High Pressure Solid State Chemistry of A 3 (VO 4 ) 2 Compounds (A: Ca, Sr, Ba)
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Abstract The purpose of this study is to explore the potential of high pressure methods for preparation of new series of compounds in the A3 (VO 4 )2 systems (A: Ca, Sr, Ba). In this study, we present our in situ vibrational and X-ray diffraction data on the behavior of the A3 (VO 4 ) 2 compounds at high pressure and room temperature. Upon compression up to 290 kbar, there is no phase change in Ba 3 (VO 4) 2 . Sr 3(VO 4 ) 2 undergoes a first order phase transition to an olivinelike structure at about 150 kbar. In both the ambient pressure and olivine structures of Sr 3(VO 4 ) 2 , oxygen atoms form a hexagonal close packing. The packing in the olivine structure is distorted from this due to loss of 3-fold axis. Ca 3 (VO 4 ) 2 amorphizes at about 100 kbar. The high pressure behavior of the compounds studied here is related to the size of the A( 2) 2+ cations. Small Ca( 2 )2+ cations hinder the completion of crystal-to-crystaltransformations in Ca 3 (VO 4) 2 . Introduction The alkaline earth metal orthovanadates, A3 (VO 4) 2 (Me: Ca, Sr, Ba), have attracted much attention owing to their interesting optical, transport, and ferroelectric properties [1-7]. They exhibit intense rare-earth activated luminescence and can be used as luminophors and host materials for lasers [1]. The intensity of their luminescence can be additionally enhanced through the formation of the Ca 3(VO 4 ) 2-Sr 3(VO 4 ) 2 and Ca 3(VO 4 )2 -Ba 3 (VO 4) 2 solid solution series [2]. Also, intense luminescence can be obtained by tuning the composition in the solid solution series between the isostructural orthovanadates and orthophosphates. For that instance, the intensity of luminescence in the system (Ca,Sr,Y) 3 [(V,P)O 4] 2 :Sm 3รท approaches that of commercially used luminophors [1]. Recently, near-IR laser action of A3(VO 4 ) 2 :Mn 5+ has been reported by Merkle et al. [3] and Buijsse et al. [4]. Ca 3 (VO 4 )2 is a high temperature ferroelectric (Tc = 1383 K) [1] with additional anomalies in pyroelectric and dielectric properties and electronic thermal emission spectra at the 550 - 800 K temperature range. The defect structure of calcium orthovanadate along with the presence of V4 + ions or rare-earth doping accounts for its high electronic conductivity [1]. Further investigations of A3(VO 4 )2 orthovanadates are of interest to possibly develop more efficient optical compounds. The luminescence and transport properties are determined by point defects and impurity ions whose valence state is different from that of the ions in a host lattice (e.g., rare earth elements). Hence, high pressure formation of the defect structures with vacancies and injection of controlled quantities of impurities should be a convenient tool for studying luminescence in these materials. A systematic high pressure study of structures and properties of different rare earth containing compounds, like orthovanadates and orthophoshates, is useful to assess the extent to which the optical parameters of rare-earth ions can be varied and possibly enhanced in various hosts. In this
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