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.Comment: PRB, in pres

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We present the first low frequency (0.1-50 Hz) measurements of the complex elastic constant on C6o single crystals. We have measured a negative dip anomaly of 25% at T\ =261 K, which did not show up in previous elastic studies at 10 4 Hz. The slow relaxation observed near T\ is supposed to be due to the very slow dynamics of the order parameter fluctuations. In addition it is shown that the elastic anomalies observed at 10 4 Hz and 160 K originate from the same physical process as the "glass 1 ' transition at 80 K observed in thermal expansivity.PACS numbers: 62.20. Dc, 05.40.+J, 64.70.Kb, 64.70.Pf The synthesis of the icosahedral carbon molecule Cm [1], the preparation of large quantities of material [2], and the growth of single crystals [3,4] have enabled a number of investigations revealing exciting results even for the undoped material. At room temperature the crystal structure is face centered cubic with space group Fm3m [5], and the nearly spherical C6o molecules are orientationally disordered with a reorientational correlation time of 9.2x10 ~1 2 s [61. At ri=260 K a ^-type order-disorder transition [7] leads the crystal to a simple cubic phase Pa3. Below T\ the reorientational correlation time increases abruptly to 2xl0~9 s [6]. Detailed neutron scattering studies [8] have shown that between 90 and 260 K the C6o molecules shuffle between two nearly degenerate orientations differing in energy by ui -WA meV and separated by an energy barrier of «i=290 meV. With decreasing temperature the population of the energetically less favorable orientation decreases, but it does not vanish completely. Below T\\=90 K 16.7% of the molecules were found to be frozen in the energetically less favorable minority orientation [8]. This freezing of the misoriented molecules was shown to be strongly time (or equivalently frequency) dependent and is reflected in different quantities, like specific heat [9], thermal expansion [8], thermal conductivity [10], and dielectric permittivity [11]. Sound velocity measurements [12,13] performed in the kHz region exhibit anomalies around 260 and 160 K; the latter was shown to be consistent with a strong temperature dependent relaxation process. At 260 K an 8% upwards jump was observed in the Young's modulus E. This appears surprising, since the PippardBuckingham-Fairbank relationship [14] E ~](T) cca(T) x(dT\/dP) (a is the thermal expansivity [7])-which also holds in the presence of the third order invariant of the order parameter in the free energy-yields a negative jump of the elastic modulus at T\.In this Letter we report the first low frequency (0.6-50 Hz) elastic constant measurements on single-crystal C60 performed in a temperature range between 100 and 300 K. Two characteristic results are obtained which are essential for the understanding of the transitions at T\ and T\\. The low frequency elastic relaxations observed above T\ may originate from the appearance and growth of short-range ordered clusters of the low symmetry phase (order parameter fluctuations) with a characterist...

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Data obtained by dielectric spectroscopy reveal a strongly polydispersive, non-Debye dispersion appearing in potassium dihydrogen phosphate on cooling below the paraelectric-ferroelectric phase transition temperature Tc. Characterized by a nearly frequencyindependent dielectric loss and a real part of the permittivity decreasing in an almost linear way with frequency on a logarithmic scale, the corresponding relaxation mechanism is discussed in terms of dielectric contributions of elastic domain walls weakly pinned on quenched disorder.