Influence of initial charge composition and growth/annealing atmospheres on the structural parameters of Czochralski-grown (NaxGd1−x)MoO4crystals

Abstract

The composition of microparts of sodium-gadolinium molybdate crystals, grown by the Czochralski method in different atmospheres from stoichiometric ((Na0.5Gd0.5)MoO4; group I) and nonstoichiometric ((Na2/7Gd4/71/7)MoO4 (II.1) and (Na6/15Gd8/151/15)MoO4 (II.2); group II) initial charge compositions (where = vacancies), was refined by X-ray diffraction analysis. All the crystals under investigation were found to crystallize in the space group I41/a without any signs of structural modulation and the refined crystal compositions were different from the initial charge compositions. Crystals from group I had a gadolinium excess and sodium deficiency without vacancies in the dodecahedral site. Vacancies were found in the Mo and O sites of the structure of the dark-gray micropart of the crystal grown in an Ar atmosphere, with the composition of the near-colorless micropart of the same crystal being close to stoichiometry. Microparts of the crystals from group II contain vacancies in both cation sites in (Na0.348(8)Gd0.5280.124)(Mo6+0.996(3)0.004)O4 (II.1) and in the dodecahedral and oxygen sites in (Na0.300(8)Gd0.5760.124)Mo6+(O3.880(10)0.120) (II.2). It was revealed that an additional annealing in air of the crystals grown in a slightly oxidizing atmosphere reduces the oxygen vacancies content. The possibility of growing crystals with an ordered structure depending on the ratio of the formula coefficients of components in the dodecahedral site of scheelite structure is thus discussed herein.