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Soft-mode behavior and incipient ferroelectricity in Na 1 / 2 Bi 1 / 2 Cu 3 </mml…

2010; American Physical Society; Volume: 81; Issue: 22 Linguagem: Inglês

10.1103/physrevb.81.224112

1550-235X

Matthew C. Ferrarelli, D. Nuzhnyy, Derek C. Sinclair, S. Kamba,

Multiferroics and related materials

The correlation between crystal structure and intrinsic dielectric properties of the unusual cubic perovskite-related family of compounds $A{\text{Cu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$, where $A=\text{Ca}$, Cd, ${\text{La}}_{2/3}$, ${\text{Bi}}_{2/3}$, ${\text{Na}}_{1/2}{\text{La}}_{1/2}$, ${\text{Na}}_{1/2}{\text{Bi}}_{1/2}$, etc., (space group $Im\overline{3}$ ) has been a controversial topic for several years, especially for the most studied member of the family ${\text{CaCu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$ (CCTO). ${\text{Na}}_{1/2}{\text{Bi}}_{1/2}{\text{Cu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$ (NBCTO) is isostructural with CCTO and retains centrosymmetric and cubic symmetry (space group $Im\overline{3}$) down to 4 K. NBCTO ceramics exhibit high intrinsic relative permittivity of $\ensuremath{\sim}145$ at 300 K that rises on cooling and levels off below $\ensuremath{\sim}50\text{ }\text{K}$ with a value of $\ensuremath{\sim}260$ at 10 K without any signature of a structural transition. Such behavior is a typical feature of incipient ferroelectrics such as the perovskite ${\text{CaTiO}}_{3}$. Infrared and terahertz spectroscopy on NBCTO ceramics are presented and reveal a room-temperature soft polar optic mode at $\ensuremath{\sim}34\text{ }{\text{cm}}^{\ensuremath{-}1}$ that softens on cooling and is responsible for the temperature dependence of the relative permittivity. The temperature dependence of the soft-mode frequency obeys the Barrett formula with the onset of quantum fluctuations ${T}_{1}$ occurring near 85 K and a hypothetical Curie temperature ${T}_{0}$ of $\ensuremath{\sim}\ensuremath{-}155\text{ }\text{K}$. A fitted vibrational zero-point energy $({k}_{\text{B}}{T}_{1})/2$ is in good agreement with the measured soft-mode frequency. The crystal structure-intrinsic dielectric properties of NBCTO and possibly all cubic $A{\text{Cu}}_{3}{\text{Ti}}_{4}{\text{O}}_{12}$ phases are, therefore, consistent with that of other untilted/tilted ${\text{TiO}}_{3}$-based centrosymmetric perovskites such as ${\text{CaTiO}}_{3}$ and ${\text{SrTiO}}_{3}$. The ``so-called'' giant permittivity values of $>1000$ reported for CCTO, NBCTO, and related phases from radio-frequency capacitance measurements near room temperature are an extrinsic effect associated with the semiconducting nature of these materials, as opposed to an intrinsic effect associated with their rather unusual perovskite-type crystal structure.