CC BY
17LS-PS-1
Cation-deficient sodium-gadolinium molybdates: structure, modeling, energy transformation
E. Zharikov1, K. Subbotin1, V. Dudnikova2, D. Lis1, A. Titov1
1Prokhorov General Physics Institute RAS, Laser Crystals, Moscow, Russian Federation 2Lomonosov Moscow State University, Geology Faculty, Moscow, Russian Federation
Sodium-gadolinium molybdate (NGM) is representative of a large family of compounds crystallized into the scheelite CaWO4 structure (tetragonal syngony, space group I41/a) with a statistical distribution of sodium and gadolinium ions at calcium sites. Ca2+ in prototype powellite CaMoO4 replaced for Na+ and Gd3+ in NGM. Scheelite-like NGM has a cation-deficient structure and, according to its nominal composition, contains up to 1/7 unoccupied cation sites. General formula for charge compositions is NaxGd(2-x)/3(i-2x)/3MoO4, with x value varied from 0,2 to 0,6. The average amount of vacancies in crystals grown by Czochralski technique is about 0.07 f.u., which is equal to 7 % of empty cation sites in the sublattice.
The simulation was performed by the method of interatomic potentials using the GULP 4.0.1 code (General Utility Lattice Program), which is based on minimizing the energy of the crystal structure. Atomistic simulation considered two kinds of cation sites distribution: the statistical distribution of sodium, gadolinium, and unoccupied cation positions in the I4/a structure and their partial ordering. As a result of the simulation, structural characteristics of NGM agreed well with the known experimental data. In addition, a number of important elastic and thermodynamic properties of these compounds are predicted.
NGM single crystal is a promising laser host. It provides the broad and strong luminescence bands of Ln3+ dopants, appropriate for femtosecond laser pulses generation in mode-locked regime as well as for tunable lasing. NGM doped by different rare earths are also promising phosphors for use in white light-emitting diodes, optical displays, and as scintillators, photocatalysts, etc. Yb-doped scheelite-like NGM is the efficient down-converter, enhancing the efficiency of crystalline silicon photovoltaic cells (Eg = 1,14 eV). The efficient excitation of 1 |m Yb3+ emission by 'soft' UV light (260-400 nm) occurs due to quantum cutting. It runs via donor-acceptor interaction: donor optical centers absorb the UV quanta and non-radiatively transfer the excited state energy to a doubled number of acceptors via the mechanism of cooperative down-conversion. Then the acceptor emits the energy as the secondary quanta. Ytterbium ions are the acceptor in these crystals, while the structure of donors is not known so far. The possible donor nature is discussed.
This work has been supported by Russian Scientific Fund (grant # 18-12-00517)
