Научная статья на тему 'Influence of Ca3(VO4)2 crystal structural transformations on spectroscopic properties of transition metal ions '

Influence of Ca3(VO4)2 crystal structural transformations on spectroscopic properties of transition metal ions Текст научной статьи по специальности «Химические науки»

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Текст научной работы на тему «Influence of Ca3(VO4)2 crystal structural transformations on spectroscopic properties of transition metal ions »

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LS-I-11

LASER SYSTEMS AND MATERIALS

Influence of Ca3(VO4)2 crystal structural transformations on spectroscopic

properties of transition metal ions

Calcium orthovanadate Ca3(VO4)2 (CVO) is a multifunctional optical material whose multifunctionality is primarily caused by the disordered whitlockite-like structure without an inversion center. Being well-known ferroelectric material CVO has also received special attention due to its potential use as a host medium for diode-pumped and tunable lasers. The most simple and effective tool to modify functional properties of the material is introduction of dopants or impurities into the crystalline matrix. In present work we consider the spectroscopic properties of CVO crystals doped with TM metals (Mn,Co) in their relation to structural transformations connected to introduction impurities into the matrix.

The main research method for statistical structure was X-ray structural analysis with using diffractometers and synchrotron; and X-ray absorption spectroscopy (synchrotron) was used for determination of local structure. Dopant ions were added into the melt (Czochralski method) over CVO stoichiometry and introduced into the crystalline plates by high-temperature diffusion doping. The full composition for CVO unit cell can be written as [Ca1(18)Ca2(18) Ca3(18)Ca4(6)Ca5(2.65)Ca5^f0.35)][(V1(18)V2(18)V3(6)]O168. It was found that Mn ions in green CVO:Mn crys-tal(as-grown) occupy the Ca3, Ca4, Ca5, Ca5^ crystallographic sites, (CN Mn3+ = 6 and CN V5+ = 4);Mn ions in yellow-orange CVO:Mn crystal (after air annealing) promote the appearance of Mn4+ and Mn(3+ S )+ ions having octahedral coordination in the Ca3 and Ca5 sites in the structures, respectively; Mn ions in blue-green CVO:Mn crystal (after high-temperature diffusion annealing) demonstrate the number of sites (Ca2, Ca3, Ca4) occupied by a greater number of Mn(2+S)+ ions with variable formal charge and tetrahedral local environment. Thus, according to the structural studies, in the CVO:Mn structure,manganese ions Mn(2+S)+ with a mixed FC can be located in all calcium sites except for Ca1. Co ions in violet CVO:Co3O4 (after high-temperature diffusion annealing) demonstrate the appearance of Co2+ ions, which partially replace the Ca2+ ions in the Ca2, Ca3, Ca4, Ca5, and Ca5^ crystallographic sites. According to single-crystal-X-ray diffraction, single-crystal synchrotron X-ray diffraction, and powder synchrotron X-ray diffraction methods, a gradual decrease in cell parameters can be observed for Mn and Co doped CVO crystals with increasing of dopant concentration (rCa> rV> rCo> rMn; r, Â are ionic radii).

The measured absorption spectrum of doped materials is characterized by an absorption bands specific for each dopant or their combination which intensity was observed to increase linearly with doping ions concentration. Quite complex absorption spectra of CVO crystals doped with Mn were measured at room temperature within a broad wavelength range (350-1200 nm). The absorption band peaking at 460 nm was assigned to the absorption of Mn2+ ions; absorption about 600 nm is most likely associated with Mn3+ ions while the broad band with maximum at 750 nm and a weak band at 1160 nm can be assigned to Mn5+ ions. Low-temperature site-selective spectroscopy also allowed to confirm the presence of manganese ions in Mn2+and Mn3+ states, which, in general, does not contradict the structural studies. Though, specific lines in low-temperature fluorescence and excitation spectra which are attributed in literature to Mn5+ state (not detected in structural studies) were also observed.

Absorption spectrum of CVO:Co corresponded well to Co2+ ions in tetrahedral sites with absorption band maximum 640 nm. Measured fluorescence spectrum under excitation at 640 nm was broad with maximum about 850 nm. Decay curve is seen to be double exponential with decay times of 8.5 and 30 ^s. At least two positions of Co2+ ions with sufficiently different environment (intermediate between tetrahedral and octahedral) can be observed in the crystal. A fundamentally different structural behavior of dopant ions (concentration, formal charge, and coordination environment) in the CVO was defined depending on doping method and special conditions.

L.I.Ivleva1, G.M.Kuzmicheva2, M.E.Doroshenko1

1-Prokhorov General Physics Institute, RAS, 38 Vavilova str., Moscow 119991, Russia 2-MIREA - Russian Technological University, 78 Vernadsky ave., Moscow 119454, Russia.

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