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Dive into the research topics where Nick P. Blake is active.

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Featured researches published by Nick P. Blake.


Journal of Chemical Physics | 2001

Band structures and thermoelectric properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30

Nick P. Blake; Susan E. Latturner; J. Daniel Bryan; Galen D. Stucky; Horia Metiu

Density functional calculations in the generalized gradient approximation are used to study the transport properties of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30. The band structures of these clathrates indicate that they are all semiconductors. Seebeck coefficients, conductivities and Hall coefficients are calculated, to assess the effects of carrier concentration on the quantity S2σ/τ (where S is the Seebeck coefficient, σ is the conductivity, and τ the electron relaxation time) which is proportional to the thermoelectric power factor. In each compound we find that both p- and n-doping will significantly enhance the thermoelectric capabilities of these compounds. For p-doping, the power factors of all four clathrates are of comparable magnitude and have similar temperature dependence, while for n-doping we see significant variations from compound to compound. We estimate that room-temperature ZT values of 0.5 may be possible for optimally n-doped Sr8Ga16Ge30 or Ba8In16Sn30; a...


Journal of Chemical Physics | 2001

Structure and stability of the clathrates Ba8Ga16Ge30, Sr8Ga16Ge30, Ba8Ga16Si30, and Ba8In16Sn30

Nick P. Blake; Dan Bryan; Susan E. Latturner; Lone Møllnitz; Galen D. Stucky; Horia Metiu

We use density functional calculations and single-crystal x-ray diffraction measurements to study structure and bonding in the solid state clathrates Ba8Ga16Ge30, Ba8Ga16Si30, Sr8Ga16Ge30, and Ba8In16Sn30. The structures calculated by minimizing the energy provided by the density functional theory agree well with those determined by x-ray scattering. The preferred stoichiometry is found to always have 8 group II, 16 group III, and 30 group IV elements. The resultant structures are shown to be substantially more stable than the constituent elements in their standard states at room temperature and pressure. Calculations show that the group III elements prefer to be located in the six rings of the structure and are distributed to avoid bonding to one another. Motion of the group II atom (the guest) within the cages is facile, with estimated frequencies for vibration ranging from 40 to 100 cm−1. While these results may suggest a weak guest-frame bond, we find that the binding energy is over 4 eV per guest. We...


Journal of Chemical Physics | 1999

Why clathrates are good thermoelectrics: A theoretical study of Sr8Ga16Ge30

Nick P. Blake; Lone Mo; llnitz; Georg Kresse; Horia Metiu

Recent measurements have shown that the inorganic clathrate Sr8Ga16Ge30 has good thermoelectric properties. This discovery has caused intense experimental activity to synthesize and test other compounds in this class. It has been conjectured that clathrates may be good thermoelectrics if they satisfy several conditions. The Sr atoms, trapped inside the clathrate cages, scatter phonons efficiently, leading to low thermal conductivity. Electric conductivity takes place mostly through the clathrate frame and the conduction electrons are not scattered by Sr vibrations. The compounds, being made of atoms that are semiconductors in the solid state, may have a high Seebeck coefficient. There has been no direct evidence, experimental or theoretical, for this scenario. By performing density functional calculations we show that these ideas are correct. The Sr atoms are weakly bound to the cage and do undergo large-amplitude motion. An analysis of conductivity shows that the largest contribution comes from a band in...


Journal of Applied Physics | 2002

Nonstoichiometry and chemical purity effects in thermoelectric Ba8Ga16Ge30 clathrate

J. Daniel Bryan; Nick P. Blake; Horia Metiu; Galen D. Stucky; Bo B. Iversen; Rasmus D. Poulsen; Anders Bentien

Zone melting purification experiments have been carried out on the clathrate, Ba8Ga16Ge30. The impurities present have been identified and their approximate concentrations measured. Trace impurities were determined to be approximately 240 parts per million (ppm) in the most impure sample to 17 ppm in the most pure sample. The temperature-dependent Seebeck coefficient, thermal conductivity, and electrical conductivity are reported as a function of sample purity as well as the room-temperature Hall coefficient. Microprobe analysis suggests that the samples are nonstoichiometric with excess Ge relative to Ga, and there are indications of the presence of defects. Single-crystal x-ray investigations as well as synchrotron powder diffraction measurements support the presence of defects, but the x-ray data cannot accurately determine the relative amounts of Ga and Ge. Band-structure calculations in the generalized gradient approximation show that the measured Hall and Seebeck coefficients are consistent with a d...


Journal of Chemical Physics | 2002

Effects of morphology on the electronic and transport properties of Sn-based clathrates

Lone Møllnitz; Nick P. Blake; Horia Metiu

Density-functional calculations are used to study the electronic structure and transport properties of the type-I clathrates, K8Sn46, and K8Sn44□2, (□ is a missing Sn atom), and the type-III clathrate K8Sn25. We show K8Sn44□2 to be more stable than the defect-free K8Sn46, with K8Sn46 being metallic and K8Sn44□2; semimetallic. K8Sn25 is a zintl-phase semiconductor with a band gap of 0.5 eV. It has flatter bands than the type-I clathrates and can be expected to exhibit a smaller electrical conductivity but much larger Seebeck coefficient than the almost zintl K8Sn44□2. The figure of merit of the type-III clathrate is an order of magnitude larger than its type-I counterparts. This would make a suitably doped K8Sn25 at least as good a thermoelectric as the best Ge-based type-I clathrates, and potentially better, depending on the momentum relaxation time for carriers in this material.


Journal of Chemical Physics | 1994

Absorption spectrum calculations for a system having a few quantum and many ‘‘classical’’ degrees of freedom

Nick P. Blake; Horia Metiu

We examine here several methods for calculating the photon absorption spectrum in a system consisting of an electron coupled to a large number of heavy particles. Past work has shown that calculations of the absorption cross section which treat the heavy particles classically and the electron quantum mechanically lead to qualitative errors. It has been suggested that a Gaussian wave packet description of the nuclear motion will remove these qualitative errors; moreover it will give accurate results in the case when the system has many ‘‘heavy’’ degrees of freedom or the spectrum is taken with low frequency resolution. Here we examine several implementations of this suggestion and show that it fails if a time‐dependent Hartree method is used for computing the dynamics but it works very well if one treats the electron motion adiabatically or one uses the time‐dependent Hartree equations with an adiabatic initial electronic state. Both methods lead to very efficient computational schemes in which the electro...


Journal of Chemical Physics | 1996

AN INVESTIGATION OF THE ELECTRONIC AND OPTICAL PROPERTIES OF DEHYDRATED SODALITE FULLY DOPED WITH NA

Nick P. Blake; V. I. Srdanov; Galen D. Stucky; Horia Metiu

Prolonged exposure of colorless dry sodalite to alkali vapor causes the material to gradually turn blue, dark blue, and finally black. The blue color observed at low sodium uptake appears because the absorbed sodium atoms are spontaneously ionized. The electron produced by ionization is shared by the four sodium ions present in the sodalite cage (three initially there and the fourth originating from the absorbed atom). The color center created in this way is represented by the formula (Na+)4eF3−. Here, e stands for the electron and F3− for the negatively charged frame surrounding a zeolite cage. At the highest loading, when each cage contains an absorbed alkali atom, the color centers are arranged in a body‐centered cubic lattice, allowing the electrons associated with the centers to form bands. This may explain the black color observed at high concentration. In this paper we present measurements of the absorption coefficient of the black sodalite for photon energies between 0 and 3 eV, and interpret them...


Journal of Physics A | 1989

Three-dimensional rotational averages in radiation-molecule interactions: an irreducible cartesian tensor formulation

David L. Andrews; Nick P. Blake

The authors present a new method for the calculation of the rotational averages which arise in the theory of spectroscopic radiation-molecule interactions in fluid media. Based upon the principles of irreducible cartesian tensor analysis, the method presented allows them to express results either in the usual reducible form, or directly in terms of linearly independent sets of irreducible tensor products. For interactions up to and including rank 3 in the molecular response (or non-linear susceptibility) tensor, the rotational averages cast in terms of irreducible tensor products are considerably simpler in structure than the corresponding results expressed in reducible form.


Journal of Chemical Physics | 1998

SELF-INTERACTION-CORRECTED BAND STRUCTURE CALCULATIONS FOR INTRACAVITY ELECTRONS IN ELECTRO-SODALITE

Nick P. Blake; Horia Metiu

The zeolite sodalite is a crystalline compound consisting of cages. The windows connecting the cages are large enough to allow small molecules to be absorbed. The material studied here, called sodium electro-sodalite (SES), is prepared by absorbing one Na atom in each cage. Because of the large electric fields inside the cages, each alkali atom is ionized and the donated electron is shared among several ions. A noninteracting electron model, used in earlier work, calculated the absorption spectrum and the temperature dependence of the Al and Si NMR shifts and found good agreement with experiment. The model predicted that the material is a metal. However, recently published low-temperature magnetic susceptibility measurements show that the ground electronic state is antiferromagnetic. This is incompatible with a noninteracting electron model. In this paper we study the electronic properties of this material by using various levels of spin–density functional (local density approximation, local spin–density ...


Journal of Chemical Physics | 1999

The importance of self-interaction and nonlocal exchange corrections to the density functional theory of intracavity electrons in Na-doped sodalites

Nick P. Blake; Horia Metiu

Electrons that are confined to zeolite cavities are modeled using a simplified pseudopotential scheme to represent the interaction of the electrons with both the sodalite framework and the Na+ ions. By comparing theory with recent experimental studies of G centers in Na-doped NaBr-SOD it is demonstrated that restricted forms of density functional theory, where two electrons are forced to pair in the same Kohn–Sham orbital, fail to correctly predict the true nature of the singlet, (spin unpolarized), G center. Electron confinement leads to generalized gradient corrections to the exchange of 0.74 eV and self-interaction corrections (SIC) of 0.7 eV over calculations performed in the local spin density approximation (LSDA). Only the self-interaction corrected generalized gradient approximation and the unrestricted Hartree–Fock approximation are in accord with experiment for the relative stability of the triplet (spin polarized) state. The unrestricted Hartree–Fock method is used to show that G-center absorpti...

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Horia Metiu

University of California

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Lone Møllnitz

University of California

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D. E. Cox

Brookhaven National Laboratory

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