Incommensurate cooperative tilting modes of MnF6 octahedra relatedto the structural phasetransitions in BaMnF4

The structural phase transitions of the layer compound BaMnF₄ were studied by high-resolution X-ray diffraction using synchrotron radiation. The intensities and profiles of two kinds of superlattice reflections having incommensurate reduced wave vectors q ₁= (～ ± 1/5,0,0) _p and q ₂ =(～ ± 2/5,1/2,1/2)_p, respectively, were measured as a function of temperature from 25 K to 280 K. These temperature dependencies show that incommensurate structural phase transitions of second order occur at 234 K and 244 K. These structural phase transitions are interpreted as successive condensations of a folding-screen-like incommensurate plane-distortion mode and a commensurate anti-ferro-distortive tilting mode of the MnF₆ octahedra around the primitive a₀ - and b₀ -axes, when cooled down. It is also found that there is another structural phase transition at about 45 K related to a precursor structural distortion for the antiferromagnetic transition occurring at about 26 K.     

Phase transitions in LiKSO4: Low-temperature neutron diffraction results

Results of a detailed crystallographic study of the structural phase transitions in LiKSO₄, carried out using single crystal neutron diffraction at ambient and below-ambient temperatures (300–100 K), are presented. The study indicates two clear phase transitions at 205 K (from space group P6₃ to space group P31c), at 189 K (to space group Cc) and a third transition at about 130 K in the cooling cycle. The corresponding temperatures are 250 K, 200 K and 138 K in the heating cycle. The existence of twins, mixed phases, and extremely sluggish kinetics for the various phase transitions, as well as the structural details of the various phases in this mixed salt, can explain the apparent inconsistencies in the results of several experimental investigations reported in the literature.     

Ferroic phase transitions in β-LiNH4SO4

Abstract

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in β-LiNH₄SO₄ and its deuterated analogue. β-LiNH₄SO₄ undergoes succesive phase transitions: a paraelectric - ferroelectric phase transition at T₁ ? 462 K, a ferroelectric - ferroelastic phase transition at T₂ ? 283 K and a transition from one ferroelastic phase to the other at T₃ ? 28 K. Attention is focused on the influence of the order of phase transitions on the pattern of ferroelectric and ferroelastic domain structure, and also on the role played by the dynamics of molecular groups in the mechanism of transitions. The pre-transition effect connected with the ferroelectric-paraelectric transition: heterophase, capable of accounting for anomalies in different physical properties present 1-3 K below T₁ is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.     

X-ray study on the phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals

The structural phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals below room temperature have been studied by X-ray diffraction. Three phases are observed in the temperature range from 25°C down to — 160°C. The space groups in three different phases are identified as C2/c, P2/n (or Pn), and C2 for (NH₄)₃H(SO₄)₂ and (ND₄)₃D(SO₄)₂ crystals. No isotope effect on the structural phase transitions in these crystals could be detected by these studies. The occurrence of structural phase transitions caused by the reorientation of SO₄ groups and/or the shift of oxygen atoms from the sulfate atom in the SO₄ group are suggested from the diffraction photographs.     

Anharmonic phonons and central peaks at structural phase transitions

Recent light scattering studies of solids undergoing structural phase transitions clearly demonstrate that the “central peak” phenomenon is not one, but several processes (both static and dynamic) contributing in different degrees to the spectra observed for different types of transition. Our own work on Pb₅Ge₃O₁₁, SrTiO₃, K₂SeO₄ and BaMnF₄ is reviewed and the studies by other on KDP, KH₃(SeO₃)₂, KD₃(SeO₃)₂ are discussed briefly.     

Laser Raman study of phase transformations in [N(CH3)4]2ZnCI4 single crystals

We report temperature-dependent Raman studies on single crystals of [N(CH₃)₄]₂ZnCI₄ from 300 to 10 K. The observed spectral features suggest that both the N(CH₃)₄ ⁺ and ZnCl^2- ₄ ions are distorted from their regular tetrahedral structure and occupy sites of C_s symmetry in the lattice at room temperature. From the variation of line width of some selected Raman bands and other spectral changes as a function of temperature, it is inferred that both the ZnCl^2- ₄ and—CH₃ groups have high motional freedom at room temperature and the different phase transitions up to 160 K are triggered by the gradual freezing-in of orientational freedom of these groups, while the N—C₄ tetrahedra do not play any significant role in these phase transitions. The monoclinic to orthorhombic superlattice phase transitions at 159 K is triggered by freezing-in of the orientational motions of both the ZnCl^2- ₄ and N(CH₃)⁺ ₄ groups in the lattice.     

Phase transition in Cu2+-doped RbH2 PO4 as observed by EPR

The electron paramagnetic resonance (EPR) spectra of Cu²⁺-doped RbH₂ PO₄ at elevated temperatures indicate a phase transition at 358 K. The EPR-silent state at this temperature is attributed to a so-called polymeric phase transition. After the transition when the temperature is lowered to 293 K, the EPR signal does not appear; therefore, the transition is irreversible. This result seems to be in agreement with the other observations. The EPR spectra for the sample indicate the presence of two sites for Cu²⁺, and the values of EPR parameters are in accord with the literature on Cu²⁺-doped single crystals. Any other phase transitions could not to be observed at low temperatures down to 113 K.     

Symmetry breaking at the phase transitions in ferroelectric pyridinium salts

The phase transitions in ferroelectric pyridinium tetrafluoroborate (PyBF₄) and pyridinium perchlorate (PyClO₄) have been characterized by structural studies. The continuous ferroelectric phase transition at 238.7 K in PyBF₄ appears to be a unique case for multiaxial ferroelectrics, while the first order phase transition in PyClO₄ at 248 K is consistent with the Landau theorem for deducing the character of phase transitions from symmetry considerations. The phase transition in both materials is caused by ordering of the pyridinium cation as well as tetrahedral anions.     

Some stereochemical criteria concerning the structural stability of A2BX4 compounds of type β-K2SO4

In A₂BX₄ structures which are isostructural to β-K₂SO₄ (with A being a monoatomic cation) there are two crystallographically independent cations surrounded by 11 and 9 X-atoms. The 11-coordinated cation is less firmly bound and the arrangement of its five closest neighbours is irregular. One of these contacts is approximately parallel to the pseudohexagonal axis of the structure and is often shorter than the sum of the corresponding ionic radii. A survey of available structural data indicates that the low-temperature structural instability of a good number of these compounds is related to the coordination of this 11-coordinated cation and especially to the bonding strength of this short bond, which is often the shortest cation-anion contact in the structure. Typically, the relative contribution of this contact to the bond-valence sum of the 11-coordinated cation is larger in the compounds which undergo phase transitions at lower temperatures. The presence of this short contact is correlated with the ratio of the lattice parameters a/b (Pnma-setting). In general, the Pnma phase is unstable at low temperatures in those compounds where this ratio is smaller. On the other hand, the value of a/b can be related to the ratio of the effective sizes of cations and BX₄ tetrahedra, so that typical low-temperature instabilities of the β-K₂SO₄ structure occur for smaller values of the ratio between cation radius and the sum of the ionic radii of atoms A and X. In most cases, the resulting phase transitions stabilize modulated structures (frequently incommensurate), with slight distortions with respect to the β-K₂SO₄ structure. However, when the bond valence sum of the eleven-coordinated cation is extremely low, more drastic (first-order) structural changes are observed (e.g. phase transitions into the Sr₂GeS₄ structure type). In addition, the survey indicates, especially in complex oxides, that low-temperature phase transitions are more probable in those structures with looser packing. Considering the criteria proposed, a set of compounds is indicated where low-temperature phase transitions are plausible.     

Some characterizations of a new metal–organic framework (n-C14H29NH3)2CdCl4 and the role of hydrogen bonding

A cadmium-based organic–inorganic hybrid (n-C₁₄H₂₉NH₃)₂CdCl₄ is synthesized and characterized, thermally and dielectrically. The differential scanning calorimetery (DSC) and the differential thermal analysis (DTA) thermograms were performed in a suitable range of temperature. The general feature of each thermogram indicates that the hybrid undergoes two structural phase transitions at T_major ≈ 351.5?K and T_minor ≈ 344.66?K in addition to an intermediate temperature which is located at ≈346.8?K. For further confirmation of the observed phase transitions, the complex dielectric permittivity ε* (ω,T)?=?ε′(ω,T) – iε″(ω,T) and ac conductivity σ_ac(ω,T) were accurately measured in the wide range of temperature 100?K<T<400?K at some suitable range of frequencies. The data evidenced the existence of such transitions. Comparison with other hybrids reveals the absence of the odd–even effect whereas the transition temperature increases with the increase of the chain length. The mechanism of proton transfer and kink defects was outlined.     

Evidence for vacancy-driven phase transitions in BaMnF 4 at elevated temperatures

Anomalies near 370 K and/or 400 K in dielectric constant, diffuse x-ray scattering, C₁₁ elastic coefficient, electrical conductivity, and piezoelectric resonance have been reported for BaMnF ₄. Particularly curious is the dielectric constant along the ac-diagonal, which rises from 15 at 300 K to a sharp peak of 73 at 400 K. These anomalies are discussed in terms of 2D ionic conduction mechanisms (fluorine vacancy hopping), and the disappearance of short-range antiferroelectric order at temperatures above 400 K. At 650 K the crystals lose all piezoelectricity, which is interpreted as a 3D vacancy-driven C_2v – D_2h, transition.     

First-principle study of strain-driven phase transition in incipient ferroelectric SrTiO3

First-principle calculations were carried out to investigate the epitaxial strain dependence of the structural stability and related properties of SrTiO₃. The P4mm-to-P4/mmm-to-Amm2 phase transitions in SrTiO₃ with the epitaxial strain changing from compressive to expansive have been identified by analysis of the soft-mode eigendisplacement. The zero-temperature critical strains for the P4mm-to-P4/mmm and P4/mmm-to-Amm2 phase transitions are −0.72% and 0.83%, respectively, which is consistent with the previous predictions based on the phenomenological thermodynamic model and the parameterized total-energy model. The influences of the epitaxial strain on the dielectric tensor and spontaneous polarization along the phase transition path are discussed in terms of charge transfer. Further investigations on the vibration modes show unusual softening behaviors of the A₁ and B₂ modes in the Amm2 phase, which may be associated with the soft phonon modes away from Γ point and possibly result in other phase transitions unreported by any previous studies.     

A CCD based detector for quick detection of pressure induced phase transitions

Abstract

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn², AuGa² and low Z scatterer adamantane (C₁₀H_l6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In_0.25Sn_0.75 and AuGa₂. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.     

Thermal,Electrical and Optical Studies of Phase Transitions in Ag2CdI4 Solid Electrolyte

The article deals with the preparation method of bulk Ag₂CdI₄ superionic material and with investigations of structural phase transitions (PT) which take place in the 100÷450 K temperature range. The phase transitions were studied by thermal, electrical and luminescence methods. The temperature dependencies of thermal expansion, heat capacity, thermal conductivity, electric conductivity and excitation luminescence spectra show that in the temperature range 100 ÷ 450 K Ag₂CdI₄ possesses a sequence of structural changes at 180 ÷190 K, 320 ÷ 330 K, 350 ÷ 370 K and at T>400K. The nature and mechanisms of the observed structural changes are discussed.     

BiFeO_3结构性质与相转变的第一性原理研究

采用基于密度泛函理论的第一性原理计算,系统地研究了BiFeO3的7种不同空间群(R3c,R3m,P4mm,Cm,Pm3m,R3m和R3c)结构及其转变关系.结果表明,铁电相R3c结构是基态,不同结构之间也存在着一定的转变关系,其变化主要包括两种形式,在[111]方向上Bi3+相对FeO6八面体存在一定的位移和FeO6八面体绕[111]极化轴的反铁扭曲旋转.此外,还得出BiFeO3的薄膜结构受到衬底结构的作用会导致其从三方相(R3c)向四方相(P4mm)转变.     

X-ray study of structural phase transitions in nanocrystalline LaMnO3+δ perovskite

The nanocrystalline LaMnO_3+δ perovskite was synthesized by the microwave-assisted glycothermal method and calcined at several temperatures up to 1500°C. The prepared samples were examined by the X-ray powder diffraction with the aim to show that LaMnO_3+δ exhibits the size-induced structural phase transitions. The as-received nanocrystals of LaMnO_3+δ are of tetragonal, pseudo-cubic symmetry not known for bulk material. The samples calcined at temperatures 750–1100°C have trigonal symmetry known from the high-temperature phase of LaMnO₃ single crystal. The samples calcined from 1200°C to 1500°C have two phases: trigonal and orthorhombic. Thus, the observed phase sequence is inverted with respect to that of the bulk material, which is the characteristic of the size-induced mechanism of phase transitions in the nanocrystals. The critical crystallite sizes for both structural transitions were evaluated as 20 and 100?nm.     

Structural and magnetic phase transitions in (Chloroanilinium)2CuX4 (X = Cl, Br)

Successive structural phase transitions of (4-ClC₆H₄NH₃)₂CuCl₄, which occur in a very narrow temperature range were reinvestigated by Fourier transform nuclear quadrupole resonance (FT NQR) measurements. The phase transitions at 275.5 and 277.0 K were confirmed. The effect of the deuteration of the ammonium end on these transitions was studied. The³⁵Cl NQR frequencies of organic cation were observed to decrease by about 4 kHz and the phase transition temperatures to decrease by about 2 K by the deuteration, suggesting that the ?NH₃ ⁺ … Cl hydrogen bond is weakened by the deuteration. The magnetic phase transition temperature of 8.6 K showed no remarkable change within experimental error by the deuteration. It was found that the magnetically ordered state is broken by the radio-frequency magnetic field of about 15–35 Oe usually employed in pulsed NQR. However, in the deuterated compound (4-ClC₆H₄ND₃)₂CuCl₄, the ordered state was found to be stabler for the usual radiofrequency power. By combining with the NQR data of (4-ClC₆H₄NH₃)₂CuBr₄ and (3,5-Cl₂C₆H₃NH₃)₂CuCl₄, the possibility is discussed of tuning the interlayer interaction between the organic cation layer and the inorganic complex anion layer by the halogen substitution in the organic cation as well as by the halogen replacement in the inorganic complex anion.     

Investigation of the phase transitions of ferroelectric KIO3 and KNbO3 crystals by optical diffraction

Optical diffraction is reviewed as a technique for investigation of the phase transitions in crystals with a multidomain structure. It has been used to study the phase transitions in KIO₃ and KNbO₃ single crystals. Strong optical diffraction bands resulted from electric domains in KNbO₃ crystals and their change with temperature were observed when a laser beam passed through the crystals. The diffraction patterns observed changed abruptly at 427°C, 223°C, and -50°C respectively, at which KNbO₃ crystals undergo structural phase transitions. It is considered that the change of the diffraction patterns with temperature is due to change of the electric domains in the crystals.     

Magnetic phase transitions in Nd7Rh3 single crystal

Magnetic phase transitions in rare earth intermetallic compound Nd₇Rh₃ have been investigated using a single crystal. Measurement results of magnetization, magnetic susceptibility, specific heat, and electrical resistivity reveal that Nd₇Rh₃ has two magnetic phase transitions at T_N=34 K, T_t2=9.1 K and a change of the magnetic feature at T_t1=6.8 K in the absence of an external magnetic field. Antiferromagnetic orderings exist in all the three magnetic states; a large magnetic anisotropy between the c-axis and the c-plane is observed. In the magnetic phase below T_t2, an irreversible field-induced magnetic phase transition takes place in the c-plane; after removing external magnetic field, a coexistence state of ferro- and antiferromagnetic ordering or a ferrimagnetic state having a remanent magnetization M_R is stabilized. The M_R decays to a certain value for several hours after the first process; a magnetic field cooling effect was also observed in the c-plane below T_t2. In the antiferromagentic state above T_t2, the irreversibility disappears and an ordinary antiferromagnetic state takes place. As the origin of this phenomenon, a kind of martensitic structural transition that is observed in Gd₅Ge₄ can be considered.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.