Why Do Relative Intensities of Charge Transfer and Intra-4f Transitions of Eu³⁺ Ion Invert in Yttrium Germanate Hosts? Unravelling the Underlying Intricacies from Experimental and Theoretical Investigations

Abstract

The dominant intensity of parity-forbidden intra-4f transitions of europium(III) over O → Eu charge-transfer band (CTB) intensity is against common perceptions, yet this trend is observed in many germanate hosts and has not been rationalized so far. In search of a plausible explanation for this unusual trend, present work reports an experimental and theoretical investigations in conjunction on two sibling germanate host, namely, Y 2 GeO 5 and Y 2 Ge 2 O 7 having dopant Eu 3+ in their respective YO 7 polyhedra. Whereas for Y 2 GeO 5 :Eu 3+ , the CTB is more intense than the intra-4f transitions in the excitation spectrum, in the case of Y 2 Ge 2 O 7 :Eu 3+ , the relative intensities of CTB and intra-4f transitions are reversed. Comparative structural analysis reveals that Eu 3+ present in YO 7 of Y 2 GeO 5 has a greater number of tetra-coordinated oxygen (O tetra ) and yttrium atom as first and second neighbors, respectively (Eu 3+ −O tetra −Y 3+ linkages). Conversely, in Y 2 Ge 2 O 7 host, the Eu 3+ ion mostly has tricoordinated oxygen (O tri ) as its nearest neighbor and germanium ions next to O tri (Eu 3+ −O tri −Ge 4+ linkage). Theoretical calculations reveal that while Y 2 GeO 5 :Eu has O tetra (4Y) dominating at the Fermi level and the 4f state of Eu 3+ remains inert toward mixing, in Y 2 Ge 2 O 7 :Eu, the Fermi level has major contribution from O tri (2Y + 1Ge) with significant mixing with 4f states of Eu. The dominant control of Eu 3+ −O tri −Ge 4+ linkages in geometrical and electronic structure of Y 2 Ge 2 O 7 :Eu owing to the GeO 4 surrounding has been attributed to relative poor intensity of O → Eu CTB. Siege of Eu 3+ by GeO 4 and subsequent occurrence of Eu 3+ −O tri −Ge 4+ linkages play a dual role: First, it induces electronic rigidity to hinder excitation of electron at bridging (O tri ) oxygen by highly charged small Ge 4+ cation; second, the covalent character in Eu−O bond is achieved by intermixing of Eu's 4f and O tri 2p orbital which facilitates relaxing of the parity-selection rule thus enhancing the probability of intra-4f transitions. The inferences drawn remain valid when extrapolated to other inorganic oxides having EuO x polyhedra surrounded by covalent units like PO 4 , SiO 4 , etc. and have a prevailing number of low-coordinated oxygen atoms and highly charged small cation in the first and second coordination shells, respectively. The optical basicity concept is also found to endorse our explanation. These remarkable generic inferences will pave the rational way for designing efficient phosphors for solidstate lighting.

Strong and pure red-emitting Eu3+-doped phosphor with excellent thermal stability for warm WLEDs

“…These bands result from the charge transfer band, 7 F 0 → 5 H 6 , 7 F 0 → 5 H 3 , 7 F 0 → 5 D 4 , 7 F 0 → 5 G 2 , 7 F 0 → 5 L 6 , 7 F 0 → 5 D 3 , 7 F 0 → 5 D 2 , 7 F 0 → 5 D 1 , and 7 F 1 → 5 D 1 transitions of the Eu 3+ ions. 26 Under 395 nm light irradiation, the emission spectrum displays some distinct transition bands belonging to the 5 D 0 → 7 F J (J=0, 1, 2, 3, 4) transitions of the Eu 3+ ions with centers of 582, 595, 613, 658, and 709 nm. 27 It is important to note that the excitation peak at 395 nm has the maximum absorption and produces the strongest luminescence comparison with the other excitations, as seen in Fig.…”

Revealing Eu3+-doped yttrium pyrogermanate as a soft UV excitable phosphor: retaining the pros of the commercial phosphor and compensating for the cons

“…The influence of Ge 4+ ions in Y 2 Ge 2 O 7 (YGO) resulting in suppression of CTB is surprising with respect to intra-4f transitions. 59 The corresponding Diffuse Reflectance spectrum (DRS) is shown in Fig. S3 of the supporting information.…”

“…Germanates in recent times have become a potential host for rare earth ions due to their particular advantages of single-phase synthesis, high thermal stability, and low phonon energy. [52][53][54][55][56][57][58][59] Most importantly, many germanate hosts with Eu-dopant show dominant excitation efficiency for intra-4f transition suitable for n-UV-LEDs. 59 Motivated by these facts, in the present work we have carried out a systematic investigation on the yttrium pyrogermanate, i.e.…”

“…[52][53][54][55][56][57][58][59] Most importantly, many germanate hosts with Eu-dopant show dominant excitation efficiency for intra-4f transition suitable for n-UV-LEDs. 59 Motivated by these facts, in the present work we have carried out a systematic investigation on the yttrium pyrogermanate, i.e. Y 2 Ge 2 O 7 :Eu 3+ , phosphor with different Eu 3+ doping concentrations.…”

The composition engineering of red‐emitting Eu3+‐doped Ca10.5(PO4)7‐type solid solution phosphors and application in LED

“…The excited-state electrons decay to the lowest excited-state ( 5 D 0 ) via nonradiative transition, and subsequently radiative transition of 5 D 0 -7 F J (J = 0-4) exists with characteristic emissions at 578, 590, 612, 652, and 698 nm. [37][38][39] As the x and y value change, the luminescence intensity line diagram of Eu 3+ -doped CNYP-I (0 ≤ x ≤ 1/2) and CNYP-II (0 ≤ y ≤ 1) phosphors is indicated in Figure 5D. When the values of x and y increases, the intensity of the phosphors at 612 nm shows a decreasing trend; but at y = 1, the PL intensity is obviously higher than CNYP-II (y = 0, 1/3, and 2/3) increases.…”