Second-order Birch-Murnaghan Equation Of State Research Articles

A high-pressure structural transition of norsethite-type BaFe(CO3)2: Comparison with BaMg(CO3)2 and BaMn(CO3)2

Abstract Investigations on the phase stability of the norsethite-type family [BaMg(CO3)2, BaMn(CO3)2, BaFe(CO3)2] under high-pressure conditions are of great significance for understanding the structure and metal cationic (Mg2+, Fe2+, Mn2+) substitution mechanism in double divalent metal carbonates. The structural evolution and equation of state of BaFe(CO3)2 were studied at high pressure up to ~7.3 GPa by synchrotron X-ray diffraction (XRD) in diamond-anvil cell (DAC) in this study. BaFe(CO3)2 undergoes a reversible phase transition from R3m (α-phase) to C2/c (γ-phase) space groups at ~3.0 GPa. The fitted elastic parameters are V0 = 377.79(2) Å3 and K0 = 40.3(7) GPa for α-BaFe(CO3)2, V0 = 483.24(5) Å3 and K0 = 91.2(24) GPa for γ-BaFe(CO3)2 using second-order Birch-Murnaghan equation of state (BM2-EoS). Besides, the vibrational properties and structural stability of complete norsethite-type minerals were also investigated first by Raman spectroscopy combined with DAC up to 11.1 GPa. Similar structural phase transitions occur in BaMg(CO3)2, BaFe(CO3)2, BaMn(CO3)2 at 2.2–2.6, 2.6–3.7, and 3.7–4.1 GPa, respectively. The onset phase transition pressures of the norsethite-type family are much lower than that of dolomite-type Ca(Mg,Fe,Mn)(CO3)2 and calcite-type (Mg,Fe,Mn)CO3 carbonates. These results provide new insights into the divalent cation substitution effects on the stability and structural evolution of carbonates under high-pressure conditions.

P-V-T equation of state of α-ScOOH.

ABSTRACT In this study, an in situ X-ray diffraction study of α-ScOOH, scandium oxyhydroxide with a diaspore-type structure, was conducted at a pressure and temperature of up to 4.12 GPa and 700 K, respectively, to determine the pressure-volume-temperature (P-V-T) equation of state (EoS). Using a least-squares fit of the second-order Birch–Murnaghan EoS to the P-V-T data, the EoS parameters of α-ScOOH were determined as bulk modulus KT 0 = 101(1) GPa, (∂KT /∂T) P = −0.009(5) GPa K−1, and thermal expansion coefficient at zero pressure α = 3.12(6) 10−5 K−1. α-ScOOH exhibits anisotropic compression and thermal expansion behaviors, which are consistent with those of previous studies on diaspore-type oxyhydroxides. α-ScOOH is more compressible than other diaspore-type oxyhydroxides, and the product of the bulk modulus and volume is approximately constant among diaspore-type M3 + OOH (M = Al, Sc, and Fe).

Pressure Tuning the Jahn-Teller Transition Temperature in NaNiO2.

NaNiO2 is a layered material consisting of alternating layers of NaO6 and Jahn–Teller-active NiO6 edge-sharing octahedra. At ambient pressure, it undergoes a broad phase transition from a monoclinic to rhombohedral structure between 465 and 495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO2 as a function of pressure and temperature from ambient pressure to ∼5 GPa between 290 and 490 K. The 290 and 460 K isothermal compressions remained in the monoclinic phase up to the maximum pressures studied, whereas the 490 K isotherm was mixed-phase throughout. The unit-cell volume was fitted to a second-order Birch–Murnaghan equation of state, where B = 119.6(5) GPa at 290 K. We observe at 490 K that the fraction of the Jahn–Teller-distorted phase increases with pressure, from 67.8(6)% at 0.71(2) GPa to 80.2(9)% at 4.20(6) GPa. Using this observation, in conjunction with neutron diffraction measurements at 490 K on removing pressure from 5.46(9) to 0.342(13) GPa, we show that the Jahn–Teller transition temperature increases with pressure. Our results are used to present a structural pressure–temperature phase diagram for NaNiO2. To the best of our knowledge, this is the first diffraction study of the effect of pressure on the Jahn–Teller transition temperature in materials with edge-sharing Jahn–Teller-distorted octahedra and the first variable-pressure study focusing on the Jahn–Teller distortion in a nickelate.

Thermoelastic parameters of Mg-sursassite and its relevance as a water carrier in subducting slabs

Abstract We report the synthesis, at 7 GPa and 923 K, and the thermoelastic characterization, up to 16 GPa and 850 K, of a single crystal of Mg-sursassite, Mg5Al5Si6O21(OH)7. In situ high-pressure and high-temperature single-crystal diffraction allowed the study of structural variation at non-ambient conditions and the determination of bulk elastic properties. The refined parameters of a second-order Birch-Murnaghan equation of state (BM-II EoS) are V0 = 446.02(1) Å3 and KT0 = 135.6(7) GPa. The thermal expansion coefficients of a Berman-type EoS are α0 = 3.14 (5) × 10−5 K−1, α1 = 2.50(16) × 10−8 K−2, and V0 = 445.94(3). For comparison, the P-V EoS is determined for a natural sursassite sample, ideally Mn4Al6Si6O22(OH)6. The refined parameters of BM-II EoS [V0 = 470.2(3) Å3, KT0 = 128(4) GPa] indicate that composition has a minimal effect on elastic properties. The similarity of density and bulk properties of Mg-sursassite if compared to olivine and other anhydrous mantle minerals suggests that this phase could be overseen by geophysical methods.

Compressibility and blue-shifting O–H stretching bands of magnesium hydroxyfluoride Mg(OH)F up to 20 ​GPa

In-situ high-pressure synchrotron X-ray diffraction (XRD) and Fourier transform infrared absorption spectroscopy (FTIR) measurements have been conducted up to ~20 ​GPa at room temperature using diamond anvil cells (DAC), to investigate compressibility and spectroscopic properties of magnesium hydroxyfluoride, Mg(OH)F (Pnma, Z ​= ​4). Without any phase transition being observed, the pressure-volume correlation is fitted by the second-order Birch-Murnaghan equation of state, yielding K0 ​= ​80 (±2) GPa, K0′ ​= ​4.0 (fixed), and V0 ​= ​145.2 (±0.3) Å3. In accordance with other isostructural compounds crystallizing in the diaspore-type structure, higher contraction occurs along the c axis of Mg(OH)F. Positive pressure dependence of the O–H stretching mode frequencies initially observed at 3537 (1.56 ​cm−1/GPa), 3641 (0.67 ​cm−1/GPa), and 3680 ​cm−1 (2.85 ​cm−1/GPa) at ambient pressure indicates that the O–H⋯F hydrogen bond does not strengthen under compression.

Pressure-Induced Phase Transition in Mn(Ta,Nb)2O6: An Experimental Investigation and First-Principle Study.

The high-pressure and high-temperature behaviors of manganotantalite Mn(Ta,Nb)2O6 have been investigated by single-crystal X-ray diffraction and Raman spectroscopy combined with diamond anvil cell technique, as well as first-principle calculations. A pressure-induced reversible phase transition of manganotantalite occurs at 9.5 GPa and room temperature, accompanied by a large volume collapse (∼7.0%) and drastic color change from brownish-yellow to red. The space groups of low-pressure (LP) and high-pressure (HP) phases are the same (Pbcn), but the coordination numbers of Mn increase from six to eight and Ta increase from six to seven, respectively. The band gap becomes narrow from 2.37 to 1.59 eV. We determined the P-T phase diagram of manganotantalite with a positive Clapeyron slope of dP/dT = 0.0073 GPa/K. The P-V data were fitted to a second-order Birch-Murnaghan equation of state with B0 = 149(4) GPa for the LP phase and B0 = 188(3) GPa for the HP phase. The isothermal Grüneisen parameters were determined to be 0.23∼2.03 of the LP phase and 0.59∼0.86 of the HP phase for Raman modes. High-pressure behaviors of Mn(Ta,Nb)2O6 indicate that this kind of material is a potential effective pressure sensor to monitor pressure change or warn pressure abnormality.

Pressure-induced reversible phase transition on Mo2Ga2C

The newly carbide α-Mo2Ga2C with P63/mmc space group presents a unique top-packed Ga double layer structure. Pressure-induced phase transition of Mo2Ga2C has been investigated by in-situ high-pressure synchrotron radiation X-ray diffraction, Raman spectroscopy combined with diamond anvil cell technique, and first-principle calculations. Both experimental and theoretical results show that a first-order phase transition from P63/mmc to P3¯m1 occurs at ∼22 GPa, where the high-pressure phase named β-Mo2Ga2C has a close-packed Ga double layer zig-zag structure. The isothermal pressure-volume relationship of Mo2Ga2C is described by the second-order Birch-Murnaghan equation of state, yielding K0 = 139(2) GPa, K0′ = 4(fixed), and V0 = 144.5(1) Å3 for α-Mo2Ga2C; K0 = 187(3) GPa, K0′ = 4(fixed), and V0 = 137.9(2) Å3 for β-Mo2Ga2C. The instability caused by the repulsive force between the Ga atoms in the top-packed double-layer under pressure mainly accounts for the phase transition. The newly discovered β-Mo2Ga2C reveals that the MAX-like M2A2X type ternary compounds with different compositions may form various structures with different space groups.

Ambient-temperature high-pressure-induced ferroelectric phase transition in CaMnTi2O6

The ferroelectric to paraelectric phase transition of multiferroic CaMnTi$_2$O$_6$ has been investigated at high pressures and ambient temperature by second harmonic generation (SHG), Raman spectroscopy, and powder and single-crystal x-ray diffraction. We have found that CaMnTi$_2$O$_6$ undergoes a pressure-induced structural phase transition ($P4_2mc \rightarrow P4_2/nmc$) at $\sim$7 GPa to the same paraelectric structure found at ambient pressure and $T_c$ = 630 K. The continuous linear decrease of the SHG intensity that disappears at 7 GPa and the existence of a Raman active mode at 244 cm$^{-1}$ that first softens up to 7 GPa and then hardens with pressure, are used to discuss the nature of the phase transition of CaMnTi$_2$O$_6$ for which a d$T_c$/d$P = -48$ K/GPa has been found. Neither a volume contraction nor a change of the normalized pressure on the eulerian strain are observed across the phase transition with all the unit-cell volume data following a second order Birch-Murnaghan equation of state with a bulk modulus of $B_0$ = 182.95(2) GPa.

Compressibility and high-pressure structural behavior of Mg2Fe2O5

The compressibility and structural behavior of the novel Mg 2 Fe 2 O 5 oxide has been investigated by in situ single-crystal X-ray diffraction in a diamond-anvil cell up to a pressure of 17 GPa. The bulk compressibility of Mg 2 Fe 2 O 5 can be described using a second-order Birch-Murnaghan equation of state (BM2 EoS) with V 0 = 352.4(2) A 3 and K 0 = 171(4) GPa. Three linear BM2 EoS were used to describe the axial compressibility of Mg 2 Fe 2 O 5 , which was found to be highly anisotropic. The a and b lattice parameters have very similar compressibilies, with a 0 = 2.8917(11) A and linear modulus M a = 572(16) GPa and b 0 = 9.736(3) A and linear modulus M b = 583(15) GPa, respectively. The c -axis is the most compressible direction as indicated by the smaller linear modulus [ c 0 = 12.520(15) A and M c = 404(28) GPa]. The Mg 2 Fe 2 O 5 structure consists of edge-sharing octahedra alternating with layers of trigonal prisms. The compression behavior of the M-O bonds of the M1 and M2 octahedra and of the M3 prisms depend on their location in either an edge-sharing environment, which makes them stiffer, or a corner-sharing environment where they have more freedom to distort and compress. The main compression mechanism consists of a polyhedral tilting around the M2-O1-M2 angle, which decreases with increasing pressure. Mg 2 Fe 2 O 5 has recently been added to the list of stable end-members of phases with M 4 O 5 stoichiometry, making it a potentially relevant phase in the Earth’s upper mantle and transition zone. To develop thermodynamic activity-composition models for high-pressure phases, it is crucial to know the accurate elastic parameters of each individual end-member. Currently these have only been measured for Mg 2 Fe 2 O 5 (this study) and Fe 4 O 5 .

로소나이트(Lawsonite)의 압력에 따른 등방성 압축거동 연구

자연산 로소나이트(Ca-Lawsonite, <TEX>$CaAl_2Si_2O_7(OH)_2{\cdot}H_2O$</TEX>) 분말의 압력증가에 따른 변화와 구조적 특성을 소형 압력 발생장치인 다이아몬드 앤빌셀과 실시간 싱크로트론 X-선 회절실험을 통하여 확인하였다. 본 연구에서 시료에 적용한 압력은 1 기압에서 8 GPa 까지였으며 메탄올, 에탄올, 물의 혼합 용액을 압력 매개체로 하여 상온조건에서 실험하였고, 특이한 상전이나 압력하 부피변화의 이상현상은 발견되지 않은 가역적 압축특성을 관찰하였다. 최고압력까지 로소나이트는 결정성을 잃지 않는 것으로 확인되었고 압력증가에 따른 체적감소를 통해 체적탄성률(<TEX>$B_0$</TEX>)을 계산해 본 결과 146(6) GPa로 도출되었다. 체적탄성률과 함께 계산된 선형압축률은 a-, b-, c-축에서 각각 0.0022, 0.0024, <TEX>$0.0020GPa^{-1}$</TEX>로 압력에 의한 부피감소는 대체적으로 3축이 같은 비율로 감소하는 것을 확인하였다. Powder samples of natural lawsonite (Ca-lawsonite, <TEX>$CaAl_2Si_2O_7(OH)_2{\cdot}H_2O$</TEX>) was studied structurally up to 8 GPa at room temperature using monochromatic synchrotron X-ray powder diffraction and a diamond anvil cell (DAC) with a methanol : ethanol : water (16 : 3 : 1 by volume) mixture solution as a penetrating pressure transmitting medium (PTM). Upon pressure increase, lawsonite does not show any apparent pressure induced expansion (PIE) or phase transition. Pressure-volume data were fitted to a second-order Birch-Murnaghan equation of state using a fixed pressure derivative of 4 leading to a bulk modulus (<TEX>$B_0$</TEX>) of 146(6) GPa. This compression is further characterized to be isotropic with calculated linear compressibilities of <TEX>${\beta}^a=0.0022GPa^{-1}$</TEX>, <TEX>${\beta}^b=0.0024GPa^{-1}$</TEX>, and <TEX>${\beta}^c=0.0020GPa^{-1}$</TEX>.

Experimental and theoretical identification of a high-pressure polymorph of Ga2S3 with α-Bi2Te3-type structure

Since the discovery of α-phase Bi2Te3, Sb2Te3, and Bi2Se3 as 3D topological insulators, many experimental and theoretical studies of A2B3-type chalcogenides have been performed to search for new materials with interesting elastic and electric properties at ambient and extreme conditions. In this study, high-pressure properties of Ga2S3 have been characterized by in situ synchrotron X-ray diffraction (XRD), X-ray absorption near edge structure measurements, and Density-functional theory (DFT) calculations. At ∼16.0 GPa, a phase transition of α′-Ga2S3 (Cc and Z = 4) is observed experimentally to a new polymorph, which is indentified to be the tetradymite-type or α-Bi2Te3-type crystal structure (R3¯m and Z = 3) by laser-annealing XRD experiments and DFT calculations. The isothermal pressure-volume relationship of Ga2S3 is well described by the second-order Birch-Murnaghan equation of state with K0 = 59(2) GPa and K0′ = 4 (fixed) for the α′-Ga2S3, and K0 = 91(3) GPa, and K0′ = 4 (fixed) for the tetradymite-type phase. In addition, band gap of α′-Ga2S3 decreases on compression and the tetradymite-type Ga2S3 exhibits metallization based on DFT calculations. The pressure-induced phase transition accompanying by changes of elastic and electrical properties may give some implications to other chalcogenides under high pressure.

Equations of state of ice VI and ice VII at high pressure and high temperature.

High-pressure H2O polymorphs among which ice VI and ice VII are abundant in the interiors of large icy satellites and exo-planets. Knowledge of the elastic properties of these pure H2O ices at high-temperature and high-pressure is thus crucial to decipher the internal structure of icy bodies. In this study we assess for the first time the pressure-volume-temperature (PVT) relations of both polycrystalline pure ice VI and ice VII at high pressures and temperatures from 1 to 9 GPa and 300 to 450 K, respectively, by using in situ synchrotron X-ray diffraction. The PVT data are adjusted to a second-order Birch-Murnaghan equation of state and give V0 = 14.17(2) cm(3) mol(-1), K0 = 14.05(23) GPa, and α0 = 14.6(14) × 10(-5) K(-1) for ice VI and V0 = 12.49(1) cm(3) mol(-1), K0 = 20.15(16) GPa, and α0 = 11.6(5) × 10(-5) K(-1) for ice VII.

Thermo-elastic behavior and P/T phase stability of TlAlSiO4 (ABW)

The P/T phase stability and the thermo-elastic behavior of synthetic TlAlSiO4 [ABW framework type, space group Pna21, a=8.2719(2), b=9.4373(2), c=5.4180(1)Å] have been investigated up to 950°C (at room-P) and up to 8GPa (at room-T) by means of in situ synchrotron powder diffraction with a diamond anvil cell and with a high-temperature furnace. The Rietveld structure refinement of TlAlSiO4 at room P/T confirms the general structure model previously reported. Only one independent Tl site, with full site occupancy and close to channel wall, represents the extra-framework population. No phase transition has been observed within the temperature- and pressure-range investigated. P–V data were fitted to a second-order Birch–Murnaghan Equation of State (BM-EoS), giving: V0=420.76(5)Å3, KT0=48.8(2)GPa. The evolution of the lattice parameters with pressure shows a significantly anisotropic compressional pattern. The elastic parameters calculated with a “linearized” second-order BM-EoS are: KT0(a)=21.96(7)GPa for the a-axis; KT0(b)=68(1)GPa for the b-axis, and KT0(c)=112(2)GPa for the c-axis. The volume thermal expansion with T was modeled by the polynomial function: V(T)/V0=1+α0·ΔT+α1·ΔT2=1+4.44(3)·10−5·ΔT−2.3(3)·10−9·ΔT2. The anisotropic thermal scheme is characterized by a negative thermal expansion along [010] (i.e. α0(b)=−8.5(1)·10−6°C−1), almost no expansion along [001] (i.e. α0(c)=0.9(1)·10−6°C−1) and a positive expansion along [100] (i.e. α0(a)=52.4(1)·10−6°C−1). A comparative analysis of the thermo-elastic behavior of the isotypic TlAlSiO4 and CsAlSiO4 is carried out.

The high-pressure behavior of balliranoite: a cancrinite-group mineral

The high-pressure elastic behavior and structure evolution of a natural balliranoite, i.e. a mineral isotypic with cancrinite belonging to the davyne subgroup, (Na4.47Ca2.86K0.11)(Si5.96Al6.04O24)Cl2.03(CO3)0.78(SO4)0.33, a = 12.680(1) Å, c = 5.3141(5) Å and V = 739.9(1) Å3, S.G. P63, have been studied by means of in-situ single-crystal X-ray diffraction with a diamond anvil cell, up to 6.77(2) GPa. No evidence of phase transition, structure collapse or change of the compressional behavior have been observed within the pressure range investigated. The unit-cell volume evolution as function of pressure has been fitted with a second-order Birch-Murnaghan equation of state (BM EoS), yielding the following refined parameters: V0 = 735.5(7) Å3, KV0 = 48.1(8) GPa. Fitting of the a vs. P and c vs. P data with linearized third-order BM-EoS leads to the following elastic anisotropy at ambient conditions: Ka0:Kc0 = 1.35:1. The P-induced structure evolution of balliranoite is mainly governed by the framework re-arrangement through tilting of quasi-rigid tetrahedra. A comparative analysis of the elastic behavior and of the structure deformation mechanisms of balliranoite and cancrinite at high-pressure are carried out.

Pressure-Induced Amorphization in Gd2O3/Er3+ Nanorods

The high-pressure phase transformation of Er3+-doped Gd2O3 nanorods was studied by synchrotron radiation X-ray diffraction (XRD) and Raman scattering spectra. The starting cubic phase is stable up to about 9 GPa, and the Gd2O3/Er3+ nanorods start to transform into an amorphous phase related to monoclinic structure at 9.4 GPa. The phase transition is different from that direct crystal crystal transformation observed in the bulk Gd2O3 and Gd2O3 nanomaterials. The bulk modulus for cubic Gd2O3/Er3+ nanorods is determined to be K-0 = 196 +/- 13.4 GPa by fitting to the second-order Birch-Murnaghan equation of state, which is larger than that of 164 +/- 3 GPa of the bulk Gd2O3/Er3+. In addition, the quenched samples almost maintain their pristine nanorod shape but with amorphous form. It is proposed that the special rodike morphology and particle size probably play important roles in the high-pressure behavior of Er3+-doped Gd2O3 nanorods.

High-pressure effect on inverse spinel LiCuVO4: X-ray diffraction and Raman scattering

The effect of external quasi-hydrostatic pressure on the inverse spinel structure of LiCuVO4 was studied in this paper. High-pressure synchrotron X-ray diffraction and Raman spectroscopy measurements were carried out at room temperature up to 35.7 and 40.3 GPa, respectively. At a pressure of about 20 GPa, both Raman spectra and X-ray diffraction results indicate that LiCuVO4 was transformed into a monoclinic phase, which remained stable up to at least 35.7 GPa. Upon release of pressure, the high-pressure phase returned to the initial phase. The pressure dependence of the volume of low pressure orthorhombic phase and high-pressure monoclinic phase were described by a second-order Birch-Murnaghan equation of state, which yielded bulk modulus values of B0 = 197(5) and 232(8) GPa, respectively. The results support the empirical suggestion that the oxide spinels have similar bulk modulus around 200 GPa.

The thermoelastic behavior of clintonite up to 10 GPa and 1,000°C

The thermoelastic behavior of a natural clintonite-1M [with composition: Ca1.01(Mg2.29Al0.59Fe0.12)Σ3.00(Si1.20Al2.80)Σ4.00O10(OH)2] has been investigated up to 10 GPa (at room temperature) and up to 960°C (at room pressure) by means of in situ synchrotron single-crystal and powder diffraction, respectively. No evidence of phase transition has been observed within the pressure and temperature range investigated. P–V data fitted with an isothermal third-order Birch–Murnaghan equation of state (BM-EoS) give V0 = 457.1(2) A3, KT0 = 76(3)GPa, and K′ = 10.6(15). The evolution of the “Eulerian finite strain” versus “normalized stress” shows a linear positive trend. The linear regression yields Fe(0) = 76(3) GPa as intercept value, and the slope of the regression line leads to a K′ value of 10.6(8). The evolution of the lattice parameters with pressure is significantly anisotropic [β(a) = 1/3KT0(a) = 0.0023(1) GPa−1; β(b) = 1/3KT0(b) = 0.0018(1) GPa−1; β(c) = 1/KT0(c) = 0.0072(3) GPa−1]. The β-angle increases in response to the applied P, with: βP = β0 + 0.033(4)P (P in GPa). The structure refinements of clintonite up to 10.1 GPa show that, under hydrostatic pressure, the structure rearranges by compressing mainly isotropically the inter-layer Ca-polyhedron. The bulk modulus of the Ca-polyhedron, described using a second-order BM-EoS, is KT0(Ca-polyhedron) = 41(2) GPa. The compression of the bond distances between calcium and the basal oxygens of the tetrahedral sheet leads, in turn, to an increase in the ditrigonal distortion of the tetrahedral ring, with ∂α/∂P ≈ 0.1°/GPa within the P-range investigated. The Mg-rich octahedra appear to compress in response to the applied pressure, whereas the tetrahedron appears to behave as a rigid unit. The evolution of axial and volume thermal expansion coefficient α with temperature was described by the polynomial α(T) = α0 + α1T−1/2. The refined parameters for clintonite are as follows: α0 = 2.78(4) 10−5°C−1 and α1 = −4.4(6) 10−5°C1/2 for the unit-cell volume; α0(a) = 1.01(2) 10−5°C−1 and α1(a) = −1.8(3) 10−5°C1/2 for the a-axis; α0(b) = 1.07(1) 10−5°C−1 and α1(b) = −2.3(2) 10−5°C1/2 for the b-axis; and α0(c) = 0.64(2) 10−5°C−1 and α1(c) = −7.3(30) 10−6°C1/2for the c-axis. The β-angle appears to be almost constant within the given T-range. No structure collapsing in response to the T-induced dehydroxylation was found up to 960°C. The HP- and HT-data of this study show that in clintonite, the most and the less expandable directions do not correspond to the most and the less compressible directions, respectively. A comparison between the thermoelastic parameters of clintonite and those of true micas was carried out.

Exploring the high-pressure behavior of superhard tungsten tetraboride

In this work, we examine the high-pressure behavior of superhard material candidate WB${}_{4}$ using high-pressure synchrotron x-ray diffraction in a diamond anvil cell up to 58.4 GPa. The zero-pressure bulk modulus, ${K}_{0}$, obtained from fitting the pressure-volume data using the second-order Birch-Murnaghan equation of state is 326 $\ifmmode\pm\else\textpm\fi{}$ 3 GPa. A reversible, discontinuous change in slope in the $c/a$ ratio is further observed at \ensuremath{\sim}42 GPa, suggesting that lattice softening occurs in the $c$ direction above this pressure. This softening is not observed in other superhard transition metal borides such as ReB${}_{2}$ compressed to similar pressures. Speculation on the possible relationship between this softening and the orientation of boron-boron bonds in the $c$ direction in the WB${}_{4}$ structure is included. Finally, the shear and Young's modulus values are calculated using an isotropic model based on the measured bulk modulus and an estimated Poisson's ratio for WB${}_{4}$.

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2) Å3 (1 Å=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.

Pressure-induced structural phase transition of the iron end-member of ringwoodite ( -Fe2SiO4) investigated by X-ray diffraction and Mossbauer spectroscopy

We have carried out X-ray diffraction and Mossbauer spectroscopy measurements on the spinel phase γ-Fe 2 SiO 4 (ringwoodite) at ambient temperature and pressures up to 66 GPa using diamond anvil cells. At pressures above 30 GPa, a previously unknown structural phase transition to a rhombohedrally distorted spinel phase has been observed (space group R 3 mR ). Mossbauer spectroscopy measurements reveal two different Fe 2+ sites at high pressure with an abundance ratio of 3:1, in agreement with the two crystallographic sites occupied by the iron in this distorted spinel structure. The unit-cell volume of the low-pressure spinel phase as a function of pressure results in a bulk modulus of K 0 = 197(3) GPa using the second-order Birch-Murnaghan equation of state, and K 0 = 201(8) GPa and K′ = 3.7(7) when using a third-order equation of state. The pressure evolution of the unit-cell volume and the Mossbauer hyperfine parameters are in good agreement with previous studies, which were limited to a lower pressure range.