Kyle Michel

Thermodynamic measurements of submilligram bulk samples using a membrane-based "calorimeter on a chip"

Calorimetry offers a direct measurement of thermodynamic properties of materials, including information on the energetics of phase transitions. Many materials can only be prepared in thin film or small crystal (submilligram) form, negating the use of traditional bulk techniques. The use of micromachined, membrane-based calorimeters for submilligram bulk samples is detailed here. Numerical simulations of the heat flow for this use have been performed. These simulations describe the limits to which this calorimetric technique can be applied to the realm of small crystals (1-1000 microg). Experimental results confirm the feasibility of this application over a temperature range from 2 to 300 K. Limits on sample thermal conductivity as it relates to the application of the lumped and distributed tau 2 models are explored. For a typical sample size, the simulations yield 2.5% absolute accuracy for the heat capacity of a sample with thermal conductivity as low as 2 x 10(-5) W/cm K at 20 K, assuming a strong thermal link to the device. Silver paint is used to attach (both thermally and physically) the small samples; its heat capacity and reproducibility are discussed. Measurements taken of a submilligram single crystal of cobalt oxide (CoO) compare favorably to the results of a bulk calorimetric technique on a larger sample.

Determining dilute-limit solvus boundaries in multi-component systems using defect energetics: Na in PbTe and PbS

Defect calculations are standard practice for understanding the electronic structure of dopants and alloying elements in semiconductors and insulators. However, these calculations have untapped potential to quantitatively determine thermodynamic properties of doped semiconductor systems. We present a methodology which couples defect energetics and compound formation energies to determine defect concentrations in a host material as functions of temperature and chemical equilibrium. From these defect concentrations we find the solvus boundaries of the host phase in a multi-dimensional composition space. As an example, we present first-principles calculations of the solvus boundaries of PbTe and PbS in Na–Pb–Te and Na–Pb–S. We calculate the formation energies of compounds in Na–Pb–S–Te and the defect energetics of a large number of intrinsic and Na-containing defects in PbTe and PbS. With these, we obtain equilibrium defect concentrations and solvus boundaries for PbTe and PbS. We find vacancies are the lowest-energy intrinsic defects in PbTe and PbS. We also find Na substituted for Pb is the lowest-energy Na defect in both PbTe and PbS. We find that the PbTe solvus boundary in Na–Pb–Te is a sharply peaked function of composition. We find negative defect formation energies for Na on Pb in PbS, suggesting the existence of a ternary compound in the Na–Pb–S system. The methodology presented herein is a general and straightforward way to extend the use of defect calculations from making inferences about the electronic structure of dopants to calculating solvus boundaries in multicomponent systems.

Symmetry building Monte Carlo-based crystal structure prediction

Abstract Methods are presented that allow for the automatic increase and preservation of symmetry during global optimization of crystal structures. This systematic building of symmetry allows for its incorporation into structure prediction simulations even when the space group information is not known a priori. It is shown that simulations that build and maintain symmetry converge much more rapidly to ground state crystal structures than when symmetry is ignored, allowing for the treatment of unit cells much larger than would otherwise be possible, especially when beginning from the P1 space group.