T. Mhiri

Electrical properties of ferroelectric addition compound K2SeO4.Te(OH)6

Abstract Single crystals of K 2 SeO 4 .Te(OH) 6 , (KTSe) were obtained by slow evaporation at 300 K. The differential scanning calorimetry diagram showed three anomalies at 433, 480 and 495 K, respectively. The dielectric constant evolution as a function of frequency and temperature revealed the first two anomalies. The peak at 433 K was attributed to the ferroelectric–paraelectric phase transition and the second one to the superionic phase transition. X-ray powder diffraction data confirmed that the third anomaly at 495 K characterizes the decomposition before melting of the material.

Synthesis and crystal structure of a new form of potassium–bismuth polyphosphate KBi(PO3)4

Abstract Synthesis and complete structural characterization by X-ray diffraction, IR and Raman are given for a new form of potassium–bismuth polyphosphate. This compound is monoclinic P 2 1 / n with a structural type IV and with the following unit cell dimensions: a =10.443(4) A, b =8.989(2) A, c =10.757(4) A, β =105.76(2)°, V =971.8(5) A 3 , Z =4 and ρ cal =3.855 g/cm 3 . The structure was solved from 3390 independent reflections with R 1 =0.0420 and WR 2 =0.0910, refined with 164 parameters. The atomic arrangement can be described as a long chain polyphosphate organization. Two infinite (PO 3 ) n chains with a period of eight tetrahedra run along the [101] direction. Bismuth atoms have an eightfold coordination while potassium atoms have nine oxygen neighbours. Infrared and Raman spectra were investigated at room temperature in the frequency range, 250–1500 and 200–1600 cm −1 , respectively, showing some characteristic bands of infinite chain structure of PO 4 tetrahedra linked by a bridging oxygen.

Structural, vibrational and calorimetric study of a new ammonium dihydrogen phosphate–arsenate: NH4H2(PO4)0.52(AsO4)0.48

Mixed crystals NH4H2(PO4)1−x(AsO4)x (ADAP) of the two antiferroelectric NH4H2PO4 (ADP) and NH4H2AsO4 (ADA) were grown with x=0.48 by slow evaporation from aqueous solution at room temperature. The structural properties of the crystals were analyzed by means of X-ray diffraction, which revealed that the title compound is isostructural with the tetragonal phases of ADP and ADA. This mixed compound crystallizes in space group I42d and has lattice parameters a=7.5920(13) A, c=7.6260(2) A with four formula units in the unit cell. Two kinds of disorder can be expected in this structure: a statical or dynamical disorder of the acidic proton in the O–H…O hydrogen bond and another one which is connected with a reorientational motion of NH4+ ions. Broader peaks in the IR spectrum confirm a structural disorder in this material. Crystals of ADAP undergo two phase transitions at T1=193 K and T2=423 K which are detected by differential scanning calorimetry (DSC).

Disorder and protonic conductivity in RbH(SO4)0.81(SeO4)0.19 mixed crystals

Abstract X-ray diffraction, calorimetry, vibrational and impedance spectroscopy studies of a new protonic conductor with the RbH(SO 4 ) 0.81 (SeO 4 ) 0.19 composition are offered. Crystals of this composition undergo five phase transitions at T =245, 288, 353, 380 and 482K. The transition at 380 K is a superionic–protonic transition (SPT) related to a rapid [HS(Se)O 4 − ]reorientation and fast H + diffusion. A sudden jump in the temperature dependance of conductivity confirms the presence of this transition. Above 380 K, this high temperature phase is characterized by a high electrical conductivity (10 −3 Ω −1 cm −1 ) and low activation energy ( E a

Conductivity relaxation parameters of some H+ and K+ conducting in the polycrystalline compound KDyHP3O10

Abstract Polycrystalline samples of KDyHP 3 O 10 was obtained by heating for 12 hour at 553 K a mixture containing K 2 CO 3 , Dy 2 O 3 and H 3 PO 4 . Samples were characterized through X-ray diffraction, examined by IR vibrational spectroscopy and impedance and modulus spectroscopy techniques. The conductivity relaxation parameters of some H + and K + conducting in this compound have been determined from an analysis of ac conductivity data measured in a wide temperature range. Transport properties in this material appears as due to H + and K + ions hopping mechanism. The stretched exponential function exp[−( t / τ σ ) β ] has been used to describe the conductivity relaxation. The relaxation parameters have been investigated as a function of the nature of mobile ions. The results obtained are shown to be in good agreement with the predictions of the Ngai coupling model.

Phase transitions and vibrational study of Rb2SeO4·Te(OH)6 and Rb1.12(NH4)0.88SO4·Te(OH)6

Calorimetry, conductivity, IR and Raman studies of the two ionic-protonic conductors Rb2SeO4·Te(OH)6 (RbSeTe) and Rb1.12(NH4)0.88SO4·Te(OH)6 (RNST) have been recorded and analyzed. These compounds crystallize in the monoclinic system, the space groups being, respectively, P21/c and P21/a for (RbSeTe) and (RNST) with four formula units in their unit cells. The main feature of these atomic arrangements is the coexistence of two independent and different types of anions in the unit cell, connected by hydrogen bonds, which insure the cohesion of the crystalline edifice. Differential scanning calorimetry traces show three peaks for both the materials, corresponding to three phase transitions, at 430, 470 and 493 K for RbSeTe and at 418, 458 and 483 K for RNST. A sudden surge in the temperature dependence of conductivity confirms the presence of the third transition, which is characterized by a high conductivity. The Raman spectra of these two materials were investigated in the range 295–600 K and IR spectra achieved at room temperature between 10 and 4000 cm−1, confirm the presence of the phase transitions and show that anionic groups coexist independently in the same crystal.

X-ray single crystal, vibrational and phase transitions in the mixed Rb2(HSeO4)(H2PO4)

Ongoing studies of the RbHSeO4–RbH2PO4 system, aimed at developing novel proton conducting solids, resulted in the new compound Rb2(HSeO4)(H2PO4) (dirubidium hydrogenoselenate dihydrogenophosphate). Single-crystal X-ray diffraction (performed at room temperature) revealed Rb2(HSeO4)(H2PO4) to crystallize in space group P21/n with lattice parameters a = 7.514 (3) Å, b = 7.658 (2) Å, c = 7.696 (3) Å and β = 101.20 (3)°. The compound has a unit-cell volume of 434.4 (2) Å3 and two formula units per cell, giving a calculated density of 3.149. Six non-H atoms and three H atoms were located in the asymmetric unit, with SeO4 and PO4 groups randomly arranged on the single tetrahedral anions site. Refinement using all observed reflections yielded residuals of 0.0971 and 0.0438 based on F 2 and F values, respectively. Anisotropic temperature factors were employed for all six non-H atoms and isotropic temperature factors for the three H atoms. The structure contains zigzag chains of hydrogen bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron neighbouring chain to give a planar structure with hydrogen-bonded sheets lying parallel to The rubidium atom is coordinated by nine oxygen atoms with Rb–O range from 2.868 (4) to 3.364 (5) Å. The infrared spectra of the new compound Rb2(HSeO4)(H2PO4) recorded at room temperature in the frequency range 4000–400 cm−1, confirm the presence of and groups in the same crystal. Differential scanning calorimetry traces show two phase transitions at 382 and 665 K in this material.

Ferroelectric and electric properties of Rb0.6(NH4)0.4HSO4 single crystal

Abstract Dielectric investigations in the temperature and frequency ranges 150–450 K and 10 2 –10 5 Hz, respectively show that Rb 0.6 (NH 4 ) 0.4 HSO 4 composition is ferroelectric below T c =290 K. A.C. complex impedance measurements were performed on this ferroelectric and superconductor material at high temperatures. An impedance relaxation was observed. In addition, to the ferroelectric properties, an important level of conductivity at high temperatures attributed to the motion of H + proton at least in the off-stoichiometric material, was observed. This behavior indicates the presence of superionic protonic phase transition in this material.

Crystal structure, characterisation and vibrational study of a mixed compound Cs0.4Rb0.6H2PO4

Abstract The crystal structure of Cs1-x Rb x H2PO4, x = 0.6 (CRDP) which crystallises in space group P21/m and is isostructural with the monoclinic phase of CsH2PO4 (CDP), has been refined at room temperature using single-crystal X-ray diffractometer data. The cell parameters are a = 4.8183(9)å, b = 6.2671(6) å, c = 7.7620(10)å, β= 108.260(10)°, V = 222.58(5)å3, Z = 2, Dx = 3.009g cm−3. F(000)=187, T = 298(2)K (room-temperature phase), R = 0.0355 and wR = 0.0949 for 654 observed reflections. CRDP contains two crystallography inequivalent hydrogen bonds in the unit cell. The shorter bond (Ko – o = 2.453(7) A) links the phosphate groups into chains running along the b-axis and the longer bond (Ko – o = 2.488(6) A) which is always ordered, crosslinks the chains to form (001) layers. The phase transitions in the mixed Cs0.4Rb0.6H2PO4 (CRDP) were characterised by differential scanning calorimetry which shows two anomalies at about 293 and 525 K. The Raman and infrared spectra at room temperature were investigated in the frequency ranges 10–3500 and 200–4000 cm−1 respectively. An assignment of all the bands is given. The bands are in agreement with the monoclinic room-temperature phase implying high dynamical disorder of the acidic proton O-H s–O hydrogen bond.

Phase transitions and electrical properties of CsH(SO4)0.76(SeO4)0.24 mixed crystals

The study by X-ray diffraction, calorimetry, vibrational and impedance spectroscopy of CsH(SO4)0.76(SeO4)0.24 new solid solution is presented. Crystals of this composition undergo two phase transitions at T = 333 and 408 K. The last one at 408 K is a superionic-protonic transition (SPT) related to a rapid [HS(Se)O4−] reorientation and fast H+ diffusion. A sudden jump in the conductivity plot confirms the presence of this transition. Above 408 K, this high temperature phase is characterized by high electrical conductivity (7 × 10t-3 Ω− cm−1) and low activation energy (Ea < 0.3 eV).