Antiferroelectric properties and site occupations of R 3+ cations in Ca 8 Mg R (PO 4 ) 7 luminescent host materials

Abstract

Ca 8 MgR(PO 4 ) 7 (R = La, Pr, Nd, Sm−Lu, and Y) phosphates with a β-Ca 3 (PO 4 ) 2 related structure were prepared by a standard solid-state method in air. Second-harmonic generation, differential scanning calorimetry, and dielectric measurements led to the conclusion that all Ca 8 MgR(PO 4 ) 7 are centrosymmetric and go to another centrosymmetric phase in the course of a first-order antiferroelectric phase transition well above room temperature (RT). High-temperature electron diffraction showed that the symmetry changes from R3 ത c to R3 ത m during the phase transition.Structures of Ca 8 MgR(PO 4 ) 7 at RT were refined by the Rietveld method in centrosymmetric space group R3 ത c. Mg 2+ cations occupy the M5 site; the occupancy of the M1 site by R 3+ cations increases monotonically from 0.0389 for R = La to 0.1667 for R = Er-Lu, whereas the occupancy of the M3 site by R 3+ cations decreases monotonically from 0.1278 for R = La to 0 for R = Er-Lu.In the case of R = Er-Lu, the M3 site is occupied only by Ca 2+ cations. P1O 4 tetrahedra and cations at the M3 site are disordered in the R3 ത c structure of Ca 8 MgR(PO 4 ) 7 . Using synchrotron X-ray powder diffraction, we found that annealing conditions do not significantly affect the distribution of Ca 2+ and Eu 3+ cations between the structure positions of Ca 8 MgEu(PO 4 ) 7 .Luminescent properties of Ca 8 MgEu(PO 4 ) 7 powder samples were investigated under nearultraviolet (n-UV) light. Excitation spectra of Ca 8 MgEu(PO 4 ) 7 show the strongest absorption at about 395 nm that matches with commercially available n-UV-emitting GaN-based LED chips.Emission spectra show an intense red emission due to the 5 D 0 → 7 F 2 transition of Eu 3+ .

A novel dazzling Eu3+‐doped whitlockite‐type phosphate red‐emitting phosphor for white light‐emitting diodes

“…The XRD patterns of Ca 8 ZnLa 1− x Eu x (PO 4 ) 7 ( x = 0, 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, 1) samples are shown in Figure , with the pattern of Ca 9 Fe(PO 4 ) 7 (JCPDS card: 45‐0338) shown as a reference. It can be seen that all the diffraction peaks are in good agreement with JCPDS card 45‐0338 (β‐Ca 9 Fe(PO 4 ) 7 ) and can be assigned to a whitlockite‐type structure for β‐Ca 3 (PO 4 ) 2 exhibiting a R3c (161) space group . As the concentration of Eu 3+ is increased, the dominant characteristic diffraction peak (0210) was shifted slightly towards larger angles, indicating a smaller Eu 3+ radius (0.947 Å, CN = 6) when compared to that of La 3+ (1.032 Å, CN = 6) in the same coordination environment.…”

Influence of magnesium on dielectric properties of Ca9–xMgxBi(VO4)7 ceramics

“…Calcium phosphates and vanadates of Ca 9 R (EO 4 ) 7 ( R = rare‐earth (RE), Bi, Y; E = P, V) belong to mineral whitlockite family . Most members of this family demonstrate interesting luminescent, optical nonlinear, ionic‐conductive, ferro‐ or antiferroelectric properties . In particular, the relevant phosphates are discussed as possible materials for light‐emitting diodes and also as luminescent bio‐probes in medicine .…”

“…3 Most members of this family demonstrate interesting luminescent, 4-10 optical nonlinear, [11][12][13] ionic-conductive, [14][15][16][17] ferro- 18,19 or antiferroelectric properties. 20,21 In particular, the relevant phosphates are discussed as possible materials for light-emitting diodes [22][23][24][25][26][27] and also as luminescent bio-probes in medicine. 28 It has been proved that Czochralski grown single crystals of Ca 9 R(VO 4 ) 7 (R 3+ = Bi, La-Yb, Y) 5,6,13,[29][30][31][32][33][34][35][36][37][38] can be used for various laser optics elements.…”

“…[45][46][47][48][49] Beside affecting SHG the guest cations allow to control ferroelectric or antiferroelectric phase transition temperatures. 18,20,21,[46][47][48][49] The size and chemical characteristics of the guest cations are also important for the formation of properties in Ca 9 R(EO 4 ) 7 (R = RE, Bi, Y; E = P, V). For example, magnesium for 1 calcium substitution in ferroelectric Ca 9 R(PO 4 ) 7 18 leads to Ca 8 MgR(PO 4 ) 7 possessing center of symmetry and electric polarization of antiferroelectric type.…”

New Dy3+ Containing Whitlockite for LED Devises

“…Ca 8 MgR(PO 4 ) 7 compounds have the structure-forming RE cations, and they can be additionally doped by a large variety of luminescent cations. For this reason, Ca 8 MgR(PO 4 ) 7 (R = RE and Y) compounds have been intensively studied recently as potential luminescence materials [16].…”