Elastic and thermodynamic properties of α-Bi2O3 at high pressures: Study of mechanical and dynamical stability

Abstract

The elastic and thermodynamic properties of the monoclinic polymorph of bismuth oxide (-Bi 2 O 3 ); aka mineral bismite, have been theoretically investigated both at room pressure and under hydrostatic compression by means of first principles calculations based on density functional theory. In this work, the elastic stiffness coefficients, elastic moduli, Poisson's ratio, B/G ratio, elastic anisotropy indexes (A B , A G , A 1 , A 2 , A 3 , A U ) and directional dependence of Young modulus and linear compressibility have been obtained. Vickers hardness, and sound wave velocities have been calculated. Our simulations show that bismite has a high elastic anisotropy. -Bi 2 O 3 is a ductile material whose elastic anisotropy increases under compression and presents a stronger ability to resist volume compression than shear deformation at all pressures. Besides, it has a very small minimum thermal conductivity, which is well suited for thermoelectric applications. Finally, the mechanical and dynamical stability of bismite at high pressure has been studied and it has been found that -Bi 2 O 3 becomes mechanically unstable at pressures beyond 19.3 GPa and dynamically unstable above 11.5 GPa. These instabilities could be responsible for the amorphization of bismite observed experimentally between 15 and 20 GPa.

High‐Pressure Effect on Elastic Constants and Their Related Properties of MgCa Intermetallic Compound

“…A similar trend regarding the Vickers hardness H V versus pressure was observed in compound crystals with metallic bonding, MB 12 (M = Zr, Hf, Y, Lu) with UB 12 ‐type structure, indium antimonide semiconducting materials, cadmium telluride semiconductor compounds, and several hard and superhard materials (diamond, c‐BN, etc) . Nevertheless, an opposite trend in H V versus pressure was observed in the data of Gomis et al . for bismuth oxide (α‐Bi 2 O 3 ) material and in the data of Pan et al .…”

“…As a matter of fact, some theoretical models established for calculating the hardness of materials such as compound crystals with metallic bonding, MB 12 (M = Zr, Hf, Y, Lu) with UB 12 ‐type structure, indium antimonide semiconducting compounds, and cubic zinc‐blende cadmium telluride semiconductor compounds, as well as other approaches correlating the hardness H to the bulk modus B , showed that H increases as pressure is elevated. However, other models based on the Pugh's ratio ( G/B ) showed that H decreases with increasing pressure . The increase of H with rising pressure could be justified by two reasons: the first reason is the increase of the rigidity (due to the increase of B ) and the Debye temperature θ D with increasing pressure, and the second reason is the decrease of H with rising temperature (thermal softening) .…”

“…[52] Generally, the pressure and temperature affect the physical properties of materials in an opposite manner. In contrast, Gomis et al and Pan et al [48,49] justified the decrease of H with increasing pressure by the increase of the ductility (the decrease of Pugh's ratio (G/B) and the increase of Poisson's ratio σ) of the material under applied pressure. To examine the variation of Vickers hardness H V versus pressure, H V has been estimated from the Mukhanov's et al [47] model using Murnaghan's equation of state (EoS)…”

“…[42,43] H V is very sensitive to dislocations and other defects in the studied solids. Using different models, several works [43][44][45][46][47][48][49] studied the variation of the hardness H as a function of pressure. Some of these models correlate the hardness H and other elastic moduli (B, G, G/B, etc) using different expressions.…”

“…However, other models based on the Pugh's ratio (G/B) showed that H decreases with increasing pressure. [48,49] The increase of H with rising pressure could be justified by two reasons: the first reason is the increase of the rigidity (due to the increase of B) and the Debye temperature θ D with increasing pressure, [51] and the second reason is the decrease of H with rising temperature (thermal softening). [52] Generally, the pressure and temperature affect the physical properties of materials in an opposite manner.…”