J.E. Diosa

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.

Impedance spectroscopy studies of the polymer electrolyte based on poly(vinyl alcohol)-(NaI+4AgI)-H2O

The polymer electrolyte films based on poly(vinyl alcohol) (PVA), (NaI+4AgI) and water have been synthesized. The polymer electrolyte exhibited an ionic conductivity of ~10^-^1Scm^-^1 and shows to be stable and constant in a wide temperature range (10-70^oC) for the hydrated samples. The Impedance spectroscopy studies were obtained from 10^6 to 10^9Hz and over the temperature range 10 to 70^oC. The results reveal three dielectric relaxations that appear in both the imaginary and real part of the permittivity. These relaxations are related with the cooperative segmental movements of the macromolecular chains, normal-mode relaxation and localized motions of the structural units.

Anomalous Phase Behaviour of NH4HSO4 below Room Temperature

Impedance spectroscopy (IS) and modulated differential scanning calorimetry (MDSC) were used to study the phase behaviour of NH 4 HSO 4 (AHS) below room temperature. Phase transitions were clearly evidenced by dielectric measurements at about 270 and 154 K, but a large hysteresis of the low-temperature phase boundary was observed during thermal cycling of the samples. However, the thermal measurements at zero field (E = 0) only showed an endothermic peak at 154 K and thus appeared to be a first-order transition, but no thermal hysteresis was observed, which indicated a strong effect of the external field on the transition at this temperature. The phase transition at about 270 K, appeared to be a second-order one in which only dielectric measurements exhibited strong anomaly, but its intensity decreased remarkably on successive thermal cyclings of the sample. This behaviour is discussed in terms of disordering of the NH 4 + orientations with thermal treatments of the sample that reduce the ferroelectric activity of the intermediate phase (154 K < T < 270 K) of AHS.

Dielectric Relaxation Studies in Cs2SO4

Dielectric measurements of Cs 2 SO 4 show a distinct relaxation at low frequencies at several isotherms (T < 651°C). For example, the relaxation frequency is around 5.1 kHz at 376°C and increases to approximately 1 MHz at 651°C. The relaxation has an activation energy of 0.97 eV, which is in close agreement whit that of Cs+ transport. We suggest that the observed dielectric relaxation could be produced by the Cs+ jump and SO 4 − reorientation that cause distortion and change the local lattice polarizability inducing dipoles like CsSO 4 −.

On the high temperature phase behaviour of NH4H2AsO4

Abstract X-ray diffraction, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electrical conductivity measurements were used to study the phase behaviour of NH 4 H 2 AsO 4 (ADA) above room temperature. Our results indicate that dehydration occurs in the temperature range below the earlier reported onset of decomposition at 438 K; on heating two events are observed, at T p1 =361 K and at T p2 =423 K and hydrolysis takes place on cooling to room temperature under appropriate water vapour pressure. We suggest, in accordance with recent findings for similar compounds, that partial polymerisation sets in at reaction sites distributed on the surface of the crystals.

Heat capacity and phase transitions in (NH4)2HPO4

Abstract Heat capacity measurements on (NH 4 ) 2 HPO 4 single crystals in the 77–300 K temperature range are reported. Two distinct and reversible heat capacity anomalies at normal pressure are identified, one near 174 K and the other near 246 K. The amplitude of the heat capacity anomalies are relatively small, less than 5.0% of the total lattice contribution. No latent heat and no thermal hysteresis were detected in the transition regions, indicating that the corresponding phase transitions are characterized by gradual changes in the activation energy for ionic migration and the heat capacity.

Phase behaviour of RbH2AsO4 above room temperature

Abstract The high-temperature behaviour of RbH 2 AsO 4 (RDA) was studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and X-ray diffraction (XRD). The phase-transition like phenomena (i.e. DSC endothermic peak accompanied by a change of crystal symmetry from a tetragonal to monoclinic structure), named as a high-temperature phase transition (HTPT), have been observed in RDA around T p =150°C. Our results show that this temperature marks the onset of a thermal dehydration of RDA similar to that suggested by Lee for the KDP family compounds on heating above T p .

Oxygen diffusion in observed by resistivity measurements

In situ resistivity measurements have been used to monitor the oxygen uptake and removal for the ceramic system (YPBCO) in the temperature range 300 - 1000 K. The study of the out-diffusion of oxygen was performed by annealing the oxygenated samples in ambient air at constant heating rates. We found that, independently of x, the oxygen concentration is preserved up to 600 K, and that oxygen diffuses out of the oxides at temperatures higher than 600 K. However, the rate of oxygen removal from (PBCO) is greatly reduced as compared to the rate for pure (YBCO), and decreases with increasing x in YPBCO. The study of the in-diffusion of oxygen was performed by annealing the deoxygenated samples in ambient air at constant heating rates. We found that, independently of x, the oxygen uptake takes place in the temperature range 550 - 750 K, where the in-diffusion rate depends slightly on the Pr concentration. The observed resistivity change for all of the YPBCO samples as they interchange oxygen with the gas phase indicates that the number of majority carriers in the oxides is mainly controlled by the oxygen content and seems to be independent of the Pr concentration, at least for .

Anomalous phase behavior of (NH4) HSO4

We have studied the phase behavior at normal pressure of NH4HSO4 (AHS) over the 100–420 K temperature region using ac calorimetry on single crystal samples. From the changes in the ac-calorimetric curves recorded on successive thermal cycles of the samples, it is inferred that metastable phases can be induced in the previously reported region of its intermediate ferroelectric phase (154 K < T < 220 K) at 1 atm. An additional thermal anomaly was found to occur at 220 K that seems to indicate the coexistence of the low and intermediate phase in the 154 K < T < 220 K temperature range. No thermal anomalies were observed in the paraelectric region 270 K < T < 420 K, where it has been claimed that some proton transport occurs in AHS.

Isotope effects on phase transitions in (ND4)2DPO4

Abstract High-resolution a.c. calorimetry was used to study the phase behaviour of (ND 4 ) 2 DPO 4 and compare it with that of (NH 4 ) 2 HPO 4 in the 70–450 K temperature range. The temperature dependence of the heat capacity is similar in both salts, but there is an unexpected reverse shift in the phase transition temperatures as T 1 = 174 K and T 2 = 246 K for the proton compound to 147 K and 229 K, respectively, for the deuteron counterpart. A third phase transition is also verified near T 3 = 416 K in both salts, which shows a thermal hysteresis of about 15 K and no effect is observed on the T 3 transition temperature when substituting H for D. The phase transitions are discussed in terms of continuous (second-order) reorientations of the ND 4 + (NH 4 + ) tetrahedra over two preferred directions at T 1 and T 2 and a discontinuous (first-order) generation of mobile D + (H + ) defects at T 3 .