Size-induced Phase Transition Research Articles

Size-induced phase transitions

ABSTRACT This paper presents a holistic view of the structural phase transitions in nanocrystals. The databases for size-induced phase transitions is a main part of this paper. The database on transitions with temperature contains 603 items on 415 crystals, and database on transitions under high pressure contains 209 items in 124 crystals. Moreover, some new aspects of such transitions are presented like island diagram and new analysis of anomalous compressibility observed in some nanocrystals.

The Size Induced Phase Transition in the Rb0.95NbxMo2−xO6.475–0.5x (x=1.31–1.663) Solid Solution with β‐Pyrochlore Structure

AbstractThe size induced phase transition for non‐cubic β‐pyrochlore compounds Rb0.95NbxMo2−xO6.475–0.5x (x=1.31–1.663) has been studied in detail. The Rb0.95NbxMo2−xO6.475–0.5x (x=1.31–1.663) powders with different size particles have been prepared and investigated by X‐ray powder diffraction analysis and scanning electron microscopy with X‐ray microanalysis. The Rb0.95Nb1.5Mo0.5O5.73 and Rb0.95Nb1.375Mo0.625O5.79 powders consisted of ~20 μm crystals possess orthorhombic Pnma symmetry, whereas the same powders with particle size less 1 μm have cubic Fd‐3m and characterized by decreasing Rb content – Rb0.7Nb1.5Mo0.5O5.58 and Rb0.9Nb1.625Mo0.375O5.62. The crystal structure of Rb0.7Nb1.5Mo0.5O5.58 cubic phase refinement has been performed using the Rietveld method; and crystallographic reasons of structure reconstruction have been discussed. The electronic structure and band gap of Rb0.95NbxMo2−xO6.475–0.5x (x=1.31–1.663) compounds have been studied. Moreover the comparison of electronic structure for cubic and orthorhombic phases has been performed.

Could the negative capacitance effect be used in field-effect transistors with a ferroelectric gate?

We analyze the electric potential and field, polarization and charge, and differential capacitance of a silicon metal-oxide-ferroelectric field effect transistor (MOSFET), in which a gate insulator consists of thin layers of dielectric SiO2 and weak ferroelectric HfO2. It appeared possible to achieve a quasi-steady-state negative capacitance (NC) of the HfO2 layer, , if the layer thickness is close to the critical thickness of the size-induced ferroelectric-paraelectric phase transition. However, this effect disappears as the gate voltage increases above a certain critical value, which can be explained by the nonlinearity of the ferroelectric permittivity. The quasi-steady-state NC corresponds to a positive capacitance of the whole system. Implementation of the gate insulator NC, , can open the principal possibility to reduce the MOSFET subthreshold swing below the critical value, and to decrease the gate voltage below the fundamental Boltzmann limit. However, we failed to found the parameters for which is negative in the quasi-steady states; and thus, the negative cannot reduce the subthreshold swing below the fundamental limit. Nevertheless, the increase in , related with , can decrease the swing above the limit, reduce device heating during the operation cycles, and thus contribute to further improvements of MOSFET performances.

Size-induced structural phase transition in nanocrystalline CaYTiNbO7:Eu: comments on the paper by Mahesh and Rao (J. Mater. Sci.: Mater. Electron. 31, 20,847 (2020))

Size-induced structural phase transition in nanocrystalline CaYTiNbO7:Eu: comments on the paper by Mahesh and Rao (J. Mater. Sci.: Mater. Electron. 31, 20,847 (2020))

Size and phase dependent thermal conductivity of TiO2-water nanofluid with theoretical insight

Nanomaterial based heat transfer fluid has become one of the optimistic technologies that ushered a new horizon in the heat transfer process. In this work, TiO2 powder (Degussa P25) was subjected to mechanical milling for 0–16 h and the average particle size was varied from 21.8–13.3 nm. A size induced phase transition of the TiO2 nanoparticles from anatase to rutile was observed and confirmed from both the XRD and RAMAN analysis. It was found that the rutile content increased gradually compared to anatase phase with decreasing particle size. Aqueous TiO2 based nanofluid systems of different milled samples were prepared and heat transfer properties were investigated experimentally. Particle size and phase of the TiO2 nanoparticles both affected the observed thermal conductivity. From the density functional theory based ab initio calculations it was found that the variation in lattice thermal conductivity became negligible when the dimension of the sample is greater than ~23 Å and ~26 Å for rutile and anatase TiO2 respectively, suggesting the phase to be the dominant factor for the observed variation above that size. The DFT study supports our experimental observation indicating that the thermal conductivity of rutile phase is less than that of anatase phase. This work presents for the very first time, a new investigation about the dependency of thermal conductivity on the phase content of TiO2 nanoparticles which may help in explaining various conflicting results in the literature about the thermal conductivity variation of TiO2 based nanofluids.

Comment on and complement to “Effect of calcination temperature on the degree of polymorphic transformation in Y2SiO5 nanopowders synthesized by sol–gel method” by Z.S. Khan et al. [Journal of Non-Crystalline Solids 432 (2016) 540–544

The correct values of crystallites size of Y2SiO5 powder are presented as a correction of data published in the commented paper. Additionally, the description of the size-induced phase transition is given.

X-ray study of structural phase transitions in nanocrystalline LaMnO3+δ perovskite

ABSTRACTThe nanocrystalline LaMnO3+δ perovskite was synthesized by the microwave-assisted glycothermal method and calcined at several temperatures up to 1500°C. The prepared samples were examined by the X-ray powder diffraction with the aim to show that LaMnO3+δ exhibits the size-induced structural phase transitions. The as-received nanocrystals of LaMnO3+δ are of tetragonal, pseudo-cubic symmetry not known for bulk material. The samples calcined at temperatures 750–1100°C have trigonal symmetry known from the high-temperature phase of LaMnO3 single crystal. The samples calcined from 1200°C to 1500°C have two phases: trigonal and orthorhombic. Thus, the observed phase sequence is inverted with respect to that of the bulk material, which is the characteristic of the size-induced mechanism of phase transitions in the nanocrystals. The critical crystallite sizes for both structural transitions were evaluated as 20 and 100 nm.

Emergence of ferromagnetism and metallicity in Pr0.5Sr0.5MnO3 nanoparticles

In this paper, we have studied low-temperature magnetic properties and transport dynamics of half-doped Pr0.5Sr0.5MnO3 (PSMO) nano-manganite. Nanoparticles with different average grain sizes are synthesized using chemical pyrophoric reaction route and exhibit same crystal structure. It is reported that particle size reduction in half-doped PSMO manganite interestingly leads to ferromagnetic (FM) metallic behavior which is accompanied by coupled positive magnetoresistance (PMR) (up to 9%) at fields (H ≤ 1kOe at 50K) and negative magnetoresistance (NMR) (∼59% at 100K) at 80kOe fields. Interestingly, maximum PMR is appeared at much lower than the critical field (30kOe) for the melting of insulating -antiferromagnetic (AFM) ordered state. Moreover, it is found that low-temperature AFM-FM phase transition, TN ∼ 135K is smeared out with reduction of particle size; however, the transition is prominent in bulk material. A metastable magnetic state with a mixed fraction of FM and AFM phases is observed below TN. Superparamagnetic (SPM) behavior of PSMO nanoparticles has been revealed by probing ac-susceptibility measurements. Low-temperature transport dynamics has been investigated through four probe resistivity and magneto-transport measurements. A size-induced insulating-metallic phase transition is observed on nano dimension in the range of 125-155K under high magnetic fields; however, the bulk counterpart remains insulating over the entire temperature range. The non-trivial electronic- and magnetotransport properties are explained by the inter-grain spin-polarized hopping mechanism through double exchange (DE) interactions. These issues have been revisited and discussed in the framework of enhanced surface disorder, where surface spins plays the crucial role for inter-grain transport dynamics in nanometric dimension. We believe the present field dependent MR holds excellent potential for future spintronic devices and manifests the exotic properties of strongly correlated materials.

Grain-size-induced ferroelectricity in NaNbO3

The influence of a material's length scale on its functional properties has been reported for many ferroics, where a decrease in the dielectric, ferroelectric, and piezoelectric properties is frequently observed when the size is reduced below the micrometer range. Here we demonstrate that in NaNbO3, in contrast to general expectations, a long-range ferroelectric order is induced when decreasing the size. A series of sintered, polycrystalline, NaNbO3 samples with different grain sizes was prepared and analyzed using differential scanning calorimetry, dielectric measurements, and 23Na 3QMAS nuclear magnetic resonance (NMR). A size-induced phase transition into the ferroelectric polymorph was observed when the grain size decreased below 0.27 μm. The different polymorphs were distinguished based on the local symmetry of the Na(1) sodium site. NMR was also used to determine the relative amounts of the polymorphs in the samples. The observed size-induced phase transition is attributed to the existence of intragranular stresses, induced by the decreased compensation of the ferroelastic energy during the formation of non-180° domain walls when decreasing the grain size and the large anisotropy of the thermal expansion. The results demonstrate the unique possibility of stabilizing ferroelectricity by reducing the grain size, which was not observed in other ferroic systems.

Size-Induced Structural Phase Transition at ∼6.0 nm from Mixed fcc–hcp to Purely fcc Structure in Monodispersed Nickel Nanoparticles

We have investigated the core issue of atomic lattices in monodispersed Ni nanoparticles (NPs) of sizes 3.8 nm to 10.1 nm using detailed analysis of X-ray diffraction, synchrotron radiation X-ray absorption spectroscopy (XAS) and magnetization data. This has revealed the very remarkable coexistence of atomic face-centered cubic (fcc) and hexagonal closed-packed (hcp) lattices in samples with particle size less than or equal to 6.0 nm with the prevalence of only fcc phase beyond this. They are also associated with reduced coordination number, modified electronic structure, and surface atom coordination with ligands. Magnetization data furthermore reveal coexistence of ferromagnetism and superparamagnetism at 300 K. Considered to be due to dominant roles of ligands, they are likely to open up far-reaching implications to their future applications.

Flexocoupling impact on size effects of piezoresponse and conductance in mixed-type ferroelectric semiconductors under applied pressure

Flexocoupling impact on the size effects of the spontaneous polarization, effective piezo-response, elastic strain and compliance, carrier concentration and piezo-conductance have been calculated in thin films of ferroelectric semiconductors with mixed-type conductivity under applied pressure. Analysis of the self-consistent calculation results revealed that the thickness dependences of aforementioned physical quantities, calculated at zero and nonzero flexoelectric couplings, are very similar under zero applied pressure, but become strongly different under the application of external pressure pext. At that the differences become noticeably stronger for the film surface under compression than under tension. The impact of the Vegard mechanism on the size effects is weaker in comparison with flexocoupling except for the thickness dependence of the piezo-conductance. Without flexoelectric coupling the studied physical quantities manifest conventional peculiarities that are characteristic of the size-induced phase transitions. Namely, when the film thickness h approaches the critical thickness hcr the transition to paraelectric phase occurs. The combined effect of flexoelectric coupling and external pressure induces polarizations at the film surfaces, which cause the electric built-in field that destroys the thickness-induced phase transition to paraelectric phase at h= hcr and induces the electret-like state with irreversible spontaneous polarization at h<hcr. The built-in field leads to noticeable increase of the average strain and elastic compliance under the film thickness decrease below hcr that scales as 1/h at small thicknesses h. The changes of the electron concentration by several orders of magnitude under positive or negative pressures can lead to the occurrence of high- or low-conductivity states, i.e. the nonvolatile piezo-resistive switching.

Spontaneous flexoelectric effect in nanosystems (topical review)

ABSTRACTFlexoelectric coupling and flexo-chemical effects can be crucially important for understanding and control of the nanoferroics phase diagrams, electromechanical and polar properties, and soft phonon spectra. Spontaneous flexoelectric coupling in nanoferroics can propose the way to improve and control the nanosystems' physical properties and critical sizes of the size-induced phase transitions by the synergy of flexoelectric effect and chemical strains, as well as it can help to create nanosystems with giant electromechanical response and superior polar properties, which can serve as a source of breakthrough for advanced applications in energy-saving, harvesting, sensorics, information storage and bio-applications. This review can be of interest for the scientists working in condensed matter physics, nano-bio-physics and physical chemistry.

Flexo-chemo effect in nanoferroics as a source of critical size disappearance at size-induced phase transitions

Based on Landau-Ginzburg-Devonshire approach, we explore the critical size disappearance at size induced phase transitions and reentrant phase occurrence in nanoferroics. Our calculations have shown that the physical mechanism of the exciting phenomenon can be the flexo-chemo effect, being the synergy of the spontaneous flexoelectric stresses and the chemical pressure induced by ion vacancies via Vegard effect. The flexo-chemo effect can lead to the remarkable changes of the nanoferroics phase diagrams, such as non-monotonic increase of the transition temperature and long-range order enhancement appearing under the size decrease and fulfilment of definite conditions. Since the flexo-chemo effect should exist in any nanostructured ferroics, obtained analytical results can be predictive for many of them. As a concrete example of the primary ferroics, we consider ferroelectric nanoparticles and have shown that a commonly expected transition from the ferroelectric to paraelectric phase at some small critical size is absent, so that the critical size loses its sense. Contrarily, the stabilization of the ferroelectric phase manifests itself by the enhancement of the transition temperature and polarization with the particle size decrease (ferroelectric phase reentrance), which was observed earlier in the tetragonal BaTiO3 nanospheres of radii 5–50 nm and stayed unexplained up to now.

Effect of grain size on the phase structure and electrical properties of PZT–PNZN quaternary systems

The quaternary systems 0.74Pb(Zr0.47Ti0.53)O3–0.26Pb[(Ni0.6Zn0.4)1/3Nb2/3]O3(PZT–PNZN) have been prepared using the conventional oxide mixing method in a wide temperature region between 1050 and 1250°C. All specimens present high relative density above 95%, indicating a wide sintering window for this system. Investigation of the microstructure reveals a continuous increase in grain size from 1.07 to 2.77μm when the sintering temperature is increased from 1050 to 1250°C, which obeys the common grain-growth law well. However, unlike compositional component PZN–PZT and PNN–PZT, the phase structure of quaternary PZN–PNZN is not sensitive to the grain size. The specimens with different grain size retain the morphotropic phase boundary (MPB) structure and the size induced phase transition does not appear. Excluding the influence of the phase structure, the size effect plays a dominant role in improving the piezoelectric properties, which facilitates the motion of domain walls due to the decreasing number of grain boundary. The optimum piezoelectric parameters are obtained at 1150°C: d33=307pC/N, kp=0.56.

The uncertainty in the grain size calculation from X-ray diffraction data

The phase transition, usually called size-induced phase transition (SIPT), occurs at a threshold crystal diameter – critical grain size. The first published database and classification of such phase transitions [P.E. Tomaszewski, Phase transitions in extremely small crystals, Ferroelectrics 375 (2008), pp. 74–91] listed the set of critical values for known SIPT. Now it becomes clear that the published values have a limited scientific significance due to several factors influencing the calculated values of the critical diameter and grain size. Thus, a simple question arises – is the accurate grain size evaluation possible? The answer is NO. Unfortunately, none of the known methods give the correct value for the crystallite size of nanocrystals! It is only possible to talk about the range of values. Finally, it should be emphasized that all published values for the threshold crystal size of SIPT are not correct! The way to partially solve this serious problem is to provide the detailed description of definitions, methods, etc.

Crystal structure and orthorhombic–tetragonal phase transition of nanoscale (Li 0.06Na 0.47K 0.47)NbO 3

Lead-free nano (Li 0.06Na 0.47K 0.47)NbO 3 powders, with pure perovskite structure and various grain sizes between about 30 and 60 nm, have been successfully synthesized by a novel sol–gel method, in which Nb 2O 5 was used as the Nb source. The refining XRD and Raman spectra were used in combination with TEM to investigate the evolution of lattice structure and phase transformation behavior as a function of grain size. The results demonstrated that the growth process of grains has been divided into two stages, and the distortion of unit cell apparently decreases with decreasing grain size. At around 35 nm, the phase structure of (Li 0.06Na 0.47K 0.47)NbO 3 changed from orthorhombic to tetragonal. This phenomenon is related to a grain size-induced structural phase transition. For the well accepted wisdom that niobates get super piezoelectric properties in orthorhombic–tetragonal transition region, our results suggested a critical size for the application of (Li 0.06Na 0.47K 0.47)NbO 3 in nano piezoelectric devices.

Size Dependent Structural, Vibrational, and Luminescence Properties of Nanocrystalline LiIn(WO4)2:Cr3+

X-ray, electron transmission spectroscopy, vibrational and luminescence studies of LiIn(WO4)2:Cr+ nanoparticles prepared by Pechini method are reported. On annealing the sample several structural changes were observed resulting in a creation of three new, previously unknown polymorphs. It was shown that this tungstate undergoes two size-induced phase transitions from the structure similar to LiFe(WO4)2 into the structure similar to LiYb(WO4)2 and then into the structure of LiGa(WO4)2 type. These transitions occur for the critical particle size of about 100 and 30 nm, and they could be attributed mainly to some changes in the distribution of the sites occupied by Li+ and In3+ ions. Luminescence studies revealed decrease of the covalent character of chromium environment and electron-phonon coupling strength with decreasing size of the nanoparticles.

Formation of ε-Fe 2O 3 phase by the heat treatment of α-Fe 2O 3/SiO 2 nanocomposite

Starting from a nanocomposite consisting of 4 nm hematite nanoparticles embedded in a silica matrix (30 wt.% hematite) and by applying heat treatments at increasing temperatures up to 1050 °C, gradual formation of e-Fe2O3 phase was obtained. Besides representing a new synthesis route of e-Fe2O3 phase, the results obtained in this work indicate that under certain conditions hematite nanoparticles are not thermally stable. Hence, like other ferric oxide polymorphs, nanosized hematite can undergo thermally triggered, size-induced structural phase transitions.

Thermodynamic theory of intrinsic finite-size effects in PbTiO3 nanocrystals. I. Nanoparticle size-dependent tetragonal phase stability

We propose a phenomenological intrinsic finite-size effect model for single domain, mechanically free, and surface charge compensated PbTiO3 (PT) nanocrystals with no depolarization fields, undergoing a first-order tetragonal→cubic ferrodistortive phase transition. We then derive a Landau-Devonshire type free-energy functional, which is commensurate with the solution of the free-energy equation for the polar point group 4mm, wherein the nanoparticle size (ξ) is a variable. By using experimental particle size-dependent spontaneous polarization (P⃗s) data for PbTiO3, we compute the Landau coefficients up to the sixth order in the range &amp;lt;150 nm as a function of ξ. This thermodynamic potential takes into account the size dependence of the Landau coefficients in a consistent manner, and is able to predict the size-induced phase transition as well as the metastable tetragonal phase in the cubic phase field. We then construct a free-energy density surface in ΔG-P⃗s-ξ space, which describes the decrease in tetragonal phase stability with decreasing ξ rigorously.

Examination of Size-Induced Ferroelectric Phase Transitions in Template Synthesized PbTiO3 Nanotubes and Nanofibers

Here we report on the application of sol-gel template synthesis to the investigation of size-induced ferroelectric phase transitions in lead titanate (PbTiO 3 ) nanotubes and nanofibers. A 0.8 M chelate sol-gel, made from titanium(IV) tertabutoxide and lead(II) trihydrate acetate, was applied to two different templates. Nanotubes were formed within 200-nm-pore Whatman anodisc aluminum oxide membranes, and nanofibers were prepared using 50-, 100-, and 200-nm Whatman track-etched polycarbonate membranes. Transmission electron microscopy images revealed that the tubes comprised grains of ≤20 nm and the fibers comprised individual grains ≤200 nm in width when a 200-nm pore size template is used. An examination of how the grain/crystallite size and aspect ratio of one-dimensional morphologies affect the ferroelectric phase transition was monitored through the comparison of bulk powders and the nanostructured materials using electron diffraction, X-ray diffraction, Raman spectroscopy, and differential scanning calorimetry.