Structural and dielectric properties of a new cesium-rubidium dihydrogen arsenate: Cs0.2Rb0.8H2AsO4

Accepted Manuscript Structural and dielectric properties of a new cesium-rubidium dihydrogen arsenate: Cs0.2Rb0.8H2AsO4 Samia Chouchene, Khaled Jaouadi, Tahar Mhiri, Nabil Zouari PII:

S0925-8388(17)30597-2

DOI:

10.1016/j.jallcom.2017.02.159

Reference:

JALCOM 40887

To appear in:

Journal of Alloys and Compounds

Received Date: 31 December 2016 Revised Date:

13 February 2017

Accepted Date: 14 February 2017

Please cite this article as: S. Chouchene, K. Jaouadi, T. Mhiri, N. Zouari, Structural and dielectric properties of a new cesium-rubidium dihydrogen arsenate: Cs0.2Rb0.8H2AsO4, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.02.159. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Structural and dielectric properties of a new Cesium- Rubidium Dihydrogen Arsenate: Cs0.2Rb0.8H2AsO4 Samia CHOUCHENE*, Khaled JAOUADI, Tahar MHIRI, Nabil ZOUARI Laboratoire physico-chimie de l’état solide, Faculté des Sciences de Sfax, 3038 Sfax, Tunisie

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ABSTRACT Ongoing studies of the CsH2AsO4-RbH2AsO4 system, aimed at developing novel proton conducting solids, resulted in the new compound Cs0.2Rb0.8H2AsO4 (CRDA). Single crystal X-ray diffraction (performed at room temperature) revealed Cs0.2Rb0.8H2AsO4 to

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crystallize in tetragonal space group I42d with lattice parameters a = 7.8090(1) Å and c= 7.5010(1) Å. It has a unit cell volume of 457.415(10) Å3 and four formula units per cell. The

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title compound is isostructural with the tetragonal phases of CsH2AsO4 (CDA) and RbH2AsO4 (RDA). The main feature of this structure is the coexistence of two different cations (Cs+ and Rb+) in the same crystal. In this structure, the arsenic atom as well as the rubidium and cesium ions lie on points with site symmetry S4, the oxygen atoms lies in general positions about the arsenic atoms, in a tetrahedral arrangement. The AsO4 tetrahedra are connected by O-H…O hydrogen bonds laying essentially in the a,b plane. The infrared spectrum performed at room

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temperature in the frequency range 4000– 400 cm-1 confirms the presence of structural disorder in this material. A calorimetric study of the title compound shows two peaks, they are detected at 185 and 433 K. Samples were examined by impedance and modulus spectroscopy techniques. The first transition (185 K) is attributed to a ferroelectric–paraelectric type. A

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high temperature phase transition (433 K) leading to a superionic–protonic phase was found, characterised by an unusual high conductivity. The conductivity relaxation parameters

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associated with the high-disorder protonic conduction have been determined from analysis of the M”/M”max spectrum measured in a wide temperature range. Transport properties in this material appear to be due to proton hopping mechanism.

Keywords: Inorganic materials; X-ray single crystal; Thermal behavior; ferroelectric– paraelectric transition; conductivity; proton hopping mechanism.

ACCEPTED MANUSCRIPT 1. Introduction 1 The KDP family of compounds of the general formula MH2XO4 (where M is an alkali 2 metal: K, Rb, Cs, and X is P or As) has been of great interest for researchers, thanks to its 3 structural and physical properties [1–5]. In this family of compounds, the conductivity in 4 the superprotonic state is due to the proton motion between dynamically disordered

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5 hydrogen bonds. In this process, proton transport in the superprotonic phase is facilitated by 6 the rapid reorientation of XO43- groups, and the fast proton transport is achieved mainly 7 attributable to the thermally activated hopping [1]. This crystal family shows a phase-

8 transition-like phenomenon called the high-temperature phase transition at a characteristic

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9 temperature Tp. It is known that many crystals with hydrogen bonds undergo high-

10 temperature phase transitions, in addition to the low temperature ferroelectric (FE) or

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11 antiferroelectric (AFE) phase transitions. Much interest has been focused on phosphates 12 and arsenates.

13 RbH2AsO4, a MH2AO4 group compound, is isomorphous with KH2PO4, and it under goes 14 phase transition at TC=110 K from a high-temperature paraelectric phase to a low15 temperature ferroelectric phase [6]. In the paraelectric and ferroelectric phases, the crystal 16 has a tetragonal structure with space group I42d and an orthorhombic structure with space

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17 group Fdd2 [7], respectively. The crystal CsH2AsO4 (CDA) is isomorphous with KDP and 18 has similar transition from a tetragonal (I42d) paraelectric phase to an orthorhombic 19 (Fdd2) ferroelectric phase at Tc=140 K [8]. In our study, we have shown that the 20 introduction of defects by cationic substitution in the host framework spreads out the phase

22 phases.

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21 transition, bringing together the electrical properties of the high and low-temperature

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23 In the present study, we report and discuss the results of a structural investigation, 24 concerning a new solid solution: Cs0.2Rb0.8H2AsO4 (CRDA). We have performed X-ray 25 diffraction measurements providing us information about the complete crystal structure at 26 room temperature of the new compound. This structural study is accompanied by infrared 27 and Raman measurement, and we try to evidence the phase transitions in (CRDA) by 28 differential scanning calorimetric. In order to determine the influence of the partial 29 substitution of the caesium by rubidium (Cs0.2Rb0.8H2AsO4), we have undertaken an 30 analysis of the frequency response of a.c. conductivity data in our material. This process is 31 useful in determining the complex permittivity ε* and the conductivity relaxation time. The 32 complex modulus formalism has been adopted to determine the conductivity relaxation

ACCEPTED MANUSCRIPT 33 times. 2. Experimental details 2.1. Preparation 34 Single crystals of the title compound Cs0.2Rb0.8H2AsO4 were grown from an aqueous 36 H3AsO4 with the following reaction: 37

xRb2CO3 + (1−x) Cs2CO3+ H3AsO4

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35 solution of caesium carbonate Cs2CO3, rubidium carbonate Rb2CO3 and arsenic acid 2RbxCs1-xH2AsO4 + H2O + CO2

38 The resulting solution is kept under ambient condition which was allowed to evaporate 40 (0.24 × 0.20 × 0.16) mm3 of CRDA were obtained. 2.2. Experimental

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39 slowly. Fifteen days later, colorless, transparent and plate-like crystals with a size about

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41 Single-crystal X-ray diffraction intensity data were obtained on an Enraf–Nonius Kappa 42 CCD diffractometer using MoKα radiation (λ= 0.71073 Å). The raw intensity data were 43 corrected for Lorenz and polarizing effects before proceeding to the refinement of the 44 structure. Atomic scattering factors were taken from the International Tables for X-ray 45 crystallography [10]. There were 1140 reflections collected in the whole Ewald sphere for 46 3.77 ° ≤ θ ≤ 26.97° of which 166 reflections had an intensity of I > 2σ(I). The chemical

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47 crystal composition data and the results of crystal structure determinations are listed in 48 Table 1. We solved the structure by first location the rubidium and arsenate atoms position 49 using SHELXS-97 program [9] and subsequently the remaining non hydrogen atoms were 50 deduced from difference Fourier maps during the refinement of the structure with

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51 SHELXL-97 program [10]. The H atoms were located through difference maps on the 52 basis of geometric considerations [11]. Their positions and temperature parameters being

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53 kept fixed in the structure refinement. The final cycle of refinement leads to the final 54 discrepancy R1 = 0.0512 and WR2 = 0.0942, obtained by fitting 17 parameters. The final 55 fractional atomic coordinates and the equivalent anisotropic displacement parameters for 56 all non-hydrogen atoms are given respectively in Tables 2 and 3. The structure graphics 57 were created by the Diamond program [12]. 58 Infrared spectrum was recorded in the 4000–400 cm-1 with a Perkin Elmer FT-IR 420 59 spectrophotometer, using samples in spectroscopically pure KBr pellets. Raman spectra of 60 polycrystalline samples sealed in glass tubes have been recorded on a Labram HR 800 61 instrument using 632.81 nm radiations from a physics argon ion laser. Calorimetric 62 measurements were performed between 100 and 600 K on a DSC Mettler TA 4000

ACCEPTED MANUSCRIPT 63 calorimeter. 64 Electrical properties were determined by impedance and modulus method using a 65 frequency response analyser (Hewlett-Packard 4192 A LF automatic bridge monitored by 66 a HP Vectra microcomputer). The frequency range was 100 Hz to 1 MHz and 67 measurements were carried out in vacuum between 130 and 503 K. At each interval, the

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68 sample temperature was maintained by a Herrrmann-Moritz 28480 Chassant (using a 69 Chromel/Alumel couple) controller for 0.5 h before collecting data; the stability was ±1°C. 3. Results and discussion

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3.1. Structural study

70 The unit cell parameters optimised by least-squares refinement, were calculated and 71 refined using indexation of the collected intensities and revealed Cs0.2Rb0.8H2AsO4

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72 (CRDA) to be tetragonal I42d with lattice parameters a = 7.8090(1)(Å); c = 7.5010(1)(Å) 73 and Z=4.

74 The first figure shows a perspective drawing of (CRDA) with atom labeling. Its structure 75 consists of AsO4 and (Cs/Rb) groups held together by O-H…O hydrogen bonds. The mean 76 feature of this structure, is the simultaneously coexistence of one anion AsO43- and two

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77 different cations (Cs+ and Rb+) in the same crystal. The AsO4 tetrahedra are connected by 78 O-H…O hydrogen bonds laying essentially in the a,b plane (Fig. 2). The localization 79 of the hydrogen atoms based on the As–O bond distances and indications in the difference 80 electron maps is in good agreement with the hydrogen bond net which can be deduced

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81 from the short intermolecular O…O distances. The distance between two oxygen, 82 participating in the formation of O-H…O of hydrogen-bonding, are all equal to 2.515 Å.

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83 The high-resolution neutron diffraction study of (KDP) has resolved two peaks in the 84 proton-density distribution map of the H-bond of 2.4945 Å [13, 14], this result indicates 85 that the proton in the crystallographically symmetric H-bond O-H…O in (CRDA) will also 86 certainly be disordered. In the (CRDA) structure, we observe one type of AsO4 tetrahedra. 87 The main interatomic distances and bond angles for the tetrahedra are given in Table 4. 88 It is well known, that A-OH (A = P or As) distances are in general longer than A-O 89 distances, and that A-OH distances increase as the strength of the hydrogen bond O-H…O 90 increases, that is, as O…O decreases [15]. In our investigation of this structure, we can 91 notice the equality of As-O and As-OH distances; this can be interpreted as a result of a

ACCEPTED MANUSCRIPT 92 disorder in (CRDA) compound. Similar effects have been reported in other crystals of the 93 (KDP) family [13]. In many hydrogen-bonded ferroelectrics, it is usual for the protons to 94 be disordered in the paraelectric phase and to become ordered in the ferroelectric phase. 95 The coordination sphere of Cs+/Rb+ cations is determined by eight oxygen atom neighbors 96 forming two interpenetrating tetragonal disphenoids (Fig. 3). One of the disphenoids is flat

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97 and the other is steep with respect to the c axis. Four (Cs/Rb)–O bond distances are equal 98 to 2.933(8) Å in the flat disphenoids and also four (Cs/Rb)–O bond distances are equal to 99 3.112(7) Å in the steep disphenoids (Table 4). The structure of (CRDA) has been described 100 as similar to those of (CDA) and (RDA).

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3.2. Vibrational study:

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3.2.1 IR spectroscopy investigation:

101 As IR spectroscopy is one of the major physical methods of investigation of the 102 molecular structure, we have studied the IR spectrum of the polycrystalline samples of 103 (CRDA), which is shown in Fig. 4. The low symmetry of (CRDA), as well as the 104 existence of multiple, crystallographically independent X atoms (X = As), makes a

106 meaningless.

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105 complete group factor analysis of this compound not only difficult, but also somewhat 107 The IR peaks obtained in the low-frequency region of the spectra (< 580 cm-1) most likely 108 correspond to internal bending modes of the HnXO4 groups. The IR bonds observed at 109

425 cm−1 is attributed to ν4(AsO4) mode. The peaks obtained in the region from 750 to

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110 1190 cm-1 most likely correspond to HnXO4 stretching modes and appear to be further 111 distinguished between those with frequencies ranging between 750 and 930 cm-1 and

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112 those whose frequencies range between 970 and 1190 cm-1. So, the peaks observed at 758 113 and 866 cm−1 are associated to the ν1(AsO4) mode and those noted in the region 1067114 1186 cm−1 are presumed to result from ν3(AsO4). The bond obtained at 1279 cm-1 is 115 interpreted to the X–O–H in-plane bending modes, although there may be some 116 contribution from the X–O and X–OH stretching modes. The bands observed in the high 117 frequency, appear at 2835, 2360 and 1652 cm-1, have also been observed in the spectrum 118 of CsH2PO4 [14] at 2750, 2300 and 1640 cm-1 and are interpreted as OH stretching modes 119 in Fermi resonance with combination of OH bending vibrations [15]. The frequencies for 120 the corresponding bands are given in Table 5.

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Raman spectroscopy investigation:

121 The Raman spectrum of both internal and external modes of polycrystalline samples of 122 Cs0.2Rb0.8H2AsO4 (CRDA) were recorded at room temperature (Fig.5). This study was 123 restricted to the 50 – 1300 cm-1 spectral region because it included the lattice vibrations 124 and ν(X-OH) and ν(X-O) stretching modes, which are the most sensitive to phase

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125 transition and crystal changes. In the Raman spectrum, the band appears at 78 cm-1 is 126 assigned to translational vibration of the Rb+ and Cs+ ions and the peak at 113 cm-1 is 127 attributed to δ (O–H…O). While the band observed at 147 cm-1 is corresponded to ν(O– 128 H…O). The two peaks detected at 355 cm−1 and 393 cm−1, can be attributed to ν2(AsO4)

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129 vibration and the two band observed at 435 cm-1 and 551 cm-1 are corresponding to

130 ν4(AsO4). The peak that appears at 790 cm-1, can be attributed to ν1(AsO4). While, the 132 assignments are given in Table 5. 3.3. Calorimetric study:

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131 band at 923 cm-1 corresponds to ν3(AsO4). The observed Raman bands and their

133 The thermogram of differential scanning calorimetry (DSC) shows three endothermal 134 peaks at T1= 185, T2= 433 and T3= 570 K (Fig.6). The first transition is considered to be

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135 of an order–disorder structure phase transition, the second one seems to be a superionic– 136 protonic type and the latter one is attributed to the melting point of the product. The 137 calculated transition enthalpy, for the first transition at T =185 K, were ∆H1 = 28.2 J mol1

138 and for the second transition at T =433 K, ∆H2 =143 J mol-1. These two transitions were

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139 confirmed by dielectric permittivity at different frequencies and temperatures for the first 140 transition and by conductivity measurements using impedance spectroscopy for the

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141 second.

3.4. Electrical properties 3.4.1. Dielectric study

142 The electrical data measured as an impedance, Z*, have been converted into permittivity, 143 ε*, using the relation ε*= 1/iwC0Z* where C0 is the vacuum capacitance [16]. The 144 temperature dependence of the dielectric constants ε'r and ε"r, the real and imaginary parts 145 of the dielectric permittivity (ε*=ε'r - iε"r) are presented at several frequencies inFig.7 146 and 8. These spectra present a prominent dielectric peak at Tc=185 K characterizing the 147 ferroelectric–paraelectric phase transition which is observed by differential scanning

ACCEPTED MANUSCRIPT 148 calorimetry. The evolution of this dielectric constant as a function of frequency shows an 149 increase of ε'r with decreasing frequencies. This phenomenon characterizes the presence 150 of dispersion character in this material. 151 The values of paraelectric–ferroelectric temperature phase transition does not change with 152 increasing frequency, this suggests that this compound does not present a dipolar-type ஼

154 well the Curie–Weiss law: ε′௥ = ்ି்଴

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153 relaxation in this frequency range. The dielectric data in the paraelectric state obey quite where C is the Curie–Weiss constant and T0 is

155 the Curie–Weiss temperature for the paraelectric state (fig.9). The strong decrease of ε'r 156 with increasing frequency observed in Fig. 7 is nothing but a dispersion without loss of

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157 peak resulting from charge carriers present in the material. This phenomenon is usually 158 referred to as low-frequency dispersion (LFD) [17] where the charge carrier contribution

3.4.2. Protonic conductivity

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159 is significant.

160 The crystals of CRDA have been reported to demonstrate high protonic conductivity in 161 the high temperature phase. Our measurements were achieved in the temperature range 162 300–530 K. Some complex impedance diagrams –Z” versus Z’ at various temperatures

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163 are given in Fig. 10, and show that CRDA follows the Cole–Cole law. The difference 164 between the Cole–Cole and Debye law is determined from the precedent figure (α = 165 0.19). The bulk ohmic resistance relative to experimental temperature is the intercept on 166 the real axis of the zero-phase angle extrapolation of the highest-frequency curve. These

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167 results are used elsewhere to show the evolution of the conductivity versus inverse 168 temperature log (σT) = f(1000/T) for CRDA compound (Fig. 11). In the temperature

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169 range 380–470 K, the experimental points are located on both sides of a line above and 170 below 433 K. An Arrhenius-type law (σT= σ0 exp (-∆Ea/kT)) characterise the low and the 171 high temperature domain with a sudden sharp increase of the conductivity upon 433 K. 172 The activation energy of the title compound at high temperature is 0.33 eV and the 173 conductivity increases sharply from 433 K (σ = 4.83 × 10-7 Ω-1 cm-1 at 433 K to 6.52 × 174 10-3 Ω-1 cm-1 at 438 K). 175 This result confirms the presence of a superprotonic phase transition observed previously 176 at the same temperature by differential scanning calorimetry measurements. The detected 177 transition corresponds to the structural transformation between the tetragonal phase and 178 the superprotonic phase, and leads to fast proton conduction. In Fig. 11 we show log

ACCEPTED MANUSCRIPT 179 (σT) = f(1000/T) plots for CRDA material at different temperatures. At low frequencies 180 dc conductivity plots are observed above and below the superionic–protonic phase 181 transition which occurs at 433 K. ac conductivity increases with increasing temperature. 182 At high frequencies, these curves show an increasing hard segment length. This is due to 183 the fact that charge-carrier motion occurs mostly through the soft segment phase. The

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184 increasing of the protonic conductivity is due to an increasing disorder of the hydrogen 185 atoms among partly occupied symmetry equivalent positions. Therefore, we have

186 included in this work the modulus formalism M*=1/ε* =jwC0Z*, to understand the proton 187 conduction mechanism.

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4.3. Modulus spectroscopy analysis

188 A plot of log M' and of the normalised M"/M"max imaginary part of the complex

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189 modulus versus log f is given in figures 12 and 13 at various temperatures for CRDA 190 compound.

191 At all temperatures, the value of M' reaches at high frequencies, a constant value M’∞. At 192 low frequencies, it approaches zero, which indicates that the electrode polarization 193 phenomena make a negligible contribution to M* and may be ignored when the electric 194 data are analysed in this form [18]. The M"/M"max relative to a temperature given shows

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195 an asymmetrical peak approximately centred in the dispersion region of M* (Fig. 13). 196 The region to the left of the peak is where the H+ ions are mobile over long distances; the 197 region to the right is where the ions are spatially confined to their potential wells.

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198 The frequency range where the peak occurs is indicative of the transition from short199 range to long-range mobility at decreasing frequency and is defined by the condition 200 ωτσ ≈ 1 where τσ is the most probable ion relaxation time [19]. The M"/M"max curves

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201 are asymmetric, in agreement with the non-exponential behaviour of the electrical 202 function, that is well described by the empirical stretched exponential Kohlrausch 203 function ϕ (t) = exp [- (t / τσ)β] [20–22]. 204 The full-width at half-height (FWHH) of the M"/M"max spectrum is clearly wider than 205 the breadth of a Debye-peak (1.14 decades) [23] and it results in a value β = 0.74 for the 206 Kohlrausch parameter. When temperature increases, modulus peak maxima shift to 207 higher frequencies (Fig. 13). 208 An Arrhenius-type law is shown with a high jump at about 433 K and confirm the

ACCEPTED MANUSCRIPT 209 superionic–protonic transition already observed by conductivity measurements and DSC 210 at the same temperature. At low and high temperatures, both lines of the conductivity log 211 (σT) and the modulus peak maxima log (fp) observed in the temperature studied are quasi 212 – parallel (fig.14), the activation energies deduced from the impedance (∆Eσ) and 213 modulus (∆Ef) spectra are very close (∆Eσ(l.T) = 0.51 eV; ∆Ef(l.T) = 0.49 eV; ∆Eσ(HT) =

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214 0.33 eV; ∆Ef(HT) = 0.32 eV), suggesting that the H+ protons transport in (CRDA) is

215 probably due to a hopping mechanism [24]. On the other hand, the (FWHH) width of 216 peaks corresponding to various measurement temperatures is approximately close to 1.53 217 decades. Consequently, β may be considered as independent of temperature in the

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218 temperature range studied. The value of the β parameter, is clearly smaller than 1, can be 219 attributed to the existence of a distribution of relaxation times in the material. Such an

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220 interpretation has been adopted for many solid electrolytes [25,26]. On the contrary, 221 recent studies based on the overlapping of the log M' vs log f and log M" vs log f plots 222 obtained for various temperatures have shown that this interpretation was questionable 223 [20, 27]. 5. Conclusion

224 The synthesis and structure determination of the salt CRDA were determined in a

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225 tetragonal system. This structure has been described as similar to those of (RDA) and 226 (CDA). The hydrogen bonds connect the tetrahedra in such a way as to form a three227 dimensional network. The cesium/ rubidium atoms have eight oxygen atoms neighbours. 228 The main feature of this structure is the coexistence of two different cations (Cs+ and

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229 Rb+) in the same crystal.

230 The infrared and Raman spectra of the title compound was acquired at room temperature

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231 and is characterized by the apparition of a few bonds which confirm the presence of the 232 anion AsO43-. The physical properties and phase transitions of this compound were 233 examined by different methods. The differential scanning calorimetry shows three 234 anomalies at 185, 433 and 570 K corresponding, respectively, to paraelectric– 235 ferroelectric, superionic– protonic phase transitions and to the melting point. Based on 236 a.c.-impedance measurements, we have analysed the low-frequency dispersion 237 phenomena in this compound in the 110–310 K temperature range. The ordering effect of 238 protons in the short hydrogen bonds is the key factor responsible for ferroelectricity in 239 CRDA compound. 240 The conductivity relaxation is well described by the empirical stretched exponential

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241 Kohlrausch function ϕ (t) = exp [- (t / τσ) ]. The value of the β parameter (0 < β < 1) 242 represents for the conductivity relaxation the departure from the linear exponential (β < 243 1). The fp frequency relative to M"max (fp = 1/2πτσ) is defined by the condition ωτσ ≈ 1, 244 where τσ is the most probable ion relaxation time, fp increases with increasing 245 temperature and the temperature dependence of fp is of Arrhenius type. Information about

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246 charge carrier transport mechanism is obtained by comparison of ∆Ef with ∆Eσ . These 247 activation energies for CRDA compound which issued from the impedance and modulus 248 spectra are very close, suggesting that the protonic transport above and below the

249 superionic–protonic phase transition (433 K) is probably due to a hopping mechanism.

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250 On the other hand, in the temperature range studied, β may be considered as independent 251 of temperature. The value of this parameter, clearly smaller than 1, shows the existence of

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252 a distribution of relaxation times in the CRDA material and confirms the validity of this

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253 model for protonic conducting in the KDP family.

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ACCEPTED MANUSCRIPT Table captions Table1: Main crystallographic, feature X-ray diffraction data parameters results of CRDA. Table 2: Fractional atomic coordinates and temperature factors for CRDA Table 3: Anisotropic displacement parameters of CRDA Table 4: Inter atomic bond distances (Ǻ) and angles (°) of CRDA

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Table 5: Infrared and Raman frequencies for CRDA compound.

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Crystal data Formula Cs0.2Rb0.8H2AsO4 Crystal system tetragonal Space group I42d a/Å 7.8090(1) c/Å 7.5010(1) 457.145(3) Volume/Å3 Z 4 F(000) 336 Formula weight/g mol-1 235.894 ρcal /g cm-3 3.427 Morphology: plate-like Color: colorless Maximum crystal dimensions (mm3): 0.24 x 0.20 x 0.16 Experimental details Diffractometer Kappa CCD h= -9 9 k= -9 9 l= -9 9 Refined parameters: 17 Reflections observed with F0> 4 σ (F0): 166 Total reflections 1140 R1 = [F2> 4σ (F)2] 0.0512 0.0942 WR2 (F2) ρ min/e Å-3 -0.731 ρ max/e Å-3 1.222

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Table1: Main crystallographic, feature X-ray diffraction data parameters results of CRDA.

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Y

Z

Ueq

Occupation

0.0000

0.5000

0.2500

0.0194(9)

1

Cs

0.0000

0.5000

0.7500

0.0284(2)

0.22(7)

Rb

0.0000

0.5000

0.7500

0.0284(2)

0.77(3)

O

-0.154(1)

0.5890(9)

0.1212(8)

0.0283(5)

1

H

0.1530

0.2182

0.1220

0.05

RI PT

As

U22

As

0.0190(2)

0.0190(2)

Cs

0.0316(3)

0.0316(3)

Rb

0.0316(3)

0.0316(3)

O

0.030(4)

0.025(4)

U33

EP AC C

U23

M AN U

U11

U13

U12

0.0200(5)

0.0000

0.0000

0.0000

0.0220(4)

0.0000

0.0000

0.0000

0.0220(4)

0.0000

0.0000

0.0000

0.030(3)

0.009(3)

-0.009(3)

-0.004(3)

TE D

Atoms

SC

Table 3: Anisotropic displacement parameters of CRDA

0.5

ACCEPTED MANUSCRIPT Table 4: Inter atomic bond distances/Ǻ and angles/° of CRDA (a) arsenate groups OI- As-OII= 109.0(2)

As – OI = 1.693(6)

OI - As-O= 109.0(2)

As – OII = 1.693(6)

OII- As-O= 110.4(5)

As – OIII = 1.693(6)

OI- As-OIII= 110.4(5)

RI PT

As – O = 1.693(6)

OII- As-OIII= 109.0(2) O- As-OIII= 109.0(2)

SC

(b) Hydrogen bonds

O . . . OIX= 2.515(1)

O–H(O) = 1.010(7)

H(O) . . . H(O) IX= 0.499(2 )

M AN U

H(O) . . . OIX= 1.506(7)

O–H(O) . . . OIX= 177.19(4) (c)Ceasium-Ribidium coordination Cs/Rb–OV = 2.933(8)

Cs/Rb –OI = 3.112(7)

Cs/Rb –OVI= 2.933(8)

Cs/Rb –OII =3.112(7)

Cs/Rb –OVIII = 2.933(8)

Cs/Rb –OIII= 3.112(7)

TE D

Cs/Rb –OVII=2.933(8)

Cs/Rb –OIV = 3.112(7)

Symmetry code: I -y+1/2, x+1/2, -z+1/2; IIy, -x, -z; III-x+1, y+1/2, -z+5/4; IV -x, -y, z; Vx+1/2, -y+1/2, -x, z+3/4;

AC C

0.25+z ;

VI

VII

EP

y, -z+3/4;

y+1/2, x, z+3/4;

VIII

x+1/2, y+1/2, z+1/2;

IX

0.5-y, -x, -

ACCEPTED MANUSCRIPT Table 5: Infrared and Raman frequencies for CRDA compound at room temperature

Assignement

T=300 K

T=300 K

(cm-1)

(cm-1) Lattice vibrations of (Cs+)and (Rb+ )

-

78

-

113

δ(O…O)

-

147

ν(O…O)

-

355

ν2(AsO4)

393 425

435

ν4(AsO4)

M AN U

551 758

790

ν1(AsO4)

1067

923

ν3(AsO4)

1000 -

δ(OH)

1652

-

C

-

B

-

A

AC C

EP

2835

TE D

1279

2360

RI PT

Raman

SC

IR

ν(OH)

ACCEPTED MANUSCRIPT Figures captions Fig 1: Representation of the CRDA structure. Fig.2. Projection view of the title compound in the ab-plane. Hydrogen bonds O–H…O are drawn. Fig.3. A perspective drawing of Rubidium-cesium coordination

RI PT

Fig.4. IR spectrum at room temperature of Cs0.2Rb0.8H2AsO4 compound. Fig.5. Raman spectrum at room temperature of Cs0.2Rb0.8H2AsO4 compound. Fig.6. DSC heating curve of Cs0.2Rb0.8H2AsO4 material. Fig.7. Temperature dependence of ε'r at different frequencies.

SC

Fig.8. Temperature dependence of ε"r at different frequencies.

Fig.9. Temperature dependence of the inverse of the real part of the permittivity 1/ε'r, at 21 KHz.

M AN U

Fig.10 (a,b). Complex plane plots of CRDA at various temperatures. Fig.11. Temperature dependences of log(σT) = f(103/T) for CRDA.

Fig.12. Plots of log M' versus log f for CRDA at various temperatures. Fig.13. Plots of normalized modulus (M”/M”max) versus log f for CRDA compound at various temperatures.

TE D

Fig.14. Temperature dependences of log(σT) = f(103/T) and log fp = f(103/T), where fp is the

AC C

EP

M"max peak frequency, for CRDA.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig 1: Representation of the CRDA structure.

Fig.2. Projection view of the title compound in the ab-plane. Hydrogen bonds O–H…O are drawn

SC

RI PT

ACCEPTED MANUSCRIPT

M AN U

TE D

AC C

EP

relative Intensity/a.u.

Fig.3. A perspective drawing of Rubidium-cesium coordination

4000

3000

2000

Wavenumber/cm

1000 -1

Fig.4. IR spectrum at room temperature of Cs0.2Rb0.8H2AsO4 compound.

200

400

M AN U

SC

RI PT

Relative intensity/a.u

ACCEPTED MANUSCRIPT

600

800

1000

1200 -1

Wavenumber/cm

AC C

EP

TE D

Fig.5. Raman spectrum at room temperature of Cs0.2Rb0.8H2AsO4 compound.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig.6. DSC heating curve of Cs0.2Rb0.8H2AsO4 material.

Fig.7. Temperature dependence of ε'r at different frequencies.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig.8. Temperature dependence of ε"r at different frequencies.

Fig.9. Temperature dependence of the inverse of the real part of the permittivity 1/ε'r, at 21 KHz.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig.10 (a,b). Complex plane plots of CRDA at various temperatures.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig.11. Temperature dependences of log(σT) = f(103/T) for CRDA.

Fig.12. Plots of log M' versus log f for CRDA at various temperatures.

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig.13. Plots of normalized modulus (M”/M”max) versus log f for CRDA compound at various temperatures.

Fig.14. Temperature dependences of log(σT) = f(103/T) and log fp = f(103/T), where fp is the M"max peak frequency, for CRDA.