The chemical vapor transport technique (CVT) consists in evaporation of a polycrystalline material in one part of the quarz ampoule, diffusion tranport of the vapor into another part of the ampoule, where crystallization in single crystalline form takes place. The evaporation is assisted by a chemical reaction with a transport agent (for example, tellurium tetrachloride TeCl₄ for VTeO₄ and CuSb₂O₆). Free standing well-shaped crystals of very high perfection can be grown by CVT. The quarz ampoule is positioned in a tube furnace with temperature gradient.