Structural, vibrational and dielectric properties of the ...

Solid State Sciences 6 (2004) 1393–1401 www.elsevier.com/locate/ssscie

Structural, vibrational and dielectric properties of the new mixed solution K0.84(NH4 )1.16SO4 ·Te(OH)6 L. Ktari a , M. Dammak a,∗ , A. Hadrich a,c , A. Cousson b , M. Nierlich b , F. Romain c , T. Mhiri a a Laboratoire de l’état solide (LES), faculté des sciences de Sfax, 3018 Sfax, Tunisia b Laboratoire Léon Brillouin, CEA Saclay, 91191 Gif-Sur-Yvette cedex, France c Laboratoire de dynamique, interactions et réactivité, CNRS, 2, rue Henry Dunant, 94320 Thiais, France

Received 21 June 2004; accepted 27 July 2004 Available online 25 September 2004

Abstract Synthesis, crystal structure, DSC characterization, dielectric and Raman measurements are given for a new mixed solution K0.84 (NH4 )1.16 SO4 ·Te(OH)6 (KNST). X-ray studies showed that the title compound crystallizes in the monoclinic system (P 21 /c) with the following parameters: a = 14.929(5) Å, b = 6.558(1) Å, c = 11.325(1) Å, β = 120.17(2)◦ and Z = 4. The structure can be regarded as being built of isolated TeO6 octahedra, SO4 tetrahedra and K+ /NH+ 4 cations. The main feature of this structure is the coexistence of two types of hydrogen bonds O–H. . .O and N–H. . .O ensuring the cohesion of the crystal. Crystals of K0.84 (NH4 )1.16 SO4 ·Te(OH)6 undergo two endothermic peaks at 425 and 480 K and a shoulder at 470 K. These transitions detected by DSC and analyzed by dielectric measurements using the impedance and modulus spectroscopy techniques. Raman scattering measurements on K0.84 (NH4 )1.16 SO4 ·Te(OH)6 material taken between 300 and 620 K are reported in this paper. The spectra indicate clearly two phase transitions.  2004 Elsevier SAS. All rights reserved.

1. Introduction The compounds of general formula M2 XO4 ·Te(OH)6 + + (M = Na+ , K+ , NH+ 4 , Rb , Cs and X = S, Se, P) undergo structural phase transitions and interesting physical properties such as superprotonic conduction and ferroelectricity [1–4]. The studies of the effect of the partial cationic substitution on crystal symmetry and physical properties have been reported in previous works for the mixed solution of rubidium ammonium sulfate tellurate Rb1.12 (NH4 )0.88SO4 ·Te(OH)6 [5,6]. In the same way and to continue these studies, we have synthesized a new mixed solution of potassium ammonium sulfate tellurate K0.84 (NH4 )1.16SO4 ·Te(OH)6 (KNST). The structure of the new mixed solution of potassium ammonium sulfate tellurate is different from based compounds (K2 SO4 ·Te(OH)6 and (NH4 )2 SO4 ·Te(OH)6 ) struc* Corresponding author.

E-mail address: [email protected] (M. Dammak). 1293-2558/$ – see front matter  2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.07.034

tures. The structure of K2 SO4 ·Te(OH)6 is triclinic P 1¯ whereas (NH4 )2 SO4 ·Te(OH)6 is monoclinic with space group Cc [7,8]. In the present work, the X-ray, calorimetric and dielectric studies of mixed crystal K0.84(NH4 )1.16SO4 · Te(OH)6 (KNST) were carried out in order to determine the effect of the partial substitution of potassium by ammonium cation.

2. Experimental details Transparent, colorless single crystals of the title composition were grown from aqueous solution of a mixture of K2 SO4 , (NH4 )2 SO4 and Te(OH)6 at room temperature H6 TeO6 + x(NH4 )2 SO4 + (1 − x)K2 SO4 → K2−2x (NH4 )2x SO4 ·Te(OH)6 . The formula is determined by refinement of the crystal structure and confirmed by chemical analysis.

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L. Ktari et al. / Solid State Sciences 6 (2004) 1393–1401

Table 1 Main crystallographic data for K0.84 (NH4 )1.16 SO4 Te(OH)6 Formula Formula weight (g mol−1 ) Crystal system a (Å) b (Å) c (Å) β (deg) V (Å3 ) Min/max (deg) Total reflections Reflection with I > 4σ (I ) Parameters Min, max, ρ (e Å3 ) R(F )a Rw(F )a

K0.84 (NH4 )1.16 SO4 Te(OH)6 440 Monoclinic 14.929 (5) 6.558 (1) 11.325 (1) 120.17 (2) 958.6 (9) 3.48/32.11 3184 2395 132 −2.13, +2.52 3.8 10

a R values are defined as W R = ([w(F 2 − F 2 )2 ]/[w(F 2 )2 ])1/2 and 2 o c o

R1 =

||Fo | − |Fc ||/

|Fo |.

The X-ray single crystal were carried out using KappaCCD diffractometer with graphite monochromated Mo-Kα radiation [9]. The unit cell parameters are identified and refined using a Denzo and collect programs [10]. The integrated intensities were corrected for Lorentz and polarization effects [11]. All subsequent computations were carried out using the computer program SHELX [12,13]. The structure was solved by conventional Patterson and differenceFourier techniques and refined by the full-matrix leastsquares procedure. Differential scanning calorimetric measurements were realized on DSC SETARAM 92 between 300 and 750 K. Electric impedances were measured in the range 100 Hz–13 MHz using a Hewlett-Packard 4192ALF automatic bridge monitored by a HP vectra microcomputer. Raman spectra of crystals in sealed glass cell (2 mm in diameter), were obtained between 300 and 620 K on a multichannel X–Y Dilor spectrometer equipped with a CCD detector cooled with liquid nitrogen. The 514.5 nm radiation of a spectra-physics 2000 argon ion laser was used for excitation with a power about 20 mW. A furnace built in the laboratory was used for high-temperature experiments. The resolution was between 0.5 and 2 cm−1 and the accuracy of the sample temperature was about 5 K. The increment of temperature was in the range 20–30 K between each spectrum. The details of data collection, the final atomic positions and Ueq parameters for our compound are given in Tables 1, 2 and 3.

3. Results and discussion 3.1. Calorimetric study A typical result of the calorimetric study is presented in Fig. 1. The DSC curve shows three endothermic peaks at 425, 480 and 570 K and one shoulder at 470 K. The first

Table 2 Fractional atomic coordinates and temperature factors for K0.84 (NH4 )1.16 SO4 Te(OH)6 Atoms Te(1) Te(2) S K1 N1 K2 N2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 H11 H12 H13 H14 H21 H22 H23 H24 H5 H6 H7 H8 H9 H10

X

Y

0.0000 0.0000 −0.5000 −0.5000 −0.251(7) −0.007(9) −0.144(2) −0.505(3) −0.144(2) −0.505(3) −0.355(5) 0.484(2) −0.355(5) 0.484(2) −0.296(3) 0.056(8) −0.164(2) 0.127(6) −0.218(3) −0.222(6) −0.328(3) 0.006(4) 0.135(3) −0.110(6) 0.025(3) 0.075(9) −0.042(3) −0.265(6) −0.363(2) −0.584(6) −0.449(3) −0.356(6) −0.516(3) −0.738(5) −0.1477 −0.4200 −0.0807 −0.5000 −0.1475 −0.5796 −0.2022 −0.4950 −0.3502 0.5600 −0.3496 −0.9200 −0.3585 −0.4788 −0.3844 −0.4430 0.0966 0.0013 0.0634 −0.0111 −0.1029 −0.2964 −0.6455 −0.2574 −0.5475 −0.6046 −0.4377 −0.1538 a U = 1/3 U a ∗ a ∗ a a . eq i j ij i j i j

Z

Ueq a (Å3 )

0.0000 −0.5000 −0.485(8) −0.241(3) −0.241(3) −0.702(2) −0.702(2) −0.625(4) −0.398(3) −0.472(4) −0.440(5) 0.072(4) 0.176(4) 0.021(5) −0.450(4) −0.331(3) −0.414(3) −0.2687 −0.2140 −0.2683 −0.2432 −0.7450 −0.2488 −0.6235 −0.7676 0.0678 0.2408 0.0175 −0.5548 −0.5998 −0.5376

0.013(8) 0.012(8) 0.015(2) 0.046(8) 0.046(8) 0.031(5) 0.031(5) 0.043(9) 0.033(8) 0.040(9) 0.032(9) 0.036(8) 0.054(9) 0.052(6) 0.032(7) 0.035(8) 0.033(8) 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050 0.050

Fig. 1. Differential scanning calorimetry of K0.84 (NH4 )1.16 SO4 Te(OH)6 .

and the second enthalpy changes are, respectively, H1 = 12.89 J g−1 and H2 = 327.64 J g−1 . The first transition at 425 K can be attributed to a structural phase transition which transforms our solution from paraelectric to ferroelectric phase. The endothermal peak at 480 is probably attributable to a super-protonic phase transition accompanied by the breaking of two types of hydrogen bonds (O–H. . .O and N– H. . .O) which link TeO6 and/or NH4 to SO4 , such that the


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Table 3 Anisotropic displacement parameters of K0.84 (NH4 )1.16 SO4 Te(OH)6 Atoms

U11

U22

U33

U23

U13

U12

Te1 Te2 S K1 N1 K2 N2 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10

0.0128(2) 0.0132(2) 0.0133(5) 0.0414(4) 0.0414(4) 0.0336(9) 0.0336(9) 0.039(2) 0.026(5) 0.037(8) 0.0295(8) 0.0247(5) 0.053(2) 0.050(2) 0.0229(4) 0.0414(8) 0.0340(6)

0.0132(2) 0.0133(2) 0.0174(5) 0.059(2) 0.059(2) 0.0265(9) 0.0265(9) 0.054(2) 0.040(2) 0.028(2) 0.020(2) 0.043(2) 0.080(3) 0.030(2) 0.036(2) 0.0357(9) 0.0316(8)

0.0129(2) 0.0111(9) 0.0138(4) 0.0338(9) 0.0338(9) 0.0331(9) 0.0331(9) 0.024(7) 0.024(5) 0.060(2) 0.055(3) 0.0344(8) 0.0224(2) 0.090(3) 0.0351(8) 0.0244(6) 0.0285(6)

0.0002(9) −0.0001(9) −0.0008(3) 0.0002(8) 0.0002(8) 0.0027(5) 0.0027(5) −0.0010(8) −0.0032(4) 0.0007(7) −0.0038(1) −0.0026(5) 0.000(2) 0.003(2) 0.0023(5) −0.0093(4) 0.0069(3)

0.0056(8) 0.0054(8) 0.0049(3) 0.0157(5) 0.0157(5) 0.0152(7) 0.0152(7) 0.0071(5) 0.0041(2) 0.0261(8) 0.0268(9) 0.0100(3) 0.0122(7) 0.045(2) 0.0118(3) 0.0119(4) 0.0111(3)

0.0015(9) 0.0001(1) −0.0007(3) 0.0004(8) 0.0004(8) 0.0038(5) 0.0038(5) −0.016(2) −0.011(2) 0.003(5) −0.004(1) 0.007(4) 0.036(3) −0.001(2) 0.002(3) 0.0001(5) −0.005(4)

The anisotropic displacement exponent takes the form (−2π 2

i

∗ j Uij hi hj ai aj ).

Fig. 2. Projection of K0.84 (NH4 )1.16 SO4 Te(OH)6 crystal structure at 293 K on the ac plane.

proton moves between the potential wells associated with the anionic and cationic entities. The anomaly at 570 K can be attributed to the decomposition of the salt. These tentative of attributions will be confirmed by dielectric and vibrational studies. 3.2. Structure description The KNST material crystallizes in the monoclinic system with unit cell parameters: a = 14.929(5) Å, b = 6.558(1) Å, c = 11.325(1) Å, β = 120.17(2)◦. Differently to sulfate tellurate studied, these parameters of KNST material show an increasing of the a unit cell and an elargissement of the β angle which becomes near 120◦ . This angle is of about 109◦ in the other material already studied of this family of materials. A projection of the KNST structure on the ac plane is depicted in Fig. 2. The main feature of this structure is the 2− coexistence of two different anions (TeO6− 6 and SO4 ) in the same crystal. This structure type can be the origin of the ferroelectric polar phases in this material, so what dielectric measurements were performed to justify this statement. The

structure can be regarded as being built of planes of pure SO4 tetrahedra altering with planes of pure TeO6 octahedra, parallel to the bc plane. The K+ /NH+ 4 cations are intercalated between these kinds of polyhedra. In contrast with the ammonium sulfate tellurate [8], the Te atom in TeO6 octahedra in KNST, occupies two special positions. In consequence, the structure shows two kinds of octahedra Te1 O6 and Te2 O6 , with Te–O values between 1.896 and 1.917 Å. The O–Te–O angles varying from 86.9 to 93.06◦. In comparison with the sulfate tellurate studies, the Te–O distances and O–Te–O angles show an intermediate state. Indeed, in the pure compound K2 SO4 ·Te(OH)6 , Te–O distances spread from 1.914 to 1.938 Å and they are between 1.874 and 1.944 Å in (NH4 )2 SO4 ·Te(OH)6 compound [7,8]. This behavior confirms the effect of the ammonium cation insertion in the octahedra disorder. The SO4 groups are regular as seem in Table 4. The S–O distances vary from 1.439 to 1.475 Å and the average angles O–S–O is 109◦ . Differently to the pure potassium and ammonium sulfate tellurate where the environment of the K atom in K2 SO4 ·Te(OH)6 is octahedral, and the environment of the N(1) atoms is made of 7 oxygen atoms and the second atom N(2) is coordinated by 8 oxygens in (NH4 )2 SO4 ·Te(OH)6 material [8], in the new mixed solution K0.84(NH4 )1.16SO4 · Te(OH)6 , the K/N atoms are distributed on two sites and they are nine-coordinated with (K/N)–O bonds ranging from 2.924 to 3.152 Å for K1 /N1 and from 2.829 to 3.274 Å for K2 /N2 . These values are slightly lesser than the average found for the Rb/N–O distances at room temperature and at 435 K in Rb1.12(NH4 )0.88SO4 ·Te(OH)6 [14,15]. This fact is due to the difference between K and Rb atomic radius. These distances are higher than the average of K–O found in K2 SO4 ·Te(OH)6 where K–O distances are between 2.713 and 2.987 Å. This phenomenon is a second effect of the partial cationic substitution.

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Table 4 Atomic distances (Å) and angles (deg) of K0.84 (NH4 )1.16 SO4 Te(OH)6 Distance (Å)

Distance (Å)

Potassium and ammonium coordination K1 /N1 O2 a = 2.924 (4) O3 = 2.931 (5) O8 = 2.964 (5) O7 = 3.018 (6) O7 b = 3.042 (5) O6 c = 3.151 (6) O5 b = 3.130 (5) O1 d = 3.152 (6) O6 e = 3.215 (7)

K2 /N2 O9 f = 2.829 (4) O10 c = 2.888 (4) O5 g = 2.913 (4) O4 f = 2.925 (5) O1 = 2.945 (5) O8 h = 2.949 (4) O3 h = 3.050 (5) O9 k = 3.219 (4) O10 k = 3.274 (4)

Sulfate groups S–O1 = 1.439 (4) S–O2 = 1.470 (3) S–O3 = 1.472 (4) S–O4 = 1.475 (4) Tellurate groups Te1 –O6 e = 1.896 (4) Te1 –O6 = 1.896 (4) Te1 –O5 e = 1.905 (3) Te1 –O5 = 1.905 (3) Te1 –O7 e = 1.908 (4) Te1 –O7 = 1.908 (4) Te2 –O8 = 1.901 (3) Te2 –O8 l = 1.901 (3) Te2 –O9 l = 1.912 (3) Te2 –O9 = 1.912 (3) Te2 –O10 = 1.917 (3) Te2 –O10 l = 1.91 (3)

O1 –S–O2 = 108.9 (2) O1 –S–O3 = 109.6 (3) O1 –S–O4 = 110.9 (3) O2 –S–O4 = 108.3 (2) O3 –S–O2 = 111.6 (2) O3 –S–O4 = 107.5 (2) O6 –e–Te1 –O5 e = 92.12 (6) O6 –Te1 –O5 e = 87.88 (6) O6 –Te1 –O5 = 92.11 (6) O6 e–Te1 –O7 e = 90.5 (2) O6 –Te1 –O7 e = 89.5 (2) O5 e–Te1 –O7 e = 86.94 (7) O5 –Te1 –O7 e = 93.06 (7) O6 –Te1 –O7 = 90.5 (2) O5 e–Te1 –O7 = 93.06 (7) O5 –Te1 –O7 = 86.94 (7) O8 –Te2 –O9 l = 88.66 (6) O8 –Te2 –O9 = 91.34 (6) O8 l–Te2 –O9 = 88.66 (6) O8 –Te2 –O10 l = 90.11 (6) O9 l–Te2 –O10 l = 88.99 (5) O8 –Te2 –O10 = 89.89 (5) O8 l–Te2 –O10 = 90.11 (6) O9 l–Te2 –O10 = 91.01 (5) O9 –Te2 –O10 = 88.99 (5)

Indeed, the environment of K1 /N1 atoms is made from three oxygen atoms belonging to SO4 groups, three of the first type of octahedral (Te1 O6 ), two oxygens of another Te1 O6 octahedra and only one oxygen from the second octahedral Te2 O6 . The K2 /N2 atoms are coordinated by three oxygen atoms of the SO4 tetrahedra, one oxygen from Te1 O6 octahedra, three oxygens belonging to the Te2 O6 groups and two oxygen atoms of oxygen of another octahedral of Te2 O6 . In the KNST structure, the sulfate tetrahedra are connected with tellurate octahedra thanks to two types of hydrogen bonds O–H. . .O and N–H. . .O assured, respectively, by protons belonging to hydroxide groups and ammonium ones. The distances O–H vary from 1.684 to 1.797 Å and the O–H. . .O angles values are between 162.3 and 173.6◦. These values, showing in Table 5, are slightly smaller than those found in K2 SO4 ·Te(OH)6 structure. Consequently, three oxygen atoms (O2 , O3 , O4 ) belonging to SO4 tetrahedra groups participate in the formation of the O–H. . .O hydrogen bond type. One of them (O4 ) is tied to two hydrogen

Fig. 3. Complex impedance curves of K0.84 (NH4 )1.16 SO4 Te(OH)6 at various temperatures.

atoms but the others (O2 and O3 ) are tied to one hydrogen atom each. In addition to O–H. . .O hydrogen bonding, the structure of this mixed compound is stabilized by N–H. . .O hydrogen bonding. All the hydrogen atoms belonging to the first and the second type of NH+ 4 group participate in the formation of this kind of hydrogen bond. Only H14 and H24 , each one of them makes two bonds with oxygen atoms. The obtained O–H distances vary from 2.250 to 2.518 Å and N– H. . .O angles values are between 119.20 and 174.80◦. The presence of the two types of hydrogen bonds is in the origin of the protonic conduction phase transition. 3.3. Dielectric studies Polycrystalline pellets, 13 mm in diameter and 1 mm in thickness, were obtained at room temperature under 200 MPa pressure. The pellets were sintered at 400 K for 12 h in vacuum. This processing was applied to eliminate, as much as possible, the water content in the sample and to obtain dense pellets [16–18]. Some complex impedance diagrams showing Z vs Z , i.e., Cole–Cole plots [19], recorded at different temperatures are presented in Fig. 3. The resistance was determined by extrapolation of the circular arc centered under the Z axis to zero frequency [20]. These curves show the temperature dependence of the resistance proving the superionic conduction properties of the new mixed solution KNST. The temperature dependence of the conductivity is presented in Fig. 4 in a log(σ T ) vs 1000/T . The diagram shows two regions, the first one is below 480 K. In this region, the conductivity increases considerably, such behavior indicates the protonic conduction of our material at this temperature. One anomaly is observed at about 425 K which can change the activation energy of the two regions. This fact is due to the establishment of dipolar moment which can change the proton displacement mechanism. The KNST structure is


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Table 5 Distances and hydrogen bond angles in K0.84 (NH4 )1.16 SO4 Te(OH)6 O. . .O and N. . .O distances (Å)

O. . .H distances (Å)

O–H. . .O bonds O4 a. . .O8 = 2.723 (4) O2 e. . .O6 = 2.679 (4) O4 n. . .O10 = 2.712 (4) O3 d. . .O7 = 2.677 (4)

O4 O2 O4 O3

N–H. . .O bonds N2 . . .O9 f = 2.829 (3) N2 . . .O4 f = 2.925 (4) N2 . . .O4 f = 2.925 (4) N2 . . .O9 f = 3.050 (3) N2 . . .O8 = 2.949 (3) N1 . . .O3 = 2.931 (4) N1 . . .O6 b = 3.042 (3) N1 . . .O2 a = 2.924 (3) N1 . . .O8 = 2.964 (3) N1 . . .O1 d = 3.052 (3)

O9 f. . .H21 = 2.345 (3) O4 f. . .H22 = 2.435 (4) O4 f. . .H24 = 2.518 (4) O9 f. . .H24 = 2.355 (5) O8 . . .H23 = 2.250 (3) O3 . . .H11 = 2.380 (3) O6 f. . .H12 = 2.478 (3) O2 a. . .H13 = 2.361 (3) O8 . . .H14 = 2.443 (3) O1 d . . .H14 = 2.404 (3)

a. . .H8 p = 1.684 (3) e. . .H6 = 1.828 (3) n. . .H10 q = 1.784 (3) d. . .H7 = 1.797 (2)

O. . .H–O and O. . .N–H angles (deg) O4 a. . .H8 p–O8 = 173.54 (2) O2 e. . .H6 –O6 = 163.80 (4) O4 n. . .H10 q–O10 = 166.91 (2) O3 d. . .H7 –O7 = 162.3 (2) N2 –H21 . . .O9 f = 125.45 (1) N2 –H22 . . .O4 f = 126.90 (1) N2 –H24 . . .O4 f = 136.90 (1) N2 –H23 . . .O9 f = 142.50 (1) N2 –H23 . . .O8 = 146.20 (1) N1 –H11 . . .O3 = 147.90 (1) N1 –H12 . . .O7 b = 119.20 (1) N1 –H13 . . .O2 a = 174.80 (1) N1 –H14 . . .O8 = 120.05 (1) N1 –H14 . . .O1 d = 146.75 (1)

Symmetry codes: (a) x, y − 1, z; (b) −x, −y − 1, −z; (c) x, −y − 1/2, z − 1/2; (d) x, −y − 1/2, z + 1/2; (e) −x, −y, −z; (f) x, −y + 1/2, z − 1/2; (g) −x, y + 1/2, −z − 1/2; (h) x, y + 1, z; (k) −x − 1, −y, −z − 1; (l) −x − 1, −y − 1, −z − 1.

Fig. 4. Temperature dependence of log σ T for K0.84 (NH4 )1.16 SO4 Te(OH)6 .

stabilized by two types of hydrogen bonds O–H. . .O and N– H. . .O. In consequence, the transition at 480 K detected by DSC can be characterized by the breaking of these hydrogen bonds and the proton moves between the potential wells. The second region for temperature above 480 K, has an arc form and the conductivity does not obey the Arrhenius law. This phenomenon is due to many mechanisms such as the breaking of hydrogen bonds, the reorientation of the ammonium tetrahedral and the beginning of the decomposition of the salt. In the temperature range studied, the conductivity in + the mixed compound was assured by K+ /NH+ 4 and H ions.

Fig. 5. Temperature dependence of εr as a function of frequency for K0.84 (NH4 )1.16 SO4 Te(OH)6 .

Fig. 5 illustrates the temperature dependence of the positivity εr in the range 400–600 K for KNST salt. These curves show two anomalies at 430 and 470 K. The first peak at 430 K, observed by DSC at about 425 K, can be attributed to a structural phase transition which can favor the polar phase at high temperature. This fact is confirmed by the minimum observed in the dissipation factor values at the same temperature. This behavior is one of the characteristic of the fer-

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Fig. 7. Plots of log M versus log f for K0.84 (NH4 )1.16 SO4 Te(OH)6 at various temperatures.

Fig. 6. Thermal evolution of the dissipation factor as a function of frequency for K0.84 (NH4 )1.16 SO4 Te(OH)6 .

roelectric phase transition. The most intense peak at 470 K can be the summation of two peaks which characterize the ferroelectric–paraelectric and the superionic–protonic phase transitions, respectively, because the temperatures of the two phase transitions are very near. The evolution of εr for various frequencies shows that there is a significantly variation with the frequency in this material due to the fact that the material presents a long-range ion diffusion. In consequence, two polarization mechanisms are possible and the real part of dielectric constant can be presented as + εr(carr) , εr = εr(latt) presents the lattice response due to permawhere εr(latt) nent dipole orientation or other motions which do not involve long-range displacement of mobile charge carriers. In this contribution we observe the changes caused by the ferroelectric–paraelectric transition. presents the conductivity relaxation, or carrier reεr(carr) sponse, associated with long-range migration. The second contribution is very linked to the frequency and especially to the low frequency. This part of the permittivity characterizes the conductivity mechanisms. Fig. 6 shows the dissipation factor (tan δ) evolution as a function of temperature. The values of the dissipation factor are relatively important in agreement with the important contribution of the conductivity in this material which makes

) versus log f for Fig. 8. Plots of normalized modulus (M /Mmax K0.84 (NH4 )1.16 SO4 Te(OH)6 .

this compound a ferroelectric with diffuse character [4]. On the other hand, tan δ increases from low temperature, presents a maximum then decreases and presents a minimum in the vicinity of Tc . This behavior corroborates the presence of a ferroelectric–paraelectric phase transition at Tc = 470 K [4,16]. This fact confirms that our material is ferroelectric in the 230–470 K temperature range [5,6]. The values of ferroelectric–paraelectric temperature phase transition does not change with increasing frequency, this suggests that this compound does not present a dipolar-type relaxation in this frequency range. This phase transition is detected in the mother compound K2 SO4 ·Te(OH)6 of our new mixed solution at 490 K. In consequence, the cationic substitution of ammonium group by the potassium one increases the ferroelectric–paraelectric phase transition temperature. In order to throw some additional light on the role of K+ and NH+ 4 ions, and to identify the particle contributed to the conductivity phenomenon, dielectric relaxation studies have been consequently undertaken at high temperature, between 480 and 575 K, in the complex modulus M ∗ formalism. For a given temperature and frequency, the real part M and the imaginary part M of the M ∗ complex modulus (M ∗ = M + j M ) were calculated from the complex impedance data (Z ∗ = Z − j Z ) using the relations M = ωC0 Z and M = ωC0 Z . The plots of


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Fig. 10. Raman spectra of crystalline K0.84 (NH4 )1.16 SO4 Te(OH)6 between 300 and 620 K in the range 560–1210 cm−1 (λ0 = 514.5 nm). Fig. 9. Raman spectra of crystalline K0.84 (NH4 )1.16 SO4 Te(OH)6 between 300 and 620 K in the range 10–710 cm−1 (λ0 = 514.5 nm). log M and the normalized M /Mmax imaginary part of the complex modulus of KNST versus log f are given in Figs. 7 and 8 at various temperatures. Whatever the = 1/ε ) temperature, M reaches a constant value (M∞ ∞ at high frequencies and at low frequencies it approaches zero, which indicates that the electrode polarization phenomenon makes a negligible contribution to M ∗ and may be ignored when the electric data are analyzed in this spectra relative to a given temform [21]. The M /Mmax perature shows an asymmetrical peak. The modulus peak maximum shifts to higher frequencies as temperature increases. The region of the left of the peak maximum is where the H+ protons are mobile over long distances, whereas the region of the right is where the ions are spatially confined to their potential wells. The frequency range where the peak occurs indicative of the transition from short-range to long-range mobility at decreasing frequency and is defined by the condition ωτσ = 1, where τσ is the most probable constitution proton relaxation time [22]. This phenomenon is similar to the one observed in Rb1.12 (NH4 )0.88SO4 ·Te(OH)6, which confirms that the proton transport in K0.84(NH4 )1.16 SO4 ·Te(OH)6 is probably due to a hopping mechanism [23].

3.4. Raman studies In order to gain more informations on the crystal dynamics, on the degree of disorder in the different phases and the mechanisms involved in the transitions, we have undertaken a Raman study between 300 and 620 K (Figs. 9 and 10) in the range 10–1200 cm−1 . Frequencies and assignments of the Raman peaks in the different phases are given in Table 6. The observed frequencies are interpreted on the basis of the characteristic frequencies of the Te(OH)6 and SO4 groups [25,26]. The stretching and bending vibrations for compounds containing the TeO6 group normally occur in the range of 550–750 and 350– 450 cm−1 , respectively [24]. At room temperature, the Raman spectrum of mixed compound K0.84(NH4 )1.16 SO4 ·Te(OH)6 is very resolved, which confirm the presence of the polar phase. The intense peak around 653 cm−1 is assigned to ν1 (TeO6 ) whereas the ν2 (TeO6 ) and ν4 (SO4 ) appear in the spectra at 634 and 611 cm−1 , respectively. The narrow and intense band at 982 cm−1 is attributed to ν1 (SO4 ). The weak line at 1072 cm−1 is assigned to ν3 (SO4 ). This fact confirm the 2− coexistence of the TeO6− 6 and SO4 anionic groups independently which confirm our last structural description. From the thermal evolution of Raman spectra for the mixed K0.84(NH4 )1.16 SO4 ·Te(OH)6 solution, we can deduce

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Table 6 Observed Raman frequencies (cm−1 ) and band assignments for K0.84 (NH4 )1.16 SO4 Te(OH)6 at different temperatures Raman 300 K

373 K

403 K

443 K

473 K

523 K

603 K

Assignment

1072 vw 982 s 653 vs 634 sh 611 sh 480 vw 460 w 446 w 364 m 351 sh 323 w – – 57 vw – –

1074 vw 981 s 650 vs 633 sh 611 sh 477 vw 459 w 445 vw 362 m 352 sh 323 w – – 55 vw – –

1075 vw 982 s 651 vs 633 sh 611 sh – 459 w 445 vw 359 m 353 sh 322 w – – 54 vw – –

1075 vw 980 s 649 vs – 610 sh – 458 vw 445 vw 359 m – 323 w – – 52 vw – –

1052 vw 979 vs 660 m – 618 sh – 491 m 454 w – – 337 vw 242 m 171 vw – 42 vw 23 vw

1047 vw 978 vw 659 w – 618 vw – 487 w 452 vw – – – 243 vw – – 45 vw –

– – – – 631 vw – – 455 vw – – – – – – 47 vw –

ν3 (SO4 ) ν1 (SO4 ) ν1 (TeO6 ) ν2 (TeO6 ) ν4 (SO4 ) ν2 (SO4 )

ν5 (TeO6 )

T (NH4 ) νOH. . .O T (K+ )

Vs: very strong; s: strong; m: medium; w: weak; vw: very weak; sh: shoulder.

that when temperature increases many bands present a decreasing in intensity and an increasing of width, which is in agreement of the establishment of a disorder with the paraelectric phase and confirm our last interpretation that the superprotonic phase transition observed in this salt is due to the breaking of the hydrogen bond which can favor the disorder phase. So, two phase transitions at 425 and 460 K have been evidenced in this compound by Raman spectroscopy. All of these phase transitions are reversible and hysteresis was not observed. Furthermore, the spectra never show the presence of both phases. The first phase transition presents a second character and is of order–disorder type. The second phase transition shows the proton conduction and the ferro– paraelectric transition given by dielectric and DSC measurements. These two phenomenons are illustrated by the Raman spectra of this compound at 473 K. At high temperature, the transformation of this compound was evidenced (570 K). In fact, the band at 630 cm−1 attributed to ν2 (TeO6 ) decreases in intensity on raising temperature and is masked at 443 K, whereas the line at 611 cm−1 relative to ν4 (SO4 ) decreases in intensity and becomes very broad after the first transition detected by DSC at 425 K. The bands relative to ν2 (SO4 ), at 446, 460 and 480 cm−1 decrease in intensity on raising the temperature and the last one at 480 cm−1 disappears after the first phase transition. On the other hand, the ν5 (TeO6 ) bands which appear at 323, 351 and 364 cm−1 decrease in intensity and broaden as the temperature is increased. They disappear at the second phase transition. This phenomenon confirms the presence of the first phase transition detected by both DSC and dielectric measurement at about 425 K. The line at 653 cm−1 assigned to ν1 (TeO6 ) stretching shifts by about 10 cm−1 , decreases in intensity and broadens through temperature increases after the second phase transition. The narrow peak detected at 982 cm−1 relative to ν1 (SO4 ) remains constant in intensity and in wavelength

number but it decreases in intensity and broadens near the second transition then disappear at high temperature. These modes can be considered as a soft mode accompanying the disordered paraelectric phase at high temperature of our material. The second phase transition is mainly characterized by the appearance of three bands at 242, 454 and 491 cm−1 , and for lattice mode the appearance of two new bands at 42 and 23 cm−1 and the desperation of the one at 57 cm−1 . These phenomenons confirm the presence of the first order–disorder transition at 425 K, the second and the third phase transitions, respectively, at 470 and 480 K detected by both DSC and dielectric measurements.

4. Conclusion The new mixed composition K0.84 (NH4 )1.16SO4 Te(OH)6 (KNST) was characterized at room temperature by X-ray, dielectric and vibrational investigations. It is found that crystals of KNST crystallize in the monoclinic system P 21 /c. The structure can be regarded as being built of planes of TeO6 octahedra and pure tetrahedra of SO4 . The K+ /NH+ 4 cations are intercalated between these kinds of polyhedra. The measurements of some electrical properties combined with DSC, Raman spectroscopy and X-ray examination indicate the existence of three phase transitions at 425 K which can favor the polar phase, and at 470 and 480 K attributed to the ferroelectricity and the superprotonic conduction, respectively. The peak detected by DSC at 570 K is related to the decomposition of the salt.

Acknowledgement We express our thanks to Dr. N. Zouari for his dielectric measurements and fruitful discussions and interpretations.


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