A neutron diffraction study of the high pressure structural phase transition in (CD3ND3)2MnCl4

Abstract

High-pressure neutron diffraction experiments have been performed at room temperature on a powdered sample of the perovskite type-layer compound (CD₃ND₃)₂MnCl₄. A phase transition from the orthorhombic room-temperature phase (ORT) to a new high-pressure phase (HP) is demonstrated at 20.5 ± 0.2 kbars. A monoclinic unit cell with lattice parameters a = 6.824 (5) Å; b = 7.409 (8) Å c = 17.126 (12) Å and β = 82.94(9)° has been inferred for the HP phase, consistent with a two-dimensional perovskite-type structure. The HP phase appears to be much more compact than ORT; it is characterized, in particular, by an important compression (?10%) of the inter-layer distance. Space groups P2/c or P2₁/c consistent with the experimental data have been deduced for the HP phase, after group theoretical considerations based on shear transformation and order-disorder mechanisms.     

DSC study of superionic phase transition in (NH4)4H2(SeO4)3 single crystals

DSC and complex impedance studies of the protonic conductor (NH₄)₄H₂(SeO₄)₃, which undergoes a superionic phase transition of first order at T_s = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔC_p of the crystal are proportional: d(lg σ)/dT = XΔC_p/RT_s + const, as found for MAg₄I₅ crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at T_s, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH₄)₄H₂(SeO₄)₃ crystal above T_s.     

X-ray study of the low-temperature phase transition in K2CoCl4

Precise lattice parameter measurements and intensity measurements of selected main and satellite reflections of K₂CoCl₄ have been performed in the temperature range 100 to 300 K in the vicinity of the low-temperature phase transition (commensurate-commensurate phase transition, T _c = 142 K). A broadening of the FWHM for the h01 reflections was observed below 142 K which suggests a transition from an orthorhombic phase to a monoclinic phase.     

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.     

Pressure-induced phase transformation of CsPbI3 by X-ray diffraction and Raman spectroscopy

The structural transformation of cesium lead iodine (CsPbI₃) has been investigated in diamond anvil cells up to ～15 GPa at room temperature by employing synchrotron radiation X-ray diffraction and Raman spectroscopy. One reversible transformation from orthorhombic (Pnma) to monoclinic (P2₁/m) phase has been observed at 3.9 GPa. Isothermal pressure–volume relationship of orthorhombic CsPbI₃ is well fitted by the third-order Birch–Murnaghan equation of state with K₀ = 14(3) GPa, K′₀ = 6(2) and V₀ = 891(7) ų. The ultralow value of bulk modulus K₀ demonstrates the high compressible nature of CsPbI₃, similar to those of organic–inorganic metal halide perovskites. The present results provide essential information on the intrinsic properties and stability of CsPbI₃, which may be applied in photovoltaic devices.     

Propagation of longitudinal ultrasonic waves in RbH2PO4 near its ferroelectric phase transition temperature

Ferroelectric phase transition in RbH₂PO₄ has been investigated using propagation of longitudinal acoustic waves along the polar axis near the transition temperature. The velocity of this mode is continuous across the transition temperature. Velocity data in the ferroelectric phase are analyzed in terms of coupled soft modeacoustic mode model of Pytte to obtain the temperature dependence of the soft mode frequency. The attenuation data in the ferroelectric phase show power law dependence. It follows scaling behaviour of the type predicted by Kawasaki from the mode-mode coupling theory and the dynamical scaling.     

Theoretical studies of the spin Hamiltonian parameters and local structures for WO3 doped Zn3(PO4)2ZnO nanopowders

The spin Hamiltonian parameters (SHPs) and the local structures for impurity W⁵⁺ in the Zn₃(PO₄)₂ZnO nanopowders doped with WO₃ under different concentrations are theoretically investigated using the perturbation calculations of these parameters. The exponential functions of the related quantities (cubic field parameter Dq, covalency factor N, relative tetragonal compression ratio τ and core polarisation constant κ) of concentration x with totally four adjustable coefficients a, b, c and d are adopted to fit the concentration dependences of the experimental d-d transition bands and SHPs. The impurity W⁵⁺ centres demonstrate moderate tetragonal compression ratios τ (～3.1%) due to the Jahn–Teller effect. With the increase of WO₃ concentration, Dq and N show moderately decreasing rules, while τ and κ exhibit slightly and moderately increasing tendencies with x, respectively. The mechanisms of the above concentration dependences of these quantities are analysed from the modifications of the local crystal-field strength and electron cloud density around the impurity W⁵⁺ with the variation of x. Present theoretical studies would be useful to the exploration of the structural properties and optical applications for WO₃ doped Zn₃(PO₄)₂ZnO nanopowders.     

Microscopic distortion and order parameter in langbeinite K2Cd2(SO4)3

The structural data on the orthorhombic phase of K₂Cd₂(SO₄)₃ at four different temperatures of Abraham et al. (1978) [J. Chem. Phys., 68, 1926] is reanalyzed in terms of frozen symmetry modes. The eigenvector of the primary distortion present in the orthorhombic phase is derived. The temperature variation of the amplitude of the order parameter is determined and compared with those of the amplitude of the frozen secondary distortion and the amplitude of the orthorhombic strain. In contrast with some previous literature, it is shown that, within experimental accuracy, all these magnitudes exhibit the correlation expected from a conventional Landau model.     

Pressure-induced weak moment in the two-dimensional antiferromagnet (C2H5NH3)2CuCl4

Abstract

The two-dimensional Heisenberg antiferromagnet (C₂H₅NH₃)₂CuCl₄ has the ferromagnetic intralayer exchange interaction, while the extremely weak interlayer exchange interaction is antiferromagnetic. Neutron scattering experiments under high pressures have been performed on this compound. We confirm that the spin structure changes around 1～2 GPa from the collinear alignment along the a-axis to a spin-canting one. The weak moment due to the canting is parallel to the c-axis. The results indicate that the ferromagnetic intralayer and the antiferromagnetic interlayer exchange interactions are maintained up to 1～2 GPa. Why the weak ferromagnetic moment along the c-axis occurs is due to a lowering of crystal symmetry by pressure.     

Three-dimensionally ordered composite electrode between LiMn2O4 and Li1.5Al0.5Ti1.5(PO4)3

A novel electrode system composed of three-dimensionally ordered macroporous (3DOM) Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) and LiMn₂O₄ was fabricated by the colloidal crystal templating method and sol–gel process. A LATP nanoparticle for the fabrication of 3DOM-LATP was prepared by a sol–gel process. A suspension containing polystyrene (PS) beads and the LATP nanoparticles was filtrated by using a polycarbonate filter to accumulate PS beads and LATP. The accumulated PS beads had a close-packing structure, and the void between PS beads was filled with LATP nanoparticles. 3DOM-LATP was obtained by heat treatment of the accumulated composite. Li–Mn–O sol was injected by a vacuum impregnation process into the macropores of 3DOM-LATP and then was heated to form three-dimensionally ordered composite materials consisting of LiMn₂O₄ and LATP. The formation of the composite between 3DOM-LATP and LiMn₂O₄ were confirmed with scanning electron microscopy and X-ray diffraction method. The prepared composite electrode system exhibited a good electrochemical performance. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

Oxygen vacancy in LiTiPO5 and LiTi2(PO4)3: A first-principles study

The structures of LiTiPO₅ and LiTi₂(PO₄)₃, as well as the possibility of oxygen vacancies formation in the systems are studied by first-principles calculations. It is found that oxygen vacancies can be formed in LiTiPO₅ and LiTi₂(PO₄)₃ under oxygen poor condition. The formation of oxygen vacancies introduce a defect band within their band gaps, which is expected to improve the electronic conductivity of LiTiPO₅ and LiTi₂(PO₄)₃ significantly. Meanwhile, a great concentration of oxygen vacancies may increase the discharge voltage of LiTiPO₅ and LiTi₂(PO₄)₃.     

Lithium ion conductive glass–ceramics with Li3Fe2(PO4)3 and YAG laser-induced local crystallization in lithium iron phosphate glasses

The glasses with the composition of 37.5Li₂O–(25 − x)Fe₂O₃–xNb₂O₅–37.5P₂O₅ (mol%) (x = 5,10,15) are prepared, and it is found that the addition of Nb₂O₅ is effective for the glass formation in the lithium iron phosphate system. The glass–ceramics consisting of Nasicon-type Li₃Fe₂(PO₄)₃ crystals with an orthorhombic structure are developed through conventional crystallization in an electric furnace, showing electrical conductivities of 3 × 10^− 6 Scm^− 1 at room temperature and the activation energies of 0.48 eV (x = 5) and 0.51 eV (x = 10) for Li⁺ ion conduction in the temperature range of 30–200 °C. A continuous wave Nd:YAG laser (wavelength: 1064 nm) with powers of 0.14–0.30 W and a scanning speed of 10 μm/s is irradiated onto the surface of the glasses, and the formation of Li₃Fe₂(PO₄)₃ crystals is confirmed from XRD analyses and micro-Raman scattering spectra. The crystallization of the precursor glasses is considered as new route for the fabrication of Li₃Fe₂(PO₄)₃ crystals being candidates for use as electrolyte materials in lithium ion secondary batteries.     

Influence of crystal synthesis on the incommensurate phase and lock-in transition of Rb2ZnCl4 crystals

A comparative study of several crystals of Rb₂ZnCl₄, obtained by different crystal growing methods, has allowed us to determine the influence of growth defects on the incommensurate phase and on the lock-in transition of these samples. X-ray diffraction has allowed us to complete previous dielectric measurements realized on the same samples and to relate the crystalline quality to the evolution of the modulation as a function of the temperature. The principal influence of an increasing defect density seems to be a stronger pinning of the modulated phase and this induces a lower lock-in temperature and a wider hysteresis.     

Renormalization phenomena in Ba-diluted ferroelastic lead phosphate, (Pb1-x Ba x )3(PO4)2

Abstract

In the improper ferroelastic palmierite-type lead phosphate order parameter coupling with a defect induced conjugate field leads to the renormalization of the two different critical temperatures of three order parameter components {Q ₃} and {Q ₁ Q ₂}. The influence of the lead dilution by barium on the ferroelastic domain pattern, the critical temperature of the ferroelastic transformation R m–C2/c and the development of the intermediate regime in (Pb_1–x Ba _x )₃(PO₄)₂ is studied using optical birefringence measurements, Raman and infrared spectroscopy. At the ferroelastic transition temperature T _c the orientational contribution of the three-states Potts model becomes critical. T _c is reduced from 453 K (x = 0) to zero K at x? 0.12. Modifications of the shape of zigzag needle domains as well as the angle between the monoclinic binary axis and the W walls along [031] with temperature and increasing Ba-content are reported. Above the ferroelastic transition point the component Q ₃, which corresponds to the displacive part in the Gibbs free energy, leads to dynamic short-range monoclinic deformation in the trigonal matrix. The temperature where Q ₃ shows critical behaviour is renormalized to 720 K (x = 0.12) as compared with 563 K in pure lead phosphate. For x>0.13 no monoclinic precursor clusters were found.     

Pressure-induced phase transition of B-type Y_2O_3

The synthesized monoclinic(B-type) phase of Y_2O_3 has been investigated by in situ angle-dispersive x-ray diffraction in a diamond anvil cell up to 44 GPa at room temperature. A phase transition occurs from monoclinic(B-type) to hexagonal(A-type) phase at 23.5 GPa and these two phases coexist even at the highest pressure. Parameters of isothermal equation of state are V_0= 69.0(1) ~3, K_0= 159(3) GPa, K_0= 4(fixed) for the B-type phase and V_0= 67.8(2) ~3, K_0= 156(3) GPa,K'_0= 4(fixed) for the A-type phase. The structural anisotropy increases with increasing pressure for both phases.     

A new high-pressure polymorph of Ti2O3: implication for high-pressure phase transition in sesquioxides

The post-corundum phase transition has been investigated in Ti₂O₃ on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti₂O₃ was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti₂O₃ can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a=7.6965 (19) Å, b=2.8009 (9) Å, c=7.9300 (23) Å, V=170.95 (15) Å³ at 19 GPa, and a=7.8240 (2) Å, b=2.8502 (1) Å, c=8.1209 (3) Å, V=181.10 (1) Å³ at ambient conditions. The Birch–Murnaghan equation of state yields K ₀=206 (3) GPa and K′₀=4 (fixed) for corundum phase, and K ₀=296 (4) GPa and K′₀=4 (fixed) for the post-corundum phase. The molar volume decreases by 12% across the phase transition at around 20 GPa. The structural identification was carried out on a recovered sample by the Rietveld method, and a new polymorph of Ti₂O₃ can be identified as Th₂S₃-type rather than U₂S₃-type structure. The transition from corundum-type to Th₂S₃-type structure accompanies the drastic change of the form of polyhedron: from TiO₆ octahedron in the corundum-type to TiO₇ polyhedron in the Th₂S₃-type structures.     

The monoclinic/orthorhombic phase transition in Ba2Cu(HCOO)6: An optical and x-ray powder diffraction study

α-Ba₂Cu(HCOO)₆ grown at room temperature crystallizes in space group P2₁. On heating a reversible, hysteretic, equitranslational, first-order phase transition at about 60-90°C takes place. Here we discuss the influence of thermal treatments on the occurrence, coexistence and stability ranges of the two observed phases, as studied by several techniques (optical microscopy, differential scanning calorimetry and X-ray diffraction). On cooling a single set of domain walls parallel to (001) have been observed. Taking into account experimental results and the crystal pseudosymmetry of the α phase we propose that the high-temperature β phase crystallizes in space group Pbnm.