Rattling Behavior in a Simple Perovskite NaWO₃

Area of Science:

• Solid State Chemistry
• Materials Science
• Phononics

Background:

• Rattling phenomena, characterized by low-frequency vibrations of guest atoms within cage-like structures, are known to suppress thermal conductivity.
• These phenomena are typically observed in materials with large cage structures, such as clathrates and filled skutterudites, but not in simple ABO3-type perovskites due to their ability to relieve size mismatch via octahedral rotations.

Purpose of the Study:

• To investigate the potential for rattling phenomena in stoichiometric perovskite oxides.
• To demonstrate the induction of rattling vibrations in a pristine ABO3 perovskite and its effect on thermal conductivity.

Main Methods:

• High-pressure synthesis of sodium tungstate (NaWO3).
• Structural analysis using X-ray diffraction and crystallographic site determination.
• Phonon mode analysis to identify anharmonicity and rattling behavior.
• Comparative study with potassium tungstate (KWO3) as a non-rattling reference.

Main Results:

• NaWO3, prepared under high pressure, exhibits anharmonic phonon modes indicative of low-energy rattling vibrations.
• The rattling behavior is attributed to the presence of two distinct sodium crystallographic sites (Na1 and Na2) and in-phase octahedral rotations, creating an open space of approximately 0.5 Å at the Na2 site.
• This rattling significantly suppresses the thermal conductivity of NaWO3.
• The rattling mechanism in NaWO3 differs from that in quadruple perovskites AA'3B4O12.

Conclusions:

• Stoichiometric NaWO3 exhibits intrinsic rattling behavior, challenging the notion that ABO3 perovskites cannot host such phenomena.
• The specific crystallographic structure and octahedral rotations in NaWO3 are key to generating the necessary space for rattling.
• This work provides a foundational understanding and a general strategy for inducing atomic rattling in pristine ABO3 perovskite materials, with implications for thermoelectric applications.