CC BY
1The effect of post-growth annealing on CaMoO4 optical properties and elemental composition
N.S. Kozlova1*, E.V. Zabelina1, V.M. Kasimova1, E.A. Scryleva1, O.A. Buzanov2
1- National University of Science and Technology "MISIS", 119049, Russia, Moscow, Leninskypr., 4, b. 1 2- FOMOS-MATERIALS, 107023, Russia, Moscow, Buzheninova st., 16
Synthesized calcium molybdate (povellite) CaMoO4 is a well-known material. The first works on CaMoO4 crystals growth date back to the 40s of the 20th century. CaMoO4 calcium molybdate single crystals are characterized by a scheelite-type structure, belong to tetragonal symmetry, the space symmetry group is I41/a, the point symmetry group is 4/m. The unit cell is tetragonal, volume-centered, and contains 4 formula units. Initially, CaMoO4 single crystals were used as a material for tunable acousto-optical filters, and later was considered as a material for Raman lasers. In recent years, this material was investigated due to the possibility of using isotope-enriched calcium molybdate 40Ca100MoO4 as a cryogenic phonon-scintillation detector with a metallic magnetic calorimeter (MMC) and Superconducting Quantum Interference Device (SQUID) in experiments searching for double neutrinoless beta decay. The crystals used for the manufacture of such detectors must have a large sensitive mass and exceptionally high optical quality, in particular, they must be optically homogeneous, characterized by the absence of color and structural defects, the attenuation index ^ at the maximum emission at a wavelength of 520 nm must be no more than 0.01 cm-1. The specified characteristics must be maintained throughout the entire period (years) of operation of the detector. The practical use of CaMoO4 crystals for optical purposes is complicated by the presence of defect centers in them, such as color centers, which appear as yellow, blue or bright blue color. This color of the crystals is usually associated with the presence of oxygen vacancies. The reasons of those vacancies is a evaporation of volatile molybdenum oxide MoO3 from the melt during the growth process. Isothermal annealing in an atmosphere with a controlled oxygen content leads to bleaching of such crystals. This is associated with the filling of oxygen vacancies with oxygen ions, while the electrical neutrality of the crystal is achieved, presumably, due to the recharging of molybdenum.
The purpose of this work was to study the effect of post-growth isothermal annealing in air on the optical properties and elemental composition of calcium molybdate crystals.
CaMoO4 crystals were grown at Fomos-Materials using the Czochralski method from a charge of stoichiometric composition with the addition of an excess amount of MoO3, from platinum crucibles in Kristall-3M set-up using a high-frequency heating method. The samples were studied in the initial state and after high-temperature annealing in air for 6 and 100 hours at a temperature of 1000°C, and in vacuum for 7 hours at a temperature of 1250°C.
The optical properties of the sample were investigated in the accredited tested laboratory of semiconductor materials and dielectrics "Single Crystals and Stock on their base" of National University of Science and Technology "MISIS". The spectral dependences ofthe transmittance T(^) were measured taking into account optical anisotropy and dichroism on a Cary-5000 spectrophotometer (Agilent Technologies) with a universal measurement accessory (UMA).
The elemental composition of CaMoO4 crystals in the initial state and after isothermal annealing in air for 100 hours (colorless) was studied by X-ray photoelectron spectroscopy (XPS).
Annealing of CaMoO4 crystals leads to a change in the concentrations of the main elements, however, all elements remain in charge states typical for the compound; we also did not detect a change in the charge state of molybdenum. Apparently, the generally accepted model of defect formation in calcium molybdate crystals should be revised.
This study was supported by the Ministry of Science and Higher Education of the Russian Federation and within State assignment for Higher Education Institutions no. FSME-2023-0003.
