Mechanical Spectrum Study Of Point Defects And Charge Ordering In Solids

Low frequency torsion pendulum, audio-frequency reed vibration and megahertz ultrasonic attenuation are well known experimental tools to measure the mechanical spectra which can be further used to study phase transitions and dynamics of many kinds of microscopic units, such as point defects, dislocations, grain boundaries and domain walls, etc., in the single crystal and polycrystalline. In this dissertation, we present our efforts on the study of the mechanical spectrum of point defects and charge ordering in solids by using the audio-frequency reed vibration method.The mechanical spectrum of y-Nao.66Co02 from high temperature around 450K to liquid nitrogen was measured at kilohertz frequencies. A sodium ordering transition is proposed to explain a step increase of resonance frequency around 427K with the decrease in temperature in annealed sample. Below the room temperature, internal friction is less temperature dependent above 240K and shows a continuous decrease with the decrease in temperature below 240K in annealed sample. A transition was suggested to occur around 240K, which is proposed to be a freezing transition of the mobility of sodium atoms. A similar behavior was observed in as-prepared sample. However, a superposed internal friction peak around 160K was observed in as-prepared sample, which is absent in annealed one. This peak is suggested to be sodium off-center hopping near oxygen vacancies.The mechanical spectrum of La1-x SrxFeO3 (x=0.50,0.55,0.60,0.67) ceramics was also measured from liquid nitrogen temperature to room temperature at kilohertz frequencies. An internal friction peak was observed in all samples around 240K which is most prominent in charge ordering sample. And the intensity of this internal friction peak decreases as the Sr content decreases. This peak and accompanied modulus softening process is proposed to be caused by charge ordering fluctuation in terms of charge lattice coupling. With the decrease in temperature, a step increase of modulus and accompanied sudden decrease of internal friction is observed around the charge ordering transition temperature for x=0.67 and 0.60. This anomalous behavior is shown to be an indication of the charge ordering transition. Another internal friction peak is observed for x=0.50,0.55 and 0.60 around 160K, which is most prominent in x=0.50 sample and its intensity decreases with the increase of Sr content. This peak is expected to be the freezing transition of disordered charge.The internal friction of partially Sr-substituted Pr(Ba1-xSrx)2Cu3O7-δ (x=0,0.05, 0.1,0.2,0.3, and 0.4) ceramics was measured from liquid nitrogen temperature to room temperature at kilohertz frequencies. The internal friction spectrum in Pr(Ba1-xSrx)2Cu3O7-δ is roughly similar to that in Y(Ba1-xSrx)2Cu3O7-δ, while the crystal structure shows some different variation during Sr substitution. The intensity of the internal friction peak, which appears around 250K, decreases upon Sr doping. The result is explained in terms of a possible combining structural freezing transition of CuO5 square pyramids and oxygen atoms in CuOx chains. Another internal friction peak around 134K was observed in tetragonal Pr(Ba0.6Sr0.4)2Cu307-δ which is temporarily explained by the hopping of holes in the CuO2 layers.