Solid Solution Domain Research Articles

Investigation of the crystal chemical and ferroelectric properties in the vicinity of LiNbO 3 and LiTaO 3 inside the ternary systems Li 2ONb 2O 5(TiO 2) 2 and Li 2OTa 2O 5(SnO 2) 2

A crystal chemical study has been carried out for the ternary systems Li 2O M 2O 5( M′O 2) 2 ( M = Nb, M′ = Ti; M = Ta, M′ = Sn), in order to characterize and delimit the extension of domains of solid solutions in the vicinity of LiNbO 3 and LiTaO 3. The nature of the nonstoichiometry within these regions of solid solution has been found to be of a cationic excess or deficit. No evidence for anionic deficit has been found in this study. The ferroelectric Curie temperature T C decreases as the composition deviates from LiNbO 3 or LiTaO 3. The decline in T C has been interpreted on the basis of the electrostatic interactions resulting from either cationic excess or deficit.

Phases stability and conduction properties in the Bi 2O 3-PbOPbF 2 system

The Bi 2O 3-PbOPbF 2 system was investigated by means of X-ray diffraction at temperatures between 770 and 950 K. Two solid solution domains have been evidenced. The first one exhibits a tetragonal cell (a = 7.74(1), c = 5.78(1) A ̊ for Bi(0.857)Pb(0.143)O(1.357)F(0.143)). Conductivity measurements performed by complex impedance method show that the value of σ remains lower than 10 −7 (Ω.cm) −1 at 470 K. The second domain, isostructural with A La203 (hexagonal cell, a = 4.101(2), c = 6.087(2) A ̊ for Bi(0.5)Pb(0.5)0 F(0.5)) is more attractive: σ values close to 3×10 −4 (Ω.cm) −1 are reached at 470 K. The conductivity is strongly influenced by both oxygen and fluorine contents.

Effect of homovalent (I −Br −) ion substitution on the ionic conductivity of LiI 1−x Br x systems

The study of electric conductivity in the whole range of compositions of mixed LiI 1− x Br x crystals has shown that doping of pure lithium halide (LiI or LiBr) by homovalent substitution leads to an increase in conductivity. This enhancement is essentially pronounced in the solid solution domains near the compositions of pure compounds (x < 0.15 and x > 0.90). Maximum conductivity is attained for the two phase composition LiI 0.75Br 0.25 (5 × 10 −7 S cm −1 at 293 K) compared to those of starting compounds (4 × 10 −8: LiI and 9 × 10 −9: LiBr at 293 K). An Arrhenius behaviour of the conductivity evolution versus the inverse of the temperature shows that variations in conductivity are related to those of the activation energy whose minimum is close to 8.7 Kcal mole −1. Lattice strain involved by substituting anions of different raddi could be the factor which increase the defect population.

Rutiles of the system Tio 2-FeNb 2O 6-NbO 2 : Structural, magnetic and electron transport properties

A wide domain of rutile solid solution has been isolated in the system TiO 2-FeNb 2O 6-NbO 2. The cristallographic parameters, characteristic of the classical rutile cell, have been determined by X-ray diffraction. The Mössbauer and magnetic susceptibility study shows that in the whole domain iron exhibits the divalent state. A second type of site for Fe(II) has been observed on the limit Ti 1−x IVFe II x 3 Nb V 2x 3 2 , which has been interpreted by a small loss of oxygen involving the formation of trivalent titanium. The study of the conductivity and the thermoelectric power confirms the classical band model proposed for the rucile structure, involving in this case narrow Π H bands. The evolution of σ and α as well as that of the c parameter shows that two domains, I and II, must be distinguished, in which the Π H bands involve the Ti-3d and Nb-4d orbitals, respectively. The first domain is indeed characterized by the presence simultanously of Ti IV, Ti III and Nb V and corresponds to the triangle Ti IVO 2-Fe IINb V 2O 6Ti III 0.5Nb V 0.5O 2 second domain in which all the titanium ions are in the trivalent state is characterized by the presence of Ti III, Nb IV and Nb V simultanously and corresponds to the triangle Nb IVO 2-Fe II 2Nb V 2O 6-Ti III 0.5Nb V 0.5O 2.

Phase equilibrium and ionic conductivity in the PbF 2YF 3 system

The phase diagram of the PbF 2-YF 3 system has been studied by D.T.A. and X-ray diffraction techniques. A large domain of a fluorite-type solid solution Pb 1−xY xF 2+x (0⩽x⩽0.39 at 885°C) called F has been isolated. A very small domain of a tysonite-type solid solution H only stable in a very narrow temperature range is also observed. Two ordered phases R and T, whose structures derive from the fluorite-type are identified. The transport properties of F, R and T are analyzed and compared to those of analogous phases belonging to other Pb 1−xM xF 2+x systems (M = In, Sb, Bi). The influence of the polarizability of the substituent trivalent ion on the electrical properties is particularly discussed.

Study of mixed-oxide catalysts containing bismuth, vanadium and antimony. Preparation, phase composition, spectroscopic characterization and catalytic oxidation of propene

The structural and catalytic properties of oxides in the Bi–V–Sb–O system, which are propene oxidation catalysts, have been studied. Two domains of solid solutions exist for the compounds BiV1–xSbxO4, with the scheelite structure for 0 ⩽x < 0.04 and a monoclinic structure for 0.876 ⩽x⩽ 1. Characterization by X-ray photoelectron spectroscopy (X.p.s.) shows that for a given MO (M = Bi, V or Sb) environment, the higher the average interatomic distance, the higher the cation binding energy, corresponding to an effective decrease in electronic density. From a semi-quantitative X.p.s. analysis of the surface composition of the BiV1–xSbxO4 system it is shown that for low values of x(x⩽ 0.10) the surface concentrations of vanadium and oxygen decrease sharply compared with those of the other elements. Activities and selectivities for acrolein, determined in a stirred gas–solid reactor, are measured as a function of x. Oxides having the scheelite structure exhibit a sharp increase in activity with increasing x. The larger performance found for x= 0.035 is connected with a high oxygen mobility.

Dielectric and thermocurrent studies of some fluoride and oxide fluoride compounds with a (NH 4) 3FeF 6−related structure

Dielectric and thermocurrent measurements have been carried out on (NH 4) 3AlF 6,(NH 4) 3FeF 6 and Rb 3FeF 6 ceramic samples. A maximum of permittivity is observed close to the transition temperature (T tr(NH 4) 3AlF 6 = 217 K; T tr(NH 4) 3FeF 6 = 264 K; T tr(Rb 3FeF 6) = 629 K). This non cubic-cubic transition is of the ferroelastic type. The high Curie temperature of the rubidium iron fluoride causes large dielectric losses at the transition and thermocurrent measurements cannot be performed. On the contrary, in the low-temperature phase of the ammonium compounds a polarization current of about 10-9A/cm 2 is obtained and can be reversed when the sign of the polarization field is changed. This property could correspond to a ferroelectric behavior. However no pyroelectric current is detected when the temperature decreases from T tr. Another hypothesis, based on the field-induced polarization, has been also considered. In connection with the ferroelectric properties of Rb 3MoO 3F 3, the system Rb 3MoO 3F 3Rb 3FeF 6 has been performed. Extended domains of solid solution with Rb 3(Mo 1−xFe x)O 3−3xF 3+3x formula and having a (NH 4) 3FeF 6-related structure have been pointed out. The replacement of oxygen by fluorine leads to a strong decrease of the ferroelectric Curie temperature: T C(x=0) = 538 K, T C(x=0.4) < 80 K. This result is in good agreement with the lack of ferroelectricity in A 3MF 6 fluorides. In any case, the low-temperature phase is ferroelastic and is characterized by a remanent polarization.

Phase diagram of the system CaF2-GdF3

A phase diagram of the system CaF2-GdF3 was studied by thermal and X-ray analysis. Two wide domains of solid solutions based on CaF2 and a high-temperature modification of α-GdF3 (LaF3-structural type) are present in this system. Two maxima were found on the melting curves of the Ca1−xGdxF2+x and α-(Gd1−yCayF3−y solid solutions, at 1428 ± 10‡ (5 mole % GdF3) and 1282 ± 5‡ (85 mole % GdF3), respectively. The coordinates of the eutectic are 60 mole % GdF3 and 1233 ± 5‡.

Étude du diagramme de phase dans le système Gd 2O 3-Ga 2O 3

Mass spectrometry studies of Gd2O3-Ga2O3 mixtures have resulted in partial determination of equilibrium phase diagram and Ga2O3 activity variations with composition of mixtures. Lattice parameter measurements have specified width and limits of the garnet phase solid solution domain. The congruently melting composition was found to be equal to Gd3.05Ga4.95O12. The proposed phase diagram explains all experimental results reported up to now.