Order Birch-Murnaghan Equation Of State Research Articles

The thermal equation of state of the magma Ocean

As the protolunar disk formed by the giant impact with the incoming asteroid Theia started to cool, the Earth condensed in its center as a molten planet. The outer layer, rich in lithophile oxides, formed the magma ocean, whose dynamical behavior heavily influenced the entire evolution of the early Earth. Here, we employ first-principles molecular dynamics simulations to characterize the magma ocean in its initial stage. We compute the compressibility and the viscosity of the molten pyrolite, which best approximates the bulk silicate Earth composition. With an extensive set of calculations, we span the entire relevant pressure-temperature range. We provide a detailed analysis of the athermal and thermal equations of state in various formulations. We obtain a linear temperature dependence for the bulk modulus using the 4th order Birch-Murnaghan equation of state. We show that the liquid silicate accommodates compression by the collapse of the second Si-O coordination sphere, whose corresponding coordination number passes from about 24 at the surface of the magma ocean to about 40 at its deepest point. We found that pyrolite melts have a very low viscosity, about one order of magnitude lower than that of molten basalt. We also propose a revised position of the critical point of the bulk silicate Earth, which lies around 6000 K and 1.1–1.2 kbars.

High-pressure structural behavior and elastic properties of Ce2In and Ce3In: A combined in situ synchrotron X-ray diffraction and density functional theory study

Across the cerium (Ce)-indium (In) binary phase diagram, there are five line compounds including Ce3In, Ce2In, Ce3In5, CeIn2 and CeIn3. So far, CeIn3 is the only compound whose high-pressure structural behavior and elastic properties have been studied in detail. In this work, we investigated the compression behavior of Ce3In and Ce2In at pressures up to 26.2 GPa using in situ high-pressure angle dispersive synchrotron powder X-ray diffraction (XRD). Our results indicate that Ce3In retains its Cu3Au-type structure in the pressure range investigated, whereas Ce2In undergoes a phase transformation from the Ni2In-type structure to an unidentified phase at ∼4.6 GPa. Unit-cell parameters as a function of pressure were obtained by Rietveld analysis of XRD data. Unit cell volumes were fitted to a second order Birch-Murnaghan equation of state (EOS). The experimentally determined bulk moduli (K0) of the two cerium indides are 50 ± 3 GPa for Ce3In and 39.2 ± 1.6 GPa for Ce2In. The zero-pressure compressibilites of the a-axis (βa,0) for Ce3In is 6.67 (±0.36) × 10–3 GPa–1 and of the a- and c-axis for Ce2In are βa,0 = 8.85 (±0.39) × 10−3 GPa−1 and βc,0 = 9.26 (±0.51) × 10−3 GPa−1, respectively. These results are in general agreement with those calculated by density functional theory (DFT + U) and open-source machine learning algorithms.

High pressure dissociation of CaTiO3 perovskite into CaO and CaTi2O5

In this study, we investigated phase transformations of CaTiO3 perovskite using x-ray diffraction at high pressure and high temperature up to 170 GPa and 4500 K in a laser-heated diamond-anvil cell. We report a high-pressure dissociation of CaTiO3 into CaO-B2 and CaTi2O5 with a monoclinic P2/m structure, instead of the expected transformation of the orthorhombic distorted perovskite structure into a post-perovskite phase. We propose that this transition may be favored by the B1 to B2 phase change of CaO at around 60 GPa. In order to provide additional information on the high pressure properties of CaTiO3 perovskite, we measured its melting temperature using CO2 laser heated diamond anvil cell up to 55 GPa yielding a fit of the melting curve to a Kraut-Kennedy empirical law according to: Tm (K) = 2188 ∗ [1 + 4.23 ∗ (ΔV/V0)]. To provide some further insight into the thermodynamic properties of CaTiO3, we determined the P-V-T equation of state of the orthorhombic mineral perovskite, fitted by using a third order Birch-Murnaghan equation of state and a Berman thermal expansion model. The fit of the data yields to K0 = 180.6(4) GPa, K′0 = 4 (fixed), ∂K/∂T = −0.022(1) GPa K−1, α1 = 3.25(5) x 10−5 K−1, α2 = 1.3(1) x 10−8 K−2

Structural Study of δ-AlOOH Up to 29 GPa

A structure and equation of the state of δ-AlOOH has been studied at room temperature, up to 29.35 GPa, by means of single crystal X-ray diffraction in a diamond anvil cell using synchrotron radiation. Above ~10 GPa, we observed a phase transition with symmetry changes from P21nm to Pnnm. Pressure-volume data were fitted with the second order Birch-Murnaghan equation of state and showed that, at the phase transition, the bulk modulus (K0) of the calculated wrt 0 pressure increases from 142(5) to 216(5) GPa.

Synthesis of palladium carbides and palladium hydride in laser heated diamond anvil cells

The diamond anvil cell (DAC) technique is a powerful method for the synthesis and studying of novel materials at extreme conditions. In this work we report on high-pressure high-temperature (HPHT) synthesis of palladium carbides (PdCx) and palladium hydride (PdH) in a laser heated diamond anvil cell. Formation of PdCx with a face-centered cubic (fcc) structure resulted from a chemical reaction of Pd with carbon from the diamond anvils at a pressure of about 50 GPa and temperature of 2500–3000 K. The samples were analyzed in situ using synchrotron X-ray diffraction. The compressional behavior of PdC0.19(3) and PdC0.17(3), was studied on decompression. The fit of the pressure-volume data using the 3rd order Birch-Murnaghan equation of state gave the following parameters: V0 = 65.1 (1) Å3, K0 = 241 (9) GPa and K0’ = 2.1 (3) for PdC0.19(3), and V0 = 64.51 (5) Å3, K0 = 189 (8) GPa and K0’ = 4.5 (4) for PdC0.17(3). The palladium hydride PdH was synthesized at P = 39 (2) GPa and T = 1500 (200) K through a direct reaction of Pd with paraffin oil.

Compression experiments to 126 GPa and 2500 K and thermal equation of state of Fe3S: Implications for sulphur in the Earth's core

Pressure-volume-temperature (P-V-T) experiments on tetragonal Fe3S were conducted to 126 GPa and 2500 K in laser-heated diamond anvil cells (DAC) with in-situ X-ray diffraction (XRD). Seventy nine high-T data as well as four 300-K data were collected, based on which new thermal equations of state (EoS) for Fe3S were established. The room-T data together with existing data were fitted to the third order Birch-Murnaghan EoS, which yielded, K0=126±2 GPa and K′=5.1±1 with V0 fixed at 377.0 Å3. A constant αKT term in the thermal pressure equation, Pth = αKT(T-300), fitted the high-T data well to the highest temperature, which implies that the contributions from the anharmonic and electronic terms should be minor in the thermal pressure term. The high-T data were also fitted to the Mie-Grüneisen-Debye model; γ0=1.01±0.03 with θ0 and q fixed at 417 K and 1 respectively. Calculations from the EoS show that crystalline Fe3S at 4000-5500 K is denser than the Earth's outer core and less dense than the inner core. Assuming a density reduction due to melting, liquid Fe3S would meet the outer core density profile, which however suggests that no less than 16 wt%S is needed to reconcile the observed outer core density deficit. The S-rich B2 phase, which was suggested to be a potential liquidus phase of an Fe3S-outer core above 250 GPa, namely the main constituent of its solid inner core, would likely be less dense than the Earth's inner core. As such, while the outer core density requires as much sulphur as 16 wt%, the resulting liquidus phase cannot meet the density of the inner core. Any sulphur-rich composition should therefore be rejected for the Earth's core.

Compressibility and structure behaviour of maruyamaite (K-tourmaline) from the Kokchetav massif at high pressure up to 20 GPa

The structural behaviour of maruyamaite (K-dominant tourmaline) X(K0.54Na0.28Ca0.19)Y(Mg1.3Al1.17Fe0.39Ti0.14)Z(Al5Mg)[Si5.95Al0.05O18](BO3)3V,W[O1.69(OH)2.31] from the ultrahigh-pressure metamorphic rocks of Kokchetav massif was studied using synchrotron based single-crystal diffraction up to 20 GPa. Within the whole pressure range the compression is regular and anisotropic, with the c direction being more compressible than the a direction. Fitting the V/P data with the 2nd and 3rd order Birch-Murnaghan equations of state gives: V0 = 1587.2(7) A3, K0 = 115.6(9) GPa at fixed K′ = 4, and V0 = 1588(1) A3, K0 = 112(3) GPa, K′ = 4.5(4). The bulk modulus values are slightly higher as compared to those found for dravite and cation-deficient synthetic K-dravite. The pressure evolution of the main structural parameters of K-tourmaline is similar to those of dravite. However, a minor change in the rigidity of local contacts of the X site with 6-membered ring, due to the presence of K, is apparently critical for stabilization of tourmaline structure within 15–20 GPa, which is evinced by the absence of the phase transition observed in dravite near 15.4 GPa. The stabilizing function of K becomes apparent at P > 15 GPa. The comparison of the HP structural behaviour of maruyamaite and dravite supports the recent suggestion that the large X site plays a secondary role in the elastic behaviour of tourmaline, compared to the octahedral framework. In addition, the present study reveals several new features of polyhedra distortions, which demonstrate their complex interaction on compression.

Investigation of the high pressure phase BiS2: Temperature-resolved structure and compression behavior to 60 GPa

BiS2 was synthesized using a multi-anvil large volume press at a pressure and temperatures of 5.5 GPa and 1250 °C, respectively, and was then recovered at ambient conditions. Using data collection from single crystal synchrotron X-ray diffraction experiments, the crystal structure was found to consist of two distinct distorted square-based pyramidal BiS5-units. Synchrotron powder X-ray diffraction in the 85–300 K range shows smooth thermal expansion with a modest anisotropy. Physical property measurements reveals an optical band gap of 1.10 eV and a heat capacity with no anomalies in the 2–300 K range. Debye temperatures, determined by both heat capacity and thermal motion analysis, agree well with values of around 114 and 107 K for Bi1 and Bi2 atoms, respectively. Furthermore, theoretical calculations of the electronic band structure, by density functional theory, confirm the gapped state and reveal a small degree of band inversion at the Γ-point, but calculation of parity eigenvalues show BiS2 to possess a trivial topology. The high pressure behavior up to 60 GPa was investigated by powder diffraction in a diamond anvil cell; the structure is retained to at least 35 GPa while indications of a structure transition are observed afterwards. Fitting a 3rd order Birch-Murnaghan equation of state for pressures up to 30 GPa gives a bulk modulus of K0 = 35.5(8) GPa and K0′ = 7.37(18). Finally, a short structural comparison between the high pressure phase of BiS2 and BiS2-based superconductors is presented.

Theoretical and Experimental Evidence for a Post-Cotunnite Phase Transition in Hafnia at High Pressures

Using first-principles density-functional theory (DFT) computations, we have predicted a new post-cotunnite (OII) phase of hafnia (HfO2) at high pressures. Our computations, using the generalized gradient approximation (GGA), predict a phase transition from OII to a Fe2P-type structure at ~ 120 GPa (~ 140 GPa) with a slight volume collapse at the transition pressure of ~ 0.2% (~ 0.1%) between the two phases using the second- (third-) order Birch-Murnaghan equation of state, respectively. The prediction of the new phase is consistent with recent experiments and computations performed on similar dioxides titania (TiO2) and zirconia (ZrO2) at extreme pressure-temperature (p-T) conditions. Importantly, our theoretical prediction for the OII → Fe2P transition in HfO2 is experimentally supported by the re-analysis of X-ray diffraction patterns of HfO2 at extreme pressure-temperature conditions. Additionally, the equation of state and hardness of the predicted phase have been computed and show that Fe2P-type phase while less compressible than the OII phase is nearly identical in hardness, indicating that none of the HfO2 phases qualify as superhard.

High pressure synchrotron x-ray diffraction study of the Mn0.94Ti0.06CoGe alloy

High pressure x-ray diffraction studies up to 10.4 GPa were performed on the Mn0.94Ti0.06CoGe alloy using synchrotron radiation with a diamond anvil cell. No structural phase transitions occurred in the entire range of our measurements. Unit cell parameters were determined up to 10.4 GPa and the calculated unit cell volumes were found to be well represented by a third order Birch-Murnaghan equation of state. The bulk modulus determined from the pressure – volume data was found to be, B0 = 231.72 ± 7.79 GPa. This study, employing high resolution synchrotron x-rays has helped clarify the behaviour of the Mn0.94Ti0.06CoGe alloy under high pressure.

Compressibility of the 23 Å phase under high pressure and high temperature

A new Al-bearing high-pressure hydrous Mg-silicate named 23 Å phase with a hexagonal structure, which is stable in chlorite composition and could be an important water carrier in the subducting slab, was reported by Cai et al. (2015a). Here, for the first time, we determined the equation of state of the 23 Å phase up to 10 GPa and 1073 K by energy-dispersive in situ X-ray diffraction. Fitting the P-V data to the room temperature Birch-Murnaghan equation of state yielded V0 = 538.0(3) Å3 and K0 = 111(1) GPa by fixing K′ at 4. The high temperature 3rd order Birch-Murnaghan equation of state was used to fit the P-V-T data, yielding V0 = 538.0(3) Å3, K0 = 109(1) GPa, ∂K/∂T = −0.012(5) GPa/K, and a0 = 3.0(4) × 10−5/K by fixing K′ at 4. Very little anisotropy was observed, and the axial compressibilities were 2.54(2) 10−3/GPa for the a axis and 2.68(5) 10−3/GPa for the c axis. The 23 Å phase has similar volume compressibility and density as phase A, indicating that the stability region (or the depth in the subduction zone) of these phases are close to each other. This is consistent with the observations of Cai et al. (2015a). Based on the present results, the potential effect of hydration on the physical properties of the subduction slab were discussed, and the possible formation enthalpy and entropy of the 23 Å phase were calculated.

Pressure and Temperature Dependent Structural Studies on Hollandite Type Ferrotitanate and Crystal Structure of a High Pressure Phase.

The structural stability and phase transition behavior of tetragonal (I4/m) hollandite type K2Fe2Ti6O16 have been investigated by in situ high pressure X-ray diffraction using synchrotron radiation and a diamond anvil cell as well as by variable temperature powder neutron and X-ray diffraction. The tetragonal phase is found to be stable in a wider range of temperatures, while it reversibly transforms to a monoclinic (I2/m) structure at a moderate pressure, viz. 3.6 GPa. The pressure induced phase transition occurs with only a marginal change in structural arrangements. The unit cell parameters of ambient (t) and high pressure (m) phases can be related as am ∼ at, bm ∼ ct, and cm ∼ bt. The pressure evolution of the unit cell parameters indicates anisotropic compression with βa = βb ≥ βc in the tetragonal phase and becomes more anisotropic with βa ≪ βb < βc in the monoclinic phase. The pressure-volume equations of state of both phases have been obtained by second order Birch-Murnaghan equations of state, and the bulk moduli are 122 and 127 GPa for tetragonal and monoclinic phases, respectively. The temperature dependent unit cell parameters show nearly isotropic expansion, with marginally higher expansion along the c-axis compared to the a- and b-axes. The tetragonal to monoclinic phase transition occurs with a reduction of unit cell volume of about 1.1% while the reduction of unit cell volume up to 6 K is only about 0.6%. The fitting of temperature dependent unit cell volume by using the Einstein model of phonons indicates the Einstein temperature is about 266(18) K.

Structural stability of high entropy alloys under pressure and temperature

The stability of high-entropy alloys (HEAs) is a key issue before their selection for industrial applications. In this study, in-situ high-pressure and high-temperature synchrotron radiation X-ray diffraction experiments have been performed on three typical HEAs Ni20Co20Fe20Mn20Cr20, Hf25Nb25Zr25Ti25, and Re25Ru25Co25Fe25 (at. %), having face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close-packed (hcp) crystal structures, respectively, up to the pressure of ∼80 GPa and temperature of ∼1262 K. Under the extreme conditions of the pressure and temperature, all three studied HEAs remain stable up to the maximum pressure and temperatures achieved. For these three types of studied HEAs, the pressure-dependence of the volume can be well described with the third order Birch-Murnaghan equation of state. The bulk modulus and its pressure derivative are found to be 88.3 GPa and 4 for bcc-Hf25Nb25Zr25Ti25, 193.9 GPa and 5.9 for fcc-Ni20Co20Fe20Mn20Cr20, and 304.6 GPa and 3.8 for hcp-Re25Ru25Co25Fe25 HEAs, respectively. The thermal expansion coefficient for the three studied HEAs is found to be in the order as follows: fcc-Ni20Co20Fe20Mn20Cr20 &amp;gt; bcc-Hf25Nb25Zr25Ti25 ≈ hcp-Re25Ru25Co25Fe25.

High pressure structure studies of 6H-SrIrO3 and the octahedral tilting in 3C-SrIrO3 towards a post-perovskite

The high pressure behaviors of the two perovskite structures (hexagonal 6H-SrIrO3 and orthorhombic 3C-SrIrO3) have been studied in diamond anvil cells to 43 and 60GPa, respectively, using synchrotron powder X-ray diffraction. 6H-SrIrO3 was first synthesized at ambient pressure and subsequently transformed into 3C-SrIrO3 in a large volume press at 8.8GPa and 1000°C. Both polymorphs were found to retain the initial symmetry up to the highest pressures measured, but in the case of 6H-SrIrO3, two anomalies were identified: a change in the axial compressibilities at 24GPa and a change in both the axial and volume compressibilities at 32GPa. Fitting a 3rd order Birch-Murnaghan equation of state to the obtained P-V data yielded bulk moduli of K0=151.5(12)GPa (fitted range 0<P<30GPa) for 6H-SrIrO3 and K0=187.1(9)GPa for 3C-SrIrO3. Analysis of the structural parameters for 6H-SrIrO3 aided by F-f plots suggests the anomalies are caused by changes in the compression mechanism. Comparison of the two polymorphs reveals that 6H-SrIrO3 becomes less compressible than 3C-SrIrO3 above 32GPa as a result of the mechanistic change, and a crossing of their P-V curves is avoided. For 3C-SrIrO3, analysis of the octahedral tilt angles shows that these increase monotonically from the ambient value of 7.23(6) to 23.0(2)° at 60GPa suggesting that a transition to a post-perovskite is approached.

A new high pressure and temperature equation of state of fcc cobalt

The high pressure and temperature equation of state of cobalt metal in the face-centered cubic phase was measured up to 57 GPa and 2400 K using the laser heated diamond anvil cell in conjunction with synchrotron X-ray diffraction. The measured region is bisected by a ferromagnetic to paramagnetic transition across the Curie temperature necessitating use of an equation of state that incorporates a 2nd order phase transition within its formalism. A third order Birch-Murnaghan equation of state with a Mie-Grüneisen-Debye thermal correction and a Hillert-Jarl magnetic correction is employed to describe the data above and below the Curie temperature. We find best fit parameters of V0 = 6.753 (fixed) cm3/mol, K0 = 196 (3) GPa, K′ = 4.7 (2), γ0 = 2.00 (11), q = 1.3 (5), and θ0 = 385 K (fixed).

Diamond–garnet geobarometry: The role of garnet compressibility and expansivity

We report P–V and T–V Equations of State (EoS) for synthetic single crystal pyrope (Py, Mg3Al2Si3O12) and the P–V EoS for synthetic single crystals of almandine (Alm, Fe3Al2Si3O12) as well as an intermediate composition (Py60Alm40) as measured by in-situ high-pressure and high-temperature X-ray diffraction experiments. The unit-cell volumes of the three samples were measured at room temperature and different pressures and up to about 8GPa in a diamond-anvil cell. The high-temperature experiment was carried out using a micro-furnace. The pressure-volume data were fitted to a third order Birch–Murnaghan EoS giving the following coefficients: V0=1506.15(16)Å3, KT0=163.7(1.7)GPa and K′=6.4(4) for pyrope, V0=1533.52(10)Å3, KT0=172.6(1.5)GPa and K′=5.8(5) for almandine and V0=1516.32(13)Å3, KT0=167.2(1.7)GPa and K′=5.6(5) for the intermediate Py60Alm40 composition. The unit-cell volume along the pyrope–almandine join changes linearly within the error of measurement indicating ideality in the volume of mixing behaviour for the solid solution. The first pressure derivative, K′, for all three garnets is similar within experimental uncertainty with an average value of K′=6.0(4). The thermal expansion parameters for end-member pyrope, as described using to the Kroll-type EoS, with the Einstein temperature, θE, fixed to 320K, are α(303K, 1bar)=2.543(5)∗10−5K−1 and V0=1504.64(4)Å3. The determined thermoelastic parameters were combined in a thermal-pressure type EoS (thermal-P type EoS) to calculate the entrapment pressures, Pe, for natural garnet inclusions in diamonds at mantle temperatures. A calculated pressure of 5.8GPa at T=1500K using our newly determined thermoelastic parameters appears to be more consistent with diamond-forming conditions compared to the higher pressures that range up to 6.8GPa at T=1500K, which are obtained using thermoelastic data in the literature.

Structural phase transition of ternary dielectric SmGdO3: Evidence from angle dispersive x-ray diffraction and Raman spectroscopic studies

High-pressure synchrotron based angle dispersive x-ray diffraction (ADXRD) studies were carried out on SmGdO3 (SGO) up to 25.7 GPa at room temperature. ADXRD results indicated a reversible pressure-induced phase transition from ambient monoclinic to hexagonal phase at ∼8.9 GPa. The observed pressure-volume data were fitted with the third order Birch-Murnaghan equation of state yielding zero pressure bulk modulus B0 = 132(22) and 177(9) GPa for monoclinic (B-type) and hexagonal (A-type) phases, respectively. Pressure dependent micro-Raman spectroscopy further confirmed the monoclinic to hexagonal phase transition at about 5.24 GPa. The mode Grüneisen parameters and pressure coefficients for different Raman modes corresponding to each individual phases of SGO were calculated using pressure dependent Raman mode analysis.

Multiferroic CuCrO2 under high pressure: In situ X-ray diffraction and Raman spectroscopic studies

The compression behavior of delafossite compound CuCrO2 has been investigated by in situ x-ray diffraction (XRD) and Raman spectroscopic measurements up to 23.2 and 34 GPa, respectively. X-ray diffraction data show the stability of ambient rhombohedral structure up to ∼23 GPa. Material shows large anisotropy in axial compression with c-axis compressibility, κc = 1.26 × 10−3(1) GPa−1 and a-axis compressibility, κa = 8.90 × 10−3(6) GPa−1. Our XRD data show an irreversible broadening of diffraction peaks. Pressure volume data when fitted to 3rd order Birch-Murnaghan equation of state give the value of bulk modulus, B0 = 156.7(2.8) GPa with its pressure derivative, B0′ as 5.3(0.5). All the observed vibrational modes in Raman measurements show hardening with pressure. Appearance of a new mode at ∼24 GPa indicates the structural phase transition in the compound. Our XRD and Raman results indicate that CuCrO2 may be transforming to an ordered rocksalt type structure under compression.

Petalite Under Pressure

The lithium aluminosilicate mineral petalite (LiAlSi4O10) has been studied using high-pressure single-crystal X-ray diffraction up to 5 GPa. Petalite is a layered silicate mineral. The layers comprise puckered double-sheets of corner-sharing SiO4tetrahedra. Corner-sharing AlO4tetrahedra bridge neighboring layers and complete the 3D architecture. The charge is balanced by lithium cations that reside within channels that propagate through the structure. Petalite undergoes two pressure-induced phase transitions at ca. 1.5 and 2.5 GPa. The first of these transforms the low-pressure α-phase of petalite (P2/c) to an intermediate β′ phase that then fully converts to the high-pressure β phase at ca. 2.5 GPa. The α→β transition is isomorphic with a commensurate modulation that triples the unit cell volume. Measurement of the unit cell parameters of petalite as a function of pressure, and fitting of the data with 3rd order Birch-Murnaghan equations of state, has provided revised elastic constants for petalite. The bulk moduli of the α- and β-phases are 49(1) and 35(3) GPa, respectively. These values indicate that petalite is one of the most compressible lithium aluminosilicate minerals. The α-phase structure has been refined at five different pressures, revealing a compression mechanism that is driven by the rigid body movement of the Si2O7units from which the silicate double-layers are constructed. The structure of the β′ phase was not determined. The structure of the β phase was determined at 2.71 GPa. Although the fundamental structural features of petalite are retained in the α → β phase transition, subtle alterations occur in the internal structure of the silicate double-layers.

Experimental verification of the high pressure crystal structures in NH3BH3.

A detailed high-pressure study on NH3BH3 has been carried out using in situ synchrotron X-ray diffraction (XRD) and Raman scattering with a diamond anvil cell up to 20 and 33 GPa, respectively. The Rietveld refinement based on the XRD pattern and analysis of Raman data indicate two first-order phase transitions from the ambient pressure I4 mm structure (α-NH3BH3) to a high pressure Cmc21 phase (β-NH3BH3) at 2.14 GPa, and further into a monoclinic P21 (Z = 2) phase (γ-NH3BH3) at 9.67 GPa. Fitting the measured volumetric compression data to the third order Birch-Murnaghan equation of state reveals a bulk modulus of B0 = 9.9 ± 0.5 and 17.0 ± 3.0 GPa (with fixed B0 (') = 4) for the β-NH3BH3 below and above 5 GPa, respectively. Still, with the splitting of the NBH rock mode in Raman experiment, it is concluded that a second-order isostructural phase transition occurs at 5 GPa. By analyzing the dihydrogen bonding framework, the origin of the isostructural phase transition is attributed to the number of dihydrogen bondings per molecule in the Cmc21 phase increasing from 12 to 14 at 5 GPa.