B. Cockayne

An investigation of phase stability in the Y2O3-Al2O3 system

The stability of the three known phases in the Y2O3-Al2O3 pseudo-binary system has been investigated. YAlO3 (YAP) and Y4Al2O9 (YAM) decompose at elevated temperatures, the products of the reaction being the third compound Y3Al5O12 (YAG) and an unknown phase (designated X). The decomposition is most evident in powders but can also be initiated on the surface of bulk single crystals. X-ray diffraction studies have been performed in an attempt to identify the structure and composition of the unknown phase. The thermal decomposition has been found to be surface controlled and an optical and scanning electron microscope study of the associated morphological changes in YAlO3 indicates that the reaction involves localized surface melting, probably with the loss of oxygen which effectively moves the composition off the binary join.

The relationship between crystal growth behaviour and constitution in the systems LiF-LuF3, LiF-ErF3 and LiF-YF3

The crystal growth behaviour of LiRF4-type compounds in the systems LiF-LuF3, LiF-ErF3 and LiF-YF3, have been related to the constitution of the respective systems. Crystal growth, differential thermal analysis (DTA) and microstructural studies indicate that the phases Li Lu F4 and LiErF4are clearly congruent in nature which is in agreement with previous work on these compounds. Corresponding studies for the compound LiYF4 are consistent with this phase being either just congruent or just syntectic (but definitely not peritectic) in character, its constitutional behaviour depending critically on the level of contamination.

Cracking in yttrium orthoaluminate single crystals

Several major factors which influence cracking in Czochralski-grown YAIO3 single crystals have been identified. The procedures evolved for minimizing crack formation are also described.

Ferroelectric domains and growth striae in barium sodium niobate single crystals

The relationship between the ferroelectric domain structure and growth striae is examined in barium sodium niobate (Ba2NaNb5O15) single crystals. Growth striae stabilise a particular form of domain structure and are further shown to be caused by temperature fluctuations occurring in the melts from which such crystals are grown. These fluctuations and the associated striae can be eliminated by reducing the melt level in the crucible used to contain the molten material. Evidence is presented to show that this effect cannot simply be regarded as a reduction in the Rayleigh Number of the melt system, as convective effects occurring in the gas ambient above the melt also exert an influence.

A DTA study of zone-refined LiRF4 (R=Y, Er)

The melting behaviour of zone-refined bars of LiYF4 and LiErF4 has been investigated by means of differential thermal analysis (DTA). Both compounds can be produced congruently under an atmosphere of carefully purified argon. Consideration of the constitutional variations occurring along the length of the bars has established that the congruent melting composition is non-stoichiometric, and a detailed analysis of the profiles of the thermal events indicates that both compounds possess a homogeneity range and are not line compounds as previously reported. The influence of purity on the melting behaviour is assessed and the implication of the results on the crystal growth of laser materials based on these compounds is discussed.

Barium sodium niobate: Constitutional studies and crystal growth

Phase equilibria of barium sodium niobate, stoichiometric formula Ba2 NaNb5O15 (10 Na2O, 40 BaO, 50 Nb2O5), in the ternary Na2O-BaO-Nb2O5 system have been studied and related to observations made on Czochralski grown single crystals. Extensive solubility occurs in Nb2O5-rich specimens, the liquidus and solidus surfaces being extremely flat and close together over the complete homogeneity range. Crystals grown from the non-stoichiometric congruent melting composition (7.2 Na2O, 42.2 BaO, 50·6 Nb2O5) are completely free from growth striae.

A dilatometric study of the orthorhombic-tetragonal phase transition in barium sodium niobate

The thermal expansion characteristics of the two known phases of barium sodium niobate have been studied using a dilatometric technique. Observation of discontinuities in the expansion curves along the three principal axes of the room temperature orthorhombic phase have been directly related to the crystallographic changes associated with the orthorhombic-tetragonal (O-T) phase transformation occurring at ∼ 260°C. A comparison of the behaviour of twinned and detwinned crystals has shown that microtwinning in the O-T transition is not a mechanism essential to the transformation; it only occurs in the orthorhombic phase to relieve the large elastic strains known to arise in the tetragonal phase on cooling. An apparently anomalous behaviour recorded on one of the crystal axes emphasises the importance of push rod geometry on determining expansion behaviour by this method. A simple geometrical model has been constructed to relate the magnitude and sense of the observed discontinuities to the composition dependence of the unit cell dimensions of the two phases, and the three general cases arising are discussed with respect to the present and previously published results.The expansion curves of all three axes showed a well defined discontinuity at a temperature corresponding to that of the ferroelectric Curie point indicating the possibility of a first order phase change at this temperature.

The nucleation and suppression of transformation twinning in barium sodium niobate

It has already been established that the microtwinning associated with the tetragonal to orthorhombic phase transition in barium sodium niobate can be suppressed by cooling from above the transformation temperature under an applied stress. The work reported here indicates that the quotation of specific stress values for this process is misleading since, for any specimen, there is an optimum range of applied stress over which complete suppression of the microtwinning is achieved. This range is shown to be dependent upon the particular twin density and distribution which is determined by the presence of other crystal defects and by the composition of the material. Means of reducing the twin density are also discussed.An anomalous expansion behaviour in the [001] crystal direction, detected by a dilatometric technique, is consistent with the observed tendency to crack as the material is cooled through the Curie point.

A study of the LiYF4-LiErF4 psuedo-binary system

The constitution and structure of compounds represented by the general formula LiY1−x Erx F4 (0 < x< 1) have been studied by X-ray diffraction, magnetic susceptibility and differential thermal analysis (DTA). X-ray diffraction shows that the bct Scheelite structure is retained across the system with only a very slight change in the lattice parameters. Large changes in room temperature magnetic susceptibilities as a function of x have been observed and these have been used to provide a very sensitive composition check on the compounds. DTA profile analysis shows that, for samples prepared in a carefully purified environment, the solidus-liquidus separation is so small that an approximation to congruent melting can be made for all the compounds in the system. A comparison with material prepared under unpurified conditions provides information on the effects of contamination upon the melting behaviour.

Dislocations in yttrium orthoaluminate single crystals

The dislocations present in single crystals of yttrium orthoaluminate grown by the Czochralski technique have been studied by means of etch pits and electron microscopy. They are shown to be predominantly edge in character with Burgers vectors which are principallya [1 0 0] andb [0 1 0]. In most instances, the dislocations form simple tilt boundaries. A few dislocations of a more random nature occur in association with twin boundaries. Dislocation formation is shown to be a characteristic of c-axis growth related to stresses caused by the anisotropic contraction of the material;b-axis crystals can be grown in a dislocation-free form.