Lattice Anomalies Research Articles

Vibrational Spectroscopy of Perovskite Ferroelectrics

Ferroelectric and antiferroelectric materials are technologically important by the richness of applications such as piezoelectric, pyroelectric, electro-optic, elasto-optic, and nonlinear optic effects. Especially, oxides with a perovskite structure are very important. Its chemical formula is ABO3, where A is a cation with a larger ionic radius, and B is a cation with a smaller ionic radius. Various elements are available in A- and B-sites. For example, the large piezoelectricity of well-known Pb(ZrxTi1−x)O3 (PZT) solid solutions was found in a morphotropic phase boundary (MPB). The very high dielectric constant, colossal piezoelectric effect, and large electro-optic effect are induced by ferroelectric phase transitions. Such excellent functionalities are closely related to lattice dynamical instability. The vibrational spectroscopy, i.e., Raman scattering, Brillouin scattering, far-infrared, and terahertz time-domain spectroscopy, is a powerful tool for lattice dynamical anomalies. This paper intended a brief review of vibrational spectroscopy on ferroelectric phase transitions of advanced perovskite oxides.

MnCaTa2O7-A Magnetically Ordered Polar Phase Prepared via Cation Exchange.

Reaction between the pseudo-Ruddlesden-Popper phase Li2CaTa2O7 and MnCl2 at 375 °C yields MnCaTa2O7, a paramagnetic polar phase (space group P21nm), which adopts an a-b-c+/b-a-c+ distorted, layered perovskite structure. Magnetization and neutron diffraction data show that MnCaTa2O7 adopts an antiferromagnetically ordered state below TN = 56 K and exhibits large lattice parameter anomalies and a transient increase in its polar distortion mode at TA = 50 K. However, in contrast to the related phase MnSrTa2O7, MnCaTa2O7 shows no strong signature of weak ferromagnetism and thus shows no signs of magnetoelectric coupling. The differences in physical behavior between the two MnATa2O7 phases appear to be related to their differing Mn cation-order and differing TaO6 tilting schemes and demonstrate that even subtle changes to these orderings can have large effects on the distortion-mode couplings, which drive complex behavior of this class of "hybrid improper" ferroelectric material.

Investigating the magnetoelastic properties in FeSn and Fe3Sn2 flat band metals

Topological quantum magnets FeSn and Fe$_{3}$Sn$_{2}$ were studied using neutron scattering and first-principles calculations. Both materials are metallic but host dispersionless flat bands with Dirac nodes at the $K$ point in reciprocal space. The local structure determined from the pair density function analysis of the neutron diffraction data provided no evidence for electron localization in both compounds, consistent with their metallic nature. At the same time, in FeSn, an anomalous suppression in the $c$-axis lattice constant coupled with changes in the phonon spectra were observed across T$_{N}$ indicating the presence of magneto-elastic coupling and spin-phonon interactions. In addition, it was observed that spin waves persisted well above T$_{N}$, suggesting that the in-plane ferromagnetic spin correlations survive at high temperatures. In contrast, no lattice anomaly was observed in Fe$_{3}$Sn$_{2}$. The inelastic signal could be mostly accounted for by phonons, determined from density functional theory, showing typical softening on warming.

Influence of anharmonicity on the negative thermal expansion of α−Sn

The lattice vibrational properties of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ (gray tin) were investigated experimentally by temperature-dependent x-ray diffraction and theoretically by density functional theory calculations. Similar to the other elements of group IV, $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ exhibits a lattice anomaly at low temperatures and negative thermal expansion, with a minimum at $\ensuremath{\sim}27\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and a magnitude three times larger than in Si. The influence of anharmonic effects up to fourth-order potential terms on the phonon dispersion relations, the lattice parameters, and the thermal expansion coefficient have been tested. The performed analysis gives an excellent agreement with experiment when quartic potential terms are included in the theory. We point out that negative thermal expansion in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ is not driven by the anharmonicity of the interatomic potential. This resolves the long-standing puzzle in the thermal behavior of $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$.

Noncubic local distortions and spin-orbit excitons in K2IrCl6

The cubic antifluorite ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$ has recently garnered renewed attention due to its relevance to Kitaev magnetism. Combining Raman spectroscopy with numerical calculations, we investigate its electronic structure and ensuing low-lying excitations as well as lattice instabilities. For temperatures below ${T}^{*}\ensuremath{\approx}180$ K, we observe several lattice anomalies: (i) a gradual appearance of the symmetry-forbidden phonons; (ii) a central-mode-like excitation; and (iii) a soft-mode-like behavior of the ${\mathrm{\ensuremath{\Gamma}}}^{5+}$ mode involving vibrations of the ${\mathrm{K}}^{+}$ ion relative to the ${\mathrm{Cl}}^{\ensuremath{-}}$ ion. All these features indicate the occurrence of local noncubic distortions. At high energies, we observe spin-orbit (SO) excitons made of five peaks at $\ensuremath{\omega}=0.62--0.79$ eV as well as a weak electronic excitation at $\ensuremath{\omega}=0.12--0.86$ eV. Our numerical calculations reproduce their spectral energy and shape with the electronic parameters: SO coupling $\ensuremath{\lambda}=465$ meV; Hund's coupling ${J}_{H}=300$ meV; tetragonal distortion strength ${\mathrm{\ensuremath{\Delta}}}_{t}=30$ meV and on-site Coulomb interaction $U=2.2$ eV. The multiple SO excitons are interpreted in terms of bounded SO excitons of the $|{j}_{\mathrm{eff}}=\frac{3}{2},\ifmmode\pm\else\textpm\fi{}\frac{1}{2}\ensuremath{\rangle}$ states arising from the coupling between the SO excitons and electron-hole excitations. Our results showcase that ${\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$ is on the brink of a structural phase transition.

The roles of kinematic constraint and diffusion in non-equilibrium solid state phase transformations of Ti-6Al-4V

A solid state phase transformation of Ti-6Al-4V was studied using high speed in situ x-ray diffraction measurements made during rapid cooling of a cold metal transfer arc weld bead deposited onto a water cooled substrate. Analysis of body centered cubic (BCC) and hexagonal close packed (HCP) lattices revealed an abrupt, nonlinear shift in the lattice parameters of both phases just after the HCP phase had nucleated. Postmortem transmission electron microscopy confirmed that V diffusion was mostly suppressed during cooling. Together, these results indicate that at this cooling rate of approximately 104 K/s, which is representative of cooling rates of many additive manufacturing and welding processes, kinematic coherency of the BCC–HCP interfaces gives rise to the anomalous lattice expansion and contraction behaviors of both phases during the initial nucleation and growth stages of (mostly) martensitic transformation from BCC to HCP; the role of diffusion in such lattice anomalies is shown to be minimal.

Giant Viscoelasticity near Mott Criticality in PbCrO_{3} with Large Lattice Anomalies.

Coupling of charge and lattice degrees of freedom in materials can produce intriguing electronic phenomena, such as conventional superconductivity where the electrons are mediated by lattice for creating supercurrent. The Mott transition, which is a source for many fascinating emergent behaviors, is originally thought to be driven solely by correlated electrons with an Ising criticality. Recent studies on the known Mott systems have shown that the lattice degree of freedom is also at play, giving rise to either Landau or unconventional criticality. However, the underlying coupling mechanism of charge and lattice degrees of freedom around the Mott critical end point remains elusive, leading to difficulties in understanding the associated Mott physics. Here, we report a study of Mott transition in cubic PbCrO_{3} by measuring the lattice parameter, using high-pressure x-ray diffraction techniques. The Mott criticality in this material is revealed with large lattice anomalies, which is governed by giant viscoelasticity that presumably results from a combination of lattice elasticity and electron viscosity. Because of the viscoelastic effect, the lattice of this material behaves peculiarly near the critical end point, inconsistent with any existing university classes. We argue that the viscoelasticity may play as a hidden degree of freedom behind the Mott criticality.

Growth mechanisms of vermicular graphite in cast iron

To steadily produce high-quality vermicular graphite cast iron (VGI), it is essential to have a deep understanding of the growth mechanism of vermicular graphite. The morphology and microstructure of graphite in VGI were studied using various techniques, including both experimental and theoretical methods. An abnormal graphite structure with an anomalous layer distance of 6.60 Å was observed by transmission electron microscopy (TEM) imaging, which is approximately two times larger than the normal structure (3.40 Å). The theoretical simulations of possible trace elements added (approximately 34 different elements) to the graphite basal planes were studied and analyzed individually to understand the observed phenomenon and their effects on graphite growth. The theoretical calculations (6.56 Å) show good agreement with the experimental results (6.60 Å), which proves that the lattice anomaly is caused by Mg. The abnormal graphite was believed to be the remains of the transition state during the growth of graphite. Based on the investigation, a mechanism is proposed to describe the influence of Mg during the growth process of vermicular graphite.

Magnetically induced local lattice anomalies and low-frequency fluctuations in the Mott insulator La2O3Fe2Se2

We report a study of the Mott insulator ${\mathrm{La}}_{2}{\mathrm{O}}_{3}{\mathrm{Fe}}_{2}{\mathrm{Se}}_{2}$ by means of $^{139}\mathrm{La}$ nuclear quadrupole resonance (NQR). The NQR spectra evidence a single La site in the paramagnetic phase and two inequivalent La sites, La1 and La2, in the antiferromagnetic phase. These two sites are characterized by different quadrupole couplings, indicative of distinct lattice configurations segregated in domains. The dependence of the quadrupole coupling for La2 on temperature suggests that the structural distortion is driven by the magnetic order parameter. The nuclear transverse relaxation rate $1/{T}_{2}$ evidences fluctuations in the paramagnetic phase with characteristic frequencies well below the Heisenberg exchange frequency and likely associated with nematic fluctuations.

Variable-period oscillations in optical spectra in sub-bandgap long wavelength region: signatures of new dispersion of refractive index?

Variable-period oscillations (VPOs) in various optical spectra in sub-bandgap long wavelength region, including reflection, transmission, absorption, and even luminescence spectra, are a frequently observed phenomenon in various semiconducting and dielectric films. These functional films include nitrides, oxides, silicides, sulfides, and perovskites. Although the phenomenon is widely known to be caused by optical interference, a generalized analytical model for it has not yet been established, probably due to both varying oscillation period and amplitude. In this article, we attempt to develop such a model by introducing a new concise dispersion of the sub-bandgap refractive index, i.e. containing a frictional component. In particular, we show that the VPOs in the reflectance and corresponding transmission shall have reverse maxima and minima in intensity, giving a self-consistent explanation to the existing experimental results in literature. Furthermore, the present analytical method is proven to be accurate for finding optical constants and thicknesses of films. Finally, the rationality of the new dispersion is further argued in terms of a generalized refractive index of crystal with lattice anomalies (e.g. defected and strained lattice), which was derived by Gonçalves et al from the fractional Drude–Lorentz model. In addition, the equivalency between the VPOs and Newton’s rings in energy space is discussed based on the present model.

The intricate lattice dynamics of perovskite oxides

The lattice dynamics of perovskite oxides are investigated theoretically within the polarizability model which is a nonlinear approach taking into account anharmonic electron-lattice interactions. The emphasis is on local lattice anomalies which appear in the phonon modes dispersion. Above the actual phase transition temperature finite size polarized clusters appear at an onset temperature TBH which marks the precursor formation temperature. These local anomalies appear as well experimentally and are reviewed for various perovskite compounds.

Pseudospin-lattice coupling and electric control of the square-lattice iridate Sr2IrO4

$\rm Sr_2IrO_4$ is an archetypal spin-orbit-coupled Mott insulator and has been extensively studied in part because of a wide range of predicted novel states. Limited experimental characterization of these states thus far brings to light the extraordinary susceptibility of the physical properties to the lattice, particularly, the Ir-O-Ir bond angle. Here, we report a newly observed microscopic rotation of the IrO$_6$ octahedra below 50~K measured by single crystal neutron diffraction. This sharp lattice anomaly provides keys to understanding the anomalous low-temperature physics and a direct confirmation of a crucial role that the Ir-O-Ir bond angle plays in determining the ground state. Indeed, as also demonstrated in this study, applied electric current readily weakens the antiferromagnetic order via the straightening of the Ir-O-Ir bond angle, highlighting that even slight change in the local structure can disproportionately affect the physical properties in the spin-orbit-coupled system.

High oxygen pressure floating zone growth and crystal structure of the metallic nickelates R4Ni3O10 ( R=La,Pr )

Single crystals of the metallic Ruddlesden-Popper trilayer nickelates R$_4$Ni$_3$O$_{10}$ (R=La, Pr) were successfully grown using an optical-image floating zone furnace under oxygen pressure (pO$_2$) of 20 bar for La$_4$Ni$_3$O$_{10}$ and 140 bar for Pr$_4$Ni$_3$O$_{10}$. A combination of synchrotron and laboratory x-ray single crystal diffraction, high-resolution synchrotron x-ray powder diffraction and measurements of physical properties revealed that R$_4$Ni$_3$O$_{10}$ (R=La, Pr) crystallizes in the monoclinic $P$2$_1$/$a$ (Z=2) space group at room temperature, and that a metastable orthorhombic phase ($Bmab$) can be trapped by post-growth rapid cooling. Both La$_4$Ni$_3$O$_{10}$ and Pr$_4$Ni$_3$O$_{10}$ crystals undergo a metal-to-metal transition (MMT) below room temperature. In the case of Pr$_4$Ni$_3$O$_{10}$, the MMT is found at ~157.6 K. For La$_4$Ni$_3$O$_{10}$, the MMT depends on the lattice symmetry: 147.5 K for $Bmab$ vs. 138.6 K for $P$2$_1$/$a$. Lattice anomalies were found at the MMT that, when considered together with the pronounced dependence of the transition temperature on subtle structural differences between $Bmab$ and $P$2$_1$/$a$ phases, demonstrates a not insignificant coupling between electronic and lattice degrees of freedom in these trilayer nickelates.

First-order structural transition and pressure-induced lattice/phonon anomalies in Sr2IrO4

We investigate the crystal structure and lattice vibrations of Sr$_2$IrO$_4$ by a combined phonon Raman scattering and x-ray powder diffraction experiment under pressures up to $66$ GPa and room temperature. Density functional theory (DFT) and $ab$-initio lattice dynamics calculations were also carried out. A first-order structural phase transition associated with an $8$ % collapse of the $c$-axis is observed at high pressures, with phase coexistence being observed between $\sim 40$ and $55$ GPa. At lower pressures, lattice and phonon anomalies were observed, reflecting crossovers between isostructural competing states. A critical pressure of $P_1=17$ GPa is associated with: (i) a reduction of lattice volume compressibility and a change of behavior of the tetragonal $c/a$ ratio take place above $P_1$; (ii) a four-fold symmetry-breaking lattice strain associated with lattice disorder; (iii) disappearance of two Raman active modes (at $\sim 180$ and $\sim 260$ cm$^{-1}$); and (iv) development of an asymmetric Fano lineshape for the $\sim 390$ cm$^{-1}$ mode. DFT indicates that the phase above $P_1$ is most likely non-magnetic. Exploring the similarities between iridate and cuprate physics, we argue that these observations are consistent with the emergence of a rotational symmetry-breaking electronic instability at $P_1$, providing hints for the avoided metallization under pressure and supporting the hypothesis of possible competing orders that are detrimental to superconductivity in this family. Alternative scenarios for the transition at $P_1$ are also suggested and critically discussed. Additional phonon and lattice anomalies in the tetragonal phase are observed at $P_2=30$ and $P_3=40$ GPa, indicating further competing phases that are stabilized at high pressures.

Local lattice distortions vs. structural phase transition in NdFeAsO1−xFx

The lattice properties at low temperatures of two samples of NdFeAsO1−xFx (x=0.05 and 0.25) have been examined in order to investigate possible structural phase transition that may occur in the optimally doped superconducting sample with respect to the non-superconducting low-F concentration compound. In order to detect small modifications in the ion displacements with temperature micro-Raman and high resolution synchrotron powder diffraction measurements were carried out. No increase of the width of the (220) or (322) tetragonal diffraction peaks and microstrains could be found in the superconducting sample from synchrotron XRD measurements. On the other hand, the atomic displacement parameters deviate from the expected behavior, in agreement with modifications in the phonon width, as obtained by Raman scattering. These deviations occur around 150K for both F dopings, with distinct differences among the two compounds, i.e., they decrease at low doping and increase for the superconducting sample. The data do not support a hidden phase transition to an orthorhombic phase in the superconducting compound, but point to an isostructural lattice deformation. Based on the absence of magnetic effects in this temperature range for the superconducting sample, we attribute the observed lattice anomalies to the formation of local lattice distortions that, being screened by the carriers, can only acquire long-range coherence by means of a structural phase transition at low doping levels.

Structural phase transition in La2/3Ba1/3MnO3 perovskite: Elastic, magnetic, and lattice anomalies and microscopic mechanism

The temperature dependences of the elastic and magnetic properties of polycrystalline perovskite manganite La2/3Ba1/3MnO3 were studied using ultrasonic and SQUID magnetometer techniques. The minimum of the temperature-dependent sound velocity v(T) and corresponding maximum of the decrement δ(T) were found in the vicinity of the structural phase transition R3̄c↔Imma at Ts ∼ 200 K. Large alterations of v and δ indicate a structural phase transition of the soft mode type. A high sensitivity of dc magnetization to a low uniaxial pressure caused by the softening was found in the Ts region. A negative value of the linear thermal expansion coefficient along one of the crystallographic axis was found in the Imma phase near Ts. The proposed microscopic mechanism explains the appearance of the soft mode in the vicinity of the structural phase transition temperature associated with the displacement of the manganese atom from the center of the oxygen octahedron.

Electronic, magnetic, and transport properties of the isotypic aluminides SmT2Al10 (T = Fe, Ru)

We report the results of a comprehensive physical and magnetic property study of the new isotypic aluminides SmT2Al10 (T = Fe, Ru). These two compounds are members of a rare-earth based system which has become an exemplary case study of the interplay of magnetism and correlated electron phenomena. SmFe2Al10 and SmRu2Al10 are found to order in a putative antiferromagnetic spin arrangement at TN = 14.5 K and 12.5 K, respectively. Moreover, SmRu2Al10 shows a further phase transition at TSR = 5 K which is likely due to spin reorientation. The susceptibility of SmFe2Al10 points to a valence instability of the Sm ionic state at intermediate temperatures well above TN. Electronic and thermal transport confirm that SmFe2Al10 undergoes an antiferromagnetic superzone gap formation below TN, whereas SmRu2Al10 suffers a lattice anomaly driven magnetoelastic coupling at TN. Below TN, the physical properties of SmT2Al10 (T = Fe, Ru) are governed by magnons with an antiferromagnetic spin-wave spectrum that reveals spin-gap opening. Our findings in this work have exposed a new anomalous correlated compound in the RT2Al10 series. SmFe2Al10 has a magnetic ordered ground state in spite of an unstable valence at higher temperature. This is comparable with CeRu2Al10, which is a unique and controversial Kondo insulator that orders antiferromagnetic at TN = 27 K. Among the series of rare-earth RT2Al10 compounds, the presented Sm compounds are two new members with anomalously high magnetic ordering temperatures, and it is envisaged that together with the two very well studied compounds CeRu2Al10 and CeOs2Al10 our presented studies will enable a broader approach towards understanding the fascinating properties of this materials class.

Pressure induced lattice anomalies in pnictides

Hydrostatic pressure Raman and synchrotron XRD measurements at room temperature have been carried out on a series of NdFeAsO1−xFy (Nd1111) oxypnictides in order to investigate pressure-induced lattice modifications. The synchrotron XRD data indicate that there is an increased deviation of the lattice constants from smooth pressure dependence in the superconducting compound, in close agreement with the results from SmFeAsO1−xFx (Sm1111), although the effect is less pronounced in Nd1111. As in Sm1111 the hydrostatic pressure Raman measurements show that the A1g mode of the rare earth atom deviates from the linear pressure dependence. Anomalous pressure dependence and a hysteresis is observed in the phonon width of the phonon modes. The calculated Grüneisen parameter for the Nd phonon is very similar to the corresponding value for SmFeAsO1−xFx compound and it does not vary with substitution. For the As mode it has a lower value indicating a stiffer phonon with the increased ion size. In connection with the XRD measurements the Raman data show a sudden increase of the pressure-induced lattice anomalies close to doping where the compounds become superconducting.

Temperature-driven phase transformation in Y3Co: Neutron scattering and first-principles studies

Contrary to previous studies that identified the ground state crystal structure of the entire ${R}_{3}$Co series ($R$ is a rare earth) as orthorhombic $Pnma$, we show that Y${}_{3}$Co undergoes a structural phase transition at ${T}_{t}\ensuremath{\simeq}160$ K. Single crystal neutron diffraction data reveal that at ${T}_{t}$ the trigonal prisms formed by a cobalt atom and its six nearest-neighbor yttrium atoms experience distortions accompanied by notable changes of the Y-Co distances. The formation of the low-temperature phase is accompanied by a pronounced lattice distortion and anomalies seen in heat capacity and resistivity measurements. Density functional theory calculations reveal a dynamical instability of the $Pnma$ structure of Y${}_{3}$Co. In particular, a transversal acoustic phonon mode along the $(00\ensuremath{\xi})$ direction has imaginary frequencies at $\ensuremath{\xi}<1/4$. Employing inelastic neutron scattering measurements we find a strong damping of the $(00\ensuremath{\xi})$ phonon mode below a critical temperature ${T}_{t}$. The observed structural transformation causes the reduction of dimensionality of electronic bands and decreases the electronic density of states at the Fermi level that identifies Y${}_{3}$Co as a system with the charge density wave instability.

Non-linear lattice response of Sm oxypnictides to hydrostatic pressure

Hydrostatic pressure Raman measurements at room temperature have been carried out on the SmFeAsO (Sm1111) series of oxypnictides with various substitutions (F for O and Co for Fe) and transition temperature in order to investigate lattice modifications and their connection to doping and superconductivity. Synchrotron XRD data on some of these compounds indicated that at low doping the lattice constants vary smoothly with pressure, but with further increasing of the carrier concentration there is a deviation from the normal equation of state and these effects are related with modifications in the superconducting FeAs4 tetrahedra. The hydrostatic pressure Raman measurements indicate that the A1g mode of the rare earth atom for the superconducting compounds deviates from the linear pressure dependence at the same pressures where the XRD results show pressure-induced lattice anomalies. A similar anomaly is found for the As phonon of the same symmetry. As in cuprates, the effect is diminished in the non-superconducting compounds and it is not related with the F substitution being present in the Sm(Fe1−xCox)AsO as well. The calculated Grüneisen parameters indicate a more anharmonic phonon for the Fe atom compared with the Sm and As atoms.