Maria M. R. Costa

Structural and insulator-to-metal phase transition at 50 GPa in GdMnO 3

We present a study of the effect of very high pressure on the orthorhombic perovskite GdMnO3 by Raman spectroscopy and synchrotron x-ray diffraction up to 53.2 GPa. The experimental results yield a structural and insulator-to-metal phase transition close to 50 GPa, from an orthorhombic to a metrically cubic structure. The phase transition is of first order with a pressure hysteresis of about 6 GPa. The observed behavior under very high pressure might well be a general feature in rare-earth manganites.

Detailed Structural X-Ray Study of (Betaine Phosphate)1—x(Betaine Phosphite)x Compounds

We have determined the structure of a series of mixed compounds BP 1-x BPI x of betaine phosphate (BP) and betaine phosphite (BPI) at room temperature through single crystal X-ray diffraction measurements. In agreement with previous studies the observed symmetry for all studied concentrations was P2 1 /c. The evolution of the structure as a function of BPI content and the behaviour of the unit cell parameters a and b as a function of x are analyzed. A calculation of the electric dipolar moment of the H 3 PO 4 tetrahedron was effectuated as a function of x and its relevancy is discussed. A comparison between the BPI concentration in the solution and in the crystal was also made along the series.

Dynamic and structural properties of orthorhombic rare-earth manganites under high pressure

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 (XRD), for A = Pr, Sm, Eu, and Dy. In all cases, a phase transition was evidenced by the disappearance of the Raman signal at a critical pressure that depends on the A cation. For the compounds with A = Pr, Sm, and Dy, XRD confirms the presence of a corresponding structural transition to a noncubic phase, so that the disappearance of the Raman spectrum can be interpreted as an insulator-to-metal transition. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the ac 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 smaller A cations considered in this study (Dy and Tb), the compression is nearly isotropic in the ac plane, with only small evolutions of the tilt angles and cooperative 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 tilting of octahedra chains becomes conversely less pronounced. We finally discuss our results in light of the notion of chemical pressure and show that the analogy with hydrostatic pressure works quite well for manganites with the smaller A cations considered in this paper but can be misleading with large A cations.

Magnetic X-ray scattering from samarium

Abstract X-ray diffraction studies have been made of the antiferromagnetic ordering in the rare earth metal samarium, at the Synchrotron Radiation Source, Daresbury Laboratory. We have observed diffraction peaks which reflect the magnetic order on the hexagonal sites in the lattice and which are in agreement with previous neutron measurements. We have also observed the energy dependence of the resonant enhancement at both the LIII and LII absorption edges of Sm.

Crystal Structure of the Ferroelectric Phase of (BP)0.08(BPI)0.92

We have determined the structure of (BP) 0.08 (BPI) 0.92 both in the ferrodistortive (at 294 K) and in the ferroelectric (at 146 K) phases, by single crystal X-ray diffraction measurements. A detailed comparison between the structures revealed small but significant structural distortions, consisting almost exclusively of rotations which evidence the order-disorder nature of protons in the bonds linking neighbouring molecules in the chains and confirm the increase of the hybridization of the orbitals of the O-H valence electrons in these bonds upon cooling. These observations suggest that the proton ordering may be, if not the only contribution to the macroscopic polarization, at least the mechanism triggering the para-ferroelectric phase transition in (BP) 0.08 (BPI) 0.92 .

XAFS and Dielectric Study for Pb(Fe1/2Nb1/2)O-3-PbTiO3 Crystals

Multiferroic Pb(Fe1/2Nb1/2)O3-PbTiO3 (PFNT) system exhibits the co-operative phenomena with the coexistence of electric and magnetic orders. Through the temperature-dependent dielectric measurements, powder XRD and Raman scattering studies, combining the x-ray absorption fine-structure (XAFS) investigation, the dielectric properties and structures of Pb(Fe1/2Nb1/2)1-xTixO3 (x = 0.07 and 0.48) crystals were investigated. The frequency-dependent dielectric anomaly was revealed, which is significantly correlated to the local structure differences and the Fe2+/Fe3+ hoping. The presented results will be useful for understanding and exploring the structure/property relationship beyond this material.

Magnetic X-ray diffraction from samarium

Abstract We report X-ray scattering observations on antifferomagnetic samarium, using the NSLA synchroton source. We have made new observations of the absorption-corrected energy dependence and temperature variation of the resonantly enhanced magnetic signla from the hexagonal sites near the L III edge.

Magnetic phase diagram of the dilute Cr-Ge system

Abstract The magnetic phase diagram of the dilute Cr-Ge system has been determined using neutron diffraction at ILL. The effect of alloying on the Q vector, on the ordered magnetic moment per Cr atom and its asphericity has been determined.

L-alaninium perrhenate: crystal structure and non-linear optical properties

AbstractThe crystal structure and non-linear optical properties of L-alaninium perrhenate, C3H8NO2+ ReO4−, are reported. The protonated amino acid and the perrhenate anion have their usual geometries. The three-dimensional hydrogen-bonded network can be seen as a stacking of layers parallel to the (100) planes. Each layer is formed by chains of alternating positive and negative ions along the b and c axes. Hydrogen bonding of adjacent layers forms alternating chains along the a axis. A high damage threshold and a second-harmonic generation efficiency three times that of KDP make this new material potentially useful in non-linear optics.

Development of High Curie Temperature Ferroelectric xBi(Sc-0.5,Yb-0.5)O-3-(1-x)PbTiO3 Single Crystals

Bismuth based ferroelectrics xBi(Sc0.5,Yb0.5)O3–(1-x)PbTiO3 (BYSPT) solid-solution systems had been anticipated to hold a high Curie temperature (Tc), meeting the conditions for using under high-temperature environment. In the current research, a solid state reaction processing route had been adopted to synthesize BYSPT based ceramics, and developed BYSPT crystals by a flux approach. The as-grown BYSPT perovskite crystals exhibit a Tc of about 570°C, based on measurements of dielectric and thermal properties.