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Bi2Te crystallizes in the tetragonal I-42d space group. The structure is three-dimensional. Bi1+ is bonded in a bent 150 degrees geometry to two equivalent Te2- atoms. Both Bi–Te bond lengths are 3.17 Å. Te2- is bonded in a 4-coordinate geometry to four equivalent Bi1+ atoms.
In the recent past, MnTe has proven to be a crucial component of the intrinsic magnetic topological insulator (IMTI) family [MnTe] m [Bi2Te 3 ] n , which hosts a wide range of magneto-topological properties depending on the choice of m and n. However, bulk crystal growth allows only a few combinations of m and n for these IMTIs due to the strict limitations of the thermodynamic growth conditions. One way to overcome this challenge is to utilize atomic layer-by-layer molecular beam epitaxy (MBE) technique, which allows arbitrary sequences of [MnTe]m and [Bi 2 Te 3 ] n to be formed beyond the thermodynamic limit. For such MBE growth, finding optimal growth templates and conditions for the parent building block, MnTe, is a key requirement. Here, we report that two different hexagonal phases of MnTe - nickeline (NC) and zinc-blende/wurtzite (ZB-WZ) structures, with distinct in-plane lattice constants of 4.20 ± 0.04 Å and 4.39 ± 0.04 Å, respectively - can be selectively grown on c-plane Al 2 O 3 substrates using different buffer layers and growth temperatures. Moreover, we provide the first comparative studies of different MnTe phases using atomic-resolution scanning transmission electron microscopy and show that ZB and WZ-like stacking sequences can easily alternate between the two. Surprisingly, In 2 Se 3 buffer layer, despite its lattice constant (4.02 Å) being closer to that of the NC phase, fosters the ZB-WZ instead, whereas Bi 2 Te 3 , sharing the same lattice constant (4.39 Å) with the ZB-WZ phase, fosters the NC phase. Furthermore, these discoveries suggest that lattice matching is not always the most critical factor determining the preferred phase during epitaxial growth. Overall, this will deepen our understanding of epitaxial growth modes for chalcogenide materials and accelerate progress toward new IMTI phases as well as other magneto-topological applications.
Bi2Te(IO3)O5Cl crystallizes in the monoclinic Cc space group. The structure is three-dimensional. there are four inequivalent Bi3+ sites. In the first Bi3+ site, Bi3+ is bonded in a 6-coordinate geometry to six O2- and one Cl1- atom. There are a spread of Bi–O bond distances ranging from 2.28–2.54 Å. The Bi–Cl bond length is 3.09 Å. In the second Bi3+ site, Bi3+ is bonded in a 7-coordinate geometry to six O2- and one Cl1- atom. There are a spread of Bi–O bond distances ranging from 2.29–2.66 Å. The Bi–Cl bond length is 2.89 Å. In the third Bi3+ site, Bi3+ is bonded in a 7-coordinate geometry to six O2- and two equivalent Cl1- atoms. There are a spread of Bi–O bond distances ranging from 2.28–2.83 Å. There are one shorter (2.80 Å) and one longer (3.39 Å) Bi–Cl bond lengths. In the fourth Bi3+ site, Bi3+ is bonded in a 7-coordinate geometry to seven O2- and one Cl1- atom. There are a spread of Bi–O bond distances ranging from 2.25–2.76 Å. The Bi–Cl bond length is 3.21 Å. There are two inequivalent Te6+ sites. In the first Te6+ site, Te6+ is bonded to six O2- atoms to form edge-sharing TeO6 octahedra. There are a spread of Te–O bond distances ranging from 1.92–2.03 Å. In the second Te6+ site, Te6+ is bonded to six O2- atoms to form edge-sharing TeO6 octahedra. There are a spread of Te–O bond distances ranging from 1.92–2.04 Å. There are sixteen inequivalent O2- sites. In the first O2- site, O2- is bonded in a 2-coordinate geometry to two Bi3+ and one Te6+ atom. In the second O2- site, O2- is bonded in a 3-coordinate geometry to two Bi3+ and one Te6+ atom. In the third O2- site, O2- is bonded in a 3-coordinate geometry to two Bi3+ and one Te6+ atom. In the fourth O2- site, O2- is bonded in a 3-coordinate geometry to two Bi3+ and one Te6+ atom. In the fifth O2- site, O2- is bonded in a 3-coordinate geometry to two Bi3+ and one Te6+ atom. In the sixth O2- site, O2- is bonded in a 3-coordinate geometry to two Bi3+ and one Te6+ atom. In the seventh O2- site, O2- is bonded in a 3-coordinate geometry to two Bi3+ and one Te6+ atom. In the eighth O2- site, O2- is bonded in a 3-coordinate geometry to two Bi3+ and one Te6+ atom. In the ninth O2- site, O2- is bonded in a 2-coordinate geometry to one Bi3+ and two Te6+ atoms. In the tenth O2- site, O2- is bonded in a distorted water-like geometry to one Bi3+ and two Te6+ atoms. In the eleventh O2- site, O2- is bonded in a bent 150 degrees geometry to one Bi3+ and one I5+ atom. The O–I bond length is 1.84 Å. In the twelfth O2- site, O2- is bonded in a bent 120 degrees geometry to one Bi3+ and one I5+ atom. The O–I bond length is 1.87 Å. In the thirteenth O2- site, O2- is bonded in a 1-coordinate geometry to one Bi3+ and two I5+ atoms. There are one shorter (1.84 Å) and one longer (2.62 Å) O–I bond lengths. In the fourteenth O2- site, O2- is bonded in a distorted single-bond geometry to two Bi3+ and one I5+ atom. The O–I bond length is 1.84 Å. In the fifteenth O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Bi3+ and one I5+ atom. The O–I bond length is 1.84 Å. In the sixteenth O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Bi3+ and one I5+ atom. The O–I bond length is 1.82 Å. There are two inequivalent I5+ sites. In the first I5+ site, I5+ is bonded in a 3-coordinate geometry to three O2- and one Cl1- atom. The I–Cl bond length is 3.01 Å. In the second I5+ site, I5+ is bonded in a 3-coordinate geometry to four O2- atoms. There are two inequivalent Cl1- sites. In the first Cl1- site, Cl1- is bonded in a distorted single-bond geometry to three Bi3+ atoms. In the second Cl1- site, Cl1- is bonded in a 1-coordinate geometry to two Bi3+ and one I5+ atom.
The steady-state heat-pumping capabilities of thermoelectric devices are examined for spacecraft in earth orbit, using a simple model. Thermoelectric parameters for a standard Bi2Te, alloy are fitted with fourth-order polynomials in temperature. Parameters are used which are the average for the temperature difference across the p-n couple. Internal spacecraft temperatures are varied from 270 to 330 K, and absorbed incident radiation fluxes are varied from zero to 700 W per sq m. It is found that heat rejection can be optimized with respect to thermoelectric device and radiator fin geometries, as well as with respect to input electrical power and that these devices are most useful on the surfaces with highest incident radiant power. Maximum heat fluxes which can be pumped out of the spacecraft are on the order of 50-160 W per sq m for the various thermal environments examined. Input electrical powers corresponding to these maximum heat fluxes range from about 50-300 W per sq m. However, efficiency is greatly improved by operation at lower input-power levels.
Advanced thermoelectric microdevices integrated into thermal management packages and low power, electrical source systems are of interest for a variety of space and terrestrial applications. By shrinking the size of the thermoelements, or legs, of these devices, it becomes possible to handle much higher heat fluxes, as well as operate at much lower currents and higher voltages that are more compatible with electronic components. The miniaturization of state-of-the-art thermoelectric module technology based on Bi2Te3 alloys is limited due to mechanical and manufacturing constraints for both leg dimensions (100-200 gm thick minimum) and the number of legs (100-200 legs maximum). We are investigating the development of novel microdevices combining high thermal conductivity substrate materials such as diamond, thin film metallization and patterning technology, and electrochemical deposition of thick thermoelectric films. It is anticipated that thermoelectric microcoolers with thousands of thermocouples and capable of pumping more than 200 W/sq cm over a 30 to 60 K temperature difference can be fabricated. In this paper, we report on our progress in developing an electrochemical deposition process for obtaining 10-50 microns thick films of Bi2Te3 and its solid solutions. Results presented here indicate that good quality n-type Bi2Te3, n-type Bi2Te(2.95)Se(0.05) and p-type Bi(0.5)Sb(1.5)Te3 thick films can be deposited by this technique. Some details about the fabrication of the miniature thermoelements are also described.