The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and X-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T. At low temperatures the dielectric constant shows a minor but significant dependence on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.

Correlations of structural, magnetic, and dielectric properties of undoped and doped CaCu3Ti4O12

The present work reports synthesis, as well as a detailed and careful characterization of structural, magnetic, and dielectric properties of differently tempered undoped and doped CaCu3Ti4O12 (CCTO) ceramics. For this purpose, neutron and x-ray powder diffraction, SQUID measurements, and dielectric spectroscopy have been performed. Mn-, Fe-, and Ni-doped CCTO ceramics were investigated in great detail to document the influence of low-level doping with 3d metals on the antiferromagnetic structure and dielectric properties. In the light of possible magnetoelectric coupling in these doped ceramics, the dielectric measurements were also carried out in external magnetic fields up to 7 T, showing a minor but significant dependence of the dielectric constant on the applied magnetic field. Undoped CCTO is well-known for its colossal dielectric constant in a broad frequency and temperature range. With the present extended characterization of doped as well as undoped CCTO, we want to address the question why doping with only 1% Mn or 0.5% Fe decreases the room-temperature dielectric constant of CCTO by a factor of ~100 with a concomitant reduction of the conductivity, whereas 0.5% Ni doping changes the dielectric properties only slightly. In addition, diffraction experiments and magnetic investigations were undertaken to check for possible correlations of the magnitude of the colossal dielectric constants with structural details or with magnetic properties like the magnetic ordering, the Curie-Weiss temperatures, or the paramagnetic moment. It is revealed, that while the magnetic ordering temperature and the effective moment of all investigated CCTO ceramics are rather similar, there is a dramatic influence of doping and tempering time on the Curie-Weiss constant.

The transition-metal oxide ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$ with perovskite-type structure shows characteristic properties of an intermediate-valence system. The temperature-dependent susceptibility exhibits a broad maximum around 150--160 K. At this temperature, neutron powder diffraction reveals a small but significant volume change whereby the crystal structure is preserved. Moreover, the temperature-dependent resistivity changes its slope. NMR Knight shift measurements of Ru reveal a cross-over from high temperature paramagnetic behavior of localized moments to itinerant band states at low temperatures. Additional density-functional theory calculations can relate the structural anomaly with the $d$-electron number. The different experimental and calculational methods result in a mutually consistent description of ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$ as an intermediate-valent system in the classical sense of having low-energy charge fluctuations.

/pdf/intermediate-valence-behavior-of-the-transition-metal-oxide-2b0iotf3hp.pdf

Intermediate-valence behavior of the transition-metal oxide CaCu 3 Ru 4 O 12

The magnetic and thermodynamic properties of the complete Ln2/3Cu3Ti4O12 series were investigated. Here Ln stands for the lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. All the samples investigated crystallize in the space group $Im\bar{3}$ with lattice constants that follow the lanthanide contraction. The lattice constant of the Ce compound reveals the presence of Ce4+ leading to the composition Ce1/2Cu3Ti4O12. From magnetic susceptibility and electron-spin resonance experiments it can be concluded that the copper ions always carry a spin S = 1/2 and order antiferromagnetically close to 25 K. The Curie-Weiss temperatures can approximately be calculated assuming a two-sublattice model corresponding to the copper and lanthanide ions, respectively. It seems that the magnetic moments of the heavy rare earths are weakly coupled to the copper spins, while for the light lanthanides no such coupling was found. The 4f moments remain paramagnetic down to the lowest temperatures, with the exception of the Tm compound, which indicates enhanced Van-Vleck magnetism due to a non-magnetic singlet ground state of the crystal-field split 4f manifold. From specific-heat measurements we accurately determined the antiferromagnetic ordering temperature and obtained information on the crystal-field states of the rare-earth ions.

On the magnetism of Ln2/3Cu3Ti4O12 (Ln = lanthanide)

We synthesized ACu3Ru4O12 (A = Na, Na0.5Ca0.5, Ca, Ca0.5La0.5, La) and measured their DC magnetization, AC susceptibility, specific heat, and resistivity, in order to investigate the effects of the hetero-valent substitution. A broad peak in the DC magnetization around 200 K was observed only in CaCu3Ru4O12, suggesting the Kondo effect due to localized Cu2+ ions. However, the electronic specific heat coefficients exhibit large values not only for CaCu3Ru4O12 but also for all the other samples. Moreover, the Wilson ratio and the Kadowaki-Woods ratio of our samples are all similar to the values of other heavy-fermion compounds. These results question the Kondo effect as the dominant origin of the mass enhancement, and rather indicate the importance of correlations among itinerant Ru electrons.

Heavy-Mass Behavior of Ordered Perovskites ACu3Ru4O12 (A = Na, Ca, La)

We present bulk magnetic susceptibility, specific heat, transport, NMR, and NQR results of the perovskite compound CaCu3Ru4O12. The data consistently describe a metallic state with a moderately enhanced Sommerfeld coefficient of =92 mJ /mol K2 and a spin-fluctuation temperature of about 180 K. The heat capacity below 2 K shows a logarithmic increase providing evidence for non-Fermi-liquid NFL behavior in CaCu3Ru4O12. Further signatures of NFL properties are found in NQR measurements revealing deviations from a Korringa behavior of the spin-lattice relaxation rate. The results are discussed with respect to heavyfermion and NFL properties in other transition-metal compounds and traditional f-electron systems.

Non-Fermi-liquid behavior in CaCu3Ru4O12

We present bulk magnetic susceptibility, specific heat, transport, NMR, and NQR results of the perovskite compound ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$. The data consistently describe a metallic state with a moderately enhanced Sommerfeld coefficient of $\ensuremath{\gamma}=92\text{ }\text{mJ}/\text{mol}\text{ }{\text{K}}^{2}$ and a spin-fluctuation temperature of about 180 K. The heat capacity below 2 K shows a logarithmic increase providing evidence for non-Fermi-liquid (NFL) behavior in ${\text{CaCu}}_{3}{\text{Ru}}_{4}{\text{O}}_{12}$. Further signatures of NFL properties are found in NQR measurements revealing deviations from a Korringa behavior of the spin-lattice relaxation rate. The results are discussed with respect to heavy-fermion and NFL properties in other transition-metal compounds and traditional $f$-electron systems.

Non-Fermi-liquid behavior in CaCu 3 Ru 4 O 12

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Intermediate-valence behavior of the transition-metal oxide CaCu 3 Ru 4 O 12

Non-Fermi-liquid behavior in CaCu3Ru4O12

Non-Fermi-liquid behavior in CaCu 3 Ru 4 O 12