Non-centrosymmetric, transverse structural modulation in SrAl4, and elucidation of its origin in the BaAl4 family of compounds
arxiv(2023)
摘要
At ambient conditions SrAl4 adopts the BaAl4 structure type with space group
I4/mmm. It undergoes a charge-density-wave (CDW) transition at TCDW = 243 K,
followed by a structural transition at TS = 87 K. Temperature-dependent
single-crystal X-ray diffraction (SXRD) leads to the observation of
incommensurate superlattice reflections at q = σc* with σ= 0.1116
at 200 K. The CDW has orthorhombic symmetry with the acentric superspace group
F222(00sigma)00s, where F222 is a subgroup of Fmmm as well as of I4/mmm. Atomic
displacements mainly represent a transverse wave, with displacements that are
90 deg out of phase between the two diagonal directions of the I-centered unit
cell, resulting in a helical wave. Small longitudinal displacements are
provided by the second harmonic modulation. The orthorhombic phase realized in
SrAl4 is similar to that found in EuAl4. Electronic structure calculations and
phonon calculations by density functional theory (DFT) have failed to reveal
the mechanism of CDW formation. However, DFT reveals that Al atoms dominate the
density of states near the Fermi level, thus, corroborating the SXRD
measurements. SrAl4 remains incommensurately modulated at the structural
transition, where the symmetry lowers from orthorhombic to b-unique monoclinic.
We have identified a simple criterion, that correlates the presence of a phase
transition with the interatomic distances. Only those compounds XAl4-xGax(X =
Ba, Eu, Sr, Ca; 0 < x <4) undergo phase transitions, for which the ratio c/a
falls within the narrow range 2.51 < c/a < 2.54.
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