H.C. Zeng

Correlation of PbMoO4 crystal imperfections to Czochralski growth process

Abstract Direct correlation of crystal imperfections in a Czochralski grown PbMoO4 single crystal to growth conditions has been studied using vis-transmission measurement, chemical etching technique and differential thermal analysis (DTA). Two different types of crystal imperfections are identified. It is found that vis-transmission property decreases along the crystal boule length. The density of impurity-related defects in the outer part of the crystals is about 0.65–1.25 times higher than that in the center. A slight off PbMoO4 stoichiometry is observed in surface samples of the upper part crystal due to thermal decomposition. The melting point profile of solidified crucible PbMoO4 is analyzed and its relationship to the observed crystal imperfections is addressed.

Rectangular vacancy island formation and self-depletion in Czochralski-grown PbMoO4 single crystal during heat treatment

Single crystal samples of PbMoO4 with a 〈100〉 orientation have been investigated by DTA, chemical etching, and thermal treatments at 950°C for various annealing times. Surface morphologies of chemically and thermally etched {;100}; surfaces have been studied in detail in respect of microstructure, etch pit dimension, defect type and density. It is found that both chemical and thermal etching techniques reveal a similar radial defect-distribution pattern on the {;100}; surfaces while DTA shows a small compositional variation along the radial and axial directions. The correlation between two types of etch pits has been addressed. For thermally etched samples, a kinetic study on the rectangular vacancy island (thermal etch pit) formation and self-depletion has been presented. Based on the current work, the formation rate of the vacancy islands can be expressed as D2x − D20 = k0t1.32exp(− EaRT). Mechanisms regarding the vacancy island formation and self-depletion upon the heat treatment have also been proposed. The current study on the post-growth heat treatment has also identified key processing parameters to improve overall crystal quality.

Bubble formation in Czochralski-grown lead molybdate crystals

Lead molybdate crystals were grown by the Czochralski technique under different conditions from charges of equal molar fractions of PbO and MoO3 powders. Bubbles were found to be the main scattering sites in the crystals. In general, those grown at a pull rate of 3 mm/h are free of bubbles while those grown at higher pull rates may or may not contain bubbles depending on the rotation rate and the crystal boule length grown. At higher pull rates of 4 to 5 mm/h and a high rotation rate of 25 rpm, bubbles form early in the crystal growth process. At a low rotation rate of 2.5 rpm, bubbles will not form until the crystal-melt interface extends to the bottom of the crucible leading to a stagnant layer of melt in between them. The results are attributed to the high fluidity of lead molybdate melt and the efficiency of free convection in homogenizing the layer of melt adjacent to crystal-melt interface, thus alleviating the concentration build-up of gaseous constituents ejected from the growing crystal and preventing the formation of bubbles in the adjacent melt. The early formation of bubbles at high rotation rates is explained in terms of the negating effect of forced convection in countering the homogenizing effect of free convention, enabling the concentration of gaseous constituents in the melt adjacent to the growing crystal to build up sufficiently to effect bubble formation.

Pseudo-dendritic growth in lead molybdate single crystal by Czochralski technique

A pseudo-dendritic crystal growth of lead molybdate (PbMoO4) has been studied with the Czochralski technique. The observed dendritic growth is iso-directional, projecting from the circumference of the growth front, where the content of impurities, melt stoichiometry and temperature are expected to be varied. The dendrite arm length and inter-dendrite angle are closely correlated. It is found that the grown dendrite arms are bounded only by ten low-index crystallographic planes (001), (001), (101), (101), (101), (101), (011), (011), (011), and (011). Based on this work, a facet transition pattern existing among the dendritic arms has been revealed. Further analysis for crystal grains in as-grown PbMoO4 single crystals indicates that a similar dendritic growth can also occur under conventional growth conditions. It is further suggested that the constitutional supercooling is a major factor causing the dendritic growth in PbMoO4 single crystals.

Effect of ambient water on crystal morphology and coloration of lead molybdate

Vapour phase growth of two types of lead molybdate (PbMoO4) crystals has been studied in air under normal pressure. It has been found that both PbMoO4 crystal morphology and crystal coloration depend sensitively on water content in ambient air. Using dry air, the resultant PbMoO4 is mainly in form of needles growing along 〈001〉 directions of the crystal whereas in an atmosphere of moisturized air, the product is mostly terrace-like platelets, extending towards 〈110〉 directions. The PbMoO4 needle crystals are found with light-yellow colour while the platelet crystals are light-grey or colourless. Considerations of growth have also been given to the observed two types of PbMoO4 crystals.

Purification growth of orthorhombic molybdenum trioxide crystals from alkali-metal-containing Mo-sources

Abstract Transparent and colourless α-MoO3 crystal platelets have been prepared in vapour phase from various alkali-metal-containing molybdenum sources. With XRD technique, it is evident that the α-MoO3 is the only crystalline phase in the as-grown crystals. Furthermore, FTIR and elemental analysis results reveal that alkali metals are not intercalated in the layered crystal lattice of α-MoO3. Factors that lead to the success of this purification growth are identified. In view of its simplicity, this method can be efficiently used for laboratory preparation of high quality α-MoO3 crystals from low-grade molybdenum sources.

Effects of meniscus on the directional growth of potassium niobate single crystals

The study of meniscus effects on the directional growth of single crystals of potassium niobate (KNbO3) with a [110]pc oriented seed by the top-seeded-solution-growth technique has been described. The directional growth of KNbO3 single crystals has been illustrated for different crystallographic orientations of the crystal. Experimental results show that the shape of the solution flux meniscus, which is determined by a number of growth parameters, has a significant effect on growth rates on different 100pc, 010pc and 001pc type surfaces. Findings based on this work enable one to select a dominant growth direction and hence to engineer the geometrical shaping of resultant KNbO3 single crystals through careful control of growth parameters. Maximum crystal dimensions for the grown KNbO3 along [100]pc and [001]pc achieved in this work are 34.0 and 31.2 mm, respectively.

Growth kinetic study of potassium niobate single crystal: a new method for high time-resolution kinetic data of ABO3-type materials

Abstract A new method to obtain high time-resolution kinetic data has been introduced in conjunction with a directional growth of KNbO 3 single crystal. Using the step-growth on a selected (1 0 0) pc plane, experimental conditions such as solid-liquid-vapour meniscus, solute concentration, and temperature, can be well defined for the kinetic data. Due to the selective growth employed, the growth data account truly for a crystallographic plane of interest. Because the grown steps serve as a time marker, the volume of step can be measured and assigned to a respective growth whose time resolution achieved in this study is 8.4 s. In addition, using the current method, sequential growth events can be conducted with variable pulling rates to abstract different growth information. Small growth perturbation can also be recorded sensitively, as evidenced in the growth oscillation observed. Based on these findings, it is suggested that the method can be employed to other oxide perovskite ABO 3 single crystal growths. Future applications and investigations are also proposed.

Two-level growth of potassium niobate KNbO3 single crystals : a new growth method for ABO3-type materials

Abstract Based on our recent study on the directional growth of KNbO 3 single crystals, a new growth scheme called the two-level growth (TLG) has been developed. By using the TLG method, high quality transparent KNbO 3 single crystals can be prepared with high reproducibility. The technique includes two growth stages. In level-one growth, a dimensionally defined (shaped) KNbO 3 seed with high crystal perfection is fabricated. The target crystal with desired dimensions and an architectural structure can be obtained in the second level of the growth. A process flow chart for the TLG method has been presented and six representative types of the growths have been introduced. Generally, crystals grown by TLG have much less crystal imperfections than those obtained from the conventional way. Technological merits of the TLG scheme have been studied, indicating that this growth scheme can also be employed to other oxide perovskite ABO 3 single crystal growths.