R.A. Vargas

On the high-temperature phase transitions of CsH2PO4: A polymorphic transition? A transition to a superprotonic conducting phase?

X-ray diffraction, thermogravimetric (TGA), differential scanning calorimetric (DSC), and impedance analysis were used to study the reported high-temperature phase transitions at 107, 149, 230, and 256 °C in crystals of cesium dihydrogen phosphate, CsH2PO4 (CDP). Our results show strong evidence that at all these temperatures, the observed DSC or differential thermal analysis (DTA) endothermic effects appear only as a consequence of a dehydration process starting on the surface of the crystal. Our results thus show that the reported transition at 230 °C is not a polymorphic transition. This means that the monoclinic symmetry, stable at room temperature, with space group P21/m–C2k2, is maintained up to the final decomposition. Moreover, since we have not found any evidence for the existence of a superprotonic high-temperature phase above 230 °C, the high conductivity above 230 °C is thus only a consequence of the dehydration of the crystal surface.

More studies on the PVAl+H3PO2+H2O proton conductor gels

Solid polymer proton conductors made of poly(vinyl alcohol) (PVAl), hypophosphorous acid (H3PO2) and water with conductivities as high as 0.1 S cm−1 at room temperature were studied by means of differential scanning calorimetry (DSC), X-ray diffraction and thermogravimetry (TG). The DSC curves for membranes with the highest acid concentrations show on heating a very low glass transition temperature (revealing a separate acid/water phase), a cold crystallization after this glass transition, a melting of crystallizes or of the freezing water in the polymer network and a recrystallization above ambient temperatures as the membranes lose water. The X-ray spectra for the raw samples at room temperature indicate that the amorphousness of PVAl complexes increases with the concentration of H3PO2; but its degree of crystallinity increases with the annealing time of samples above room temperature. The TG traces confirm that membranes with the highest acid concentrations have the highest water content and that the maximum rate of water removal is at about 50°C for all samples.

On the high-temperature phase transitions of some KDP-family compounds: a structural phase transition? A transition to a bulk-high proton conducting phase?

Abstract Thermal analysis, X-ray diffraction, and impedance analysis were used to study the high-temperature nature of the KDP-family compounds: KH2PO4 (KDP), RbH2PO4 (RDP) and CsH2PO4 (CDP). Our results show that the structural phase transition from tetragonal to monoclinic modification in KDP and RDP earlier reported at around Tp≈180°C and Tp≈86°C, respectively, does not exist. For CDP we found that the structural phase transition from a monoclinic to a cubic symmetry superprotonic phase above 230°C does not take place. Instead, in close agreement with Lee’s suggestion, we found evidence that at all these temperatures a partial polymerization takes place at reaction sites distributed on the surface of the samples.

Phase behaviour of the solid proton conductor CsHSO4

The sequence of previously found phase transitions in CsHSO4 at around 60, 122, and 141 °C was carefully examined by using thermo-gravimetric analysis (TGA), mass spectroscopy (MS), differential thermal analysis, ac calorimetry, impedance spectroscopy, and x-ray diffraction (XRD). Our results show evidence that at all these transition temperatures, the dehydration processes take place at or very near the surface of the crystal. As a consequence, our results support the phase of CsHSO4 above 141 °C being not a superprotonic-conducting phase but rather a mixture of CsHSO4 and Cs2S2O7 caused by a partial thermal decomposition that includes the break-up of hydrogen bonds and the formation of H2O molecules.

On the high-temperature phase transition of RbH2PO4—A polymorphic transition?

Abstract X-ray diffraction, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to study the dynamic process of dehydration and the high-temperature phase transition at T p = 98.5. ± 26.5 ° C (often stated as 86 °C) in rubidium dihydrogen phosphate (RDP). Our results indicate that the phase transition at T p in RDP is not a bulk structural transition (tetragonal to monoclinic) but the onset temperature of the formation of the double salt 2RbH 2 PO 4 -Rb 2 H 2 P 2 O 7 . Thus, our results support Lees suggestion that the term high-temperature phase transition should be replaced by the term “onset of partial polymerization at reaction sites distributed on the surface”.

Phase behavior of complexes of PVA and acid salts

Polymer electrolytes of COLBON R (BASF ) (based on poly vinyl alcohol (PVA)) complexed with KHSO 4 of different stoichiometric ratios have been characterized using high-resolution ac calorimetry, differential scanning calorimetry (DSC) and electrical conductivity measurements. These complexes have been prepared by solvent casting method using H 3 PO 2 as solvent. All complexes have been confirmed to be amorphous up to 450 K by thermal and conductivity measurements. The glass transition temperature is maintained low (around T g 213 K), without largely depressing conductivity up to the KHSO 4 weight fraction x = 0.5. The electrolyte with x = 0.09 exhibits the highest conductivity, of 1.1 x 10 -2 (Ω cm) -1 at room temperature. The activation energy of charge transport is independent of salt concentration (x < 0.5) and remains mainly constant at 0.23 eV in the 300-360 K temperature range.

High Temperature Phase Transitions of NH4H2PO4

The high temperature phase behaviour of NH4H2PO4 (ADP) was studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric (TG) analysis in the temperature range of 300 to 500 K. The DSC results show peaks at 430 and 483 K, that are associated with different phase transformations. The powdered XRD results show that when an ADP sample is heated above 430 K, additional peaks appear besides those corresponding to its tetragonal structure at room temperature. As a consequence, the observed change of symmetry of the crystal suggests a phase transition like phenomena named as high-temperature phase transition (HTPT) at the characteristic temperature Tp = 430 K. However, the observed weight loss in the sample found in the TG curve, suggests that Tp marks the onset of partial polymerization taking place on the surface of the sample, such that the phase above 430 K is a mixed phase consisting of tetragonal ADP in the bulk of the sample and an ammonium polyphosphate (i.e., (NH4)2H2P2O7) at the surface of the crystal.

Phase behaviour of RbH2PO4 and CsH2PO4 in the fast-ion regime

Abstract Results of thermal analysis in CsH 2 PO 4 (CDP) and RbH 2 PO 4 (RDP), using combined differential scanning calorimetry (DSC) and high-resolution ac calorimetry, are presented. For single crystal samples, a sequence of phase transitions in both salts above room temperature is found that is very dependent of their thermal treatments. For CDP, the ac data show a broad heat capacity anomaly near T 1=420 K for freshly prepared samples at room temperature or non-annealed samples above T 2=500 K, and an endothermic peak at T 2 on heating runs and at 470 K on cooling runs. For RDP, much more variations were observed in the region where two transitions were previously reported for different prepared samples that occured near 352 K and 380 K, respectively.

Anomalous specific heat of silver iodide

Abstract The temperature dependence of the specific heat of single crystal samples of silver iodide (AgI), which were grown from solution in the β phase, was measured over the temperature range 300–750 K using ac calorimetric methods. The samples measured were or were not subjected to previous annealing treatments in the α phase. The ac data taken on the samples in the low temperature β phase show no dependence on the thermal treatment of the samples. However, in the electrolyte α phase, differences are observed between the measurements, both in the magnitude of the specific heat and the temperature at which a controversial order-disorder transition have been proposed. Our results would seem to indicate that a residual ordering of the mobile ions remains in an intermediate α-AgI which is strongly coupled to the lattice strain modes.

Specific heat of KDP near the tetragonal-monoclinic phase transition

Abstract Specific heat measurements of KH 2 PO 4 (KDP) have been performed from 340 to 490 K using ac calorimetric methods. A steplike anomaly in the specific heat is observed with transition temperature 453 K on heating runs of single crystal samples, which shows the tetragonal → monoclinic phase transition of KDP. The results also show that the high-temperature monoclinic phase is metastable below 453 K.