Abstract
In recent years, finite strain Landau theory has been gradually developed as both a conceptual as well as a quantitative framework to study high pressure phase transitions of the group-subgroup type. In the current paper, we introduce a new version of this approach which is based on symmetry-adapted finite strains. This results in a substantial simplification of the original formulation. Moreover, it allows for replacing the clumsy use of truncated Taylor expansions by a convenient functional parametrization. Both the weaknesses of the traditional Landau approach based on infinitesimal strains as well as the major improvements made possible by our new parametrization are illustrated in great detail in an application to the ambient temperature high pressure transition of the perovskite KMnF 3 .
Highlights
Through many decades, the Landau theory (LT) of phase transitions [1,2] (PTs) has proven to be one of the most valuable conceptual tools for understanding PTs of the group-subgroup type
Effects of spontaneous strain that generally accompany temperature-driven structural PTs are sufficiently parameterized in terms of infinitesimal strain tensor components defined with respect to a baseline, which is obtained by extrapolating the generally small thermal expansion changes of the high symmetry reference phase
We focus on the antiferrodistortive high pressure phase transition of KMF from the cubic perovskite aristophase Pm3̄m to a tetragonal I4/mcm phase at room temperature which was experimentally investigated in Ref. [11] by X-ray diffraction up to 30 GPa
Summary
The Landau theory (LT) of phase transitions [1,2] (PTs) has proven to be one of the most valuable conceptual tools for understanding PTs of the group-subgroup type. Effects of spontaneous strain that generally accompany temperature-driven structural PTs are sufficiently parameterized in terms of infinitesimal strain tensor components defined with respect to a baseline, which is obtained by extrapolating the generally small thermal expansion changes of the high symmetry reference phase. Landau potential involves only terms up to harmonic order, and any temperature dependence of the relevant parameters (high symmetry phase elastic constants and other coupling constants) can usually be completely neglected [4]. Its capabilities have been demonstrated in a number of applications to HPPTs [5,6,7,8,9,10] As it stands, the numerical scheme underlying FSLT is still quite involved, and many practical workers in the field of HPPTs may be hesitant to go through the mathematical hardships it seems to pose. The enormous reduction of overall complexity of the approach as well as the vastly reduced numerical requirements of our new version of FSLT are demonstrated on the example of the HPPT in the perovskite KMnF3 (KMF)
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