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We report an investigation of hexagonal Y_(1-x)Eu_xMnO_3 ceramics with x=0, 0.1 and 0.2 using infrared and THz spectroscopies in the temperature range between 5 and 900 K. The temperature dependence of the THz permittivity reveals a kink near the antiferromagnetic phase transition temperature T_N ~ 70 K giving evidence of a strong spin-lattice coupling. Below T_N two absorption peaks were revealed in the THz spectra close to 43 and 73 cm-1. While the first peak corresponds to a sharp antiferromagnetic resonance exhibiting softening on heating towards TN, the second one may be attributed to an impurity mode or a multiphonon absorption peak. High-temperature THz spectra measured up to 900 K reveal only small gradual increase of the permittivity in agreement with a weak phonon softening observed in the infrared reflectance spectra upon heating. This corresponds to an improper ferroelectric character of the phase transition proposed from first principle calculations by Fennie and Rabe [Phys. Rev. B 72 (2005), pp. 100103(R)]., Comment: Phase Transitions, accepted

We describe the first-principles design and subsequent synthesis of a new material with the specific functionalities required for a solid-state-based search for the permanent electric dipole moment of the electron. We show computationally that perovskite-structure europium barium titanate should exhibit the required large and pressure-dependent ferroelectric polarization, local magnetic moments, and absence of magnetic ordering even at liquid helium temperature. Subsequent synthesis and characterization of Eu$_{0.5}$Ba$_{0.5}$TiO$_3$ ceramics confirm the predicted desirable properties.

The microwave dielectric, Bi 1.5 ZnNb 1.5 O 7 exhibits low-temperature dielectric relaxation. To find the origin of the dielectric relaxation of Bi 1.5 ZnNb 1.5 O 7 , we studied the structure and dielectric behavior of Bi 1.5 ZnNb 1.5 O 7 in detail. The Bi 1.5 ZnNb 1.5 O 7 is not composed of a single phase pyrochlore structure. Instead, it consists of unusual structure of Bi 1.5 Zn 0.92 Nb 1.5 O 6.92 and ZnO. The ZnO is distributed evenly in the grain and at the boundary of the Bi 1.5 Zn 0.92 Nb 1.5 O 6.92 structure. Many small voids (

Time-domain THz transmission experiment was performed on a $\rm SrBi_2Ta_2O_9$ film deposited on sapphire substrate. Temperatures between 300 and 923 K were investigated and complex permittivity spectra of the film were determined. The lowest frequency optic phonon near 28 cm$^{-1}$ reveals a slow monotonic decrease in frequency on heating with no significant anomaly near the phase transitions. We show that the dielectric anomaly near the ferroelectric phase transition can be explained by slowing down of a relaxational mode, observed in the THz spectra. A second harmonic generation signal observed in a single crystal confirms a loss of center of symmetry in the ferroelectric phase and a presence of polar clusters in the intermediate ferroelastic phase., Comment: subm. to J. Phys.: Condens. Matter

Infrared reflectivity spectra of cubic SrMnO$_{3}$ ceramics reveal 18 % stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T$_{N}$ = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first principles calculations. Moreover, polar phonons become Raman active below T$_{N}$, although their activation is forbidden by symmetry in $Pm\bar{3}m$ space group. This gives evidence that the cubic $Pm\bar{3}m$ symmetry is locally broken below T$_{N}$ due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small non-stoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers $n_{e}=$3.6 10$^{20}$ cm$^{-3}$ and the sample composition Sr$^{2+}$Mn$_{0.98}^{4+}$Mn$_{0.02}^{3+}$O$_{2.99}^{2-}$. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundaries., Comment: subm. to PRB

The phase purity and the lattice dynamics in bulk EuTiO3 were investigated both microscopically, using X-ray and neutron diffraction, 151-Eu-M\"ossbauer spectroscopy, and 151-Eu nuclear inelastic scattering, and macroscopically using calorimetry, resonant ultrasound spectroscopy, and magnetometry. Furthermore, our investigations were corroborated by ab initio theoretical studies. The perovskite symmetry, Pm-3m, is unstable at the M- and R- points of the Brillouin zone. The lattice instabilities are lifted when the structure relaxes in one of the symmetries: I4/mcm, Imma, R-3c with relative relaxation energy around -25 meV. Intimate phase analysis confirmed phase purity of our ceramics. A prominent peak in the Eu specific density of phonon states at 11.5 meV can be modelled in all candidate symmetries. A stiffening on heating around room temperature is indicative of a phase transition similar to the one observed in SrTiO3, however, although previous studies reported the structural phase transition to tetragonal I4/mcm phase our detailed sample purity analysis and thorough structural studies using complementary techniques did not confirm a direct phase transition. Instead, in the same temperature range, Eu delocalization is observed which might explain the lattice dynamical instabilities.

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We performed factor-group analysis of all phonons in possible monoclinic C2/c and C2 structures of BiMnO3 and compared it with our experimental infrared and Raman spectra. We conclude that the crystal structure is centrosymmetric C2/c in the whole investigated temperature range from 10 to 550 K, therefore BiMnO3 cannot be ferroelectric. We revealed a dielectric relaxation in THz spectra above the structural phase transition taking place at T_C1=475 K giving evidence in strong lattice anharmonicity and a large dynamical disorder of Bi cations above T_C1. Step-like dielectric anomaly observed at T_C1 in THz permittivity reminds antiferroelectric phase transition. Nevertheless, the low-temperature dielectric studies did not reveal any antiferroelectric or ferroelectric hysteresis loop. Our experimental results support theoretical paper of P. Baettig et al. (J. Am. Chem. Soc. 129, 9854 (2007)) claiming that BiMnO3 is not multiferroic, but only antipolar ferromagnet., accepted to JAP

We report measurements of the optical response of polycrystalline DyN thin films. The frequency-dependent complex refractive index in the near IR-visible-near UV was determined by fitting reflection/transmission spectra. In conjunction with resistivity measurements, these identify DyN as a semiconductor with an optical energy gap of 1.2 eV. When doped with nitrogen vacancies it shows free carrier absorption and a blue-shifted gap associated with the Moss-Burstein effect. The refractive index of 2.0 ± 0.1 depends only weakly on energy. Far infrared reflectivity data show a polar phonon of frequency 280 cm−1 and a dielectric strength of Δe=20.

X-ray diffraction, dynamical mechanical analysis and infrared reflectivity studies revealed an antiferrodistortive phase transition in EuTiO3 ceramics. Near 300K the perovskite structure changes from cubic Pm-3m to tetragonal I4/mcm due to antiphase tilting of oxygen octahedra along the c axis (a0a0c- in Glazer notation). The phase transition is analogous to SrTiO3. However, some ceramics as well as single crystals of EuTiO3 show different infrared reflectivity spectra bringing evidence of a different crystal structure. In such samples electron diffraction revealed an incommensurate tetragonal structure with modulation wavevector q ~ 0.38 a*. Extra phonons in samples with modulated structure are activated in the IR spectra due to folding of the Brillouin zone. We propose that defects like Eu3+ and oxygen vacancies strongly influence the temperature of the phase transition to antiferrodistortive phase as well as the tendency to incommensurate modulation in EuTiO3., PRB, in press

Compressively strained epitaxial (001) EuTiO${}_{3}$ thin films of tetragonal symmetry have been deposited on (001) (LaAlO${}_{3}$)${}_{0.29}$-(SrAl${}_{1/2}$Ta${}_{1/2}$O${}_{3}$)${}_{0.71}$ (LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the N\'eel temperature by 1 K with 0.9$%$ compressive strain was revealed. The polar phonons of the films have been investigated as a function of temperature and magnetic field by means of infrared reflectance spectroscopy. All three in-plane polarized infrared active phonons show strongly stiffened frequencies compared to bulk EuTiO${}_{3}$ in accordance with first-principles calculations. The phonon frequencies exhibit gradual softening on cooling, leading to an increase in static permittivity. Additional polar phonon with frequency near the TO1 soft mode was detected below 150 K. This mode coupled with the TO1 mode was assigned as the optical phonon from the Brillouin zone edge, which is activated in infrared spectra due to an antiferrodistortive phase transition and due to simultaneous presence of polar and/or magnetic nanoclusters. In the antiferromagnetic phase, we have observed a remarkable softening of the lowest-frequency polar phonon under an applied magnetic field, which qualitatively agrees with first-principles calculations. This demonstrates the strong spin-phonon coupling in EuTiO${}_{3}$, which is responsible for the pronounced dependence of its static permittivity on magnetic field in the antiferromagnetic phase.

Polarized microscope observation of ferroelastic domains in a SrBi2Ta2O9 (SBT) single crystal reveals the presence of domains up to Tc1≃770 K, which supports the ferroelasticity and Amam symmetry of the intermediate phase between ferroelectric and paraelectric phases. Far-infrared spectra of SBT ceramics, single crystal and thin films show a well underdamped optical soft mode at 28 cm−1, which partially softens to 21 cm−1 near the ferroelectric transition temperature (Tc2≅600 K). This softening does not explain the entire anomaly of low-frequency permittivity observed near Tc2. On the basis of high-frequency measurements, which do not show a significant dispersion, central-mode-type dispersion in the 10–100 GHz range is proposed as an explanation. So, the phase transition at Tc2 apparently shows a crossover behavior between the displacive and the order–disorder type.

Terahertz and far-infrared electric and magnetic responses of hexagonal piezomagnetic YMnO3 single crystals are investigated. Antiferromagnetic resonance is observed in the spectra of magnetic permeability mu_a [H(omega) oriented within the hexagonal plane] below the Neel temperature T_N. This excitation softens from 41 to 32 cm-1 on heating and finally disappears above T_N. An additional weak and heavily-damped excitation is seen in the spectra of complex dielectric permittivity epsilon_c within the same frequency range. This excitation contributes to the dielectric spectra in both antiferromagnetic and paramagnetic phases. Its oscillator strength significantly increases on heating towards room temperature thus providing evidence of piezomagnetic or higher-order couplings to polar phonons. Other heavily-damped dielectric excitations are detected near 100 cm-1 in the paramagnetic phase in both epsilon_c and epsilon_a spectra and they exhibit similar temperature behavior. These excitations appearing in the frequency range of magnon branches well below polar phonons could remind electromagnons; however, their temperature dependence is quite different. We have used density functional theory for calculating phonon dispersion branches in the whole Brillouin zone. A detailed analysis of these results and of previously published magnon dispersion branches brought us to the conclusion that the observed absorption bands stem from phonon-phonon and phonon- paramagnon differential absorption processes. The latter is enabled by a strong short-range in-plane spin correlations in the paramagnetic phase., Comment: subm. to PRB

The relaxor ferroelectric PbMg1/Nb2/3O3 was investigated by means of broad-band dielectric and Fourier Transform Infrared (FTIR) transmission spectroscopy in the frequency range from 1 MHz to 15 THz at temperatures between 20 and 900 K using PMN films on infrared transparent sapphire substrates. While thin film relaxors display reduced dielectric permittivity at low frequencies, their high frequency intrinsic or lattice response is shown to be the same as single crystal/ceramic specemins. It was observed that in contrast to the results of inelastic neutron scattering, the optic soft mode was underdamped at all temperatures. On heating, the TO1 soft phonon followed the Cochran law with an extrapolated critical temperature equal to the Burns temperature of 670 K and softened down to 50 cm-1. Above 450 K the soft mode frequency leveled off and slightly increased above the Burns temperature. A central mode, describing the dynamics of polar nanoclusters appeared below the Burns temperature at frequencies near the optic soft mode and dramatically slowed down below 1 MHz on cooling below room temperature. It broadened on cooling, giving rise to frequency independent losses in microwave and lower frequency range below the freezing temperature of 200 K. In addition, a new heavily damped mode appeared in the FTIR spectra below the soft mode frequency at room temperature and below. The origin of this mode as well as the discrepancy between the soft mode damping in neutron and infrared spectra is discussed.

Magnesium titanate (MgTiO3) powder was prepared by a chemical route (Pechini method) and different dopants were added to prepare several compositions. These pure and doped compositions were sintered in air and dense ceramics were obtained. The pure MgTiO3 samples were also subjected to different heat treatments during sintering. Complex permittivity spectra of ceramic samples were determined by various techniques in the 109–1014 Hz range. These techniques included infrared spectroscopy in transmission and reflectivity modes and microwave dielectric measurements. Extrapolation to microwave frequencies from infrared data, according to the proportionality ɛ″(v) ∝ v, agrees quite well with the microwave data measured at 8 GHz and it is a useful procedure to estimate intrinsic microwave losses. Fast cooling from high temperatures of MgTiO3 samples increased dielectric loss, probably due to a structural disorder. Dopants have two types of effect depending on whether they form a distinct second phase or a solid solution with MgTiO3. In this last case intrinsic losses are strongly affected.

International audience; The long-standing classification scheme of ferroelectrics into either relaxor or displacive ones (the phase transition is driven by a soft phonon mode) is too restrictive since a smooth crossover between them exists which even admits for a coexistence of both phenomena. This crossover and coexistence is a consequence of the varying density of polar nanoregions due to different doping levels of the respective system. The formation of polar nanoregions is attributed here to intrinsic local modes in terms of discrete breathers.

The dielectric response of the Rb0.5(NH4)0.5H2PO4 dipolar glass was determined in the spectral range 8-650 cm-1 and temperature range 10-300 K using BWO monochromatic transmission spectroscopy (8-18 cm-1) and Fourier transform reflection spectroscopy (20-650 cm-1). Above the glass transition temperature Tg approximately=100 K the low-frequency dynamics (below about 100 cm-1) is similar to that of an averaged pure crystal. The observed one- and two-mode behaviour in the phonon region confirms the earlier Raman results. Below Tg a broad background absorption in addition to phonon peaks remain present down to the lowest temperatures especially for the E perpendicular to c polarization. The authors suggest that this is caused by breaking of the quasi-momentum conservation selection rule. The appearance of new forbidden modes below Tg gives evidence of polar microregions. The low-frequency dispersion below Tg is caused by different collective proton-hopping process from the higher-frequency soft-mode-type hopping, probably because of specific diffusion of Takagi defects.

Sr_(9-x)PbxCe2Ti12O36 system is derived from the perovskite SrTiO3 and its chemical formula can be written as (Sr_(1-y)Pby)0.75Ce0.167TiO3. We investigated dielectric response of Sr_(9-x)PbxCe2Ti12O36 ceramics (x = 0-9) between 100 Hz and 100 THz at temperatures from 10 to 700 K using low- and high-frequency dielectric, microwave (MW), THz and infrared spectroscopy. We revealed that Sr9Ce2Ti12O36 is an incipient ferroelectric with the R-3c trigonal structure whose relative permittivity e' increases from 167 at 300 K and saturates near 240 below 30 K. The subsequent substitution of Sr by Pb enhances e' to several thousands and induces a ferroelectric phase transition to monoclinic Cc phase for x>=3. Its critical temperature Tc linearly depends on the Pb concentration and reaches 550 K for x=9. The phase transition is of displacive type. The soft mode frequency follows the Barrett formula in samples with x=3. The MW dispersion is lacking and quality factor Q is high in samples with low Pb concentration, although the permittivity is very high in some cases. However, due to the lattice softening, the temperature coefficient of the permittivity is rather high. The best MW quality factor was observed for x=1: Q*f=5800 GHz and e'=250. Concluding, the dielectric properties of Sr_(9- x)PbxCe2Ti12O36 are similar to those of Ba_(1-x)SrxTiO3 so that this system can be presumably used as an alternative for MW devices or capacitors., subm. to Chem. Mater

Infrared reflectance spectra of an epitaxial ${\mathrm{Bi}}_{2}\mathrm{Fe}\mathrm{Cr}{\mathrm{O}}_{6}$ thin film prepared by pulsed laser deposition on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ substrate were recorded between 10 and $900\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. No evidence for a phase transition to the paraelectric phase was observed, but some phonon anomalies were revealed near $600\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Most of the polar modes exhibit only a gradual softening, which results in a continuous increase of the static permittivity on heating. It indicates that the phase transition from ferroelectric to paraelectric phase should occur somewhere above $900\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Magnetic measurements of the partially $B$-site-ordered ${\mathrm{Bi}}_{2}\mathrm{Fe}\mathrm{Cr}{\mathrm{O}}_{6}$ thin film deposited on Nb-doped $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ substrate were performed up to $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. They revealed a possible magnetic phase transition between 600 and $800\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, but the exact critical temperature cannot be determined due to a strong diamagnetic signal from the substrate. Nevertheless, our experimental data show that the $B$-site-ordered ${\mathrm{Bi}}_{2}\mathrm{Fe}\mathrm{Cr}{\mathrm{O}}_{6}$ is one of the rare high-temperature multiferroics.

Pb{sub 1.83}Mg{sub 0.29}Nb{sub 1.71}O{sub 6.39} crystallizing in a cubic pyrochlore structure exhibits a typical feature of quantum paraelectrics--its permittivity continuously increases on cooling and levels off below {approx}30 K without any signature of a structural phase transition. Broadband dielectric spectra do not show any dielectric dispersion in the real part of permittivity up to 8.8 GHz. Terahertz and infrared spectra reveal a soft polar optic mode, which is responsible for the temperature dependence of the permittivity. The leveling off of the permittivity at low temperatures obeys the Barrett formula, and the fitted vibrational zero-point energy (1/2)k{sub B}T{sub 1} corresponds to the measured soft-mode frequency. The number of observed infrared phonons exceeds that predicted from the factor-group analysis, which indicates that the structure is at least locally noncubic.

Relaxor ferroelectric PbMg_1/3Ta_2/3O_3 ceramics and thin films were investigated by means of broad-band dielectric, time-domain terahertz (THz) and Fourier-transform infrared (IR) spectroscopy in the frequency range 100 Hz - 90 THz at temperatures 100 - 490 K, the THz and IR spectra were studied from 20 to 900K. Diffused and strongly temperature dependent peak in the complex permittivity is caused by a dielectric relaxation due to the dynamics of polar clusters. The relaxation appears below Burns temperature T_d in the THz range, slows down on cooling through the microwave and MHz range and anomalously broadens. The shortest and longest relaxation times of the distribution of relaxation times follow Arrhenius and Vogel-Fulcher law, respectively. The degree of B-site order has only a small influence on the parameters of the dielectric relaxation and almost no influence on the phonon parameters. Below T_m ~ 180K, the distribution of relaxation frequencies becomes broader than our experimental spectral range and frequency independent dielectric losses develop below 100 GHz in the spectra. Although the macroscopic crystal structure is cubic, IR spectra give evidence about the lower local symmetry which can be assigned to the presence of polar clusters below T_d. Infrared spectra above T_d still reveal more modes than predicted by selection rules in the paraelectric phase of the Fm-3m space group so that we suggest selection rules which take into account chemical inhomogeneity in the beta''-perovskite sublattice., Comment: subm. to J. Appl. Phys

Lattice dynamics for five ordered ${\mathrm{PbMg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_{3}$ supercells were calculated from first principles by the frozen phonon method. Maximal symmetries of all supercells are reduced by structural instabilities. Lattice modes corresponding to these instabilities, equilibrium ionic positions, and infrared reflectivity spectra were computed for all supercells. Results are compared with our experimental data for a chemically disordered PMN single crystal.

New relaxor ferroelectric system has been synthesized. BLTN exhibits a smeared maximum of permittivity, characteristic of classic relaxor behaviour, with a peak shift from 185 K at 100 Hz to 300 K at 1 GHz. BNTN undergoes a first order ferroelectric phase transition at 389 K and BLNTN exhibits both a ferroelectric phase transition at 274 K and relaxor behaviour at higher temperatures., Comment: subm. to J. Eur. Ceram. Soc., proceedings of Electroceramics IX conference, Cherbourg 2004

High-frequency dielectric studies of Bi_0.85Nd_0.15FeO_3 ceramics performed betweeen 100 and 900 K reveal hardening of most polar phonons on cooling below antiferroelectric phase transition, which occurs near 600 K. Moreover, a strong THz dielectric relaxation is seen in paraelectric phase. Its relaxation frequency softens on cooling towards Tc ~ 600 K, its dielectric strength simultaneously decreases and finaly the relaxation disappears from the spectra below 450 K. Both phonon and dielectric relaxation behavior is responsible for a decrease in the dielectric permittivity at the antiferroelectric phase transition. Origin of unusual strong THz dielectric relaxation in paraelectric phase is discussed. Bi_0.85Nd_0.15FeO_3 structrure lies on the phase boundary between polar rhombohedral and non-polar orthorhombic phase and owing to this the polarization rotation and polarization extension can enhance the piezoelectric response of this system. Similarities and discrepancies with lead-based piezoelectric perovskites, exhibiting morphotrophic phase boundary between two ferroelectric phases, are discussed., Comment: JAP, in print

We investigate 100 nm thick epitaxial monolayer of SrTiO3 strained by 1% in biaxial tension and grown on (110) DyScO3. Spectra within the whole far-infrared spectral range are obtained as a function of temperature and an electric bias. An appreciable tunability of high frequency polar phonons is observed upon bias. The splitting of TO4 mode clearly observed below 220 K, which is further increased upon bias, is a signature of the ferroelectricity of the film at these temperatures. (c) 2010 American Institute of Physics. [doi: 10.1063/1.3447812]

Infrared reflectance, terahertz transmittance, and microwave resonance measurements show that SrTiO3 films, strained by similar to 1% in biaxial tension by growing them on (110) DyScO3 substrates, undergo a pronounced phonon softening near 270 K. This in-plane soft-mode drives the ferroelectric transition. The appearance of two new low-frequency modes and splitting of the high-frequency TO4 mode provide evidence of an antiferrodistortive phase below similar to 180 K. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3271179]

Electron spin resonance (ESR), constant photocurrent method (CPM), photothermal deflection spectroscopy (PDS), Raman and IR spectroscopy have been used to measure microcrystalline silicon films. Besides standard defects with a g-value of 2.0055, new defects with a g-value ~2.0030 have been created during annealing this material. Proportionality between the subgap optical absorption and ESR spin density has been observed.

Temperature dependences of the optic phonons in PbSc1/2Ta1/2O3 (PST) sol-gel films deposited on sapphire substrates were studied by means of Fourier transform far-infrared transmission spectroscopy in the temperature range of 20-900 K. Four films displaying different B-site order with both ferroelectric and relaxor behavior were studied. In all cases the TO mode near 80 cm(-1) at 10 K softens on heating to approximate to 45 cm(-1) following the Cochran law with extrapolated critical temperature near 700 K which is 400 K above the temperature of dielectric maximum, T-m. Above 600 K the TO1 mode remains stabile. This mode can be assigned to the A(1) component of the ferroelectric soft-mode inside polar clusters which form below the Burns temperature near 700 K. In the ordered PST film another mode activates below T-m in the infrared spectra near 60 cm(-1), also exhibiting an anomalous temperature dependence due to its coupling with the former mode. It is assigned to the A(1) component of the F-2g Raman active mode. The central mode, which appears below the Burns temperature in the terahertz range, is assigned to the dynamics of polar clusters. It slows down on cooling and vanishes from our spectral range below T-m. Another overdamped excitation assigned to the E component of the soft mode appears near 30 cm(-1) at low temperature. (c) 2005 American Institute of Physics.

Relaxor ferroelectrics 0-2PbSc(1/2)Nb(1/2)O(3) - 0.4PbMg(1/3)Nb(2/3)O(3)-0.4PbZn(1/3)Nb(2/3)O(3) ceramics were studied by means of the Fourier transform infrared reflection and THz transmission spectroscopy in the temperature range of 20-500 K. On heating from low temperatures, the A(1) component of the strongly split TO1 mode softens towards the Burns temperature, but the softening ceases near 400 K, which could be a signature of polar cluster percolation temperature.