Whitlockite-like Structure Research Articles

Effect of chromium substitution on structural and optical properties of Ca3(VO4)2 for laser and SRS applications

Crystal structures of Czochralski-grown Ca3(VO4)2 single crystals doped with over-stoichiometric 0.01, 0.05, and 0.1 wt% Cr2O3 (CVO:Cr) are solved and refined. A whitlockite-like structure with five Ca sites, two-capped (Ca1 and Ca3) and mono-capped (Ca2) trigonal prisms, distorted octahedron (Ca4) and tetrahedron (Ca5), is determined for CVO:Cr. Single-crystal synchrotron X-ray diffraction analysis revealed the presence of chromium in Ca2, Ca3, Ca4 sites. In CVO:Cr, chromium has octahedral (Cr3+) and tetrahedral (Cr4+) coordination, which suggests structural superpositions of CaO8, CaO7, CaO6 and Cr3+O6 or Cr4+O4 polyhedra. X-ray diffraction, EPR and spectroscopic studies showed that green or yellow color of CVO:Cr is related to chromium valence state, Cr3+ or Cr4+, respectively. An increase in the Cr4+ concentration is accompanied by increased vacancy content in two of three vanadium sites. Structural behavior of transition metal ions with a variable formal charge (Co, Mn, Cr) in CVO structure is summarized, analyzed and discussed. Three absorption bands around 23900, 16300 and 14000 cm−1, typical for Cr3+ ions, are revealed in the UV-VIS spectra of green samples. An additional broad band with a maximum at 21254 cm−1 refers to Cr4+ ions, the appearance of which after annealing in air was established by X-ray diffraction analysis.

The enhancement of hydroxyapatite thermal stability by Al doping

Al-substituted hydroxyapatite (Al-HA) powders with apatite structure and particle size of 30–70 nm were obtained via precipitation method. The effect of Al content on specific surface area and morphology of powders was studied, and a formation of highly anisotropic phase due to Al doping was observed. The influence of heat treatment in 300–1400 °C range on the phase composition, lattice parameters, Fourier-transform infrared spectroscopy (FTIR) spectra and mass loss of powders was investigated. Introduction of Al in the hydroxyapatite (HA) lattice in the range 0.5–1.0 mol.% resulted in the improvement in thermal stability, which had not been reported previously. Incorporation of 5–10 mol.% of Al resulted in the formation of biphasic materials based on HA and whitlockite-like structure at 900 °C and apatite and α- tricalcium phosphate (α-TCP) phases at 1200–1400 °C. Introduction of 20 mol.% of Al resulted in the formation of isomorphic Al-substituted whitlockite phase Ca9Al(PO4)7. We have estimated the onset of Ca9Al(PO4)7 transformation into α-TCP phase at 1400 °C.

Equation of State and Amorphization of Ca9R(VO4)7 (R = La, Nd, Gd): A Combined High-Pressure X-ray Diffraction and Raman Spectroscopy Study.

Ca9R(VO4)7 (R = rare earth) multicomponent oxides of a whitlockite-related structure are under consideration for applications in optoelectronics. In this work, the Czochralski-grown Ca9R(VO4)7 crystals were investigated as a function of pressure by powder X-ray diffraction and single-crystal Raman spectroscopy. The diffraction experiments were performed at the ALBA synchrotron under pressures ranging up to 9.22(5), 10.7(1), and 8.55(5) GPa for R = La, Nd, and Gd, respectively, to determine the third order equation of state (EOS) parameters. Fitting of the Birch-Murnaghan EOS provided the isothermal bulk moduli K0 = 63(4), 63(2), and 61(5) GPa for these three orthovanadates. These values are apparently lower than that reported for structurally related tricalcium vanadate Ca3(VO4)2. The compressibility anisotropy was observed; the lattice is markedly stiffer in [001] than in [100] direction. For Ca9Nd(VO4)7, the variation of the diffractograms just above 10 GPa provides an indication on the beginning of amorphization process; during pressure release the whitlockite-like structure is recovered. Raman spectroscopy measurements for single crystals of the above-mentioned rare-earth vanadates and Ca9Y(VO4)7 were performed (the maximum pressures achieved were 16.3(1), 21.2(1), 15.3(1), and 18.6(1) GPa for R = Y, La, Nd, and Gd, respectively). These measurements reveal a partially reversible phase transition interpreted as amorphization, with an onset at the pressure of ∼9-10 GPa, characterized by broadening of the peaks and their shift to lower energies.

Novel laser crystals in Ca9Y(VO4)7-x(PO4)x mixed system

The Ca9Y(VO4)7-x(PO4)x (x = 0, 1, 3, 4) solid solution whitlockite-like single crystals have been grown by the Czochralski method from mixtures of single phase Ca9Y(VO4)7 and Ca9Y(PO4)7 starting materials produced by solid-state synthesis. Real compositions of solid solution crystals has been determined and analyzed. It is found that crystals with x = 0, 1, 4 exist with polar structure at room temperature, space group R3c. At least one of them, Ca9Y(VO4)7, demonstrates ferroelectric behaviour with phase transition at 942 °C. Unlike the others, Ca9Y(VO4)3(PO4)4 crystallizes in centrosymmetric whitlockite-like structure with space group R3¯c confirmed by single-crystal X-ray structure analysis. Similar to other whitlockite-family phosphates and vanadates, its crystal structure is described as a disordered structural version of usual whitlockites, corresponding to their high-temperature paraelectric phases. The transmission and luminescence spectra of solid solution crystals are reported, the hardness Hv and fracture toughness K1c of solid solution single crystals are also determined.

A novel pure red phosphor Ca8MgLu(PO4)7:Eu3+ for near ultraviolet white light-emitting diodes

A novel red-emitting phosphor Ca8MgLu(PO4)7:Eu3+ was synthesized by a high-temperature solid-state reaction method. Its crystal structure, photoluminescence emission and excitation spectra, and decay time were investigated in detail. X-ray diffraction (XRD) results indicate that Ca8MgLu(PO4)7 crystallizes in single-phase component with a whitlockite-like structure and the space group R3c of β-Ca3(PO4)2. The emission spectrum shows a dominant peak at 612nm due to the dipole 5D0→7F2 transition of Eu3+, and the luminescence intensity keeps increasing with increasing the content of Eu3+ to 100%. The excitation spectrum is coupled well with the emission of near ultraviolet (NUV) LED (380–410nm). The CIE coordinates of Ca8MgLu(PO4)7:Eu3+ phosphor is (0.654, 0.346), being close to the standard value of National Television Standard Committee (NTSC) for red phosphor, (0.670, 0.330). The internal quantum efficiency of the phosphor is 69% under the excitation of 394nm. The results show that Ca8MgLu(PO4)7:Eu3+ is a very appropriate red-emitting phosphor with a high ratio of red and orange for NUV-based white LEDs.

Structure and properties of Ca9 − x Pb x R(PO4)7 (R = Sc, Cr, Fe, Ga, In) whitlockite-like solid solutions

We have prepared solid solutions based on whitlockite-structure ferroelectrics, Ca9 − x Pb x R(PO4)7 (R = Sc, Cr, Fe, Ga, In), through Pb substitution for Ca. Single-phase Ca9 − x Pb x R(PO4)7 materials with a polar whitlockite-like structure (sp. gr. R3c) exist in the range 0 ≤ x ≤ 1.5 for all of the R metals studied. X-ray powder diffraction profile analysis results for R = In (x = 0.5, 0.8, and 1.0) demonstrate that the trivalent cations reside on the octahedral site M5 of the whitlockite structure, the calcium cation occupy the M1–M3 sites, and the lead cations are located predominantly on the M3 site. Differential scanning calorimetry, dielectric permittivity, and second-harmonic generation data attest to a ferroelectric phase transition, whose temperature is 580–610°C in Ca9R(PO4)7 and decreases monotonically to 480–520°C as x increases to 1.5.

Luminescence and microstructures of Eu 3+-doped in triple phosphate Ca 8MgR(PO 4) 7 (R = La, Gd, Y) with whitlockite structure

Eu 3+-doped triple phosphate Ca 8MgR(PO 4) 7 (R = La, Gd, Y) was synthesized by the general high temperature solid-state reaction. This phosphor was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and emission spectra. XRD and FT-IR analysis indicated that Ca 8MgR(PO 4) 7 (R = La, Gd, Y) crystallized in single-phase component with whitlockite-like structure (space group R3 c) of β-Ca 3(PO 4) 2. Under the excitation of UV light, the phosphors show bright red emission assigned to the transition ( 5D 0 → 7F 2) at 612 nm. The crystallographic sites of Eu 3+ ions in Ca 8MgR(PO 4) 7 (R = La, Gd, Y) host were discussed on the base of site-selective excitation and emission spectra, luminescence decay and its host crystal structure.

Ca 3 (VO 4 ) 2 –LaVO 4 Cation Conductors

Experimental data are presented on the electrical conductivity, ionic transference numbers, and phase transitions of Ca3 – x La2x/3(VO4)2 (0 ≤ x ≤ 1) vanadates, which are shown to be Ca ionic conductors. The highest Ca ion conductivity is found in Ca2La2/3(VO4)2 (palmierite structure) at low temperatures and in β"-Ca3(VO4)2 (whitlockite-like structure) at high temperatures. The mechanisms of Ca2+ transport are discussed.

Chemical and Structural Properties of a Whitlockite‐Like Phosphate, Ca9FeD(PO4)7.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Chemical and Structural Properties of a Whitlockite-like Phosphate, Ca9FeD(PO4)7

A new phosphate, Ca9FeD(PO4)7, with a whitlockite-like structure was synthesized by treating Ca9Fe(PO4)7 with D2 at 820 K. Mossbauer spectroscopy showed that the oxidation state of Fe in Ca9FeD(PO4)7 is +2. Ca9FeD(PO4)7 was paramagnetic in a temperature range from 5 to 300 K with an effective magnetic moment of 4.97 μB per Fe2+ ion. The infrared spectrum of Ca9FeD(PO4)7 exhibited a broad absorption band due to an O−D stretching vibration at 2225 cm-1 in addition to bands assigned to PO4 groups. Ca9FeD(PO4)7 was stable under an Ar atmosphere up to 840 K. Above this temperature, it decomposed to a β-Ca3(PO4)2-like phase, Ca9.333Fe1.167(PO4)7, as well as β-Ca2P2O7 with releasing D2O and keeping the oxidation state of +2 for Fe. Ca9FeD(PO4)7 liberated D2O in air above ca. 720 K to produce Ca9Fe(PO4)7. Second-harmonic-generation signals, I2ω/I2ω(SiO2), in Ca9FeD(PO4)7 (ca. 0.15) were by a factor of ca. 17 less than ca. 2.5 in Ca9Fe(PO4)7. Rietveld analysis of time-of-flight neutron diffraction data for Ca9FeD(...

Preparation, Structure Determination, and Redox Characteristics of New Calcium Copper Phosphates

New calcium copper phosphates o-Ca19Cu2(PO4)14, r′-Ca19Cu2H1.42(PO4)14, and r″-Ca19Cu2−yH2.24(PO4)14 (0.64≤y≤0.7) were synthesized and characterized. The structures of these phosphates were refined using Rietveld analysis. The compound o-Ca19Cu2(PO4)14 has β-Ca3(PO4)2-like structure (Space Group R3c, z=3). Compounds r′-Ca19Cu2H1.42(PO4)14 and r″-Ca19Cu2−yH2.24(PO4)14 (0.64≤y≤0.7) have the whitlockite-like structure (Space Group R3c, z=3). The corresponding crystal data are (i) o-Ca19Cu2(PO4)14: a=10.3633(1) Å, c=37.242(2) Å, V=3463.8(9) Å3, Rp=4.24%, Rwp=5.75%; (ii) r′-Ca19Cu2H1.42(PO4)14: a=10.3987(1) Å, c=37.300(2) Å, V= 3493.0(9) Å3, Rp=4.89%, Rwp=6.50%; (iii) r″-Ca19 Cu2−yH2.24(PO4)14 (0.64≤y≤0.7): a=10.3975(1) Å, c= 37.274(2) Å, V=3489.7(9) Å3, Rp=4.57%, Rwp=6.04%. Redox reactions in calcium/copper double phosphates were investigated by XRD, DTA, DTG, and IR spectroscopy under hydrogen-containing and oxygen-containing atmospheres. These reactions proceed reversibly in the temperature ranges 753–825 K for the reduction process and 973–1173 K for the oxidation process. The redox cycles can be repeated continuously without destroying the crystal lattice.

Synthesis and crystal structure of calcium copper phosphate, s-Ca 19Cu 2(PO 4) 14

A new s-Ca 19Cu 2(PO 4) 14 monophosphate with the whitlockite-like structure was synthesized and its structure was determined by the Rietveld analysis ( R wp = 6.15%, R p = 4.66%, R 1 = 3.44%, R F = 2.08% for 758 reflections). The crystal symmetry is trigonal ( R3 c space group, Z = 3) and the hexagonal unit cell parameters are a = 10.3638(1) A ̊ , c = 37.219(2) A ̊ , and V = 3462(2) A ̊ 3 The copper atom is octahedrally coordinated.

The new phosphates Ca 9[formula omitted](PO 4) 7 ( M = Li, Na; Ln = rare earth, Y, Bi)

The new family of triple phosphates Ca 9MLn 2 3 (PO 4) 7 ( Ln = rare earth, Y, Bi; M = Li +, Na + has been synthesized and studied by X-ray powder diffraction, IR-spectroscopy, differential thermal analyses and luminescence. All compounds have the whitlockite-like structure (space group R3c). Cell parameters, IR and emission (Eu 3+) spectra are given. The luminescence properties of trivalent europium are analyzed in Ca 9NaEu 2 3 (PO 4) 7 and Ca 9LiEu 2 3 (PO 4) 7 at 290 K. The fluorescence lifetime of the 4F 3 2 − 4I 11 2 transition of Nd 3+ is reported for the solid solutions Ca 9Nd xGd 1 − x(PO 4) 7, Ca 3 − xNd 2x 3 (PO 4) 2 and Ca 9M(Nd xGd 1 − x) 2 3 (PO 4) 7 (M = Li, Na).

Triple phosphates of calcium, sodium and trivalent elements with whitlockite-like structure

Triple phosphates of calcium, sodium and trivalent elements (Eu, Nd, Fe) in Ca 3(PO 4) 2- RPO 4-Na 3PO 4 systems were predicted on the basis of crystal chemistry data and synthesized. The obtained phases were studied by XRD, IR-spectroscopy and emission spectroscopy under 90Sr- 90Y and λ = 380 nm excitation. The only phosphates with the whitlockite-like structure appear within the range of Ca 3(PO 4) 2-Ca 9Na 1.5 R 0.5(PO 4) 7-Ca 10Na(PO 4) 7 compositi The emission spectra of phases with europium under 90Sr- 90Y excitation consist of transition lines 5D 0 → 7F j in Eu 3+ and two broad bands of inherent luminescence of the matrix with maximums at ~ 360 and ~ 510 nm.