Yuan Xu Wang

Low effective mass leading to an improved ZT value by 32% for n-type BiCuSeO: a first-principles study

BiCuSeO is an attractive material for its high temperature stability, high Seebeck coefficient, and low lattice thermal conductivity. However, its electrical conductivity is low. To enhance the thermoelectric properties of BiCuSeO further, we investigated the materials electronic structure and transport property using first-principles calculations. We determine that the low electrical conductivity may originate from strong ionic bonding and without a Cu–Se conductive pathway forming near the Fermi level. Moreover, although p-type BiCuSeO has a high thermopower due to the high density of states near the Fermi level, its electrical conductivity is relatively low because no conductive pathway for carrier transport exists along the c axis. In contrast, a conductive pathway is formed between Bi and Cu atoms at the conduction-band minimum along the c axis. This feature would lead to high electrical conductivity for n-type BiCuSeO. With relatively high electrical conductivity and slightly decreased thermopower, n-type BiCuSeO exhibits a higher power factor than that of p-type BiCuSeO. The maximum ZT value could be improved by 32% for n-type BiCuSeO compared with the prevailing p-type doping approach at 920 K.

Predicted boron-carbide compounds: A first-principles study

By using developed particle swarm optimization algorithm on crystal structural prediction, we have explored the possible crystal structures of B-C system. Their structures, stability, elastic properties, electronic structure, and chemical bonding have been investigated by first-principles calculations with density functional theory. The results show that all the predicted structures are mechanically and dynamically stable. An analysis of calculated enthalpy with pressure indicates that increasing of boron content will increase the stability of boron carbides under low pressure. Moreover, the boron carbides with rich carbon content become more stable under high pressure. The negative formation energy of predicted B5C indicates its high stability. The density of states of B5C show that it is p-type semiconducting. The calculated theoretical Vickers hardnesses of B-C exceed 40 GPa except B4C, BC, and BC4, indicating they are potential superhard materials. An analysis of Debye temperature and electronic localization function provides further understanding chemical and physical properties of boron carbide.

The high thermopower of the Zintl compound Sr5Sn2As6 over a wide temperature range: first-principles calculations

The thermoelectric properties and the electronic structure of Sr5Sn2As6 were studied according to the first principles and semi-classical Boltzmann theory. To elucidate the thermoelectric performance of Sr5Sn2As6, we simulated its carrier concentration, Seebeck coefficient, and electrical conductivity, and provided an estimated value for the thermoelectric figure of merit ZT. For pure Sr5Sn2As6, the largest Seebeck coefficient (S) is 248 (μV K−1) at 500 K, and the minimum S is 202 (μV K−1) at 1200 K. The large Seebeck coefficient over a wide temperature range most likely results from the appropriate band gap (0.55 eV) of Sr5Sn2As6. By studying the carrier concentration dependence of the transport properties, the ZT value for p-type doping was found to be ∼1.4 times that of n-type doping, which is mainly due to the larger effective mass of the valence band. Moreover, for n-type doping, both the Seebeck coefficient and the electrical conductivity along the z-direction are much larger than those along the other directions, due to the large band degeneracy and the light band, which results in a highest ZT value of 3.0 along the z-direction, with a carrier concentration of 9.4 × 1019 electrons per cm3 at 950 K. The highest ZT value for p-type along the z-direction is 1.7, with a carrier concentration of 1.2 × 1020 holes per cm3 at 950 K. Meanwhile, the minimum lattice thermal conductivity of Sr5Sn2As6 is small (0.47 W m−1 K−1), and is comparable to those of Ca5M2Sb6 (M = Al, Ga, In). Hence, good thermoelectric performance along the z-direction for n-type Sr5Sn2As6 was predicted.

Electronic structure and thermoelectric performance of Zintl compound Ca5Ga2As6

Ca5Ga2As6 gives us a chance to achieve a high thermoelectric performance with a normal bulk system. This is mainly due to the fact that its complex chemical structure can not only supply an ultra-low lattice thermal conductivity but also show a high Seebeck coefficient. In this paper, we have discussed the lattice, electronic, and transport properties of Ca5Ga2As6 for elucidating its high thermoelectric performance. In our calculations, density-functional theory and Boltzmann transport theory were used. For intrinsic Ca5Ga2As6, the valence band effective mass is larger than the conduction band effective mass, which leads to its positive sign of S over the whole temperature range. By studying the anisotropy of its transport properties, we find that the transport properties of p-type Ca5Ga2As6 are better than that of the n-type one, which mainly comes from the large anisotropy of its band structure. Moreover its transport properties along the z direction are much better than those along the x and y directions, which can be attributed to its anisotropic one-dimensional lattice structure. At different temperatures, the peak value of ZeT appears at different carrier concentrations, which makes it possible to obtain a highest efficiency under large temperature gradients. The carrier concentrations corresponding to large ZeT are in the range of 0.033–0.5 e per uc, which is equal to 5.19 × 1019 cm−3 to 7.87 × 1020 cm−3. The largest value of ZeT is equal to 0.95.

Electronic structure and thermoelectric performance of Zintl compound Sr3GaSb3: A first-principles study

By using first-principles method and Boltzmann theory, we simulated the thermoelectric transport properties of p-type and n-type Sr3GaSb3. It is found that the thermoelectric figure-of merit (ZT) of n-type Sr3GaSb3 is probably better than that of p-type, mainly due to its large band degeneracy. Moreover, a high ZT value of 1.74 at 850 K can be achieved for n-type Sr3GaSb3 along the yy direction, corresponding to the carrier concentration 3.5 × 1020 e cm−3. We propose that the high ZT value of experimentally synthesized p-type Sr3GaSb3 is originated from appearing of the larger number of band valley on the top of valence bands.

Improved thermoelectric performance of CuGaTe2 with convergence of band valleys: a first-principles study

A high value (1.4) of figure of merit (ZT) of CuGaTe2 has been experimentally discovered by T. Plirdpring et al. [Adv. Mater. 24, 3622 (2012)]. In order to further enhance its thermoelectric properties, we investigated its electronic structure and thermoelectric properties by first-principles study. Large band-valley number and high convergence of the bottom conduction bands induced a large Seebeck coefficient and a high electrical conductivity of n-type CuGaTe2. So, for n-type CuGaTe2, the maximum ZT values 2.1 may be found at 950 K by suitable carrier concentration tuning, which results in a 25% increment in the ZT value compared with p-type CuGaTe2. Band decomposed charge density calculations indicate that the transport properties are mainly determined by the Cu and Te atoms at the valence-band maximum, in contrast, transport properties are simultaneously affected by the three kind of atoms at the conduction-band minimum. At high temperature, ab initio molecular dynamics calculations demonstrate that Cu atoms precipitated from their crystal matrices lead to a decrease in thermopower. Along the high symmetry point M, the charge density of all atoms is centrosymmetric. Maybe this centrosymmetric electronic structure leads to the conduction band valley convergence at the M point.

Phase Stability and Elastic Properties of Chromium Borides with Various Stoichiometries

Phase stability is important to the application of materials. By first-principles calculations, we establish the phase stability of chromium borides with various stoichiometries. Moreover, the phases of CrB3 and CrB4 have been predicted by using a newly developed particle swarm optimization (PSO) algorithm. Formation enthalpy-pressure diagrams reveal that the MoB-type structure is more energetically favorable than the TiI-type structure for CrB. For CrB2, the WB2-type structure is preferred at zero pressure. The predicted new phase of CrB3 belongs to the hexagonal P-6m2 space group and it transforms into an orthorhombic phase as the pressure exceeds 93 GPa. The predicted CrB4 (space group: Pnnm) phase is more energetically favorable than the previously proposed Immm structure. The mechanical and thermodynamic stabilities of predicted CrB3 and CrB4 are verified by the calculated elastic constants and formation enthalpies. The full phonon dispersion calculations confirm the dynamic stability of WB2 -type CrB2 and predicted CrB3. The large shear moduli, large Youngs moduli, low Poisson ratios, and low bulk and shear modulus ratios of CrB4-PSC and CrB4-PSD indicate that they are potential hard materials. Analyses of Debye temperature, electronic localization function, and electronic structure provide further understanding of the chemical and physical properties of these borides.

The ground-state structure and physical properties of ReB3 and IrB3 predicted from first principles

ReB3 has been synthesized and was reported to have symmetry of P63/mmc [Acta Chem. Scand. 1960, 14, 733]. However, we find that this structure is not stable due to its positive formation energy. In 2009, IrB1.35 and IrB1.1 were synthesized and were considered to be superhard [Chem. Mater. 2007, 21, 1407; ACS Appl. Mater. Interfaces 2010, 2, 581]. Inspired by these results, we explored the possible crystal structures of ReB3 and IrB3 by using the developed particle swarm optimization algorithm. We predict that Pm2-ReB3 and Amm2-IrB3 are the ground-state phases of ReB3 and IrB3, respectively. The stability, elastic properties, and electronic structures of the predicted structures were studied by first-principles calculations. The negative calculated formation enthalpies for Pm2-ReB3 and P63/mmc-ReB3 indicate that they are stable and can be synthesized under ambient pressure. Their dynamical stability is confirmed by calculated phonon dispersion curves. The predicted P63/mmc-ReB3 has the highest hardness among these predicted structures. The calculated density of state shows that these predicted structures are metallic. The chemical bonding features of the predicted ReB3 and IrB3 were investigated by analyzing their electronic localization function.

Electronic structure and thermoelectric properties of the Zintl compounds Sr5Al2Sb6 and Ca5Al2Sb6: first-principles study

Previous experimental work showed that Zn-doping only slightly increased the carrier concentration of Sr5Al2Sb6 and the electrical conductivity improved barely, which is very different from the results of Zn-doping in Ca5Al2Sb6. To understand their different thermoelectric behaviors, we investigated their stability, electronic structure, and thermoelectric properties using first-principles calculations and the semiclassical Boltzmann theory. We found that the low carrier concentration of Zn-doped Sr5Al2Sb6 mainly comes from its high positive formation energy. Moreover, we predict that a high hole concentration can possibly be realized in Sr5Al2Sb6 by Na or Mn doping, due to the negative and low formation energies of Na- and Mn-doped Sr5Al2Sb6, especially for Mn doping (−6.58 eV). For p-type Sr5Al2Sb6, the large effective mass along Γ–Y induces a large Seebeck coefficient along the y direction, which leads to the good thermoelectric properties along the y direction. For p-type Ca5Al2Sb6, the effective mass along Γ–Z is always smaller than those along the other two directions with increasing doping degree, which induces its good thermoelectric properties along the z direction. The analysis of the weight mobility of the two compounds confirms this idea. The calculated band structure shows that Sr5Al2Sb6 has a larger band gap than Ca5Al2Sb6. The relatively small band gap of Ca5Al2Sb6 mainly results from the appearance of a high density-of-states peak around the conduction band bottom, which originates from the Sb–Sb antibonding states in it.

Electronic structure and thermoelectric properties of Zintl compounds A3AlSb3 (A = Ca and Sr): first-principles study

Experimentally synthesized Zn-doped Sr3AlSb3 exhibited a smaller carrier concentration than Zn-doped Ca3AlSb3, which induces a lower thermoelectric figure of merit (ZT) than Zn-doped Ca3AlSb3. We used first-principles methods and the semiclassical Boltzmann theory to study the reason for this differing thermoelectric behavior and explored the optimal carrier concentration for high ZT values via p-type and n-type doping. The covalent AlSb4 tetrahedral arrangement exhibited an important effect on the electronic structure and thermoelectric properties. p-type Ca3AlSb3 may exhibit good thermoelectric properties along its covalent AlSb4 chain due to its double band degeneracy at the valence band edge and small effective mass along its one-dimensional chain direction. Zn doping the Al site exhibited higher formation energy for Sr3AlSb3 than Ca3AlSb3, which explains the lower carrier concentration for Zn-doped Sr3AlSb3 than Zn-doped Ca3AlSb3. The double band degeneracy at the valence band edge for Ca3AlSb3 may also help to increase the carrier concentration. Sr3AlSb3 containing isolated Al2Sb6 dimers can exhibit a high thermoelectric performance via heavy p-type doping with a carrier concentration above 1 × 1020 holes per cm3. Moreover, the ZT maxima for the n-type Sr3AlSb3 can reach 0.76 with a carrier concentration of 4.5 × 1020 electrons per cm3.