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Register a new userElastic and structural instability of cubic Sn3N4 and C3N4 under pressure
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Added by
Gopal K. Pradhan Mr.
Publication date
2010
fields
Physics
and research's language is
English
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We use in-situ high pressure angle dispersive x-ray diffraction measurements to determine the equation of state of cubic tin nitride Sn3N4 under pressure up to about 26 GPa. While we find no evidence for any structural phase transition, our estimate of the bulk modulus (B) is 145 GPa, much lower than the earlier theoretical estimates and that of other group IV-nitrides. We corroborate and understand these results with complementary first-principles analysis of structural, elastic and vibrational properties of group IV-nitrides, and predict a structural transition of Sn3N4 at a higher pressure of 88 GPa compared to earlier predictions of 40 GPa. Our comparative analysis of cubic nitrides shows that bulk modulus of cubic C3N4 is the highest (379 GPa) while it is structurally unstable and should not exist at ambient conditions.
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Electronic, structural, vibrational and elastic properties of PaN have been studied both at ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. Under pressure PaN is found to undergo a structural transition from NaCl to the R-3m structure near 58 GPa. The high pressure behavior of the acoustic phonon branch along the (1,0,0) and (1,1,0) directions, and the C44 elastic constant are anomalous, which signals the structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). Therefore, this constitutes an example of GGA and LDA leading to qualitatively different predictions, and it is of great interest to examine experimentally whether this topological transition occurs.
The structural properties of Thallium (III) oxide (Tl2O3) have been studied both experimentally and theoretically under compression at room temperature. X-ray powder diffraction measurements up to 37.7 GPa have been complemented with ab initio total-energy calculations. The equation of state of Tl2O3 has been determined and compared to related compounds. It has been found experimentally that Tl2O3 remains in its initial cubic bixbyite-type structure up to 22.0 GPa. At this pressure, the onset of amorphization is observed, being the sample fully amorphous at 25.2 GPa. The sample retains the amorphous state after pressure release. To understand the pressure-induced amorphization process, we have studied theoretically the possible high-pressure phases of Tl2O3. Although a phase transition is predicted at 5.8 GPa to the orthorhombic Rh2O3-II-type structure and at 24.2 GPa to the orthorhombic a-Gd2S3-type structure, neither of these phases were observed experimentally, probably due to the hindrance of the pressure-driven phase transitions at room temperature. The theoretical study of the elastic behavior of the cubic bixbyite-type structure at high-pressure shows that amorphization above 22 GPa at room temperature might be caused by the mechanical instability of the cubic bixbyite-type structure which is theoretically predicted above 23.5 GPa.
An experimental and theoretical study of the structural properties of monoclinic bismuth oxide (alfa-Bi2O3) under high pressures is here reported. Both synthetic and mineral bismite powder samples have been compressed up to 45 GPa and their equations of state have been determined with angle-dispersive x-ray diffraction measurements. Experimental results have been also compared to theoretical calculations which suggest the possibility of several phase transitions below 10 GPa. However, experiments reveal only a pressure-induced amorphisation between 15 and 25 GPa, depending on sample quality and deviatoric stresses. The amorphous phase has been followed up to 45 GPa and its nature discussed.
First principles study of structural, elastic, and electronic properties of the cubic perovskitetype BaHfO$_3$ has been performed using the plane wave ultrasoft pseudo-potential method based on density functional theory with revised Perdew-Burke-Ernzerhof exchange-correlation functional of the generalized gradient approximation (GGA-RPBE). The calculated equilibrium lattice constant of this compound is in good agreement with the available experimental and theoretical data reported in the literatures. The independent elastic constants (emph{C}$_{11}$, emph{C}$_{12}$, and emph{C}$_{44}$), bulk modules emph{B} and its pressure derivatives $B^{prime}$, compressibility $beta$, shear modulus emph{G}, Youngs modulus emph{Y}, Poissons ratio $ u$, and Lam{e} constants ($mu, lambda$) are obtained and analyzed in comparison with the available theoretical and experimental data for both the singlecrystalline and polycrystalline BaHfO$_3$. The band structure calculations show that BaHfO$_3$ is a indirect bandgap material (R-$Gamma$ = 3.11 eV) derived basically from the occupied O 2emph{p} and unoccupied Hf 5emph{d} states, and it still awaits experimental confirmation. The density of states (total, site-projected, and emph{l}-decomposed) and the bonding charge density calculations make it clear that the covalent bonds exist between the Hf and O atoms and the ionic bonds exist between the Ba atoms and HfO$_3$ ionic groups in BaHfO$_3$. From our calculations, it is shown that BaHfO$_3$ should be promising as a candidate for synthesis and design of superhard materials due to the covalent bonding between the transition metal Hf 5emph{d} and O 2emph{p} states.
We report a high-pressure study of orthorhombic rare-earth manganites AMnO3 using Raman scattering (for A = Pr, Nd, Sm, Eu, Tb and Dy) and synchrotron X-ray diffraction (for A = Pr, Sm, Eu, and Dy). In all cases, a structural and insulator-to-metal transition was evidenced, with a critical pressure that depends on the A-cation size. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the a-c plane, and the Raman bands associated with out-of-phase MnO6 rotations and in-plane O2 symmetric stretching modes. Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smallest A-cations, the compression is nearly isotropic in the ac plane, with presumably only very slight changes of tilt angles and Jahn-Teller distortion. As the radius of the A-cation increases, the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significant as a compression mechanism, while the pressure-induced bending of octahedra chains becomes conversely less pronounced. We finally discuss our results in the light of the notion of chemical pressure, and show that the analogy with hydrostatic pressure works quite well for manganites with small A-cations but can be misleading with large A-cations.
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