Studies at high pressures and temperatures are helpful for understanding the physical properties of the solid state, including such classes of materials as, metals, semiconductors, superconductors, or minerals. In particular, the phase behaviour of ABX4 scintillating materials is a challenging problem with many implications for other fields including technological applications and Earth and planetary sciences. A great progress has been done in the last years in the study of the pressure-effects on the structural and electronic properties of these compounds. In particular, the high-pressure structural sequence followed by these compounds seems now to be better understood thanks to recent experimental and theoretical studies. Here, we will review studies on the phase behaviour of different ABX4 scintillating materials. In particular, we will focus on discussing the results obtained by different groups for the scheelite-structured orthotungstates, which have been extensively studied up to 50 GPa. We will also describe different experimental techniques for obtaining reliable data at simultaneously high pressure and high temperature. Drawbacks and advantages of the different techniques are discussed along with recent developments involving synchrotron X-ray diffraction, Raman scattering, and ab initio calculations. Differences and similarities of the phase behaviour of these materials will be discussed, on the light of Fukunaga and Yamaoka’s and Bastide’s diagrams, aiming to improve the actual understanding of their high-pressure behaviour. Possible technological and geophysical implications of the reviewed results will be also commented.

Pressure effects on the structural and electronic properties of ABX 4 scintillating crystals

http://personales.upv.es/~fjmanjon/pms53_711.pdf

Pressure effects on the structural and electronic properties of ABX4 scintillating crystals

Conceptually, it is not difficult to imagine reaching into a solid structure and carving out just that portion of the framework we desire. Solid-state reactions incorporating an ionic component into a covalent structure have long been recognized as experimental means for accomplishing just such feats. However, the method does not always succeed, and so, in an effort to extend its use in more logical approaches to solid synthesis, we herein provide an assessment of the scope and limitations of the reaction type. Dimensional reduction is set forth as a general formalism describing how the metal−anion (M−X) framework of a parent compound, MXx, is dismantled upon reaction with an ionic reagent AaX to form a child compound AnaMXx+n. The added anions serve to terminate M−X−M bridges, yielding a less tightly connected framework that retains the metal coordination geometry and polyhedron connectivity mode of the original parent structure. In most instances, the connectedness of the ensuing framework can also be p...

Dimensional Reduction: A Practical Formalism for Manipulating Solid Structures

The database on structural phase transitions is presented in the simple and easy accessible form of table. The symmetry changes and temperature for 3446 phase transitions and the references for each of 2242 registered crystalline materials with well defined stoichoimetry are the main data in this table. The rules for constructing and reading the database table are also presented.

Structural phase transitions in crystals. I. Database

The dynamical behaviour of systems exhibiting structural phase transitions is reviewed, with particular emphasis on the problems posed by the simultaneous existence of quasi-elastic and phonon features in the dynamic response of the soft coordinates. The results of scattering experiments revealing this phenomenon are surveyed in detail, and are used in an assessment of current theories, subdivided into three categories: those which seek to describe the phenomenon within an extended phonon-based theory; those which attempt to trace the effect to the formation of long-lived clusters of precursor order; and those which associate the quasi-elastic scattering with crystal imperfections.

Structural phase transitions III. Critical dynamics and quasi-elastic scattering

2014 The temperature dependence of the structural parameters (ions coordinates and ions temperature factors) in the vicinity of the cubic to tetragonal structural phase transition of RbCaF3 at 193 K is reported together with the structure of this compound below 50 K. The neutron powder profile refinement method has been used to carry out this study. The same space group (Pnma-D162h) has been found at 20 K and 4.5 K. Equality of the CaF6 tilt angles about each of the three 100 ) cubic axes in the low temperature phase and the behaviour of the temperature factors in the cubic and tetragonal phases indicate an order-disorder character of the 193 K and 50 K phase transitions. J. Physique 41 (1980) 87-96 JANVIER 1980, Classification Physics Abstracts 64.70K 61.12’

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The temperature dependence of the structures of RbCaF3, and the determination of the low temperature phases

2014 High resolution inelastic neutron scattering constant Q scans have been performed near the 193 K cubic (O1h) to tetragonal (D184h) phase transition in RbCaF3. A divergent temperature dependent central peak was observed at q = qR. The parameters 03BB1, 03BB2 and 03BB3 characterizing the anisotropy of the dispersion map in the vicinity of the R25 mode have been measured at different temperatures. Taking into account the large anisotropy of the dynamical structure factors of the 3 modes belonging to the R25 soft mode, the 4 unknown parameters (A, 03C92~, 0393, 03B42) of the spectral response used by Shapiro et al. in SrTiO3 were determined by fitting the calculated scans with the experimental ones. The temperature dependence of 03C92~ et 03B42 is reported ; 03B42 is found to decrease when (T Tc) increases. The validity of the q and T dependence of the spectral response function is discussed on the basis of our results. LE JOURNAL DE PHYSIQUE TOME 38, NOVEMBRE 1977, Classification Physics Abstracts 63.20 64.70K

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The 193 k phase transition in RbCaf3 : II. critical neutron scattering

In KCaF3 the sequence of soft mode condensation is the reverse of that in RbCaF3 1). The M3 and X' modes condense together at 560 K to give an ortho-rhombic Bbmm cell with tilts (a-b+c°). TheΤ25 mode then condenses at 551 K to give the usual Puma structure with tilts (a-b+a-). The intermediate phase is not then uniaxial (tetragonal) as in RbCaF3, which explains the different linear birefringence results. At room temperature the tilt angles are large and equal, as in RbCaF3, indicating ordering on well defined potential minima.

Structural phase transitions in Kcaf3 - DSC, birefringence and neutron powder diffraction results

We report some new crystallographic and Raman scattering data on the 193 K phase transition in RbCaF3 Below Tc, the temperature dependence of the order parameter deduced from the relative distortion (c - a)/a is well described by a critical exponent β = 033

J. Nouet

Papers

The temperature dependence of the structures of RbCaF3, and the determination of the low temperature phases

The 193 k phase transition in RbCaf3 : II. critical neutron scattering

Structural phase transitions in Kcaf3 - DSC, birefringence and neutron powder diffraction results

The 193 K phase transition in RbCaF3

Structural and vibrational study of VF3