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A hydrothermal route to single crystals of rare earth stannates RE2Sn2O7 (RE = La–Lu) in the cubic pyrochlore structure is reported. Growth reactions were performed in aqueous fluids at 700 °C and 200 MPa with CsF mineralizers in concentrations ranging from 0 to 30 M, with 20 M CsF providing the most consistent results. Single crystals of the entire range of lanthanides were grown and characterized by single crystal X-ray diffraction and found to be isostructural in the Fd3m space group. The unit cell sizes range from 10.7106(16) A for La2Sn2O7 to 10.3005(9) A for Lu2Sn2O7. Both the unit cell size and RE–O distances are found to be essentially linear with respect to the ionic radius of the rare earth ion. The high quality diffraction data strongly suggests that there is very little site disorder or lattice defects in the sample. Of particular interest is the synthesis and single crystal growth of Ce2Sn2O7, which represents one of the few f1 pyrochlore samples. Specific heat measurements were obtained dow...

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) A, b = 11.538(2) A, c = 5.2128(10) A, β = 130.900(3)°, Z = 4, R1 = 0.0162, wR2 = 0.0405, and LuNbO4: a = 6.9805(6) A, b = 10.8271(8) A, c = 5.0406(4) A, 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 o...

An investigation into the crystal chemistry of the alkali thorium silicates resulted in the formation of five novel materials by hydrothermal synthesis. Of these, Rb4Th2Si12O30, and K2ThSi4O10F2 are new structure types crystallizing in space groups P1 and Pnma, respectively. The structure of Rb4Th2Si12O30 reveals a silicate sheet structure with eight-, six-, five-, and four-membered silicate rings. Crystals of a related composition, Cs2ThSi6O15, were also synthesized. They crystallize in space group Cmc21 and have a silicate sheet containing eight-, six-, and four-membered silicate rings. The structure of K2ThSi4O10F2 consists of a tubular silicate column comprised of eight-, six-, and four-membered silicate rings and features chains of fluorine-bridged Th atoms. Two new wadeite analogues Cs2ThSi3O9 and Rb2ThSi3O9 were also discovered and constitute the first actinide representatives in this mineral family. All products were crystallized from reactions of ThO2 and silica with alkali fluorides or hydroxid...

The growth of Lu3Al5O12 (LuAG) as high quality single crystals using the hydrothermal method is described. The growth protocol is similar to that of Y3Al5O12 (YAG) using binary oxides (Lu2O3 and sa...

The hydrothermal growth of bulk single crystals of lutetium oxide (Lu 2 O 3 ) is reported. Crystals were grown at 600― 650 °C at 2 kbar in water with high concentrations (10―20 M) of KOH as a mineralizer. Under these conditions crystals formed spontaneously and could also be transported to suitable seeds to form large (3—8 mm/edge) single crystals. Experiments were done to optimize transport conditions and the quality of the crystals for the first time. The crystals were doped with trivalent laser active ions such as Er 3+ and Yb 3+ . Absorption spectra of the doped materials were determined. Thermal conductivities of the pure single crystals as well as of the Yb-doped crystals were determined between room temperature and 77 K. The conductivity of the pure crystals increases significantly at lower temperatures as expected, and is significantly greater than that of YAG. The Yb-doped crystals have a thermal conductivity almost unchanged from the pure host, which is promising for use in high-energy laser applications.

Single crystals of ThO2 exceeding 8 mm in length have been produced by hydrothermal synthesis in aqueous cesium fluoride solutions at 750 °C. Solubility studies have confirmed ThO2 is congruently saturating in aqueous cesium fluoride with positive solubility with respect to temperature and increasing mineralizer concentration. Bulk transport growth of ThO2 was performed to determine the viability of large-scale production of ThO2 single crystals for possible nuclear fuel applications. The thermal conductivity and thermal expansion properties of single crystal ThO2 were measured and are reported here for the first time. The single crystal sample, in regard to nuclear fuel applications, showed improved thermal performance compared with previously reported polycrystalline values.