Structure and Properties of Reduced Early Transition Metal Oxide Single Crystals Grown from Borate Fluxes

Reduced ternary oxides of the early transition metals exhibit a variety of interesting physical and structural properties. Superconductivity (LiTi2O4 [1], tetragonal and hexagonal Mo and W bronzes [2]) and charge density wave behavior (Mo ’blue’ bronzes [3]) have been observed in these systems. Structures range from simple perovskites like SrxNbO3 [4] to complicated structures involving metal-metal bound clusters [5]. Single crystals are highly desirable for a better understanding of structure-property relations. In systems having low-melting high oxidation state ternary precursors (like A2MO4 and A2M2O7; A = alkali metal, M = Mo, W) molten salt electrolysis provides a convenient route to single crystals. For other systems (particularly niobates) other methods have to be employed. Here we describe the use of borate fluxes to produce single crystals of various reduced ternary niobates and titanates in inert atmosphere. Borate fluxes have been regularly employed in the crystal growth of ferroelectrics and garnets [6], but rarely used for reduced compounds. An interesting aspect of the borates AnOm ·xB2O3 is that they can act both as a source and as a buffer for AmOn , allowing the product formation to be influenced by the flux composition when the A-cation is also part of the reduced ternary system. An advantage of the Ba, Sr, and La-borates is their low volatility, which allows their use under high-vacuum conditions.