Entropically Stabilized Local Dipole Formation in Lead Chalcogenides

Abstract

We report the observation of local structural dipoles that emerge from an undistorted ground state on warming, in contrast to conventional structural phase transitions in which distortions emerge on cooling. Using experimental and theoretical probes of the local structure, we demonstrate this behavior in binary lead chalcogenides, which were believed to adopt the ideal, undistorted rock-salt structure at all temperatures. The behavior is consistent with a simple thermodynamic model in which the emerging dipoles are stabilized in the disordered state at high temperature due to the extra configurational entropy despite the fact that the undistorted structure has lower internal energy. Our findings shed light on the anomalous electronic and thermoelectric properties of the lead chalcogenides. Similar searches may show that the phenomenon is more widespread.

Anharmonicity and atomic distribution of SnTe and PbTe thermoelectrics

“…36 It had been previously conjectured that this anomaly could be related to proposed atomic off-centerings causing an additional localized optical mode, beyond the dispersions of the rock-salt structure. 1,11 However, our first-principles calculations of the temperature-dependent spectral density functions showed how the double-peak arises from the shape of the anharmonic phonon self-energy, whose imaginary part contains a pronounced peak, owing to nesting of the phonon dispersions. 37 Calculations of the zone-center TO phonon spectral function at zone center in PbTe based on molecular dynamics also reproduced the effect.…”

“…28,29 Recently, an asymmetry of peaks was reported in the pair-distribution-function (PDF) or radial distribution function (RDF) of PbTe, PbS, and SnTe, which was interpreted in terms of atomic off-centerings increasing with temperature. 1,2,11,30 However, it is important to note that these x-ray PDF/RDF measurements did not filter out the phonon contribution to the scattered intensity, or thermal diffusive scattering (TDS). In addition, the interpretation of PDF/RDF asymmetry in terms of atomic off-centerings was put in question by molecular dynamics simulations, which reproduced the asymmetry in the measurements, but found no time-averaged offcentering on a time scale of 12 picosecond or longer.…”

Thermoelectric performance of tellurium-reduced quaternary p-type lead–chalcogenide composites

“…3 4 However, they cannot be used for high volume applications, so nanostructuring should be utilized in bulk materials. A large number of recent studies [5][6][7][8][9][10] have focused on the so-called mid-range temperature (600-900 K) thermoelectric materials and specifically PbTe, where the majority of broad-based applications benefit. Recent studies 7-9 11 12 have improved PbTe thermoelectric performance significantly through tuning the electronic structure and/or nanostructuring of bulk materials.…”

Chemical composition tuning in quaternary p-type Pb-chalcogenides – a promising strategy for enhanced thermoelectric performance

“…[2][3][4][5][6][7][8] The search for high performance materials from naturally abundant elements has continued to improve the relatively low conversion efficiency of such materials, which is determined by the figure of merit, zT = S 2 Ts/(k E + k L ), where, S, s, T, k L , and k E are the Seebeck coefficient, electrical conductivity, absolute temperature, and the lattice and electronic components of the thermal conductivity, respectively.…”