T. Sekiya

Preparation and properties of LiTaO3-based solid solutions with cation vacancies

It has been found that nonstoichiometric solid solutions (Li1−xM2+x2□x2)TaO3 with M=Zn, Ni, Mg and Ca exist over wide ranges. In the case of the Zn-system, especially, a LiTaO3 structure was still stable even at x=0.5, where 25% of the Li sites are vacant. A new series of solid solutions shows ferroelectric properties. The Curie temperature is closely related with the axial ratio c/a of the hexagonal cell.

Processing and properties of (1 − x)Pb(Mg1/3Nb2/3)O3·xPbTiO3 solid solutions from PbO- and MgO-excess compositions

Abstract Perovskite solid solutions (1 − x)Pb(Mg 1/3 Nb 2/3 )O 3 ·xPbTiO 3 with x between 0.06 and 0.14 were prepared by heat treatment followed by acid treatment of mixtures containing both PbO and MgO in amounts greater than in the perovskite composition. For example, heat treatment of 4PbO + 1.5MgO + 0.9Nb 2 O 5 + 0.3TiO 2 mixture at 900°C for 1 h produced the perovskite with x = 0.1 without the formation of pyrochlore phase. The Pb(Mg 1/3 Nb 2/3 )O 3 –PbTiO 3 (PMN–PT) solid solutions were crystallized into discrete cubes several micrometers in size. After sintering these particles at 1290°C for 2 h in a PbO atmosphere, the relative density of the ceramics was 93% or more. The ceramics with x = 0.08–0.1 showed a large electric field–induced strain near room temperature.

Processing and properties of relaxor ferroelectric PNN-PT perovskite ceramics

Abstract Relaxor-type ferroelectric perovskites, (1-x)PNN · xPT, were prepared through process of heat treatment and then acid treatment of oxide mixtures containing PbO in amount greater than in the perovskite composition. For obtaining the perovskites without formation of pyrochlore phase, it was essential that the amount of PbO is in excess of the mixtures 5PbO+(1-x)NiO+(1-x)Nb2O5+3xTiO2 and heating temperature is 1000°C or more. In the PNN-PT system, morphotropic phase boundary (MPB) from rhombohedral to tetragonal symmetry was observed near x=0.3. The ceramics exhibited very high dielectric constant and high electrostriction near the MPB composition. The electrostriction was almost twice higher than that of well-known PMN-PT ceramics. (PNN: Pb(Ni1/3Nb2/3)O3, PT: PbTiO3, PMN: Pb(Mg1/3Nb2/3)O3)

Elaboration et photoconduction des films de Bi2O3-MO3 (M = Mo OU W) obtenus par hypertrempe

Abstract The Bi2O3-MO3 films (M = Mo or W) were prepared by a rapid quenching technique using a twin-roller type equipment. The quenched films containing from 2.5 to 5 mol% MoO3 or WO3 precipitated only a tetragonal phase. By further increase of these oxide additives, a cubic phase (δ-phase) was formed coexisting with the tetragonal one. The lowest content of the additives to form singly the δ-phase was 12.5% for MoO3 and 10% for WO3 respectively. The lattice parameters of the tetragonal phase tended to increase with increasing amount of additives. The quenched films, although consisting either of the tetragonal phase, the cubic phase or their mixture, decomposed to form both α-Bi2O3 and tetragonal phase, on annealing at a temperature between 350 and 550°C. As a result of the determination of IR absorption spectra, it was suggested that a certain amount of pentavalent Bi ions was contained in the quenched phase. The quenched films were also characterized by a high photoconductivity. The highest photoconductivity appeared when a beam of about 500 nm in wavelength was irradiated on the film. The photoconduction was observed in all films containing less than 12.5% additives.

Crystallization in rapid quenching of La0.33NbO3 melt

Abstract Transparent La0.33NbO3 flakes were fabricated by a rapid quenching technique. The quenched La0.33NbO3 phase takes a simple cubic perovskite structure and cation vacancies are randomly distributed on the A sites. The cubic La0.33NbO3 phase is a new modification which is different from the well-known orthorhombic phase with ordered A-site vacancies. Transparent flakes were found to show the predominant orientation of (110) and (220) planes.

Structural and dielectric properties in the system LiTaO3-WO3

Phase transition in the (1-x)LiTaO3-xWO3 solid solution system has been studied by means of X-ray diffraction and dielectric measurements. Nonstoichiometric solid solutions with a LiTaO3 structure are formed in the range of 0 < x ≤ 0.4, and the introduction of WO3 into LiTaO3 causes cation vacancies in the Lisites. The axial ratio (c/a) of the hexagonal cell and the ferroelectric Curie temperature decrease with the increase of x. A trirutile compound LiTaWO6 (x=0.5) exhibits photochromism.

Preparation and photoconduction of rapidly quenched films in the Bi2O31bM2O5 system (M=V, Nb and Ta)

Abstract The Bi 2 O 3 1bM 2 O 5 films (M=V, Nb and Ta) were prepared by a rapid quenching technique using a twin-roller type equipment. Quenched film of pure Bi 2 O 3 partially precipitated a tetragonal phase which differs from well-known β-Bi 2 O 3 in structure. In every (Bi 2 O 3 ) 0.975 (M 2 O 5 ) 0.025 composition (2.5% M 2 O 5 ), the tetragonal phase was obtained as single phase, and the tetragonality became greater than that of the pure Bi 2 O 3 . Beyond this composition, the tetragonality decreased to form δ-phase with a fcc structure with increasing the M 2 O 5 content. The lowest V 2 O 5 content forming a δ-phase was 5%, while a slight tetragonality still remained in the samples containing 10% Nb 2 O 5 and 10% Ta 2 O 5 , respectively. From the measurement of the photoconductivities of the quenched films, it was proved that a photoconduction appeared in the tetragonal phase while no photoconductivity was observed in the cubic phase. The characteristic of the photoconduction was that a remarkable decrease in the resistivity was observed when a light of ca. 500 nm in wave length was irradiated on the films.

Preparation and properties of rapidly quenched glasses in the V2O5-CdO system

Rapidly quenched glassy films in the V2O5-PbO system were prepared using a twin-roller type apparatus. The glass formation region was 0–60 mol% PbO. The first crystallization temperature increased from 210 ‡C for the V2O5 glass to 292 ‡C for the glass of 60 mol% PbO. The metastable unknown phase was first crystallized in the glasses around 50 mol% PbO. Infrared absorption spectra of the glasses were measured. Electrical conductivity of the glasses decreased with increasing of PbO content. The electrical behaviour of the glasses is discussed based on the glass structure estimated from the infrared spectral data.

Preparation and properties of rapidly quenched films in the PbONb2O5 system

Rapidly quenched films in the PbO-Nb/sub 2/O/sub 5/ system (10 to 66.7 mol% Nb/sub 2/O/sub 5/) were prepared by the use of a twin-roller type apparatus. The rapid quenching of melts in this system resulted in the formation of several new kinds of metastable phases. The films containing 10% Nb/sub 2/O/sub 5/ precipitated PbO solid solution of tetragonal form. In 15% Nb/sub 2/O/sub 5/, crystal phase having a face centered tetragonal structure was produced coexisting with another tetragonal metastable phase. In the compositions with 20 to 40% Nb/sub 2/IO/sub 5/, crystal phase with a defect fluorite structure was obtained as the major product. Beyond 50% Nb/sub 2/O/sub 5/, there was no great difference in the phase constitutions between rapidly quenched films and solid state reacted samples. Annealing of the films at higher temperatures was also characterized by the appearance of metastable phases. The phase change by annealing was accompanied by a sharp increase in the dielectric constant.

Preparation and properties of glassy films in the V2O5ZnO system

Abstract Rapidly quenched films in a given system were prepared using a twin-roller type apparatus. It was found that glassy films were obtained in a wide range of 0–75 mol% ZnO content, except for the vicinity of 60ZnO·40V 2 O 5 composition. Of four binary compounds, glassy films of ZnV 2 O 6 and Zn 2 V 2 O 7 crystallized directly without changing the composition. The other glassy films showed a complicated process. The first crystallization temperature rose with an increase in ZnO content. In addition, the IR-spectral and electrical properties of quenched films were investigated. On the basis of the data, the structural aspect of Zn-vanadate glasses was discussed.