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.     

Preparation and magnetic properties of Bi substituted single crystal rare earth orthoferrites

Single crystal rare earth orthoferrites, with the rare earth partially substituted by bismuth, have been prepared from a flux mixture of PbO, Bi₂0₃ and B₂0₃. Magnetic torque data are presented for Sm_1−xBi_x Fe0₃ and Er_1−xBi_xFe0₃ where the large effects of Bi substitution on the magnetic reorientation temperature are compared to the effect of a non magnetic substitution such as yttrium.     

Microhardness and brittle fracture of garnet single crystals

Hardness and fracture toughness were measured using the Vickers microhardness test in the low load range from 25 to 100 g near to the fracture threshold for near-perfect single crystals of garnets. The influence of crystal growth parameters, calcium impurity content and crystallographic orientation of Gd₃Ga₅O₁₂ (GGG) and Ca₃Ga₂Ge₃O₁₂ (CaGeGG) samples was investigated. Fracture starts with radial cracking from indent corners followed by lateral fracture of two distinct modes. The mean hardness of [111] oriented GGG isH=13 GN m^–2, for [111] oriented CaGeGG it is 12 GN m^–2, the average fracture toughness beingK _c=1.2 and 0.8 MN m^–3/2, respectively for the two crystals. Impurity doping slightly increases the strength of the material. Among the investigated crystals (111) faces are the least strong, the (100) face has maximumH andK _c values for CaGeGG. The constraint factor,, and yield stress,Y, were deduced from the measured hardness data, giving=2.2 andY about 7 GN m^–2.     

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.     

Microhardness studies on calcium hydrogen phosphate (brushite) crystals

Vicker's and Knoop microhardness studies were carried out on grown calcium hydrogen phosphate dihydrate (CaHPO₄·2H₂O) crystals over a load range of 10-50 g. The Vickers (H_V) and Knoop (H_K) microhardness numbers for the above loads were found to be in the range of 94-170 kg/mm² and 28-35 kg/mm² respectively. It was also found that these numbers increased with increase in load. The Mayer's index (n) was found to be greater than 1.6 showing soft-material characteristics. The fracture toughness values (K_c), determined from measurements of crack length, were estimated to be 6 ± 0.5 × 10³ kg m^−3/2 and 4.5 ± 0.5 × 10³ kg m^−3/2 at 25 g and 50 g respectively. The brittleness indices (B_i) were found as 2.3 ± 0.1 × 10⁴ m^−1/2 for 25 g and 3.7 ± 0.1 × 10⁴ m^−1/2 for 50 g. Using Wooster's empirical relation, the elastic stiffness coefficient (c₁₁) has been calculated from Vicker's hardness values as 4.8 ± 0.5 × 10¹⁵ Pa for 10 g, 9.7 ± 0.5 × 10¹⁵ Pa for 25 g and 13.3 ± 0.5 × 10¹⁵ Pa for 50 g. The Young's modulus was calculated as 1.5 ± 0.1 × 10¹⁰ N m⁻² from Knoop microhardness values.     

Microstructural,etching and hardness studies on flux-grown KNiF3 crystals

Surface structures on {100} faces of flux-grown KNiF₃ crystals are reported. Etching experiments establish HNO₃ to be a dislocation etchant for the crystals. The etching behaviour of the HNO₃-KNiF₃ surface system is investigated. The results obtained on the effect of etching time and etchant concentration on lateral extension and depth of dislocation etch pits are reported. It is observed that the etchant is rendered passive after some period of initial etching. Indentation-induced hardness testing studies suggest a Vickers microhardness value in the range of (2.93 to 3.50)×10² kg mm^–2, and the response of indentation to load is in accordance with Kick's Law.     

Microhardness studies of SnI2 and SnI4 single crystals

The variation in the microhardness of tin-di-iodide (SnI₂) and tin-tetra-iodide (SnI₄) crystals has been determined using Vicker’s microhardness indentor. It is observed that the microhardness of the crystals depends on the applied load and is independent of the duration of loading. Vickers Hardness Numerals (vhn) for SnI₂ is found to be greater than that of SnI₄ crystals. Mayer’s equation and implications have been discussed.     

Microhardness anisotropy of single crystals of calcite, dolomite and magnesite on their cleavage planes

Knoop indentation microhardness measurements were made on the (10 1)cleavage planes of three structurally related single crystals: calcite, dolomite and magnesite. The effective-resolved-shear-stress concept was applied to explain the observed microhardness profiles. The deformation mechanisms of calcite, which involved both twinning and dislocation slip, were considered. Finally, the indentation load/size effect on the microhardness was addressed by both forms of Meyer's Law.     

Reactions of rare earth acetates with aluminum isopropoxide in ethylene glycol: Synthesis of the garnet and monoclinic phases of rare earth aluminates

The reactions of mixtures of rare earth (RE) acetates and aluminum isopropoxide (Al/RE = 3/5) in ethylene glycol at 300°C afforded amorphous products. The IR spectra of these products were essentially identical with each other and indicated the presence of an acetate group and ethylene glycol moiety. When the content of RE (La–Gd) acetate in the starting mixture was increased, a crystalline glycol complex of RE oxide acetate was also formed together with the amorphous phase, but the reactions of acetates of the smaller RE ions (Tb–Lu) yielded amorphous products with wider compositions (Al/RE = 5/3–1/6). Phase-pure RE3Al5O12 (garnet) and RE4Al2O9 (monoclinic) were obtained by calcination of the amorphous products with the corresponding stoichiometric compositions at low temperatures.     

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 anisotropy on the (010) cleavage plane of single crystals of Bi2S3 and Sb2S3

Knoop microhardnesses were measured on the (0 1 0) cleavage planes of Bi₂S₃ and Sb₂S₃ single crystals for orientations from the [0 0 1] to the [1 0 0]. The [0 0 1] hardnesses were nearly twice the level of the [1 0 0] values, about 150-75 kg mm^–2. The experimental microhardness profile was consistent with the calculated effective resolved shear stress (ERSS) diagram for the (0 1 0) [0 0 1] primary slip system. The magnitude of the hardness appears to be related to the crystal structure, particularly the metal-sulphur chains parallel to the [0 0 1], the crystal growth direction.     

Lattice disorder and electrical conductivity of flux-grown Ca(WO4) x (MoO4)1−x crystals

The growth of solid solutions of Ca(WO₄) _x (MoO₄)_1–x in the rangeX=0.072 to 0.86 has been successfully accomplished for the first time, employing a flux technique. The fact that proper mixing occurs is established by observing the variation in unit cell parameters and density of the crystals. The electrical conductivity of the pelletized samples is interpreted in terms of Schottky and Frenkel defect concentrations. The role of oxygen ion vacancies is stressed for the intrinsic region of conductivity.     

Spherulitic growth of single (Gd & Pr) and mixed (Di) rare earth heptamolybdates in silica gels

Single (Gd and Pr) and mixed rare earth (Di—a mixture of four rare earths La, Pr, Nd & Sm) heptamolybdates grow as platelet, multifacetted crystals and spherulites when corresponding rare earth chloride ions are made to react with ammonium paramolybdate. Various spherulitic formations are illustrated. It is shown that the spherulitic formations may be due to either fibres radially diverging from multiple nuclei or agglomeration of tiny crystallites orienting and accommodating themselves in a spherical space or intergrowth of multiple crystals which may include multiple twinning of interpenetrating type.     

Accurate measurements of the acoustical physical constants of LiNbO (3) and LiTaO(3) single crystals

The acoustical physical constants (elastic constant, piezoelectric constant, dielectric constant, and density) of commercial surface acoustic wave (SAW)-grade LiNbO(3) and LiTaO(3) single crystals were determined by measuring the bulk acoustic wave velocities, dielectric constants, and densities of many plate specimens prepared from the ingots. The maximum probable error in each constant was examined by considering the dependence of each constant on the measured acoustic velocities. By comparing the measured values of longitudinal velocities that were not used to determine the constants with the calculated values using the previously mentioned constants, we found that the differences between the measured and calculated values were 1 m/s or less for both LiNbO(3) and LiTaO(3) crystals. These results suggest that the acoustical physical constants determined in this paper can give the values of bulk acoustic wave velocities with four significant digits.