Importance of tetrahedral coordination for high-valent transition metal oxides: YCrO4_4 as a model system

We have investigated the electronic structure of the high oxidation state material YCrO$_4$ within the framework of the Zaanen-Sawatzky-Allen phase diagram. While Cr$^{4+}$-based compounds like SrCrO$_3$/CaCrO$_3$ and CrO$_2$ can be classified as small-gap or metallic negative-charge-transfer systems, we find using photoelectron spectroscopy that YCrO$_4$ is a robust insulator despite the fact that its Cr ions have an even higher formal valence state of 5+. We reveal using band structure calculations that the tetrahedral coordination of the Cr$^{5+}$ ions in YCrO$_4$ plays a decisive role, namely to diminish the bonding of the Cr $3d$ states with the top of the O $2p$ valence band. This finding not only explains why the charge-transfer energy remains effectively positive and the material stable, but also opens up a new route to create doped carriers with symmetries different from those of other transition-metal ions.Comment: 6 pages, 6 figure

A comparative three-dimensional neutron depolarization study on RCrO4 oxides (R=Y, Er, Tm, Yb)

Three-dimensional neutron depolarization experiments have been performed on RCrO4 (R=Y, Er, Tm, Yb) powder samples in order to gain insight into their magnetic domain structure in the submicrometer range. The temperature evolution of both the average domain size and the net magnetization of each compound has been studied for different applied magnetic fields. The largest average domain size at zero external magnetic field was found in YbCrO4. The effect of an applied magnetic field on the magnetic domain structure is relatively small in ErCrO4 and TmCrO4, when compared to YCrO4 and YbCrO4 where the average domain size even surpasses the average particle size determined by Scanning Electron Microscopy studies.</p

Inhomogeneous electronic structure probed by spin-echo experiments in the electron doped high-Tc superconductor Pr_{1.85}Ce_{0.15}CuO_{4-y}

63Cu nuclear magnetic resonance (NMR) spin-echo decay rate (T_2^{-1}) measurements are reported for the normal and superconducting states of a single crystal of Pr_{1.85}Ce_{0.15}CuO_{4-y} (PCCO) in a magnetic field B_0=9T over the temperature range 2K<T<200K. The spin-echo decay rate is temperature-dependent for T<55K, and has a substantial dependence on the radio frequency (rf) pulse parameters below T~25K. This dependence indicates that T_2^{-1} is strongly effected by a local magnetic field distribution that can be modified by the rf pulses, including ones that are not at the nuclear Larmor frequency. The low-temperature results are consistent with the formation of a static inhomogeneous electronic structure that couples to the rf fields of the pulses.Comment: 4 pages, 4 figure

Structural and magnetic characterization of zircon-type PrCrO4 oxide

PrCrO4 has been synthesized as a single phase using oxidizing conditions, with a strong control on temperature and time. The zircon-type structure that this compound adopts has been refined from X-ray powder diffraction by the Rietveld method, showing tetragonal symmetry with space group I41/amd, lattice parameters a=7.341(7) Å, c=6.427(1) Å and Z=4. Magnetic susceptibility measurements in the temperature range of 1.9-300 K reveal the presence of incipient ferromagnetic interactions in the Cr5+ sublattice below 12 K. When the temperature decreases, the superexchange Pr3+-O-Cr5+-O-Pr3+ antiferromagnetic interactions become predominant, giving rise to a net maximum at lower temperatures. The estimated Néel temperature is field-dependent, taking the value of 9 K when the magnetic field strength is 50 Oe. The M vs. H plots, at 2 and 8 K, indicate the existence of a metamagnetic transition, having obtained a critical field as low as 399 and 149 Oe respectively at the temperatures mentioned above.</p

Synthesis, structural characterization and magnetic properties of RCrO4 oxides, R=Nd, Sm, Eu and Lu

This paper reports the specific conditions used in the preparation of RCrO4 oxides, R = Nd, Sm, Eu and Lu, the structural determination, and the study of their magnetic behaviour. The structure of these compounds has been refined from X-ray powder diffraction by the Rietveld method, assigning the zircon type, space group I41/amd. A linear decrease has been observed in the lattice parameters from NdCrO4 to LuCrO4, according to the lanthanide contraction. Magnetic susceptibility measurements reveal the existence of an antiferromagnetic ordering in which both Cr5+ and R3+ sublattices are involved. In the case of NdCrO4, the estimated Neel temperature appears to be lower than 2 K; for the remaining oxides, it is 14.9 K, 15.9 K and 9.9 K for Sm, Eu and Lu, respectively. A superexchange mechanism has been proposed to explain such a magnetic behaviour. The pathways through which these interactions take place have also been analyzed, taking into account the structural features that these oxides present. The Cr5+ plays an important role as a promoter of these interactions in the R3+ sublattice.</p

Field-induced magnetic properties in RCrO4 oxides (R=Pr, Gd, Tb, Tm and Yb)

RCrO4 oxides (R=Pr, Gd, Tb, Tm, and Yb) have been synthesized at 773K using the corresponding nitrates as precursors. X-ray diffraction data reveal that these samples are single phases and crystallize with the zircon-type structure, showing tetragonal symmetry, space group I41/amd. All the compounds are antiferromagnetic and the Néel temperature, which depends on the R3+ ion, takes values lower than 30 K. The presence of a canting appears to be responsible for the negative values of the magnetic susceptibility found below the compensation temperature. This uncommon phenomenon is named reversal of magnetization. It is field-dependent, being suppressed at 500 Oe for the TmCrO4 compound. The highest value of the compensation temperature (24K) corresponds to the YbCrO4 oxide. A metamagnetic transition has been observed in all cases at critical fields ranging from 225Oe (GdCrO4) to 1600 Oe (YbCrO4).</p

Magnetic behaviour of ErCrO4 oxide

ErCrO4 has been synthesized as a single phase by the nitrate precursor method under an oxygen flow at 753 K. The structure of this compound has been refined from X-ray and neutron powder diffraction data, showing the zircon-type structure, S.G. I41/amd and lattice parameters a=7.062(1) Å, c=6.201(3) Å. Magnetic susceptibility and magnetization measurements reveal the existence of incipient ferromagnetic interactions in the Cr5+ sublattice below 25 K. When the temperature decreases, the superexchange Er3+-O-Cr5+-O-Er3+ antiferromagnetic interactions become predominant. The estimated Néel temperature is 15 K. A metamagnetic transition has been detected at the critical field of 375 Oe. Neutron diffraction data show a three-dimensional antiferromagnetic ordering in which both Cr5+ and Er3+ sublattices are involved. Such ordering takes place with a propagation vector k=[0].</p

The vibrational spectrum of Nd<SUB>2</SUB>BaCuO<SUB>5</SUB>

Compounds of general composition Ln2BaCuO5 (Ln=lanthanide) adopt different structural types. Two series of structures have been reported for LnzBaCuO5 compounds. For lanthanide ions smaller than Nd(III) (Y and Sm-Lu) the so-called "green phases" (orthorhombic, space group Pbnm) are stabilized. They are one of the main impurities generated during the synthesis of the LnBa2Cu3OT_x-type high-T¢ superconducting oxides. A different structural type (tetragonal, space group P4/mbm) becomes stabilized for Ln = La or Nd, although in the case of the lanthanum-containing material the totally stoichiometric composition does not seem to be attainable.Facultad de Ciencias Exacta