ABSTRACT
In previous work, molecular dynamics simulations based on a first-principles-derived effective Hamiltonian for Pb1-X(Sc1/2Nb1/2)O3-X (PSN), with nearest-neighbor Pb-O divacancy pairs, was used to calculate X[Pb-O] vs. T, phase diagrams for PSN with: ideal rock-salt type chemical order; nanoscale chemical short-range order; and random chemical disorder. Here, we show that the phase diagrams should include additional regions in which a glassy relaxor-phase (or state) is predicted. With respect to phase diagram topology, these results strongly support the analogy between relaxors and magnetic spin-glass-systems.
ABSTRACT
We have studied the phase equilibria of three ceramic quasibinary systems Ti1-x Zr x N, Ti1-x Hf x N and Zr1-x Hf x N (0 ⩽ x ⩽ 1) with density functional theory, cluster expansion and Monte Carlo simulations. We predict consolute temperatures (T C), at which miscibility gaps close, for Ti1-x Zr x N to be 1400 K, for Ti1-x Hf x N to be 700 K, and below 200 K for Zr1-x Hf x N. The asymmetry of the formation energy ΔE f(x) is greater for Ti1-x Hf x N than Ti1-x Zr x N, with less solubility on the smaller cation TiN-side, and similar asymmetries were predicted for the corresponding phase diagrams. We also analyzed different energetic contributions: ΔE f of the random solid solutions were decomposed into a volume change term, [Formula: see text], and a chemical exchange and relaxation term, [Formula: see text]. These two energies partially cancel one another. We conclude that [Formula: see text] influences the magnitude of T C and [Formula: see text] influences the asymmetry of ΔE f(x) and phase boundaries. We also conclude that the absence of experimentally observed phase separation in Ti1-x Zr x N and Ti1-x Hf x N is due to slow kinetics at low temperatures. In addition, elastic constants and mechanical properties of the random solid solutions were studied with the special quasirandom solution approach. Monotonic trends, in the composition dependence, of shear-related mechanical properties, such as Vickers hardness between 18 to 23 GPa, were predicted. Trends for Ti1-x Zr x N and Ti1-x Hf x N exhibit down-bowing (convexity). It shows that mixing nitrides of same group transition metals does not lead to hardness increase from an electronic origin, but through solution hardening mechanism. The mixed thin films show consistency and stability with little phase separation, making them desirable coating choices.
ABSTRACT
A first principles phase diagram calculation, that included van der Waals interactions, was performed for the 3D bulk system (1 - X) · MoS2 - (X) · MoTe2. Surprisingly, the predicted phase diagram has at least two ordered phases, at X ≈ 0:46, even though all calculated formation energies are positive; in a ground-state analysis that examined all configurations with 16 or fewer anion sites. The lower-temperature I-phase is predicted to transform to a higher-temperature I'-phase at T ≈ 500 K, and I' disorders at T ≈ 730 K. Both these transitions are predicted to be first-order, and there are broad two-phase fields on both sides of the ordered regions. Both the I- and I'-phases are predicted to be incommensurate, i.e., aperiodic: I-phase in three dimensions; and I'-phase in two dimensions.
ABSTRACT
Molecular dynamics simulations of first-principles-based effective Hamiltonians for Pb(Sc{1/2}Nb{1/2})O(3) under hydrostatic pressure and for Pb(Mg{1/3}Nb{2/3})O(3) at ambient pressure show clear evidence of a relaxor state in both systems. The Burns temperature is identified as the temperature below which dynamic nanoscale polar clusters form, pinned to regions of quenched chemical short-range order. The effect of pressure in Pb(Sc{1/2}Nb{1/2})O(3) demonstrates that the stability of the relaxor state depends on a delicate balance between the energetics that stabilize normal ferroelectricity and the average strength of random local fields which promote the relaxor state.
ABSTRACT
New data are presented on the phase equilibria and crystal chemistry of the binary systems CaO-Bi2O3 and CaO-CuO and the ternary CaO-Bi2O3-CuO. Symmetry data and unit cell dimensions based on single crystal and powder x-ray diffraction measurements are reported for several of the binary CaO-Bi2O3 phases, including corrected compositions for Ca4Bi6O13 and Ca2Bi2O5. The ternary system contains no new ternary phases which can be formed in air at ~700-900 °C.
ABSTRACT
New data are presented on the phase equilibria and crystal chemistry of the binary systems Sr0-Bi203 and SrO-CuO and the ternary system SrO-Bi2O3-CuO. Symmetry data and unit cell dimensions based on single crystal and powder x-ray diffraction measurements are reported for all the binary SrO-Bi2O3 phases, including a new phase identified as Sr6Bi2O9. The ternary system contains at least four ternary phases which can be formed in air at ~900 °C. These are identified as Sr2Bi2CuO6, Sr8Bi4Cu5O19+x , Sr3Bi2Cu2O8 and a solid solution (the Raveau phase) which, for equilibrium conditions at ~900 °C, corresponds approximately to the formula Sr1.8-x Bi2.2+x Cu1±x/2O z .(0.0⩽x⩽~0.15). Superconductivity in this phase apparently occurs only in compositions that correspond to negative values of x. Compositions that lie outside the equilibrium Raveau-phase field often form nearly homogeneous Raveau-phase products. Typically this occurs after relatively brief heat treatments, or in crystallization of a quenched melt.
