Structural, vibrational, electronic, and luminescence properties of the cyclotetravanadatesA2M(VO3)4(A=Na,Ag;M=Ca,Sr)

The physical properties of the family of cyclotetravanadates ${A}_{2}M{({\text{VO}}_{3})}_{4}$, where $A=\text{Na},\text{Ag}$ and $M=\text{Ca},\text{Sr}$, have been studied by means of x-ray powder diffraction, neutron diffraction, electron diffraction, infrared, Raman, NMR, photoexcitation and pulse cathode beam excitation, and x-ray photoelectron spectroscopies, and band structure calculations. The differences between the structural, vibrational, luminescence, and electronic properties of the alkali metal-containing [${\text{Na}}_{2}\text{Ca}{({\text{VO}}_{3})}_{4}$ and ${\text{Na}}_{2}\text{Sr}{({\text{VO}}_{3})}_{4}$] and the $d$ metal-containing cyclotetravanadates [${\text{Ag}}_{2}\text{Ca}{({\text{VO}}_{3})}_{4}$ and ${\text{Ag}}_{2}\text{Sr}{({\text{VO}}_{3})}_{4}$] are analyzed. ${\text{Na}}_{2}\text{Ca}{({\text{VO}}_{3})}_{4}$, ${\text{Ag}}_{2}\text{Ca}{({\text{VO}}_{3})}_{4}$, ${\text{Na}}_{2}\text{Sr}{({\text{VO}}_{3})}_{4}$, and ${\text{Ag}}_{2}\text{Sr}{({\text{VO}}_{3})}_{4}$ have tetragonal structures, $P4/nbm$, with $a=10.438\text{ }49(6)$, $10.445\text{ }24(5)$, $10.634\text{ }49(4)$, and $10.625\text{ }74(6)$, and $c=4.938\text{ }73(5)$, $4.968\text{ }45(5)$, $4.962\text{ }05(4)$, and $4.979\text{ }30(4)\text{ }\text{\AA{}}$, respectively. The main structural feature of ${A}_{2}M{({\text{VO}}_{3})}_{4}$ is the tetracyclic $[{\text{V}}_{4}{\text{O}}_{12}]$ units. The hybridized $\text{O}\text{ }2p--\text{V}\text{ }3d$ states of the tetracyclic $[{\text{V}}_{4}{\text{O}}_{12}]$ units have a dominant influence on the electronic structure of these compounds. The compounds are semiconducting with a local density approximation band gap increasing from 1.85 eV for ${\text{Ag}}_{2}\text{Ca}{({\text{VO}}_{3})}_{4}$ to 3.02 eV for ${\text{Na}}_{2}\text{Ca}{({\text{VO}}_{3})}_{4}$. The prospects of these compounds as advanced materials for detectors of photon and corpuscular radiation as well as for color correction of light emission sources such as lamp and light emitting diode sources are discussed.