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September 3, 2003

Giant Dielectric Effect in CaCu₃Ti₄O₁₂ and its Cd Analogue

C.C. Homes¹, T. Vogt¹, S.M. Shapiro¹, S. Wakimoto^1,2, M.A. Subramanian³, and A.P. Ramirez^4
1Department of Physics, Brookhaven National Laboratory, Upton, NY; ²Department of Physics, Massachusetts Institute of Technology, Cambridge, MA; ³DuPont Central Research & Development Experimental Station, Wilmington, DE; ⁴Materials Integration Science Laboratory, K764, Los Alamos National Laboratory, Los Alamos, NM

The perovskite-related material CaCu₃Ti₄O₁₂ has a very high static dielectric constant ε₀~104 at room temperature, which drops to about 100 at low temperature. Substituting Cd for Ca reduces the room temperature value by over an order of magnitude. The origin of the large ε₀ is not fully understood, but may be due to an internal barrier layer capacitance (IBLC) effect. Infrared measurements on the Ca and Cd compounds show dramatic changes in the nature of the normal modes at low temperature, suggesting that increasing electronic localization may lead to a breakdown of the IBLC effect.

Materials with large dielectric constants are highly sought after in the microelectronics industry for use in memory devices; the static dielectric constant ε₀ acts as a scaling factor and ultimately determines the level of miniaturization. One of the most commonly used dielectric materials is silicon nitride with ε₀~7; materials with ε₀>7 are generally referred to as high-dielectric constant materials. It was recently noted that the cubic perovskite-related material CaCu₃Ti₄O₁₂ shown in Figure 1 had a tremendously high dielectric constant at room temperature, ε₀~104, which showed little temperature dependence until about 100 K, below which it decreased by nearly two orders of magnitude to ε₀~100, shown in Figure 2a. The substitution of Cd for Ca results in a large reduction of ε₀, shown in Figure 2b. It was initially suggested that this large value for ε₀ was a purely extrinsic effect due to Maxwell-Wagner-type depletion layers forming between the sample and the metal contacts used in the capacitive measurements of ε₀ in the kHz range. This effect has indeed been observed in a number of different materials. However, the addition of a thin a buffer layer of a dielectric with well-known electrical properties (aluminum oxide) has demonstrated that the large value for ε₀ persists in these materials, and is therefore a property of the bulk. The dramatic drop of ε₀ at low temperature is curious because it is not accompanied by any change in the long-range crystallographic structure when probed by high-resolution x-ray and neutron powder diffraction. In ferroelectric materials, large changes in ε₀ are usually accompanied by a structural distortion and soft-mode condensation.

Infrared (IR) spectroscopy is a powerful tool to study the lattice vibrations in insulators, in particular soft-mode condensation. The large change in ε₀ and the absence of any structural distortion suggested that the normal modes should be examined. The temperature dependence of the reflectance was determined over a wide frequency range using the spectrometer at NSLS beamline U10A and the complex optical properties determined from a Kramers-Kronig analysis. The real part of the dielectric function ε₁ is shown in Figures 3a and 3b for CaCu₃Ti₄O₁₂ and CdCu₃Ti₄O₁₂, respectively, at a variety of temperatures in the infrared region.

The dielectric function is dominated by the infrared active lattice modes. (The units of cm^-1 are commonly used in IR spectroscopy, 1 THz = 30 cm^-1.) While these experiments are done in the THz range, the low-frequency value for ε₁ agrees with the values for ε₀ (kHz) at low temperature. It is clear from Figure 3 that ε₁ at low frequency is also increasing at low temperature. Fits to simple Lorentzian oscillators describe these features quite well and indicate that many of the vibrational modes are gaining strength at low temperature, a violation of the partial f-sum rule for oscillators. From this result, it may be inferred that the Born effective charge on the atoms is changing and that the material is becoming more ionic. We (along with others) have speculated that the large value of ε₀ is due to an internal barrier layer capacitance (IBLC) effect due to extensive twinning or grain boundaries.

This view is consistent with impedance spectroscopy that indicates these materials may be described as semiconducting regions separated by insulating barriers. The increase in ionicity implies increasing charge localization at low temperatures. This localization may lead to an increase in the size of the insulating regions and a commensurate reduction of ε₀ within the IBLC picture, suggesting that the large value for ε₀ is, at least in part, due to “extrinsic” effects.

BEAMLINE
U10A

FUNDING
Department of Energy

PUBLICATION
C. C. Homes et al., “Charge transfer in the high dielectric constant materials CaCu₃Ti₄O₁₂ and CdCu₃Ti₄O₁₂”, Phys. Rev. B 67, 092106 (2003).