Papers by Keyword: Structural Phase Transition

Abstract: This paper study the effect of structural behaviour and electrical properties in cubic NaZn13-type La (FeSi)₁₃ compounds annealed at different high temperatures from 1000oC to 1200oC. The Raman spectroscopy measurement revealed a different level of coexistence of La (FeSi)₁₃-type phase and α-Fe in 1000oC, 1100oC and 1200oC annealed samples. The different level coexistence of the phases suggested that the samples show a significant phase change with different annealing temperature methods. This finding has also supported by impedance analyzer measurement, where the pattern shows that the 1000oC and 1100oC samples have similar behaviour waveform pattern compared to the 1200oC, which has different behaviour. Besides, the impedance pattern clearly shows that the propagation of the signal for the 1000oC and 1100oC before it arrives in the conductor state (R=0.00) has high resistive values compared to 1200oC annealed samples. Further investigation on the electrical properties was done with the conductivity and tan delta measurements. The results show that 1200oC annealed sample have a high value of conductivity (S=18μ S/m) compares with 1100oC (S= 0.1μ S/m) and 1000oC (S=4.89n S/m). The tan delta measurements found that samples annealed at 1200oC temperature has low value of resistivity (tan ɗ = 0.117) compares with 1100oC (tan ɗ = 0.335) and 1000oC (tan ɗ = 0.482) respectively.

Abstract: In this work, the effect of different annealing treatments on the phase transition, structural behaviour and thermal analysis of MnCoGe alloys has been analysed. The changes in the transition temperatures have been studied by Simultaneous Thermal Analysis (STA). The results show that the structural transition temperature (T_str) depends on the annealing treatment of the samples preparation. However, under the same heat treatment no significant change is observed on the curie temperature (T_c). The thermal analysis reveals endothermic peak which corresponds to the structural phase transition of the compounds. The microstructural evolution has been monitored using in-situ X-ray diffraction which made known this compound produce three type of structures which are hexagonal, orthorhombic and mix structure (hexagonal and orthorhombic).

Abstract: Since the discovery of magnetocaloric effect (MCE), numbers of method in producing magnetocaloric materials has been studied. Among those methods, ball milling has been shown as a very versatile technique with several advantages compared to other preparation methods. In this work, the effect of ball milling preparation technique on the phase structural behaviour and electrical properties of MnCoGe alloys has been analysed. The changes in the structural behaviour have been studied by X-ray powder diffraction (XRD) and Raman Spectroscopy. The results suggest that the samples show significant structural changes with different method of ball milling running time. This finding has also been supported by electrical properties where the measurement found that the frequency also plays important role in the structure changes. The absolute impedance value,|Z| (Ω) suggest that structure start to change at initial frequency structure of hexagonal at point 3.22 Ω and 44.1 MHz region. The permittivity and dielectric loss (tan delta) graph that corresponds to a frequency (Hz) up to 100 kHz shown that the 2-hours milling time MnCoGe compound has the lowest permittivity value which make it had lower energy and required more frequency to react.

Abstract: We report a phenomenological model based calculation of pressure-induced structural phase transition and elastic properties of ZrN compound. Gibb’s free energy is obtained as a function of pressure by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. From the present study, we predict a structural phase transition from NaCl structure (B1) to the CsCl structure (B2). The variations of elastic constants with pressure follow a systematic trend identical to that observed in others compounds of NaCl type structure family.

Abstract: The high-pressure technique is useful to understand physical properties because the technique can directly control bond length and phase transition. As a general trend, the pressure-induced phase transition causes an increase of coordination number with a drastic change of their physical properties. Here, we attempt to explore the pressure-induced phase transitions from the sixfold-coordinated NaCl structure (B1) to the eightfold-coordinated CsCl structure (B2) in Mg_xCd_1−xO by applying an effective interionic interaction potential, which includes the long range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction upto second-neighbor ions within the Hafemeister and Flygare approach. Assuming that both the ions are polarizable, the Slater-Kirkwood variational method is employed to estimate the vdW coefficients for parent compounds. The estimated values of the phase transition pressure (P_t) increase with Mg concentration. The vast volume discontinuity in pressure volume phase diagram identifies the structural phase transition from B1 to B2 structure. The results obtain from the present calculations requires the complete understanding of many physical interactions that are essential to ternary oxides, containing elements with size and chemical mismatch, will lead to a consistent explanation of the documented structural properties.

Abstract: The tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA) is used to study the electronic structure and high pressure behaviour of thulium compounds TmX (X= P, As, S, and Se). We also predict a structural phase transition from NaCl to CsCl-type structure. The transition pressures were found to be 40.0, 31.0, 58.0 and 49.0 GPa, for TmP, TmAs, TmS and TmSe respectively. Apart from this, the lattice parameter (a₀), bulk modulus (B₀), band structure and density of states are calculated. From energy band diagram, it is observed that these compounds exhibit weak metallic character. The calculated values of lattice parameters and bulk modulus are of reasonable agreement with available data.

Abstract: The density functional theory within generalized gradient approximation (GGA) has been used to calculate lattice parameter, total energy, phase transition pressure and electronic properties of neptunium sulphide (NpS). From our calculations we observe that NpS is stable in NaCl – type structure under ambient pressure. For this compound, the phase transition pressure was found to be 29.5 GPa. The nature of metallic behaviour is remarked from energy band diagram in NpS. All properties obtained for this compound are in good agreement with available results.

Abstract: A theoretical study of the elastic behavior in IrN compound using effective interionic interaction potential is carried out. The estimated values of phase transition pressure and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zinc blende (B3) to CsCl structure (B2). C₁₁, C₁₂ and C₄₄ increase nearly linearly with pressure. At phase transition pressure IrN has shown a discontinuity in second order elastic constants, which is in accordance with the first-order character of the phase transition.

Abstract: The electronic, elastic and mechanical properties of neptunium bismuthide have been studied systematically using first principles density functional theory within generalized gradient approximation. Ground state properties such as lattice constant (a₀), bulk modulus (B), its pressure derivative (B′) and elastic constants are calculated. The present results are in good agreement with the experimental and other available theoretical results. Poisson’s ratio (σ), Young’s moduli (E), shear moduli (G_H) and the ratio of elastic anisotropy factor (A) are also estimated.

Abstract: The electronic structure and high pressure structural phase transition of SmTe and SmPo have been studied by using tight binding linear muffin-tin-orbital (TB-LMTO) method within the local density approximation (LDA). The total energy as a function of volume is obtained and it is found that these compounds are stable in NaCl-type (B₁-phase) structure and transform to CsCl-type (B₂-type) structure. The transition pressure of SmTe and SmPo are found to be 6.6 GPa and 8.6 GPa respectively. We have also calculated lattice parameter (a₀), bulk modulus (B₀), band structure (BS) and density of states (DOS). From energy band diagram, we observed that these compounds exhibit weakly metallic behaviour. The calculated values of lattice parameter and bulk modulus agree well with the available data.