Rare-earth niobates and tantalates are refractory materials that have been exploited in applications involving ion conductivity, photo-catalysis, and luminescence, both in doped and un-doped forms.
In general, refractory oxides have high melting points therefore synthesis of single crystals are more challenging. For example LnNbO4 can be grown readily by melt techniques such as Czochralski pulling ~1300 ˚C. However during the cooling crystal quality degraded resulting low quality crystals therefore high temperature melting techniques are not suitable for bulk single crystals growth.
Sanjeewa, Liurukara D.; Fulle, Kyle; McMillen, Colin D.; and Kolis, Joseph W., "Hydrothermal Single Crystal Growth and Characterization of Novel Rare Earth Niobates and Tantalates: LnNbO4 (Ln = La-Lu , Y), La2TaO5(OH) and Ln3Ta2O9(OH)" (2016). Chemistry Annual Research Symposium. Paper 1.