Phase transitions in K₁₋x(NH₄)xH₂PO₄ single crystals

Bromberek, Marek (2008) Phase transitions in K₁₋x(NH₄)xH₂PO₄ single crystals. PhD thesis, Memorial University of Newfoundland.

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Abstract

An apparatus for growing single crystals from aqueous solution by the slow evaporation method was constructed. Mixed crystals of K₁₋x (NH₄) x H₂PO₄ (KADP) with (NH₄)H₂PO₄ (ADP) fractions of 0.021 and 0.12 were successfully grown. Their composition was determined by means of the x-ray powder diffraction method. This analysis also suggests that those crystals are a mixture of two phases even at room temperature. The details of the structure of the additional phase could not be determined. -- The dielectric constant ε of the mixed crystals as well as pure KH₂PO₄ (KDP) was measured along the polar axis in the temperature range from 20 K to 300 K. The frequency range of the applied electric field was from 100 Hz to 10 MHz. The data were analyzed by means of the standard Landau theory of phase transitions with coupling terms reflecting the interaction between the electric and elastic degrees of freedom. The temperature dependence of ε follows a typical Curie-Weiss behavior in the range of approximately 30 K above Tc for all crystals studied. The value of the critical temperature decreases with increasing ammonium ion content in agreement with previously published results. The analysis of the frequency dependence of the dielectric susceptibility clearly indicates the existence of two dispersion processes: resonant and relaxational. The former is attributed to the piezoelectric activity of all the crystals studied. The latter is the result of the response of the permanent dipole moments present in ferroelectric crystals to the applied electric field. In the case of mixed crystals the relaxational dispersion is characterized by a distribution of relaxation times. Its mean relaxation time as well as its width increases with decreasing T. This is a typical behavior for mixed crystals for which the two end members of the solid solution in their pure form undergo ferroelectric and antiferroelectric phase transitions, respectively. This behavior is a result of competing long range interactions. -- New results for the temperature dependence of the CE/66 and CE/11 elastic constants of the crystals were obtained by means of the Brillouin spectroscopy. In addition to that the soft acoustic mode behaviour was also studied by means of the piezoelectric resonance method. Both of them gave qualitatively similar results which also correlate very well with the dielectric measurements. All major differences could be explained by assuming an inhomogeneous distribution of the ammonium ions in the KDP matrix. The Brillouin results were also analyzed by means of the Landau theory. It approximates very well the temperature dependence of the CE/66 elastic constant in the paraelectric phase region for all crystals studied. The measured values of CE/66 below Tc are in general lower than those predicted by the model. The greater the NH₄ content the bigger the discrepancy. -- Moreover, the analysis of the temperature dependence of the CE/66 elastic constant indicates the existence of a new phase in the mixed materials. Its stability region broadens with increasing NH₄ concentration. For the crystal with the highest ammonium concentration studied it coincides with the dielectric plateau region between 70 K and 55 K. At temperatures below the plateau region the K₀.₈₈(NH₄)₀.₁₂H₂PO₄ sample scattered the laser light very strongly. This is most likely caused by defects and sample inhomogeneities but annealing the sample for two hours at 400 K does not result in any significant change.

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