Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4)

ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 68 (2007) 1281–1292 www.elsevier.com/locate/jpcs

Review

Structural, vibrational and dielectric properties of new potassium hydrogen sulfate arsenate: K4(SO4)(HSO4)2(H3AsO4) M. Amria, N. Zouaria,, T. Mhiria, S. Pechevb, P. Gravereaub, R. Von Der Muhllb a

Laboratoire de l’Etat Solide (L.E.S), Faculte´ des Sciences de Sfax, Route de Soukra, 3038 Sfax, Tunisia Institut de Chimie de la Matie`re Condense´e de Bordeaux (ICMCB-CNRS), 87 Avenue Schweitzer, 33608 Pessac Cedex, France

b

Received 18 September 2006; received in revised form 14 February 2007; accepted 2 March 2007

Abstract A new compound, K4(SO4)(HSO4)2(H3AsO4) was synthesized from water solution of KHSO4/K3H(SO4)2/H3AsO4. This compound crystallizes in the triclinic system with space group P1¯ and cell parameters: a ¼ 8.9076(2) A˚, b ¼ 10.1258(2) A˚, c ¼ 10.6785(3) A˚; a ¼ 72.5250(14)1, b ¼ 66.3990(13)1, g ¼ 65.5159(13)1, V ¼ 792.74(3) A˚3, Z ¼ 2 and rcal ¼ 2.466 g cm3. The refinement of 3760 observed  reflections (I42s(I)) leads to R1 ¼ 0.0394 and wR2 ¼ 0.0755. The structure is characterized by SO2 4 , HSO4 and H3AsO4 tetrahedra  2  connected by hydrogen bridge to form two types of dimer (H(16)S(3)O4 ?S(1)O4 and H(12)S(2)O4 ?H3AsO4). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)–H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H3AsO4 group to build the chain S(1)O4?H3AsO4 which are parallel to the ‘‘a’’ direction. The potassium cations are coordinated by eight oxygen atoms with K–O distance ranging from 2.678(2) to 3.354(2) A˚. Crystals of K4(SO4)(HSO4)2(H3AsO4) undergo one endothermic peak at 436 K. This transition detected by differential scanning calorimetry (DSC) is also analyzed by dielectric and conductivity measurements using the impedance spectroscopy techniques. The obtained results show that this transition is protonic by nature. r 2007 Published by Elsevier Ltd. Keywords: C. IR spectroscopy; D. Crystal structure; D. Electrical properties

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. X-ray structure determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Description of the structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. IR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Calorimetric study and impedance spectroscopy analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1281 1282 1282 1282 1283 1283 1287 1288 1290 1291

1. Introduction Corresponding author. Tel.: +216 74 274923; fax: +216 74 274437.

E-mail address: [email protected] (N. Zouari). 0022-3697/$ - see front matter r 2007 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2007.03.007

The structure of many metal hydrogen sulfates, selenates, phosphates and arsenates has been investigated in

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detail using X-ray and neutron diffraction methods. In the last few years, new mixed hydrogen sulfate phosphate [1–5], hydrogen selenate phosphate [6–8] and hydrogen sulfate arsenate [9] have been synthesized and structurally characterized. Some of these compounds are known to undergo phase transitions to superprotonic or ferroelectric phases [10,11]. An examination of the literature has shown so far no structural study of the title compound. Many solid acids in this compounds family have been examined as superprotonic conductors. Proton transport in the superionic phase is facilitated by the rapid reorientation of SO4 groups [12,13]. Owing to the limited number of structural types known as exhibiting superprotonic behavior, it has been difficult to assess the role of the hydrogen bond network and/or local hydrogen bond geometry in the proton transport and transformation mechanisms. Following the first investigation concerning the synthesis and the crystal structure of Cs4(SeO4)(HSeO4)2(H3PO4) compound [14], we describe in this paper the synthesis and crystal structure of a new potassium hydrogen sulfate arsenate, K4(SO4)(HSO4)2(H3AsO4) which is the first compound belonging to the family of compound with general formula MI4(SO4)(HSO4)2(H3AsO4) (MI ¼ Cs+, Rb+, K+, Na+, NH+ 4 , y). We demonstrate here through some results of calorimetric analysis and through measurements of dielectric permittivity and conductivity using the complex impedance method, all carried at high temperature, that the transition in K4(SO4)(HSO4)2(H3AsO4) at 436 K is protonic and accompanied by breaking of the hydrogen bonds. At room temperature, the K4(SO4)(HSO4)2(H3AsO4) material crystallizes in the triclinic space group P1¯ and its unit cell contains two chemical units giving a calculated density of 2.466. In this work, we try to describe the structure of this compound, which is clearly different to Cs4(SeO4)(HSeO4)2(H3PO4) [14], K4(HSO4)3(H2PO4) [15] and Rb4(HSO4)3(H2PO4) [16]. 2. Experimental 2.1. Synthesis In a first step, KHSO4 [17] and K3H(SO4)2 [18] are synthesized by reaction of K2CO3 (Merck) and sulfuric acid 95%, respectively, with a molar ratio 1:2 and 5:16. Crystals of the title compound are produced from an aqueous solution with 0.68 g of KHSO4, 1.55 g of K3H(SO4)2 and 0.71 g of orthoarsenic acid, H3AsO4, by the following reaction: KHSO4 þ K3 HðSO4 Þ2 þ H3 AsO4 H2 O

! K4 ðSO4 ÞðHSO4 Þ2 ðH3 AsO4 Þ. Just enough deionised water was added to a mixture of the salts and acid to cause dissolution. This resulting solution was kept under room conditions which allowed it to evaporate slowly. A few days later, colorless, transparent,

Table 1 Results of chemical analysis for K4(SO4)(HSO4)2(H3AsO4) Weight percentage

k

S

As

Calculated Experimental

26.57 26.39a

16.34 15.05b

12.73 13.44c

a

Determined by atomic absorption. Determined by gravimetry. c Determined by argentimetric method. b

and parallelepipedic single crystals of K4(SO4)(HSO4)2 (H3AsO4) were obtained. The formula of this compound is determined by chemical analysis (Table 1) and confirmed by refinement of the crystal structure. The compound is stable in air. 2.2. X-ray structure determination Crystal of K4(SO4)(HSO4)2(H3AsO4) suitable for X-ray structure analysis was selected under a polarisation microscope. Data collection on single crystal at room temperature was performed on a KappaCCD diffractometer (Bruker–Nonius), using graphite-monochromatized MoKa radiation (l ¼ 0.71073 A˚). The cell parameters obtained from single-crystal diffractometer measurement are: a ¼ 8.9076(2) A˚, b ¼ 10.1258(2) A˚, c ¼ 10.6785(3) A˚; a ¼ 72.5250(14)1, b ¼ 66.3990(13)1, g ¼ 65.5159(13)1 with space group P1¯ . The raw intensity data were corrected for Lorenz and polarising effects before refinement of the structure. Eight thousand six hundred and fifty-two reflections were collected in the whole Ewald sphere for 3.32oyo30.041 of which 4533 reflections are independent and 3760 had an intensity of I42s(I). The heavy-atom method was used to solve the structure. Atomic scattering factors were taken from the International Tables for X-ray Crystallography [19]. Patterson methods for structure resolution were used. The structure was successfully developed in the centrosymmetric space group P1¯ . Potassium and arsenic atom positions were located using SHELXS-97 [20], whereas the atom positions of sulfur and oxygen were deduced from difference Fourier maps during the refinement of the structure with an adapted version of SHELXL-97 program [21]. There are two formula units in the unit cell and all the atoms are in general positions. The H atoms were located through difference Fourier maps with the aid of calculated bond distances and angles. The five acidic hydrogen atoms required in a unit cell by the chemical formula also appeared in general positions. The least-squares refinement, including isotropic hydrogen atoms, converge to an acceptable final agreement factors R1 ¼ 3.94% and wR2 ¼ 7.55%, obtained by fitting 238 parameters. A last difference Fourier series gave only intensity peaks inferior or equal to 0.703 e A˚3 with respective distances from S(1) and As atoms equal to 0.37 and 0.67 A˚.

ARTICLE IN PRESS M. Amri et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1281–1292 Table 2 Main crystallographic features, X-ray diffraction data collection parameters and final results for K4(SO4)(HSO4)2(H3AsO4) Formula Weight Color Crystal system Space group Temperature a b c a b g V Z Diffractometer (Bruker–Nonius) y range l (Mo Ka) Maximun crystal dimensions rcal rexp m Monochromator hmin; kmin; lmin hmax; kmax; lmax Reflections collected Independent reflections Reflection with I42s(I) Parameters R1a [F242s(F2)] wR2a (F2) S W ¼ 1/[s2(F2o)+(0.0323p)2+0.45p] Dr(max) and Dr(min) a

R1 ¼

P

K4(SO4)(HSO4)2(H3AsO4) 588.54 g mol1 Colorless Triclinic P1¯ 293(2) K 8.9076(2) A˚ 10.1258(2) A˚ 10.6785(3) A˚ 72.5250(14)1 66.3990(13)1 65.5159(13)1 792.74(3) A˚3 2 KappaCCD 3.32–30.041 0.71073 A˚ 0.05  0.08  0.21 mm3 2.466 g cm3 2.490 g cm3 3.668 mm1 Graphite 12; 14; 15 12; 14; 15 8652 4533 [R(int) ¼ 0.0186] 3760 238 0.0394 0.0755 1.063 p ¼ ðF 2o þ 2F 2c Þ=3 0.703 and 0.588 e A˚3

 P P ½jF o jjF c j= jF o j and wR2 ¼ð ½W ðF 2o F 2c Þ2 =½W ðF 2o Þ2 Þ1=2 .

The chemical crystal data and the results of crystal structure determination are summarized in Table 2. The final atomic coordinates and the equivalent anisotropic thermal parameters are given in Tables 3 and 4. 3. Results and discussion 3.1. Description of the structure The crystal structure of K4(SO4)(HSO4)2(H3AsO4) (denoted KSHSAs) projected onto the ac plan is shown in Fig. 1. In the unit cell, four crystallographically independent potassium atoms are present, as well as three crystallographically different S tetrahedra and one As tetrahedron. The whole of atoms in this structure are located in general positions. All potassium atoms have eight K–O contacts: K(1), 2.7124(18)–3.3530(20) A˚; K(2), 2.7494(18)–2.8959(19) A˚; K(3), 2.7235(17)–3.0084(18) A˚ and K(4), 2.6783(18)–3.2700(20) A˚. Their environments are shown in Fig. 2. Based on the differences between K–O bond distances, the K(1)O8 and K(4)O8 polyhedra are assumed to be very irregular. The S(1) atom is surrounded by four oxygen atoms at S–O distances in a very narrow

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Table 3 Fractional atomic coordinates and temperature factors for K4(SO4) (HSO4)2(H3AsO4)

As S(1) S(2) S(3) K(1) K(2) K(3) K(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15) O(16) H(2) H(3) H(4) H(12) H(16)

x

Y

Z

U(eq)

0.47268(3) 0.03308(6) 0.04671(7) 0.46564(7) 0.22108(8) 0.23325(7) 0.24383(7) 0.26526(7) 0.4769(2) 0.4440(2) 0.3364(2) 0.3145(2) 0.0337(2) 0.1395(2) 0.0706(2) 0.1689(2) 0.0549(3) 0.0681(2) 0.0127(2) 0.2283(2) 0.4743(3) 0.4325(2) 0.4990(2) 0.2756(3) 0.348(4) 0.274(4) 0.226(5) 0.3102 0.1974

0.31694(2) 0.19629(5) 0.68405(6) 0.82301(6) 0.54424(6) 0.07040(5) 0.42684(5) 0.97775(6) 0.4837(2) 0.2597(2) 0.8081(2) 0.2976(2) 0.0455(2) 0.6978(2) 0.2240(2) 0.2188(2) 0.5693(2) 0.1725(2) 0.6461(2) 0.7037(2) 0.9329(2) 0.3255(2) 0.8582(2) 0.8194(2) 0.246(4) 0.771(3) 0.271(4) 0.6213 0.8964

0.22080(2) 0.27103(5) 0.18600(6) 0.31583(6) 0.50471(6) 0.59421(5) 0.90033(5) 0.00787(6) 0.1824(2) 0.1017(2) 0.7624(2) 0.3736(2) 0.2978(2) 0.7364(2) 0.3818(2) 0.1365(2) 0.1282(2) 0.8538(2) 0.3343(2) 0.1198(2) 0.3702(2) 0.6405(2)) 0.1676(2) 0.3849(2) 0.129(3) 0.755(3) 0.366(4) 0.1335 0.3626

0.0190(1) 0.0128(1) 0.0189(1) 0.0189(1) 0.0310(1) 0.0232(1) 0.0232(1) 0.0272(2) 0.0311(4) 0.0271(4) 0.0303(4) 0.0274(4) 0.0270(4) 0.0249(4) 0.0245(4) 0.0239(3) 0.0430(5) 0.0385(5) 0.0333(4) 0.0356(5) 0.0390(5) 0.0366(5) 0.0301(4) 0.0343(4) 0.045(9) 0.051(10) 0.065(11) 0.050 0.050

PP Ueq ¼ 1/3 i jUijai*aj*aiaj. Standard deviations are given in parentheses.

range of 1.4651(16)–1.4903(17) A˚. All oxygen atoms in the S(1)O4 tetrahedron are involved in hydrogen bonds as acceptors (Figs. 3 and 4). The S–O distances in the S(2) and S(3) tetrahedra are strongly differentiated, with one distance in each longer than 1.5 A˚, which can be attributed to the typical S–OH bond. The main interatomic distances and bond angles for the four independent tetrahedra SO4 and AsO4, and the four independent polyhedra around K(1), K(2), K(3) and K(4) are given in Table 5. The structure of K4(SO4)(HSO4)2(H3AsO4) is built up from discrete arsenate and sulfate groups connected by O–H?O hydrogen bonds and through the electrostatic actions of the K+ cations. The overall structure of this compound can be described as a sequence of isolated AsO4 and SO4 tetrahedra alternating with the potassium atoms in the b direction. A projection on the ab plane is depicted in Fig. 3. It consists of SO4 and AsO4 tetrahedra which develop as chains including S(1)O4 alternating with AsO4 along the [0 1 0] direction (Fig. 4). It can be regarded that this structure is characterized by the presence of SO2 4 , HSO and H AsO tetrahedra connected by hydrogen 4 3 4 2 bridge to form two types of dimer (H(16)S(3)O 4 ?S(1)O4

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Table 4 Anisotropic displacement parameters (in 103 A˚2) for K4(SO4)(HSO4)2(H3AsO4)a Atomes

U11

U22

U33

U23

U13

U12

As S(1) S(2) S(3) K(1) K(2) K(3) K(4) O(1) O(2) O(3) O(4) O(5) O(6) O(7) O(8) O(9) O(10) O(11) O(12) O(13) O(14) O(15) O(16)

0.0156(1) 0.0094(2) 0.0159(2) 0.0161(2) 0.0367(3) 0.0226(2) 0.0228(2) 0.0317(3) 0.0228(8) 0.0212(8) 0.0220(9) 0.0274(9) 0.0202(8) 0.0183(8) 0.0205(8) 0.0239(8) 0.0439(12) 0.0321(10) 0.0346(10) 0.0211(9) 0.0366(11) 0.0336(10) 0.0313(9) 0.0196(8)

0.0173(1) 0.0132(2) 0.0188(3) 0.0189(3) 0.0229(3) 0.0223(2) 0.0227(2) 0.0204(2) 0.0169(8) 0.0416(10) 0.0178(8) 0.0399(10) 0.0177(8) 0.0191(8) 0.0359(9) 0.0332(9) 0.0442(11) 0.0330(10) 0.0368(10) 0.0257(9) 0.0413(11) 0.0277(9) 0.0345(10) 0.0327(10)

0.0264(2) 0.0177(3) 0.0218(3) 0.0211(3) 0.0343(3) 0.0218(3) 0.0223(3) 0.0302(3) 0.0562(12) 0.0243(9) 0.0566(13) 0.0205(9) 0.0450(11) 0.0400(10) 0.0219(9) 0.0201(8) 0.0526(13) 0.0334(11) 0.0221(9) 0.0526(12) 0.0491(13) 0.0348(11) 0.0210(9) 0.0415(11)

0.0025(1) 0.0014(2) 0.0051(2) 0.0051(2) 0.0064(2) 0.0014(2) 0.0023(2) 0.0032(2) 0.0004(8) 0.0076(7) 0.0006(8) 0.0062(7) 0.0008(7) 0.0035(7) 0.0042(7) 0.0048(7) 0.0276(10) 0.0047(8) 0.0021(8) 0.0101(8) 0.0256(9) 0.0053(8) 0.0025(7) 0.0092(8)

0.0092(1) 0.0063(2) 0.0082(2) 0.0080(2) 0.0211(3) 0.0066(2) 0.0062(2) 0.0158(2) 0.0191(9) 0.0043(7) 0.0226(9) 0.0036(7) 0.0138(8) 0.0152(7) 0.0076(7) 0.0044(7) 0.0103(10) 0.0174(8) 0.0116(8) 0.0131(8) 0.0124(9) 0.0175(9) 0.0087(7) 0.0103(8)

0.0061(1) 0.0043(2) 0.0022(2) 0.0021(2) 0.0005(2) 0.0069(2) 0.0080(2) 0.0043(2) 0.0072(7) 0.0172(8) 0.0059(7) 0.0196(8) 0.0077(7) 0.0039(6) 0.0135(7) 0.0169(7) 0.0168(10) 0.0111(8) 0.0055(8) 0.0108(7) 0.0110(9) 0.0089(8) 0.0087(8) 0.0102(8)

a

The anisotropic displacement exponent takes the form exp [2p2SiSjUijhihjai*aj*].

Fig. 1. Projection of K4(SO4)(HSO4)2(H3AsO4) crystal structure along the b-axis.

and H(12)S(2)O 4 ?H3AsO4). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)–H(3)?O(6). They are also linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H3AsO4 group to build the chain S(1)O4?H3AsO4 which are parallel to the [1 0 0] direction. In this structure, the cationic layers (K) are located between the sheets; the interaction between oxygen and potassium atoms ensures the cohesion of the structure. A geometric description of the H bonds is presented in Table 6.

The S(3) tetrahedron is hydrogen-bonded to the S(1) tetrahedron by the hydrogen bond O(16)?O(5)v, and the S(2) tetrahedron is attached to the As tetrahedron by O(12)?O(1)i giving rise to two-dimensional layers (Fig. 3). The arsenic tetrahedron has one shorter As–O distance (As–O(1), 1.6262(16) A˚) and three elongated As–O distances (1.6866(17)–1.6974(17) A˚). Taking into account the short distances for the S(1) tetrahedron, one can assume that all three elongated As–O distances correspond to the hydrogen donor functions of O atoms. Therefore, the

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Fig. 2. Local coordination of potassium cations.

K4 O13 K2 H16 S3 O15 O10 O16 S2 O12 O14 H12 O11 K3 O9 K1 O1 O7 H4 S1 O8

O4 H2 O2

AS

H3

O6

O3

O5 b a Fig. 3. Projection of K4(SO4)(HSO4)2(H3AsO4) crystal structure along the c-axis.

composition of this compound should be described as K4(SO4)(HSO4)2(H3AsO4) rather than K4(HSO4)3(H2AsO4). In the unit cell of both structures K4(HSO4)3(H2PO4) and Rb4(HSO4)3(H2PO4) [15,16], there are only two different types of tetrahedra chains are present. One chain is formed by S tetrahedra exclusively; the other one consists of both S and

P tetrahedra. For the first type, the sulfur atom laid in general position with full occupancy of their site. In contrast, in the disordered type, the position of sulfur and phosphorus is statistically disordered and the occupations by these two 2 atoms are equal; there is only HSO 4 and H2PO4 coexisting in both structures, while in KSHSAs it is apparently to

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Fig. 4. Projection of K4(SO4)(HSO4)2(H3AsO4) crystal structure along the a-axis.

consider it the first documented case of a coexistence of  SO2 4 , HSO4 and H3AsO4 in the same structure. However, according to the preliminary X-ray study of the (NH4)2SO4–H3AsO4 system, the 1:1 compound contains both SO2 4 and H3AsO4 species, (NH4)2(SO4)(H3AsO4) [22]. In spite of their same formula, K4(SO4)(HSO4)2(H3AsO4) and Cs4(SeO4)(HSeO4)2(H3PO4) compounds have two different structures. This is on account of the difference between potassium and caesium ionic ray. When the cationic rays become bigger, the structures pick up to a space group having the high symmetry. H atoms positions were identified on the basis of geometric considerations (the combination of O?O and X–O lengths [23]; where X ¼ S and As). An examination of intertetrahedra oxygen distances and X–O lengths suggest the existence of five hydrogen sites, the first H(2) linking O(2) and O(8), the second H(3) linking O(3) and O(6), the third H(4) linking O(4) and O(7), the fourth H(12) linking O(12) and O(1)i and the fifth H(16) linking O(16) and O(5)v. The O?O bridges with distances between 2.517(2) and 2.642(2) A˚, without O(2)?O(3)ii (2.639(2) A˚)belong to the strong hydrogen bonds [24].

Table 5 Bond distances (A˚) and angles (1) in K4(SO4)(HSO4)2(H3AsO4) Tetrahedra around As As O(1) O(1) 1.6262(16) O(2) 116.05(9) O(3)ii 111.51(8) O(4) 109.78(9)

O(2) 2.810(2) 1.6866(17) 102.89(9) 108.37(9)

O(3)ii 2.740(2) 2.639(2) 1.6883(16) 107.78(9)

O(4) 2.719(2) 2.744(2) 2.735(2) 1.6974(17)

Tetrahedra around S(1) S(1) O(5) O(5) 1.4651(16) O(6)iv 109.92(9) O(7) 111.06(10) O(8) 108.79(10)

O(6)iv 2.414(2) 1.4831(15) 108.67(10) 109.34(10)

O(7) 2.426(2) 2.405(2) 1.4777(16) 109.04(9)

O(8) 2.403(2) 2.426(2) 2.417(2) 1.4903(17)

Tetrahedra around S(2) S(2) O(9) O(9) 1.4357(19) O(10)iv 114.07(13) O(11) 112.04(12) O(12) 107.17(12)

O(10)iv 2.415(3) 1.4432(17) 112.47(11) 102.70(11)

O(11) 2.390(3) 2.402(2) 1.4467(18) 107.61(11)

O(12) 2.409(3) 2.344(2) 2.424(3) 1.5562(18)

Tetrahedra around S(3) S(3) O(13) O(13) 1.44072(19)

O(14)ii 2.419(3)

O(15) 2.401(3)

O(16) 2.416(3)

ARTICLE IN PRESS M. Amri et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1281–1292 Table 5 (continued ) O(14)ii O(15) O(16)

113.90(12) 112.47(12) 106.91(11)

1.4450(17) 112.81(11) 102.32(11)

2.409(24) 1.4468(18) 107.52(11)

2.346(2) 2.431(3) 1.5658(18)

Polyhedra around K(1) K(1)–O(14) 2.7124(18) K(1)–O(11) 2.7949(18) K(1)–O(7)iv 2.8037(18) K(1)–O(16) 2.8489(19) K(1)–O(4) 2.8865(19) K(1)–O(6) 3.0000(18) K(1)–O(11)iv 3.0790(20) K(1)–O(1) 3.3530(20) Polyhedra around K(2) 2.7494(18) K(2)–O(3)v K(2)–O(13)ii 2.7650(20) K(2)–O(5)vii 2.7683(17) 2.7720(20) K(2)–O(13)v K(2)–O(10) 2.8480(20) K(2)–O(4) 2.8509(18) K(2)–O(7) 2.8821(17) K(2)–O(11)iv 2.8959(19) Polyhedra around K(3) 2.7235(17) K(3)–O(1)ii K(3)–O(9)x 2.7660(20) K(3)–O(9)iv 2.7810(20) K(3)–O(6) 2.7862(17) K(3)–O(8)x 2.8050(17) K(3)–O(14) 2.8500(19) K(3)–O(15)ii 2.9244(18) 3.0084(18) K(3)–O(2)x Polyhedra around K(4) 2.6783(18) K(4)–O(10)vi K(4)–O(12) 2.7726(18) K(4)–O(8)iii 2.8129(17) 2.8209(19) K(4)–O(2)i K(4)–O(15) 2.8680(18) K(4)–O(15)xi 2.9237(19) K(4)–O(5)iii 3.0492(19) 3.2700(20) K(4)–O(3)viii With standard deviations in parentheses. Note: See Table 6 for symmetry codes.

3.2. IR spectroscopy Infrared (IR) absorption spectra of suspensions of crystalline powders in KBr were recorded at room temperature using an IR-470 Shimadzu spectrophotometer over the wavenumber range from 4000 to 400 cm1. Although IR spectroscopy is one of the major physical methods of investigation molecular structure, we were able to use it in studying the infrared spectra of the polycrystalline samples of KSHSAs at room temperature, which is shown in Fig. 5. The low symmetry of KSHSAs, as well as the existence of multiple crystallographically independent X atoms (X ¼ S or As), renders a complete group factor analysis of this compound not only difficult, but also somewhat meaningless. To make a qualitative assignment of IR peaks to vibrational modes, most of which are tied to XO4 group vibrations, we examine the modes and

1287

frequencies observed for HnXO4 anions [25]. The ideal, five atoms AsO4 or SO4 tetrahedra group exhibits Td symmetry. Thus, there are nine vibrational modes, only four of which are independent: n1(A1) ¼ XO4 symmetric stretch; n2(E  2) ¼ X–O–X bend; n3(F1  3) ¼ XO4 asymmetric stretch; and n4(F2  3) ¼ X–O–X bend. The influence of the addition of protons to the anion can be understood through a two-step process. First, in HXO4 one of the oxygen atoms is replaced with a hydroxyl group (X–OH) with point mass. This reduces the symmetry from Td to C3v, and eliminates degeneracies such that there are six rather than four independent vibrational modes. Second, the vibrations associated with the hydroxyl group are considered. There should be three such modes: O–H stretching, X–O–H in-plane bending, and X–O–H out-ofplane bending (or torsion). Similarly, the H2XO4 (ion or molecule) can be considered a group with C2v symmetry and therefore nine independent internal modes. In addition, there are six modes associated with the hydroxyl groups, as the X(OH)2 stretching and bending modes may now be either symmetric or asymmetric. The case of H3XO4, which displays C3v symmetry, is similar to that of HXO4. However, there are additional O–H modes. To apply these concepts to the infrared spectra of KSHSAs compound, it is necessary to first identify the HnXO4 groups present in the structure. The IR spectra of KSHSAs that we obtained consist of a number of distinct and well-separated groups of bands, and can be divided into two frequency regions: 400–1200 cm1, AsO4 and SO4 internal modes; and 1200–3800 cm1, high-frequency hydrogen modes [26–29]. The weak and the shoulder bands observed, respectively, near 413 and 435 cm1 are associated with n4 of AsO3 4 tetrahedra. The IR peaks obtained in the low-frequency region of the spectra between 440 and 630 cm1 most likely correspond to n4 and n2 of SO2 4 tetrahedra (n44n2). The two shoulder bands around 769 and 780 cm1 can be assigned to n1 (AsO4). The IR peaks obtained in the region from 820 to 970 cm1 most likely correspond to HnXO4 stretching modes appear to be further distinguished into those with strong frequencies between 820 and 935 cm1 and the very weak peak which frequency around 970 cm1. The low-frequency stretches are interpreted to correspond to X–OH stretches, in particular, to the S–OH stretch of the HSO4 ions and symmetric and asymmetric As–(OH)2 stretches of the H3AsO4 molecule. The higher-frequency stretching modes are believed to reflect modes involving S–O3 stretch of HSO4 ion. The strong peak near 1047 cm1 can be attributed to n1(SO4). We distinguish the n3(SO4) by the shoulder and the intense infrared bands near 1107 and 1122 cm1. The d(As–O–H) bending vibration mode is assigned to the very strong infrared peaks observed at 1207 and 1223 cm1 and the broad infrared band observed at 1295 cm1. The weak broad band around 1409 cm1 is tentatively attributed to the hydrogen bending modes in the plane (d(OH)). Other information which can be obtained from the vibrational spectra concerns the strength and the

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Table 6 Atomic coordinates of proton in K4(SO4)(HSO4)2(H3AsO4) and geometry of the associated hydrogen bonds Atoms

O neighbors

d(O?O) (A˚)

d(O–H) (A˚)

d(O?H) (A˚)

OHO angle (1)

H(2) H(3) H(4) H(12) H(16)

O(2)–O(8) O(3)–O(6) O(4)–O(7) O(1)–O(12) O(16)–O(5)v

2.517(2) 2.569(3) 2.537(2) 2.553(2) 2.642(3)

0.84(3) 0.83(4) 0.97(4) 0.871(2) 0.848(2)

1.70(4) 1.74(4) 1.58(5) 1.699(2) 1.800(2)

164.1(40) 178.2(38) 170.1(47) 165.98(14) 171.68(14)

Relative Intensity

Note: Symmetry code: (i) x+1, y+1, z; (ii) x+1, y+1, z+1; (iii) x, y+1, z; (iv) x, y+1, z+1; (v) x, y1, z; (vi) x, y+1, z1; (vii) x, y, z+1; (viii) x, y, z1; (ix) x, y1, z+1; (x) x, y, z+1; (xi) x, y+1, z1; (xii) x+1, y, z+1; (xiii) x, y, z1.

4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 Wavenumber [cm-1] Fig. 5. IR spectrum at room temperature of K4(SO4)(HSO4)2(H3AsO4) in the frequency range 400–4000 cm1.

type of O–H?O hydrogen bonds, which remain practically the same in the strongly hydrogen-bonded crystals such as KH2PO4, NaH2PO4, CsHSO4 and CsH2PO4 [30–33]. The corresponding OH stretching vibration gives rise to characteristic broad triobands of ABC type, associated with strongly hydrogen-bonded systems. The broad lines observed at 2810, 2358 and 1600 cm1 can be assigned, respectively, to the ABC bonds of OH stretching vibrations of both the short and the long hydrogen bonds in KSHSAs compound. The broadness of the ABC bands is not due to structural disorder of any kind, but is an intrinsic property of a strong hydrogen bond associated with vibrational anharmonicity of the OH group and the particular shapes of the O–H?O potential curves in the fundamental and excited states [34]. The frequencies for the corresponding bands are given in Table 7. 3.3. Calorimetric study and impedance spectroscopy analyses The differential scanning calorimetry (DSC) has been performed on a DSC 204 t-sensor/E between 300 and 573 K. The thermogram showed one endothermal peak at 436 K (Fig. 6). This peak was also characterized by impedance and dielectric permittivity measurements and

was attributed to the protonic phase transition of the materials. Ionic conductivity of K4(SO4)(HSO4)2(H3AsO4) was determined by employing ac impedance measurements. Specimen for the conductivity measurements was prepared from powder sample by pressing it into disk with 13 mm in diameter and 1.2 mm in thickness. On both sides of the disk surface, gold electrode with 6.5 mm in diameter was deposited to form thin films as blocking electrodes. The ac impedance measurements were made using HP 4194 impedance analyser in the range from 103 to 107 Hz and measurements were carried out in vacuum over thermal intervals 300–550 K. The sample was maintained few minutes at each temperature before collecting data; the temperature stability was 71 K. The capacity Cp (F) and the dielectric losses tan d were measured with a computer interfaced. Four different formalisms are generally employed for analysing ac response of our materials which are: the complex dielectric constant e* (e* ¼ e0 je00 ), complex electric modulus M* (M* ¼ M0 +jM00 ), complex resistively r* (r* ¼ r0 +jr00 ) and complex impedance Z* (Z* ¼ Z0 jZ00 ). We have calculated the real part of the permittivity 0r from the capacity values and using the formula 0r  tan d ¼ 00r , we have determined the imaginary

ARTICLE IN PRESS M. Amri et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1281–1292 Table 7 Infrared frequencies (cm1) of K4(SO4)(HSO4)2(H3AsO4) Ia

IR

Assignments 

413 435 441 451 494 510 589 623

w sh sh m m w s s

769 780 820 864 931 970 1047 1107 1122 1207 1223 1295 1409 1600 2358 2810

sh sh s s s vw s sh s vs vs wb wb mb ‘‘C’’ wb ‘‘B’’ vb ‘‘A’’

2990 3700 3776

vb vb b

n4(AsO4)

o

n2(SO4)

9 > > =

n4(SO4)

> > ; o

n1(AsO4)

9 > =

n(S–OH) and n(As–(OH)2)

> ; o

S–O3 stretch of HSO 4 n1(SO4) n3(SO4)

9 > =

d(As–O–H)

> ; 9 > =

d(OH) n(OH)

> ; 9 > =

n(OH) of free H2O of KBr

> ;

Endothermic

a Relative intensities: sh, shoulder; w, weak; vw, very weak; wb, weak broad; m, medium; mb, medium broad; b, broad; vb, very broad; s, strong; vs, very strong.

350

T =436 K

400

450

500

550

600

T (K) Fig. 6. Differential scanning calorimetry of K4(SO4)(HSO4)2(H3AsO4).

part 00r . From the following relation of the complex dielectric constant,  ¼ 1=joC 0 Z ¼ 1=M  , we have determined the real and the imaginary part of the impedance (Z0 , Z00 ). The ionic conductors (IC) with point defects lead to a conductivity ranging up to 105 O1 cm1 whereas the

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superionic conductors (SIC) result in a conductivity of at least 104 O1 cm1. The main difference between these two groups of materials concerns the activation energy (DEs): in the case of SIC, DEs is lower than 0.4 eV while in IC, values varying between 0.6 and 1.2 eV are usually observed [35]. The SIC have thus a high conductivity far below the melting point. This fundamental difference is principally due to the particular structures of SIC. Some complex impedance diagrams (Z00 ) versus (Z0 ) recorded at various temperatures are given in Fig. 7(a,b). These impedance spectrums show one non-ideal semicircle arc, which attributed to the bulk boundary properties and whose center is displaced below the real axis. This shows that K4(SO4)(HSO4)2(H3AsO4) compound follow the Cole–Cole law and their extrapolation gives rise to an [a(p/2)] dispersion angle where a ¼ 0.45 is an empirical parameter (0pap1). This angle represents the difference between the Cole–Cole law and the Debye model and is constant when the temperature various for all given impedance curve. The bulk ohmic resistance relative to each experimental temperature is the intercept on the real axis of the zero-phase angle extrapolation of the highest-frequency curve. The observed non-ideal semicircle is modeled using an equivalent circuit that contains one subcircuit, which is consisting of a resistance Rg, a capacitance Cg and a constant phase element (CPE) connected in parallel which represents the grain response of the sample. These curves show the temperature dependence of the resistance proving a remarkable improvement of the protonic conduction of K4(SO4)(HSO4)2(H3AsO4) material at high temperature which increases from 6.70  109 O1 cm1 at 440 K to 5.17  107 O1 cm1 at 510 K. The temperature dependence of the conductivity between 300 and 550 K of the title compound is represented in Fig. 8, in a log (sT) versus 1000/T plot. The diagram shows two regions, the first one is below 440 K. In this region, the protonic conductivity increases considerably and can be well described by the Arrhenius relation: sT ¼ s0 exp(DEs/kT) with an activation energy DEs ¼ 0.46 eV, where s0 is the pre-exponential factor, DEs is the activation energy for ion migration and k is Boltzmann’s constant. One anomaly is observed at 440 K, which can change the activation from 0.46 to 0.114 eV above 510 K. At the temperature 440 K, Fig. 8 shows a remarkable increasing in conductivity and decreasing in the activation energy by almost four orders of magnitude, such behavior shows the protonic conduction transition phase of K4(SO4)(HSO4)2 (H3AsO4) compound. The second region for temperature above 510 K is seen as a linear segment which follows the Arrhenius law. The detected transition observed at 440 K corresponds to the structural transformation between the triclinic phase and the high-temperature protonic phase. Therefore, this transition which is revealed by DSC measurement is well related to the breaking of the hydrogen bonds in which the proton moves between the potential wells. The drastic increase in conductivity by almost 77 orders of magnitude on going from 440 to 510 K is thus related to the disorder of both sublattices.

ARTICLE IN PRESS M. Amri et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1281–1292

1290

2.50E+07 T = 480 K T = 470 K

-Z"(Ω)

2.00E+07

T = 460 K T = 450 K T = 420 K T = 400 K

1.50E+07

1.00E+07

5.00E+06

0.00E+00 0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07 2.50E+07 3.00E+07 3.50E+07 4.00E+07 Z'(Ω)

2.50E+06 T = 550 K T = 540 K T = 530 K T = 520 K T = 510 K T = 500 K T = 490 K T = 480 K

2.00E+06

-Z"(Ω)

1.50E+06

1.00E+06

5.00E+05

0.00E+00 0.00E+00 5.00E+05 1.00E+06 1.50E+06 2.00E+06 2.50E+06 3.00E+06 3.50E+06 4.00E+06 Z' (Ω) Fig. 7. (a, b) Complex impedance diagrams Z00 versus Z0 for K4(SO4)(HSO4)2(H3AsO4) at various temperatures.

In order to characterize the protonic phase transition and to give more information on the crystal dynamics (short interaction distance), we have undertaken a dielectric study between 300 and 550 K. Fig. 9(a, b) illustrates the thermal variation of the real part of the permittivity 0r , the dielectric losses tan d and the imaginary part of the permittivity 00r for K4(SO4)(HSO4)2(H3AsO4) compound. These curves show that at the protonic phase transition which is detected at about 440 K by DSC, there is an increasing of the dielectric response 0r and 00r which are achieved by a peak at around 510 K. The protonic transition may be interpreted by a dynamic order–disorder as resulted by breaking of the hydrogen bond. These figures show that for all frequencies given, the dielectric response increase from room temperature, present a maximum at 510 K and then decrease in the disorder phase. The evolution of the dielectric constants (0r and 00r ) as a function of frequency shows an increase of these constants with decreasing frequency. This behavior suggests the

presence of several dispersion mechanisms for the title compound. In Fig. 9(a), we observe at 510 K, a maximum of 0r corresponding to a maximum of tan d, this dielectric behavior rules out the existence of a ferroelectric phase at high temperature. 4. Conclusion The structure of the salt K4(SO4)(HSO4)2(H3AsO4) (KSHSAs) can be described in the triclinic system P1¯ . This structure is built up from discrete arsenate and sulfate groups connected by O–H?O hydrogen bonds and through the electrostatic actions of the K+ cations. It can be regarded that this structure is also characterized by  the presence of SO2 4 , HSO4 and H3AsO4 tetrahedra connected by hydrogen bridge to form two types of dimers 2 (H(16)S(3)O and H(12)S(2)O 4 ?S(1)O4 4 ?H3AsO4). These dimers are interconnected along the [1¯ 1 0] direction by the hydrogen bonds O(3)–H(3)?O(6). They are also

ARTICLE IN PRESS M. Amri et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1281–1292

linked by the hydrogen bridge assured by the hydrogen atoms H(2), H(3) and H(4) of the H3AsO4 group to build the chain S(1)O4?H3AsO4 which are parallel to the [1 0 0] direction. All potassium atoms are coordinated by eight oxygen atoms involving a very irregular K(1)O8 and K(4)O8 polyhedra. The cationic layers (K) are located between the sheets assuring an interaction between oxygen and potassium atoms and ensure the cohesion of the structure. The physical properties and phase transitions of this compound were examined by different methods. The DSC analysis shows one non-energetic anomaly at 436 K. This transition is detected by impedance complex and dielectric permittivity. At around 440 K, the conductivity increases sharply and the activation energy decreases from 0.46 to 0.114 eV. This dynamic order–disorder transition is in agreement with a protonic conductor behavior, which is due to the breaking of the hydrogen bonds linking the SO4, HSO4 and AsO4 ions.

-3 T = 510 K

log (σT) (KΩ-1 cm-1)

-4

-5

T = 440 K

-6

-7 1.6

1.8

2 2.2 1000/T (K-1)

2.4

2.6

Fig. 8. Temperature dependences of log (sT) ¼ f(103/T) for K4(SO4) (HSO4)2(H3AsO4).

70 60

ε'r tg δ

ε'r

50

10

f = 50000 Hz f = 10000 Hz T = 510 K f = 2000 Hz f = 50000 Hz f = 10000 Hz f = 2000 Hz

8 6

40 T = 440 K

30

4

20

Dielectric loss: tg δ

80

2 10 0 300

ε"r

600

350

400

f = 10000 Hz

400

f = 2000 Hz

500

550

0 600

T = 510 K

f = 50000 Hz

500

450 T (K)

300 200

T = 440 K

100 0 300

350

400

450 T (K)

500

550

1291

600

Fig. 9. (a,b). Thermal variation of 0r , 00r and tan d for K4(SO4) (HSO4)2(H3AsO4).

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