Synthesis and magnetic properties of 12L-perovskites Ba4LnIr3O12 (Ln=lanthanides)

New quadruple perovskite oxides Ba₄LnIr₃O₁₂ (Ln=lanthanides) were prepared and their magnetic properties were investigated. They crystallize in the monoclinic 12L-perovskite-type structure with space group C2/m. The Ir₃O₁₂ trimers and LnO₆ octahedra are alternately linked by corner-sharing and form the perovskite-type structure with 12 layers. The Ln and Ir ions are both in the tetravalent state for Ln=Ce, Pr, and Tb compounds , and for other compounds (Ln=La, Nd, Sm-Gd, Dy-Lu), Ln ions are in the trivalent state and the mean oxidation state of Ir ions is . An antiferromagnetic transition has been observed for Ln=Ce, Pr, and Tb compounds at 10.5, 35, and 16 K, respectively, while the other compounds are paramagnetic down to 1.8 K.     

Magnetic properties of orthorhombic fluorite-related oxides Ln3SbO7 (Ln=rare earths)

Ternary rare earth antimonates Ln₃SbO₇ (Ln=rare earths) were prepared and their structures were determined by X-ray diffraction measurements. They crystallize in an orthorhombic superstructure of cubic fluorite (space group Cmcm for Ln=La, Pr, Nd; C222₁ for Ln=Nd-Lu), in which Ln³⁺ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). Their magnetic properties were characterized by magnetic susceptibility and specific heat measurements from 1.8 to 400 K. The Ln₃SbO₇ (Ln=Nd, Gd-Ho) compounds show an antiferromagnetic transition at 2.2-3.2 K. Sm₃SbO₇ and Eu₃SbO₇ show van Vleck paramagnetism. Measurements of the specific heat down to 0.4 K for Gd₃SbO₇ and the analysis of the magnetic specific heat indicate that the antiferromagnetic ordering of the 8-coordinated Gd ions occur at 2.6 K, and the 7-coordinated Gd ions order at a furthermore low temperature.     

Magnetic properties of EuLn2O4 (Ln=rare earths)

Ternary rare earth oxides EuLn₂O₄ (Ln=Gd, Dy-Lu) were prepared. They crystallized in an orthorhombic CaFe₂O₄-type structure with space group Pnma. ¹⁵¹Eu Mössbauer spectroscopic measurements show that the Eu ions are in the divalent state. All these compounds show an antiferromagnetic transition at 4.2-6.3 K. From the positive Weiss constant and the saturation of magnetization for EuLu₂O₄, it is considered that ferromagnetic chains of Eu²⁺ are aligned along the b-axis of the orthorhombic unit cell, with neighboring Eu²⁺ chains antiparallel. When Ln=Gd-Tm, ferromagnetically aligned Eu²⁺ ions interact with the Ln³⁺ ions, which would overcome the magnetic frustration of triangularly aligned Ln³⁺ ions and the EuLn₂O₄ compounds show a simple antiferromagnetic behavior.     

Structures, magnetic, and thermal properties of Ln3MoO7 (Ln=La, Pr, Nd, Sm, and Eu)

Ternary lanthanide-molybdenum oxides Ln₃MoO₇ (Ln=La, Pr, Nd, Sm, Eu) have been prepared. Their structures were determined by X-ray diffraction measurements. They crystallize in a superstructure of cubic fluorite and the space group is P2₁2₁2₁. The Mo ion is octahedrally coordinated by six oxygens and the slightly distorted octahedra share corners forming a zig-zag chain parallel to the b-axis. These compounds have been characterized by magnetic susceptibility and specific heat measurements. The La₃MoO₇ shows complex magnetic behavior at 150 and 380 K. Below these temperatures, there is a large difference in the temperature-dependence of the magnetic susceptibility measured under zero-field-cooled condition and under field-cooled condition. The Nd₃MoO₇ show a clear antiferromagnetic transition at 2.5 K. From the susceptibility measurements, both Pr₃MoO₇ and Sm₃MoO₇ show the existence of magnetic anomaly at 8.0 and 2.5 K, respectively. The results of the specific heat measurements also show anomalies at the corresponding magnetic transition temperatures. The differential scanning calorimetry measurements indicate that two phase-transitions occur for any Ln₃MoO₇ compound in the temperature range between 370 and 710 K.     

X-ray and neutron powder diffraction study of the double perovskites Ba2LnSbO6 (Ln=La, Pr, Nd and Sm)

The crystal structures of Ba₂LnSbO₆ (Ln=La, Pr, Nd and Sm) at room temperature have been investigated by profile analysis of the Rietveld method using either combined X-ray and neutron powder diffraction data or X-ray powder diffraction data. It has been shown that the structure of Ba₂LnSbO₆ with Ln =La, Pr and Nd are neither monoclinic nor cubic as were previously reported. They are rhombohedral with the space group . The distortion from cubic symmetry is due to the rotation of the LnO₆/SbO₆ octahedra about the primitive cubic [111]_p-axis. On the other hand, the structure of Ba₂SmSbO₆ is found to be cubic. All compounds contain an ordered arrangement of LnO₆ and SbO₆ octahedra.     

Magnetic and electrical properties of quadruple perovskites with 12 layer structures Ba4LnM3O12 (Ln=rare earths; M=Ru, Ir): The role of metal-metal bonding in perovskite-related oxides

Structures and magnetic and electrical properties of quadruple perovskites containing rare earths Ba₄LnM₃O₁₂ (Ln=rare earths; M=Ru, Ir) were investigated. They crystallize in the 12L-perovskite-type structure. Three MO₆ octahedra are connected to each other by face-sharing and form a M₃O₁₂ trimer. The M₃O₁₂ trimers and LnO₆ octahedra are alternately linked by corner-sharing, forming the perovskite-type structure with 12 layers. For Ln=Ce, Pr, and Tb, both the Ln and M ions are in the tetravalent state (Ba₄Ln⁴⁺M⁴⁺₃O₁₂), and for other Ln ions, Ln ions are in the trivalent state and the mean oxidation state of M ions is +4.33 (Ba₄Ln³⁺M^4.33+₃O₁₂). All the Ba₄Ln³⁺Ru^4.33+₃O₁₂ compounds show magnetic ordering at low temperatures, while any of the corresponding iridium-containing compounds Ba₄Ln³⁺Ir^4.33+₃O₁₂ is paramagnetic down to 1.8 K. Ba₄Ce⁴⁺Ir⁴⁺₃O₁₂ orders antiferromagnetically at 10.5 K, while the corresponding ruthenium-containing compound Ba₄Ce⁴⁺Ru⁴⁺₃O₁₂ is paramagnetic. These magnetic results were well understood by the magnetic behavior of M₃O₁₂. The effective magnetic moments and the entropy change for the magnetic ordering show that the trimers Ru^4.33+₃O₁₂ and Ir⁴⁺₃O₁₂ have the S= ground state, and in other cases there is no magnetic contribution from the trimers Ru⁴⁺₃O₁₂ or Ir^4.33+₃O₁₂.Measurements of the electrical resistivity of Ba₄LnM₃O₁₂ and its analysis show that these compounds demonstrate two-dimensional Mott-variable range hopping behavior.     

Crystal structures, magnetic and thermal properties of Ln3IrO7 (Ln=Pr, Nd, Sm, and Eu)

Ternary iridium oxides Ln₃IrO₇ (Ln=Pr, Nd, Sm, and Eu) were prepared and their crystal structures, magnetic and thermal properties were investigated. Powder X-ray diffractions (XRDs) were measured for all samples and neutron diffraction (ND) measurements were performed for Pr₃IrO₇. All the profiles were refined with space group Cmcm (No. 63). The lattice parameters for Pr₃IrO₇ refined by using ND data are a=10.9782(13) Å, b=7.4389(9) Å, and c=7.5361(9) Å. From specific heat and differential thermal analysis (DTA) measurements, Ln₃IrO₇ (Ln=Pr, Nd, Sm, and Eu) show thermal anomalies at 261, 342, 420, and 485 K, respectively. The results of powder high-temperature XRD and ND measurements indicate that these anomalies are due to the structural phase transition. Magnetic susceptibilities of these compounds were measured in the temperature range between 1.8 and 400 K. Nd₃IrO₇ shows an antiferromagnetic transition at 2.6 K. A specific heat anomaly has also been observed at the same temperature. For Ln₃IrO₇ (Ln=Pr, Sm, and Eu), no magnetic anomalies have been found in the experimental temperature range.     

Crystal structures and magnetic properties of fluorite-related oxides Ln3NbO7 (Ln=lanthanides)

Crystal structures and magnetic properties of the ternary oxides Ln₃NbO₇ (Ln=La, Pr, Nd, Sm-Lu) are reported. Their powder X-ray diffraction measurements and Rietveld analyzes show that they have the fluorite-related structures with space group Pnma (Ln=La, Pr, Nd), C222₁ (Ln=Sm-Tb), or Fm-3m (Ln=Dy-Lu). Magnetic susceptibility measurements were carried out from 1.8 to 400 K. The Ln₃NbO₇ compounds for Ln=Pr, Gd, Dy-Yb show Curie-Weiss paramagnetic behavior, and Sm₃NbO₇ and Eu₃NbO₇ show van Vleck paramagnetism. On the other hand, two magnetic anomalies were observed for both Nd₃NbO₇ (0.6 and 2.7 K) and Tb₃NbO₇ (2.0 and 3.2 K). From the results of specific heat measurements, it was found that these anomalies are due to the antiferromagnetic ordering of Ln ions in two different crystallographic sites (the 8-coordinated and 7-coordinated sites).     

Structural phase transition and magnetic properties of double perovskites Ba2CaMO6 (M=W, Re, Os)

Structures and magnetic properties for double perovskites Ba₂CaMO₆ (M=W, Re, Os) were investigated. Both Ba₂CaReO₆ and Ba₂CaWO₆ show structural phase transitions at low temperatures. For Ba₂CaReO₆, the second order transition from cubic to tetragonal I4/m has been observed near 120 K. For Ba₂CaWO₆, the space group of the crystal structure is I4/m at 295 K and the transition to monoclinic I2/m has been observed between 220 K. Magnetic susceptibility measurements show that Ba₂CaReO₆ (S=1/2) and Ba₂CaOsO₆ (S=1) transform to an antiferromagnetic state below 15.4 and 51 K, respectively. Anomalies corresponding to their structural phase transition and magnetic transition have been also observed through specific heat measurements.     

Magnetic properties of lanthanide rhenium oxides Ln3ReO7 (Ln=Sm, Eu, Ho)

Ternary lanthanide rhenium oxides Ln₃ReO₇ (Ln=Sm, Eu, Ho) were prepared and their structures were determined by X-ray diffraction measurements. They crystallize in an orthorhombic superstructure of cubic fluorite (space group Cmcm for Ln=Sm, Eu; C222₁ for Ln=Ho). The magnetic properties were characterized by magnetic susceptibility and specific heat measurements from 1.8 to 400 K. The Sm₃ReO₇ shows an antiferromagnetic transition at 1.9 K. The Eu₃ReO₇ indicates a magnetic anomaly at 12 K. On the other hand, the results of the specific heat measurements indicate that both Sm₃ReO₇ and Eu₃ReO₇ undergo a structure transition at 270 and 350 K, respectively. The Ho₃ReO₇ is paramagnetic down to 1.8 K.     

Structural and magnetic properties of the quaternary oxides Ba6Ln2Fe4O15 (Ln=Pr and Nd)

The crystal structures and magnetic properties of the quaternary lanthanide oxides Ba₆Ln₂Fe₄O₁₅ (Ln=Pr and Nd) are reported. They crystallize in a hexagonal structure with space group P6₃mc and have the “Fe₄O₁₅ cluster” consisting of one FeO₆ octahedron and three FeO₄ tetrahedra. Measurements of the magnetic susceptibility, specific heat, and powder neutron diffraction reveal that this cluster behaves as a spin tetramer with a ferrimagnetic ground state of S_T=5 even at room temperature. The cluster moments show a long-range antiferromagnetic ordering at 23.2 K (Ln=Pr) and 17.8 K (Nd), and the magnetic moments of the Ln³⁺ ions also order cooperatively. By applying the magnetic field (∼2 T), this antiferromagnetic ordering of the clusters changes to a ferromagnetic one. This result indicates that there exists a competition in the magnetic interaction between the clusters.     

Studies on magnetic susceptibility and specific heat for 6H-perovskite-type oxides Ba3LnIr2O9 (Ln=La, Nd, Sm-Yb)

Magnetic properties of the 6H-perovskite-type oxides Ba₃LnIr₂O₉ (Ln=La and Nd: monoclinic; Ln=Sm-Yb: hexagonal symmetry) were investigated. For all the title compounds, a specific heat anomaly was found at 5.3-17.4 K. At the corresponding temperatures, the magnetic susceptibilities show a slight variation in its gradient. These magnetic anomalies suggest the magnetic ordering of the magnetic moments (S=1/2) remaining in the Ir^4.5+₂O₉ face-shared bioctahedra. In addition, the Ln³⁺ ions show the onset of the antiferromagnetic ordering around these temperatures. The Ba₃NdIr₂O₉ only shows a ferromagnetic behavior below 17.4 K with a remnant magnetization of 1.25 μ_B. This behavior may be due to the ferromagnetic ordering of the Nd³⁺ moments.     

New insight into the symmetry and the structure of the double perovskites Ba2LnNbO6 (Ln=lanthanides and Y)

Structures of the double perovskites Ba₂LnNbO₆ (Ln=La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, and Y) at room temperature have been re-examined by Rietveld profile analysis of X-ray diffraction data. It was shown that the correct phase sequence across the lanthanides is I2/m (Ln=La, Pr, Nd, and Sm), I4/m (Ln=Eu, Gd, Tb, and Dy), and (Ln=Ho and Y), respectively. All phases can be derived from the ideal cubic perovskite by ordering the Ln(III) and Nb(V) ions and by out-of-phase tilting the LnO₆/NbO₆ octahedra around either the primitive two-fold [110]_p-axis (I2/m) or the four-fold [001]_p-axis (I4/m). The monoclinic P2₁/n structure that contains both out-of-phase and in-phase tilt around the primitive [110]_p- and [001]_p-axis, respectively, has not been observed for this series of compounds.     

Magnetic properties and structural transitions of orthorhombic fluorite-related compounds Ln3MO7 (Ln=rare earths, M=transition metals)

Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln₃MO₇ (Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln₃MoO₇ (Ln=La-Gd). They crystallize in an orthorhombic superstructure of cubic fluorite with space group P2₁2₁2₁, in which Ln³⁺ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). All of these compounds show a phase transition from the space group P2₁2₁2₁ to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility measurements from 1.8 to 400 K and specific heat measurements from 0.4 to 400 K. Gd₃MoO₇ shows an antiferromagnetic transition at 1.9 K. Measurements of the specific heat for Sm₃MoO₇ and the analysis of the magnetic specific heat indicate a “two-step” antiferromagnetic transition due to the ordering of Sm magnetic moments in different crystallographic sites, i.e., with decreasing temperature, the antiferromagnetic ordering of the 7-coordinated Sm ions occur at 2.5 K, and then the 8-coordinated Sm ions order at 0.8 K. The results of Ln₃MoO₇ were compared with the magnetic properties and structural transitions of Ln₃MO₇ (M=Nb, Ru, Sb, Ta, Re, Os, or Ir).     

Crystal growth of a series of lithium garnets Ln3Li5Ta2O12 (Ln=La, Pr, Nd): Structural properties, Alexandrite effect and unusual ionic conductivity

We report the single crystal structures of a series of lanthanide containing tantalates, Ln₃Li₅Ta₂O₁₂ (Ln=La, Pr, Nd) that were obtained out of a reactive lithium hydroxide flux. The structures of Ln₃Li₅Ta₂O₁₂ were determined by single crystal X-ray diffraction, where the Li⁺ positions and Li⁺ site occupancies were fixed based on previously reported neutron diffraction data for isostructural compounds. All three oxides crystallize in the cubic space group (No. 230) with lattice parameters a=12.7735(1), 12.6527(1), and 12.5967(1) Å for La₃Li₅Ta₂O₁₂, Pr₃Li₅Ta₂O₁₂, and Nd₃Li₅Ta₂O₁₂, respectively. A UV-Vis diffuse reflectance spectrum of Nd₃Li₅Ta₂O₁₂ was collected to explain its unusual Alexandrite-like optical behavior. To evaluate the transport properties of Nd₃Li₅Ta₂O₁₂, the impedance data were collected in air in the temperature range 300?T(°C)?500.     

Crystal growth, structure and magnetic properties of the double perovskites Ln2MgIrO6 (Ln=Pr, Nd, Sm-Gd)

Single crystals of double-perovskite type lanthanide magnesium iridium oxides, Ln₂MgIrO₆ (Ln=Pr, Nd, Sm-Gd) have been grown in a molten potassium hydroxide flux. The compounds crystallize in a distorted 1:1 rock salt lattice, space group P2₁/n, consisting of corner shared MO₆ (M=Mg²⁺ and Ir⁴⁺) octahedra, where the rare earth cations occupy the eight-fold coordination sites formed by the corner shared octahedra. Pr₂MgIrO₆, Nd₂MgIrO₆, Sm₂MgIrO₆, and Eu₂MgIrO₆ order antiferromagnetically around 10-15 K.     

Raman scattering study and electrical properties characterization of elpasolite perovskites Ln2(BB′)O6 (Ln=La, Sm…Gd and B,B′=Ni, Co, Mn)

Two series of elpasolite perovskites Ln₂CoMnO₆ and Ln₂NiMnO₆ (Ln=La, Pr, Nd, Sm, Gd) have been prepared. The electronic band gap and magnetic Curie temperature vary systematically as a function of the rare earth cation size within both series. Here we used Raman scattering spectroscopy along with the results of previous structural studies to show that there is little change in octahedral distortion but significant changes in the octahedral tilting angle upon decreasing lanthanide ionic radius. The data indicate differences in the orbital overlap and bond strengths between the two series of materials that allow us to understand variations in the magnetic and electrical properties within and between the two perovskite series.     

Crystal structure of Ln1/3NbO3 (Ln=Nd, Pr) and phase transition in Nd1/3NbO3

The crystal structure of the A-site deficient perovskite Ln_1/3NbO₃ (Ln=Nd, Pr) at room temperature has been determined, for the first time, as orthorhombic in space group Cmmm using high-resolution neutron powder diffraction. Pertinent features are the alternation of unoccupied layers of A-sites and layers partly occupied by Ln cations, as well as out-of-phase tilting of the NbO₆ octahedra around an axis perpendicular to the direction of the cation/vacancy ordering. The phase transition behaviour of Nd_1/3NbO₃ has also been studied in situ. This compound undergoes a continuous phase transition at around 650 °C to a tetragonal structure in space group P4/mmm due to the disappearance of the octahedral tilting. The analysis of spontaneous strains shows that this phase transition is tricritical in nature.     

Magnetic properties of ternary sodium oxides NaLnO2 (Ln=rare earths)

Magnetic properties of ternary sodium oxides NaLnO₂ (Ln=rare earths) are investigated. Their crystal structures are grouped into three types of structures, which are α-LiFeO₂, β-LiFeO₂, and α-NaFeO₂, depending on the size of rare earths. Their magnetic susceptibilities and specific heats have been measured from 1.8 to 300 K. Among them, NaGdO₂, NaDyO₂, and NaHoO₂ show antiferromagnetic transitions at 2.4, 2.2, and 2.4 K, respectively, and NaNdO₂ transforms to the ferromagnetic state below 2.4 K. NaSmO₂, NaErO₂, and NaYbO₂ exhibit a magnetic anomaly below 1.8 K.