.
Many transition-metal oxides show very large (“colossal”) magnitudes of the dielectric constant and thus have immense potential for applications in modern microelectronics and for the development ...of new capacitance-based energy-storage devices. In the present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially emphasising effects generated by external and internal interfaces, including electronic phase separation. In addition, we provide a detailed overview and discussion of the dielectric properties of CaCu
3
Ti
4
O
12
and related systems, which is today’s most investigated material with colossal dielectric constant. Also a variety of further transition-metal oxides with large dielectric constants are treated in detail, among them the system La
2−x
Sr
x
NiO
4
where electronic phase separation may play a role in the generation of a colossal dielectric constant.
Single crystals of La2RuO5 were obtained from a BaCl2 flux. The structure was determined by single crystal x-ray diffraction and compared to earlier x-ray and neutron powder diffraction results. The ...local structures of Ru and La/Ln in pure La2RuO5 and the rare earth substituted La2−xLnxRuO5 (Ln=Pr, Nd, Sm, Gd, Dy) polycrystalline samples were determined from the extended x-ray absorption fine structures (EXAFS) of the K- and LIII-edges of Ru, La, and Ln, respectively. A four shell model was developed to reduce the number of refinable parameters in the fit of the EXAFS spectra. The distribution of the Ln-ions in the layered crystal structure was determined by a comparison of the coordination spheres obtained from the Ln-edges with the unsubstituted La2RuO5. Interatomic distances were compared to the values obtained from the single crystal diffraction and were found to agree very well.
The crystal structure of La2−xLnxRuO5 (Ln=Pr, Nd, Sm, Gd, Dy) is shown viewed along the c-axis. The alternating stacking of LaRuO4 and LnO layers leads to the formation of zig-zag layers of corner sharing RuO6 octahedra. The La sites in the LaRuO4 layers are represented by light blue spheres, while the La/Ln sites in the LnO layers are colored dark blue. EXAFS investigations reveal a cationic ordering with roughly 65% of the substituting Ln ions occupying the LnO layers. Display omitted
► Structure determination of La2RuO5 by single crystal x-ray diffraction. ► Ru-K EXAFS investigations of La2−xLnxRuO5 powder samples. ► EXAFS of La and Ln in La2−xLnxRuO5 powder samples using K- and LIII-absorption edges. ► Good agreement of EXAFS results and crystal structure data despite low symmetry space group. ► Observed partial cationic ordering of the inserted Ln on the two La sites.
Three charge-ordering lanthanum nickelates La
2−
x
A
x
NiO
4
, substituted with specific amounts of A = Sr, Ca, and Ba to achieve commensurate charge order, are investigated using broadband ...dielectric spectroscopy up to GHz frequencies. The transition temperatures of the samples are characterized by additional specific heat and magnetic susceptibility measurements. We find colossal magnitudes of the dielectric constant for all three compounds and strong relaxation features, which partly are of Maxwell-Wagner type arising from electrode polarization. Quite unexpectedly, the temperature-dependent colossal dielectric constants of these materials exhibit distinct anomalies at the charge-order transitions. This phenomenon is ascribed to a variation of intrinsic material properties affecting the formation of depletion layers at the electrode-sample interfaces.
The oxygen stoichiometries of pure and rare-earth substituted La2RuO5 have been investigated by thermogravimetry (TG) in reducing atmosphere. Assuming that the observed total weight loss is caused by ...the reduction of Ru4+ to Ru-metal, remarkable oxygen deficiencies were calculated. These would correspond to ruthenium oxidation states significantly lower than the ones experimentally observed by XANES. To explain this discrepancy we investigated the reduction products by X-ray absorption spectroscopy (XAS). EXAFS measurements at the Ru–K edge revealed the presence of an X-ray amorphous ruthenium oxide, indicating an incomplete reduction. The apparent oxygen deficiencies obtained for pure and rare-earth substituted samples correlate with the amount of remaining ruthenium oxide. The presence of a ruthenium oxide species was furthermore verified by Ru–LIII XANES investigations. Our results show that the determination of oxygen contents by thermogravimetry might fail even for the easily reducable nobel metal oxides and therefore has to be applied with caution if the reaction products cannot be identified unambiguously.
Display omitted
Quantum Criticality in Transition-Metal Oxides Büttgen, N.; Krug von Nidda, H.-A.; Kraetschmer, W. ...
Journal of low temperature physics,
10/2010, Letnik:
161, Številka:
1-2
Journal Article
Recenzirano
We report on experiments of the bulk susceptibility
χ
(
T
), heat capacity
C
(
T
)/
T
, resistivity
ρ
(
T
) and nuclear resonances (NMR and NQR) in order to review evidence of quantum critical ...behaviour in some metallic transition-metal oxides. In analogy to the conventional 4
f
- and 5
f
-electron based heavy-fermion compounds, the prerequisites of quantum criticality, i.e. a magnetic phase transition at
T
=0 accompanied by non-Fermi liquid behaviour, are observed in LiV
2
O
4
which was the first transition-metal oxide showing heavy-fermion properties. Furthermore, we investigate the large group of copper-ruthenates of the composition
A
Cu
3
Ru
x
Ti
4−
x
O
12
which also demonstrate heavy-fermion and non-Fermi liquid properties. For LaCu
3
Ru
x
Ti
4−
x
O
12
we establish a magnetic phase diagram where the substitution of titanium by ruthenium cations suppresses long-range magnetic order in favour of a magnetic quantum critical point, which comes along with a metal-to-insulator transition. Our experiments elucidate open questions concerning the nature of the heavy-fermion properties, intermediate valence, and the origin of the metal-to-insulator transition. Specifically, the role of the ruthenium and copper
d
-electrons is addressed: the metal-to-insulator transition turns out to correlate with the increasing itinerancy of local moments at the Cu site in our doping experiments.
La2RuO5 shows a magneto-structural phase transition at 161 K with spin dimerization and concomitant formation of a non-magnetic singlet ground state. To gain a deeper insight into the origin of this ...transition systematic substitution of Ru by Ti has been carried out. Polycrystalline samples have been synthesized by thermal decomposition of citrate precursors leading to La2Ru1−yTiyO5 (0 ≤ y ≤ 0.45). The crystal structure was investigated by x-ray powder diffraction at room temperature and at 100 K. The valences of Ti and Ru were obtained from x-ray absorption near edge structure spectroscopy at the Ti-K and the Ru-LIII absorption edges, respectively. The magnetic phase transition was investigated by magnetic susceptibility measurements as a function of Ti substitution, revealing a decreasing transition temperature on increasing the level of substitution. The step-like feature in the magnetic susceptibility reflecting the Ru-Ru spin dimerization transition becomes smeared out close to y = 0.3 and completely vanishes at y = 0.45, indicating complete suppression of spin-dimer formation. Additional specific-heat measurements show a continuous decrease of the magnetic entropy peak with increasing Ti substitution mirroring the reduced number of spin dimers due to the magnetic dilution. A magnetic anomaly of the dimerization transition can hardly be detected for y ≥ 0.3. Density functional theory calculations were carried out to study changes of the electronic band structure caused by the substitution. A possibly preferred distribution of Ti and Ru and the magnetic interactions as well as the change of the density of states close to the Fermi level are investigated. Based on these experimental results a detailed (y,T) phase diagram is proposed.
.
We report on magnetic and electronic properties of various perovskite-type oxides containing 4d- and 5d-transition metals. The compounds under investigation crystallize in (distorted) cubic, ...layered, and hexagonal perovskite-related structures. These changes in structural dimensionality are reflected by different ordering phenomena. (Pseudo-) cubic perovskites ACu
3
B
4
O
12
(with A = alkali, alkaline earth or rare earth; B = Ru, Ti) possess an A-site ordered structure with copper on modified A-positions. Structural investigations as well as XANES (X-ray absorption near edge structure) measurements indicate a valence degeneracy, which is keeping the oxidation state of Ru close to +4. Upon replacing Ru by Ti, the itinerant magnetism and metallic conductivity of the pure ruthenates successively change to a localized magnetic moment and a semiconducting behavior. The pure titanates like Ln
2/3
Cu
3
Ti
4
O
12
or CaCu
3
Ti
4
O
12
are insulators with colossal dielectric constants. The cation-deficient Cu
2+x
Ta
4
O
12+δ
shows a large compositional flexibility with 0.125 ≤ x ≤ 0.500. Both copper content and cooling speed have a strong impact on the crystal structure and the observed magnetic ordering. This behavior can be explained by uncompensated Cu
2+
-moments resulting from different site occupations. Quasi-2D La
2
RuO
5
undergoes a structural and magnetic phase transition at roughly 160 K, leading to a diminishing magnetic moment and a semiconductor-semiconductor transition. LDA calculations reveal an antiferromagnetic coupling within pairs of neighboring Ru
4+
-ions, leading to a spin-Peierls like transition. New hexagonal perovskites containing Ru, Ir, and Pt crystallize in the AO
1+δ
A
2
BO
6
structure type and contain peroxide ions (O) in the AO
1+δ
layers. La
1.2
Sr
2.7
IrO
7.33
exhibits a small temperature-independent paramagnetism, which can be explained on basis of the crystal-field splitting and the strong spin-orbit coupling. The isostructural La
1.2
Sr
2.7
RuO
7.33
shows a frustrated magnetic ordering at roughly 6 K. The frustration results from the alignment of Ru
5+
-ions, which form elongated, edge-sharing Ru
4
-tetrahedra. Substituting La
3+
by the smaller Nd
3+
results in shorter Ru–Ru distances and leads to an increase of the frustrated magnetic interaction.
PDF
