Crystal growth of ReCa3Mn3O3(BO3)4 (Re = Gd, Y) gaudefroyite: Phase sequence and equilibrium study in multi-component fluxes

The work is aimed at studying the crystallization of ReCa3Mn3O3(BO3)4 (Re = Gd, Y) borates with the gaude-froyite structure in different flux systems. Gaudefroyites are promising materials in the field of spin ice and magnetocaloric effect. The study has been performed on systems based on the following solvents: Bi2Mo3O12-Na2B4O7-B2O3, Bi2O3-Na2B4O7-B2O3, Bi2O3-BaO-Na2B4O7-B2O3, Li2WO4-Li2O-B2O3. The influence of the solvent components on the sequence of high-temperature crystallizing phases has been studied. The feature of Mo-and W-containing fluxes consisting in the formation of stable chemical bonds of the M2+Mo(W)O4 (M = Ca, Mn) molybdate or tungstate types has been revealed which prevents the formation of gaudefroyite type bonds. An approach has been suggested for changing the sequence of the crystallizing phases which consists in the super saturation of molybdate (tungstate) bonds by Na+ or Li+ cations up to the formation of delafossite-type bonds (Li (Na)-Mn-O phase) with higher manganese valence states Mn3+ and Mn4+. The optimization of the growth conditions of ReCa3(MnO)3(BO3)4 (Re = Gd, Y) gaudefroyite crystals has allowed one to obtain single crystal samples with the size up to 0.8 x 0.8 x 5 mm3. The structural characterization and chemical composition of all the obtained phases are also presented.