Surface structural investigations on (100) and (110) faces of flux-grown lanthanum borate crystals

Results of topographical studies carried out on (100) and (110) faces of lanthanum borate crystals grown from the PbO-B₂O₃ flux system are illustrated and discussed. The habit faces display the formation of cavities, microcrystals, elliptical etch pits, elliptical hillocks, circular hillocks and irregular structures. Also described are a various number of elevated structures identified as impurity phases. Energy dispersive X-ray (EDAX) studies have confirmed that these impurity phases in the growth of lanthanum borate (LaBO₃) crystals are enriched by lead (Pb). The habit faces also exhibit some elevated regions which are reported to be more imperfect in comparison with others. It is inferred that independent growth on the habit faces has taken place during the last stage of crystal growth by a two-dimensional nucleation mechanism.     

Formation of secondary phases and microdiscs on flux-grown HoFeO3 crystals

Results of optical and scanning electron microscopic and EDAX studies, carried out on structures exhibited by HoFeO₃ crystals grown from the PbO-PbF₂-B₂O₃ flux system, are reported. Aluminium and silicon are present as impurities in the crystals studied. EDAX of certain structures indicate formation of magnetoplumbite (PbO · 6Fe₂O₃) during the flux growth of HoFeO₃. Crystallization of HoOF on the HoFeO₃ crystal surfaces is also indicated; the process taking place almost at the end of HoFeO₃ crystal growth. A variety of microdisc patterns on HoFeO₃ crystal surfaces are illustrated. Their formation is attributed to the covering process of impurity phases by the rapidly advancing growth fronts on the HoFeO₃ crystal surfaces. Experimental evidence in support of this is offered by exposing the impurity phase buried under the microdisc. Precipitation of the secondary phases during the flux growth of HoFeO₃ crystals and their influence on the latter is discussed.     

Microhardness measurements on single crystals of flux-grown rare earth perovskites (orthoferrites,orthochromites and aluminates)

The results of indentation-induced microhardness testing studies of flux-grown single crystals of rare earth orthoferrites, RFeO₃ (R=Gd to Er and Yb), rare earth orthochromites RCrO₃ (R=La, Eu and Dy), and rare earth aluminates RAlO₃ (R=La, Sm, Gd, Eu and Ho) are presented. The variation in the value of microhardness with load is observed to be non-linear in the case of all these materials. It is found that the results are not in accordance with Kick's law. The results have been analysed and the applicability of the idea of materials resistance pressure in the modified law as proposed by Hays and Kendall [Metallography 6 (1973) 275] is discussed.     

Improved flux growth of rare-earth arsenates,RAsO4

The tetragonal rare-earth arsenates (R = Lu to Sm) have previously been grown as small crystals from Pb₂As₂O₇ as flux.Attempts to prepare larger crystals by modifying the flux and controlling evaporation by means of a crystalline seal are described. The crystal size was increased by 1 to 2 orders of magnitude.The preparation of the monoclinic arsenates (R = Nd to La) is also described.     

The flux growth of some rare-earth and iron group complex oxides

The flux growth of crystals of the following rare-earth complex oxides is reported here for the first time: (Nd, Pb)MnO₃, (Eu, Pb)MnO₃. Tm₂Ge₂O₇ and Pr₂MoO₆. Starting compositions for the flux growth of RBO₃ (R=Nd, Pr, La), (La, Pb)MnO₃, Pb₃Mn₇O₁₅, R₂Ti₂O₇ (R=Tm to Pr), R₂SiO₅ (R=Er, Ho, Dy) and LiCoPO₄ are also given; these compositions have yielded larger crystals than previously reported. In many cases, only a small number of crystals was obtained by spontaneous nucleation.     

Flux growth of some complex oxide materials

This paper describes the preparation of the following complex oxide crystals which have not previously been grown by the flux method: FeNbO₄, MnWO₄, CoWO₄, NiWO₄, RMn₂O₅ (R = Er to Sm, and Y), RMnO₃ (R = Er to Gd), LaOCl, La₂Ti₂O₇₁, Bi₂Sn₂O₇, and PbTiP₂O₈. Single crystal X-ray diffraction data are reported for the latter material, the preparation of which has not previously been reported, and for La₂Ti₂O₇ and Bi₂Sn₂O₇₁, for which only powder data were previously available. Improved methods for the growth of Mn₃O₄, LaCoO₃, RPO₄ (R = Yb to Gd), RCrO₃ (R = Lu, Yb), ErOF and NaNbO₃ are also reported.     

Secondary phases and their role in the development of microdiscs on flux-grown ErFeO3 crystal surfaces

The results of scanning electron microscope and qualitative element analysis studies, carried out on some typical structures exhibited by flux-grown ErFeO₃ crystals grown from PbO-PbF₂-B₂O₃ flux systems, are reported. Aluminium is present as an impurity in the crystals studied. Qualitative analysis of certain structures indicates the formation of magnetoplumbite (PbO · 6Fe₂O₃) during the flux growth of ErFeO₃. Microdisc patterns are interpreted as resulting from the covering of such formations by the rapidly advancing growth fronts on the ErFeO₃ crystal surfaces. The crystallization of ErOF and ErBO₃ crystals on the ErFeO₃ crystal surfaces is also indicated by the qualitative analysis. Precipitation of the impurity phases during the flux growth of ErFeO₃ crystals and its effects on the development of the latter is discussed.     

Microhardness of flux grown pure doped and mixed rare earth aluminates and orthochromites

The results of microhardness measurements on flux-grown crystals of (i) single (pure) rare earth aluminates RAlO₃ (R = Eu, Gd, Dy, Er) and rare earth orthochromites RCrO₃ (R = Y, Gd, Yb), (ii) rare earth aluminates doped with neodymium, erbium, ytterbium and holmium, and (iii) mixed rare earth aluminate crystals of the type (La_1–x ) Pr_(x)AlO₃ (x=0, 0.25, 0.75 and 1.00) are presented. The variations in the microhardness value with load are non-linear in all cases. Kick's law fails to explain the observed variations. Instead, they are best explained by the application of the idea of materials resistance pressure in the modified law proposed by Hays and Kendall. The results indicate that the doping does not increase the hardness value of crystals in all cases. The hardness instead depends on the composition of the parent material as well as the dopant entering into the crystal lattice. Mixed rare earth aluminate crystals are shown to be harder than those of single rare earth aluminates.     

Scintillation and optical properties of Pb-doped YCa4O(BO3)3 crystals

This communication reports optical properties and radiation responses of Pb²⁺ 0.5 and 1.0 mol%-doped YCa₄O(BO₃)₃ (YCOB) single crystals grown by the micro-pulling-down (μ-PD) method for neutron scintillator applications. The crystals had no impurity phases according to the results of X-ray powder diffraction. These Pb²⁺-doped crystals demonstrated blue-light luminescence at 330 nm because of Pb²⁺¹S₀-³P_0,1 transition in the photoluminescence spectra. The main emission decay component was determined to be about 250-260 ns under 260 nm excitation wavelength. When irradiated by a ²⁵²Cf source, the relative light yield of 0.5% Pb²⁺-doped crystal was about 300 ph/n that was determined using the light yield of a reference Li-glass scintillator.     

The flux growth of some new rare earth and iron group complex oxides

The flux growth of crystals of a number of new rare-earth and transition metal compounds is reported. The following empirical formulae were in good agreement with electron probe microanalysis (EPMA): (Ba, Gd)TiO₃, R₄Ba₆(BO₃)₉(R = Ho to La), Ni₃Nb₂O₈, Ni₈NbB₃O₁₅, Ni₂V₂PbO₈, Ta₂Co₄O₉ and PbCr_2.3Ti_2.3O₉. X-ray powder pattern data are given. The growth of crystals of GdVO₃, Pb₂CrO₅ and Ni₃V₂O₈, which have previously been prepared only as powders, is also reported.     

Optical and scanning electron microscopic and energy-dispersive X-ray analytical studies on some typical surface structures of flux-grown DyFeO3 crystals

The results of optical, scanning electron microscope and qualitative analysis studies conducted on surface structures displayed by flux-grown DyFeO₃ crystals are reported. The crystals were grown from PbO-PbF₂ B₂O₃ flux under various conditions. Magnetoplumbite (PbO · 6Fe₂O₃) is the most favoured secondary phase. Crystallization of DyOF and DyBO₃ on the DyFeO₃ crystal surfaces also takes place, almost at the end of DyFeO₃ crystal growth. Macro- and micro-disc patterns on DyFeO₃ and a rare observation of an elliptical disc of material containing lead are illustrated. Metallic platinum deposited on flux-grown DyFeO₃ is reported when the crystals were finally cooled in contact with the flux. Additional secondary phases (material containing lead, platinum, PbO · 6Fe₂O₃, DyBO₃) and other imperfections (inclusions, cavities, microcrystals) also occur. The addition of V₂O₅ to the flux leads to incorporation of traces of vanadium in the DyFeO₃ crystals and DYVO₄ as a secondary phase.     

Crystal growth and characterization of YAl3(BO3)4 doped with Sc, Ga, Pr, Ho, Tm, Yb

New data on flux growth of mixed crystals (Y,R)(Al,M)₃(BO₃)₄ (R=Pr, Ho, Yb, Tm; M=Sc, Ga) have been obtained. The average R distribution coefficients were found to be 0.53–1.02 for different rare earth elements as well as for scandium and gallium. Two morphological types of (Tm,Y)Al₃(BO₃)₄ crystals were characterized. The visible luminescence spectra of (Pr,Y)Al₃(BO₃)₄ and PrAl₃(BO₃)₄ at 10 K and room temperature were measured.     

Pyroelectric and dielectric properties of some heavy rare-earth orthochromites

The measurement of pyroelectric coefficient (p) and dielectric constant (K) of rare-earth orthochromites RCrO₃, where R = Tb, Dy, Ho, Er and Yb are reported for the temperature range 300–600 K. Pyroelectric data show that all the studied orthochromites have ferroelectric phase in this temperature range. The dielectric data in some cases support this conclusion. Spontaneous polarization (P _s) and Curie temperature (T _c) have also been evaluated. The maximum value of P _s varies from 0.041 Cm^–2 to 0.30 Cm^–2 in the studied orthochromites, which is very small.     

Phase equilibrium and ionic conductivity in the PbF2YF3 system

The phase diagram of the PbF₂-YF₃ 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.     

New rare earth silicate crystals: Dy2MoSi2Al4O16, compounds in the systems R2O3-SiO2-PbO,and a new form of R2SiO5

The growth of crystals of a new cubic compound, Dy₂MoSi₂Al₄O₁₆, and of R₂SiO₅ (R=Dy, Tb, Gd) with a new structure is reported. Flux growth studies in the systems R₂O₃-SiO₂-PbO are also described. These have yielded single crystals of two new families of compounds: PbR₄Si₅O₁₇, with a monoclinic structure, and apatite-related phases of composition between Pb_1.4Er_2.93Si_3.6O₁₃ and Pb_1.8Er_2.5Si_3.7O₁₃ for R=Er. The compounds are characterized by X-ray powder patterns, single crystal X-ray data, and EPMA.     

Preparation and structural characterization of the lead oxide iodide Pb3O2I2

Single crystals of orthorhombic Pb₃O₂I₂ could be prepared by annealing a mixture of 2PbO + PbI₂ at 400 °C. Cell parameters are a = 17.862(3), b = 5.954(1), and $\text{c}\text{= 7.487(2)}\text{A}\text{?}$; space group is Pnma. The structure was determined from 1121 independent reflections and refined to R = 3.16%. It is built up by edge-shared Pb₄O tetrahedra, forming Pb₆O₄ double chains parallel to [010], which are connected by iodine. Three anion coordinations of lead can be distinguished: Pb(1) forms a bicapped triangular prism with 4 oxygen and 4 iodine atoms (4+4), Pb(3) a monocapped prism (2+5), whereas Pb(2) is octahedrally (2+4) surrounded; all coordination polyhedra are distorted. Structural relationships to Pb₃O₂Cl₂ exist.     

Luminescence properties for the high-pressure polymorphs of CaSiO3:Pb2+ and SrSiO3:Pb2+

High-pressure phases of CaSiO₃:Pb²⁺ and SrSiO₃:Pb²⁺ phosphors were synthesized at 40–55 kbar and 1000°C, $\text{viz}$. δ-CaSiO₃:Pb²⁺, δ-SrSiO₃:Pb²⁺, and δ′-SrSiO₃:Pb²⁺, and their luminescence properties were investigated. Among them, δ-CaSiO₃:Pb²⁺ was found to give a strong violet-blue emission ($\text{ca}$. 341 nm) as well as β-CaSiO₃:Pb²⁺ (an atmospheric phase), and the emission intensity of SrSiO₃:Pb²⁺ drastically increased when the host lattice transformed into high-pressure phases (δ and δ′ forms). These results were discussed by considering their structures and quenching temperatures of luminescences.     

Interfacial reactions and properties of Y3Fe5O12/Ba1−x PbxTiO3 composites

We have revealed interfacial chemical interaction in Y₃Fe₅O₁₂/Ba_1?x Pb_xTiO₃ magnetoelectric mixture composites during high-temperature firing. The interaction leads to the formation of pyrochlore impurity phases (x > 0.2) and yttrium orthoferrite (x = 0, t = 1300°C). Electrical and piezoelectric characterization results indicate that the best piezoelectric performance is offered by the Y₃Fe₅O₁₂/Ba_0.1Pb_0.9TiO₃ composites, which experience no chemical changes during sintering.     

Influence of heat treatment conditions on the phase composition and superconducting properties of Bi1.6Pb0.4Sr1.98K0.02Ca2Cu3F0.8Oy ceramic

The influence of synthesis temperature and time on the properties and formation of superconducting phases in Bi_1.6Pb_0.4Sr_1.98K_0.02Ca₂Cu₃F_0.8O_y samples has been studied by x-ray phase analysis and by measuring the electrical resistance. It is found that the ratio of the 2223 and 2212 phases formed during synthesis at 845 °C for 240 h remains almost constant in the range 845–855 °C, regardless of the additional synthesis time. Synthesis at higher temperatures leads to breakdown of the 2223 phase and enhances the content of the 2212 phase and impurity phases. The highest values T _c(R=0)=112.8 K were obtained for samples synthesized at 845 °C for 240 h. Pis’ma Zh. Tekh. Fiz. 23, 30–34 (July 26, 1997)