An Ab Initio Study of Pressure-Induced Reversal of Elastically Stiff and Soft Directions in YN and ScN and Its Effect in Nanocomposites Containing These Nitrides

0498978 - ÚFM 2019 RIV CH eng J - Článek v odborném periodiku
Friák, Martin - Kroupa, P. - Holec, D. - Šob, Mojmír
An Ab Initio Study of Pressure-Induced Reversal of Elastically Stiff and Soft Directions in YN and ScN and Its Effect in Nanocomposites Containing These Nitrides.
Nanomaterials. Roč. 8, č. 12 (2018), č. článku 1049. E-ISSN 2079-4991
Grant CEP: GA ČR(CZ) GA16-24711S
Institucionální podpora: RVO:68081723
Klíčová slova: YN * ScN * pressure * elasticity * ab initio * stability * nanocomposites
Obor OECD: Condensed matter physics (including formerly solid state physics, supercond.)
Impakt faktor: 4.034, rok: 2018

Using quantum-mechanical calculations of second- and third-order elastic constants
for YN and ScN with the rock-salt (B1) structure, we predict that these materials change the
fundamental type of their elastic anisotropy by rather moderate hydrostatic pressures of a few
GPa. In particular, YN with its zero-pressure elastic anisotropy characterized by the Zener anisotropy
ratio AZ = 2C44/(C11 􀀀 C12) = 1.046 becomes elastically isotropic at the hydrostatic pressure of
1.2 GPa. The lowest values of the Young’s modulus (so-called soft directions) change from h100i
(in the zero-pressure state) to the h111i directions (for pressures above 1.2 GPa). It means that the
crystallographic orientations of stiffest (also called hard) elastic response and those of the softest
one are reversed when comparing the zero-pressure state with that for pressures above the critical
level. Qualitatively, the same type of reversal is predicted for ScN with the zero-pressure value of
the Zener anisotropy factor AZ = 1.117 and the critical pressure of about 6.5 GPa. Our predictions
are based on both second-order and third-order elastic constants determined for the zero-pressure
state but the anisotropy change is then verified by explicit calculations of the second-order elastic
constants for compressed states. Both materials are semiconductors in the whole range of studied
pressures. Our phonon calculations further reveal that the change in the type of the elastic anisotropy
has only a minor impact on the vibrational properties. Our simulations of biaxially strained states
of YN demonstrate that a similar change in the elastic anisotropy can be achieved also under stress
conditions appearing, for example, in coherently co-existing nanocomposites such as superlattices.
Finally, after selecting ScN and PdN (both in B1 rock-salt structure) as a pair of suitable candidate
materials for such a superlattice (due to the similarity of their lattice parameters), our calculations
of such a coherent nanocomposite results again in a reversed elastic anisotropy (compared with the
zero-pressure state of ScN).

Trvalý link: http://hdl.handle.net/11104/0293615