Hydrothermal phase equilibria studies in Ln2O3H2O systems and synthesis of cubic lanthanide oxides

Phase diagrams for the systems Ln₂O₃H₂O (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y) studied at 5000 to 10,000 psi and temperature range of 200–900°C, show that Ln(OH)₃ hexagonal and LnOOH monoclinic are the only stable phases from Nd to Ho. The cubic oxide phase (CLn₂O₃) is stable for systems of Er, Tm, Yb and Lu, with no evidence of its equilibrium in the systems of lighter lanthanides. Using strong acids, HNO₃ and HCOOH, as mineralisers the cubic oxides could be stabilised from Eu down to Lu. Solid solution phases of CeO₂Y₂O₃ and Eu₂O₃Y₂O₃ have also been synthesised with HNO₃ as mineraliser, since these compounds have promising use as solid electrolyte and phosphor materials respectively.     

Structure and electrical conductivity of Ln2+x Hf2−x O7−x/2 (Ln = Sm-Tb; x = 0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln_2+x Hf_2?x O_7?δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln₂Hf₂O₇ (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm_2.096Hf_1.904O_6.952 is shown to undergo pyrochlore-disordered pyrochlore-pyrochlore (P-P1-P) phase transformations in the temperature range 1200–1670°C. The former transformation leads to a rise in 840°C conductivity from 10^?4 to 3 × 10^?3 S/cm in the samples synthesized at 1600°C, and the latter leads to a drop in 840°C conductivity to 6 × 10^?4 S/cm in the samples synthesized at 1670°C. The reduction in the conductivity of Sm_2.096Hf_1.904O_6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu_2+x Hf_2?x O_7?δ (x = 0.096) and Ln₂Hf₂O₇ (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu_2.096Hf_1.904O_6.952, Gd₂Hf₂O₇, and Tb₂Hf₂O₇ synthesized at 1670°C: 7.5 × 10^?3, 5 × 10^?3, and 2.5 × 10^?2 S/cm, respectively.     

Synthesis and characterization of ultrafine Ln2Ti2O7 (Ln = Sm,Gd, Dy,Er) pyrochlore oxides by stearic acid method

Stearic acid method (SAM) was developed to synthesize series of pyrochlore Ln₂Ti₂O₇ (Ln = Sm, Gd, Dy, Er) nanocrystals. The synthesis process was monitored by X-ray diffraction, Thermal–gravimetric–differential thermal analysis and Fourier Transform InfraRed methods. Comparing with traditional solid-state reaction (SSR), Ln₂Ti₂O₇ can be synthesized at relatively low temperature (700–800 °C) with shortened reaction time (2–4 h). The average particle size of Ln₂Ti₂O₇ was greatly reduced (ca. 40 nm) and the BET surface area was increased (ca. 12 m²/g) by using SAM. From the X-ray diffraction patterns, we found that Ln has an effect on the crystal structure of Ln₂Ti₂O₇, every lattice peak shifted to larger angle slightly with the increasing atomic number of Ln. Also, the lattice constant of Ln₂Ti₂O₇ was calculated by Jade.5 and found it decreased along with the decrease of ionic radius of Ln³⁺. The morphology of obtained Ln₂Ti₂O₇ was determined by transmission electron microscopy technique. Results showed that the obtained Ln₂Ti₂O₇ were all square-like and the interplanar distance of Ln₂Ti₂O₇ (Ln = Sm, Gd, Dy, Er) according to (111) plane was 0.65, 0.64, 0.63, and 0.62 nm respectively, which was measured from High Resolution Transmission Electron Microscopy images. Possible reason for this phenomenon was presented.     

Etude cristallochimique et dielectrique des oxyfluorures de terres rares Na4Ln(WNb2)O9F5

Six new compounds with formula Na₄Ln(WNb₂)O₉F₅ (Ln = Y, Nd, Eu, Gd, Dy, Lu) have been synthetized. The corresponding room temperature phases have a tetragonal symmetry and a chiolite-type structure. At low temperature a ferroelectric-paraelectric transition is detected for each compound. The Curie temperature increases with the size of the Ln³⁺ ion.     

Surface characterization and catalytic activity of Ln2Ti2O7 (Ln=Y,Sm, Gd and Tb)

The surface of the catalyst, Ln₂Ti₂O₇ where Ln=Y, Sm, Gd and Tb was analysed by X-ray photoelectron spectroscopy. Terbium is found to be present in 3⁺ and 4⁺ states while Y, Sm and Gd are present in the 3⁺ state. The catalytic activity of these oxides was tested using decomposition of N₂O as the test reaction and the catalytic activity was correlated to the surface properties.     

Electrical properties of Ln2Mo2O7 pyrochlores (Ln=SmYb,Y)

Cubic oxide pyrochlores, Ln₂Mo₂O₇, Ln=SmYb,Y, (Nd_1?xA_x), A=Er and Yb; 0 < x < 1.0, have been synthesized and electrical properties were examined in the range 77-600K. Semiconductor behavior is observed for Ln=EuYb, Y but the resistivity and E_a are low. Sm₂Mo₂O₇ and Nd_1?xYb_xMo₂O₇ (0.05 ≤ x ≤ 0.1) exhibit semimetallic or metallic behavior. The mechanism of conduction in these compounds is explained on the basis of band model proposed by Sleight et al.     

Synthesis and oxygenation behavior of RBaCo4O7 + δ (R = Y, Dy-Lu)

We have studied phase formation processes in the R₂O₃-BaO-CoO = 0.5: 1: 4 (R = Y, Gd-Lu) systems at temperatures from 900 to 1100°C, optimized solid-state synthesis conditions for RBaCo₄O_{7 + δ} (R114), and obtained single-phase samples for R = Y, Dy, Ho, Er, Tm, Yb, and Lu. The behavior of R114 (R = Y, Dy, Lu) materials during thermal cycling and oxygenation has been investigated.     

Oxygen interstitial and vacancy conduction in symmetric Ln2 ± x Zr2 ± x O7 ± x/2 (Ln = Nd,Sm) solid solutions

We have compared (Ln_{2 ? x} Zr _x )Zr₂O_{7 + x/2} (Ln = Nd, Sm) pyrochlore-like solid solutions with interstitial oxide ion conduction and Ln₂(Zr_{2 ? x} Ln _x )O_{7 ? δ} (Ln = Nd, Sm) pyrochlore-like solid solutions with vacancy-mediated oxide ion conduction in the symmetric systems Nd₂O₃-ZrO₂ (NdZrO) and Sm₂O₃-ZrO₂ (SmZrO). We have studied their structure, microstructure, and transport properties and determined the excess oxygen content of the (Sm_{2 ? x} Zr _x )Zr₂O_{7 + x/2} (x = 0.2) material using thermal analysis and mass spectrometry in a reducing atmosphere (H₂/Ar-He). The Ln_{2 ± x} Zr_{2 ± x} O_{7 ± x/2} (Ln = Nd, Sm) solid solutions have almost identical maximum oxygen vacancy and interstitial conductivities: (3–4) × 10^?3 S/cm at 750°C. The lower oxygen vacancy conductivity of the Ln₂(Zr_{2 ? x} Ln _x )O_{7 ? δ} (Ln = Nd, Sm; 0 < x ≤ 0.3) solid solutions is due to the sharp decrease in it as a result of defect association processes, whereas the interstitial oxide ion conductivity of the (Ln_{2 ? x} Zr _x )Zr₂O_{7 + x/2} (Ln = Nd, Sm; 0.2 ≤ x < 0.48) pyrochlore-like solid solutions is essentially constant in a broad range of Ln₂O₃ concentrations.     

Etude de quelques composes Ln2Ge2O7 (Ln = La,Nd, Sm,Eu, Gd)

Some Ln₂Ge₂O₇ compounds (Ln = La, Nd, Sm, Eu, Gd) have been reinvestigated. A preliminary crystallographic study shows that the compounds Nd₂Ge₂O₇ and Sm₂Ge₂O₇ belong to a new structural family. Tetragonal form has also been found for Gd₂Ge₂O₇ below 1250°C.     

Influence of composition on the T→M transformation in the systems ZrO2-Ln2O3 (Ln=La,Nd, Sm,Eu)

The systems ZrO₂-Ln₂O₃ have been studied on samples annealed at 600,1170,1450°C in the 0–15 mol % Ln₂O₃ (where Ln is the rare-earth La, Nd, Sm or Er) range using X-ray diffraction, thermal analysis and dilatometry. The microstructure of annealed samples was examined mainly by electron microscopy. It was found that rare-earth oxides-doped zirconia formed monoclinic, tetragonal, cubic and pyrochlore-type phases. The existing region of the tetragonal phase is 1–15 mol % Ln₂O₃, which is independent of the species but dependent on the dopant content and temperature. The equilibrium phase diagrams and non-equilibrium diagrams have been deduced. The temperature and composition of eutectoid ZrO_2,ss (T)→ZrO_2,ss (M) + Py, as well as interconnection between grain size, Ln₂O₃ content and the martensitic transformation temperature, (M _s), were determined.     

Order–Disorder Transformations in Ln2Ti2O7(Ln = Lu, Yb, Tm, Gd)

The ordering processes in the rare-earth titanates Ln₂Ti₂O₇ with Ln = Lu, Yb, Tm, and Gd are studied by x-ray diffraction, thermal analysis, IR spectroscopy, electron microscopy, and electrical conductivity measurements. The compounds are prepared via hydroxide coprecipitation, followed by freeze-drying and heat treatment in the temperature range 350–1700°C. The compounds Ln₂Ti₂O₇ with Ln = Lu, Yb, and Tm are found to have the fluorite structure between 600 and 800°C. Above 800°C, they undergo a fluorite-to-pyrochlore transformation, with cation disordering and the formation of Ln_Ti + Ti_Ln antistructure pairs. Gd₂Ti₂O₇ has the pyrochlore structure over the entire temperature range studied and contains no antistructure defects. In contrast to Gd₂Ti₂O₇, the compounds Ln₂Ti₂O₇ with Ln = Lu, Yb, and Tm undergo a high-temperature pyrochlore-to-fluorite phase transition around 1700°C. The 750°C conductivity of Ln₂Ti₂O₇ (Ln = Lu, Yb, Tm) samples sintered at 1700°C is 5 × 10^–3 to 10^–2 S/cm, which is two orders of magnitude higher than that of ceramics of the same composition prepared at lower temperatures (950 or 1400°C). The conductivity of the Gd₂Ti₂O₇ ceramic prepared at 1500°C is two orders of magnitude lower than that of Ln₂Ti₂O₇ with Ln = Lu, Yb, and Tm.     

Polycrystalline t′-ZrO2(Ln2O3) formed by displacive transformations

ZrO₂ (3 mol% Y₂O₃) tetragonal and t-ceramics (displacively formed ceramics) were compared with ZrO₂ ceramics (3 mol% Ln₂O₃, where Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, Er, or Yb) processed in an identical manner. Sintering at 1500 °C for 2 h produced mainly tetragonal polytypes for the dopants with smaller ionic radii than Dy(i.e., Er, Y and Yb) but when ZrO₂ was reaction sintered with dopants with larger ionic radii excessive monoclinic phase transformation and associated microcracking resulted. High-temperature annealing in the cubic stability regime and rapid cooling through the tetragonal stability regime was used to fabricate t-composites of ZrO₂ doped with Y, Yb, Er or Dy. Room-temperature fracture toughness and strength values are explained on the basis of a ferroelastic-cubic-to-tetragonal transformation. The domain structure was viewed by transmission optical microscopy (TOM) or transmission electron microscopy (TEM).     

Physicochemical properties of ZrF4-BaF2-LnF3 glasses (Ln=Y or rare-earth elements)

The influence of lanthanoid fluorides on physicochemical properties of fluorozirconate glasses has been examined on 60ZrF₄ · 30BaF₂ · 10LnF₃ glasses (Ln=Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb or Lu). Physicochemical properties such as ion packing, hardness, molecular refraction, thermal stability and fluoride-ion conduction have been evaluated on the glasses by the density, refractive index, differential thermal analysis, thermomechanical analysis, Vickers hardness and electrical conductivity measurements. The variation of these properties with lanthanoid species and the relationships between them have been discussed to characterize lanthanoid fluoride-containing fluorozirconate glasses.     

Phase relations in the K<Subscript>2</Subscript>MoO<Subscript>4</Subscript>–Ln<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript>–Zr(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> (Ln = La–Lu,Y) systems

We have studied phase relations in the K₂MoO₄–Ln₂(MoO₄)₃–Zr(MoO₄)₂ (Ln = La–Lu, Y) systems by the method of “intersecting cuts,” identified pseudobinary joins in their composition triangles, and constructed their phase compatibility diagrams. The systems have been shown to contain new ternary molybdates with the general formula K₅LnZr(MoO₄)₆ (Ln = Dy–Lu and Y). The thermal characteristics of the synthesized compounds have been studied by differential scanning calorimetry in the temperature range 25–700°C. The new ternary molybdates crystallize in a trigonal structure (sp. gr. R\(\bar 3\)c, Z = 6).     

Facile hydrothermal crystallization of NaLn(WO4)2 (Ln=La-Lu,and Y), phase/morphology evolution,and photoluminescence

Abstract

Hydrothermal reaction of Ln nitrate and Na₂WO₄ at pH=8 and 200 °C for 24 hours, in the absence of any additive, has directly produced the scheelite-type sodium lanthanide tungstate of NaLn(WO₄)₂ for the larger Ln³⁺ of Ln=La-Dy (including Y, Group I) and an unknown compound that can be transformed into NaLn(WO₄)₂ by calcination at the low temperature of 600 °C for the smaller Ln³⁺ of Ln=Ho-Lu (Group II). With the successful synthesis of NaLn(WO₄)₂ for the full spectrum of Ln, the effects of lanthanide contraction on the structural features, crystal morphology, and IR responses of the compounds were clarified. The temperature- and time-course phase/morphology evolutions and the phase conversion upon calcination were thoroughly studied for the Group I and Group II compounds with Ln=La and Lu for example, respectively. Unknown intermediates were characterized by elemental analysis, IR absorption, thermogravimetry, and differential scanning calorimetry to better understand their chemical composition and coordination. The photoluminescence properties of NaEu(WO₄)₂ and NaTb(WO₄)₂, including excitation, emission, fluorescence decay, and quantum efficiency of luminescence, were also comparatively studied for the as-synthesized and calcination products.     

Predicting the Phase Stability of Sc<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ln<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>PO<Subscript>4</Subscript> (Ln = Gd–Lu,Y) Zircon-Structured Solid Solutions

The energies of mixing (interaction parameters) in the Sc_1–xLn _x PO₄ (Ln = Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y) systems have been calculated using Urusov’s crystal energy theory of isomorphous substitutions. The decomposition (stability) temperatures of the solid solutions have been plotted against the atomic number of the rare-earth elements at x = 0.01, 0.03, 0.05, 0.10, 0.20, and 0.50. The present results can be helpful in a search for host and activator compositions for new luminescence, laser, and other materials based on the zircon-structured rare-earth orthophosphates.     

Synthesis and magnetic properties of Cr2Ln6S11 (Ln = Gd, Tb, Er, Ho)

The CrS-Er₂S₃ phase diagram was studied, and the stability limits of CrEr₂S₄, CrEr₄S₇, Cr₂Er₆S₁₁, and Cr₃Er₈S₁₅ were determined. The compounds Cr₂Ln₆S₁₁ (Ln = Gd, Tb, Er, Ho) were prepared, and their magnetic properties were investigated. All these sulfides were found to be paramagnetic in the temperature range 80–300 K     

Composition ranges of Ln<Subscript>2 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3 ± δ</Subscript> (Ln = Y,Ho, Er) solid solutions between 900 and 1400°C in air

The metal stoichiometry ranges of the Ln_{2 − x} Mn _x O_{3 ± δ} (Ln = Y, Ho, Er) manganites have been determined using X-ray diffraction analysis of ceramic samples prepared by reacting oxide mixtures in air at temperatures from 900 to 1400°C. The results are represented as partial phase diagrams of the Ln-Mn-O systems in air. Comparison of the phase diagrams demonstrates that the phase boundaries of the manganites are determined not only by the effective cation radius of the rare-earth metal. The solubilities of the binary oxides Y₂O₃, Ho₂O₃, and Mn₃O₄ in yttrium and holmium manganites increase with temperature, with significant quantitative distinctions. The Er₂O₃ and Mn₃O₄ solubilities in erbium manganite remain unchanged over the entire temperature range studied. The LnMn₂O₅ solubility in LnMnO_{3 ± δ} is an intricate function of temperature. We analyze the possible causes of the Ln₂O₃, Mn₃O₄, and LnMn₂O₅ solubility in the LnMnO_{3 ± δ} (Ln = Y, Ho, Er) manganites.     

New Members of the Pb-based 1222 Superconductor Containing Phosphorus: (Pb0.75P0.25)Sr2(Ln1.9?x Ce x Sr0.1)Cu2O z (Ln=Sm and Gd)

We previously reported on the synthesis of new Pb-based 1222 superconductor containing phosphorus in the (Pb_0.75P_0.25)Sr₂(Eu_1.9−x Ce _x Sr_0.1)Cu₂O _z system. Recently, we have discovered new members of the Pb-based 1222 superconductor containing phosphorus in the (Pb_0.75P_0.25)Sr₂(Ln_1.9−x Ce _x Sr_0.1)Cu₂O _z (Ln=Sm and Gd) systems. In each system, it is found that the samples of almost the single 1222 phase can be obtained for a wide composition area of 0.3≤x≤1.0. After annealing under 143 atm O₂ atmosphere at 400 °C, each sample of Ln=Sm and Gd with x=0.3 is found to show an onset of resistivity-drop at about 28 K and at about 24 K, respectively. Moreover, each samples of Ln=Sm and Gd is found to show an onset of diamagnetic signal at about 25 K and about 22 K, respectively. From these facts, these samples of Ln=Sm and Gd are found to be new members of the Pb-based 1222 superconductor containing phosphorus.     

Structural and luminescent properties of the Ln2Hf2O7-type rare earth hafnates

All Ln₂Hf₂O₇-type rare earth hafnates (including those of Sc, Y and La) were prepared by high temperature solid state techniques, and their lattice constants are reported. While those where Ln = La to Tb are ordered, those from Dy to Lu crystallize in a disordered CaF₂ structure.The luminescence of such activators as Eu⁺³, Tb⁺³, and Ti⁺⁴ in La₂Hf₂O₇ is described.