Oxygen stoichiometry and the problem of the growth of (VO)2P2O7 single crystals

The crystal growth of (VO)2P2O7 single crystals is discussed with respect to the formation of a high-temperature nonstoichiometric phase (VO)2P2O7+x. The interaction between vanadyl pyrophosphate and oxygen was investigated in the temperature region 600–900°C. It was found that the decay of the high-temperature nonstoichiometric phase at lower temperature led to imperfections in crystals grown in an atmosphere with oxygen concentration higher than 0.2%. Two mechanisms for the decay are considered. The oxygen content of the crystals may be reduced at the surface by evolution of gaseous oxygen, whereas in the bulk the formation of an oxygen-enriched solid phase is observed.