K.V. Govindan Kutty

Thermal expansion behaviour of some rare earth oxide pyrochlores

Oxide pyrochlores of the composition Ln[sub 2]M[sub 2]O[sub 7] (Ln = La - Gd; M = Zr, Hf) have been prepared and their thermal expansion coefficients determined in the temperature range 298--1,500 K by means of high temperature x-ray powder diffractometry. It is seen that for a given M atom, the expansion coefficient increases with the atomic number of the lanthanide element. Between the Zr and Hf series of compounds, the zirconates are found to be more expansive. The results support the ionic model for this class of compounds.

Thermal expansion studies on the Sodium Zirconium Phosphate family of compounds A1/2M2 (PO4)3: effect of interstitial and framework cations

Thermal expansion of the sodium zirconium phosphate (NZP) family of compounds A1/2M2(PO4)3 (A = Ca or Sr; M = Ti, Zr, Hf or Sn) has been measured in the temperature range 298–1273 K by high-temperature X-ray powder diffractometry. Some of the compounds in the series (calcium zirconium phosphate and calcium hafnium phosphate) display the typical thermal expansion behaviour of NZP compounds, namely expansion along the hexagonal c axis and contraction along the a axis. The other compounds, depending on their interstitial and framework composition, behave differently. The observed axial thermal expansion and contraction behaviour is explained on the basis of the crystal chemistry of the compounds. Low-expansion compounds in this series are identified and their expansion anisotropy examined. Infared spectra of the compounds are reported. Differential scanning calorimetry measurements on the tin compounds indicate the occurrence of a diffuse phase transformation at high temperatures.

Oxide ion conductivity in some substituted rare earth pyrozirconates

Abstract This paper reports oxide ion conductivity measurements on pyrozirconates of neodymium and gadolinium in which the rare earth elements were partially substituted by strontium, and zirconium partially substituted by aluminium. 5–10 atom% Sr and very little Al were found to enter the pyrochlore lattice, ac impedance measurements have shown an enhancement of conductivity as a result of the incorporation of small amounts of Sr in both the pyrochlores. In the Gd2Zr2O7 system, this could be traced to an improvement of the grain interior conductivity, by resolving the grain interior and grain boundary contributions to the total conductivity. Such a separation of the bulk and grain boundary conductivities could not be achieved in the Nd2Zr2O7-based compositions. The observed variations in conductivity are explained in terms of the pyrochlore crystal chemistry.

Effect of variation in frame work composition on the thermal expansivity of NZP phases

Two series of NZP compounds, NaM[sub 2](PO[sub 4])[sub 3] and NbM(PO[sub 4])[sub 3] [M = Ti, Zr, Hf, Sn and Th] have been synthesized and their thermal expansion coefficients in the temperature range 298--1,273 K determined by using high temperature x-ray powder diffractometry. In the first series, the influence of the M-O bond is clearly seen even in the presence of the large contribution of the Na-O bond to thermal expansion; the expansively of the lattice is found to vary with the ionic radius of the M atom. This trend is also reflected in the infra-red absorption frequencies of these compounds. In the second series, while the expansion behavior is found to be dependent on M, the variation of expansively with M is different from that found in the first series. The tin compounds in both the series display anomalous behavior. The authors also report here, for the first time, a phase transformation in NbSn(PO[sub 4])[sub 3] around 673 K characterized by a maximum and a minimum in the lattice parameters a and c respectively.

Combustion synthesis and thermal expansion measurements of the rare earth-uranium ternary oxides RE6UO12 (Re = La, Nd and Sm)

Abstract Rare earth–uranium ternary oxides were synthesized by a solution combustion route. The starting materials were the corresponding metal nitrates and urea. In these preparations, the metal nitrates act as oxidizer and urea as fuel. Highly exothermic decomposition of the metal nitrate–urea complexes on heating at about 500 K leads to a combustion process yielding RE 6 UO 12 fine powders. Thermal expansion measurements of these compounds were carried out in the temperature range of 298–1173 K by high temperature X-ray powder diffractometry. The observed axial thermal expansion behaviour is explained on the basis of the crystal chemistry of the compounds.

Studies on the structural and ionic transport properties at elevated temperatures of La1−xMnO3±δ synthesized by a wet chemical method

Abstract Structural transformation and ionic transport properties are investigated on wet-chemically synthesized La 1− x MnO 3 ( x =0.0–0.18) compositions. Powders annealed in oxygen/air at 1000–1080 K exhibit cubic symmetry and transform to rhombohedral on annealing at 1173–1573 K in air/oxygen. Annealing above 1773 K in air or in argon/helium at 1473 K stabilized distorted rhombohedral or orthorhombic symmetry. Structural transformations are confirmed from XRD and TEM studies. The total conductivity of sintered disks, measured by four-probe technique, ranges from 5 S cm −1 at 298 K to 105 S cm −1 at 1273 K. The ionic conductivity measured by blocking electrode technique ranges from 1.0×10 −6 S cm −1 at 700 K to 2.0×10 −3 S cm −1 at 1273 K. The ionic transference number of these compositions ranges from 3.0×10 −5 to 5.0×10 −5 at 1273 K. The activation energy deduced from experimental data for ionic conduction and ionic migration is 1.03–1.10 and 0.80–1.00 eV, respectively. The activation energy of formation, association and migration of vacancies ranges from 1.07 to 1.44 eV.

Calorimetric study of selected NZP-type of ceramic materials

Sodium zirconium phosphate, NZP, ideally NaZr (PO ) , is a potential host matrix for nuclear waste. Thermophysical properties of 24 3 these materials are necessary to assess the thermal stability of these compounds. This paper presents the synthesis and calorimetric measurements of some NZP-type compounds containing alkaline earth elements. It also describes the indigenously developed room temperature drop calorimeter for low temperature calorimetric measurements. Calibration of the instrument is done with standard reference materials. Enthalpy increment of these materials is measured using this setup in the temperature range 373-873 K, and specific heat data are derived. Temperature dependence of enthalpy increment and specific heat of these materials, and the influence of the chemical composition on these properties are discussed.  2002 Elsevier Science B.V. All rights reserved.

Effect of aliovalent ion substitution on the oxide ion conductivity in rare-earth pyrohafnates RE2−xSrxHf2O7−δ and RE2Hf2−x AlxO7−δ (RE=Gd and Nd; x=0, 0.1 and 0.2)

Abstract Oxide ion conductivity of the pure and aliovalent ion substituted rare-earth pyrohafnates in the series RE 2− x Sr x Hf 2 O 7 and RE 2 Hf 2− x Al x O 7 (RE=Gd and Nd; x =0–0.2) has been explored in the temperature range 400°C–700°C for the first time. It is seen that, conductivity is enhanced by doping 5 atom% Sr at the rare–earth site in these systems. Well defined impedance plots due to grain interior and grain boundary resistances were obtained in the Gd pyrohafnate with Sr substitution. The results of the conductivity variation for the pure, Sr and Al doped phases are explained on the basis of pyrochlore structure.

Encapsulation of heterovalent ions of two simulated high-level nuclear wastes and crystallization into single-phase NZP-based wasteforms

Preliminary data were obtained on the immobilization of various heterovalent ions present in the two selected high-level nuclear waste compositions. The entire set of ions present in the waste compositions were immobilized into sodium zirconium phosphate (NZP) structure. The waste loading was in the range 5–25%. The two types of wasteforms loaded with the simulated high-level waste compositions were characterized by powder X-ray diffraction, FT-IR, TGA/DTA, and scanning electron microscopy. The difference in the compositions of the two simulated wastes was reflected in the waste loading percentage and the crystallization of the wasteforms into NZP structure. Up to 12% waste loading, single phase isostructural with NZP was obtained as a major product in the case of the first waste composition. An increase in the waste loading led to the segregation of ZrP2O7 as a secondary phase. With the second waste composition, an NZP-like phase was obtained as the major product even at 25% waste loading.

Self-assembled three-dimensional epitaxial ionic fluorite Gd2Zr2O7 nanorods on (001) LaAlO3

Self-organized, three-dimensional (3D), coherent fluorite Gd2Zr2O7 nanorods were synthesized with pulsed-laser ablation. Transmission electron microscopy studies reveal that the 3D nanorods are epitaxially grown on (001) LaAlO3 with (001)GZO‖(001)LAO, [110]GZO‖[100]LAO and [11¯0]GZO‖[010]LAO. The nanorods have in-plane dimensions of about 20–35 nm, and their length can extend for the entire film thickness. The 3D nanorods have a vertical prism shape with side surfaces exhibiting a serrated morphology composed of {111} planes. The formation of such self-organized nanorods is consistent with a preferential growth mechanism and surface energy minimization considerations.