Abstract
Modification of lattice thermal conductivity (κL) of a solid by means of hydrostatic pressure (P) has been a crucially interesting approach that targets a broad range of advanced materials from thermoelectrics and thermal insulators to minerals in mantle. Although it is well documented knowledge that thermal conductivity of bulk materials normally increase upon hydrostatic pressure, such positive relationship is seriously challenged when it comes to ceramics with complex crystal structure and heterogeneous chemical bonds. In this paper, we predict an abnormally negative trend dκL/dP < 0 in Y2Si2O7 silicate using density functional theoretical calculations. The mechanism is disclosed as combined effects of slightly decreased group velocity and significantly augmented scattering of heat-carrying acoustic phonons in pressured lattice, which is originated from pressure-induced downward shift of low-lying optic and acoustic phonons. The structural origin of low-lying optic phonons as well as the induced phonon anharmonicity is also qualitatively elucidated with respect to intrinsic bonding heterogeneity of Y2Si2O7. The present results are expected to bring deeper insights for phonon engineering and modulation of thermal conductivity in complex solids with diverging structural flexibility, enormous bonding heterogeneity, and giant phonon anharmonicity.
Highlights
Phonon engineering, in terms of controlling heat flow within the lattice by manipulating phonon behavior, has became a powerful paradigm in the design and optimization of advanced materials with optimal thermal conductivity
Unexpected anomalous decrement of κL is observed in pressured lattice; and the physical mechanism is disclosed as combined effects of 1) slightly decreased acoustic phonon group velocity and 2) significantly augmented scattering of acoustic phonons owing to pressure-induced enhancement of phonon anharmonicity
We find that longitudinal acoustic phonons are more effective heat-carriers in Y2Si2O7 and dominate the magnitude of decrement of total κL under pressure; whereas transverse acoustic phonons are more intensely scattered by low-lying optic phonons and yield negligible contribution to thermal conduction
Summary
In terms of controlling heat flow within the lattice by manipulating phonon behavior, has became a powerful paradigm in the design and optimization of advanced materials with optimal thermal conductivity. Ouyang[19] observed a dκL/dP < 0 relationship in HgTe with partially ionic atomic bonds, in which the reduction of relaxation time of transverse acoustic phonons considerably overwhelms the less enhancement of group velocity of longitudinal acoustic and optic modes Such mechanism, cannot directly guide the investigations of complicated-structured ceramics with multiple chemical compositions and covalent and heterogeneous bonding features. Our density functional theoretical (DFT) calculations on high-pressure Raman spectroscopy of γ-Y2Si2O7 (referred as Y2Si2O7 for brevity) have identified obvious softening of low-frequency optic phonon modes This is much likely to account for significant acoustic-optic coupling, which has been recognized in other complex-structured materials with similar structural characteristics[20,21,22,23,24]. We find that longitudinal acoustic phonons are more effective heat-carriers in Y2Si2O7 and dominate the magnitude of decrement of total κL under pressure; whereas transverse acoustic phonons are more intensely scattered by low-lying optic phonons and yield negligible contribution to thermal conduction
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