Benjamin P. Burton

Lattice dynamics inPbMg1∕3Nb2∕3O3

Lattice dynamics for five ordered PMN supercells were calculated from first principles by the frozen phonon method. Maximal symmetries of all supercells are reduced by structural instabilities. Lattice modes corresponding to these instabilities, equilibrium ionic positions, and infrared reflectivity spectra were computed for all supercells. Results are compared with our experimental data for a chemically disordered PMN single crystal.

First-principles-based simulations of relaxor ferroelectrics

The phenomenology of Pb(B,B′)O3 perovskite-based relaxor ferroelectrics (RFE) is reviewed, with emphasis on the relationship between chemical short-range order and the formation of polar nanoregions in the temperature range between the “freezing” temperature, T f, and the Burns temperature, T B. Results are presented for first-principles-based effective Hamiltonian simulations of (PSN), and simulations that were done with empirically modified variants of the PSN Hamiltonian. Arbitrarily increasing the magnitudes of local electric fields, caused by an increase in chemical disorder, broadens the dielectric peak, and reduces the ferroelectric (FE) transition temperature; and sufficiently strong local fields suppress the transition. Similar, but more dramatically glassy results are obtained by using the PSN dielectric model with a distribution of local fields that is appropriate for (PMN). The results of these simulations, and reviewed experimental data, strongly support the view that within the range , polar nanoregions are essentially the same as chemically ordered regions. In PSN a ferroelectric phase transition occurs, but in PMN, a combination of experimental and computational results indicate that pinning from local fields is strong enough to suppress the transition and glassy freezing is observed.

First principles phase diagram calculations for the wurtzite-structure systems AlN–GaN, GaN–InN, and AlN–InN

First principles phase diagram calculations were performed for the wurtzite-structure quasibinary systems AlN–GaN, GaN–InN, and AlN–InN. Cluster expansion Hamiltonians that excluded, and included, excess vibrational contributions to the free energy, Fvib, were evaluated. Miscibility gaps are predicted for all three quasibinaries, with consolute points, (XC,TC), for AlN–GaN, GaN–InN, and AlN–InN equal to (0.50, 305 K), (0.50, 1850 K), and (0.50, 2830 K) without Fvib, and (0.40, 247 K), (0.50, 1620 K), and (0.50, 2600 K) with Fvib, respectively. In spite of the very different ionic radii of Al, Ga, and In, the GaN–InN and AlN–GaN diagrams are predicted to be approximately symmetric.

A low-temperature phase diagram for ilmenite-rich compositions in the system Fe2O3-FeTiO3

Abstract An approximate low-temperature, metastable phase diagram is drawn for the system (1 - X) Fe2O3-(X)FeTiO3. It is based on published and new magnetic data from nine synthetic samples with bulk compositions in the range 0.6 < X < 1.0. Fields are plotted for (1) the paramagnetic phase (PM); the Fe2O3-rich ferrimagnetic phase (FM); (2) the FeTiO3-rich antiferromagnetic phase (AF); and (3) a re-entrant spin-glass phase (RSG). In addition, two subfields are plotted: (1) FM′, a subfield of the FM-phase, which occurs below a characteristic temperature TK, at which the magnetic susceptibility drops sharply on cooling, and (2) PM′, a subfield of the PM-phase (traditionally called superparamagnetic) forms below a sharp rise in susceptibility at TS, and exhibits measurable dispersion in the magnetic susceptibility at T < TS. The diagram is drawn with a bicritical point, Tλλ′, at X ≈ 0.87, T ≈ 39 K, which is the intersection of second-order magnetic phase boundaries for the paramagnetic → ferrimagnetic [PM(PM′) → FM] transition, TC(X), and the PM(PM′) → AF transition, TN(X). In addition, the RSG phase is plotted as one of four stable phases at Tλλ′, a construction that is not required by the phase rule, but is strongly favored by the physics of competition between the incompatible magnetically ordered structures of the FM- and AF-phases. These phase relations are at such low temperature as to be of little consequence for terrestrial magnetism, however, they may well be essential for interpreting the magnetism of the Moon, Mars, and other cold planets. These phase relations are also essential for the characterization of fine natural and synthetic intergrowths, and for understanding magnetic materials for low-temperature technological applications.

Origin of diffuse scattering in relaxor ferroelectrics

High-pressure and variable temperature single-crystal synchrotron x-ray measurements combined with first principles based molecular-dynamics simulations were used to study diffuse scattering in the relaxor ferroelectric system

Multicritical Phase Relations in Minerals

{\text{PbSc}}_{1/2}{\text{Nb}}_{1/2}{\text{O}}_{3}

Why Pb(B1/3B′2/3)O3 perovskites disorder more easily than Ba(B1/3B′2/3)O3 perovskites and the thermodynamics of 1:1-type short-range order in PMN

. Constant temperature experiments show a pressure-induced transition to the relaxor phase, in which butterfly- and rod-shaped diffuse scattering occurs around the {h00} and {hh0} Bragg spots. Simulations qualitatively reproduce the observed diffuse scattering features as well as their pressure-temperature behavior and show that they arise from polarization correlations between chemically ordered regions, which in previous simulations were shown to behave as polar nanoregions. Simulations also exhibit radial diffuse scattering [elongated toward and away from

Models for composition dependence

\mathbf{Q}=(000)

Sr2Bi2O5: A structure containing only 3-coordinated bismuth

] that persists even in the paraelectric phase; consistent with previous neutron experiments on

Structural and calorimetric studies of order-disorder in CdMg(CO3)2

{\text{PbMg}}_{1/3}{\text{Nb}}_{2/3}{\text{O}}_{3}