Thermoelectric properties and crystallographic shear structures in titanium oxides of the Magnèli phases

Abstract

The thermoelectric properties of Magnèli phase titanium oxides Ti n O 2n−1 ͑n =2,3,...͒ have been investigated, paying special attention to how the thermoelectric performance can be altered by changing the microstructure. Dense polycrystalline specimens with nominal composition of TiO 2−x ͑x = 0.05, 0.10, 0.15, and 0.20͒ prepared by conventional hot-pressing are all identified to be one of the Magnèli phases, in which crystallographic shear planes are regularly introduced according to the oxygen deficiency. Electrical conduction is n-type for all specimens and the carrier concentration increases with the increase in the oxygen deficiency. The values of lattice thermal conductivity, on the other hand, decrease with the increase in the oxygen deficiency, which can be attributed to phonon scattering at the crystallographic shear plane. The largest value of thermoelectric figure of merit Z, 1.6ϫ 10 −4 K −1 was obtained at 773 K for the hot-pressed specimen of TiO 1.90 .

Assessing the Thermoelectric Properties of Sintered Compounds via High-ThroughputAb-InitioCalculations

“…The thermoelectric properties of micrograined polycrystalline titanium oxides have been measured, giving P $ 2 W=cmK 2 [55]. Using the value of bulk e estimated above, our calculations for TiO 2 yield P values in the same order of magnitude.…”

Possible “Magnéli” Phases and Self-Alloying in the Superconducting Sulfur Hydride

“…In a wide variety of materials such as metal oxides, multiple crystalline phases with slightly different stoichiometries emerge depending on the bonding between the unit complexes [35,36]. One of such examples is the Magnéli phases in molybdenum- [20,37,38], tungsten- [20,39], titanium- [40][41][42][43], and vanadium-oxides [44][45][46][47], where two-dimensional defects (or crystallographic shear [48]) of edge-or face-sharing bonding between metal-oxide complexes are periodically formed. On the analogy to them, here we consider possible crystalline phases formed by the structural H 2 S unit.…”

TinO2n−1Magnéli phases studied using density functional theory

“…This structure can be viewed as infinite planes of rutile, n-TiO 6 octahedra thick along the (121) direction of the rutile crystal, limited by a plane of defects (oxygen vacancies), which characterizes a crystallographic shear structure [19,29]. The unit cells obtained from crystallographic databases were triclinic and presented space group P1 for the three known phases: high-(HT), intermediate-(IT), and low-temperature (LT).…”

“…Those structures can be regarded as oxygen-deficient TiO 2 , where the concentration of oxygen vacancies (V O ) is such that those defects become organized in a shear-plane structure. The formation of these extended defects is exemplified by the operation (121) 1 2 [011] in the rutile structure, where the first three indices refer to a plane in the rutile structure and the last three to a displacement vector in the same structure [18,19]. An example of this structure is presented in Fig.…”