Local Symmetry Breaking in the High Temperature Regime of SnTe

The term emphanisis [1] has been coined to define the appearance of local off-centering displacements of ions from a high-symmetry ground state on warming, as recently discovered in PbTe [2]. Such a phenomenon is unusual because, in the canonical view of structural transformations, a low-symmetry ground state evolves into a higher symmetry state on warming. Although it is not uncommon for remnants of a low-symmetry phase to appear as spatial fluctuations at high temperature, the emergence of a locally broken symmetry state from a high symmetry ground state is quite rare. Emphanisis may be behind some long-known, but poorly understood anomalies seen in the lead chalcogenides. However, the origin and nature of emphanisis are still the subject of controversy. Several explanations for emphanisis have been suggested, including a simple response to an underlying anharmonic potential [3], a dynamic ferroelectric-like off-centering [2], and a temperature-dependent competition between ionicity and covalency [1], but an understanding remains elusive. In this talk I will report on atomic pair distribution function (PDF) measurements of the lead-free compound SnTe, which is isostructural to PbTe at high T but with a ferroelectric phase below Tc ~ 100K. Our data show that SnTe also exhibits an emphanitic response, but with an onset temperature well above Tc and a symmetry that is distinct from that of the ferroelectric phase. Taken together these results suggests that the emphanitic and ferroelectric responses are quite distinct.

Recrystallization behavior of amorphous and crystalline lactose from TSPDF

Lactose is a disaccharide sugar of galactose and glucose that is most commonly associated with milk. Its importance to the food and animal product industry cannot be overstated, as it is involved in aspects as diverse as baking, confectionary, and infant products. The other major use of lactose is in the pharmaceutical industry. The mildly sweet and loosely bland flavor of lactose has lent to its use as a stabilizer and excipient in pharmaceutical products. Despite the wide range of applications for this material, there is still much to be studied. While it is known to exist in several crystalline forms and as two anomers, α and β, characterized by the flipping of a hydroxide group on the glucose ring, the amorphous form of lactose is less understood. Yet it is this amorphous form which may play a crucial role in the physicochemical stability of amorphous drug dispersions. In order to fully understand the structural changes which lactose undergoes when converted from a crystalline to amorphous material, total scattering experiments coupled with PDF analysis for structural identification of amorphous lactose of different origins were undertaken with the goal of understanding the recrystallization behavior of this versatile material. Samples measured included commercial forms of lactose, crystalline and amorphous, and amorphous forms obtained by melt quenching and lyophilization. Recrystallization was followed for the amorphous forms by measuring characteristic samples aged at 40 ⁰C/75% RH, which is a standard condition for stressing pharmaceutical materials to extrapolate shelf-life. By fitting the PDF curves to a structural model of lactose, and refining with the characteristic function for a sphere of radius r, an estimate of the coherence length of atom-atom correlations for a give sample provides a measure of the growth progression, from single molecule to crystallite for the lactose samples. Coupled with data from NMR spectroscopy, TSPDF analysis is teasing out the nuances of the recrystallization behavior of lactose.

Size-dependent non-space filling atomic packing in metallic nanoparticles

Ni-Pd nanoparticles synthesized for CO catalysis are characterized by transmission electron microscopy and total X-ray scattering. The sizes of these nanoparticles can be tuned to size with great control over the monodispersity of the samples. The pair distribution functions of the reveal a local ordering within the highly disordered atomic structure within the nanoparticles. The PDFs show a size-dependent deviation from typical bulk face centered cubic (fcc) structure for these materials. The long-range isotropic disorder within these non-fcc nanoparticles can be fitted using an exponentially damped single-mode sine wave. Below a diameter of 5 nm, the Ni-Pd nanoparticles exhibit local ordering of atoms as found in typical icosahedral clusters. The transition from fcc to non-space filling atomic packing of icosahedral clusters in a nanoparticle is modeled to show the structural origin of the observed PDFs. Understanding this type of disorder can give insight into structure-property relations for applications in heterogeneous catalysis.

Formation of γ-Fe2O3 in hydrothermal synthesis: In situ total scattering studies

The properties of metal oxide nanoparticles are highly dependent on particle characteristics such as size, crystallinity, and structural defects. To obtain particles with tailormade properties, it is crucial to understand the mechanisms that govern these characteristics during material synthesis. For this purpose, in situ studies of particle synthesis have proven powerful.[1] Here, in situ Total Scattering (TS) combined with in situ PXRD studies of the hydrothermal synthesis of γ-Fe2O3 (maghemite) from ammonium iron citrate will be presented. In situ TS with Pair Distribution Function (PDF) analysis has recently shown to be an efficient tool for understanding the fundamental chemical processes in particle crystallization.[2,3] The full γ-Fe2O3 crystallization process from ionic complexes over nanoclusters to crystalline particles is followed and material formation mechanisms are suggested. The study shows that the local atomic structure of the precursor solution is similar to that of the crystalline coordination polymer [Fe(H2citrate)(H2O)]n where corner sharing [FeO6] octahedra are linked by citrate. As hydrothermal treatment of the solution is initiated, clusters of edge sharing [FeO6] units form. Tetrahedrally coordinated iron subsequently appears in the structure and as the synthesis continues, the clusters slowly assemble into nanocrystalline maghemite. The primary transformation from amorphous clusters to nanocrystallites takes place by condensation of the large clusters along corner sharing tetrahedral iron units. The crystallization process is related to large changes in the local structure as the interatomic distances in the clusters change dramatically with cluster growth. The local atomic structure is size dependent, and particles below 6 nm are highly disordered. Whole Powder Pattern Modelling of the PXRD data shows that the final crystallite size (<10 nm) is dependent on synthesis temperature and that the size distribution of the particles broadens with synthesis time.

Nanoscale phase coexistence at the metal-insulator transition in Cu(Ir_1-xCr_x)₂S₄

Increasingly, nanoscale phase coexistence and hidden broken symmetry states are being found in the vicinity of metal-insulator transitions (MIT), for example, in high temperature superconductors, heavy fermion and colossal magnetoresistive materials, but their importance and possible role in the MIT and related emergent behaviors is not understood. Despite their ubiquity, they are hard to study because they produce weak diffuse signals in most measurements. Here we propose Cu(Ir1-xCrx)2S4 as a model system for studying nanoscale phase coexistence at the MIT, where robust local structural signals lead to key new insights. We demonstrate by x-ray scattering measurements and atomic pair distribution function approach a hitherto unobserved coexistence of a Ir4+ charge-localized dimer phase and Cr-ferromagnetism. The resulting phase diagram that takes into account the short range dimer order, is highly reminiscent of a generic MIT phase diagram similar to the cuprates. The results represent the first observation of nanoscale phase coexistence in iridates [1]. We suggest that the presence of quenched strain from dopant ions acts as an arbiter deciding between the competing ground states.

Nanostructure studies using complex modeling method

Accurate characterization of nanostructures is an open difficult problem. Nanoparticle sizes are too small to form a sizable periodic order in atom positions that would give rise to resolved Bragg peaks. A significant number of atoms lie at the surface where their positions can relax and distort. Experimental data from nanoparticles are thus in general more noisy and less signal-rich than from crystals, but their structures have less symmetry and require more variables for their characterization. To overcome these difficulties, we have developed Complex Modeling (CM) approach, which combines multiple experimental and theoretical probes in a common structure optimization routine. The Complex Modeling method is based on the DiffPy software library of forward calculators for pair distribution function, bond valence sums, hard-sphere atom overlaps and third-party routines for small angle scattering simulations and structure restraints for bond lengths and bond angles. The top-level SrFit framework allows flexible setup of optimization schemes by tying available experimental inputs and theoretical and chemical constraints or restraints on the structure. The presentation will provide several examples of Complex Modeling studies of concrete nanostructure systems, such as determination of atomic structure and shape of CdSe quantum dots from PDF and SAXS, structure determination of Au nanoparticle from PDF data and surface optimization, and determination of molecule packing in organic crystals from PDF and chemical constraints.

Magnetic pair distribution function analysis: introduction and applications

Short-range magnetic correlations play a crucial role in a variety of condensed matter phenomena, yet they remain notoriously difficult to investigate experimentally. Quantitative analysis of the diffuse scattering of neutrons from local magnetic correlations represents a viable but challenging route toward revealing short-range magnetic order in complex materials. Reverse Monte Carlo techniques that iteratively fit randomly generated structural models in momentum space have been used successfully [1], demonstrating that diffuse magnetic scattering can be rich in information. Recently [2], we developed a real-space approach to investigating local magnetic correlations, which we call magnetic pair distribution function (mPDF) analysis in analogy to the more familiar atomic pair distribution function. This experimentally accessible quantity reveals magnetic correlations directly in real space and places diffuse and Bragg scattering on equal footing, thereby gaining sensitivity to both short- and long-range magnetic order. Here we present the basic theory behind mPDF analysis and provide several examples of its utility using both simulated and experimentally measured data on several interesting magnetic systems, including a canonical antiferromagnetic, a spin glass, and a spin ice. We discuss the potential impact that mPDF methods may have on current and future research interests in magnetism.