Abstract: A lead-containing complex oxide containing substantially no alkali metal is produced under hydrothermal conditions in the absence of alkali metals using a Pb source which serves also as a mineralizer for precipitating the lead-containing complex oxide. Preferred Pb source acting as the mineralizer is lead oxide.

Abstract: A starting material composed of calcium carbonate is dissolved in a nitrate aqueous solution containing either Ca(NO.sub.3).sub.2 or NH.sub.4 NO.sub.3 under application of heat and pressure. The starting material is hydrothermally synthesized within the nitrate aqueous solution to effect the rapid growth of calcium carbonate single crystal.

Abstract: A starting material composed of calcium carbonate is dissolved in a nitrate aqueous solution containing alkali nitrate under application of heat and pressure. The starting material is hydrothermally synthesized within the nitrate aqueous solution to effect the rapid growth of calcium carbonate single crystal.

Abstract: The present invention is the method of manufacturing BaPb.sub.1-x Bi.sub.x O.sub.3 single crystal utilized as a super-conductor. In this method, the starting material and the growing substrate are mounted inside an autoclave, in which an aqueous solution comprised of acid, alkali aqueous solution or peroxide is charged. With the hydrothermal synthesis that applies necessary temperature and pressure to the starting material, the BaPb.sub.1-x Bi.sub.x O.sub.3 single crystal is obtained and the composition reappearance characteristic is improved. The strain is removed by growing at a temperature below the transition point. Also, by removing such defects as heat strain, the crystallization ability and super-conducting are improved.

Abstract: Calcium carbonate single crystal, which can be optically used and industrialized, is produced by hydrothermal synthesis method in which the single crystal over species crystal is grown by using NaCl or KCl or LiCl aqueous solution under fixed temperature and pressure.