The phase diagram for praseodymium (Pr) has been determined for pressures between 5 and 60 GPa and temperatures between 295 and 830 K using both in situ energy- and angle-dispersive x-ray diffraction with externally heated diamond-anvil cells. Mineral oil and argon were alternatively used as pressure media in order to compare conflicting results in the literature and to ensure the validity of mineral oil as an inert medium. Evidence for the presence of an, as yet, unidentified phase (denoted Pr-VI) above 675 K has been observed, whereas no compelling evidence has been observed for the existence of the recently reported monoclinic phase (Pr-V). The new constraints of the phase diagram, therefore, suggest that the phase transitions occur as Pr-I(dhcp)\ensuremath{\rightarrow}Pr-II(fcc)\ensuremath{\rightarrow}Pr-V$\stackrel{\ensuremath{\rightarrow}}{\mathrm{I}}$Pr-IV(\ensuremath{\alpha}-U) above approximately 700 K. Additionally, there is a Pr-III(distorted fcc), Pr-VI, and Pr-IV triple point at approximately 675 K and 23.8 GPa. Temperature-dependent equations of state have been determined, allowing the temperature-dependent volume collapse at the transition between Pr-III and Pr-IV to be calculated. We report a linear decrease of the volume collapse along the Pr-III to Pr-IV boundary with temperature, $\ensuremath{\Delta}V/V$ $(%)=16.235\ensuremath{-}0.0156[T(\mathrm{K})];$ the extrapolation indicates that the volume collapse should vanish well below the melting point. With the temperature-dependent equation of state data and new phase diagram we demonstrate that the volume collapse can be accounted for by a change in the multiplicity of Pr atoms as the f electrons go from localized to itinerant.

Phase diagram and equation of state of praseodymium at high pressures and temperatures

The lanthanide metal Pr has been studied by x-ray powder diffraction from 5 to 23 GPa with the aim of resolving the long-standing disagreements about the identity of the high-pressure phases, transitions, and structures of this element. We find that the distorted-fcc d-fcc phase observed above 7.4 GPa has the rhombohedral hR24 structure reported previously by Hamaya et al. and can definitely rule out the monoclinic structure proposed by other authors. At 13.7 GPa, the d-fcc phase starts to transform to another phase, Pr-VII, the structure of which is body-centered orthorhombic or a very slight distortion thereof. Pr-VII transforms to the -uranium phase at 20.55 GPa. In addition to identifying the phase transitions and structures, we have also resolved many of the disagreements between previous diffraction studies and show that almost all previous work is in agreement with the present study. The likely existence of the Pr-VII phase in a second lanthanide, Nd, suggests that it might be an additional member of the sequence of phases observed across the lanthanide elements.

/pdf/phase-transitions-in-praseodymium-up-to-23-gpa-an-x-ray-cqe1ji0nfk.pdf

Phase transitions in praseodymium up to 23 GPa: An x-ray powder diffraction study

The trigonal (P-3 m1) modification of Cs 3 Bi 2 Br 9 and Cs 3 Sb 2 I 9 have been studied using NQR, X-ray single crystal and powder pattern methods. Moreover, the heat capacity was measured in a wide temperature interval: 4-300 K. In Cs 3 Bi 2 Br 9 a second-order phase transition was found at T C = 96 K. The low-temperature phase is monoclinic (C12/c1), with the unit cell doubled along the [001] direction. Cs 3 Sb 2 I 9 has a sequence of phase transitions at T C = 85 K, T i = 78 K and T L = 72.1 K. The monoclinic structure below 85 K is isomorphic with the low-temperature structure of Cs 3 Bi 2 Br 9 . According to calorimetric data the lock-in transition at 72.1 K is discontinuous.

LOW-TEMPERATURE PHASE TRANSITIONS IN THE TRIGONAL MODIFICATION OF Cs3Bi2Br9 AND Cs3Sb2I9

The displacive transition in La is studied in the pressure range up to 26 GPa and under temperatures up to 630 K with angular dispersive X-ray diffraction at the ESRF and with energy dispersive X-ray diffraction in HASYLAB to elucidate further details of this transition with an extension of the transition line up to 22.5(5) GPa and 590(10) K and a determination of the order parameter down to a level of η ≈=5· 10−4.

Characterization of the fcc-distorted fcc-structural transition in lanthanum in an extended pressure and temperature range

The recent advances of structural chemistry of organoselenium and organotellurium compounds are reviewed. The basic concepts of structural chemistry of the compounds are well summarized in Chapter 3 of Volume 1 in this series. About 9,000 structures for the compounds have been reported since then, therefore only selected results can be discussed here. Such compounds are expected to contain at least one CE (E = Se or Te) bond, although those with no CE bonds are also discussed, as necessary. Compounds containing CE (E = Se and Te) are discussed in some detail. Structures of unstable and/or reactive species are explained in detail, which are stabilized by the formation of dimers or higher oligomers or using large protecting groups. Bis-selenides and bis-tellurides and the derivatives at the naphthalene 1,8-positions and the related systems are discussed in detail. The compounds containing hypervalent interactions are also discussed in detail. The idea is applied to the intramolecular and intermolecular linear interactions in crystals. In this connection, the extended hypervalent interactions are also explained in detail for the linear interactions longer than three, exemplified by typical cases and unique cases in crystals.


Keywords:

extended hypervalent bonds;
halides;
hypervalent bonds;
molecular complexes;
organoselenium compounds;
organotellurium compounds;
T-shaped compounds;
structures;
QC calculations;
X-ray crystallographic analysis

Recent Advances of Structural Chemistry of Organoselenium and Organotellurium Compounds

Phase transitions in praseodymium and lanthanum under pressure have been studied using a synchrotron powder X-ray diffraction technique. A structure refinement of the distorted fcc phase of Pr using diffraction data collected with an imaging plate (IP) detector demonstrate that among some possible structures the rhombohedral structure with space group R3m best reproduces the observed diffraction pattern. The distorted fcc-fcc phase transition in La was observed as a function of the temperature at 23 GPa using a CCD-based detector. A five-minute exposure sufficiently long to measure the intensities of very weak superlattice reflections from the distorted fcc phase, which has been found to transform to the fcc phase at 550 K. The performance of the IP and a CCD-based detector are compared and their future developments discussed.

Performance of different types of detectors in a high-pressure X-ray study of phase transition

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Performance of different types of detectors in a high-pressure X-ray study of phase transition