•Fe incorporated visible light active pyrochlore photocatalyst is synthesized.•UV–vis driven H2 generation demonstrated using Fe-Bi2Ti2O7 in methanol–water.•Fe concentration dependent red shift up to 200nm in visible light absorbance.•1wt% Fe shows 75% higher H2 yield compared to Fe-free Bi2Ti2O7.•Activity increase with 1wt% Fe is attributed to a 12.5% reduction in bandgap.

Pair distribution function (PDF) analysis is a long-established technique for studying the local structure of amorphous and disordered crystalline materials. In today's increasingly complex materials landscape, the coexistence of amorphous and crystalline phases within single samples is not uncommon. Though a couple of reports have been published studying samples with amorphous and crystalline phases utilizing PDF analysis, to date little has been done to determine the sensitivity that the method currently has in resolving such contributions. This article reports a series of experiments that have been conducted on samples with known ratios of crystalline quartz and amorphous glassy silica to examine this question in detail. Systematic methods are proposed to obtain the best possible resolution in samples with unknown phase ratios and some problems that one might encounter during analysis are discussed.

Resonant ultrasound spectroscopy has been used to characterize elastic softening and anelastic dissipation processes associated with the transition in single crystal and ceramic samples of LaAlO3. Softening of the cubic structure ahead of the transition point is not accompanied by an increase in dissipation but follows different temperature dependences for the bulk modulus, , and the shear components, and C44, as if the tilting instability contains two slightly different critical temperatures. The transition itself is marked by the complete disappearance of resonance peaks (superattenuation), which then reappear below ∼700 K in spectra from single crystals. Comparisons with low frequency, high stress data from the literature indicate that the dissipation is not due to macroscopic displacement of needle twins. An alternative mechanism, local bowing of twin walls under low dynamic stress, is postulated. Pinning of the walls with respect to this displacement process occurs below ∼350 K. Anelasticity maps, analogous to plastic deformation mechanism maps, are proposed to display dispersion relations and temperature/frequency/stress fields for different twin wall related dissipation mechanisms. These allow comparisons to be made of anelastic loss mechanisms under mechanical stress with elastic behaviour observed by means of Brillouin scattering at high frequencies which might also be related to microstructure.

Much of the power of the Resonant Ultrasound Spectroscopy (RUS) technique is the ability to make mechanical resonance measurements while the environment of the sample is changed. Temperature and magnetic field are important examples. Due to the common use of piezoelectric transducers near the sample, applied electric fields introduce complications, but many materials have technologically interesting responses to applied static and RF electric fields. Non-contact optical, buffered, or shielded transducers permit the application of charge and externally applied electric fields while making RUS measurements. For conducting samples, in vacuum, charging produces a small negative pressure in the volume of the material--a state rarely explored. At very high charges we influence the electron density near the surface so the propagation of surface waves and their resonances may give us a handle on the relationship of electron density to bond strength and elasticity. Our preliminary results indicate a charge sign dependent effect, but we are studying a number of possible other effects induced by charging. In dielectric materials, external electric fields influence the strain response, particularly in ferroelectrics. Experiments to study this connection at phase transformations are planned. The fact that many geological samples contain single crystal quartz suggests a possible use of the piezoelectric response to drive vibrations using applied RF fields. In polycrystals, averaging of strains in randomly oriented crystals implies using the "statistical residual" strain as the drive. The ability to excite vibrations in quartzite polycrystals and arenites is explored. We present results of experimental and theoretical approaches to electric field effects using RUS methods.

Inhomogeneities such as grains or plastically deformed regions in materials locally change sound wave velocities and introduce scatter in the pattern of expected resonance frequencies. The accompanying variation in the components of the free energy may promote, or block, nucleation and growth of new phases near transition temperatures. Lanthanides and martensites are two groups of materials with complex phase structures where control of the phase transformation is important in technological applications. Complex oxide minerals with a wide variety of phase structures are important in understanding the behavior of the earth’s crust. The influence on mechanical resonances of both the micro (meso) structures and the appearance and growth of phases near transition temperatures will be discussed. Experimental data on resonance behavior in inhomogeneous materials (largely lanthanides, transition metals, and silicates), where attempts have been made to control the scale and type of microstructure as they approach phase transitions will be presented. [This work receives support from DOE Grant No. DE-FC52-06NA27616 through the University of Nevada Terawatt Facility.]

Resonant ultrasound spectroscopy (RUS) is capable of determining the bulk elastic properties of a solid from its characteristic vibration frequencies, given the dimensions, density and shape of the sample. The model used for extracting values of the elastic constants assumes perfect homogeneity, which can be approximated by average–isotropic polycrystals. This approximation is excellent in the small grain regime assumed for most averaging procedures, but for real samples with indeterminate grain size distributions, it is not clear where the approximation breaks down. RUS measurements were made on pure copper samples where the grain size distribution was changed by progressive heat treatments in order to find a quantitative limit for the loss of homogeneity. It is found that when a measure of the largest grains is 15% of the sample's smallest dimension, the deviation in RUS fits indicates elastic inhomogeneity.

Octahedral tilting transitions in perovskites are usually identified by the significant lattice distortions which accompany them. The underlying mechanism of coupling between the tilts and the macroscopic strain also gives rise to large anomalies in single crystal and bulk elastic moduli. Landau theory provides an effective framework for describing these different changes in properties and relating them, quantitatively. to the evolution of the driving order parameter for the transition. This approach has been used to analyse the overall elastic behaviour of perovskites belonging to the CaTiO3-SrTiO3 (CST) solid solution, which is expected to be closely analogous to the behaviour of silicate perovskites at higher pressures and temperatures. Pm (3) over barm <-> I4/mcm and I4/mcm <-> Pnma transitions in CST perovskites are marked by changes in the shear modulus of similar to 10-30%. The evolution of the order parameter and, hence, of the octahedral tilt angles through these can be followed through the variations of spontaneous strains extracted from high resolution lattice parameter data. Contributions to the elastic softening which are due to strain/octahedral tilt coupling have been calculated using a fully parameterised Landau model of the Pm (3) over barm <-> I4/mcm transition as a function of temperature, pressure and composition. Differences between calculated elastic constants and experimental data from Dynamical Mechanical Analysis, pulse-echo ultrasonics and Resonant Ultrasound Spectroscopy suggest that a proportion of the total softening in tetragonal samples may be due to anelastic effects. The anelastic contributions are observed at frequencies of both a few Hz and 10's of MHz, and can be understood in terms of strain contributions arising from movements of transformation twin walls in response to an externally applied shear stress. Similar transitions in other perovskites are likely to display small anomalies in the bulk modulus. due to weak coupling between octahedral tilts and volume strain, but much larger anomalies in the shear modulus. The elastic properties of tetragonal and orthorhombic structures are likely to be quite different due to different anelastic contributions from twin wall displacements.

Deviations from linear elasticity in consolidated rocks have been measured by acoustic methods and by quasistatic application of stress since the early 1900s. We have used elastic neutron scattering to probe a large volume (on a grain-size scale) of a variety of consolidated rocks under dynamic and quasi-static conditions. Differences between macroscopic external strain and average atomic-scale strain were determined. Nonlinear behavior is intimately related to strain magnitudes so the microscopic distribution of strain is important. Our results indicate that a very small volume (< 5%) of the rock is responsible for almost all of the nonlinear response of the bulk rock. We discuss our results and possible future experiments.