Formation of ferroelectric phases in KNbO<Subscript>3</Subscript> and other niobates with perovskite structure

The Curie-Weiss temperatures T _CW of NaNbO₃ and AgNbO₃ are determined for the first time by extrapolating the temperature dependence of the reciprocal permittivity from the cubic phase. It is established that the values of T _CW of ANbO₃ perovskites (A = Na, Ag, K) are practically equal, i.e., that the cubic phases of all the compounds are potentially unstable to an almost equal extent with respect to polarization. From an analysis of the EXAFS spectra of the NaNbO₃ and KNbO₃ cubic phases, it follows that the potential-energy surfaces of the Nb cation in both compounds are similar and exhibit eight minima shifted by 0.16 and 0.19 Å, respectively, from the oxygen-octahedron center along the [111]-type directions. Based on these facts, it is inferred that the ANbO₃ instability with respect to the occurrence of polarization is due to the tendency of the Nb cations to be ordered over the eight minima and that the phase transitions induced by this instability are of the order-disorder type. A statistical model capable of describing the sequence of ferroelectric phase transitions occurring in KNbO₃ and in (Ba, Sr)TiO₃ is proposed and studied.     

Influence of crystallite-boundary potential barriers on the thermopower and peltier effect in polycrystalline ferroelectric semiconductors

The influence of local charged states at crystallite boundaries on the thermoelectric effects in polycrystalline ferroelectrics is considered. It is shown that the differential thermopower and the Peltier coefficient depend on the height of the crystallite-boundary potential barriers. The possible manifestation of an anomalous behavior of the thermopower in the vicinity of the Curie ferroelectric point is demonstrated.     

Characteristics of the “strong metal-carrier interaction” using the example of influence of vanadium oxide on catalytic activity of platinum metals in CO hydrogenation

The specific catalytic activities of metals M (M = Ru, Rh, Pd, Ir, Pt) in the hydrogenation of CO change under the influence of vanadium oxide addition. The nature of the change is determined by the ability of M to donate electrons to the oxide or to take them from the oxide. The higher the electron donor capacity the greater the degree to which the bond between CO and the M surface is weakened and the higher the catalytic activity.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 5, pp. 612–616, September–October, 1988.     

Thermal and physical properties of sodium niobate ceramics over a wide temperature range

The temperature dependences of the heat capacity C _p (T) and thermal expansion coefficient α(T) of NaNbO₃ ceramic samples have been investigated in the temperature range from 2 to 800 K. In addition to the anomalies associated with the known phase transitions at temperatures T ₆ ≈ 265 K, T ₅ ≈ 638 K, T ₄ ≈ 760 K, and T ₃ ≈ 793 K, anomalies in the behavior of C _p (T) and α(T) have been observed near T _5″ ≈ 500 K and T _5′ ≈ 600 K. It has been found that all the observed structural transformations, according to the values of the entropy change, are not related to the ordering of structural elements. It has been shown that, with an increase in the temperature, the unit cell volume during the phase transitions near 265, 515, 604, and 638 K decreases. The specific features of the transition to the phase R3c have been examined. Two possible scenarios of the sequence of phase transformations in the temperature range between T ₅ and T ₆ have been analyzed.     

Mössbauer Study of the Effect of Cation Substitutions on the Magnetic Phase Transitions in BiFe1 – xCrxO3 and (1 – x)BiFeO3–xPbFe0.5Sb0.5O3 Solid Solutions

Crystallography Reports - The concentration dependences of the magnetic phase transition temperature TN, determined based on the changes in Mössbauer spectra, were investigated for a BiFeO3...     

Physicochemical foundations and specificity of the practical application of supercritical water oxidation to the destruction of persistent organic pollutants

The theoretical and practical aspects of application of one of the most promising methods for the destruction of persistent organic pollutants, supercritical water oxidation, are discussed. The current state of research in the field of analysis and prediction of the critical parameters of systems with an arbitrary number of components, the behavior of the components in supercritical water-organic pollutant binary systems, and the specificity of supercritical fluids are considered. Various types of phase diagrams of binary systems are examined, and theoretical foundations for calculating the critical physicochemical parameters of multicomponent systems are presented. By the example of hexachlorocyclohexane (Lindane), one of the most toxic pesticides, calculations of the material and heat balance of the oxidation of pesticides with oxygen and hydrogen peroxide are performed. The calculations can be used in designing technological installations for the destruction of pollutants by the supercritical water oxidation.