recorded from 10to 300 K at an applied field of 100 Oe. The inset is the zoom of the twopeaks in the ZFC curve at 14 and 40 K respectively.

FIG. 4. �Color online� Variation in inverse of molar susceptibility withtemperature. The solid line is the mathematical fit according to the Curie–Weiss law.

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was observed at 40 K with very small magnetization �Msat�and coercivity �HC� values. The observed values of Msat,Mspont, and Hc of STMO1573 and STMO1623 samples havebeen listed in Table II. Figure 6 is the comparison of the M-Hloops of STMO1573 and STMO1623. The difference in theobserved parameters is rationalized based on the mixed oxi-dation states of Mn in 3+ and 4+ under different sinteringtemperatures. At 10 K, the hysteresis loop indicates a typicalferromagnetic behavior although with very small Msat valuesconsiderably less than �4 �B /Mn cation �expected in caseall Mn3+ /Mn4+ spins are oriented in the direction of the ap-plied field�. Therefore, this hysteresis behavior is attributedto the ferromagnetic clusters present in the predominantlyantiferromagnetic lattice or can be considered as a case ofcanted antiferromagnetic behavior where the spins do notperfectly cancel each other giving rise to a very low mag-netic moment value. Moreover, magnetization exhibiting alinear behavior even at higher fields ��5 T� is contrary to anideal ferromagnetic behavior. The low coercive field andremnant magnetization values are characteristic features offrustrated systems such as cluster glass/spin glass materialsbut investigation of ac magnetic susceptibility around thetransition temperature is required to confirm such a behavior.Figure 7 shows the ac magnetic susceptibility behavior of

STMO at three different frequencies that include 42, 100,and 420 Hz at a very low applied ac magnetic field of 1 Oe.Broad maximum was observed at 45 K with another rela-tively weak anomaly occurring around 14 K in the imaginarypart of susceptibility ���� which is in agreement with thatobserved in the dc magnetization measurements. Accordingto Mydosh,21 a quantitative assessment of magnetically frus-trated systems can be carried out using the factorTm /Tm�log���, where Tm=Tm

1 −Tm2. Here Tm

1 and Tm2

represent the temperatures at which the maximum of thepeak occurs in the temperature dependent susceptibility be-havior for different frequencies such as 1 and 2. Thisquantitative assessment returns a very high value for super-paramagnetic systems �with a characteristic blocking tem-perature� whereas values in the range of 0.01 to 0.003 havebeen observed for classic spin glass systems. Therefore, thedata collected from few frequencies might be enough toidentify a superparamagnetic or a spin glass system. In thecase of STMO, we obtained a value of �0.0087 for the datarecorded in the frequency range of 42 to 420 Hz. However, astudy of aging effects below the magnetic transition tempera-ture is necessary to establish a true spin glass behavior.22 In

TABLE III. The calculated and observed magnetic moment per Mn cationfor various spin state configurations.

Spin states �x��so �spin only�a

��B� �effb

1 3.870.9 3.980.8 4.090.7 4.21 4.21�1�c

0.6 4.300.5 4.410.4 4.51 4.52�2�c

0.3 4.61

a�so= �x�Mn24++ �1−x��Mn

23+�1/2 �Mn3+ is assumed to be in high spin configura-tion�.b�eff is the experimentally observed value.c�1� STMO1573 and �2� STMO1623.

FIG. 5. �Color online� M-H hyteresis loops for STMO recorded at varioustemperatures such as 10, 40, and 100 K.

FIG. 6. �Color online� M-H hysteresis loops recorded at 5 K for STMOsamples sintered at two different temperatures 1573 K and 1623 K.

FIG. 7. �Color online� ac susceptibility curves of STMO at different fre-quencies such as 42, 100, and 420 Hz in the temperature range of 2 to 100K.

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the case of ideal spin glass systems, the susceptibility mustexhibit a slow decline below the transition temperature de-pending on the time spent before cutting the field. Thereforewe do not have enough experimental evidence to prove thespin glass behavior of STMO beyond any doubt.

VI. DIELECTRIC AND CONDUCTIVITY PROPERTIES

The giant dielectric response of STMO in a narrow tem-perature range of 180–300 K was studied by our group indetail12 and the origin of the very high permittivity valueswas explained using the interlayer barrier capacitance model,where the extrinsic contribution to the dielectric constantfrom the grain/grain boundary interface, sample/electrode in-terface was separated out. In the present study, dielectric be-havior of STMO was studied at temperatures down to 10 Kcovering its magnetic ordering temperatures to probe anypossible spin-phonon coupling behavior or low temperaturestructural phase transitions. The dielectric properties of theSTMO ceramics were studied at various frequencies between1 Hz and 1 MHz. Figures 8�a� and 8�b� show the dielectricpermittivity �r� and tan � behavior as a function of tempera-ture at several frequencies. Unlike in the case of pureSrTiO3, where the dielectric constant increases with decreas-ing temperature and saturates below �30 K due to polarphonon softening on cooling and subsequent saturation as aconsequence of quantum fluctuations below �30 K, the gi-

ant extrinsic dielectric constant of STMO remarkably de-creases with decreasing temperature. Finally �r� reaches in-trinsic values of around 80 below 100 K and thecorresponding value of tan ��0.008 were obtained. Note thelow temperature �r�=80 is much lower than in SrTiO3 ��r��300 at 300 K and �25 000 at 4 K�. It means that Mncation reduces the polarizability of STMO lattice and sup-presses the incipient ferroelectric properties. The overall di-electric relaxation behavior of STMO mimics that of a typi-cal paraelectric magnetic oxide where the short rangehopping of ions/defects such as Ti4+↔Ti3+ andMn4+↔Mn3+ gives rise to relaxation peaks observed asmaximum in tan �. The inverse relaxation time 1 / �i.e re-laxation frequency�, taken from the relaxation peak positionsof the temperature dependent tan � behavior, has been plot-ted as a function of the inverse of temperature �inset of Fig.8�b�� and this dependence was fitted with Vogel–Fulcher for-mula

1

=

1

0exp� − UVF

kB�T − TVF�� . �1�

Unfortunately this fit gives unphysical parameters with nega-tive Vogel–Fulcher temperature TVF� =−76 K, activation en-ergy UVF=3693 K �0.32 eV� and 0=10−14 s. This unphysi-cal result is caused by strong influence of conductivity ondielectric relaxation. Both these mechanisms can be distin-guished from the fit of frequency dependency of complexpermittivity �r

� using the Cole–Cole formula with the con-ductivity term

���� = �� +�

1 + �i �� + i �0

�0 , �2�

where ��, �, �, �0, and �0 represents the high-frequencypermittivity, dielectric strength, parameter describing distri-bution of relaxation times, static conductivity, and permittiv-ity of vacuum, respectively. In our case, � values lies be-tween 0.6 and 0.7 depending on the temperature, givingevidence to a large distribution of relaxation times. The baredielectric relaxation frequency R=1 / obtained from the

FIG. 9. �Color online� Variation in conductivity recorded at 1 Hz withinverse of temperature in low temperature range �55 to 150 K�. The straightline is a linear fit according to the VRH model.

FIG. 8. �Color online� Temperature dependencies of �a� dielectric permittiv-ity �r� and �b� tan � from 300 down to 10 K. The inset shows the Vogel–Fulcher and Arrhenius plot of the temperature dependent inverse relaxationtime.

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Cole–Cole fit is plotted by triangles in the inset of Fig. 8�b�and its temperature dependence follow the Arrhenius formula = 0 exp�Ua /kBT� with 0=1.15�10−9 s and Ua=1202 K�0.1 eV�. The electrical conductivity behavior recorded at 1Hz and at low temperatures �55–150 K� was found to obeythe Mott-type variable range hopping model given by ��exp�−�TM /T�1/4� as shown in Fig. 9. In Mott-type behav-ior, the parameter TM is given by 18 / �kBD�EF��3�, whereD�EF� is the density of states and � is the correlationlength.23 For STMO, D�EF��3�1.7439�10−22 /cm3, wherethe correlation length � is generally in the order of the atomicradii of the cations.

VII. XPS CHARACTERIZATION OF SR2MnO6

Figure 10�a� shows the survey of the spectra collected

from 1400 to 10 eV and all the peaks have been identifiedwith the excitations of the cations present in STMO. XPSspectra of Sr 3d, Ti 2p, Mn 2p, Mn 3s, and O 1s wereseparately collected for further analysis. The correction tothe peak positions was applied based on C 1s peak. Thepeak shape and position was at first used to decipher theoxidation states of cations, in particular, the transition metalcations like Ti and Mn present in STMO. For this purpose,the deconvolution of XPS spectra was carried out using XPS-

PEAK �version4.1� software where 50% Lorentzian and 50%Gaussian function was used to fit the spectra. The back-ground of the different spectra was fit with Shirley functionand subsequently subtracted from the raw spectra. Figure10�b� is the Ti 2p spectra where the two peaks at 462.6 eVand 457.01 eV which correspond to Ti 2p1/2 and Ti 2p3/2

FIG. 10. �Color online� �a� Total survey spectra, �b� XPS spectra of Ti 2p, �c� Mn 2p, �d� Mn 3s, �e� O 1s, and �f� valence band spectra.

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states, respectively. Figure 10�c� shows the Mn 2p spectracomprising both low and high spin state peaks. TheMn 2p3/2 spectra could be deconvoluted into two peaks po-sitioned at 641.95 and 640.45 eV indicating the presence oftwo different valence states of Mn with a similar deconvolu-tion of Mn 2p1/2 spectra into two peaks at 653.8 and 652.4eV. Table IV lists the Mn 2p1/2 and Mn 2p3/2 binding en-ergy values for STMO1573 and STMO1623 along with thecorresponding standard values of Mn2O3 and MnO2 givenfor comparison. The magnitude of core level splitting inMn 3s photoemission spectra is a more accurate indicator ofthe oxidation state of Mn.24,25 The splitting of the 3s corelevel spectra in transition metal compounds originates fromthe exchange coupling between the 2p hole created by theexcitation of the valence electron and the free 3d electrons.The spectral width is larger in the case of doped manganiteswhere there are mixed oxidation states of Mn present in thelattice.25 Figure 10�d� shows the broad Mn 3s spectra ofSTMO at room temperature. Though the Mn 3s peak isclearly observed at 82.94 eV, high intensity excitations fromother elements have completely overlapped with the splittingof the Mn 3s peak making it difficult to determine the Mnoxidation state. Further investigation of O 1s peak is re-quired to confirm the accommodation of vacancies in thelattice. According to Serrano et al.,26 the O 1s component ofLaTixCo1−xO3 consists of peak at 528.8–529.3 eV which cor-responds to O2− in the lattice and a second peak at 531.3–531.5 eV that can be assigned to the surface oxyhydrides. Inthe present case, the broad O 1s signal in Fig. 10�e� wasdeconvoluted to three different peaks at 528.93 eV, 530.92eV, and 532.9 eV, respectively. The peak at 528.93 eV rep-resents the O2− in the lattice whereas the peaks at 530.92 eVand 532.9 eV correspond to the surface oxyhydrides andother adsorbed O2− species, respectively. Figure 10�f� showsthe valence band spectrum of STMO deconvoluted into threepeaks. According to Galakhov et al.,27 the valence bandspectrum of LiMnO2 has a shoulder A near the Fermi level ofthe photoemission spectrum that originates from the Mn eg

states which exactly matches with the calculated band struc-ture. The other peaks B and C were attributed to the t2g statesand further confirmed by comparing with Mn K� and O K�

spectrum. In a similar way, the valence band of STMO couldbe deconvoluted into three distinct peaks, namely, A, B, andC, where A is a shoulder peak very close to the Fermi level.Drawing parallel to the valence band spectra analysis ofLiMnO2, one can assign the peak A to the eg states of Mnand the other two peaks B and C to the t2g states of Mnwhich also correlates with the mixing of O 2p orbitals with

metal 3d states. Any further meaningful analysis can be car-ried out based on theoretical estimation of the band structureof STMO. Nevertheless, the valence band spectrum furtherconfirms a high Mn3+ /Mn4+ ratio by the presence of eg statesnear the Fermi level.

VIII. CONCLUSIONS

Refined x-diffraction data revealed the space group ofSr2TiMnO6 synthesized by solid-state synthesis route to be

Fm3̄m. However IR and Raman spectra gave evidence forlocal lower crystal symmetry. In order to understand the na-ture of magnetic ordering at 14 K and the influence of anti-site disorder on the same, one needs to probe further usingneutron diffraction. The existence of multiple oxidationstates of Mn �in 4+ and 3+ states� was established via themagnetization studies carried out on the samples sintered attwo different temperatures. The dielectric studies carried outdown to 10 K at various frequencies corroborated its intrinsicnature below 100 K and the conductivity, at higher tempera-tures, follows a Mott-type variable range hopping �VRH�behavior. XPS investigation on STMO further confirmed theexistence of multiple oxidation states apart from throwinglight into its valence band structure.

ACKNOWLEDGMENTS

The authors would like to thank JATP—Defense Re-search and Development laboratory, Govt. of India andCzech Science Foundation �Project Nos. 202/09/0682,AVOZ 1010052, and SVV-2010-261303� for the financialsupport. One of the authors K.R.S.P. would like to thank Dr.Anil Kumar and Mr. Viswanathan of the Department ofPhysics and Prof. G. Mohan Rao of the Department of In-strumentation and Applied Physics, Indian Institute of Sci-ence Bangalore for the AC susceptibility measurements, XPSmeasurements and other fruitful discussions.

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Compound2p3/2�eV�

2p1/2�eV�

Mn2O3 641.7 653.4MnO2 642 653.85STMO1573 642 653.6STMO1623 642.1 653.7

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