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Solid solutions, (Eu1−x La x )2O3 (0 ≤ x ≤ 1), of the rare earth sesquioxides Eu2O3 and La2O3 have been prepared by a simple soft chemistry approach. The composition and morphology of the as-synthesized oxides have been characterized using energy-dispersive spectroscopy and scanning electron microscopy. The particles are of irregular shape and submicrometre size. In order to understand the structural evolution as a function of composition, angle-dispersive X-ray diffraction measurements have been carried out and the structural parameters have been obtained through Rietveld refinement. A structural phase transition from the cubic (C-type) to the monoclinic (B-type) structure and subsequently to the hexagonal (A-type) structure was observed with an increasing substitution of La. A detailed analysis of the transition boundaries in terms of the average cationic radius, R RE, shows that the onset of the C → B transition is at R RE = 0.980 Å, whereas the B → A transition occurs at R RE = 1.025 Å. A biphasic region of cubic and monoclinic structures is observed for 0.2 ≤ x ≤ 0.4 and one of monoclinic and hexagonal structures is observed for 0.5 ≤ x ≤ 0.6. The microstrain induced by the difference in size of the rare earth cations introduces a substitutional disorder in the crystal structure, which is a plausible cause of the observed phase transitions in these oxides.

Anisotropic behaviour of the A1g vibrational mode of MoS2 at the edges has been reported. Polarized Raman spectroscopy is used to identify edges, and decipher angles between them have been discussed. The aberrant appearance of the A1g mode at the edges has been attributed to breaking of back-scattering geometry. The direction-dependent intensity of the A1g mode is used to characterize the angle between edges, thus finding the chirality of the edges.

The intriguing functional nature of ceramics containing rare earth sesquioxide (RES) is associated with the type of polymorphic structure they crystallize into. They prefer to be in the cubic, monoclinic or hexagonal structure in the increasing order of cation size, RRE. Since the functional properties of these ceramics varies with RRE, temperature and pressure, a systematic investigation delineating the cation size effect is indispensable. In the present work we report the structural stability and compressibility behaviour of the RES ceramics, (Eu1−xLax)2O3, of RESs with dissimilar structure and significant difference in cationic radii. The selected compositions of (Eu1−xLax)2O3 have been studied using the in-situ high pressure synchrotron X-ray diffraction and the structural parameters obtained through Rietveld refinement. The cubic structure, which is stable for 0.95 Å $$\le$$≤ RRE$$<0.98 Å at ambient temperature and pressure (ATP), prefers a cubic to hexagonal transition at high pressures. The biphasic region of cubic and monoclinic structure, which is stable for 0.98 Å $$\le$$≤ RRE$$<1.025 Å at ATP, prefers a cubic/monoclinic to hexagonal transition at high pressures. Further, in the biphasic region of monoclinic and hexagonal structure, observed for 1.025 Å $$\le$$≤RRE$$<1.055 Å, the monoclinic phase is found to be progressing towards the hexagonal phase with increasing pressure. The pure hexagonal phase obtained for 1.055 Å $$\le$$≤ RRE$$\le$$≤ 1.10 Å is found to be structurally stable at high pressures. The bulk moduli are obtained from the Birch–Murnaghan equation of state fit to the compressibility data and its dependance on the cation size is discussed. The microstrain induced by the difference in cation size causes an internal pressure in the crystal structure leading to a reduction in the bulk modulus of $$\textit{x}=0.2$$x=0.2 and 0.6. A pressure–concentration (P–x) phase diagram upto a pressure of 25 GPa is constructed for (Eu1−xLax)2O3. This would provide an insight to the fundamental and technological aspects of these materials and the RESs in general.

Sm6UO12 has been synthesized by urea-combustion method and characterized using synchrotron x-ray source. The compound is found to be in single phase crystallizing in rhombohedral lattice with lattice parameters a = 10.153 A and c = 9.625 A. High-pressure experiment at ambient temperature reveals the lattice to be stable up to 27.0 GPa. The bulk modulus of the compound is found to be 157.0 GPa. At higher pressures, reflections broaden out as a result of disorder in the system, leading to amorphization at further higher pressures. High pressure and high temperature (HP-HT) x-ray diffraction studies have been carried out up to 6.0 GPa and 673 K. A softening is seen in the material at HP-HT resulting in lower bulk modulus at high temperature. Volume thermal expansion coefficients are found to be 22 × 10−6 K−1 and 17.7 × 10−6 K−1 in the temperature range 298 K–473 K and 298 K-673 K, respectively.

High-pressure X-ray diffraction studies on MgCu2 type phase of UOs2 have been carried out up to 36 GPa. The compound remains in its parent phase up to 12 GPa with bulk modulus 261 GPa. Beyond 12 GPa, a first order phase transformation is observed. The phase transition is sluggish in nature and completes at 20.1 GPa. The high-pressure phase is found to be a hexagonal cell with lattice parameters a = 3.013 A and c = 4.267 A. Charge density calculations show that uranium tetrahedra in the lattice are responsible for the phase transition. The density of state plots at Fermi level, wherein a pseudogap originates at high pressures, confirms the existence of a high pressure phase and uranium site is found to contribute significantly to such changes in density of state. The retrievable high pressure phase is found to be the least compressible among uranium intermetallic compounds with bulk modulus of 366 GPa.

First principles calculations on Sc 2 CrB 6 crystal revealed the existence of Dirac Cone in its electronic structure for the first time. The compound has an interesting crystal structure with 2D boron layers with hexagonal, pentagonal, heptagonal boron rings sandwiched between the metal layers. Calculations show that the compound has a mixed covalent and metallic nature of the bonding. Also, ground state ferromagnetic behaviour is predicted for the metallic bonded Sc 2 CrB 6 . Here the magnetization arises mostly from the Cr atom with enhanced contribution from the remaining elements. Interestingly, band crossing is seen at the Fermi level with spin polarization indicating the existence of Dirac cone in Sc 2 CrB 6 . Apart from this, high-pressure calculations up to about 67 GPa reveal a pressure-induced transition from ferromagnetic state to non-magnetic state around 30.3 GPa.

Angle dispersive x-ray diffraction measurements on thorium-praseodymium mixed oxides, (Th 1- y Pr y )O 2- x [ y = 0, 0.15, 0.4 and 1], have been carried out up to a pressure of around 36 GPa at ambient temperature. A structural transformation from cubic fluorite type to an orthorhombic α-PbCl 2 type structure has been observed for y = 0, 0.4 and 1, at a pressure of P t = 28.8 GPa, 28.8 GPa and 30.1 GPa respectively. No such phase transition could be observed for y = 0.15 oxide. This is attributed to the nano-crystallinity of the sample. In support of this, studies on nano-crystalline ThO 2 shows no phase transition up to 42 GPa, indicating the increased structural stability with a decrease in crystallite size. The observed discontinuity in the pressure volume curve of nano ThO 2 indicates a possible isostructural transition around 15 GPa. The choice of the equation of state to describe the compression data and derive the bulk moduli of these oxides is discussed. An increase in the bulk modulus with ThO 2 substitution was observed for the bulk oxides. However, the nano-crystalline mixed oxides (y = 0.15) showed a reduction in the bulk modulus compared to their bulk counterpart.

Structural stability and phase transitions in mixed rare earth sesquioxides (Eu 1− x Ho x ) 2 O 3 crystallizing in the cubic bixbyite structure were investigated under high pressure using angle dispersive X-ray diffraction technique. Studies on various compositions of the mixed oxide show that when the average cationic radius, R RE (where, R RE = x R Ho +(1− x )R Eu ) is equivalent to or below 0.9164 A, the system undergoes a cubic to monoclinic transition as a function of pressure, whereas, when R RE equivalent to or above 0.9220 A it is observed to prefer a transition from cubic to hexagonal structure. Based on our detailed investigations, a pressure-concentration (P− x ) phase diagram for (Eu 1− x Ho x ) 2 O 3 up to a pressure of 15 GPa is constructed. The bulk moduli for the parent and high pressure phases are calculated from the Birch-Murnaghan equation of state fit to the experimental pressure volume data and are reported. Structural analysis based on refinement has revealed that, an increasing structural rigidity of the cubic phase with decreasing R RE leads to an increase in the transition pressure and bulk moduli except for 0.4 ≤ x ≤ 0.6. The observed significant reduction in bulk moduli and transition pressure for 0.4 ≤ x ≤ 0.6 is due to the increased micro strain/internal pressure developed upon doping.

The evolution of crystal structure and the ground state properties of Fe2B and FeB have been studied by performing high pressure X-ray diffraction up to a pressure of ∼24 GPa and temperature dependent (4.2–300 K range) high-pressure resistivity measurements up to ∼ 2 GPa. While a pressure induced reversible structural phase transition from tetragonal to orthorhombic structure is observed at ∼6.3 GPa in Fe2B, FeB has been found to be stable in its orthorhombic phase up to the pressure of 24 GPa. In the case of Fe2B, both parent and daughter phases coexist beyond the transition pressure. The bulk modulus of FeB and Fe2B (tetragonal) have been found to be 248 GPa and 235 GPa respectively. First principle electronic structure calculations have been performed using the present experimental inputs and the calculated ground state properties agree quite well with the major findings of the experiments. Debye temperature extracted from the analysis of low temperature resistivity data is observed to decrease with pressure indicating softening of phonons in both the systems.

Monoclinic Eu 2 O 3 was investigated in a Mao-Bell type diamond anvil cell using angle dispersive x-ray diffraction up to a pressure of 26 GPa. Pressure induced structural phase transition from monoclinic to hexagonal phase was observed at 4.3 GPa with 2% volume collapse. Birch –Murnaghan equation of state fit to the pressure volume data yielded a bulk modulus of 159(9) GPa and 165(6) GPa for the monoclinic and hexagonal phases respectively. Equation of state fitting to the structural parameters yielded an axial compressibility of β a > β c > β b for the parent monoclinic phase, showing the least compressibility along b axis. Contrary to the available reports, an anomalous lattice compressibility behavior is observed for the high pressure hexagonal phase, characterized by pronounced hardening of a axis above 15 GPa. The observed incompressible nature of the hexagonal a axis in the pressure range 15–25 GPa is found to be compensated by doubling the compressibility along the c axis.

UIr 2 was investigated using high pressure x-ray diffraction technique up to 55 GPa. The compound remains stable in its cubic structure (Space Group: Fd- 3 m ) up to 40 GPa. Above 40 GPa x-ray diffraction pattern indicates the emergence of a yet unidentified phase. The large structural stability of cubic MgCu 2 type structure, predicted empirically, is not exhibited in this compound. Zero pressure bulk modulus and its pressure derivative are found to be 284 GPa and 7.9 respectively. The high bulk modulus value obtained is indicative of itinerant nature of 5- f electrons which is also supported by Hill limit criteria. Electronic structure calculations indicate that uranium tetrahedral network is less rigid as compared to iridium tetrahedral network. Valley in density of states at Fermi level deepens with pressure vindicating observation of a more stable phase at high pressure.

High pressure-high temperature (HPHT) synthesis of a new phase of chromium carbide has been carried out using laser heated diamond anvil cell (LHDAC) at about 5 ​GPa and 1500 ​K. The reproducibility of the formation has been confirmed. The HPHT synthesized phase was retrieved to ambient condition and characterized by X-ray diffraction and micro-Raman spectroscopy. Various candidate structures obtained from experimental analysis have been evaluated by carrying out ab initio electronic structure calculations and the phase was identified to be an off-stoichiometric CrC which crystallizes in hexagonal lattice with space group (S. G.) P−6m2 (187). The lattice parameters of CrC were found out to be a ​= ​2.7496(2) A, c ​= ​8.7010(4) A and the bulk modulus was estimated to be 269 ​GPa. Study of density of states, band structure and charge density distribution indicate that CrC is metallic owing to the Cr-Cr metal bonds and there exists a p-d hybridization between the carbon and chromium electrons. The Cr-C bonds show covalent nature.

The compound was synthesized using arc-melting technique and high-pressure X-ray diffraction studies were carried out up to 25 GPa. It is found to remain in its parent AuCu3 type structure up to the highest pressure studied. Birch-Murnaghan equation of state fit of pressure-volume data gives the bulk modulus to be 262 GPa while the bulk modulus of the similar isoelectronic compound – URh3 is 133 GPa. Rh and Ir being isoelectronic transition metals, their compounds with uranium metal show a dramatic change in the compressibility of the material. First principle computations indicate a significantly large contribution of 5-f electrons of uranium in UIr3. A strong f-d hybridization in UIr3, as compared to URh3, results in high bulk modulus values of UIr3. The high structural stability of UIr3 is also predicted from the density of state plots as compared to URh3. Computed elastic constants of UIr3 indicate the mechanical stability of the lattice.

In this paper the structural parameters of mixed rare earth sesquioxides (Eu1-xHox)2O3 (0 ≤ x ≤ 1) is reported for the first time. The oxide samples are characterized by angle dispersive X-ray diffraction (ADXRD) and Raman scattering techniques. ADXRD measurements confirmed the formation of single phase in the cubic bixbyite structure with space group Ia-3. Complete miscibility of the two components is confirmed by the Vegard's law. A random distribution of the two cations in the two rare earth (RE) ion sites is observed. Symmetry modes are assigned for the observed 13 out of 22 expected Raman bands and the dependency of average cationic radii, RRE, is discussed. A low frequency band shows an anomalous mode softening with decrease in Eu content. This mode disappears above x = 0.8, which coincides with a distinct slope change observed in the RE(24d) positional coordinate estimated from refined x-ray diffraction data.

In this paper, results obtained from high pressure-high temperature X-ray diffraction study of Dy 6 UO 12 are reported. X-ray diffraction (XRD) studies at ambient temperature on Dy 6 UO 12 reveals that the rhombohedral structure is stable up to 21.6 GPa. Beyond 21.6 GPa the peaks broaden out substantially indicating emergence of disorder in the system. Bulk modulus and its pressure derivative is 144 GPa and 7.0 respectively. High Pressure and High Temperature (HP-HT) XRD studies up to ∼ 11 GPa and ∼673 K was carried out using novel combination of membrane cell DAC coupled to a high flux micro-focus X-ray generator. At ambient pressure, thermal expansion coefficient comes out to be 14.5 × 10 −6 K −1 at 400 K. Further, at 1 GPa and 2.6 GPa the thermal expansion coefficients are 21.4 × 10 −6 K −1 and 32 × 10 −6 K −1 respectively, in the temperature range ∼293–673 K. The thermal expansion coefficient shows an increasing trend with pressure.

We report the structural stability and compressibility behavior of 5B transition metal (TM) monoborides VB and TaB under high pressure. For the study, VB was synthesized in single phase by arc melting method followed by repeated annealing treatments and intermediate grinding; and TaB was procured. High pressure X-ray diffraction studies were carried out using synchrotron X-ray radiation up to 37.5 and 40.5 GPa for VB and TaB, respectively. The ambient orthorhombic lattices were stable for the monoborides in the pressure range studied. The bulk moduli were estimated to be 301(5) and 367(4) GPa, respectively. The axial compressibility was highest along the a direction for both VB and TaB due to predominant presence of metallic TM–TM bonds and absence of covalent B–B bonds along this direction. However, the least compressibility occurred along the c axis for TaB, and along the b axis for VB. The experimental results were substantiated with ab initio electronic structure calculations and the electronic and elastic properties were studied. Both VB and TaB were metallic due to the metallic nature of TM–TM bonds. Study of the density of states (DOS) and charge density distribution revealed that the TM–B bonds exhibited both ionic and covalent nature. The charge transfer from Ta to B was greater than that from V to B. The higher bulk modulus of TaB can be attributed to the stronger B–B covalent bond and enhanced charge transfer between Ta and B. Presence of a pseudogap at Fermi level in both their DOS plots inferred high structural stability. The cause of pseudogap formation was discussed. The study of elastic properties showed that both VB and TaB were mechanically stable in the pressure range studied. Small Pugh and Poisson's ratio indicated that VB and TaB were not ductile or malleable in nature.

In this paper, we report for the first time formation of novel manganese monocarbide (MnC) using laser-heated diamond anvil cell (LHDAC). The synthesis was carried out at high pressure-high temperature (HPHT) and subsequently quenched to ambient condition. The formation and reproducibility have been confirmed in the pressure range of 4.7 to 9.2 GPa. Employing contribution of different probes viz.X-ray diffraction (XRD), selected area electron diffraction (SAED), and ab initio electronic structure calculation, the structure of MnC was found to be ZnS type i.e. a cubic lattice with a = 4.4294(2) A. The bulk modulus has been determined to be 170(5) GPa from in situ high-pressure X-ray diffraction (HPXRD). Hardness of ZnS type MnC is estimated from an empirical relation to be about 40 GPa, making it a potential superhard material.

In the present work, we report a combined experiment and theoretical study on high pressure structural stability of Al2Cu type- Mo2B up to ∼40 GPa. Experiments using rotating anode x-ray source indicate that the ground state tetragonal structure of Mo2B (space group I4/mcm) to be stable up to the highest pressure studied. In addition, the experimental results were complemented by first principles density functional calculations within the projector augmented wave method. The calculated structural parameters are in excellent agreement with present experiments as well as previous reports. The estimated bulk modulus using Birch-Murnaghan equation of state from both experiment (302 GPa) and theory (310 GPa) reveals the ultra-incompressible nature of Mo2B. The much higher bulk modulus of this semi boride as compared to elemental Mo is discussed in terms of Mo-Mo bond distance in their crystal structures. The calculated electronic structure of Mo2B shows a strong Fermi surface nesting along P-P direction, which might also trigger lattice instability and is addressed in detail.

High-pressure structural stability studies have been carried out on tungsten boride W2B5 up to maximum pressure of 36 GPa using a Mao-Bell diamond-anvil cell at beamline BR-12 of the ELETTRA synchrotron facility (λ = 0.68881 A). The hexagonal phase (S.G:P63/mmc) of W2B5 is stable up to the maximum pressure studied. The bulk modulus is estimated to be ~347 GPa using the Birch–Murnaghan equation of state. The variation of lattice parameters and bond lengths B–B and W–B have been studied and the c-axis is seen to be marginally more compressible than the a-axis.

High pressure structural stability studies were carried out on YGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ~35 GPa using both laboratory based rotating anode and synchrotron X-ray sources. An isostructural transition with reduced c/a ratio, was observed at ~6 GPa and above ~17.5 GPa, the compound transformed to orthorhombic structure. Bulk modulus B0 for the parent and high pressure phases were estimated using Birch–Murnaghan and modified Birch–Murnaghan equation of state. Electronic structure calculations based on projector augmented wave method confirms the experimentally observed two high pressure structural transitions. The calculations also reveal that the ‘Ga’ networks remains as two dimensional in the high pressure isostructural phase, whereas the orthorhombic phase involves three dimensional networks of ‘Ga’ atoms interconnected by strong covalent bonds.

Raman spectroscopic and x-ray diffraction measurements on NaZr2(PO4)3 were carried out up to 30 GPa at close intervals of pressure, revealing two structural phase transformations around 5 and 6.6 GPa. The second phase at 5.4 GPa is indexed to R3 space group similar to that of RbTi2(PO4)3. Bulk modulus decreases abruptly from 53 GPa (B′=4) to 36 GPa (B′=4) in the second phase above 5 GPa. The structure of the phase III at 8.2 GPa is indexed as orthorhombic similar to the case of high temperature phase of monoclinic LiZr2(PO4)3. Bulk modulus of this phase III is found to be 65 GPa (B′=4), which is higher than that of the ambient phase. In high pressure Raman studies, modes corresponding to 72 and 112 cm−1 soften in the ambient phase whereas around 5 GPa, the ones at 60, 105, 125 and 190 cm−1 soften with pressure contributing negatively to overall thermal expansion.

UZr2 has been synthesized using the arc-melting technique. At ambient the compound is found to crystallize in a modified C32, AlB2 type structure, called δ-structure and it remains stable up to 20 GPa. However, an anomalous decrease of c/a ratio has been observed in different pressure intervals at ambient temperature. At elevated temperature c/a ratio, shows a marginal increase with pressure. The bulk modulus and its pressure derivative at ambient temperature are found to be 108.3 GPa and 5.0, respectively. High-pressure and high-temperature studies reveal a softening in the material at higher temperatures. The thermal expansion coefficients are found to decrease from 4.6 × 10−5 K−1 at 1.2 GPa to 2.7 × 10−5 K−1 at 6.6 GPa in the temperature range 300 K–473 K.

The phase stability of nanocrystalline Pr2O3 has been investigated under pressure by in-situ high pressure X-ray diffraction using Mao-Bell type diamond anvil cell. The ambient structure and phase of the praseodymium oxide have been resolved unambiguously using x-ray diffraction, SEM and TEM techniques. Under the action of pressure the cubic phase of the system is retained up to 15 GPa. This is unusual as other isostructural rare earth oxides show structural transformations even at lower pressures. From the best fit to the P–V data with the Murnaghan equation of state yields a bulk modulus of 171 GPa.

The compound Ni2B stabilizes in tetragonal structure at NTP with space group I4/mcm (No. 140), and lattice parameters; a= 0.499 nm and c=0.424 nm. The B–B bond distance is of about 0.212 nm and the B–B interaction is expected to be very small. High pressure x-ray diffraction study on this compound was carried out up to 28 GPa. It was seen that the compound remained stable throughout out this pressure range and the B–B distance decreased by about ~4%. Electronic structure calculations were carried out using the Full Potential Linear Augmented Plane Wave (FP-LAPW) method to understand its structural stability with respect to pressure. It is seen that the Fermi level lies on a plateau region with very low density of states, despite the compound being metallic in nature. Further, the computations at reduced volumes revealed that the density of states at EF almost remained constant with pressure, which has been attributed to its structural stability under pressure.

High pressure X-ray diffraction studies on rare-earth gallide LaGa was carried up to a pressure of ∼28 GPa using synchrotron, as well as rotating anode X-ray source in an angle dispersive mode. LaGa exhibits CrB (B33)-type orthorhombic structure (space group Cmcm) at ambient pressure. It undergoes a reversible structural phase transition from orthorhombic to tetragonal structure (Space. Group P4/mmm) at ∼5 GPa. Both the phases coexist up to the highest pressure studied. The transition to tetragonal phase has implications in predicting the new intermediate phase in the generally observed B1 to B2 transition sequence in AB type of rare-earth intermetallics. This investigation reveals that the LnX-type compounds may exhibit the structural sequence B1→ B33→ P4/mmm→ B2 under pressure. Considering the variation of lattice parameters a, b, c and also various bond lengths with pressure, the B33 to P4/mmm transition in LaGa may be displacive in nature. The zero pressure bulk modulus and its derivative for parent p...

ThAl 2 and ThGa 2 exhibit quite different structural and compressibility behaviour under pressure: whereas ThAl 2 undergoes a series of structural transitions under pressure up to 30 GPa, ThGa 2 remains stable up to 62 GPa. The relative structural stability of ThAl 2 and ThGa 2 have been compared and contrasted in terms of their electronic structure at different pressures. Their electronic structure calculations were carried out using the Full Potential Linear Augmented Plane Wave (FP-LAPW) method with spin–orbit interaction as a function of reduced volume. In ThAl 2 the f -bands lie about 2.5 eV above Fermi level ( E F ) and are populated with mostly the dp hybridised states. The E F lies on a shallow valley with very low density of states (DOS). In ThGa 2 also, the f -bands lie about 2.3 eV above E F , but the E F lies on a steep valley with very low DOS. However, on closer examination the change in the density of states on the bonding and antibonding sides of the Fermi level seems to be much steeper than that in ThAl 2 . Further, the computations for reduced volume revealed that the bands around E F and hence the density of states in both the systems responded very differently to pressure. This is discussed in the context of their structural stability under pressure.

The physical properties of the extraterrestrial materials, meteorites, though important but are not well studied. We present here structural and electrical properties of Lohawat meteorite under high-pressure up to 9 GPa, using in situ high-pressure powder X-ray diffraction, 57 Fe Mossbauer spectroscopic and electrical conductivity techniques. The studied meteorite sample fell at Lohawat (Rajasthan) India is classified as a Howardite based on geochemical and mineralogical studies. Electron Probe Microanalysis (EPMA) indicates that Lohawat meteorite composed mainly of orthopyroxene (Fs 0.65 En 0.34 Wo 0.09 ) and plagioclase (An .946 Ab .053 ). All the three experimental studies independently confirm pressure-induced structural changes of Lohawat meteorite at ∼2.8 GPa and at 5.6 GPa representing irreversible amorphization and reversible crystallization respectively. The observed experimental results are explained based on the high-pressure behavior of orthopyroxene and anorthite, which are dominant mineral phases of the Lohawat meteorite. The observation of structural phase transition at lower pressure for the meteorite sample compared to their analogous pure end member mineral indicates that the meteorite sample preserved a residual stress in the sample. The present study, therefore, may help in estimating the residual peak metamorphic shock pressure experienced by the parent body transforming into differentiated achondrite meteorite.

URh 3 stabilizes in the cubic AuCu 3 type structure at normal temperature and pressure. High-pressure angle-dispersive X-ray diffraction experiments were performed on URh 3 up to 25 GPa using a diamond-anvil cell. URh 3 remains in its cubic AuCu 3 type structure up to the maximum pressure studied. The Birch–Murnaghan equation of state fit to the P – V data yields the bulk modulus to be 133 GPa. The Villars structural stability map gives a clue of a possible high pressure phase transition to a Ni 3 Sn type structure. The electronic structure calculations were carried out for both the ambient AuCu 3 type cubic phase and the expected Ni 3 Sn type hexagonal high pressure phase. However, the total energy curves of these two structures do not intersect even at pressure as high as 360 GPa, removing the possibility of transition to Ni 3 Sn type structure.

Raman and infrared spectroscopic measurements were carried out on 40 mol% Fe 2 O 3 :60 mol% P 2 O 5 iron phosphate glass samples to study the effect of increasing external pressure on the local structure of both anhydrous and water trapped glasses. In-situ high‐pressure Raman measurements, done up to 24.2 GPa on anhydrous samples, revealed hardening of the phonon modes accompanied by a loss of Raman intensity and ultimately leading to complete smearing of modes at very high pressures. These changes were found to be reversible upon quenching the sample back to ambient pressure. Raman and infrared spectroscopy of pressure quenched IPG samples containing trace amounts of water have revealed definite signatures of devitrification at room temperature.

To understand and compare the structural stability of UAl2 and UGa2 at room pressure, calculation of electronic structure using full potential linear augmented plane wave (FP-LAPW) method have been performed. The results of calculations with and without including spin–orbit interaction were compared and contrasted. In general, it has been observed that the density of states at the Fermi level EF increased when the calculations are performed with the inclusion of spin orbit interaction. The spin polarized calculations for UGa2 have shown that the Fermi level EF is positioned near a sharp peak in the density of states curve on the antibonding side. On the other hand, in the case of UAl2, the EF lies on the slope of a peak in the density of states curve in the bonding side. The position of the Fermi level and the structural stability under high pressure are correlated.

The f-electron based intermetallic compounds (f-IMCs) have unique nature of the f-electrons which are tuned in a controlled manner by changing the interatomic distances by applying external or chemical pressure. Among the f-IMCs, uranium intermetallics (U-IMCs) hold a very special position because of their technological importance. In this paper the phase transitions and structural stability investigations under pressure on uranium binary intermetallics are reviewed. The importance of magnetism in U-IMCs is briefly discussed. Under pressure, the UX type (where X could be a transition metal or a chalcogen) compounds generally transform from B1 phase to B2 phase, whereas some of the UX2 compounds display series of structural transitions. UX3 compounds show large range of stability under pressure. Our own experiments and calculations on few typical uranium compounds are given as examples. Wherever needed, the electronic structure calculations are discussed and correlated to the observed behavior of the compounds under pressure. The importance and the relevance of the Hill plot are examined for uranium binaries. The need to build structural stability maps for predicting high pressure phases is emphasized.

Determination of melting temperature by spectroradiometric technique, aided by visual observation of onset of melting by track formation method employing an infrared CO2 laser, was carried out. Melting temperatures of SrTiO3 and MgO single crystals and silicon wafer samples obtained by this novel technique are reported. Melting temperatures obtained by this method are within 2% of the reported standard values. These values are used to validate temperature measurements carried out in laser heated diamond anvil cell experiments.

High-pressure angle-dispersive X-ray diffraction experiments were performed on UGa 3 up to 30 GPa within a diamond-anvil cell. UGa 3 remains in its cubic AuCu 3 type structure up to the maximum pressure studied and does not show any structural phase transition. To understand the structural stability of UGa 3, band structure calculations were performed as a function of reduced volume using the full-potential linear augmented plane wave (FP-LAPW) method. The results show that the Fermi level coincides with a deep valley in the density of states (DOS) curve in the antiferromagnetic state, whereas it lies near a valley (towards the bonding side) in the nonmagnetic state. At high pressures, the DOS near EF does not show much variation in both the cases. The experimental and theoretical equation of state, bulk modulus, and its pressure derivative values are also reported. The pressure dependence of magnetic moment shows a linear decrease at the rate of dμ/dP = -0.027 μ B / GPa .

High-pressure structural stability studies have been carried out on LaGa2 (AlB2 type structure at NTP, space group P6/mmm) up to a pressure of ∼60 GPa. A structural phase transition was initiated at a pressure of ∼12 GPa and a complete transformation to the daughter phase occurred at ∼28 GPa. The high-pressure phase was identified to be orthorhombic with lattice parameters: a = 12.79, b = 5.09 and c = 5.28 A at 60 GPa. The bulk modulus B 0 and its derivative for the parent phase were found to be 100 ± 16 GPa and 5 ± 4, respectively. The bulk modulus Br and its derivative for the high-pressure phase were found to be 166 ± 18 GPa and 17 ± 4, respectively.

X-ray diffraction studies on mixed rare-earth oxides Pr2O3 and Sm2O3 quenched at various temperatures were carried out and their phase relationship was analyzed. Mixtures of Pr2O3 and Sm2O3 at different compositions were heated at various temperatures in the range 500–1200 °C and then air quenched. The structures of these quenched samples were analyzed using X-ray diffraction and Raman spectroscopy. Pure Pr2O3 and Sm2O3 exhibit C-type cubic phase at NTP. While Pr2O3 retains its cubic phase up to 1200 °C, the Sm2O3 transformed to B-type monoclinic phase above 800 °C. However, their mixtures when heated at different temperatures showed interesting structural behaviour. For the mixture up to 40 wt.% of Sm2O3 in Pr2O3, no significant change was observed at lower temperatures (∼800 °C), and the C-type cubic phase was observed. As the ratio was gradually increased above 40%, a diphasic region in the range 50–70% was observed, with a combination of both C-type cubic and B-type monoclinic phases. When the temperature was raised to 1200 °C, the 80% of Sm2O3 shows a complete structural transition from cubic to monoclinic phase. Based on the analysis of our results various crystal structures and stability regimes of mixed oxide have been reported.

When a solid is subjected to external pressure, it can undergo either structural transformation or remain stable in its parent structure. The sequence of structural transformations, when mapped for similar materials, viz., isostructural, isoelectronic and so on, can be used to create a map showing the evolution of structures under pressure for such materials. Such maps are useful in predicting high pressure phases. The structural transitions and the stability of materials as a function of pressure are intricately connected to their electronic structure. Many a times it is advantageous to know the stability of the material under pressure just by calculating its electronic structure. This can be accomplished only if several homologues materials are studied and the stability criteria arrived at by correlating their electronic structure with their structural stability under pressure. Further, as a function of pressure, the electronic structure changes can lead to enhancement in certain desired electronic, physical or mechanical properties. Several examples are known, wherein, pressure tuning of the band structure leads to improved properties. In this paper, we have discussed the above mentioned areas and presented a perspective of the above using the results of our own studies on f-electron based intermetallics (f-IMCs).

Zinc cyanide is an interesting negative thermal expansion (NTE) material exhibiting cubic structure at ambient pressure and temperature. We have investigated the structural stability of zinc cyanide under high pressure up to 5.2 GPa by performing X-ray powder diffraction in a diamond anvil cell. Under very low pressure of about 0.6 GPa, the diffraction peaks drastically reduce in intensity, indicating possible onset of disorder in the structure. In this paper, its high pressure structural and compressibility behaviour, bulk modulus and the pressure derivative of bulk modulus are reported.

The large structural stability regime of LaAl 2 and LaAl 3 as a function of pressure is investigated by the band structure calculations using the FP-LAPW method. An earlier experimental study has revealed that there is no structural phase transition at ∼35 and ∼30 GPa for LaAl 2 and LaAl 3 , respectively. Our calculations indicate that in the density of states curve of LaAl 2 , the Fermi level ( E F ) lies in a slope between bonding maxima and antibonding minima. At high pressures the E F moves slightly towards the valley, but this shifting does not affect its structural stability. In LaAl 3 , the E F falls in a flat region in the density of states and does not move even up to 33 GPa. The band dispersion curves for both the compounds show movement of bands under the influence of pressure. Some of them cross the Fermi level leading to so called Lifshitz transitions. However, it is seen that these electronic changes do not manifest into any volume anomaly in LaAl 3 under pressure. Our study clearly shows that the density of states behavior for LaAl 2 and LaAl 3 satisfies the Yamashita–Asano criterion for structural stability. The theoretical equations of state, bulk modulus and its pressure derivative values are compared with the experimental values.