R. E. Alonso

Solids under Pressure. Ab Initio Theory

(a) Institute of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark(b) Department of Physics and Astronomy, Northwestern University, Evanston,IL 60208, USA(c) Departamento de Fisica, Universidad Nacional de La Plata, La Plata 1900, Argentina(d) Instituto de Fisica de Liquidos y Sistemas Biolo´gicos, Grupo Fisica del So´lido,Casilla de Correo 565, La Plata 1900, Argentina(Received August 14, 1998)Parameter-free calculations based on the density-functional theory are used to examine high-pres-sure phases of solids, mainly semiconductors. For the elemental semiconductors, as represented bySi, the diamond!b-tin!Imma sequence is examined, and for III–V semiconductors the optimi-zation of the structural parameters of the Cmcm and Imm2 phases is described. The structuralenergy differences are in several cases very small, and in some too small to allow a safe structureprediction on the basis of the calculations. In that context we also discuss ways to go beyond thelocal density approximation (LDA). We show that the predicted high-pressure phases may be sig-nificantly affected by inclusion of (generalized) gradient corrections (GGA). Elemental Zn (h.c.p.)is further taken as an example where we find that the simple LDA leads to poor results.

Temperature dependence of the nuclear quadrupole interaction at Zr-Ti sites in in the Zr-rich rhombohedral and cubic phases

In this paper, the hyperfine quadrupole interaction at the Zr-Ti site of (PZT) polycrystalline samples is studied for the first time. Powders for x = 0, 0.1, 0.2, 0.4, 0.6, 0.8, 0.9 and 1 were prepared and characterized by XRD. The lattice constants and space groups were obtained as functions of Ti concentration. PAC analyses were done for ferroelectric PZT for x = 0.9 and 0.8 as a function of temperature, and for x = 0.6 at RT. Also antiferroelectric was analysed. Low-concentration nuclei were used as probes. In the ferroelectric and paraelectric phases of PZT compounds two sites were occupied by probes. For each site the quadrupole frequency, asymmetry and relative distribution width parameters were obtained as functions of temperature above and below the Curie temperature. For only one site was detected with a static quadrupole interaction below and a nuclear spin-relaxation process above it. Furthermore, the temperature dependence of the hyperfine parameters, the possible origin of both quadrupole interactions in PZT and the perturbation that characterizes the cubic phases of and compounds are discussed.

Electron bonds and hyperfine electric field gradient in the Sr1-xBaxHfO3 family

The study by x-ray diffraction and perturbed angular correlation spectroscopy of Sr1?xBaxHfO3 compounds for x = 0.12, 0.25, 0.5, 0.75 and 0.88 is presented. The hyperfine parameters are analysed in terms of ionic radii, ionic or covalent bonds, and cation?oxygen distances. A simple explanation of the behaviour of these parameters with composition is provided.

Hyperfine Interactions in the Cubic Phase of BaTi x Hf 1− x O 3

The hyperfine quadrupole interaction at probes in sites with cubic point group symmetry was measured in many perovskite-type compounds. This interaction is commonly associated to the presence of oxygen vacancies close to probes. The effect of this point defect on 181 Ta probe in BaTi x Hf 1 m x O 3 for x = 0.70, 0.50, 0.30, 0.10, 0.05, and 0.01, is studied. The lattice constant of these oxides at room temperature were measured using XRD technique. The quadrupole parameters corresponding to probe-defect interaction were obtained by means of Perturbed Angular Correlation (PAC) spectroscopy as functions of both, composition and temperature. A static, asymmetric and distributed quadrupole interaction was fitted to all PAC data. This interaction was observed to be strongly composition dependent. These results and those corresponding to compositions 0.75 h x < 1 were compared to the ones obtained using the point charge method for the calculation of the electric field gradient. The proposed model took into account polarized oxygen vacancies, different covalence of Ti--O and Hf--O bonds and using computational simulation for cations and oxygen vacancies lattice positions.

Phase transitions in the SrHfO3 perovskite measured with 111In/111Cd PAC

The temperature dependence of the hyperfine parameters in SrHfO3 powder samples has been investigated by means of Perturbed Angular Correlation spectroscopy using implanted 111In probes. Three quadrupole interactions have been established, with the largest fraction showing a pronounced dynamic interaction. We assign this fraction to 111In / 111Cd probe atoms on substitutional Hf sites. The temperature dependence of the dynamic interaction has been associated to the Pnma↔Imma phase transition at ∼700 K. We discuss the results in relation to those obtained for 181Hf / 111Ta-probes in AHfO3 (A=Ba, Sr, Ca) and for 111In / 111Cd-probes in PbZrO3 and BaTiO3.

The orthorhombic to tetragonal phase transition in Bi2-xTexSrNb2-xHfxO9

The necessity of producing materials with better performances than those observed in ferroelectric perovskites has induced the investigation of new oxides, especially those belonging to the Aurivillius family which exhibit better performance than PZT in non-volatile ferroelectric memories (FeRAM). In this contribution a Raman analysis of Bi2-xTexSrNb2-xHfxO9 (x = 0.10) is carried out to investigate the vibrational spectra and the ferro-paraelectric phase transition of these compounds.

Tetragonal to Cubic Phase Transition in Sr 0.75 Ba 0.25 HfO 3

Using powder X-Ray Diffraction and Perturbed-Angular-Correlation spectroscopy, the crystal structure and the hyperfine interaction at 181 Ta probe in Sr 0.75 Ba 0.25 HfO 3 have been studied. A tetragonal (I4/mcm) to cubic (Pm &3macr;m) phase transition was observed. A two-phase field region, where the tetragonal and cubic phases coexist, was found between 673 and 923 K. The spin precession curves were measured from room temperature up to 1373 K. A static, asymmetric and disordered electric field gradient model was used to fit these time spectra. The hyperfine parameters corresponding to the tetragonal structure changed as the temperature increases: y T Q diminishes, m T is almost constant and i T diminishes. This interaction was observed to be unique up to 673 K. Above this temperature, a second interaction appeared and these two phases coexist up to 923K. Above this temperature, the hyperfine parameters corresponding to the cubic structure was only observed. The tetragonal to cubic transformation is characterized by an activation energy of about 0.62 eV.

Hyperfine interactions in cubic perovskites

ABO3 perovskites display several physical properties determined by the characteristics of A and B cations. These compounds have cubic structure at high temperature. Lower symmetry cells that are distorted cubes are found at low temperature. Defects modify the properties of these compounds. Under standard conditions oxygen vacancies are produced. Cation substitution also alters the characteristics of perovskites. These materials have been studied by Perturbed Angular Correlation (PAC) spectroscopy and other hyperfine techniques. In this way abundant information is available to determine charge distributions close to probes. In the cubic phase perturbations were detected that are produced by the interaction of probes with defects. To show up these effects we analyze the quadrupole interaction at 181Ta in several compounds: ABO3 with A=Ca, Sr and Ba, BaTi1−xHfxO3 and PbZr1−xTixO3 for 0⩽x⩽1. Three different quadrupole interactions were found and are interpreted in terms of distinct probe-oxygen vacancy configurations.

Hyperfine Interactions in the Sr0.88Ba0.12HfO3 Compound

The temperature dependence of the hyperfine parameters in a powder sample of the Sr0.88Ba0.12HfO3 compound has been investigated for the first time using perturbed angular correlation spectroscopy. The time spectra were measured as a function of the temperature from 293 to 1273 K. Activated 181Ta nuclei were used as hyperfine probes at the Hf sites. The analysis of the time spectra indicates the presence of two different surroundings for the Ta probes. The most populated site (f1≈75% at laboratory temperature) was fitted with the usual static quadrupole hyperfine interactions found in most perovskite-type compounds. The resulting quadrupole frequency is ωQ≈24 Mrad/s at this temperature. This frequency continuously decreases to nearly 9 Mrad/s at 1273 K. Its line width temperature dependence displays three regions and the changes observed probably correspond to phase transitions. The other site has a large and temperature independent quadrupole frequency ωQ≈120 Mrad/s that reversibly transforms into the first at high temperatures and probably originates from some defect in the structure.

The stability of PbZr0.52Ti0.48O3 prepared by the sol-gel method

Samples of PbZr 0.52 Ti 0.48 O 3 (PZT) were fabricated by the Sol-Gel method and characterized by X-ray diffraction analysis. The Rietveld method was used to obtain lattice parameters and grain sizes as a function of the annealing temperature. A powder sample previously annealed at 873 K was irradiated to activate the 181 Ta probe. Then, Perturbed Angular Correlation (PAC) spectra were measured at RT and temperatures above 873 K to study the thermal behaviour on samples capsulated in SiO 2 tubes. The hyperfine parameters observed as a function of temperature show changes in this PZT compound that would correspond to thermal evolution of defects. Furthermore, the reaction of PZT with the substrate was observed at 1173 K, obtaining at this temperature the hyperfine parameters corresponding to SiZrO 4 . At this temperature no trace of the PZT was left.