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DFT calculations with six LDA, GGA, and hybrid functionals have been performed using the CRYS-TAL09 code to describe the crystal structure and vibrational spectra of Y 2 CaGe 4 O 12 cyclotetragermanate, a new optical host. Two space groups P4/nbm and Cmme have been considered. The former corresponds to a mixed (0.5 Ca + 0.5 Y) distribution at the octahedral sites found from the results of Rietveld refinement of room temperature powder XRD pattern; the latter refers to the model of crystallographically nonequivalent calcium and yttrium atomic setting in distorted oxygen octahedrons. The most accurate geometry description has been obtained with the WC1LYP and PBE (n = 6) hybrid functionals, while the B3LYP calculation provides the best agreement between the recorded infrared and Raman spectra and their computed counterparts. Assignments of most of the observed bands to vibrational modes are given. The comparison between calculated and experimental frequencies shows a general good agreement for the spectra below 600 cm −1 . The relationship between selected infrared bands and Raman lines, internal vibrations of the [Ge 4 O 12 ] unit, and external modes is briefly discussed.

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The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescence properties. Ag x Eu3+ (2–x)/3□(1–2x)/3WO4 and Ag x Gd(2−x)/3−0.3Eu3+ 0.3□(1−2x)/3WO4 (x = 0.5–0) scheelite-type phases were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder synchrotron X-ray diffraction. Transmission electron microscopy also revealed the (3 + 1)­D incommensurately modulated character of Ag x Eu3+ (2–x)/3□(1–2x)/3WO4 (x = 0.286, 0.2) phases. The crystal structures of the scheelite-based Ag x Eu3+ (2–x)/3□(1–2x)/3WO4 (x = 0.5, 0.286, 0.2) red phosphors have been refined from high resolution synchrotron powder X-ray diffraction data. The luminescence properties of all phases under near-ultraviolet (n-UV) light have been investigated. The excitation spectra of Ag x Eu3+ (2–x)/3□(1–2x)/3WO4 (x = 0.5, 0.286, 0.2) phosphors show the strongest absorption at 395 nm, which matches well with the commercially available n-UV-emitting GaN-based LED chip. The excitation spectra of the Eu2/3□1/3WO4 and Gd0.367Eu0.30□1/3WO4 phases exhibit the highest contribution of the charge transfer band at 250 nm and thus the most efficient energy transfer mechanism between the host and the luminescent ion as compared to direct excitation. The emission spectra of all samples indicate an intense red emission due to the 5D0 → 7F2 transition of Eu3+. Concentration dependence of the 5D0 → 7F2 emission for Ag x Eu(2–x)/3□(1–2x)/3WO4 samples differs from the same dependence for the earlier studied Na x Eu3+ (2–x)/3□(1–2x)/3MoO4 (0 ≤ x ≤ 0.5) phases. The intensity of the 5D0 → 7F2 emission is reduced almost 7 times with decreasing x from 0.5 to 0, but it practically does not change in the range from x = 0.286 to x = 0.200. The emission spectra of Gd-containing samples show a completely different trend as compared to only Eu-containing samples. The Eu3+ emission under excitation of Eu3+(5L6) level (λex = 395 nm) increases more than 2.5 times with the increasing Gd3+ concentration from 0.2 (x = 0.5) to 0.3 (x = 0.2) in the Ag x Gd(2−x)/3−0.3Eu3+ 0.3□(1−2x)/3WO4, after which it remains almost constant for higher Gd3+ concentrations.

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