High-pressure and high-temperature studies on oxide garnets

Optical pressure and temperature sensing properties of Nd3+:YTaO4

The pressure- and temperature-dependent luminescence properties of M'-phase Nd³⁺:YTaO₄ synthesized by a molten salt method are presented. Ten near-infrared emission lines originating from the transitions between the two Stark levels R_1,2 of the ³F_3/2 state and the five Stark levels Z_1,2,3,4,5 of the ⁴I_9/2 state for the doped Nd³⁺ ions can be clearly identified. All these emission lines are found to shift linearly with pressure in a range up to ∼11 GPa. The R_2,1 → Z₅ emission lines have larger pressure sensitivities, which are 16.44 and 14.27 cm^-1 GPa^-1. The intensities of all the emission lines evolve with pressure non-monotonically, and peak at ∼1 GPa. The R₁ → Z_4,5 and R₂ → Z₁ emission lines can be obviously narrowed under the hydrostatic pressure, and broadened under the non-hydrostatic pressure, indicating their potential capability for reflecting the characteristic of a pressure environment. The intensity ratio of the R_2,1 → Z₅ emission lines exhibits a large temperature dependence, with a relative sensitivity between 0.129% and 0.108% K^-1 in the physiological temperature range of 290-320 K. Thermal variations of the spectral positions and widths of the R_2,1 → Z₅ emission lines are also investigated. A high thermal stability for the position of the R₂ → Z₅ emission line is revealed. Based on the experimental results, the advantages and potential of Nd³⁺:YTaO₄ as a multi-functional sensor for pressure and temperature are discussed.

Efficient Nd3+ sensitized Yb3+ emission and infrared-to-visible energy conversion in gallium nano-garnets

We studied the structural and luminescence properties of nanocrystalline RE₃Ga₅O₁₂ (RE = Gd, Y and Lu) garnets co-doped with 1 mol% of Nd³⁺ and 10 mol% of Yb³⁺ ions. The Nd³⁺ sensitized Yb³⁺ emission at 1025 nm is observed due to efficient Nd³⁺ to Yb³⁺ energy transfer.

High-pressure phase transition in Y3Fe5O12

Yttrium iron garnet (YIG, Y3Fe5O12) was examined up to 74 GPa and 1800 K using synchrotron x-ray diffraction in a diamond anvil cell. At room temperature, YIG remained in the garnet phase until abrupt amorphization occurred at 51 GPa, consistent with earlier studies. Upon laser heating up to 1800 K, the material transformed to a single-phase orthorhombic GdFeO3-type perovskite of composition (Y(0.75)Fe(0.25))FeO3. No evidence of decomposition of the sample was observed. Both the room-temperature amorphization and high-temperature transformation to the perovskite structure are consistent with the behaviour of other rare earth oxide garnets. The perovskite sample was compressed between 28-74 GPa with annealing to 1450-1650 K every 3-5 GPa. Between 46 and 50 GPa, a 6.8% volume discontinuity was observed without any accompanying change in the number or intensity of diffraction peaks. This is indicative of a high-spin to low-spin electronic transition in Fe(3+), likely in the octahedrally coordinated B-site of the perovskite. The volume change of the inferred spin transition is consistent with those observed in other rare earth ferric iron perovskites at high pressures.

Structural, elastic and vibrational properties of nanocrystalline lutetium gallium garnet under high pressure

An ab initio study of the structural, elastic and vibrational properties of the lutetium gallium garnet (Lu₃Ga₅O₁₂) under pressure has been performed in the framework of the density functional theory, up to 95 GPa.

Garnet-to-perovskite transition in Gd3Sc2Ga3O12 at high pressure and high temperature

The structural phase transition of gadolinium-scandium-gallium garnet (Gd(3)Sc(2)Ga(3)O(12), GSGG) has been studied at high pressure and high temperature using the synchrotron X-ray diffraction technique in a laser-heated diamond anvil cell. The GSGG garnet transformed to an orthorhombic perovskite structure at approximately 24 GPa after laser heating to 1500-2000 K. The garnet-to-perovskite phase transition is associated with an ∼8% volume reduction and an increase in the coordination number of the Ga(3+) or Sc(3+) ion. The orthorhombic perovskite GSGG has bulk modulus B(0) = 194(15) GPa with B(0)' = 5.3(8), exhibiting slightly less compression than the cubic garnet structure of GSGG with B(0) = 157(15) GPa and B(0)' = 6.5(10). Upon compression at room temperature, the cubic GSGG garnet became amorphous at ∼65 GPa. Coupled with the amorphous-to-perovskite phase transition in Y(3)Fe(5)O(12) and Gd(3)Ga(5)O(12) at high-pressure-temperature conditions, we conclude that amorphization should represent a new thermodynamic state resulting from hindrance of the garnet-to-perovskite phase transition, whereas the garnet-to-amorphous transition in rare-earth garnets should be kinetically hindered at room temperature.

Optical pressure and temperature sensor based on the luminescence properties of Nd3+ ion in a gadolinium scandium gallium garnet crystal

Hypersensitivity to pressure and temperature is observed in the near-infrared emission lines of the Nd(3+) ion in a Cr(3+),Nd(3+):Gd(3)Sc(2)Ga(3)O(12) crystal, associated to the R(1,2)((4)F(3/2))→Z(5)((4)I(9/2)) and R(1,2)((4)F(3/2))→Z(1)((4)I(9/2)) transitions. The former emissions show large linear pressure coefficients of -11.3 cm(-1)/GPa and -8.8 cm(-1)/GPa, while the latter show high thermal sensitivity in the low temperature range. Thus this garnet crystal can be considered a potential optical pressure and/or temperature sensor in high pressure and temperature experiments up to 12 GPa and below room temperature, used in diamond anvil cells and excited with different UV and visible commercial laser due to the multiple Cr(3+) and Nd(3+) absorption bands.

Delocalization of Electrons in Strong Insulators at High Dynamic Pressures

Systematics of material responses to shock flows at high dynamic pressures are discussed. Dissipation in shock flows drives structural and electronic transitions or crossovers, such as used to synthesize metallic liquid hydrogen and most probably Al₂O₃ metallic glass. The term “metal” here means electrical conduction in a degenerate system, which occurs by band overlap in degenerate condensed matter, rather than by thermal ionization in a non-degenerate plasma. Since H₂ and probably disordered Al₂O₃ become poor metals with minimum metallic conductivity (MMC) virtually all insulators with intermediate strengths do so as well under dynamic compression. That is, the magnitude of strength determines the split between thermal energy and disorder, which determines material response. These crossovers occur via a transition from insulators with electrons localized in chemical bonds to poor metals with electron energy bands. For example, radial extents of outermost electrons of Al and O atoms are 7 a₀ and 4 a₀, respectively, much greater than 1.7 a₀ needed for onset of hybridization at 300 GPa. All such insulators are Mott insulators, provided the term “correlated electrons” includes chemical bonds.

Lattice dynamics and Born instability in yttrium aluminum garnet, Y3Al5O12

We report lattice dynamics calculations of various microscopic and macroscopic vibrational and thermodynamic properties of yttrium aluminum garnet (YAG), Y3Al5O12, as a function of pressure up to 100 GPa and temperature up to 1500 K. YAG is an important solid-state laser material with several technological applications. Garnet has a complex structure with several interconnected dodecahedra, octahedra and tetrahedra. Unlike other aluminosilicate garnets, there are no distinct features to distinguish between intramolecular and intermolecular vibrations of the crystal. At ambient pressure, low energy phonons involving mainly the vibrations of yttrium atoms play a primary role in the manifestations of elastic and thermodynamic behavior. The aluminum atoms in tetrahedral and octahedral coordination are found to be dynamically distinct. Garnet's stability can be discerned from the response of its phonon frequencies to increasing pressure. The dynamics of both octahedral and tetrahedral aluminum atoms undergo radical changes under compression which have an important bearing on their high pressure and temperature properties. At 100 GPa, YAG develops a large phonon bandgap (90-110 meV) and its microscopic and macroscopic physical properties are found to be profoundly different from that at the ambient pressure phase. There are significant changes in the high pressure thermal expansion and specific heat. The mode Grüneisen parameters show significant changes in the low energy range with pressure. Our studies show that the YAG structure becomes mechanically unstable around P = 108 GPa due to the violation of the Born stability criteria. Although this does not rule out thermodynamic crossover to a lower free energy phase at lower pressure, this places an upper bound of P = 110 GPa for the mechanical stability of YAG.

Transition to a virtually incompressible oxide phase at a shock pressure of 120 GPa (1.2 Mbar): Gd3Ga5O12

Cubic, single-crystal, transparent Gd(3)Ga(5)O(12) has a density of 7.10 g/cm(3), a Hugoniot elastic limit of 30 GPa, and undergoes a continuous phase transition from 65 GPa to a quasi-incompressible (QI) phase at 120 GPa. Only diamond has a larger Hugoniot elastic limit. The QI phase of is more incompressible than diamond from 170 to 260 GPa. Electrical conductivity measurements indicate the QI phase has a band gap of 3.1 eV. Gd(3)Ga(5)O(12) can be used to obtain substantially higher pressures and lower temperatures in metallic fluid hydrogen than was achieved previously by shock reverberation between Al(2)O(3) disks.