First-principles study of electronic structure, optical and phonon properties of α-ZrW2O8

Abstract

ZrW 2 O 8 exhibits isotropic negative thermal expansions over its entire temperature range of stability, yet so far its physical properties and mechanism have not been fully addressed.In this article, the electronic structure, elastic, thermal, optical and phonon properties of α-ZrW 2 O 8 are systematically investigated from first principles. The agreements between the generalized gradient approximation (GGA) calculation and experiments are found to be quite satisfactory. The calculation results can be useful in relevant material designs, e.g., when ZrW 2 O 8 is employed to adjust the thermal expansion coefficient of ceramic matrix composites. Ⅰ. IntroductionThe zirconium tungstate (ZrW 2 O 8 ) exhibits isotropic negative thermal expansion (relatively large, -9×10 -6 K -1 ) over its entire temperature range of stability (from close to absolute zero up to the decomposition temperature around 1500 K). The isotropy of the expansion is back by the fact that the cubic structure of ZrW 2 O 8 remains at these temperatures. 1,2 This feature makes ZrW 2 O 8 not only an important example to study this type of unusual lattice dynamics, but also potentially well suited for applications in composite materials in order to reduce the composites' overall thermal expansion to near zero. [3][4][5][6][7] In experiments, the phase transition, specific heat, thermal expansion of ZrW 2 O 8 have been studied by some groups [8][9][10][11] , especially for the temperature-and pressure-induced phase transitions, and the temperature-dependence of the thermal. 12-14 For example, it has long been noticed that α-ZrW 2 O 8 has a negative coefficient of thermal expansion, α= -9.07 ×10 -6 K within the temperature range 2-350 K 8 , and it undergoes a phase transition from P213 to Pa3 at 448 K 8 or 428K 12 ,

Ultralow Thermal Conductivity of Highly Dense ZrW2O8 Ceramics with Negative Thermal Expansion

“…From the measured physical properties, we estimated the κ lat values of ZWO using the Slack model from the following relation [ 46 ] where is the average mass of the atoms in the crystal, δ 3 is the volume per atom, n cell is the number of atoms in primitive cell, and A is a collective physical constant ( A = 3.1 × 10 −6 when κ lat is in W m −1 K −1 , in atomic mass units, and δ in angstroms). ZWO has a large bandgap of over 2.0 eV, [ 47,48 ] and it is considered as an insulator. Therefore, its electronic thermal conductivity is negligibly small, meaning that κ lat is almost identical to κ .…”

“…[ 57 ] C P at T = 300–550 K was retrieved from the literature, [ 41 ] but there was no reported data for a value at T > 500 K. At high temperature, C P of ZWO was predicted to approach the Dulong–Petit limit and become nearly constant. [ 48 ] Thus, we fixed C P as constant at T = 550–823 K. The sound velocities were measured by an ultrasonic pulse‐echo method (NIHON. MATECH Echometer 1062) at T = 300 K in air.…”