Growth and investigations of certain oxide crystals exibhiting phase transitions

Abstract

A simple and efficient procedure has been devised to separate the grown YBa?Cu?O?.? crystals from the flux matrix. The extracted YBCO crystals have clear surfaces free from flux. Thermal shock and consequent cracking of crystals have been eliminated by draining the flux in situ within the isothermal region of the closed furnace. X-ray topography reveals not only the dislocations present in the crystal but also manifestations of plastic-like deformation suffered by the crystal as a result of ferroelastic transition.

Crystals of YBCO doped homogeneously with cobalt were grown by an improved flux method. Tc > 85 K is realized at low concentrations of cobalt without external oxygen anneal. Optimal doping, leading to Tc as high as 86 K, has been achieved in this study. When the cobalt doping concentration is raised, Tc is drastically reduced. A similar dependence of Tc with cobalt content is also observed in bulk pellets. Crystal growth is a slow and systematic process, and by virtue of this, a large amount of oxygen is incorporated in situ. Consequently, a higher transition temperature is attained in the low doping regime. When doping exceeds a limit, strong decoupling of chains and planes occurs, leading to systematic reduction in Tc. Variation of Tc with cobalt doping has been discussed with reference to the apical oxygen model. The role of apical oxygen in charge transfer and change in Tc is highlighted. Uniform oxygen distribution and high Tc values can be obtained even for very large crystals by optimal doping, which is difficult to achieve by annealing alone. Thus, optimal cobalt doping can be adopted as an alternate route to extensive oxygen anneal for enhancing the Tc of YBCO single crystals.

Pb-doped lanthanum manganite crystals are grown at relatively lower temperatures. Heating and cooling programs are modified accordingly to facilitate growth. High quality of grown crystals is reflected in the sharp magnetic or metal-insulator transitions exhibited by these crystals. Results pertaining to the growth of single crystals of Nd and Pb co-doped lanthanum manganite are reported for the first time in this study. The phase transition temperature shifts closer to room temperature (275 K) by the addition of Nd along with Pb in the lanthanum site. Tunability of the transition temperature is considered a significant technological feature. Another important aspect is the possibility of enhancement of magnetoresistance at a relatively low applied magnetic field. A typical value of 65% is realized in Pb, Nd co-doped lanthanum manganite crystals at 3 Tesla. This is relevant to practical applications since production of a large magnetic field is expensive as well as cumbersome. These results highlight the role of the ionic radius of the substituent cation in modifying the spin and electronic structure of these crystals.

Good quality single crystals of CuGeO? are grown by the optimized flux method. Large-sized crystals are obtained when Bi?O? is used along with CuO as flux, and the growth is enabled at lower temperatures. Single crystals of doped CuGeO? are also grown by a similar method. The in-chain site is doped with magnetic impurity cobalt and the off-chain site with gallium. Size, color, and morphology of the crystals are changed by the presence of the dopant. Susceptibility studies of doped crystals showed a decrease in T?? with increasing dopant concentration. The role of substituents in interruption of Cu chains leading to lowering of T?? is evident. Study of gallium and cobalt-substituted CuGeO? single crystals confirms that in-chain doping has a more pronounced effect on the spin-Peierls transition as compared to off-chain doping, especially when the substituents have comparable ionic radii. T?? is rapidly reduced and suppressed when the in-chain substituent is magnetic with a higher spin value like cobalt. Addition of spin-3/2 cobalt to the CuGeO? system makes a non-magnetic ground state impossible.

This chapter summarizes the results obtained on the growth and studies of three oxide crystals, namely yttrium barium cuprate (YBCO), lanthanum manganite, and copper metagermanite. These crystals exhibit various phase transitions and other interesting physical properties. Substitutional studies carried out on these oxides have given a good insight into the mechanisms involved. The conclusions derived from various experimental investigations are briefed here.