Hydrothermal Chemistry and Growth of Fergusonite-type RENbO4 (RE = La–Lu, Y) Single Crystals and New Niobate Hydroxides

A convenient hydrothermal route to high quality single crystals of rare earth orthoniobates RENbO4 (RE = Y, La–Lu) is reported. The title compounds can be grown as single crystals in sizes exceeding 2 mm for most of the rare earths using 30 M KOH at temperatures of 700 °C and pressures of 2 kbar. Single crystal X-ray diffraction shows that they all form in the fergusonite structure type, and the structures were solved in space group C2/c. Examples based on the largest and smallest rare earth ions in the present study include LaNbO4: a = 7.3576(15) Å, b = 11.538(2) Å, c = 5.2128(10) Å, β = 130.900(3)°, Z = 4, R1 = 0.0162, wR2 = 0.0405, and LuNbO4: a = 6.9805(6) Å, b = 10.8271(8) Å, c = 5.0406(4) Å, β = 131.676(3)°, Z = 4, R1 = 0.0178, wR2 = 0.0489. Although domain structures are clearly present in the bulk crystals, the domains are much larger than normally reported from high temperature growth methods, and individual single domain crystals on the order of 0.1–1 mm in size can be selected. Rocking curves of these crystals have narrow fwhm and are Gaussian shaped. Several new phases were also isolated as part of the development of the crystal growth chemistry and were characterized by single crystal X-ray diffraction including such examples as K3YNb2O7(OH)2: P63/mmc, a = 5.9770(12) Å, c = 14.901(2) Å, Z = 2, R1 = 0.0303, wR2 = 0.0765 and CsNb2O5(OH): Fd3̅m, a = 10.5869(14) Å, Z = 8, R1 = 0.0336, wR2 = 0.0741.