Rare-earth Radius Research Articles

Proton and Oxygen-Ion Conductivity of the Pure and Lanthanide-Doped Hafnates with Pyrochlore Structure

In this work, a high-density ceramics Ln2Hf2O7 (Ln = La, Nd, Sm, Eu, Gd) were synthesized by mechanical activation followed by high-temperature synthesis at 1600°C (3–10 h) and their transport properties were compared with those of Ln2.1Hf1.9O6.95 (Ln = La, Nd, Sm, Eu) doped solid solutions. The total conductivity of ceramics was studied using impedance spectroscopy and dc four-probe method; for Ln2Hf2O7 (Ln = Sm, Eu), by determining the total conductivity as a function of oxygen partial pressure. The maximum oxygen-ion conductivity was observed for Gd2Hf2O7 (~1 × 10–3 S/cm at 700°C); it was shown to approach the conductivity of Gd2Zr2O7 (~2 × 10–3 S/cm at 700°C) for the first time. Thus, the gadolinium hafnate can be a promising material for further doping in order to obtain highly conductive electrolytes. Among pure rare-earth hafnates, the proton conductivity was reliably observed for Nd2Hf2O7 only; however, ac measurements detected low-temperature proton conductivity in the Gd2Hf2O7 up to 450°С as well. With a decrease in the lanthanide ionic radius, the oxygen-ion conductivity increased in the Ln2Hf2O7 (Ln = La, Nd, Sm, Gd) series. Although the conductivity of samarium hafnate is an order of magnitude lower than that of Gd2Hf2O7, it has a wide range of oxygen-ion conductivity (~10–18–1 atm at 700, 800°C); there is no contribution from hole conductivity in air, in contrast to Eu2Hf2O7. Among doped Ln2.1Hf1.9O6.95 pyrochlore solid solutions (Ln = La, Nd, Sm, Eu), the proton conductivity of ~8 × 10−5 S/cm at 700°C was shown in Ln2.1Hf1.9O6.95 (Ln = La, Nd). With a decrease in the lanthanide ionic radius, the proton conductivity disappeared; the oxygen-ion one, increased.

Enhanced rare-earth separation with a metal-sensitive lanmodulin dimer

Technologically critical rare-earth elements are notoriously difficult to separate, owing to their subtle differences in ionic radius and coordination number1–3. The natural lanthanide-binding protein lanmodulin (LanM)4,5 is a sustainable alternative to conventional solvent-extraction-based separation6. Here we characterize a new LanM, from Hansschlegelia quercus (Hans-LanM), with an oligomeric state sensitive to rare-earth ionic radius, the lanthanum(III)-induced dimer being >100-fold tighter than the dysprosium(III)-induced dimer. X-ray crystal structures illustrate how picometre-scale differences in radius between lanthanum(III) and dysprosium(III) are propagated to Hans-LanM’s quaternary structure through a carboxylate shift that rearranges a second-sphere hydrogen-bonding network. Comparison to the prototypal LanM from Methylorubrum extorquens reveals distinct metal coordination strategies, rationalizing Hans-LanM’s greater selectivity within the rare-earth elements. Finally, structure-guided mutagenesis of a key residue at the Hans-LanM dimer interface modulates dimerization in solution and enables single-stage, column-based separation of a neodymium(III)/dysprosium(III) mixture to >98% individual element purities. This work showcases the natural diversity of selective lanthanide recognition motifs, and it reveals rare-earth-sensitive dimerization as a biological principle by which to tune the performance of biomolecule-based separation processes.

Exploring the influence of rare earth ions at A-site on the properties of LnBaCo2O5+δ cathode for solid oxide fuel cells

The influence of rare earth ions on layered perovskite (AA'B2O5+δ) is the key to the development of highly active layered perovskite cathode. Herein, we make a systematic explanation through experiment and first-principles calculation. The lattice constant decreases with the decrease of Ln3+ ionic radius, resulting in a smaller space of interstitial oxygen position and increasing oxygen Frenkel energy. The electrochemical activity of LnBaCo2O5+δ (Ln = Pr, Nd and Sm) electrodes decreases with the decrease of Ln3+ ionic radius, which is mainly attributed to a lower oxygen Frenkel energy. Electrochemical impedance spectra tests show that there are significant differences in the surface exchange process of oxygen because of different surface oxygen vacancy concentration for LnBaCo2O5+δ. For Ln = Pr, Nd, and Sm cathodes, the maximum power densities of the anode-supported Ni-YSZ/YSZ/GDC/LnBaCo2O5+δ single cells reach 1.48, 1.36 and 1.06 W/cm2 at 800 °C, respectively, indicating that PrBaCo2O5+δ with the maximum rare earth ion radius exhibits better oxygen reduction reaction. These findings provide guidance for the design and fabrication of highly active layered perovskite electrode materials.

Solvent Extraction and Complexation Studies of Pyridine-di-Phosphonates with Lanthanides(III) in Solutions

ABSTRACT In this work, we studied the complex formation (1H, 31P NMR-titration, UV–vis titration, luminescent titration) and solvent extraction of lanthanides with pyridine diphosphonates. The stoichiometry of the complexes was determined: ML and ML2 forms are present. This is confirmed by all of the above methods. It was found that the pyridine ring and P=O groups are involved in the coordination of the metal cation. The coordination environment of the Eu(III) cation was studied more thoroughly using the EXAFS method in solution. Coordination numbers and distances were determined for the complex in solution. The influence of the lanthanide radius on the value of the stability constant was shown. The change in extraction efficiency in the series of lanthanide is described. A new pattern, unusual for other phosphorus-containing ligands, was obtained. To explain the change in the parameters of complexation depending on the system, DFT calculations were carried out. The effect of various initial states of the extracted cations was shown. The initial state with a large amount of nitrates corresponds to a two-phase system during extraction, and with a smaller amount, to a single-phase system with acetonitrile. Additionally, the luminescent properties of the complexes were described in detail as one more applied aspect of the work. .

The effect of the rare earth A-site cation on the structure, morphology, physical properties, and application of perovskite AFeO3

In this study, the influence of the rare earth (RE) ions on the structure, morphology, magnetic behavior, dielectric properties, and application of AFeO3 perovskite was studied. The perovskite nanoparticles AFeO3 (A: La, Nd, and Sm) were synthesized using the citrate auto combustion method. The x-ray diffraction (XRD) confirmed that the perovskites have an orthorhombic structure. The samples' crystallite size ranges from 13 to 51 nm. The high-resolution transmission electron microscope (HRTEM) assures that the perovskites are synthesized in the nanoscale. AFeO3 multiferroics have antiferromagnetic behavior. The maximum magnetization (Mm) of the NdFeO3 is two times that of the SmFeO3, owing to the variation in the number of electrons in 4f. The dielectric constant values (ε/) are strongly influenced by the rare earth (RE) element ionic radii, with ε/ increasing as the RE ionic radii increase. The investigated perovskites have semiconductor like behavior. The toxic heavy metal Cr6+ is efficiently removed from water by AFeO3 (A: La, Nd, and Sm). The optimum pH value is observed to be 6, at which the maximum removal efficiencies are 88%, 84%, and 53% for NdFeO3, LaFeO3, and SmFeO3, respectively. The Langmuir and Freundlich isotherm models have been used to fit the experimental data.

Influence of Rare-Earth Ion Radius on Metal-Metal Charge Transfer in Trinuclear Mixed-Valent Complexes.

We report the synthesis and characterization of a highly conjugated bisferrocenyl pyrrolediimine ligand, Fc2PyrDIH (1), and its trinuclear complexes with rare earth ions─(Fc2PyrDI)M(N(TMS)2)2 (2-M, M = Sc, Y, Lu, La). Crystal structures, nuclear magnetic resonance (NMR) spectra, and ultraviolet/visible/near-infrared (UV/vis-NIR) data are presented. The latter are in good agreement with DFT calculations, illuminating the impact of the rare earth ionic radius on NIR charge transfer excitations. For [2-Sc]+, the charge transfer is at 11,500 cm-1, while for [2-Y]+, only a d-d transition at 8000 cm-1 is observed. Lu has an ionic radius in between Sc and Y, and the [2-Lu]+ complex exhibits both transitions. From time-dependent density functional theory (TDDFT) analysis, we assign the 11,500 cm-1 transition as a mixture of metal-to-ligand charge transfer (MLCT) and metal-to-metal charge transfer (MMCT), rather than pure metal-to-metal CT because it has significant ligand character. Typically, the ferrocenes moieties have high rotational freedom in bis-ferrocenyl mixed valent complexes. However, in the present (Fc2PyrDI)M(N(TMS)2)2 complexes, ligand-ligand repulsions lock the rotational freedom so that rare-earth ionic radius-dependent geometric differences increasingly influence orbital overlap as the ionic radius falls. The Marcus-Hush coupling constant HAB trends as [2-Sc]+ > [2-Lu]+ > [2-Y]+.

Single crystal growths and magnetic properties of hexagonal polar semimetals RAuGe (R = Y, Gd–Tm, and Lu)

We study structural and magnetic properties of rare-earth based semimetals RAuGe (R = Y, Gd–Tm, and Lu) using flux-grown single crystals. These compounds belong to the space group P63mc, which is associated with the noncentrosymmetric polar point group 6mm. We confirm the systematic structural evolution at room temperature as a function of ionic radius of rare earths to clarify the isopointal crossover between two polar structures: three-dimensional LiGaGe-type and quasi-two-dimensional NdPtSb-type. Magnetism shows a characteristic anisotropy in reasonable agreement with the crystal electric field (CEF) theory; the easy-plane-type anisotropy for R = Tb and Dy turns into the Ising-type anisotropy for R = Er and Tm. We evaluate the CEF parameters based on the Stevens operators to reasonably reproduce the temperature dependence of magnetic susceptibilities and specific heat for RAuGe (R = Tb–Tm). The estimated energy scale of the Ising gap (∼ 11 meV) in TmAuGe is consistent with an excitation observed in an inelastic neutron scattering experiment. These findings suggest an opportunity for interplay between conduction electrons and nontrivial spin structures in the family of magnetic polar semimetals RAuGe.

Thermochemical Investigations of Bismuth, Dysprosium, Samarium, and Niobium Oxide Compounds

Ceramic Bi1.4Dy0.6O3 and Bi3Nb0.2Sm0.8O6.2 samples were prepared by solid-phase synthesis. The compounds have cubic structures (space group Fm3m). Their standard enthalpies of formation were determined by solution calorimetry, and their lattice enthalpies were calculated. The lattice enthalpies of Bi3Nb0.2R0.8O6.2 compounds decrease in magnitude when erbium is replaced by samarium, due to the lanthanide radius increasing from erbium to samarium. The lattice enthalpy of Bi1.4Dy0.6O3 has a greater magnitude than the lattice enthalpy of Bi1.2Gd0.8O3.

Noble-Free Nanophotocatalyst of TixFeyLamOz for Efficient Photocatalytic C–N Cross-Coupling Reactions under Visible Light

Nanophotocatalysts are applied for different applications, and their usage has been enhanced remarkably since a decade. On the other hand, cross-coupling reactions of C–N bonds have found increasing popularity for coupling derivatives of organic compounds such as aniline using diverse catalysts. Nanoheterogeneous photocatalysts can offer suitable platforms for the C–N coupling of aniline derivatives. However, the design of a photocatalyst on the nanoscale with remarkable activity and selectivity, made of noble-free elements, is highly welcome. In this study, the half-filled electronic configuration and shallow trapping properties of Fe were employed to reduce the energy band gap of the system. Moreover, the large ionic radius of lanthanide leads to large surface activity. Thus, bi- and trinuclear nano-oxide nanophotocatalysts were synthesized based on the titanium, iron, and lanthanide elements (TixFeyOz, TixFeyLamOz) to obtain logically designed photocatalysts with high selectivity, durability, and operation speed under visible light. In addition, designing photocatalysts in the nanoscale was the ultimate target of this study to use the highest benefit of the nanoscale, which is high surface activity. The developed nanophotocatalysts were used for coupling aniline and its 10 derivatives under visible light. The mechanism of the migration of electrons and the C–N reaction was also investigated in detail. In addition, both obtained nanophotocatalysts were applied in five different solvents and eight different conditions so the best condition for the reaction could be determined. Furthermore, the photocatalytic mechanism was studied in detail. The final nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDX), inductively coupled plasma mass spectrometry (ICP-MS), Brunauer–Emmett–Teller (BET) surface activity, Barrett–Joyner–Halenda (BJH) method, UV–vis spectroscopy, photoluminescence emission spectra, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), NMR spectra, mass spectra, and CHN elemental analysis. Both nanocomposites exhibited high photocatalytic activities in coupling aniline and its derivatives under visible light. However, the TixFeyLamOz nanophotocatalyst showed a higher activity (90% conversion to obtain 2,4 dinitro-n-phenylaniline) than the TixFeyOz nanophotocatalyst (68% conversion to obtain 2,4 dinitro-n-phenylaniline), which is mainly due to its higher surface activity because of its lanthanide content.

New Rare Earth-Doped Bilayered Perovskite Oxide Photocatalysts Sr2La0.5R0.5FeMnO7 (R = La, Nd, Sm, Gd, Dy) for the Degradation of Highly Toxic Methylene Blue Dye in Wastewater under Visible Light: Structural, Optical, and Magnetic Properties.

This paper presents the rare earth doping effect on the structural, optical, and magnetic properties of bilayered Ruddlesden-Popper oxides Sr2La0.5R0.5FeMnO7 (R = La, Nd, Sm, Gd, Dy). Moreover, we are reporting for the first time a new rare earth-doped bilayered perovskite oxide series for the highly toxic methylene blue dye degradation in wastewater under visible light. Structural analysis of the PXRD data using the Rietveld refinements confirms the formation of the phases in tetragonal symmetry with the I4/mmm space group. The unit cell lattice parameters (a & c) and the cell volume (V) decrease monotonically from La- to Dy-doped samples owing to the decrease in the lanthanide ionic radii. The X-ray photoelectron spectroscopy analysis indicates the existence of the Mn ions in the mixed valence state. The DRS study shows that the energy band gap value decreases on moving from La to Gd substitution; however, it further increases for the Dy-doped sample. The magnetic measurements reveal that all the phases exhibit dominant anti-ferromagnetic interactions with Neel temperature (T N) observed at 150, 147, 138, 113, and 117 K for La-, Nd-, Sm-, Gd-, and Dy-substituted phases, respectively. However, the presence of an unsaturated hysteresis loop observed in the isothermal magnetic field (H) vs magnetization (M) plot also indicates the existence of weak ferromagnetic interactions. The investigation of the photocatalytic activity of the synthesized samples was done by carrying out photo-oxidative degradation of methylene blue (MB) dye pollutants. The results show that the photodegradation enhances by doping with heavier rare earth ions with the exception of the Dy-doped sample. The Gd-doped catalyst shows the maximum degradation efficiency of 99.03% in 50 min under visible light irradiation. The scavenging experiments confirmed that the·OH was the main/dominant oxidizing agent involved in the degradation of the MB dye.

Structural and Spectroscopic Features of the Bixbyite-Type Yttrium Scandate Doped by Rare-Earth Ions

Yttrium scandate crystal fiber has been obtained through laser-heated pedestal growth. The crystal belongs to a bixbyite crystal structure and crystallizes in Ia3¯ space group. X-ray diffraction method shows a lattice parameter of a = 10.228(1) Å. Factor-group analysis of YScO3 Raman spectra points to high degree of disorder in crystal structure of the new compound. Spectral-kinetic investigation of the crystal fibers doped by rare-earth ions points to the presence of two independent active optical centers of rare-earth ions. Moreover, the character of rare-earth impurities’ distribution is independent on a rare-earth ionic radius size.

Improving the dielectric properties of aluminoborosilicate glasses for packaging ceramics by optimizing structure

Aluminoborosilicate glasses doped with different types and amounts of rare earth oxides (Dy2O3, Eu2O3, and La2O3) were synthesized via the sol-gel method. The impacts of doping these rare earth oxides on the glass structure, thermal, and dielectric properties were discussed. IR analyses showed that glasses doped with 2 wt.% La2O3 contained more [BO4] and [AlO4], which optimized the polymerization degree of the glass network, and strengthened the structure. This glass had a lower dielectric constant (εr = 2.27) and dielectric loss (tanδ = 0.0019) at 13 GHz. Additionally, the glass transition temperature (Tg), initial crystallization temperature (Tc), and thermal stability (△T) all increased with higher rare earth ionic radius (Dy3+, Eu3+, La3+), due to decreased non-bridging oxygen levels (NBO) of [SiO4] and increased [BO4] and [AlO4] in the glass grid. However, co-doped rare earth oxide glasses inhibited conversion from [BO3] to [BO4] when equal amounts of La2O3 and Dy2O3 or Eu2O3 were added, which resulted in a looser glass network and a higher dielectric constant. As La2O3 gradually replaced Dy2O3 or Eu2O3, the dielectric constant initially increased and then decreased, ascribed to the shift polarization of RE3+ in the glass structure.

The effects of R2O3 (R=La, Yb, Gd) on the microstructure, dielectric, ferroelectric, and energy storage properties of Ba0.65Bi0.07Sr0.245TiO3 relaxor ferroelectric ceramics

(1-x)Ba0.65Bi0.07Sr0.245TiO3-xR2O3 (BBST-xR) ceramics embellished by lanthanides (La, Yb, Gd) were synthesized in this study, and the effects in properties were ascertained. The results show that lanthanides with smaller ionic radius differences and larger relative mass will effectively limit mass transport of other cations and restrain grain growth. It is also found that the paraelectric phase of ceramics can be stabilized by decreasing the ionic radius of lanthanides. In particular, the addition of La unfolds the exceptional recoverable storage density (Wrec) up to 2.23 J/cm3 and efficiency (η) of ∼76.7% under 280 kV/cm. Finite element simulation and PFM observation also further explain the reasons for the excellent performance. Concurrently, BBST-0.015La ceramic exhibits outstanding thermostability from 30 °C to 150 °C and frequency stabilization from 1 Hz to 300 Hz which indicates the potentiality of application in actual surroundings.

Bismuth niobates doping by lutetium and dysprosium: Synthesis, formation enthalpies, heat capacity

Bismuth niobates doped by lutetium and dysprosium (Bi3Nb0.2Lu0.8O6.2, Bi3Nb0.2Dy0.8O6.2) were synthesized by solid state reaction. Bi3Nb0.2Lu0.8O6.2 was prepared for the first time. Compound Bi3Nb0.2R0.8O6.2 is one of the perspective materials for oxygen ceramic generators, photocatalytic processes, etc. For the first time, the standard formation enthalpies for compounds Bi3Nb0.2Lu0.8O6.2, Bi3Nb0.2Dy0.8O6.2 were studied by reaction calorimetry, and lattice enthalpies were calculated. It was found that standard formation enthalpies and lattices enthalpies for compounds Bi3Nb0.2R0.8O6.2 (R ​= ​Dy, Lu) were increasing in absolute value with decreasing radius of rare-earth element. To predict thermodynamic characteristics of uninvestigated compounds of Bi3Nb0.2R0.8O6.2 with other rare-earth elements, formulas were derived that related lattice enthalpies and standard formation enthalpies with radii of rare-earth elements at low doping. It was shown that dependences were linear. The heat capacity for Bi3Nb0.2Dy0.8O6.2 was measured by DSC calorimetry in the temperature range of 167–674 ​K. It was found that compound Bi3Nb0.2Dy0.8O6.2 had phase transition at temperatures about 500 ​K. It was shown that Bi3Nb0.2Lu0.8O6.2 was most thermodynamically stable and preferred for application.

Structural evolution of single-crystal RECrO3 (RE = Y, Eu–Lu) orthochromates

In-house X-ray powder diffraction has been employed to study the room-temperature structural evolution of orthochromate RECrO3 (RE ​= ​Y, Eu–Lu) single crystals grown by the floating-zone method with a laser-diode furnace. Structural refinements reveal an anomaly in the evolution of lattice parameters versus RE ionic radius. This anomaly occurred to the LuCrO3 single crystal and may be attributed to the disappearance of magnetostriction effect due to the loss of both spin and orbital angular moments of Lu3+ ions. Moreover, we have monitored the evolution of Bragg (0 2 0), (1 1 2), (2 0 0), and (0 2 1) peaks, which has varied in a regular basis with rare-earth ionic radius indicative of changes in lattice constants.

Electrochemical Oxidation and Speciation of Lanthanides in Potassium Carbonate Solution

Increasing lanthanide demand to support clean energy goals drives the need to develop more efficient approaches to separate adjacent lanthanides. Most approaches for lanthanide separations are not very selective and are based on small differences in lanthanide ionic radii. Concentrated potassium carbonate media has shown some potential to enable oxidation of praseodymium (Pr) and terbium (Tb) to their tetravalent states, which could ultimately enable a separation based on differences in oxidation states, but very little is known regarding the system’s chemistry. This work completes a detailed examination of cerium (Ce) redox chemistry in concentrated carbonate media to support the development of Pr and Tb oxidation studies. The half-wave potential (E1/2) of the Ce(III)/(IV) redox couple is evaluated under various solution conditions and computational modeling of carbonate coordination environments is discussed. Cyclic voltammetry shows higher carbonate concentrations and temperatures can lower the potential required to oxidize Ce(III) by 54 mV (3.5 to 5.5 M) and 39 mV (from 10 °C to 70 °C). Chronoabsorptometry shows Ce(III) and Ce(IV) carbonate complexes are chemically stable and reversible. Computational modelling suggests the most likely coordination environment for the Ce(IV) complex is Ce(CO3)4(OH)5− which is less entropically favorable than the lowest energy Ce(III) complex, Ce(CO3)4 5−.

Proton Conductive Lanthanide-Based Metal-Organic Frameworks: Synthesis Strategies, Structural Features, and Recent Progress.

In the fields of proton exchange membrane fuel cells as well as impedance recognition, molecular sieve, and biochemistry, the development of proton conductive materials is essential. The design and preparation of the next generation of proton conductive materials-crystalline metal-organic framework (MOF) materials with high proton conductivity and excellent water stability-are facing great challenges. Due to the large radius and high positive charge of lanthanides, they often interact with organic ligands to exhibit high coordination numbers and flexible coordination configurations, resulting in the higher stability of lanthanide-based MOFs (Ln-MOFs) than their transition metal analogues, especially regarding water stability. Therefore, Ln-MOFs have attracted considerable attention. This review offers a view of the latest progress of proton conductive Ln-MOFs, including synthesis strategy, structural characteristics, and advantages, proton conductivity, proton conductive mechanism, and applications. More importantly, by discussing structure-property relationships, we searched for and analyzed design techniques and directions of development of Ln-MOFs in the future. The latest progress of synthesis strategy, structural characteristics, proton conductive properties and mechanism and applications on Ln-MOFs. Ln-MOFS Lanthanide-based MOFs, MOF metal-organic framework, PEMFC proton exchange membrane fuel cells.

Phase Stability and Mechanical Properties of the Monoclinic, Monoclinic-Prime and Tetragonal REMO4 (M = Ta, Nb) from First-Principles Calculations

YTaO4 and the relevant modification are considered to be a promising new thermal barrier coating. In this article, phase stability and mechanical properties of the monoclinic (M), monoclinic-prime (M′), and tetragonal (T) REMO4 (M = Ta, Nb) are systematically investigated from first-principles calculations method based on density functional theory (DFT). Our calculations show that M′-RETaO4 is the thermodynamically stable phase at low temperatures, but the stable phase is a monoclinic structure for RENbO4. Moreover, the calculated relative energies between M (or M′) and T phases are inversely proportional to the ionic radius of rare earth elements. It means that the phase transformation temperature of M′→T or M→T could decrease along with the increasing ionic radius of RE3+, which is consistent with the experimental results. Besides, our calculations exhibit that adding Nb into the M′-RETaO4 phase could induce phase transformation temperature of M′→M. Elastic coefficient is attained by means of the strain-energy method. According to the Voigt–Reuss–Hill approximation method, bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio of T, M, and M’ phases are obtained. The B/G criterion proposed by Pugh theory exhibits that T, M, and M’ phases are all ductile. The hardness of REMO4 (M = Ta, Nb) phases are predicted based on semi-empirical equations, which is consistent with the experimental data. Finally, the anisotropic mechanical properties of the REMO4 materials have been analyzed. The emerging understanding provides theoretical guidance for the related materials development.

Dynamic lanthanide exchange between quadruple-stranded cages: the effect of ionic radius differences on kinetics and thermodynamics

[Ln2L4]2− helicates easily undergo transmetalation. The lanthanide ionic radius difference (ΔEIR) strongly affects the kinetics of the dynamic process while the thermodynamics is independent and mainly entropy driven.

Role of rare earth sites and vacancies in the anomalous compression of modulated scheelite tungstates RE2 ( WO4)3

X-ray powder diffraction experiments at high pressures combining conventional sources and synchrotron radiation, together with theoretical simulations have allowed us to study the anomalous compression of the entire \ensuremath{\alpha}-${RE}_{2}$(${\mathrm{WO}}_{4}{)}_{3}$ ($\mathit{RE}$ = La-Ho) family with modulated scheelite structure (\ensuremath{\alpha} phase). The investigated class of materials is of great interest due to their peculiar structural behavior with temperature and pressure, which is highly sought after for specialized high-tech applications. Experimental data were analyzed using full-profile refinements and were complemented with computational methods based on density functional theory (DFT) total energy calculations for a subset of the samples investigated. An unusual change in the compression curves of the lattice parameters $a, c$, and \ensuremath{\beta} was observed in both the experiments and theoretical simulations. In particular, in all the studied compounds the lattice parameter $a$ decreased with pressure to a minimum value and then increased upon further compression. Pressure evolution of the experimental x-ray diffraction (XRD) patterns and cell parameters is correlated with the ionic radius of the rare earth element: (1) the lighter La-Nd tungstates underwent two phase transitions, and both transition pressures decreased as the rare earth's ionic radius increased. The XRD patterns of the first high pressure phase could be indexed with propagation vectors parallel to the $a$ axis (tripling the unit cell). At higher pressures, the lattice parameters for the second phase (referred to as the preamorphous phase) showed little variation with pressure. (2) The heavier tungstates, from Sm to Dy, undergo a transition to the preamorphous phase without any intermediate phase. The reversibility of both phase transitions was investigated. DFT calculations support this unusual response of the crystal structures under pressure and shed light on the structural mechanism of negative linear compressibility (NLC) and the resulting softening. The pressure dependence of the structural modifications is related to tilting, along with small elongation and alignment, of the ${\mathrm{WO}}_{4}^{2\ensuremath{-}}$ tetrahedrons. These changes correlate with those in the alternating RE\dots{}RE\dots{}RE chains and blocks of cationic vacancies arranged along the $a$ axis. Possible stacking defects, which emerge between them, helped to explain this anomalous compression and the pressure induced amorphization. Such mechanisms were compared with other ferroelastic families of molybdates, niobates, vanadates, and other compounds with similar structural motifs classified as having ``hinge frames.''