L.G. Shcherbakova

Studies of New Order-Disorder Structural Transitions in Ln2M2O7 (Ln = Lu, Gd; M = Ti)

Phase formation processes at T = 350–1000°C in Lu(Gd)-Ti-O samples obtained by co-precipitation method are studied. Thermal processing of freeze dried co-precipitation products results in formation of Lu2Ti2O7 with fluorite structure at 650°C. The phase transition of fluorite to pyrochlore occurs at 750–800°C. At T > 800°C Lu2Ti2O7 possesses the structure of the disordered pyrochlore with antistructural defects. Further heating leads to the ordering of pyrochlore at T > 900°C. This process is accompanied by the appearance of a maximum at the temperature dependence of dielectric permeability of Lu2Ti2O7. The grain size of Lu2Ti2O7 ceramics annealed at 950°C was found to be 13–15 nm. The electric conductivity of this ceramics at 200°C (10−5 S/cm) is 5 orders higher than the values usually observed for Lu2Ti2O7 fabricated by a common solid state synthesis method. In Gd-Ti-O system only pyrochlore crystallization process at 740–900°C was observed.

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 LnTi + TiLn 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.

STRUCTURE AND ELECTRICAL CONDUCTIVITY OF LN2+XHF2-XO7-X/2 (LN = SM-TB; X = 0, 0.096)

Data are presented on the evolution of the pyrochlore structure in the Ln2+xHf2−xO7−δ (Ln = Sm, Eu; x = 0.096) solid solutions and Ln2Hf2O7 (Ln = Gd, Tb) compounds prepared from mechanically activated oxide mixtures. Sm2.096Hf1.904O6.952 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 Sm2.096Hf1.904O6.952 is accompanied by the disappearance of the assumed superstructure. In the range 1300–1670°C, Eu2+xHf2−xO7−δ (x = 0.096) and Ln2Hf2O7 (Ln = Gd, Tb) have a disordered pyrochlore structure. The highest 840°C conductivity is offered by Eu2.096Hf1.904O6.952, Gd2Hf2O7, and Tb2Hf2O7 synthesized at 1670°C: 7.5 × 10−3, 5 × 10−3, and 2.5 × 10−2 S/cm, respectively.

Effect of heterovalent substitution on the electrical conductivity of (Yb1-xMx)2Ti2O7 (M = Ca, Ba; x = 0, 0.05, 0.1)

Abstract(Yb1−xCax)2Ti2O7 and (Yb1−xBax)2Ti2O7 (x = 0, 0.05, 0.1) have been synthesized using hydroxide coprecipitation and mechanical activation of oxide mixtures, and their electrical conductivity has been measured from 350 to 1000°C. The pyrochlore titanate (Yb0.9Ca0.1)2Ti2O7 synthesized at 1400°C from a mechanically activated oxide mixture has the highest conductivity, ∼0.1 S/cm at 1000°C, among the oxygen-ion-conducting pyrochlores studied so far. The (Yb0.95Ca0.05)2Ti2O7 and (Yb0.9Ca0.1)2Ti2O7 samples prepared by reacting coprecipitated powder mixtures at 1400°C have a lower conductivity, as do the (Yb1−xBax)2Ti2O7 (x=0.05, 0.1) samples prepared using mechanical activation.

Mechanism of structure formation in samarium and holmium titanates prepared from mechanically activated oxides

We have studied the formation mechanism and phase transitions of samarium and holmium titanates prepared from mechanically activated oxide mixtures with the overall compositions Sm2(Ho2)Ti2O7 and Sm2TiO5. Mechanical activation of oxide mixtures leads to the formation of amorphous solid phases which crystallize in a distorted pyrochlore-like structure and contain OH groups on the oxygen site and structural vacancies up to 1000°C. In the range 800–1000°C, Sm2−xTi1−yO5−δ(OH)n (x < 0.02; y < 0.08; δ, n < 0.19) converts to a distorted orthorhombic phase as a result of the relaxation of internal stress and removal of OH groups. Above 1000°C, the phases studied have the compositions Sm2(Ho2)Ti2O7 and Sm2TiO5 and ordered pyrochlore-like and orthorhombic structures, respectively. The lattice parameters of the titanates have been measured in the range 800–1350°C. The internal stress produced by mechanical activation in the phases studied here fully relaxes by ∼1300°C.

Phase composition of lithium aluminosilicate glass-ceramics

The phase composition of glass-ceramics produced by heat-treating lithium aluminosilicate glass at temperatures from 600 to 1300°C has been determined by x-ray diffraction and electron probe x-ray microanalysis. The materials heat-treated in the range 1000–1230°C have been shown to contain an Si0.65Al0.19Ti0.09Sb0.013Na0.032K0.013Ce0.008As0.013O1.864 glass phase and crystalline solid solutions: tetragonal spodumene-like Li1 − 4x − 2yTixZnyAlSi3O8 (x, y ≤ 0.05), sillimanite-based Al1.92Ti0.08SiO5.04, and rutile-based Ti0.99Sb0.01O2.005. The structures and lattice parameters of the crystalline phases have been determined.

Polymorphism in the family of Ln6−xMoO12−δ (Ln = La, Gd–Lu; x = 0, 0.5) oxygen ion- and proton-conducting materials

The formation of Ln6−xMoO12−δ (Ln = La, Gd, Dy, Ho, Er, Tm, Yb, Lu; x = 0, 0.5) rare-earth molybdates from mechanically activated oxide mixtures has been studied in the range 900–1600 °C. The morphotropy and polymorphism (thermodynamic phase and kinetic (growth-related) transitions) of the Ln6−xMoO12−δ (Ln = La, Gd–Lu; x = 0, 0.5) molybdates have been analyzed in detail. As a result we have observed two new types of oxygen ion- and proton-conducting materials with bixbyite (Ia, no. 206) and rhombohedral (R, no. 148) structures in the family of Ln6−xMoO12−δ (Ln = La, Gd–Lu; x = 0, 0.5) molybdates. The heavy rare-earth molybdates Ln6−xMoO12−δ (Ln = Er, Tm, Yb; x = 0, 0.5) have been shown for the first time to undergo an order–disorder (rhombohedral–bixbyite) phase transition at 1500–1600 °C, and we have obtained compounds and solid solutions with the bixbyite structure (Ia). The stability range of the rhombohedral phase (R) increases with decreasing Ln ionic radius across the Ln6−xMoO12−δ (Ln = Er, Tm, Yb, Lu) series. We have detected a proton contribution to the conductivity of the rhombohedral La5.5MoO11.25 (2 × 10−4 S cm−1 at 600 °C in wet air) and high-temperature polymorph Yb6MoO12−δ bixbyite structure, (Ia) below 600 °C. At these temperatures, rhombohedral (R) Yb6MoO12 seems to be an oxygen ion conductor (Ea = 0.53–0.58 eV). The total conductivity of rhombohedral (R) Yb6MoO12 exceeds that of bixbyite Yb6MoO12−δ by more than one order of magnitude and is 3 × 10−5 S cm−1 at 500 °C. According to their high-temperature (T > 600 °C) activation energies, the lanthanum and ytterbium molybdates studied here are mixed electron–ion conductors.

Structural Order–Disorder Transitions in Ln2Ti2O7 (Ln = Lu, Gd)

The phase generation in the Lu(Gd)–Ti–O systems is studied at 20–1000°С using a co-precipitation method. During a thermal treatment of co-precipitation products after a sublimation dehydration, for a composition with the Lu : Ti cation ratio of 1 : 1, an Lu2Ti2O7 phase with a fluorite structure forms at 650°С. At 730–750°C the phase undergoes a fluorite ⇔ pyrochlore transition. Above 750°C its structure is that of disordered pyrochlore, in which antistructural defects occur in Lu and Ti positions (up to 18%). Above 900°C the structure of pyrochlore becomes ordered, and the number of defects in Lu and Ti positions decreases, which affects the temperature dependence of permittivity of Lu2Ti2O7. In Gd–Ti–O system, Gd2Ti2O7 is crystallized, which has a pyrochlore structure only at 740–900°С. Electroconductivity and permittivity of Lu2Ti2O7 and Gd2Ti2O7 are measured.

Acceptor doping of Ln{sub 2}Ti{sub 2}O{sub 7} (Ln = Dy, Ho, Yb) pyrochlores with divalent cations (Mg, Ca, Sr, Zn)

New LANTIOX high-temperature conductors with the pyrochlore structure, (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn; x = 0, 0.01, 0.02, 0.04, 0.07, 0.1), have been prepared at 1400-1600 8C using mechanical activation, co-precipitation and solid-state reactions. Acceptor doping in the lanthanide sublattice of Ln2Ti2O7 (Ln = Dy, Ho, Yb) with Ca 2+ ,M g 2+ and Zn 2+ increases the conductivity of the titanates except in the (Ho1� xCax)2Ti2O7� d system, where the conductivity decreases slightly at low doping levels, x = 0.01-0.02. The highest conductivity in the (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho, Yb; A = Ca, Mg, Zn) systems is offered by the (Ln0.9A0.1)2Ti2O7� d and attains maximum value for (Yb0.9Ca0.1)2Ti2O6.9 and (Yb0.9Mg0.1)2Ti2O6.9 solid solutions:� 2 � 10 � 2 and 9 � 10 � 3 Sc m � 1 at 750 8C, respectively. Ca and Mg are best dopants for Ln2Ti2O7 (Ln = Dy, Ho, Yb) pyrochlores. Using impedance spectroscopy data, we have determined the activation energies for bulk and grain-boundary conduction in most of the (Ln1� xAx)2Ti2O7� d (Ln = Dy, Ho; A = Ca, Mg, Zn) materials. The values obtained, 0.7-1.05 and 1-1.4 eV, respectively, are typical of oxygen ion conductors. We have also evaluated defect formation energies in the systems studied.

Magnetic properties of the ultrafine-grained HTSC

Peculiarities of magnetization M(H) for fine-grained and ultrafine-grained HTSC ceramics having grain size D comparable to penetration depth 1 are studied.