Research Of The Ferroelectric Phase Transition In The Polycrystalline And Powder Of NanO₂

Ferroelectric physics is an important research field in condensed matter physics, especiallythe wide application of ferroelectric materials inspired people to explore the new ferroelectricmaterials and new physical effects. The researches of ferroelectric physics involve many importantissues of solid state physics and materials science. For example, the structural phase transition,lattice dynamics, nonlinear effects, as well as the interaction of phonons and electrons, etc. In thepast few decades, studies of ferroelectric have played a significant role in promoting fundamentalscience and technology, at the same time raised some important physics concepts, solved someimportant scientific issues.NaNO2is one of the simplest structure of ferroelectrics, however, single crystal sodium nitriteexists abnormal phenomenon in1K temperature range above the critical temperature offerroelectric phase transition, which is not well understood, and the polycrystalline material playan important role in researches of ferroelectric material. So it is necessary to carry out the researchof the powder and polycrystalline material of NaNO2, to explore the single crystal materialswhether the properties and changing rules of which are the same or not to single crystal’s, such asthe abnormal phenomenon in1K temperature range above the critical temperature of ferroelectricphase transition. This paper using Simultaneous thermal analysis, scanning electron microscope,X-ray detection method and so on are employed to measure the NaNO2and the measure resultsare analyzed, the conclusions are showed as follows.(1) The structure information of NaNO2powder samples are obtained by X-ray diffraction,its crystal structure is orthorhombic, space group is Im2m(No.44), and the lattice parametersa=3.57924, b=5.59407, c=5.41181, crystal axial angle α=90°, β=90°, γ=90°, Cell volume is108.363and grain size is69.67. The NaNO2polycrystalline samples crystal is orthorhombic,space group is Im2m(No.44), and the lattice parameters a=3.63149, b=5.64968, c=5.38749,crystal axial angle α=90°, β=90°, γ=90°, Cell volume is110.533and grain size is49.39.(2) Thermal properties of the NaNO2powders and polycrystalline samples are measuredusing simultaneous thermal analyzer, the results indicate that the TG curves of NaNO2powder andpolycrystalline samples have not much changed. Heating rate is5K/min, NaNO2powder samplehave appeared a peak near438.51K as thermal change. Heating rate is0.1K/min. NaNO2powdersample have appeared a peak near437.41K as thermal change, there is also appeared such achange in438.66K. The conclusion seems to be coincided with the single crystal NaNO2. Heatingrate is5K/min, NaNO2polycrystalline sample have appeared a peak near434.14K as thermalchange. Heating rate is0.5K/min. NaNO2polycrystalline sample have appeared a peak near434.17K as thermal change, there is also appeared such a change in434.92K. The conclusionseems to be coincided with the single crystal NaNO2.(3) NaNO2powders and polycrystalline samples are measured using dielectric spectroscopy.The complex dielectric constant changes of NaNO2powder and polycrystalline material aremainly caused by the ionic diffusion at high temperature (above420K) and low-frequency (less than100Hz). The real parts of the complex dielectric constants of high-frequencies (105,106Hz)are almost independent of frequency near the phase transition temperature; and which can reflectthe polarization and structure information of NaNO2powder and polycrystalline material. In therange of443K-450K, the activation energy of NaNO2powder is0.70ev, while the activationenergy of NaNO2polycrystalline is1.58ev; the activation energy of NaNO2polycrystalline isgreater than the powder sample’s, which shows that the ionic diffusion of the powder andpolycrystalline of NaNO2may caused by the intrinsic defects. The ferroelectric phase transitionprocess in NaNO2powder is more obvious than the one in NaNO2polycrystalline on the curves ofcomplex dielectric constants. The curve of the real part of the complex dielectric constant ofNaNO2polycrystalline versus temperature deviates from the Curie-Weiss law in the range of437Kto440K above the critical temperature Tc, which indicates that the ferroelectric phase transitionprocess has the abnormal phenomenon consistent with the results of single crystal NaNO2.(4) NaNO2powder samples are measured using thermal expansion instrument, NaNO2powder samples dL/L0has been an increasing trend change with the increase of the temperature T.the results showed that NaNO2powder samples appeared swelling in the range of experimentaltested temperature, at the temperature of437.66K, the thermal expansion curve has undergone a"bend", it descript there occur introversion between the two phases near437.66K, in the process ofphase transitions, has a volume mutation.(5) Thermal conductivity and thermal diffusivity of the NaNO2powder emerge the similartrend as the temperature, firstly a trend of decrease then increase and decrease again. Heat Specificrender increase and then decrease with temperature increases. These changes of thermalparameters is corresponding to437K, indicating the occurrence of phase transition near the pointof temperature.