Challenges in Earth sciences: the 21st century

The 21st century poses many challenges for global sustainability. Among them, most importantly, the human race will encounter scarcity of raw materials and conventional energy resources. And, India may have to take the brunt of these problems as it is going to be the most populated region of the world with concomitant increase in energy demand and requirement of other resources. India will be the testing ground for introducing newer ways of green technology and innovative principles of resource management and utilization. With the vagaries of potential climate change gathering clouds in the background, Earth sciences will have a special and predominant role in guiding the society in prioritizing our resource discovery, utilization and their consumption and the upkeep of environment. On the fundamental level, Earth sciences are going through a most exciting phase of development as a born-again science. Technological breakthroughs including the satellite-based observations augur well for gaining new insights into Earth processes. A set of exciting fundamental problems that are globally identified will set the stage for an exhilarating period of new discoveries. Improvements in numerical and computer-based techniques will assist in modelling of Earth processes to unprecedented levels. India will have to take special effort in improving the existing experimentation facilities in the Earth science departments of the country, and also the general level of Earth science education to meet the global standards. This article presents an Earth science vision for the 21st century in an Indian context.

High-temperature reactions of alkali and plagioclase feldspars with alkali and alkaline-earth chlorides-formation of celsian

High-temperature reactions (Ca 900-degrees-C) involving albite, K-feldspar or plagioclase and K, Ba-or K, Sr chlorides were experimentally studied. These experiments reveal that the reaction between K-exchanged albite, potash feldspar, or plagioclase and Ba-chloride/Ba-K chloride results in the formation of celsian by the breakdown of the starting feldspar structure above 800-degrees-C. Sr-feldspar does not form under similar conditions. A size-effect of the large M-site cation appears to be responsible for the formation of celsian. The reaction between K-feldspar and barium chloride may be used as a method for synthesizing celsian.

Magnetic properties of rare earth nickelates of the Ln(2)BaNiO(5) family

Rare-earth nickelates Ln(2)BaNi(1-x)Cu(2)O(5), Ln = Nd and Dy, and Dy2-xYxBaNiO5 have been synthesized in order to investigate the effect of substitution of Ni by Cu and Dy by nonmagnetic Y on the magnetic properties of the nickelates. In Ln(2)BaNi(1-x)Cu(x)O(5), the nickelate structure (x=0.0) changes to the cuprate structure (x=1.0) at a specific composition (x=0.3). The Neel temperature of Nd2BaNi1-xCuxO5 decreases continuously with increase in x upto x=0.3 (T-N = 18K); when x > 0.3, the materials are paramagnetic down to 20K. The mu(eff) in Nd2BaNi1-xCxO5 essentially corresponds to the contribution of the Nd ions. In Dy2-xYxBaNiO5, the Neel temperature decreases from 40K when x=0.0 to 24K when x=1.5. The compositions with 1.5 less than or equal to x less than or equal to 2 (including the x=1.95 composition) are paramagnetic down to 20K, unlike Y2BaNiO5 (x=2.0) which exhibits a T-N of 370K. Even the smallest concentration of paramagnetic Dy seems to destroy the antiferromagnetic Ni-O-Ni chains in Y2BaNiO5.

New uniform solid catalyst for the low-temperature oxidation of carbon monoxide: A triple-layered rare earth perovskite containing Co and Cu ions

Oxygen reactivity and catalytic activity of the cobalt-containing layered defect perovskites, YBa2Cu2CoO7+delta and LaBa2Cu2CoO7+delta, in comparison with LaBa2Cu3O7-delta have been investigated employing temperature-programmed desorption (TPD) and temperature-programmed surface reactions (TPSR) in the stoichiometric and catalytic mode using carbon monoxide as a probe molecule. TPD studies showed evidence for the presence of two distinct labile oxygen species, one at (0 0 1/2) sites and the other at (0 1/2 0) sites in LaBa2Cu2CoO7+delta against a single labile species at (0 1/2 0) in the case of two other oxides. The activation energies for the catalytic oxidation of carbon monoxide by oxygen over LaBa2Cu3O7-delta, YBa2Cu2CoO7+delta, and LaBa2Cu2CoO7+delta have been estimated to be 24.2, 15.9, and 13.6 kcal/mol, respectively. The reactivity and catalytic activity of the oxide systems have been interpreted in terms of the structural changes brought about by substituents, guided by a directing effect of the larger rare earth cation. TPSR profiles, structural analysis, and infrared spectroscopic investigations suggest that the oxygen present at (0 0 1/2) sites in the case of LaBa2Cu2CoO7+delta is accessible to catalytic oxidation of CO through a Mars-Van Krevelen pathway. Catalytic conversion of CO to CO2 over LaBa2Cu2CoO7+delta occurs at 200 degrees C. The enhanced reactivity is explained in terms of changes brought about in the coordination polyhedra around transition metals, enhanced basal plane oxygen diffusivity, and redox potentials of the different transition metal cations.

Synthesis, structure and IR absorption studies of LnBaCuCoO(5) (Ln equal earth) oxides

A series of oxides LnBaCuCoO(5) (Ln = Pr, Nd, Sm, Dy, Gd, Ho and Er) have been synthesized by ceramic method. The oxides crystallize in a tetragonal structure, isostructural to YBaCuCoO5. All the oxides in the series are semiconducting. IR spectra of these oxides show distinct absorption bands at 630 cm(-1), 550 cm(-1) and 330 cm(-1) which are assigned to E, A(2) and A(1) modes respectively. Doping of holes in these oxides, by calcium substitution in Er1-xCaxBaCuCoO5-x (up to x similar or equal to 0.3) was done but, these oxides did not show metallic behaviour.

Synthesis and properties of rare earth doped lamp phosphors

Red, blue and green emitting lamp phosphors such as EU(3+) doped Y2O3 (red phosphor), EU(2+) doped Ba0.64Al12O18.64, BaMgAl10O17 and BaMg2Al16O27 (blue phosphors) and Ce0.67Tb0.33MgAl11O19 and Eu2+, Mn2+ doped BaMgAl10O17 (green phosphors) have been prepared by the combustion of the corresponding metal nitrates (oxidizer) and oxalyl dihydrazide/urea/carbohydrazide (fuel) mixtures at 400 degrees-500 degrees C within 5 min. The formation of these phosphors has been confirmed by their characteristic powder X-ray diffraction patterns and fluorescence spectra. The phosphors showed characteristic emission bands at 611 nm (red emission), 430-450 nm (blue emission) and 515-540 nm (green emission). The fine-particle nature of the combustion derived phosphors has been investigated using powder density, particle size and BET surface area measurements.

The nature of charge ordering in rare earth manganates and its strong dependence on the size of the A-site cations

Charge ordering in rare earth manganates of the type Ln(0.5)A(0.5)MnO(3) (Ln = rare earth, A = alkaline earth) is highly sensitive to the average radius of the A-site cations, [r(A)]. Tn the small [r(A)] regime (e.g., Y0.5Ca0.5MnO3), charge ordering occurs in the paramagnetic state, the transformation to an antiferromagnetic state occurring at still lower temperatures. At moderate [r(A)] values (e.g., Nd0.5Sr0.5MnO3), a ferromagnetic metallic state transforms to a charge-ordered antiferromagnetic state with cooling. These two distinct types of charge ordering and associated properties are explained in terms of the variation of the exchange couplings J(FM) and J(AFM) with [r(A)] and the invariance of the single-ion Jahn-Teller energy with [r(A)]. A qualitative temperature-[r(A)] phase diagram, consistent with the experimental observations, has been constructed to describe the properties of the manganates in the different [r(A)] regimes. (C) 1997 Academic Press.

Nonlinear electrical conduction and broad band noise in the charge-ordered rare earth manganate Nd0.5Ca0.5MnO3

Measurements of the dc transport properties and the low-frequency conductivity noise in films of charge-ordered Nd0.5Ca0.5MnO3 grown on Si substrate reveal the existence of a threshold field in the charge-ordered regime beyond which strong nonlinear conduction sets in along with a large broad band conductivity noise. Threshold-dependent conduction disappears as T --> T-CO, the charge-ordering temperature. This observation suggests that the charge-ordered state gets depinned at the onset of the nonlinear conduction. (C) 1999 American Institute of Physics. [S0003-6951(99)05247-X].

Studies on strontium substituted rare earth manganites

Sintering, electrical conductivity and thermal expansion behaviour of combustion synthesised strontium substituted rare earth manganites with the general formula Ln(1-x)Sr(x)MnO(3) (Ln = Pr, Nd and Sm; x = 0, 0.16 and 0.25) have been investigated as solid oxide fuel cell cathode materials. The combustion derived rare earth manganites have surface area in the range of 13-40 m(2)/g. Strontium substitution increases the electrical conductivity values in all the rare earth manganites. With the decreasing ionic radii of rare earth ions, the conductivity value decreases. Among the rare earth manganites studied, (Pr/Nd)(0.75)Sr0.25MnO3 show high electrical conductivity ( > 100 S/cm). The thermal expansion coefficients of Pr0.75Sr0.25MnO3 and Nd0.75Sr0.25MnO3 were found to be 10.2 x 10(-6) and 10.7 x 10(-6) K-1 respectively, which is very close to that of the electrolyte (YSZ) used in solid oxide fuel cells. (C) 1999 Elsevier Science B.V. All rights reserved.

Grain size effects on charge-ordering, phase segregation and related properties of rare-earth manganates, Nd(0.5)A(0.5)MnO(3) (A = Ca or Sr)

Grain size has marked effects on charge-ordering and other properties of Nd(0.5)A(0.5)MnO(3) (A=Ca or Sr). Thus, the anti-ferromagnetic (AFM) transition in Nd0.5Ca0.5MnO3 is observed distinctly only in samples sintered at 1273 K or higher. The sample with a small grain size (sintered at 1173 K) shows evidence for greater ferromagnetic (FM) interaction at low temperatures, probably due to phase segregation. The FM transition as well as the charge-ordering transition in Nd0.5Sr0.5MnO3 becomes sharper in samples sintered at 1273 K or higher. The sample sintered at 1173 K does not show the AFM-CO transition around 150 K and is FM down to low temperatures; the apparent T-c-T-co gap decreases with the increase in the grain size. The samples sintered at lower temperatures (<1673 K) show evidence for greater segregation of the AFM and FM domains. (C) 2002 Elsevier Science Ltd. All rights reserved.

Removal of As(V) by adsorption onto mixed rare earth oxides

Arsenic pollution of water is a major problem faced worldwide. Arsenic is a suspected carcinogen in human beings and is harmful to other living beings. In the present study, a novel adsorbent was used to remove arsenate [As(V)] from synthetic solutions. The adsorbent, which is a mixture of rare earth oxides, was found to adsorb As(V) rapidly and effectively. The effect of various parameters such as contact time, initial concentration, pH, and adsorbent dose on adsorption efficiency was investigated. More than 90% of the adsorption occurred within the first 10 min and the kinetic rate constant was found to be about 3.5 mg min(-1). Adsorption efficiency was found to be dependent on the initial As(V) concentration, and the adsorption behavior followed the Langmuir adsorption model. The optimum pH was found to be 6.5. The presence of other ions such as nitrate, phosphate, sulphate, and silicate decreased the adsorption of As(V) by about 20-30%. The adsorbed As(V) could be desorbed easily by washing the adsorbent with pH 12 solution. This study demonstrates the applicability of naturally occurring rare earth oxides as selective adsorbents for As(V) from solutions.

Seismic passive earth pressure coefficients using the method of characteristics

The method of characteristics was used to generate passive earth pressure coefficients for an inclined wall retaining cohesionless backfill material in the presence of pseudostatic horizontal earthquake body forces. The variation of the passive earth pressure coefficients K-pq and K-pgamma with changes in horizontal earthquake acceleration coefficient due to the components of soil unit weight and surcharge pressure, respectively, has been obtained; a closed-form solution for K-pq is also provided. The passive earth resistance has been found to decrease sharply with an increase in the magnitude of horizontal earthquake acceleration. The computed passive earth pressure coefficients were found to be the lowest when compared to all of the previous solutions available in the literature.

An infrared spectroscopic study of the low-spin to intermediate-spin state (1A1–3T1) transition in rare earth cobaltates, LnCoO3 (Ln=La, Pr and Nd)

Low-spin (LS) to intermediate-spin (IS) state transitions in crystals of LnCoO3 (Ln=La, Pr and Nd) have been investigated by variable temperature infrared spectroscopy. The spectra reveal the occurrence of the transition around 120, 220 and 275 K, respectively, in LaCoO3,PrCoO3 and NdCoO3, at which temperatures the intensities of the stretching and the bending modes associated with the LS state decrease, accompanied by an increase in the intensities of the bands due to IS state. The characteristic frequencies of both the spin states decrease with increase in temperature, showing anomalies around the transition.

An infrared spectroscopic study of the low-spin to intermediate-spin state ((1)A(1)-T-3(1)) transition in rare earth cobaltates, LnCoO(3) (Ln = La, Pr and Nd)

Low-spin (LS) to intermediate-spin (IS) state transitions in crystals of LnCoO(3) (Ln = La, Pr and Nd) have been investigated by variable temperature infrared spectroscopy. The spectra reveal the occurrence of the transition around 120, 220 and 275 K, respectively, in LaCoO3,PrCoo(3) and NdCoO3, at which temperatures the intensities of the stretching and the bending modes associated with the LS state decrease, accompanied by an increase in the intensities of the bands due to IS state. The characteristic frequencies of both the spin states decrease with increase in temperature, showing anomalies around the transition. (C) 2001 Published by Elsevier Science B.V.