Linear nonreciprocal dichroism in boracite Co3B7O13I

Linear nonreciprocal dichroism that is odd in magnetic field B was observed in the transverse geometry and studied for cubic (symmetry class T _d ) phase of boracite Co₃B₇O₁₃I. Nonreciprocal dichroism was observed in the range of absorption bands corresponding to the transitions of the Co²⁺ ion in the energy range ΔE=1.2–3.2 eV. The sign and magnitude of nonreciprocal dichroism depend on the mutual orientation of the magnetic field and crystallographic axes. Nonreciprocal dichroism refers to the phenomena of magnetically induced spatial dispersion, and its anisotropy is typical for the manifestation of the second-order magnetoelectric susceptibility in the optical range.     

A phenomenological theory of possible sequences of ferrotoroidal phase transitions in boracites

A phenomenological theory of the sequence of two second-order phase transitions with close temperatures is considered; such transitions occur in the Ni-Br boracite. The thermodynamic potential is written as a function of polarization P _i, magnetization M _i, and toroidal moment T _i vectors and fields E _i and H _i; T _i is treated as an order parameter. It is assumed that only one coefficient of T _i ² passes through zero as T decreases. The possibility of a sequence of two proper ferrotoroidal phase transitions along the T ₁ and T ₂ components is demonstrated. Spontaneous T _i, P _i, and M _i vector values and equations for susceptibility tensors (dielectric χ _ij = dP _i/dE _j, magnetic k _ij = dM _i/dH _j, and magnetoelectric α_ij = dP _i/dH _j = dM _j/dE _i) were obtained for three phases. Some of these values have well-defined anomalies in the vicinity of transitions. All possible sequences of ferrotoroidal phase transitions in boracites are considered. Depending on two potential coefficient values, these sequences may consist of one, two, or three such transitions.     

Nonreciprocal light birefringence in the R 3B7O13 X Boracites (R=Co, Cu, Ni; X=I, Br)

The field and angular dependences of nonreciprocal birefringence (NB), which is linear in magnetic field B and is due to magnetic-field-induced spatial dispersion, have been studied in the cubic (symmetry class T _d) paraelectric phase of the R ₃B₇O₁₃ X boracites (R=Co, Cu, Ni; X=I, Br) at a wavelength λ=633 nm. It is shown that the NB in crystals with different 3d and halogen ions exhibits the same anisotropy. The relation between the A and g parameters, A=2g, which determine the NB anisotropy, suggests that the microscopic mechanism of the NB is the manifestation of second-order magnetoelectric susceptibility at optical frequencies.     

Br NQR relaxation and successive phase transitions of CH3NH3HgBr3

The measurement of ⁸¹Br NQR in CH₃NH₃HgBr₃ has been carried out in the temperature range between 80 and 300 K using a pulse NQR method. The temperature dependence of ⁸¹Br NQR frequencies in CH₃NH₃HgBr₃ has revealed that it undergoes three characteristic successive phase transitions at T?=?123, 184 and 239 K. The phase transition temperature at T?=?239 K is the second-order type, whereas those at T?=?184 and 123 K are the first-order nature of the phase transitions. Each phase transition seems to be closely related to the motions of methyl ammonium cation as a partial or whole. The enhancement of 1/T ₁ at T?=?239 K indicates the onset of the molecular motion of the cation as a whole with increasing temperatures.     

Modelling of the phase transitions sequence in KNbO3 and BaTiO3

Interatomic potentials are determined in the framework of the nonlinear oxygen polarizability model to describe the structural behavior of KNbO₃ and BaTiO₃ as a function of temperature. To this purpose, the adiabatic potential is evaluated for different ferroelectric distortions and the model potential parameters are improved by comparing with total energies from full-potential LMTO calculations. Phonon dispersion curves are computed to test if the model reproduces the pronounced two-dimensional character instabilities found by first-principles lattice dynamics using a linear response approach. Finally, the phase diagram for KNbO₃ is obtained through a constant-pressure molecular dynamics simulation.     

Structure and phase transitions in Cu2P2O7

X-ray diffraction studies of the temperature variation of the Cu₂P₂O₇ crystal structure were performed in the vicinity of the transition from the α-phase (C2/c space group) to the β-phase (C2/m space group). These results as well as those of Raman spectroscopy studies and temperature variation of the electric capacitance point to the existence of an intermediate phase in the temperature range 347–363?K.     

Theory for phase transitions in insulating V2O3.

We show that the recently proposed S = 2 bond model with orbital degrees of freedom for insulating V2O3 not only explains the anomalous magnetic ordering but also other mysteries of the magnetic phase transition. The model contains an additional orbital degree of freedom that exhibits a zero temperature quantum phase transition in the Ising universality class.     

Structural disorder in the paraelectric phase of the Fe<Subscript>3</Subscript>B<Subscript>7</Subscript>O<Subscript>13</Subscript>Br boracite

A structural model of the cubic paraelectric phase of a Fe₃B₇O₁₃Br crystal belonging to the boracite family has been developed using the data obtained by single-crystal X-ray diffraction with due regard for the results of extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the best agreement between the data obtained by these two methods is achieved within a model assuming a disorder in the arrangement of both the Fe and Br atoms and a high degree of correlation of their displacements. It has been found that, during the phase transition from the rhombohedral ferroelectric phase to the cubic paraelectric phase, no significant transformation of the structure is observed on a local level. In this case, a change in the macroscopic symmetry occurs predominantly as a result of the variation in the set of possible spatial orientations of stable structural fragments, which is characteristic of order-disorder phase transitions.     

Photoinduced phase transitions by time-resolved far-infrared spectroscopy in V2O3

Using time-resolved far-infrared spectroscopy, we observe multiple routes for photoinduced phase transitions in V(2)O(3). This includes (i) a photothermal antiferromagnetic to paramagnetic transition and (ii) an incipient strain-generated paramagnetic metal to paramagnetic insulator transition, which manifests as coherent oscillations in the far-infrared conductivity. The ～100 ps conductivity oscillation results from coherent acoustic phonon modulation of the bandwidth W. Our results indicate that poor metals are particularly amenable to coherent strain control of their electronic properties.     

Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd₂Nb₂O₇ single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at T _c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below T _c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the F_1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter. Received 17 July 2000     

Comment on the paper “Nature of low-temperature phase transitions in CaMn7O12”

We contend that the anomalies observed in the magnetization, specific heat, and thermal expansion of CaMn₇O₁₂ at T _S = 89 K and T _M = 49 K reported in O. Volkova et al., Pis’ma v ZhETF 82, 498 (2005) [JETP Lett. 82, 444 (2005)] and mentioned in Pis’ma v ZhETF 82, 724 (2005) [JETP Lett. 82, 642 (2005)] are both of magnetic origin. The text was submitted by the authors in English.     

Structural phase transitions in KMnF3

The structural transitions of the perovskite KMnF₃ are studied with an energy-dispersive X-ray diffractometer. It is found that the 83 K transition is of first order, though the transition related to the condensation of a M₃-soft phonon mode is considerably affected by crystallographic domain-walls occurring below the 186 K transition. The latter transition is observed at 88.0 K (first-order), and 92.0 K (second-order) in different single crystals, respectively. The difference of the transition temperature and the transition order is interpreted in terms of inner strains appeared in the domain walls.     

Structural phase transitions in CdTiO3

The reconstructive phase transition from the ilmenite-like CdTiO₃ modification to the perovskite modification is investigated thoroughly. It is revealed that the reconstructive transition is determined by size effects, results in the formation of the closest packing of the ilmenite CdTiO₃ structure, and is irreversible with a decrease in temperature. The perovskite CdTiO₃ modification undergoes displacive structural phase transitions at temperatures of 110, 220, and 380°C. The first displacive phase transition is isostructural, whereas the second and third transitions are associated with rotations of oxygen octahedra.     

Effect of plastic deformation on phase transitions in a Ti50Ni47Fe3 alloy

The results of x-ray structural studies of the effect of previous plastic deformation by rolling on the occurrence of phase transitions in a TiNi(Fe) alloy are presented in the present work. The temperature dependences of the Bragg reflection intensities and half-widths and the dependence of the rhombic angle in the R phase with various degrees of deformation were obtained. Analysis of the results found gave the following rules. Plastic deformation substantially shifts the characteristic temperatures of the martensitic transitions (MT) R-B19′ and B19′-B19″, increases the temperature range of the transformation and can result in a “stepwise” transformation. It was observed that deformation weakly affects the temperature T_R for the B2-R transformation. Tomsk State Architecture and Construction Academy. V. D. Kuznetsov Siberian Physicotechical Institute at Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 11–20, July, 1996.     

Dielectric permittivity and AC conductivity investigations of the structural phase transitions in (NH3)2(CH2)7CdCl2Br2

The AC dielectric permittivity as a function of temperature (100–400 K) at different frequencies (between 80 Hz and 20 kHz) for the layered alkylene diammonium insulator containing Cd, namely, [(NH₃)₂(CH2)₇CdCl₂Br₂] has been measured. The formation of the compound was confirmed by microchemical analysis and IR absorption spectrometry. X-ray powder diffraction indicates an orthorhombic unit cell of dimensions: a = 10.219(2) Å, b = 9.168(2) Å and c = 38.694(4) Å. The AC conductivity ([sgrave]) is presented as a function of temperature and frequency. The conductivity and permittivity results indicate the presence of first-order phase transitions at 317 and 345 K. This has been confirmed by thermal analysis techniques. The activation energies were of values ranging between 0.15 and 0.62 eV depending upon the temperature range and the applied frequencies.     

Existence of an incommensurate phase in a Li2B4O7 crystal

The x-ray diffraction spectra of Li₂B₄O₇ single crystals are investigated in the temperature range 80–300 K, and the lattice parameter c is determined in the same temperature range in the presence of a periodically varying temperature field. An incommensurate phase is not observed anywhere in the temperature range investigated, regardless of whether the crystals are subjected to a periodic temperature field. Fiz. Tverd. Tela (St. Petersburg) 39, 1461–1463 (August 1997)