A new form of MgTa2O6 obtained by the molten salt method

Using molten salt route (with NaCl/KCl as the salt) we have been able to synthesize a new form of magnesium tantalate at 850°C. Powder X-ray diffraction data could be inde_xed on an orthorhombic unit cell with lattice parameters, ‘a’ = 15.36(1) ?, ‘b’ = 13.38(1) ? and ‘c’ = 12.10(1) ?. High resolution transmission electron microscopy and electron diffraction studies confirm the results obtained by X-ray studies. Energy dispersive X-ray spectroscopy helps ascertain the composition of MgTa₂O₆. The title compound shows a dielectric constant of ∼24 with a low dielectric loss of 0.006 at 100 kHz at room temperature. Dielectric constant is nearly unchanged with rise in temperature while the loss shows a very marginal increase (0.007 at 300°C). Dedicated to Prof J Gopalakrishnan on his 62nd birthday.     

Electric properties of solid solutions based on strontium tantalate with perovskite-type structure. Oxygen-ionic conductivity

Total conductivity of Sr_{6 − 2x} Ta_{2 + 2x} O_{11 + 3x} (0 ≤ x ≤ 0.33) solid solutions with a cryolite structure is studied in the atmosphere with a low water vapor content under variation of the temperature (500 < T < 1000°C) and oxygen activity in the gas phase (10⁻¹⁸ < aO₂ < 0.21). Conductivity is divided into components. It is found that both the oxygen-ionic conductivity and the mobility of oxygen ions increase and the percentage of p-type electronic conductivity decreases at an increase in the concentration of oxygen vacancies. It is shown that compositions with a high strontium oxide content (0 ≤ x < 0.15) and accordingly high concentration of oxygen vacancies correspond to the maximum values of the oxygen-ionic conductivity and low activation energies. The Sr₆Ta₂O₁₁ and Sr_5.92Ta_2.08O_11.12 compositions in a wide range of aO₂ are characterized by negligibly low fractions of electronic conductivity component. For the compositions of x > 0.15, a transition occurs from the electrolytic region to mixed oxygen-hole conductivity character (at aO₂ = 0.21) at an increase in the oxygen activity.     

Synthesis and X-ray diffraction study of complex oxides of the Zn2 ? x(ZraSnb)1 ? xFe2xO4 composition

A multicomponent system of complex refractory oxides of the composition Zn_{2 − x} (Zr _a Sn _b )_{1 − x} Fe_2x O₄ (a + b = 1; a: b = 1: 5, 1: 4, 1: 3, 1: 2, 1: 1, 2: 1, 3: 1, 4: 1; x = 0−1.0; Δx = 0.05) was studied by X-ray diffraction. The samples were prepared from oxides of appropriate metals by low-temperature plasma synthesis (hydrogen-oxygen flame). Two phases with wide homogeneity ranges were identified: α phase crystallized in the crystal system of inverse cubic spinel and β phase with the structure of tetragonal spinel. The phase boundaries were found. Structural data are presented for about 100 solid solutions.     

Electric properties of solid solutions based on strontium tantalate with perovskite-type structure. Protonic conductivity

Conductivity of the Sr_{6 − 2x} Ta_{2 + 2x} O_{11 + 3x} (0 ≤ x ≤ 0.33) solid solutions with the cryolite structure was studied in the atmosphere with a high content of water vapors under temperature and oxygen activity variation in the gas phase. Appearance of the protonic conductivity component was proved at the temperatures below 700°C. It was found that protonic conductivity increases at a decrease in parameter x in the composition series, which is due to an increase of both the concentration of protonic defects formed in the structure and of their mobility. In the case of compositions with x < 0.15 at the temperatures below 550°C, the protonic transport becomes predominant.     

The energetics of lanthanum tantalate materials

Lanthanum tantalates are important refractory materials with application in photocatalysis, solid oxide fuel cells, and phosphors. Soft-chemical synthesis utilizing the Lindqvist ion, [Ta₆O₁₉]⁸⁻, has yielded a new phase, La₂Ta₂O₇(OH)₂. Using the hydrated phase as a starting material, a new lanthanum orthotantalate polymorph was formed by heating to 850 °C, which converts to a previously reported LaTaO₄ polymorph at 1200 °C. The stabilities of La₂Ta₂O₇(OH)₂ (LaTa−OH), the intermediate LaTaO₄ polymorph (LaTa-850), and the high temperature phase (LaTa-1200) were investigated using high-temperature oxide melt solution calorimetry. The enthalpy of formation from the oxides were calculated from the enthalpies of drop solution to be −87.1±9.6, −94.9±8.8, and −93.1±8.7 kJ/mol for LaTa−OH, LaTa-850, and LaTa-1200, respectively. These results indicate that the intermediate phase, LaTa-850, is the most stable. This pattern of energetics may be related to cation-cation repulsion of the tantalate cations. We also investigated possible LnTaO₄ and Ln₂Ta₂O₇(OH)₂ analogues of Ln=Pr, Nd to examine the relationship between cation size and the resulting phases.     

Phase formation in the V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> system

Solid-phase interactions in the V₂O₅-Ta₂O₅-MoO₃ system were studied. The formation of com- pounds TaVO₅ and VTa₉O₂₅ in the V₂O₅-Ta₂O₅ binary system was verified. Tetragonal VTa₉O₂₅-base solid solutions of the general formula Ta_{5 + 4x} V_{5 − 4x} O₂₅ (x = 0.25–1) and TaVO₅-base solid solutions of the general formula Ta _x Mo_{1 − x} V_{2 − x} O_{8 − 3x} (x = 0.625–1) were found to form. Subsolidus phase equilibria in the V₂O₅-Ta₂O₅-MoO₃ were determined.     

Structural and magnetic anomalies among the spin-chain compounds, Ca3Co1+xIr1?xO6

The results of X-ray diffraction, andac anddc magnetisation as a function of temperature are reported for a new class of spin-chain oxides, Ca₃Co_1+x Ir_1−x O₆. While thex = 0.0, 0.3, 0.5 and 1.0 are found to form in the K₄CdCl₆-derived rhombhohedral (space group ) structure, thex = 0.7 composition is found to undergo a monoclinic distortion in contrast to a literature report. Apparently, the change in the crystal symmetry withx manifests itself as a change in the sign of paramagnetic Curie temperature for this composition as though magnetic coupling sensitively depends on such crystallographic distortions. All the compositions exhibit spin-glass anomalies with an unusuallylarge frequency dependence of the peak temperature inac susceptibility in a temperature range below 50 K, interestingly obeying Vogel-Fulcher relationship even for the stoichiometric compounds. Dedicated to Professor C N R Rao on his 70th birthday     

A high energy X-ray diffraction study of sol–gel derived (Ta2O5) x (SiO2)1? x glasses ( x =?0.05, 0.11 and 0.25)—elucidating the role of tantalum in silica

High energy X-ray diffraction (HEXRD) has been used to study the atomic structure of (Ta₂O₅) _x (SiO₂)_1−x (x = 0.05, 0.11 and 0.25) xerogels; the direct interpretation of the resultant data has been augmented using reverse Monte Carlo (RMC) modelling. For the first time in this type of material, two Ta–O correlations have been identified (at ∼1.8 ? and 2.0 ?). The RMC modelling approach explicitly used MAS-NMR data to define its constraints; when combined with HEXRD data, it helps to confirm the more directly determined total coordination of five for the two Ta–O correlation distances and suggests Ta···Si and Ta···Ta coordination distances of ∼3.3 ? and ∼3.8 ?, respectively. The O···O and Si···Si distances and coordination numbers associated with the host silica network suggest that the Ta (V) is acting as a network modifier. The way in which the Ta–O correlations are affected by composition and calcination temperature suggest some phase separation in the (Ta₂O₅)_0.25(SiO₂)_0.75 sample. However, in general, the results indicate good mixing of the component oxides.      

Structure and properties of (1−x)(0.6Pb(Zn1/3Nb2/3)O3-0.4Pb (Mg1/3Nb2/3)O3)-xPbTiO3 thin films with perovskite phase promoted by polyethylene glycol

The preferential formation of a pyrochlore structure is a knotty problem in the preparation of Pb(Zn_1/3Nb_2/3)O₃ (PZN)-based thin film materials and its presence is significantly detrimental to the dielectric and piezoelectric properties. In this study, 40 mol% of PZN was replaced with Pb(Mg_1/3Nb_2/3)O₃ (PMN) for obtaining a perovskite composition around a morphotropic phase boundary (MPB), (1−x)(0.6PZN-0.4PMN)-xPT ((1−x)PZMN-xPT, PT: PbTiO₃) where x = 0.23. The thin films with this composition were prepared with a polyethylene glycol (PEG) modi-fied sol-gel method on LaAlO₃ substrates. The microstructural evolution of the films on heat treatment was examined with X-ray diffraction. With the aid of PEG, the formation of the pyrochlore phase was suppressed and the perovskite phase formed directly from the amorphous gel film. The multilayer films with a thickness around 0.25 μm showed a single perovskite phase without any detectable pyrochlore structure. Microscopic images showed uniform grain size of a few tens of nanometers. The role of the polymer dramatically promoting the perovskite phase was investigated with the aid of X-ray photoelectron spectroscopy and thermal analysis. The dielectric constant of the obtained film was 4160 at 1 kHz. The film demonstrated typical ferroelectric hysteresis loops and exhibited excellent piezoelectric performance.     

Chemical solution deposited silver tantalate niobate, Ag x (Ta0.5Nb0.5)O3? y , thin films on (111)Pt/Ti/SiO2/(100)Si substrates

Silver tantalate niobate films are candidates for temperature stable microwave dielectrics. In this work, a chemical solution deposition synthesis method was developed for Ag _x (Ta_0.5Nb_0.5)O_3−y films on Pt-coated Si substrates. Stable solutions with a range of silver stoichiometries were prepared using 2-methoxyethanol and pyridine as solvents, from AgNO₃ and Nb and Ta ethoxide precursors. It was extremely difficult to prepare phase-pure perovskite films of Ag(Ta_0.5Nb_0.5)O₃ on Pt-coated Si subtrates; instead a mixture of perovskite and natrotantite phases was identified. Such mixed phase films had dielectric constant ɛ _r and dielectric loss tanδ values ranging from 200±20 to 270±25 and 0.006±0.002 to 0.002±0.001 at 100 kHz, respectively, depending on the firing temperature. For Ag₂(Ta_0.5Nb_0.5)₄O₁₁, Ag_0.8(Ta_0.5Nb_0.5)O_2.9, Ag_0.85(Ta_0.5Nb_0.5)O_2.925 and Ag_0.9(Ta_0.5Nb_0.5)O_2.95 films, mainly the natrotantite phase was observed. The ɛ _r values of these films were between 70±10 and 130±15 with tan δ values of 0.008±0.002 at 100 kHz.