Eu3+-doped (1% and 3%) γ-Ca3(PO4)2 was synthesized by high-pressure and high-temperature experimental method and the samples were characterized by X-ray diffraction. The luminescence properties of samples were investigated by emission and excitation spectra. The excitation spectra of Eu3+-doped γ-Ca3(PO4)2 showed that samples were mainly attributed to Eu3+–O2− charge-transfer band at 270 nm, and some sharp lines were also attributed to Eu3+ f–f transitions in near-UV regions with the strongest peaks at 395 nm. Under the 395 nm excitation, the intense red emission peak at 611 nm was observed. The strongest line (395 nm) in excitation spectra of those phosphors matched well with the output wavelength of UV InGaN-based light-emitting diodes (LEDs) chip. The luminescent properties suggested that Eu3+-doped γ-Ca3(PO4)2 might be regarded as a potential red phosphor candidate for near-UV LEDs.Highlights► The γ-Ca3(PO4)2:Eu3+ was synthesized by high-pressure and high-temperature experimental method. ► Under the 395 nm excitation, the intense red emission peak at 611 nm was observed. ► The Eu3+-doped γ-Ca3(PO4)2 might be regarded as a potential red phosphor candidate for near-UV LEDs.

The ββ-tricalcium phosphate phase, β-Ca3(PO4)2, has been studied at high pressures up to 15.4 GPa and room temperature in a diamond-anvil cell using angle-dispersive X-ray diffraction. No evidence of a phase transformation was observed in the present pressure range at 300 K. By fitting the pressure-volume data to a second-order Birch–Murnaghan equation of state, the bulk modulus, B0B0, was determined to be 79.5 ±± 2.0 GPa.

Highlights•Drop-solution enthalpies and heat capacities of β- and γ-Ca3(PO4)2 were measured.•The formation enthalpy of γ-Ca3(PO4)2 from β-Ca3(PO4)2 was determined.•The phase boundary between β- and γ-Ca3(PO4)2 was calculated thermodynamically.•The calculated boundary is used to discuss the formations of tuite and whitlockite in meteorites.γ-Ca3(PO4)2, naturally known as tuite, is regarded as an important potential reservoir for rare earth elements and large ion lithophile elements. It is a high-pressure polymorph of β-Ca3(PO4)2 whitlockite and a decomposed product of apatite under high-pressure and temperature. Drop-solution enthalpies of β- and γ-Ca3(PO4)2 were obtained as 298.59 ± 3.02 and 278.74 ± 2.98 kJ/mol, respectively, by the drop-solution calorimetry with 2PbO·B2O3 solvent at 978 K. Thus the enthalpy of transition from β- to γ-Ca3(PO4)2 at 298 K (ΔHtr,298o) was 19.85 ± 4.24 kJ/mol. The isobaric heat capacities of β- and γ-Ca3(PO4)2 were measured at temperature range of 300–770 K by differential scanning calorimetry, and compared with the results calculated from the Kieffer model. The equilibrium phase boundary between β- and γ-Ca3(PO4)2 was calculated using present measured data combined with other available thermochemical and thermoelastic data. The calculated boundary gave a phase transition boundary with a dP/dT slope of 4.7 ± 0.2 MPa/K in the temperature range of 900–2000 K. Based on the phase relationship, the occurrences of tuite and whitlockite in meteorites are discussed.

The tried and tested multianvil apparatus has been widely used for high-pressure and high-temperature experimental studies in Earth science. As a result, many important results have been obtained for a better understanding of the components, structure and evolution of the Earth. Due to the strength limitation of materials, the attainable multianvil pressure is generally limited to about 30 GPa (corresponding to about 900 km of the depth in the Earth) when tungsten carbide cubes are adopted as second-stage anvils. Compared with tungsten carbide, the sintered diamond is a much harder material. The sintered diamond cubes were introduced as second-stage anvils in a 6–8 type multianvil apparatus in the 1980s, which largely enhanced the capacity of pressure generation in a large volume press. With the development of material synthesis and processing techniques, a large sintered diamond cube (14 mm) is now available. Recently, maximum attainable pressures reaching higher than 90 GPa (corresponding to about 2700 km of the depth in the Earth) have been generated at room temperature by adopting 14-mm sintered diamond anvils. Using this technique, a few researches have been carried out by the quenched method or combined with synchrotron radiation in situ observation. In this paper we review the properties of sintered diamond and the evolution of pressure generation using sintered diamond anvils. As-yet unsolved problems and perspectives for uses in Earth Science are also discussed.