Temperature-dependent X-ray Diffraction Research Articles

Foldable and scrollable graphene paper with tuned interlayer spacing as high areal capacity anodes for sodium-ion batteries

Bulk graphitic carbon materials with expanded interlayer spacing and abundant defects may serve as efficient anodes with high areal capacities for sodium-ion batteries (SIBs). However, obtaining long-range order in bulk graphitic carbon materials with expanded interlayer spacing is extremely difficult. Herein, an explosive decomposition method is demonstrated to realize thermally reduced graphene oxide (rGO) paper with tuned interlayer spacing. The in-situ temperature-dependent X-ray diffraction (td-XRD) was utilized to establish the correlation between heating rate and interlayer spacing. Based on td-XRD results, free-standing graphene paper (FSG) was synthesized at 450°C under a high heating rate of 70°C/min. The FSG exhibits a highly porous structure made of graphene flakes with abundant surface epoxy groups with an overall interlayer spacing of 0.38–0.39 nm. The rGO flakes in FSG are separated by large voids that provide fast mass transport pathways and allow expansion/contraction of crystallites upon reversible Na intercalation. The FSG was utilized directly as anodes without any binder and conductive agent and delivered an excellent reversible capacity of 290 mAh/g at a current density of 50 mA/g. Moreover, the FSG exhibited excellent foldability and rollability to create high areal density anodes, delivering an areal capacity of 0.62 mAh/cm2 at a current density of 100 μA/cm2. These results open the door to fabricate graphene material-based carbon anodes with tunable interlayer spacing for high-performance SIBs.

Robust Barium Phosphonate Metal–Organic Frameworks Synthesized under Aqueous Conditions

The design and discovery of three-dimensional crystalline metal–organic frameworks (MOFs) from linkers with phosphonate coordinating groups and even alkaline earth metals is largely undeveloped. Herein, we report a strategy for realizing new, stable, and robust barium phosphonate MOFs, termed Empa-1 and Empa-2. The two-dimensional (2D) Empa-1 or three-dimensional (3D) Empa-2 could be realized by way of systematically modulating the ratio of Ba2+ with a tetratopic phosphonate-based linker that was crafted to incorporate nitrogen-rich triazine units bridged by a fixed piperazine core. In addition to this synthetic approach, temperature-dependent synchrotron-radiation powder X-ray diffraction analysis demonstrated that the 2D Empa-1 undergoes an irreversible phase transition upon heating and subsequent dehydration to form the 3D Empa-2. Given the presence of uncoordinated phosphonic acid moieties within the structure of 3D Empa-2, the CO2 sorption capabilities are reported. We believe our ability to link the alkaline earth metal barium with a novel tetratopic phosphonate linker, as evidenced by the robust structures of Empa-1 and -2, paves the way for further exploration and discovery of new crystalline, porous frameworks with greater structural diversity, stability, and wide-scale practical applicability.

Ultrastable Lead-Free CsAgCl2 Perovskite Microcrystals for Photocatalytic CO2 Reduction.

Nowadays, there is much attention focusing on lead halide perovskite because of its admirable performances in optoelectronic applications. However, the notorious toxicity and long-term instability are two main factors limiting its widespread applications. The findings of this work demonstrate a facile synthesis process for novel lead-free CsAgCl2 perovskite microcrystals with no organic ligand involved. The fundamental properties of the CsAgCl2 microcrystals are revealed by applying temperature-dependent X-ray diffraction and photoluminescence measurements from 77 to 300 K. Furthermore, the CsAgCl2 microcrystals exhibit excellent air (60 days), thermal (100 °C), and light stability. Meanwhile, the CsAgCl2 microcrystals have shown exciting potential applications in the fields of photocatalysis and photoelectrochemistry.

Distorted octahedra induced anisotropic strain and local disorder in delafossite CuCrO2

Delafossites (ABO2) are of great interest since they exhibit interesting structural and magnetic transitions. Edge shared geometry of octahedra in delafossites gives rise to distortion in octahedra. These distortions can influence the structural and physical properties of these materials. In the present study, the temperature dependence of structural changes related to CrO6 octahedra in CuCrO2 (a member of the delafossite family) has been investigated. Rietveld analysis of room temperature XRD data indicates that anisotropic strain, along with preferred orientation, is present in the sample. At room temperature, the CrO6 octahedra were found distorted. Careful analysis of temperature-dependent (300–610K) synchrotron X-ray diffraction data indicates that the degree of distortion of octahedra decreases with increasing temperature. Bond length Cu–O shows negative thermal expansion, which was driven by the reduction in the distortion of octahedra with increasing temperature. The existence of distorted octahedra has been proposed to be the reason for breaking the local symmetry, which is anticipated to be responsible for the presence of experimentally observed forbidden modes in the Raman spectra.

Direct observation of intermediate twinning in the phase transformations of ferroelectric potassium sodium niobate

In this study, temperature-dependent X-ray diffraction and transmission electron microscopy have been combined to reveal the mechanisms in the first-order phase transitions of ferroelectric K0·5Na0·5NbO3. Hereby, abnormal grain growth has been suppressed in the bulk material since it is a feature known to inhibit measurement reproducibility due to the introduced inhomogeneities. It is shown that phase transitions in this ceramic are multi-step processes, which feature an intermediate twinning stage. As perovskite-type ceramics can be regarded as model systems with respect to displacive transformations, the presented experiments give an insight into nucleation, propagation path and twinning mechanisms in ceramic materials.

The origin of chemical inhomogeneity in lead-free potassium sodium niobate ceramic: Competitive chemical reaction during solid-state synthesis

Excellent electromechanical properties have been reported in lead-free potassium sodium niobate (KNN)-based piezoelectric ceramics over the past 15 years. However, poor reproducibility of the electromechanical properties of KNN has been a major barrier to industrial development, due to a lack of full understanding of some of the processing aspects. Chemical inhomogeneity is one of the most critical challenges in processing, as the properties of KNN-based ceramics are strongly composition-dependent. In the present study, in-situ temperature-dependent X-ray diffraction and thermogravimetric analysis were employed to investigate the chemical reactions that occur during solid-state synthesis. Chemical homogeneity was found to be sensitive to the competition among reactants during synthesis. The phenomenon can reasonably explain the chemical inhomogeneity in KNN and possibly other lead-free piezoelectric ceramic systems prepared via solid-state synthesis.

Temperature-induced structural phase transitions in RERhSn (RE = Y, Gd-Tm, Lu)

Abstract The structures of the equiatomic stannides RERhSn with the smaller rare earth elements Y, Gd-Tm and Lu were reinvestigated on the basis of temperature-dependent single crystal X-ray diffraction data. GdRhSn crystallizes with the aristotype ZrNiAl at 293 and 90 K. For RE = Y, Tb, Ho and Er the HP-CeRuSn type (approximant with space group R3m) is already formed at room temperature, while DyRhSn adopts the HP-CeRuSn type below 280 K. TmRhSn and LuRhSn show incommensurate modulated variants with superspace groups P31m(1/3; 1/3; γ) 000 (No. 157.1.23.1) (γ = 3/8 for TmRhSn and γ = 2/5 for LuRhSn). The driving force for superstructure formation (modulation) is a strengthening of Rh–Sn bonding. The modulation is expressed in a 119Sn Mössbauer spectrum of DyRhSn at 78 K through line broadening.

Spin–phonon interaction and magnetoelastic coupling associated with unusual cluster-glass states in YFe0.9Cr0.1O3

The magnetic, optical phonon behavior of the polycrystalline YFe0.9Cr0.1O3 (YFC-10), prepared by solid-state reaction route, had been studied via static and ac magnetization measurements and Raman spectroscopic techniques respectively. The results indicated that YFe0.9Cr0.1O3 undergoes unusual cluster-glass transitions at low temperatures viz. Tf1 (SG1) = 108 K (Freez.1) and Tf2 (SG2) = 30 K (Freez.2) coexisting with the long-range weak ferromagnetic (WFM) state. Raman spectroscopic (RS) measurements revealed 12 distinct modes of vibrations at all temperatures with 83–433 K. Interestingly, the freezing transition at Tf1 is found to be accompanied by significant phonon renormalization. From the detailed analysis of the line-shape function of the Raman modes, the anomalous T-variation of the peak position and the linewidths suggests the existence of spin-phonon interaction in the present solid solution. Moreover the temperature dependent synchrotron X-ray diffraction studies between 10 and 300 K revealed the anomalous thermal variation of the lattice parameters and microscopic structural parameters such as super-exchange angles and (Fe/Cr)O6 octahedral bond lengths and bond angles at both the freezing transitions. Overall, the system exhibits the strong magnetoelastic and spin phonon coupling over a broad thermal regime viz. from Liquid. N2 to temperatures above room temperature (RT = 300 K) which makes YFe0.9Cr0.1O3 reliable for application purpose.

Tailoring growth modes by excess alkali addition in magnetron sputtered potassium sodium niobate thin films

This work reports on the deposition of thin film potassium sodium niobate sputtered from equimolar and excess alkali targets, showing that the growth mode can be shifted towards recrystallized grain growth without substrate heating, while structural and electrical material properties are improved. In particular, we investigate the influence of abnormal grain growth and its attenuation in KNN thin films. This is accompanied by a comprehensive crystallographic analysis of the obtained crystal structures involving wide-range reciprocal space mapping, temperature-dependent X-ray diffraction and elemental analysis associated with the different growth modes. Moreover, we report on the phase segregation within the processed thin film discovered by scanning transmission electron microscopy, which does not show in conventional elemental analysis of the film surface. Verification of the ferroelectric properties of the films is given by piezoresponse force microscopy, which also enables a study of the ferroelectric domains.

Surface and bulk ferroelectric phase transition in super-tetragonal BiFeO3 thin films

The temperature-dependent ferroelectric properties of super-tetragonal ${\mathrm{BiFeO}}_{3}$ are investigated using surface-sensitive low-energy electron microscopy (LEEM). We use epitaxial oxide ${\mathrm{BiFeO}}_{3}/{\mathrm{Ca}}_{0.96}{\mathrm{Ce}}_{0.04}{\mathrm{MnO}}_{3}$ bilayers grown by pulsed laser deposition on ${\mathrm{YAlO}}_{3}$ substrates. Ferroelectric, micrometer-scale domains are written by piezoresponse force microscopy and subsequently observed by LEEM from room temperature up to about 950 K. Kelvin probe force microscopy and LEEM spectroscopy reveal that the surface potential is efficiently (>50%) screened by adsorbates that are only released after annealing above 873 $\ifmmode\pm\else\textpm\fi{}$ 50 K in ultrahigh vacuum. The surface structure and chemistry of the ferroelectric thin films are analyzed using scanning transmission electron microscopy, electron energy loss spectroscopy, and x-ray photoelectron spectroscopy, discarding the occurrence of a putative ``skin layer'' effect. While its magnetic and structural transitions were reported in the literature, the true, ferroelectric Curie temperature of super-tetragonal ${\mathrm{BiFeO}}_{3}$ has not been determined so far. Here, we measure a Curie temperature of 930 $\ifmmode\pm\else\textpm\fi{}$ 30 K for the super-tetragonal ${\mathrm{BiFeO}}_{3}$ surface and corroborate it with volume-sensitive, temperature-dependent x-ray diffraction measurements. These results suggest that LEEM can be used as a powerful tool to probe surface charge and ferroelectric transitions in ultrathin films.

The mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics

(K,Na)NbO3 based ceramics are considered to be one of the most promising lead-free ferroelectrics replacing Pb(Zr,Ti)O3. Despite extensive studies over the last two decades, the mechanism for the enhanced piezoelectricity in multi-elements doped (K,Na)NbO3 ceramics has not been fully understood. Here, we combine temperature-dependent synchrotron x-ray diffraction and property measurements, atomic-scale scanning transmission electron microscopy, and first-principle and phase-field calculations to establish the dopant–structure–property relationship for multi-elements doped (K,Na)NbO3 ceramics. Our results indicate that the dopants induced tetragonal phase and the accompanying high-density nanoscale heterostructures with low-angle polar vectors are responsible for the high dielectric and piezoelectric properties. This work explains the mechanism of the high piezoelectricity recently achieved in (K,Na)NbO3 ceramics and provides guidance for the design of high-performance ferroelectric ceramics, which is expected to benefit numerous functional materials.

Large electrocaloric effect in lead-free ferroelectric Ba0.85Ca0.15Ti0.9Zr0.1O3 thin film heterostructure

A large electrocaloric effect is reported in a strain-engineered Ba0.85Ca0.15Ti0.9Zr0.1O3 (BCZT) thin film heterostructure driven by the near room-temperature electro-structural phase transition. An epitaxial BCZT/La0.7Sr0.3MnO3 (BCZT/LSMO) heterostructure was grown on a single-crystal SrTiO3 (100) substrate using pulsed laser deposition. In-depth x-ray diffraction and x-ray spectroscopic analyses revealed the single-crystalline nature and stoichiometric growth of the heterostructure. Both temperature dependent x-ray diffraction and dielectric measurements revealed a broad second-order-type phase transition near 430 K in the BCZT/LSMO heterostructure. From detailed theoretical analyses of the experimental data, it was confirmed that the phase transition around 430 K is second-order in nature, unlike the first-order transition observed in bulk BCZT materials. Thermodynamic analyses of polarization revealed an unprecedently large adiabatic temperature change of 13.5 K at 430 K under a field change of 1000 kV cm−1, hitherto unobserved in a lead-free material. Extremely broad adiabatic temperature change ΔT(T) curves over a wide working range of temperatures (330 K < T < 480 K) resulted in enhanced relative cooling powers, which are higher than those reported so far in most electrocaloric materials. We propose that an interfacial strain-induced enhanced tetragonal distortion of the BCZT layer gives rise to these large electrocaloric effects in the BCZT/LSMO heterostructure system. The demonstration of a large electrocaloric effect in the lead-free BCZT thin film may open up new pathways toward the design of artificial heterostructures for eco-friendly solid-state cooling applications.

Discotic liquid crystals of silicon phthalocyanine-based singledeckers and doubledeckers: Influence of the axial substituent groups on their mesomorphism

Three kinds of silicon phthalocyanine derivatives axially substituted by different groups were prepared and Singledecker-C[Formula: see text]PcSiCl2 (1a), Singledecker-C[Formula: see text]PcSi2(OH)4 (3a, b) and Doubledecker-(C[Formula: see text]PcSi)2Cl2 (4a, b) were obtained in order to investigate their mesomorphism for [Formula: see text] = 8 (a) and 12 (b). The mesomorphism was established by using polarizing microscopic observations, differential scanning calorimetric measurements, temperature-dependent electronic absorption spectra and temperature-dependent X-ray diffraction analysis. From the results, each of the derivatives, 1a, 3a, 3b, 4a and 4b showed columnar mesophases; 1a: Col[Formula: see text](P21/a), pseudo Col[Formula: see text] and Col[Formula: see text]; 3a, b: Col[Formula: see text](P21/a) and Col[Formula: see text]; 4a, b: two kinds of Col[Formula: see text]. Interestingly, each of the virgin samples of singledeckers, C8PcSiCl2 (1a), C8PcSi2(OH)4 (3a) and C8PcSi2(OH)4 (3a), were green in color but changed into blue over ca.180–190 [Formula: see text]C by polymerization. On the other hand, each of the double-deckers, (C[Formula: see text]PcSi)2Cl2derivatives (4a and 4b), decomposed over ca. 180–240[Formula: see text]C. Thus, the influence of the axial groups is so large on their mesomorphism that they exhibit unusual mesomorphic behaviors accompanied by color change.

Confinement of pyrrolidinium-based ionic liquids [CnMPyrr]+[Tf2N]− with long cationic alkyl side chains (n = 10 and 16) to nanoscale pores: Dielectric and calorimetric studies

Ionic liquids (ILs) with long cationic side chains have shown potential applications in wide fields such as phase-change materials for seasonal energy storage, environmentally friendly herbicides, components of ideal electrode material for supercapacitor devices, and more. In this paper, we investigated the glass transition dynamics, charge transport and structure properties of two pyrrolidinium-based ILs, [CnMPyrr]+[Tf2N]− with n = 10 and 16. Differential scanning calorimetry studies demonstrated the poor glass forming ability (GFA) of the tested materials, especially [C16MPyrr]+[Tf2N]−. Temperature-dependent X-ray diffraction measurements suggested the layered structures in the crystalline and liquid phases of both ILs. Through the dielectric measurements of [C10MPyrr]+[Tf2N]−, we noticed its higher conductivity than the homologues with shorter cationic alkyl chains, in the supercooled liquid state. Additionally, the derivative spectra, εder′′ = (−π/2)[∂ε′/∂ ln (f)], of [C10MPyrr]+[Tf2N]− at temperatures above Tg were examined, and no evidence of the slow sub- α mode associated with the self-aggregation of alkyl chains is detected. Next, we checked how [C10MPyrr]+[Tf2N]− and [C16MPyrr]+[Tf2N]− behaved when geometrically confined in alumina nanopores with various sizes ranging from 10 nm to 120 nm. With respect to the bulk material, confined [C10MPyrr]+[Tf2N]− samples are featured by the drastically enhanced GFA but up to 300-fold decrease in ionic conductivity. In addition, for [C16MPyrr]+[Tf2N]−, vitrification is unfulfilled even when confined into 10 nm nanopore. Nonetheless, nanoscale constraints bring pronounced melting temperature depression and enhanced conductivity to the crystalline confined samples relative to the bulk counterpart. The results of this study contribute to a more comprehensive picture of various confinement effects on ILs and will benefit the rational design of ILs-containing nanodevices.

Low-Temperature Crystal Structure and Mean-Field Modeling of ErxDy1−xAl2 Intermetallics

Low-temperature crystal structure of the ErxDy1−xAl2 alloys with x = 0.45, 0.67, 0.90 was examined using temperature-dependent powder X-ray diffraction. The Er-rich sample, Er0.9Dy0.1Al2, exhibits a rhombohedral distortion associated with the magnetic ordering that occurs around 20 K. The rhombohedral distortion is suppressed in Er0.67Dy0.33Al2, while a weak low-temperature tetragonal distortion is observed in Er0.45Dy0.55Al2. The mean-field theory supports the correlation between the type of structural distortion and the variable easy magnetization axis in ErxDy1−xAl2 intermetallics.

Lattice dynamics of the topological Dirac semimetal LaAgSb2 with charge density wave ordering

LaAgSb$_{2}$ is a rare material, which offers the opportunity to investigate the complex interplay between charge density wave (CDW) ordering and topology protected electronic band structure. As both of these phenomena are governed by the structural symmetries, a comprehensive study of the lattice dynamics is highly desirable. In this report, we present the results of temperature and pressure dependent Raman spectroscopy and x-ray diffraction in single crystalline LaAgSb$_{2}$. Our results confirm that Raman spectroscopy is a highly sensitive tool to probe CDW ordering phenomenon, particularly the low-temperature second CDW transition in LaAgSb$_{2}$, which appears as a very weak anomaly in most experiments. The crystal orientation-dependent measurements provide the evolution of Raman modes with crystallographic symmetries and can be further studied through group symmetry analysis. The low-temperature x-ray diffraction data show the emergence of structural modulations corresponding to the CDW instability. The combined high-pressure Raman spectroscopy and synchrotron x-ray diffraction reveal multiple structural phase transitions through lowering of crystalline symmetries, which are also expected to lead to electronic topological transitions.

Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal15R−BaMnO3

Spin-phonon coupling, the interaction of spins with surrounding lattice is a key parameter to understand the underlying physics of multiferroics and engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal BaMnO3 are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of 15R-BaMnO3 at 230 K, 280 K, and 330 K. Magnetic measurements reveal the presence of a long-range and a short-range magnetic ordering in 15R-BaMnO3 at 230 K and 330 K, respectively. The Raman spectrum shows the appearance of new Raman modes in the magnetically ordered phases. Furthermore, an additional Raman phonon appears below ~ 280 K, possibly arising from a local lattice-distortion due to the displacement of Mn-ions, that exhibits anomalous shift with temperature. The origin of the observed renormalization and phonon anomalies in Raman spectra are discussed based on the evidences from temperature- and magnetic-field-dependent Raman spectra, temperature-dependent x-ray diffraction, magnetization, and specific heat measurements. Our results indicate the presence of magnetostriction and spin-phonon coupling in 15R-BaMnO3 thus suggesting that the optical phonons are strongly correlated to its magnetoelectric properties.

Enhanced Low-Temperature Thermoelectric Performance in (PbSe)1+δ(VSe2)1 Heterostructures due to Highly Correlated Electrons in Charge Density Waves.

We explore the effect of charge density wave (CDW) on the in-plane thermoelectric transport properties of (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 heterostructures. In (PbSe)1+δ(VSe2)1 we observe an abrupt 86% increase in the Seebeck coefficient, 245% increase in the power factor, and a slight decrease in resistivity over the CDW transition. This behavior is not observed in (PbSe)1+δ(VSe2)2 and is rather unusual compared to the general trend observed in other materials. The abrupt transition causes a deviation from the Mott relationship through correlated electron states. Raman spectra of the (PbSe)1+δ(VSe2)1 material show the emergence of additional peaks below the CDW transition temperature associated with VSe2 material. Temperature-dependent in-plane X-ray diffraction (XRD) spectra show a change in the in-plane thermal expansion of VSe2 in (PbSe)1+δ(VSe2)1 due to lattice distortion. The increase in the power factor and decrease in the resistivity due to CDW suggest a potential mechanism for enhancing the thermoelectric performance at the low temperature region.

Anomalous thermal expansion in K0.55WO3.275

Monoclinic K0.55WO3.275 has been synthesized by solid state reaction method. The space group is P2/m with cell parameters a = 15.9753(2) Å, b = 7.73149(8) Å, c = 10.9393(1) Å and β = 108.477(1) ° at 313 K. The intrinsic linear coefficients of thermal expansion calculated by temperature-dependent X-ray diffraction data are 14.70 × 10−6 K−1 (253–323 K), 3.38 × 10−6 K−1 (323–363 K) and 11.52 × 10−6 K−1 (363–433 K), respectively, which are consistent with the apparent ones obtained by thermal dilatometer. The a axis shows positive thermal expansion from 253–433 K. The b and c axes display negative thermal expansion in the range of 323–363 K, and positive thermal expansion in two ranges of 253–323 K and 363–433 K. Three W–O–W bending modes with negative Grüneisen parameter may be responsible for the low thermal expansion in the range of 323–363 K.

Structural features of solid-solid phase transitions and lattice dynamics in U3O8

Triuranium octoxide $({\mathrm{U}}_{3}{\mathrm{O}}_{8})$ undergoes an orthorhombic to hexagonal structural phase transition near ${T}_{s}=305{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, and a separate nonstructural phase transition at ${T}_{c}=210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. The later transition has previously been associated with temperature-induced fluctuations in the uranium oxidation state. A discontinuity in the slope of electrical conductivity versus temperature measurement at $210{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ has supported this idea. The orthorhombic phase has three crystallographic sites in two distinct oxidation configurations [2 U(V) and 1 U(VI)], whereas the hexagonal phase has one distinct uranium site. High-resolution x-ray diffraction measurements eliminate the possibility of superlattice Bragg reflections to less than 0.2 ${\mathrm{e}}^{\ensuremath{-}}$ scattering power and ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ is not metallic; consequently, the presence of oxidation fluctuations is required for charge balancing. Interestingly, the order-to-disorder transition occurs at a much lower temperature than the structural transition. Using temperature-dependent x-ray diffraction and Raman spectroscopy, we show anisotropic lattice expansion in the in-plane $b$ and $c$ lattice constants. A specific discontinuity in the temperature derivatives of the $b$ and $c$ lattice constants are the first reported structural signatures of the order-to-disorder transition, suggestive of a change in local U--O coordination. Phonon frequencies of ${\mathrm{U}}_{3}{\mathrm{O}}_{8}$ measured by Raman spectroscopy show significant temperature-dependent dynamics. Redshifting of several modes between 40 and $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ cannot be explained by unit cell expansion alone because the unit cell volume decreases in this region. Instead, we show that phonon frequencies are highly correlated with the anisotropic lattice expansion/contraction along specific crystallographic directions.