Coordination Chemistry of Mercury(II) in Liquid and Aqueous Ammonia Solution and the Crystal Structure of Tetraamminemercury(II) Perchlorate

Chemical Insights into PbSe- x%HgSe: High Power Factor and Improved Thermoelectric Performance by Alloying with Discordant Atoms

Thermoelectric generators can convert heat directly into usable electric power but suffer from low efficiencies and high costs, which have hindered wide-scale applications. Accordingly, an important goal in the field of thermoelectricity is to develop new high performance materials that are composed of more earth-abundant elements. The best systems for midtemperature power generation rely on heavily doped PbTe, but the Te in these materials is scarce in the Earth's crust. PbSe is emerging as a less expensive alternative to PbTe, although it displays inferior performance due to a considerably smaller power factor S²σ, where S is the Seebeck coefficient and σ is electrical conductivity. Here, we present a new p-type PbSe system, Pb_0.98Na_0.02Se- x%HgSe, which yields a very high power factor of ∼20 μW·cm^-1·K^-2 at 963 K when x = 2, a 15% improvement over the best performing PbSe- x%MSe materials. The enhancement is attributed to a combination of high carrier mobility and the early onset of band convergence in the Hg-alloyed samples (∼550 K), which results in a significant increase in the Seebeck coefficient. Interestingly, we find that the Hg²⁺ cations sit at an off-centered position within the PbSe lattice, and we dub the displaced Hg atoms "discordant". DFT calculations indicate that this feature plays a role in lowering thermal conductivity, and we believe that this insight may inspire new design criteria for engineering high performance thermoelectric materials. The high power factor combined with a decrease in thermal conductivity gives a high figure of merit ZT of 1.7 at 970 K, the highest value reported for p-type PbSe to date.

Structure of solvated mercury(II) halides in liquid ammonia, triethyl phosphite and tri-n-butylphosphine solution

Liquid ammonia, trialkyl phosphites, and especially trialkylphosphines, are very powerful electron-pair donor solvents with soft bonding character. The solvent molecules act as strongly coordinating ligands towards mercury(ii), interacting strongly enough to displace halide ligands. In liquid ammonia mercury(ii) chloride solutions separate into two liquid phases; the upper contains tetraamminemercury(ii) complexes, [Hg(NH(3))(4)](2+), and chloride ions in low concentration, while the lower is a dense highly concentrated solution of [Hg(NH(3))(4)](2+) entities, ca. 1.4 mol dm(-3), probably ion-paired by hydrogen bonds to the chloride ions. Mercury(ii) bromide also dissociates to ionic complexes in liquid ammonia and forms a homogeneous solution for which (199)Hg NMR indicates weak bromide association with mercury(ii). When dissolving mercury(ii) iodide in liquid ammonia and triethyl phosphite solvated molecular complexes form in the solutions. The Raman nu(I-Hg-I) symmetric stretching frequency is 132 cm(-1) for the pseudo-tetrahedral [HgI(2)(NH(3))(2)] complex formed in liquid ammonia, corresponding to D(S) = 56 on the donor strength scale. For the Hg(ClO(4))(2)/NH(4)I system in liquid ammonia a (199)Hg NMR study showed [HgI(4)](2-) to be the dominating mercury(ii) complex for mole ratios n(I(-)) : n(Hg(2+)) > or = 6. A large angle X-ray scattering (LAXS) study of mercury(ii) iodide in triethyl phosphite solution showed a [HgI(2)(P(OC(4)H(9))(3))(2)] complex with the Hg-I and Hg-P bond distances 2.750(3) and 2.457(4) A, respectively, in near tetrahedral configuration. Trialkylphosphines generally form very strong bonds to mercury(ii), dissociating all mercury(ii) halides. Mercury(ii) chloride and bromide form solid solvated mercury(ii) halide salts when treated with tri-n-butylphosphine, because of the low permittivity of the solvent. A LAXS study of a melt of mercury(ii) iodide in tri-n-butylphosphine at 330 K resulted in the Hg-I and Hg-P distances 2.851(3) and 2.468(4) A, respectively. The absence of a distinct I-I distance indicates flexible coordination geometry with weak and non-directional mercury(ii) iodide association within the tri-n-butylphosphine solvated complex.