Ab initio study of phase transition and bulk modulus of NaH

The phase transition of NaH from NaCl- to CsCl-type structure is investigated by an ab initio plane-wave pseudopotential density functional theory method with the norm-conserving pseudopotential scheme in the frame of the generalized gradient approximation correction; the isothermal bulk modulus and its first and second pressure derivatives of the NaCl- and CsCl-type structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The phase transition obtained from the usual condition of equal enthalpies occurs at the pressure of 32 GPa, which is consistent with the experimental and other calculated values. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of cell volume V and lattice constant a on temperature T at zero pressure, the isothermal bulk modulus B0 and its pressure derivatives B0′and B0″ on pressure P along isotherms 0, 300, and 600 K, are also successfully obtained.

The isothermal bulk modulus BT, adiabatic bulk modulus BS, primitive cell volume V, and lattice constant a of an NaH with an NaCl-type structure as a function temperature T at zero pressure. When T<100 K, B nearly keeps constant; when T>100 K, B decreases dramatically as T increases. Correspondingly, when T<100 K, the primitive cell volume and lattice parameter of an NaH with an NaCl-type structure have a little change; when T>100 K, the primitive cell volume and lattice parameter changes rapidly as T increases. It is the rapid volume or lattice parameter variation that makes the bulk modulus B rapidly decrease.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► The transition phase of an NaH from NaCl to CsCl structure has been investigated. ► The relationship between B and T for an NaH at zero pressure has been given. ► The BT and its pressure derivatives of an NaH at high pressure have been obtained.