Spin-induced symmetry breaking in orbitally ordered NiCr_2O_4 and CuCr_2O_4

At room temperature, the normal oxide spinels NiCr_2O_4 and CuCr_2O_4 are tetragonally distorted and crystallize in the I4_1/amd space group due to cooperative Jahn-Teller ordering driven by the orbital degeneracy of tetrahedral Ni$^{2+}$ ($t_2^4$) and Cu$^{2+}$ ($t_2^5$). Upon cooling, these compounds undergo magnetic ordering transitions; interactions being somewhat frustrated for NiCr_2O_4 but not for CuCr_2O_4. We employ variable-temperature high-resolution synchrotron X-ray powder diffraction to establish that at the magnetic ordering temperatures there are further structural changes, which result in both compounds distorting to an orthorhombic structure consistent with the Fddd space group. NiCr_2O_4 exhibits additional distortion, likely within the same space group, at a yet-lower transition temperature of $T$ = 30 K. The tetragonal to orthorhombic structural transition in these compounds appears to primarily involve changes in NiO_4 and CuO_4 tetrahedra

Enhanced Tetragonality in (\u3cem\u3ex\u3c/em\u3e)PbTiO\u3csub\u3e3\u3c/sub\u3e-(1-x)Bi(\u3cem\u3eB′B″\u3c/em\u3e)O\u3csub\u3e3\u3c/sub\u3e systems: Bi(Zn\u3csub\u3e3/4\u3c/sub\u3eWN\u3csub\u3e1/4\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e

Solid solutions in the (x)PbTiO3–(1−x)Bi(Zn3/4W1/4)O3 system have been examined by x-ray diffraction, dielectric measurements, and thermal analysis. Bi(Zn3/4W1/4)O3increases the tetragonality and Curie temperature of PbTiO3 which reach a value of 1.08 and 530 °C, respectively, at the limit of the single-phase perovskite forming region (x~0.8). The observation of a sustained increase in the tetragonality in this system is similar to the behavior of the (x)PbTiO3–(1−x)Bi(Zn1/2Ti1/2)O3 system and highlights the unique properties of Bi-based systems when the B sites contain high concentrations of highly polarizable cations

Predicting morphotropic phase boundary locations and transition temperatures in Pb- and Bi-based perovskite solid solutions from crystal chemical data and first-principles calculations

Using data obtained from first-principles calculations, we show that the position of the morphotropic phase boundary (MPB) and transition temperature at MPB in ferroelectric perovskite solutions can be predicted with quantitative accuracy from the properties of the constituent cations. We find that the mole fraction of PbTiO$_3$ at MPB in Pb(B$'$B$''$)O$_3$-PbTiO$_3$, BiBO$_3$-PbTiO$_3$ and Bi(B$'$B$''$)O$_3$-PbTiO$_3$ exhibits a linear dependence on the ionic size (tolerance factor) and the ionic displacements of the B-cations as found by density functional theory calculations. This dependence is due to competition between the local repulsion and A-cation displacement alignment interactions. Inclusion of first-principles displacement data also allows accurate prediction of transiton temperatures at the MPB. The obtained structure-property correlations are used to predict morphotropic phase boundaries and transition temperatures in as yet unsynthesized solid solutions.Comment: Accepted for publication in J. Appl. Phy

Potential and Impedance Imaging of Polycrystalline BiFeO\u3csub\u3e3\u3c/sub\u3e Ceramics

Electrostatic-force-sensitive scanning probe microscopy (SPM) is used to investigate grain boundary behavior in polycrystalline BiFeO3 ceramics. Scanning surface potential microscopy (SSPM) of a laterally biased sample exhibits potential drops due to resistive barriers at the grain boundaries. In this technique, the tips acts as a moving voltage probe detecting local variations of potential associated with the ohmic losses within the grains and at the grain boundaries. An approach for the quantification of grain boundary, grain interior, and contact resistivity from SSPM data is developed. Scanning impedance microscopy (SIM) is used to visualize capacitive barriers at the grain boundaries. In SIM, a dc-biased tip detects the variations of local potential induced by the lateral ac voltage applied to the sample. Unlike the traditional dc and ac transport measurement, both of these techniques are sensitive to the variation of local potential (SSPM) or local voltage oscillation amplitude and phase (SIM), rather than to current. Therefore, special attention is paid to the relationship between SSPM and SIM images and data obtained from traditional impedance spectroscopy and dc transport measurements. For BiFeO3 ceramics excellent agreement between the local SIM measurements and impedance spectroscopy data are demonstrated

Structure and dielectric response in the high TcT_c ferroelectric Bi(Zn,Ti)O3_3-PbTiO3_3 solid solutions

Theoretical {\em ab initio} and experimental methods were used to investigate the $x$Bi(Zn,Ti)O$_3$-(1-$x$)PbTiO$_3$ (BZT-PT) solid solution. We find that hybridization between Zn 4$p$ and O 2$p$ orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to large polarization, strong tetragonality and an elevated ferroelectric to paraelectric phase transition temperature. nhomogeneities in local structure near the 90$^\circ$ domain boundaries can be deduced from the asymetric peak broadening in the neutron and x-ray diffraction spectra. These extrinsic effects make the ferroelectric to paraelectric phase transition diffuse in BZT-PT solid solutions

Structure and Polarization in the High T\u3csub\u3ec\u3c/sub\u3e Ferroelectric Bi(Zn,Ti)O\u3csub\u3e3\u3c/sub\u3e-PbTiO\u3csub\u3e3\u3c/sub\u3e Solid Solutions

Theoretical ab initio and experimental methods are used to investigate the [Bi(Zn1/2Ti1/2)O3]x [PbTiO3]1-x solid solution. We find that hybridization between Zn 4s and 4p and O 2p orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to unusually large polarization, strong tetragonality, and an elevated ferroelectric to paraelectric phase transition temperature

Anisotropic atom displacement in Pd nanocubes resolved by molecular dynamics simulations supported by x-ray diffraction imaging

Nearly identical Pd nanocubes yield an x-ray powder diffraction pattern with interference fringes affording access to unprecedented structural details of nanocrystal size, shape, and complex atomic displacement for a billion-sized population. The excellent agreement between diffraction data and molecular dynamics (MD) provides strong experimental validation of MD simulations and the proposed data-interpretation paradigm. These results show that individual atomic displacements within the nanocubes are not only a function of disrupted bonds and the crystallographic plane of the adjacent surface, but are complex strain gradients extending across all surfaces of the particle strongly influenced by atomic displacements. This observation of nonuniform surface strain and the manner in which it is affected by different sizes, shapes, and locations within each facet could be the key to understanding many surface related properties of shaped nanocrystals including those associated with important catalysis applications