Medium and high-energy x-ray diffraction has been used to study the atomic structure of pure amorphous Si prepared by MeV Si implantation into crystalline silicon. Both as-implanted and annealed samples were studied. The inelastically scattered x rays were removed by fitting the energy spectrum for the scattered x rays. The atomic scattering factor of silicon, previously known reliably up to 20 Å Ϫ1 , has been extended to 55 Å Ϫ1. The radial distribution function of amorphous Si, before and after annealing, has been determined through an unbiased Fourier transformation of the normalized scattering data. Gaussian fits to the first neighbor peak in these functions shows that scattering data out to at least 40 Å Ϫ1 is required to reliably determine the radial distribution function. The first-shell coordination number increases from 3.79 to 3.88 upon thermal annealing at 600°C, whereas that of crystalline Si determined from similar measurements on a Si powder analyzed using the same technique is 4.0. Amorphous Si is therefore under coordinated relative to crystalline Si. Noise in the distribution function, caused by statistical variations in the scattering data at high-momentum transfer, has been reduced without affecting the experimental resolution through filtering of the interference function after subtracting the contribution of the first-neighbor peak. The difference induced by thermal annealing in the remainder of the radial distribution functions, thus revealed, is much smaller than previously believed. ͓S0163-1829͑99͒00943-1͔ I. INTRODUCTION

The structure factor S͑Q͒ of high purity amorphous Si membranes prepared by ion implantation was measured over an extended Q range (0.03 55 Å 21). Calculation of the first neighbor shell coordination (C 1) as a function of maximum Q indicates that measurement of S͑Q͒ out to at least 40 Å 21 is required to reliably determine the radial distribution function (RDF). A 2% change in C 1 and subtle changes in the rest of the RDF were observed upon annealing, consistent with point defect removal. After annealing at 600 ± C, C 1 3.88, which would explain why amorphous Si is less dense than crystalline Si.

The disorder inherent to doping by cation substitution in the complex oxides can have profound effects on collective-ordered states. Here, we demonstrate that cation-site ordering achieved through digital-synthesis techniques can dramatically enhance the antiferromagnetic ordering temperatures of manganite films. Cation-ordered (LaMnO3)m/(SrMnO3)2m superlattices show Néel temperatures (TN) that are the highest of any La(1-x)Sr(x)MnO3 compound, approximately 70 K greater than compositionally equivalent randomly doped La(1/3)Sr(2/3)MnO3. The antiferromagnetic order is A-type, consisting of in-plane double-exchange-mediated ferromagnetic sheets coupled antiferromagnetically along the out-of-plane direction. Through synchrotron X-ray scattering, we have discovered an in-plane structural modulation that reduces the charge itinerancy and hence the ordering temperature within the ferromagnetic sheets, thereby limiting TN. This modulation is mitigated and driven to long wavelengths by cation ordering, enabling the higher TN values of the superlattices. These results provide insight into how cation-site ordering can enhance cooperative behaviour in oxides through subtle structural phenomena.

Spin-density-wave antiferromagnetism of Cr in Fe/Cr(001) superlat 'c 9 P*cEIVpD SFP t g fg@

Inelastic neutron-scattering spectra were measured to obtain the phonon density of states ͑DOS͒ of nanocrystalline fcc Ni 3 Fe. The materials were prepared by mechanical alloying, and were also subjected to heat treatments to alter their crystallite sizes and internal strains. In comparison to material with large crystallites, the nanocrystalline material shows two distinct differences in its phonon DOS. The nanocrystalline DOS was more than twice as large at energies below 15 meV. This increase was approximately proportional to the density of grain boundaries in the material. Second, features in the nanocrystalline DOS are broadened substantially. This broadening did not depend in a simple way on the crystallite size of the sample, suggesting that it has a different physical origin than the enhancement in phonon DOS at energies below 15 meV. A damped harmonic oscillator model for the phonons provides a quality factor Q u , as low as 7 for phonons in the nanocrystalline material. The difference in vibrational entropy of the bulk and nanocrystalline Ni 3 Fe was small, owing to competing changes in the nanocrystalline phonon DOS at low and high energies.

Phonon density-of-states curves were obtained from inelastic neutron scattering spectra from the three crystalline phases of uranium at temperatures from 50 to 1213 K. The a-phase showed an unusually large thermal softening of phonon frequencies. Analysis of the vibrational power spectrum showed that this phonon softening originates with the softening of a harmonic solid, as opposed to vibrations in anharmonic potentials. It follows that thermal excitations of electronic states are more significant thermodynamically than are the classical volume effects. For the a-b and b-g phase transitions, vibrational and electronic entropies were comparable. DOI: 10.1103/PhysRevLett.86.3076 PACS numbers: 63.20.-e, 64.30.+t, 78.70.Nx Although first known for its unusual nuclear properties, uranium exhibits several unusual solid-state properties that may originate with electronic instabilities. The thermally induced softening of the phonon density of states (DOS) for most elements originates with anharmonicity [1,2]. For the actinides, however, a distinction between the normal anharmonic softening and harmonic softening arising from a temperature-dependent harmonic potential has been suggested [3]. In a detailed assessment of the thermodynamic data on the six crystalline phases of Pu, it was concluded that the anharmonic and electronic contributions to the equation of state could not be separated [4]. The origin of this phonon softening is a fundamental issue for the equation of state. In this Letter, we use the power spectrum of atom motions to show that the thermal softening of the phonon DOS in a-U originates with the weakening of force constants in a harmonic solid, as opposed to the typical softening in an anharmonic potential. Temperature alters the electronic structure sufficiently to change the lattice dynamics. This Letter also addresses the entropy of phonons, and by deduction the entropy of electrons, for the three low-pressure phases of crystalline uranium metal.Previous lattice dynamics studies on uranium have been performed at room temperature and below [5,6], motivated in part by the discovery of several charge density wave transitions at low temperatures [7,8]. Independently, there has been recent interest in the vibrational entropy contribution to the high temperature phase stability of metals and alloys [9][10][11], motivated by the discovery that vibrational entropy plays a larger role in phase stability than previously expected [12]. Other experimental and theoretical work has shown that electronic contributions to the entropies of high temperature phase transitions can also be significant [13][14][15].Diffraction measurements on a-U at ambient pressure have shown that the Debye temperature decreases dramatically with increasing temperature [3,16]. This softening is consistent with decreases in the elastic constants [17,18]. Specifically, the Debye temperature was expressed by u Х ͑306 2 0.158T ͒ K, where T is temperature [3]. The magnitude of this softening suggests that the Debye temperature decreases...

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Diffuse scattering around the ͑110͒ reciprocal lattice point has been investigated by elastic neutron scattering in the paraelectric and relaxor phases of the disordered complex perovskite crystal Pb(Zn 1/3 Nb 2/3 )O 3 ͑PZN͒. The appearance of a diffuse intensity peak indicates the formation of polar nanoregions at temperature T*, approximately 40 K above T c ϭ413 K. The analysis of this diffuse scattering indicates that these regions are in the shape of ellipsoids, more extended in the ͗111͘ direction than in the ͗001͘ direction. The quantitative analysis provides an estimate of the correlation length , or size of the regions, and shows that ͗111͘ ϳ1.2 ͗001͘ , consistent with the primary or dominant displacement of Pb leading to the low-temperature rhombohedral phase. Both the appearance of the polar regions at T* and the structural transition at T c are marked by kinks in the ͗111͘ curve but not in the ͗001͘ one, also indicating that the primary changes take place in a ͗111͘ direction at both temperatures.Many of the relaxor ferroelectrics known today are leadbased compounds with perovskite structure. In addition to the characteristic frequency dispersion of their dielectric constant, several of them exhibit remarkable piezoelectric or electrostrictive properties that are finding important applications, e.g., as transducers and actuators. The Pb 2ϩ -containing relaxor perovskites such as Pb(R 1/3 Nb 2/3 )O 3 (RϭMg 2ϩ , Zn 2ϩ ) have a common ABO 3 cubic perovskite structure in which the B site can be occupied by 1 3 R 2ϩ and 2 3 Nb 5ϩ . Because of the different atomic radii and valences of the B-site cation, PMN and PZN exhibit short-range chemical ordering. 1,2 Over the years, these relaxors and their properties have been described in a variety of ways, most often in terms of the formation of polar micro-or nanoregions. 3,4 However, these models are primarily based on indirect experimental evidence for such regions and more direct evidence is necessary in order to elucidate the true origin of the relaxor behavior. For this purpose, neutron and x-ray techniques are most suitable, as they can provide evidence of local structural ordering. Obtaining such evidence is crucial if the formation of polar regions is indeed responsible for the relaxor behavior.PZN is a prototype relaxor ferroelectric. It exhibits a large dielectric dispersion and a broad dielectric maximum that depend on both frequency and temperature. Earlier studies also reported a structural phase transition from cubic to rhombohedral symmetry near 413 K, 5,6 which falls in the temperature region of the maximum of the dielectric peak. More recent studies only mention the coexistence of cubic and rhombohedral phases, with polar nanodomains growing into polar microdomains 7 and their volume fraction increasing with decreasing temperature. Strain appears to play an important role in the nanodomain-to-microdomain phase transition, which therefore resembles a martensitic phase transformation. 7 In the last few years, PZN has also been intensively investiga...