The critical role of stereochemically active lone pair in introducing high temperature ferroelectricity
Rafikul Ali Saha, Anita Halder, Desheng Fu, Mitsuru Itoh, Tanusri Saha-Dasgupta, Sugata Ray
aa r X i v : . [ c ond - m a t . m t r l - s c i ] F e b The critical role of stereochemically active lone pair inintroducing high temperature ferroelectricity
Rafikul Ali Saha , Anita Halder , Desheng Fu , MitsuruItoh , Tanusri Saha-Dasgupta , and Sugata Ray ∗ School of Materials Sciences, Indian Association for the Cultivation of Science,2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India Department of Condensed Matter Physics and Material Sciences,S. N. Bose National Centre for Basic Sciences,Block JD, Sector 3, Saltlake, Kolkata 700106, India Department of Electronics and Materials Science,and Department of Optoelectronics and Nanostructure Science,Graduate School of Science and Technology, Shizuoka University,3-5-1 Johoku, Naka-ku, Hamamatsu 432-8561, Japan and Materials and Structures Laboratory, Tokyo Institute of Technology,4259 Nagatsuta, Yokohama 226-8503, Japan
Abstract
In this paper a comparative structural, dielectric and magnetic study of two langasite compoundsBa TeCo P O (absence of lone pair) and Pb TeCo P O (Pb s lone pair) have been carriedout to precisely explore the development of room temperature spontaneous polarization in presenceof stereochemically active lone pair. In case of Pb TeCo P O , mixing of both Pb 6 s with Pb6 p and O 2 p help the lone pair to be stereochemically active. This stereochemically active lonepair brings a large structural distortion within the unit cell and creates a polar geometry, whileBa TeCo P O compound remains in a nonpolar structure due to the absence of any such effect.Consequently, polarization measurement under varying electric field confirms room temperatureferroelectricity for Pb TeCo P O , which was not the case of Ba TeCo P O . Detailed studywas carried out to understand the microscopic mechanism of ferroelectricity which revealed theexciting underlying activity of poler TeO octahedral unit as well as Pb-hexagon. ∗ [email protected] . INTRODUCTION The stereochemically active cationic lone pair driven ferroelectricity within a polar unitcell has always been an important point of investigation in materials physics . On top of it,the room temperature ferroelectricity is even more fascinating in terms of both fundamentalunderstanding and modern technological applications. In addition, when the systems also ac-commodate magnetic cations, possibility of multiferroicity arises. In quite a few systems thismechanism seems to work, e.g., ferroelectric perovskites (PbTiO , BiMnO , BiFeO , SnTiO etc) , double perovskites (Pb ScTi . Te . )O , Pb ScSc . Te . O , Pb MnWO ), as wellas α -PbO , SnO, BiOF , Bi WO , CsPbF , BiMn O etc.Surprisingly, langasite family of compounds have been ignored in this context until re-cently . Thus in order to explore the role of lone pair in bringing ferroelectricity within lan-gasite systems, here in this work we have dealt with two such compounds: Pb TeCo P O (PTCPO), with lone pair and Ba TeCo P O (BTCPO), without lone pair. Then we moveon to establish that only the presence of Pb s lone pair is not sufficient to bring polariza-tion in the system, the stereochemical activity of the lone pair is the driving force to induceferroelectricity in the Pb compound. The stronger Pb-O covalent interaction, considerablehybridization of Pb 6 s with 6 p and Pb 6 s with O 2 p , eventually facilitate the stereochemicalactivity of the lone pair.In this paper, we have discussed the structural difference between BTCPO and PTCPO,while BTCPO is noncentrosymmetric and nonpolar, PTCPO is polar. The stereochemicallyactive lone pairs of Pb distort the general non polar structure into a polar one. Further,the Te-O covalency creates dissimilar Te-O bond lengths within TeO octahedra of PTCPO,helping to enhance the polarization in the Pb containing compound. In-depth magnetizationstudy of both the compounds clearly reveals the presence of antiferromagnetic transition atlow temperature. Further polarization studies under varying electric field at room temper-ature confirm development of finite polarity in PTCPO. Additionally, the stereochemicalactivity of the Pb lone pair for the PTCPO compound has been corroborated through spinpolarized density functional theory (DFT), crystal orbital Hamiltonian population (COHP)and electron localization function calculations.2 I. METHODOLOGYA. Experimental techniques
Polycrystalline samples of Ba TeCo P O (BTCPO) and Pb TeCo P O (PTCPO)have been prepared by using conventional solid state reaction techniques. BTCPO was syn-thesized by taking stoichiometric amounts of high purity BaCO (Sigma-Aldrich 99.999%),TeO (Sigma-Aldrich 99.9995%), Co O (Sigma-Aldrich 99.99%) and NH H PO (Sigma-Aldrich 99.999%), while starting materials for PTCPO were PbO (Sigma-Aldrich 99.999%),TeO , Co O and NH H PO . In case of BTCPO, the mixtures were calcined at 600 ◦ C for12 hours and finally sintered at 800 ◦ C for 96 hours under flowing oxygen, while PTCPOcompound was prepared by heating the mixture of the starting materials at 600 ◦ C for 12hour followed by 700 ◦ C for 48 hours in oxygen atmosphere in a covered alumina crucible inoxygen atmosphere.Room temperature structural characterizations for both compounds were carried out byusing synchrotron x-ray radiation facility of MCX Beam-line at the Elettra ScnchrotronCentre, Italy with x-ray wave length of 0.827 ˚A. Temperature dependent x-ray diffraction(XRD) were carried out at RIGAKU Smartlab (9KW) XG equipped with Cu K α to probe thepresence of temperatutre dependent structural phase transitions. The x-ray diffraction datawere analyzed via Rietveld refinement using FULLPROF program . X-ray photoelectronspectroscopy (XPS) experiments were done in an Omicron electron spectrometer, equippedwith SCIENTA OMICRON SPHERA analyzer, Al K α monochromatic source (Model no:XM 1000) and 7 channel channeltron detector. The dc magnetization measurements in thetemperature range 2-300 K and in magnetic fields upto ± ac level 1 V mm − . A cryogenic temperaturesystem (Niki Glass LTS-250-TL-4W) was used to control the temperature within the rangeof 4 - 450 K. The dielectric hysteresis loops were measured using a ferroelectric measurementsystem (Toyo Corporation FCE-3) equipped with an Iwatsu ST-3541 capacitive displacementmeter having a linearity of 0.1% and a resolution of 0.3 nm.3 . Theoretical Techniques All the first principles density function theory (DFT) calculations have been performedusing plane wave pseudopotential method as implemented in Vienna Ab initio SimulationPackage (VASP) . The exchange correlation functional has been considered within gen-eralized gradient approximation (GGA) and to capture the strong electronic correlationat transition metal sites beyond GGA, additional GGA+ U calculations have been carriedout as prescribed in Liechtenstein formulation . The U = 5 eV is considered at Co sitefollowing the commonly used U value for late transition metals and the value of Hund’scoupling J H is 0.8 eV. The results have been cross checked by varying the U value within1-2 eV. The projected augmented wave(PAW) potential has been used and the kineticenergy cut off, set at 600 eV, has been found to provide a good convergence of total energy.Reciprocal-space integration has been carried out with a k -space mesh of 2 × ×
8. Toobtain the energy resolved visualization of chemical bonding, the Crystal Orbital HamiltonPopulation (COHP) scheme has been considered. A custom software has been employedfor this purpose which processes the plane wave calculations by taking the projection intoan auxiliary linear combination of atomic orbitals (LCAO) basis and can calculate projectedCOHP. The tetrahedron method has been used to perform integration in reciprocal space toobtain an energy-resolved COHP plot. To visualize the lone pair arising from the 6 s orbitalof Pb, we have calculated electron localization functions . The ELF distribution ζ ( −→ r )can be expressed as, ζ ( −→ r ) = 1 / [1 + χ ( −→ r )] where χ ( −→ r ) = P i |−→∇ ψ i | −
18 ( −→∇ ρ ) ρ (3 π ) / ρ / where ρ is the electron density and ψ i is Kohn-Sham wavefunctions. ELF measures theprobability of finiding an electron in the vicinity of another electron with same spin and it isused to visualize the atomic shells. The ELF can have value between 0 and 1, where perfectlocalization is obtained for ELF = 1. 4 II. RESULTS AND DISCUSSIONS:
Rietveld refined room temperature X-ray diffraction (XRD) patterns of both BTCPOand PTCPO compounds are shown in Fig. 1(a) and (b) respectively. BTCPO is fitted byconsidering a noncentrosymmetric and nonpolar space group P P .Refined lattice parameters and goodness factor for both compounds are listed in the tableI. The atomic positions of BTCPO and PTCPO are given in the Table-S1 and S2 in thesupplementary material respectively . Unit cells of both the crystal structures are shown inthe Fig. 1(c) and (d) respectively. Although, like general langasite, Te forms octahedral net-work, Co and P are in tetrahedral coordination while Pb/Ba are in decahedral arrangement,Pb brings large structural distortion in PTCPO (relative to BTCPO one) by accommodat-ing Pb 6 s lone pair within the structure. This indeed causes following structural differencesbetween the two compounds:1. BTCPO takes the simple trigonal P
321 structure with the Ba-Te layers defining theunit cell boundaries (see the a − c plane of the unit cell in Fig. 2(a)). For PTCPO, the Pblone pair brings forth distortion in the unit cell where few of the Co and Pb ions move awayslightly along the c axis, thereby reducing the symmetry and a polar, monoclinic P a − b plane, but in case ofPTCPO the isosceles triangles of Pb and Co undergo a relative tilting with respect to eachother, as indicated in Figs. 2(e) and (f). The tilting of the triangles away from the a − b plane (up or down of the plane) depends on the movement of Pb and Co atoms along c -axis(’+’ or ’-’ ve c -direction).4. Unlike BTCPO, the centre of mass of anionic network (0, 0.334, 1) within the TeO octahedra does not coincide with the position of Te (0, 0.336, 1) in PTCPO.As a result of these subtle structural differences, PTCPO takes much bigger unit cell (sixtimes) compared to BTCPO. However, it is primarily important to confirm the existenceof stereochemically active Pb lone pair at room temperature. In the context of Pb lone5air, the degree of stereochemical activity of lone pair is measured by calculating the valuesof △ E (Difference between the shortest distances Pb-O from the spheres I and II) and △ E (difference between the shortest distance Pb-O in the polyhedron of the compoundunder study and the known shortest distance in compounds containing these elements) . △ E = 0.32 ˚A and △ E = 0.05 ˚A (considering the minimal distance d (Pb -O) = 2.40 ˚A(as in Pb V O ) specify the high degree of stereochemical activity of Pb lone pair inPTCPO. In addition, nonuniform Te-O bond distances within TeO octahedra of PTCPOsuggest presence of charge sharing variations between Te and O, further helping to get apolar structure.Further x-ray photoelectron spectroscopy (XPS) technique is used to probe the properoxidation states of the elements of both the compounds. We probed the core level Pb 4 f ,Te 3 d , V 2 p , P 2 s , Co 2 p and O 1 s for both samples and the corresponding fitting have beenperformed. The energy positions of the respective features clearly indicate the expectedcharge states of all the cations for both these samples (not shown here). While O 1 s corelevel spectra of BTCPO shows a clear singlet, PTCPO possesses asymmetric broadening inthe higher binding energy side of the O 1 s spectrum, as shown in Figs. 3(a) and (b). Thisspectrum has been fitted by considering two singlets, which signifies strong variation in theelectron density of the oxygens of this compound, which results in two distinctly differentO 1 s signals, compared to BTCPO .We have carried out density functional theory (DFT) calculations in order to assess thecovalency effect in the context of stereochemical activity of the Pb lone pair. Since the goal ofour first-principles calculation is to reveal the covalency effect between different cations andoxygen, it is realized that the analysis of electronic structure should be carried out within therealm of spin-polarized ferromagnetic calculations. The antiferromagnetic structure, whichis the ground state magnetic structure is complex and leads to cancellation of covalencyeffect due to opposite alignment of spins, as ascertained in vanishing moment of oxygencompared to its finite value in fully spin-polarized calculation. The calculations have beencarried out both for the BTCPO (nonpolar P P
2) structures andthe detailed density of states (DOS) are shown in Figs. 4(a) and (b), respectively. DFTcalculation shows presence of Ba s partial density of states (PDOS) above the Fermi energyand absence of PDOS below the Fermi energy signifying the presence of pure Ba in thesystem. In case of PTCPO, Pb 6 p and 6 s PDOS are seen above (3 to 8 eV) and below (down6o -9 eV) the Fermi level respectively, supporting the presence of Pb state. Interestingly,presence of both Pb 6 s and 6 p PDOS in the energy range -6 to -1 eV indicates the Pb 6 s -6 p mixing in the compound. The stereochemical activity of lone pair active ions like Bi orPb within oxide structures has been argued to originate from the hybridization between6 s , 6 p orbitals at Bi/Pb site and 2 p orbitals at O site . The 6 p -2 p hybridization turnsout to be the governing factor in giving rise to directionality of the lone pair and drivingthe off-centric distortion of the unit cell . Here, strong hybridization between Pb 6 s , 6 p and O 2 p gets confirmed, as indicated in the cyan shaded region. Additionally, presenceof finite DOS from Pb 6 s , Te 6 p and O 2 p in the energy range of -6.5 to -8 eV signifiesthe hybridization of Pb and Te with same O. But in case of BTCPO, magenta shadedregion signifies relatively weaker hybridization of Te and Co with the oxygen. Further, thishas been quantitatively confirmed from our COHP (Crystal Orbital Hamilton Population)calculation where the integrated COHP (ICOHP) values have been found to become largerfor the cation-oxygen connectivity while going from BTCPO ( P P
2) (seeFigs. 4(c)-(d) and Table II). In this context, the elemental magnetic moment presented anoticeable change in the PTCPO structure relative to the BTCPO, in line with the strongercovalency effect in PTCPO (see Table II). Such an effect should obviously enhance thedegree of Pb lone pair activity in the compound at lower temperature. The charge densitydistributions of BTCPO confirm spherical symmetric distribution of Ba, while lobe likestructures are observed for PTCPO, as shown in Figs. 4(e) and (f). This lobe like structureof Pb 6 s lone pair appears due to the mixing of Pb 6 s with 6 p and Pb 6 s with O p ,signifying the stereochemical activity of Pb lone pair . In addition, asymmetric Te lobesmight indicate non uniform Te-O covalent interactions, and consequently non uniform Te-Obond distances. As a result, we may conclude that the covalency effect of both Pb-O andTe-O may give rise to spontaneous polarization in the system. Further, hybridization of Pband Co with same O and also Pb and Te with same O firmly suggest the stereochemicalactive Pb lone pair to be responsible for the movement of Co triangles and TeO octahedrain this sample, as indicated in Fig. 2(a)-(f).The temperature dependence of the real part of the dielectric constant ( ε ′ / ε ) and dielec-tric loss (tan δ ) have been carried out in the temperature range of 5 to 350 K for BTCPOand 5 to 400 K for PTCPO at different frequencies (20 kHz-1000 kHz), as shown in Fig.5(a) and (b) respectively. In case of PTCPO, a frequency independent anomaly near 3657 appears in the temperature dependent dielectric constant and loss (tan δ ) data, which isfurther supported by the respective first order derivative curves (see Fig. S1 of the supple-mentary materials ). On the other hand, there is no anomaly in the measured temperaturerange of BTCPO. In order to identify the origin of dielectric anomaly, temperature depen-dent X-ray diffraction measurements of PTCPO compound have been performed over a widetemperature range of 4-400 K. The collected XRD patterns have been fitted using mono-clinic space group P
2. An anomaly near 365 K is observed in the temperature dependentlattice parameters as well as unit cell volume variations, which coincide with the anomalyof temperature dependent dielectric variation (see Fig. 5(c)) signifying the development offerroelectricity below that temperature. The expanded view of high temperature (above365 K) and low temperature XRD patterns are shown in the Fig. S2 of the supplemen-tary material which reveals development of many small peaks below T C . However, due toonly minor changes in position coordinates across the phase transition, the present Rietveldrefinements could not conclusively determine the space group of high temperature phase.This is not very uncommon in presence of weak lattice distortion . Further polarization( P ) with varying electric field ( E ) at room temperature (300 K) has been measured forboth BTCPO and PTCPO compounds, as highlighted in Fig. S3 of the supplementarymaterial and Fig. 6(a) respectively. Clear P − E loop with remnant polarization of 0.35 µ C/cm and a butterfly loop in strain versus electric field curve are observed in PTCPOwhich support the true ferroelectric ordering at room temperature. It is to be noted thatPTCPO is a good insulator because the sample can bear a high field of 130 kV/cm, whichensures no leakage. The microscopic origin of the electrical dipoles in the PTCPO systemcan be clearly understood in terms of the strength ( △ E ) of stereochemically active Pb lonepair and local polar geometry of TeO octahedra. The value of △ E becomes 0.17 at 400K, which is significantly much smaller than the same at the 300 K (0.32). This impliesweakening of the stereochemical activity of Pb lone pair above ferroelectric T C . Moreover,at 300 K the center of mass of Te cation (0, 0.336, 1) moves away from the anionic centerof mass (0, 0.334, 1) along the y -coordinates, this difference in center of masses between Te(0, 0.3279, 1) and the anions (0, 0.3277, 1) becomes negligible at 400 K. The developmentof spontaneous room temperature polarization in PTCPO could be attributed firstly to thestereochemically active lone pair of Pb ( ns ) in Pb-Pb hexagon and secondly to the polargeometry of TeO octahedra. Therefore, a resultant dipole moment gets developed within a8ingle Pb motif (where Te and Pb are present in the a − b plane but tilted along c direction)along the b − c plane, as shown in Fig. 6(b). However, the small measured spontaneouspolarization ( 0.5 µ C/cm ), structural data and the type of dielectric anomaly as well asmultiplication of the unit cell might indicate that the ferroelectric transition in PTCPO isimproper. On the other hand, in case of BTCPO, the weaker Co-O covalency within CoO tetrahedra, and the consequent reduced charge distribution among the other cation-oxygenbonds, together oppose the geometric polar structure in BTCPO. This eventually negatesthe room temperature ferroelectricity in this compound, unlike in PTCPO.Next we have investigated the temperature dependent dc magnetization of BTCPO andPTCPO compounds. The zero field cool (ZFC) and field cool (FC) 500 Oe magnetic sus-ceptibility data of both BTCPO and PTCPO in the temperature range 2-300 K, are shownin Fig. 7(a) and (b) respectively. Both order antiferromagnetically, with Neel tempera-ture ( T N ) of around 21 K and 14 K for BTCPO and PTCPO respectively, consistent withprevious study . However, no perceptible dielectric anomaly at the antiferromagnetictransition indicates a weak magnetoelectric coupling in PTCPO. Curie-Weiss (CW) fit ( χ = χ + C /( T - θ CW )); where χ is the temperature independent paramagnetic succeptibility, C is Curie constant related to the effective moment, and θ CW being Weiss temperature) onthe 500 Oe field cooled susceptibility data in the temperature range T = 100 - 300 K havebeen performed for both the samples and the subsequent results are, shown in the insetof Fig. 7(a) and (b). The negative Weiss temperatures ( θ CW = -25.12 K and -23.1 K forBTCPO and PTCPO respectively), consistent with previous studies , indicate the presenceof antiferromagnetic interactions between Co spins, possibly due to dominant Co-O-O-Cosuper-superexchange pathways within neighboring Co-triangles . The effective paramag-netic moments, µ eff = 4.74 and 4.43 µ B /Co for BTCPO and PTCPO respectively, obtainedfrom the C-W analysis, are higher compared to the theoretically calculated spin only Co-moment value (3.87 µ B ), suggesting incomplete quenching of Co-orbital moments. IV. CONCLUSION
In summary, detailed structural, dielectric and magnetic investigations as well as first-principles electronic structure calculations have been performed on two langasite compoundsBTCPO and PTCPO. We have confirmed through detailed structural analysis that BTCPO9without lone pair) is non-polar, but PTCPO (lone pair) is polar at room temperature.Stronger Co-O covalency in PTCPO helps to redistribute the charges among the othercation-oxygen bonds, which not only affects the individual elemental magnetic moments butalso facilitates the lone pair activity of Pb by enhancing the mixing of Pb 6 s with Pb 6 p andO 2 p . On top of it, strong Te-O covalency causes significant charge disproportionation andthus creates nonuniform Te-O bond distances within a TeO octahedra. The combinatorialeffect of stereochemically active lone pair and Te-O covalency help to induce spontaneouspolarization at room temperature in the PTCPO. V. ACKNOWLEDGEMENT
R.A.S thanks CSIR, India and IACS for a fellowship. S.R thanks DST, India for funding(project no.CRG/2019/003522), Technical Research Center of IACS, Indo-Italian POC forsupport to carry out experiments in Elettra, Italy and Laboratory for Materials and Struc-tures, Collaborative Research Projects for providing experimental facilities. A.H and T.S.Dacknowledge the computational support of Thematic Unit of Excellence on ComputationalMaterials Science, funded by Nano-mission of Department of Science. A. Walsh, D. J. Payne, R. G. Egdell and G. W. Watson, Chem. Soc. Rev. , 4455 − X. He and K. Jin, Phys. Rev. B , 224107 (2016). N. A. Hill and K. M. Rabe, Phys. Rev. B , 8759 (1999). R. Seshadri and N. A. Hill, Chem. Mater , 2892-2899 (2001). L. M. Volkova and D. V. Marinin, J Supercond. Nov. Magn. , 2161-2177 (2011). K. C. Pitike, W. D. Parker, L. Louis and S. M. Nakhmanson, Phys. Rev. B , 035112 (2015). S. A. Larregola, J.A. Alonso, M. Alguero, R. Jimenez, E. Suard, F. Porcher and J.C. Pedregosa,Dalton Trans., S. A. Ivanov, A. A. Bush, A. I. Stash, K. E. Kamentsev, V. Ya. Shkuratov, Y. O. Kvashnin, C.Autieri, I. D. Marco, B. Sanyal, O. Eriksson, P. Nordblad, and R. Mathieu, Inorg. Chem. ,2791 − G. W. Watson and S. C. Parker, Phys. Rev. B , 8481 (1999). R. Seshadri, Proc. Indian Acad. Sci. (Chem. Sci.) , 487-496 (2001). C. E. Mohn, and S. Stølen, Phys. Rev. B , 014103 (2011). E. H. Smith, N. A. Benedek, and C. J. Fennie, Inorg. Chem. , 8536 − D. K. Shukla, S. Mollah, Ravi Kumar, P. Thakur, K. H. Chae, W. K. Choi, and A. Banerjee,J. Appl. Phys. , 033707 (2008). R. A. Saha, A. Halder, T. Saha-Dasgupta, D. Fu, M. Itoh, and S. Ray, Phys. Rev. B ,180406(R) (2020). J. Rodriguez Carvajal, Physica B , 55 (1993). G. Kresse and J. Furthm¨ u ller, Phys. Rev. B , 11169 − J. P. Perdew,K. Burke and M. Ernzerhof, Phys. Rev. Lett. , 3865 − A. I. Liechtenstein, V. I. Anisimov and J. Zaanen, Phys. Rev. B , R5467-5470 (1995). A. Halder, D. Nafday, P. Sanyal and T. Saha-Dasgupta, npj Quantum Materials , 17 (2018). P. E. Bl¨ o chl, Phys. Rev. B , 17953-17979 (1994). R. Dronskowski and P. E. Blochl, J. Phys. Chem. , 8617 (1993). S. Maintz, V. L. Deringer, A. L. Tchougreeff and R. Dronskowski, J. Comput. Chem. , 1030(2016). B. Silvi and A. Savin, Nature (London) , 683 (1994). A. Savin, R. Nesper, S. Wengert and T. F. F¨ a ssler, Angew. Chem., Int. Ed. Engl. , 1808(1997). J.W. Krizan, C. de la Cruz, N.H. Andersen, R.J. Cava, Journal of Solid State Chemistry ,310–320(2013). H. J. Silverstein, A. Z. Sharma, A. J. Stoller, K. Cruz-Kan, R. Flacau, R. L. Donaberger, H.D. Zhou, P. Manuel, A. Huq, A. I. Kolesnikov and C. R. Wiebe, J. Phys. Condens. Matter ,246004 (2013). A. Bandyopadhyay, S. K. Neogi, A. Paul, C. Meneghini, I. Dasgupta, S. Bandyopadhyay andS. Ray, Phys. Rev. B A. Paul, A. Mukherjee, I. Dasgupta, and T. Saha-Dasgupta, Phys. Rev. Research G. W. Watson and S. C. Parker, J. Phys. Chem. B Supplementary Material. T. Basu, V. V. R. Kishore, S. Gohil, K. Singh, N. Mohapatra, S. Bhattacharjee, B. Gonde, N.P. Lalla, P. Mahadevan, S. Ghosh and E. V. Sampathkumaran, Sci. Rep. , 5636 (2014). J. M. Rondinelli, A. S. Eidelson and N. A. Spaldin, Phys. Rev. B ABLE I. Lattice parameter and goodness factor of BTCPO and PTCPO compounds.
Sample Space group a (˚A) b (˚A) c (˚A) α β γ χ BTCPO P
321 8.470 8.470 5.316 90 90 120 2.81PTCPO P TABLE II. ICOHP values and magnetic moments of BTCPO and PTCPO compounds.
Value of ICOHP (eV/bond) Value of magnetic moment in µ B Sample Space group Ba/Pb-O Co-O Te-O Co Ba/Pb OBTCPO P
321 -0.069 0.338 0.272 2.733 0.002 0.034PTCPO P
10 20 30 40 50
Space Group: P P I n t e n s i t y ( a . u . ) BTCPO
300 K XRD300 K XRD
PTCPO I n t e n s i t y ( a . u . ) (in degree) Ba Te Co P O Pb Co Te P O (a) (b) (c) (d) P P FIG. 1. Rietveld refined XRD of (a) BTCPO and (b) PTCPO. Black open circles represent theexperimental data and red line represents the calculated pattern. The blue line represents thedifference between the observed and calculated pattern and green lines signify the position ofBragg peaks. (c) and (d) are the crystal structure of BTCPO and PTCPO respectively. (a) (b) BTCPO ( P P
2) (c) (f) (d) (e) Co Pb Pb
FIG. 2. (a) The positions of Pb, Te and Co in BTCPO unit cell. Pb and Te are in the side line of theunit cell, while all Co are exactly in the middle line (red) of the two boundaries. (b) The positionsof Pb and Co in PTCPO unit cell. Some Pb are shifted towards positive or negative c axis fromboundary line, represented by brown arrow, and some Co are shifted towards positive or negative c axis from middle line (red), represented by cyan coloured arrow. (c) Gray and cyan colouredtriangles are the Ba equilateral triangles and Co equilateral triangles of BTCPO respectively. (d)Gray and cyan coloured triangles are the Pb isosceles triangles and Co isosceles triangles of PTCPOrespectively. (e) a portion of PTCPO which contains two Pb and one Co isosceles triangles. (f)Schematic representation of two tilted Pb and one tilted Co triangles of PTCPO.
534 532 530 528 526 I n t e n s i t y ( a . u . ) B.E (eV) O1 s BTCPO
534 532 530 528
B.E (eV) I n t e n s i t y ( a . u . ) O1 s PTCPO (a) (b)
FIG. 3. (a) and (b) The Shirley background corrected experimental data (shaded black circles)along with the theoretical fitting (solid red line) and difference between experimental data andtheoretically fitted data (blue dotted line) are shown for Oxygen (O 1 s ) of BTCPO and PTCPOrespectively. Ba Te (e) (c) (a)
Pb lone pair Te (f) (d) (b)
FIG. 4. (a) and (b) Partial density of state of BTCPO and PTCPO respectively. (c) and (d)COHP and ICOHP curve of BTCPO and PTCPO respectively. (e) and (f) Electron localizationfunction within a unit cell (The isosurfaces are vizualized for a value of 0.3) of BTCPO and PTCPOrespectively. t a n T (K) PTCPO ' / T C a ( ) lattice parameter a lattice parameter b b () lattice parameter c c () lattice parameter ( d e g r ee ) volume of unit cell V V ( ) T (K)
365 K (a) (b) (c) ' / BTCPO t a n T (K) PTCPO ' / T C FIG. 5. (a) and (b) Temperature dependence of real part of dielectric constant ε ′ / ε and tan δ lossdata of BTCPO and PTCPO respectively. (c) Thermal variation of real part of dielectric constant,lattice parameters and volume of unit cell of PTCPO. -150 -75-1.50-0.750.000.751.50 Polarization P ( C / c m ) E (a) p Pb (b) Polarization E (kV/cm) S t r a i n ( % ) Strain p Te FIG. 6. (a) Electric field variation polarization and strain of PTCPO at room temperature. (b)Pb hexagon and TeO octahedra of PTCPO structure. Pb hexagon and TeO octahedra givenet dipole moment.
100 200 3001421 T (K) - ( m o l - O e / e m u ) ( e m u / m o l - O e ) T (K) BTCPO