Structural evolution in thermoelectric zinc antimonide thin films studied by in situ X-ray scattering techniques.

Zinc antimonides have been widely studied owing to their outstanding thermoelectric properties. Unlike in the bulk state, where various structurally unknown phases have been identified through their specific physical properties, a number of intermediate phases in the thin-film state remain largely unexplored. Here, in situ X-ray diffraction and X-ray total scattering are combined with in situ measurement of electrical resistivity to monitor the crystallization process of as-deposited amorphous Zn-Sb films during post-deposition annealing. The as-deposited Zn-Sb films undergo a structural evolution from an amorphous phase to an intermediate crystalline phase and finally the ZnSb phase during heat treatment up to 573 K. An intermediate phase (phase B) is identified to be a modified ß-Zn8Sb7 phase by refinement of the X-ray diffraction data. Within a certain range of Sb content (∼42-55 at%) in the films, phase B is accompanied by an emerging Sb impurity phase. Lower Sb content leads to smaller amounts of Sb impurity and the formation of phase B at lower temperatures, and phase B is stable at room temperature if the annealing temperature is controlled. Pair distribution function analysis of the amorphous phase shows local ordered units of distorted ZnSb4 tetrahedra, and annealing leads to long-range ordering of these units to form the intermediate phase. A higher formation energy is required when the intermediate phase evolves into the ZnSb phase with a significantly more regular arrangement of ZnSb4 tetrahedra.

In situ powder X-ray diffraction study of the hydro-thermal formation of LiMn2O4 nanocrystallites.

In situ measurements of the hydrothermal formation of LiMn2O4 (LMO) nanocrystallites reveal that the reaction progresses in steps, each creating a different crystalline phase. The reaction route is summarized as KMnO4→disordered δ-MnO2→(ordered δ-MnO2)→LiMn2O4→(γ-Mn2O3)→Mn3O4. The phase purity of LMO can be controlled by reaction time and temperature where phase pure LMO is obtained after 150-210 seconds at 220 °C or 45-140 seconds at 260 °C. It is also concluded that production of phase pure LMO by this method comes at the price of reduced reaction yield. From the observed reaction route an alternative way to control the phase purity is proposed by changing the amount of reducing agent. This hypothesis is rejected by a set of in situ measurements showing that the reaction kinetics of subsequent reaction steps hinders the formation of phase pure LMO. From the observation of unit cell changes as function of the transformation from LMO to Mn3O4 three distinct reaction parts are observed. This indicates that the reaction is a solid-solid reaction with a phase boundary. The in situ measurements reveal that LMO first appears in the reaction solution as thin platelets with sizes ranging from 3-13 nm. As the reaction progresses the crystallites grow faster along the [111] direction giving rod-like shaped crystallites in the end. The LMO crystallites start off with the same shape at all temperatures investigated indicating that they form from δ-MnO2 crystallites.

Understanding the formation and evolution of ceria nanoparticles under hydrothermal conditions.

Supercritical growth: The formation and evolution of ceria nanoparticles during hydrothermal synthesis was investigated by in situ total scattering and powder diffraction. The nucleation of pristine crystalline ceria nanoparticles originated from previously unknown cerium dimer complexes. The nanoparticle growth was highly accelerated under supercritical conditions.

Watching nanoparticles form: an in situ (small-/wide-angle X-ray scattering/total scattering) study of the growth of yttria-stabilised zirconia in supercritical fluids.

Understanding nanoparticle-formation reactions requires multi-technique in situ characterisation, since no single characterisation technique provides adequate information. Here, the first combined small-angle X-ray scattering (SAXS)/wide-angle X-ray scattering (WAXS)/total-scattering study of nanoparticle formation is presented. We report on the formation and growth of yttria-stabilised zirconia (YSZ) under the extreme conditions of supercritical methanol for particles with Y(2)O(3) equivalent molar fractions of 0, 4, 8, 12 and 25 %. Simultaneous in situ SAXS and WAXS reveals a quick formation (seconds) of sub-nanometre amorphous material forming larger agglomerates with subsequent slow crystallisation (minutes) into nanocrystallites. The amount of yttria dopant is shown to strongly affect the crystallite size and unit-cell dimensions. At yttria-doping levels larger than 8 %, which is known to be the stoichiometry with maximum ionic conductivity, the strain on the crystal lattice is significantly increased. Time-resolved nanoparticle size distributions are calculated based on whole-powder-pattern modelling of the WAXS data, which reveals that concurrent with increasing average particle sizes, a broadening of the particle-size distributions occur. In situ total scattering provides structural insight into the sub-nanometre amorphous phase prior to crystallite growth, and the data reveal an atomic rearrangement from six-coordinated zirconium atoms in the initial amorphous clusters to eight-coordinated zirconia atoms in stable crystallites. Representative samples prepared ex situ and investigated by transmission electron microscopy confirm a transformation from an amorphous material to crystalline nanoparticles upon increased synthesis duration.