Temperature dependence of the heat of formation of MgH₂

Summary:

The temperature dependent heat of formation of MgH₂ was calculated. Formally, the heat of formation is defined as the heat produced by the reaction Mg^solid + H₂^gas ® MgH₂^solid. Counting electronic (el) and lattice vibrational contributions, the heat of formation can be expressed as D_fH = DE_el + DE_vib,T=298 + pDV

Task:

For (MgH₂)₂ (P42-NMN symmetry, e.g. ICSD#26624) a VASP structure optimization followed by PHONON calculations yield the following properties:

Structure optimization:

	The equilibrium lattice parameters, cell volume, density, etc.
	The electronic contribution to the heat of formation

Phonon calculation:

	PHONON spectrum and phonon density of states
	Zero point energy of the  MgH2 lattice
	Thermodynamic functions: Vibrational energy, entropy

The temperature dependent heat of formation is derived by combining the results from the structure optimization and PHONON calculations.

Tools

MedeA, InfoMaticA, VASP, PHONON

Results:

Structure optimization:

	Calculated Lattice parameters are within 0.3% of the experimental  values(1)
	The electronic contribution to the heat of formation is -53.67 kJ/mol

PHONON calculations:

	Zero-point energy =    38.26 kJ/mol
	Vibrational contribution to the energy at 300K: 5.67 kJ/mol

Taking into account the zero-point energy contributions and vibrational contributions at room temperature for H2 and Mg in their standard states (calculation not shown here) yields the standard heat of formation of MgH2, i.e. the reaction energy for:

Mg (element, standard state) + H₂(element, standard state) = MgH₂

1. Chase, M.W., Jr., "NIST-JANAF Themochemical Tables, Fourth Edition", J. Phys. Chem. Ref. Data, Monograph 9, 1998, 1-1951 as given in http://webbook.nist.gov

Time to results:

45 min on a AMD Opteron cluster using 4 CPUs