Antiferrodistortive phase transition in EuTiO3

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

39K spin-lattice relaxation study of the antiferroelectric phase transition in KSCN

The temperature dependences of the 39K spin-lattice relaxation times in the laboratory (T1)and rotating (T1ϱ) frame have been studied in the vicinity of the antiferroelectric transition in KSCN. The results show that the mean reorientation time for the SCN− 180° flips is of the order of 10−9 just below Tc and that short range ordered domains persist on the NMR time scale in a relatively large temperature range above Tc. Thermally activated single particle kinetics is masking the critical effects

Non-Debye domain wall response in

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 T_\ab{c}. Characterized by a nearly frequency-independent 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

Elastic and anelastic anomalies associated with the antiferromagnetic ordering transition in wüstite, Fe xO

The elastic and anelastic properties of three different samples of Fe xO have been determined in the frequency range 0.12MHz by resonant ultrasound spectroscopy and in the range 0.150Hz by dynamic mechanical analysis in order to characterize ferroelastic aspects of the magnetic ordering transition at T N195K. No evidence was found of separate structural and magnetic transitions but softening of the shear modulus was consistent with the involvement of bilinear coupling, λe 4q, between a symmetry-breaking strain, e 4, and a structural order parameter, q. Unlike a purely ferroelastic transition, however, C 44 does not go to zero at the critical temperature" due to the intervention of the magnetic ordering at a higher temperature. The overall pattern of behaviour is nevertheless consistent with what would be expected for a system with separate structural and magnetic instabilities, linearquadratic coupling between the structural (q) and magnetic (m) driving order parameters, ?qm 2, and . Comparison with data from the literature appears to confirm the same pattern in MnO and NiO, with a smaller difference between T N and in the former and a larger difference in the latter. Strong attenuation of acoustic resonances at high frequencies and a familiar pattern of attenuation at low frequencies suggest that twin walls in the rhombohedral phase have typical ferroelastic properties. Acoustic dissipation in the stability field of the cubic phase is tentatively attributed to anelastic relaxations of the defect ordered structure of non-stoichiometric wüstite or of the interface between local regions of wüstite and magnetite, with a rate controlling step determined by the diffusion of iron

Elastic and anelastic anomalies associated with the antiferromagnetic ordering transition in wüstite, Fe x O

The elastic and anelastic properties of three different samples of Fex O have been determined in the frequency range 0.1–2 MHz by resonant ultrasound spectroscopy and in the range 0.1–50 Hz by dynamic mechanical analysis in order to characterize ferroelastic aspects of the magnetic ordering transition at TN ∼ 195 K. No evidence was found of separate structural and magnetic transitions but softening of the shear modulus was consistent with the involvement of bilinear coupling, λe4 q, between a symmetry-breaking strain, e4 , and a structural order parameter, q. Unlike a purely ferroelastic transition, however, C44 does not go to zero at the ∗ critical temperature, Tc , due to the intervention of the magnetic ordering at a higher temperature. The overall pattern of behaviour is nevertheless consistent with what would be expected for a system with separate structural and magnetic instabilities, linear–quadratic coupling between the structural (q) and magnetic (m) driving order parameters, λqm2 , and ∗ TN > Tc . Comparison with data from the literature appears to confirm the same pattern in ∗ MnO and NiO, with a smaller difference between TN and Tc in the former and a larger difference in the latter. Strong attenuation of acoustic resonances at high frequencies and a familiar pattern of attenuation at low frequencies suggest that twin walls in the rhombohedral phase have typical ferroelastic properties. Acoustic dissipation in the stability field of the cubic phase is tentatively attributed to anelastic relaxations of the defect ordered structure of non-stoichiometric w ̈ stite or of the interface between local regions of w ̈ stite and magnetite, u u with a rate controlling step determined by the diffusion of iron