Photoluminescence properties of Sm-doped BaBr2 under hydrostatic pressure

Photoluminescence spectra of Sm²⁺-doped BaBr₂ have been measured under hydrostatic pressures up to 17 GPa at room temperature. In the low pressure range a red-shift of the broad 5d-4f transition of −145 cm⁻¹/GPa is observed. From 5 to 8 GPa a phase mixture of the initial orthorhombic phase and the high-pressure monoclinic phase gives rise to two 5d-4f bands, which are strongly overlapping. Above 8 GPa the crystal is completely transformed to its high-pressure phase where two different Sm²⁺ sites exist, but only one broad 5d-4f transition is detected. It exhibits a red-shift of −36 cm⁻¹/GPa. In addition, the line shifts of the ⁵D₀→⁷F_J (J=0, 1, 2) transitions are investigated. Linear shifts of −19 cm⁻¹/GPa for J=0, 2 and of −13 cm⁻¹/GPa for J=1 are observed in the pressure range from 0 to 5 GPa.     

A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

The Mössbauer effect of SnI4 at high pressures

Tetravalent tin iodide is a molecular crystal composed of SnI₄ tetrahedra loosely packed into a cubic configuration. Under pressure SnI₄ becomes metallic at about 15 GPa. We report a Mossbauer effect study of¹¹⁹Sn and¹²⁹I in SnI₄ to pressures of 26 GPa. The spectra exhibit dramatic changes with pressure starting at about 10 GPa and show large pressure hysteresis effects upon reducing the pressure from 26 GPa. In the intermediate region tin exists in both Sn⁴⁺ and Sn²⁺ states, and iodine exists in two nonequivalent sites characterized by a different symmetry and different sign and magnitude of the electric field gradient.Supported by US DOE, BES-Materials ScienceVisiting Scientist, Tel-Aviv University, 69978 Ramat Aviv, IsraelNow at Teledyne Brown Engineering, Huntsville AL 35807 USA     

High-pressure crystal chemistry of scheelite-type tungstates and molybdates

Unit-cell parameters and crystal structures of CaWO₄(scheelite) and CaMoO₄ (powellite) have been determined at several pressures to 5 8 GPa, and unit-cell parameters of PbMoO₄ (wulfemte), PbWO₄ (stolzite) and CdMoO₄ have been measured at pressures to 6 0 GPa All five tetragonal scheelite-type compounds compress anisotropically, with the (itc) axis 1 2 to 1 9 times more compressible than a. In both CaWO₄ and CaMoO₄ the cation tetrahedra (with W⁶⁺ or Mo⁶⁺) behave as rigid structural elements with no observed cation-oxygen compression (W-O and Mo-O bond compression < 0001 GPa^?1) Compression of the eight-coordinated calcium polyhedron, on the other hand, is comparable to bulk compression of the compounds (Ca-O bond compression = 0005 ± 0 001 GPa^?1) Amsotropies in the pressure response of the calcium polyhedron, which is more compressible parallel to c than perpendicular to c, result in the amsotropic unit-cell compression Bulk moduli of the five compounds (with K' assumed to be 4) are CaWO₄ (68 ± 9 GPa), CaMoO₄ (81 5 ± 0 7 GPa), PbWO₄ (64 ± 2 GPa), PbMoO₄ (64 ± 2 GPa), and CdMoO₄ (104 ± 2 GPa) No reversible transitions to the monoclinic (fergusomte) distortion of scheelite were observed in these compounds Pressure-volume data for PbWO₄, however, display strong positive curvature (K'_calc) = 23 ± 2) up to about 5 GPa, at which pressure crystals appear to undergo a first-order phase transition The relatively large curvature may be a premonitory effect pnor to a reconstructive transition Structural changes in these compounds with increasing pressure are qualitatively similar to changes that result from isobanc cooling or substitution of a smaller cation in the eight-coordinated site.     

Study of phase transition on nanocrystalline (La, Sr)(Mn, Fe)O system using high-pressure Mössbauer spectroscopy and high-pressure X-ray diffraction

Pressure-induced structural changes on nano-crystalline La_0.8Sr_0.2Mn_0.8Fe_0.2O₃ were studied using high-pressure Mössbauer spectroscopy and high-pressure X-ray diffraction. Mössbauer measurements up to 10 GPa showed first order transition at 0.52 GPa indicating transformation of Fe^4?+? to high spin Fe^3?+?, followed by another subtle transition at 3.7 GPa due to the convergence of two different configurations of Fe into one. High-pressure X-ray diffraction measurements carried up to 4.3 GPa showed similar results at 0.6 GPa as well as 3.6 GPa. Attempts were made to explain the changes at 0.6 GPa by reorientation of grain/grain boundaries due to uniaxial stress generated on the application of pressure. Similarly variation at 3.6 GPa can be explained by orthorhombic to monoclinic transition.     

Investigations of meta-stable and post-spinel silicon nitrides

Since the discovery of post-spinel Si₃N₄, its fundamental physical properties are highly required. In this paper, theoretical calculations are performed to investigate the structural and elastic properties of the β-, γ-, wII- and post-spinel Si₃N₄ polymorphs. The calculated ground-state properties compare well with available experiments. The phase transformations of the β-, γ-, wII- and post-spinel phases are investigated by the famous plane-wave pseudo-potential density functional theory. From the elastic constants obtained, we find that β-, γ- and wII-Si₃N₄ are stable at 0 GPa and the post-spinel phase is unstable/stable at 0 GPa/160 GPa. When the high-temperature β→γ transformation is bypassed due to kinetic reasons, β-Si₃N₄ is predicted to undergo a first-order phase transition to a new phase (wII-Si₃N₄). It is found that the transition pressures of β→wII and γ→post-spinel transitions are 20.8 GPa and 152.5 GPa, respectively. The phase boundary of the γ→post-spinel transition can be described as P=152.3631−6.39×10⁻³T+2.01062×10⁻⁵T²−1.93962×10⁻⁹T³. Through the quasi-harmonic approximation, the dependences of heat capacity, entropy, thermal expansion coefficient and the Debye temperature on temperature, are also successfully predicted.     

Luminescence of BaBr2:Eu under hydrostatic pressure

Luminescence spectra of BaBr₂:Eu²⁺ have been measured under pressures up to 27 GPa at room temperature. In the low-pressure range a red-shift of the 5d-4f transition of −225 cm⁻¹/GPa is observed. From 3 to 10 GPa a phase mixture of the original orthorhombic phase and the high-pressure monoclinic phase gives rise to two luminescence bands. Above 10 GPa the crystal is completely transformed to its high-pressure phase where two different Eu²⁺ sites exist of which, however, only one 5d-4f transition is detected. This transition exhibits a red-shift of −200 cm⁻¹/GPa. The shifts are compared with other literature data.     

GdOBr∶Eu晶场参数与自由离子参数的高压研究

在GdoBr∶Eu的常压和高压荧光谱实验数据基础上，对晶场能级及其重心进行了拟合计算，得到了晶场参数和自由离子参数（Slater参数和自旋 轨道耦合参数）在高压下的变化情况．计算结果表明，随压力增大，晶场参数B⁴₀，B⁶₀增大，B⁶₄减小，B²₀，B⁴₄的变化有些起伏．晶场强度在8GPa以 关键词：     

57Fe Mössbauer study of SrFeO3 under ultra-high pressure

⁵⁷Fe Mössbauer absorption spectra under ultra-high pressure up to 53 GPa have been measured using a diamond anvil cell for SrFeO_2.97 which is one of the typical Fe⁴⁺ oxides having a cubic perovskite structure. External high pressure up to 53 GPa makes no indication of structural transformation and does not show any change in valence state of iron, however the Néel temperature of 131 K at 0 GP increases to 300 K and the⁵⁷Fe magnetic hyperfine field decreases from 32.9 T at 0 GPa and 6.5 K to 23.3 T at 53 GPa and 300 K.     

SrB4O7:Sm2+: an optical sensor reflecting non-hydrostatic pressure at high-temperature and/or high pressure in a diamond anvil cell

A systematic investigation on the fluorescent spectra of SrB₄O₇:Sm²⁺ was performed in detail at high-temperature up to 623?K and/or high pressure up to 23.2?GPa with different pressure-transmitting media (PTMs), respectively. Combined with experiment data of previous research, the change of the ⁷D₀–⁵F₀ line (0–0 line) full width at half maximum (FWHM) of SrB₄O₇:Sm²⁺ under different pressure environments was specifically discussed. The results indicate that the FWHM of 0–0 line is sensitive to the non-hydrostatic pressure environment in 2-propanol, and methanol and ethanol mixture (ME) PTMs at ambient temperature. The first-order and the second-order derivation of the temperature dependence of 0–0 line FWHM at ambient pressure are 1.48(±0.21)?×?10^?4?nm/K and 9.63(±0.63)?×?10^?7?nm²/K² below 623?K. The 0–0 line FWHM is also sensitive to the non-hydrostatic pressure environment in ME at high-temperature and high pressure simultaneous, the non-hydrostatic transition pressures are 9.6?GPa at 323?K, 11.0?GPa at 373?K, 14.4?GPa at 423?K, respectively. SrB₄O₇:Sm²⁺ is recommended as an optical sensor to reflect the change of pressure environment in liquid media at high-temperature and/or high pressure.     

Pressure and temperature induced phase transitions in LiInSe2

Abstract

At 4.1 GPa LiInSe₂ transforms from the β-NaFeO₂ - type structure to the NaCl-type structure LiInSe₂-hpI (cubic; Fm3m; a=546.4(3)pm, Z=2, D _x =5.75g/cm³; 4.1GPa) which remains metastable at normal conditions. Heating to 210°C at 1.8 GPa causes ordering of the cations and a phase transition from LiInSe₂-hpI to the α-NaFeO₂ - type structure LiInSe₂-hpII (rhombohedral; R3m; a=393.4pm, c=1919.7pm, Z=3, D _x =5.53g/cm³; 1.8GPa). Heating to 210°C at 0.27 GPa results in a phase transformation from LiInSe₂-hpII to the chalcopyrite-type phase LiInSe₂-hpIII (tetragonal; 142d; a=580.7(8)pm, c=1181.0(31)pm, Z=4, D _x =4.66g/cm³; 0.27 GPa).     

A high pressure Raman study of KTP and pressure induced phase transitions

Potassium titanate orthrophosphate KTiOPO₄ (KTP) has been studied by high pressure Raman technique to 17 GPa using a diamond cell. The Raman data reveal that two phase transitions occur in the system: one near 5.5 GPa and another near 10 GPa. The Lower-pressure transition is definitely first-order but appears to be driven by the phonon mode near 56 cm⁻¹, which exhibits marked softening. A mean field-like behavior is observed. It is hypothesized that this transition is likely to be from ferroelectric (FE) to an antiferroelectric (AF) phase. The 10 GPa transition may be due to AF-PE (paraelectric) transition driven by pressure from high temperature to room temperature. From the larger pressure responce of the PO₄ vibrational modes it is believed that the PO₄ polyhedral compression is larger than the TiO₆ polyhedral compression. leading to polyhedral tilt transitions.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

Pressure evolution of two polymorphs of Tb 2(MoO 4)3

We have studied the high pressure behavior of the α and β^′-phases of Tb ₂(MoO ₄)₃ using a combination of powder X-ray diffraction and ab initio calculations. The α-Tb ₂(MoO ₄)₃ phase did not undergo any structural phase transition in the pressure range from 0 up to the maximum experimental pressure of 21 GPa. We observed line broadening of the diffraction patterns at pressures above 7 GPa, which may be due to non-hydrostatic conditions. The complete amorphization of the sample was not reached in the pressure range studied, as expected from previous Raman studies. The behavior under pressure of the β^′-Tb ₂(MoO ₄)₃ phase is similar to that of other rare-earths trimolybdates with the same structure at room temperature. A phase transition was observed at 2 GPa. The new phase, which can be identified as the δ-phase, has never been completely characterized by diffraction studies. A tentative indexation has been performed and good refined cell parameters were obtained. We detect indications of amorphization of the δ-Tb ₂(MoO ₄)₃ phase at 5 GPa.     

Equation of state and electrical resistivity of the heavy fermion superconductor CeCoIn<Subscript>5</Subscript> to 51 GPa

We have used X-ray diffraction to study the structural phase of CeCoIn₅ in external pressure. Using high-pressure X-ray diffraction, we find that the crystalline phase is stable in the P4/mmm phase for pressures ≤51.2 GPa. From our measured equation of state, we find a bulk modulus given by B ₀ = 72.8 ± 2.9 GPa and a first pressure derivative of B ^′ = 5.1 ± 0.3. Measurement of the electrical resistivity of CeCoIn₅ to pressures as high as 34.4 GPa shows the existence of a peak in resistivity at p ^? = 8.2 ± 0.2 GPa.     

Mössbauer study of CaFeO3 under external high—Pressure

⁵⁷Fe Mössbauer and X-ray diffraction measurements have been performed on a perovskite CaFeO₃ under external high pressure upto 50 GPa at room temperature using a diamond anvil cell. Above 29 GPa the⁵⁷Fe magnetic hyperfine splitting appears superimposing with usual paramagnetic pattern of CaFeO₃. Magnitude of hyperfine field is 16 T and much smaller than 33 T of typical Fe⁴⁺ in SrFeO₃ suggesting a transition from high-spin S=2 to low-spin S=1 state in CaFeO₃.     

A pressure induced phase transition in pyroxene

Abstract

Two monoclinic pyroxenes of composition Ca(Fe,Mg)Si₂O₆ were studied up to 10 GPa using X-ray powder diffraction and ⁵⁷Fe Mössbauerspectroscopy. The results are indicative of a phase transition at 4 GPa.     

Optical properties of methane to 288 GPa at 300 K

Optical properties of solid methane (CH₄) were studied at high pressure and room temperature using a diamond anvil cell. Reflectivity and transmission measurements were used to measure the refractive index to 288 GPa. Fabry-Perot interferometery was used to measure the sample thickness to 172 GPa. This data was fitted to the derived expression of thickness vs. pressure that was then used to calculate the thickness to 288 GPa. This in turn was combined with optical absorption experiments to obtain the absorption coefficient and hence the extinction coefficient k^*. From combined reflection and absorption experiments the refractive index n=n_s+ik^* was obtained. The index of refraction and the ratio of molar refraction to molar volume showed a large increase between 208 and 288 GPa. This behavior indicated that a phase transformation of insulator-semiconductor might have occurred in solid CH₄ by 288 GPa.     

Optical absorption of zinc selenide doped with cobalt (Zn1−xCoxSe) under hydrostatic pressure

Abstract

The optical absorption of the diluted magnetic semiconductor Zn_1?xCO_xSe (x = 0.02) has been measured at room temperature under hydrostatic pressure up to 14GPa in a membrane diamond-anvil cell. We found two absorption features: (i) an absorption structure in the energy range 1.6?1.8eV, with a negligible pressure shift (i.e., 0.45 ± 0.05 meV/GPa) which we have identified as the Co²⁺(3d⁷) internal transition ⁴A₂(F)→+⁴T₁(P) and (ii) an onset in the energy range 2?2.7eV which redshifts with pressure (?8.1±0.6meV/GPa). We have attributed such absorption edge to charge transfer between the ZnSe valence band and the Co²⁺(3d⁷) levels.