Abstract
Mixed crystals Cs0.55(NH4)0.45H2(PO4)0.39(AsO4)0.61 (CADPA) of the two antiferroelectric NH4H2PO4 (ADP) and NH4H2AsO4 (ADA) and of the two ferroelectric CsH2PO4 (CDP) and CsH2AsO4 (CDA) have been prepared by slow evaporation from aqueous solution at room temperature. The structural properties of the crystal were characterized by X-ray single analysis (performed at room temperature), which revealed that (CADPA) crystallizes in space group I42d with lattice parameters: a = 7.6473(1) Å, c = 7.6803(1) Å and Z = 4. The title compound is isostructural with the tetragonal phases of (ADP) and (ADA). This structure is characterized by two kinds of disorder: 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. Thermal analysis of the (CADPA) were characterized by differential scanning calorimetry (DSC), showed that the title compound undergo two phase transitions at T = 204 K and T = 407 K.
Highlights
IntroductionAt room temperature (ADP) and (ADA) are paraelectric and crystallize in the tetragonal system I42d
The KDP family compounds (MH2XO4 with M = NH4, K, Rb, Cs and X = P, As) have been of continuing interest due to their structural and physical properties [1,2,3,4,5,6]
The phosphorus and the arsenic atom as well as the ammonium and cesium ions lie on points with site symmetry S4, the oxygen atoms lie in general positions about the phosphorus–arsenic atoms, in a tetrahedral arrangement (Fig. 1)
Summary
At room temperature (ADP) and (ADA) are paraelectric and crystallize in the tetragonal system I42d The structures of these materials are formed of (H2PO4-) or (H2AsO4-) groups linked by short hydrogen bonds parallel to (a, b) plane [5]. Both compounds became antiferroelectric in two phases below room temperature, at 148 K and 216 K and show a further high temperature phase transitions at 433 K and 355 K, respectively [6,7,8]. The aims of this work were: (i) to examine the influence of a partial anionic and cationic substitutions over crystalline symmetry and physical properties, (ii) to characterise spectroscopically this phase, (iii) to evidence the phase transitions in (CADPA) and (iv) to determine the dynamic of proton in the O–H...O hydrogen bonds
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