Preparação e caracterização estrutural e elétrica de cristais mistos de K1-x(NH4)xH2PO4

AUTOR(ES)
DATA DE PUBLICAÇÃO

2009

RESUMO

The preparation and structural and electrical characterization of KH2PO4 (KDP), (NH4)H2PO4 (ADP) and K1-x(NH4)xH2PO4 (KADPx) mixed crystals grown by the method of solvent evaporation at 40 C and with x = 0, 0.076, 0.118, 0.357, 0.857, 0.942, 1.0 was carried out. Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) was performed in order to determinate x for each mixed crystal. Plates with the dominant face of (100) were obtained and prepared to impedance measurements. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) measurements have been carried out on crystals in the temperature range of 30 C 450 C. Our results suggest that, in the decomposition process the shifts of starting weight loss in the TGA curves are dependent on x in mixed crystals. As compared with the TGA, the direct reflection of the exchange of constituent cations (NH4 + and K+), the lattice parameters, and cell volume of KADPx crystals, increase with the addition ammonium. This behavior concerning the matching of the exchange agrees with the results of investigations of the XRD results. The measured ac impedance data are analyzed as a function of frequency in the temperature range between 20 C and 160 C. An equivalent circuit model based on two parallel GC circuits was adopted to describe the complex impedance data obtained from the crystals. The conduction is attributed to the hopping of proton among hydrogen vacancies. Additional defects can be created by breaking the hydrogen bond in ammonium groups. Potassium and ammonium ions in the mixed crystals are suggested to have the contribution to the electrical conduction at high temperatures. The activation energies of migration were obtained in different temperature ranges. KADPx crystals are dielectric-type crystal that progresses to an ionic crystal one as the temperature is raised. At ADP-rich region the conductivity is higher than that of the KDP-rich region. In the intermediate compositions x the conductivity is lower than of the rich regions.

ASSUNTO(S)

1. cristais - crescimento - teses. 2. impedanciometria - teses. 3. raios x - difração - teses. i. universidade federal de ouro preto. ii. título. engenharias

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