First-order Structural Phase Transition Research Articles

Crystal structure and characterization of the novel [formula omitted] hydrogen bonded polar crystal [formula omitted

Dielectric properties and phase transitions of the piperazinium tetrafluoroborate ([NH2(CH2)4NH][BF4], abbreviated as PFB) crystal are related to the one-dimensional arrangement of the cations linked by the bistable NH+⋯N hydrogen bonds and molecular motions of the [BF4]− units. The crystal structure of [NH2(CH2)4NH][BF4] is monoclinic at room temperature with the polar space group Pn. The polar/acentric properties of the room temperature phase IV have been confirmed by the piezoelectric and pyroelectric measurements. DSC measurements show that the compound undergoes three first-order structural phase transitions: at 421/411K (heating/cooling), at 386/372K and at 364/349K. 1H and 19F NMR measurements indicate the reorientational motions of [BF4]− anions and piperazinium(+) cations as well as the proton motion in the hydrogen-bonded chains of piperazine along the [001] direction. Over the phase I the isotropic reorientational motions or even self-diffusion of the cations and anions are expected. The conductivity measurements in the vicinity of the II–I PT indicate a superionic phase over the phase I.

Suppression of geometric frustration by magnetoelastic coupling in AuCrS2

We studied the structural, magnetic, and electronic properties of the geometrically frustrated layered AuCrS${}_{2}$ system by means of x-ray and neutron powder diffraction, specific heat, dc magnetization, and dc electrical resistivity measurements. The room-temperature structural refinement is consistent with a hexagonal centrosymmetric $R$-3$m$ symmetry and with formal valence states Au${}^{+}$ and Cr${}^{3+}$, where the Cr${}^{3+}$ ions form a regular triangular lattice within the hexagonal planes. On cooling, we observe a first-order structural phase transition to a monoclinic $C$2/$m$ symmetry concomitant to an antiferromagnetic order at ${T}_{N}$ $=$ 47 K. The atomic displacements associated with this transition stretch the triangular lattice, thus suppressing the geometric frustration. This accounts for the magnetic order observed and gives evidence of a large magnetoelastic coupling. The refined magnetic structure is commensurate and consists of double ferromagnetic chains along the stretching direction with \ensuremath{\mu} $=$ 2.54 ${\ensuremath{\mu}}_{B}$/Cr${}^{3+}$; the residual frustration stabilizes an elegant pattern of alternate ferromagnetic and antiferromagnetic intra- and interplane couplings between adjacent chains. The electrical transport of our sintered powder samples is found to be semiconducting-like with ${\ensuremath{\rho}}_{300\mathrm{K}}$ \ensuremath{\sim} 157 \ensuremath{\Omega}cm and an activation energy of 0.38 eV.

Light-induced phase transition in AlD3at high pressure

Trivalent aluminum hydride in the rhombohedral $\ensuremath{\alpha}$ phase ($R$$\overline{3}$$c$ space group) was studied at high pressures in a diamond-anvil cell by means of Raman scattering, x-ray diffraction, observation of optical transmission, and the density functional simulations. At $P\ensuremath{\approx}53$ GPa the heavier isotope AlD${}_{3}$ undergoes a first-order structural phase transition which was found to be stimulated by the laser irradiation used for the Raman-scattering measurements. In the new high-pressure phase Al atoms form a lattice with a monoclinic unit cell ($P{2}_{1}/c$ space group) over which a superstructure is developed when pressure is varied. The superstructure is formed by regular displacements of the Al atoms with the period over three unit cells; the propagation vector is ${\mathbf{k}}_{2}=(\frac{1}{3}\frac{1}{3}\frac{1}{3})$. The undistorted $P{2}_{1}/c$ lattice itself appears as superstructure over the rhombohedral $R$$\overline{3}$$c$ one resulting from the displacive structure transformation with the propagation vector ${\mathbf{k}}_{1}=(\frac{1}{2}0\frac{1}{2})$. The band gap as given by the density functional calculations and evidenced from the sample transparency behavior at high pressures remains greater than the laser photon energy used (${E}_{ph}=2.41$ eV). That indicates that bond weakening/breaking due to electron excitation across the band gap is not the cause of the phase transition. A likely mechanism of the light action is that structure transformation is driven by phonons, which are excited due to strong electron-phonon coupling in the $\ensuremath{\alpha}$ phase.

Structural Phase Transition in the S = 1/2 Kagome System Cs2ZrCu3F12 and a Comparison to the Valence-Bond-Solid Phase in Rb2SnCu3F12

The crystal structure of the S = 1/2 kagome system Cs2ZrCu3F12 has been determined at both 295 and 125 K via single-crystal X-ray diffraction. A first-order structural phase transition is seen near 225 K, confirmed by variable-temperature synchrotron powder diffraction studies, in which the structure transforms from rhombohedral (R3m) to monoclinic (P21/m). A corresponding abrupt change in dielectric constant is observed near the same temperature. The phase transition is driven by a dramatic change in coordination around the Zr site, which changes from regular octahedral in the rhombohedral phase to a seven-coordinate environment in the monoclinic phase. This leads to a severe buckling of the copper fluoride kagome layers and significant changes in the geometry around the Cu sites, which correlates with both the observed dielectric anomaly and a previously observed anomaly in magnetic susceptibility. It is suggested that this structural phase transition ultimately permits long-range antiferromagnetic ord...

Low-temperature specific heat of the [formula omitted] oxide superconductors under high pressure

We report the results of the low-temperature specific heat measurements of the single crystalline β -pyrochlore oxide superconductors AOs 2O 6 (A=K, Rb, and Cs) under high pressure up to 13 GPa. We find that superconducting transition temperature ( T c ) monotonically increases for CsOs 2O 6 and RbOs 2O 6, while the one for KOs 2O 6 decreases by applying the pressure. With further increasing the pressure, T c is suddenly suppressed at the same lattice volume for all compounds, concomitant with the first-order structural phase transition.

The first-order structural phase transition of YSb

The high-pressure induced phase transition of YSb has been studied using the density functional theory method within the generalized gradient approximation. It was found that the first-order structural phase transition began to occur at 30.8 GPa, agreeing well with available experiments and theoretical calculations. In this phase transition, we do not find changes in the heat capacity and thermal expansion coefficients at lower and higher temperature, but the transition pressures decrease with temperature. The bulk modulus and Debye temperature decrease with increasing temperature, while they increase with increasing pressure. Also, the density of states and band structure of these two compounds with B1 and B2 structures have been presented and analyzed.

Pressure-induced spin-Peierls to incommensurate charge-density-wave transition in the ground state of TiOCl

The ground state of the spin-Peierls system TiOCl was probed using synchrotron x-ray diffraction on a single-crystal sample at T = 6 K. We tracked the evolution of the structural superlattice peaks associated with the dimerized ground state as a function of pressure. The dimerization along the b axis is rapidly suppressed in the vicinity of a first-order structural phase transition at Pc = 13.1(1) GPa. The high-pressure phase is characterized by an incommensurate charge density wave perpendicular to the original spin chain direction. These results show that the electronic ground state undergoes a fundamental change in symmetry, indicating a significant change in the principal interactions.

Superconductivity in Iron Telluride Thin Films under Tensile Stress

By realizing in thin films a tensile stress state, superconductivity of 13 K was introduced into FeTe, a nonsuperconducting parent compound of the iron pnictides and chalcogenides, with a transition temperature higher than that of its superconducting isostructural counterpart FeSe. For these tensile stressed films, superconductivity is accompanied by a softening of the first-order magnetic and structural phase transition, and also, the in-plane extension and out-of-plane contraction are universal in all FeTe films independent of the sign of the lattice mismatch, either positive or negative. Moreover, the correlations were found to exist between the transition temperatures and the tetrahedra bond angles in these thin films.

Influence of polytypism on structural phase transformations in TlGaSe2 crystals

The polytypism is found to have a significant influence on the structural transformation of the crystal lattice of the TlGaSe2 ferroelectric with variations in temperature. In the 2C-polytype, unlike the C-polytype, a first-order structural phase transition, which leads to a change in the translational symmetry along the C axis, is not observed in the temperature range T = 90–300 K and a second-order phase transition due to the formation of an incommensurate phase occurs at a higher temperature.

Evolution of the bulk properties, structure, magnetic order, and superconductivity with Ni doping inCaFe2−xNixAs2

Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray diffraction have been used to characterize the properties of single-crystalline ${\text{CaFe}}_{2\ensuremath{-}x}{\text{Ni}}_{x}{\text{As}}_{2}$ as a function of Ni doping for $x$ varying from 0 to 0.1. The combined first-order structural and magnetic phase transitions occur together in the undoped system at 172 K with a small decrease in the area of the $a\text{\ensuremath{-}}b$ plane along with an abrupt increase in the length of the $c$ axis in the orthorhombic phase. With increasing $x$ the ordered moment and transition temperature decrease but the transition remains sharp at modest doping while the area of the $a\text{\ensuremath{-}}b$ plane quickly decreases and then saturates. Warming and cooling data in the resistivity and neutron diffraction indicate hysteresis of $\ensuremath{\approx}2\text{ }\text{K}$. At larger doping the transition is more rounded and decreases to zero for $x\ensuremath{\approx}0.06$. The susceptibility is anisotropic for all values of $x$. Electrical resistivity for $x=0.053$ and 0.06 shows a superconducting transition with an onset of nearly 15 K which is further corroborated by substantial diamagnetic susceptibility. For the fully superconducting sample there is no long-range magnetic order and the structure remains tetragonal at all temperature but there is an anomalous increase in the area of the $a\text{\ensuremath{-}}b$ plane in going to low $T$. Heat-capacity data show that the density of states at the Fermi level increases for $x\ensuremath{\ge}0.053$ as inferred from the value of Sommerfeld coefficient $\ensuremath{\gamma}$. The regime of superconductivity is quite restrictive with a maximum ${T}_{C}$ of 15 K and an upper critical field ${H}_{c2}=14\text{ }\text{T}$. Superconductivity disappears in the overdoped region.

Structural behavior of the kagome antiferromagnetTmBaCo4O7: Neutron diffraction study and group-theoretical consideration

The first-order structural phase transition at ${T}_{S}\ensuremath{\sim}240\text{ }\text{K}$ in the extended kagome antiferromagnet ${\text{TmBaCo}}_{4}{\text{O}}_{7}$ has been studied by neutron powder diffraction. In order to comprehend the microscopic origin of the transition, a detailed symmetry analysis is performed, based on crystallographic parameters obtained by Rietveld analysis of the neutron data. The results are consistent with the $P31c\ensuremath{\rightarrow}Pna{2}_{1}$ symmetry lowering and support a displacive nature of the phase transition. The complex tilting pattern of ${\text{CoO}}_{4}$ tetrahedra in both triangular and kagome sublattices is described based on symmetry-adapted pseudovector distortion modes of the parent $P{6}_{3}mc$ hexagonal structure. Our analysis reveals that the unusual topology of the crystal structure does not allow ${\text{CoO}}_{4}$ tetrahedra to rotate as rigid units, resulting in their inevitable distortions, whatever the combination of rotational modes considered. A possible analogy between polyhedral distortions and spin frustration in this system is discussed.

Structural Phase Transformations of Mg3N2 at High Pressure: Experimental and Theoretical Studies

The structural behavior of Mg(3)N(2) has been investigated up to 40.7 GPa at room temperature by means of angle-dispersive X-ray diffraction. A reversible, first-order structural phase transition from the ambient cubic phase (Ia3) to a high-pressure monoclinic phase (C2/m) is found to start at approximately 20.6 GPa and complete at approximately 32.5 GPa for the first time. The equation of state determined from our experiments yields bulk moduli of 110.7(2) and 171.5(1) GPa for the cubic and monoclinic phases, respectively, indicating higher incompressibility of the high-pressure phase of Mg(3)N(2). First-principles calculations reproduced the phase stability and transition pressure determined in our experiment. In addition, a second phase transition from the monoclinic phase to a hexagonal phase (P3m(1)) was predicted around 67 GPa for Mg(3)N(2). The electronic band structures of three phases of Mg(3)N(2) are also calculated and discussed.

Pressure-induced structural, electronic, and magnetic phase transitions inFeCl2studied by x-ray diffraction and resistivity measurements

High-pressure (HP) synchrotron x-ray diffraction (XRD) studies were carried out in ${\text{FeCl}}_{2}$ $({T}_{N}\ensuremath{\approx}24\text{ }\text{K})$ together with resistivity $(R)$ studies at various temperatures and pressures to 65 GPa using diamond-anvil cells. This work follows a previous HP $^{57}\text{F}\text{e}$ M\"ossbauer study in which two pressure-induced (PI) electronic transitions were found interpreted as: (i) quenching of the orbital-term contribution to the hyperfine field concurring with a tilting of the magnetic moment by $55\ifmmode^\circ\else\textdegree\fi{}$, and (ii) collapse of the magnetism concurring with a sharp decrease in the isomer shift. The $R(P,T)$ studies affirm that the cause of the collapse of the magnetism is a PI $p\text{\ensuremath{-}}d$ correlation breakdown, leading to an insulator-metal transition at $\ensuremath{\sim}45\text{ }\text{GPa}$ and is not due to a spin crossover $(S=2\ensuremath{\rightarrow}S=0)$. The structure response to the pressure evolution of the two electronic phase transitions starting at low pressures (LP), through an intermediate phase (IP) 30--57 GPa, and culminating in a high-pressure phase, $P>32\text{ }\text{GPa}$, can clearly be quantified. The IP-HP phases coexist through the 32--57 GPa range in which the HP abundance increases monotonically at the expense of the IP phase. At the LP-IP interface no volume change is detected, yet the $c$ axis increases and the $a$ axis shrinks by 0.21 and $0.13\text{ }\text{\AA{}}$, respectively. The fit of the equation of state of the combined LP-IP phases yields a bulk modulus ${K}_{0}=35.3(1.8)\text{ }\text{GPa}$. The intralayer Cl-Cl distances increase but no change is observed in Fe-Cl bond length nor are there substantial changes in the interlayer spacing. The pressure-induced electronic IP-HP transition leads to a first-order structural phase transition characterized by a decrease in Fe-Cl bond length and an abrupt drop in $V(P)$ by $\ensuremath{\sim}3.5%$ accompanying the correlation breakdown. In this transition no symmetry change is detected and the XRD data could be satisfactorily fitted with the ${\text{CdI}}_{2}$ structure. The bulk modulus of the HP phase is practically the same as that of the LP-IP phases suggesting negligible changes in the phonon density of state.

The theoretical study of the cationic conductivity of AgBr

AgCl and AgBr are a special type of superionic conductor, whose superionic phase transition does not accompany any first-order structural phase transition. In the paper, the ionic conductivity of AgBr is investigated by the first principles calculations with VASP code. The calculated results show that the isolated charged interstitial or vacancy is always more stable than the neutral one whether the system is doped or not; the interstitial hops along <001> direction, and the potential barrier is 0.29 eV; the vacancy hop mainly along <111> direction, and the barrier is 0.16 eV; the migration barrier of vacancy is obviously lower than that of interstitial, and then the ionic conductivity is mainly due to the diffusions of vacancies.

Structural and magnetic phase transitions of kagome-like compounds REBaCo 4O 7 (RE=Dy, Ho, Er, Tm, Yb, Lu)

In a temperature range 5–300 K the specific heat C( T) on a new mixed valence cobalt oxides REBaCo 4O 7 (RE=Dy, Ho, Er, Tm, Yb, Lu) was investigated. The first-order structural phase transitions from hexagonal P6 3 mc to orthorhombic Cmc 2 1 phase was indicated by a peak-like anomaly in C( T) curves at T S∼160, 178, 224, and 280 K for RE=Lu, Yb, Tm, and Er correspondingly. The magnetic phase transitions was indicated as the changes of slope on the C( T) curves were found at corresponding temperatures: T N∼50, 74, 98, and 98 K for RE=Lu, Yb, Tm, and Er, correspondingly.

Absence of superconductivity in single-phaseCaFe2As2under hydrostatic pressure

Recent high-pressure studies found that structural/magnetic phase transitions are very pressure sensitive in ${\text{CaFe}}_{2}{\text{As}}_{2}$ and that superconductivity can be achieved under modest pressure, although details of the sharpness and temperature of transitions vary between liquid medium and gas medium measurements. To better understand this issue, we performed high-pressure susceptibility and transport studies on ${\text{CaFe}}_{2}{\text{As}}_{2}$, using helium as the pressure medium. The signatures of the transitions to the low-temperature orthorhombic and collapsed tetragonal phases remained exceptionally sharp, and no signature of bulk superconductivity was found under our hydrostatic conditions. Our results suggest that superconductivity in ${\text{CaFe}}_{2}{\text{As}}_{2}$ is associated with a low-temperature, multicrystallographic-phase sample that is the result of nonhydrostatic conditions associated with the combination of a first-order structural phase transition and frozen liquid media.

Lattice and electronic anomalies ofCaFe2As2studied by Raman spectroscopy

We report inelastic light-scattering experiments on ${\text{CaFe}}_{2}{\text{As}}_{2}$ in the temperature range of 4--290 K. In in-plane polarizations two Raman-active phonon modes are observed at 189 and $211\text{ }{\text{cm}}^{\ensuremath{-}1}$, displaying ${A}_{1g}$ and ${B}_{1g}$ symmetries, respectively. Upon heating through the tetragonal-to-orthorhombic transition at about ${T}_{S}=173\text{ }\text{K}$, the ${B}_{1g}$ phonon undergoes a discontinuous drop of frequency by $4\text{ }{\text{cm}}^{\ensuremath{-}1}$, whereas the ${A}_{1g}$ phonon shows a suppression of the integrated intensity. Their linewidth increases strongly with increasing temperature and saturates above ${T}_{S}$. This suggests (i) a first-order structural phase transition and (ii) a drastic change in charge distribution within the FeAs plane through ${T}_{S}$.

Structural changes in nanocrystalline mackinawite (FeS) at high pressure

The high-pressure behavior of nanocrystalline mackinawite (FeS) with particle sizes of 6, 7 and 8 nm has been investigated by high-energy X-ray total scattering and pair distribution function analysis. An irreversible first-order structural phase transition from tetragonal mackinawite to orthorhombic FeS-II was observed at about 3 GPa. The transition is induced by the closure of the van der Waals gap in the layered mackinawite structure. A grain size effect on the transition pressure and the compressibility was observed.

Anomalous transport properties of Co-site impurity doped NaxCoO2

The effect of substitution of Cu, Zn, Ga, Mn, and Ni for Co in layered NaxCoO2 (x∼0.85) on its resistivity (ρ), magnetization (M), and thermopower (S) has been investigated. At low temperature, ρ remains metallic (dρ/dT&amp;gt;0) for Cu, Zn, and Ga impurities up to a maximum doping of 15%, while for Mn and Ni a metal-insulator transition occurs at 3% and 6% doping, respectively. For the Cu-doped samples, ρ(T) shows a strong anomaly and a hysteresis around 280 K where a first-order structural phase transition occurs due to long-range ordering of the Na-vacancy clusters. Well above the antiferromagnetic transition temperature, the magnetic susceptibility exhibits the Curie–Weiss law. The Neel temperature is insensitive to impurity while both the Curie constant and Weiss constant (θ) depend on the nature of impurity. A small enhancement in magnetization above 280 K and an increase of θ with Cu doping are the signatures of superior Na-vacancy ordering in the Cu-doped samples. In contrast to ρ and M, S changes systematically with doping and temperature, and does not show any anomalous behavior around 280 K.

Polaron liquid-gas crossover at the orthorhombic-rhombohedral transition of manganites

High-resolution synchrotron x-ray powder diffraction in ${\text{La}}_{0.7}{\text{Ca}}_{0.3}{\text{MnO}}_{3}$ shows in detail a first-order structural phase transition from orthorhombic (space-group $Pnma$) to rhombohedral (space-group $R\overline{3}c$) crystal structures near ${T}_{S}=710\text{ }\text{K}$. Magnetic susceptibility measurements show that the rhombohedral phase strictly obeys the Curie-Weiss law as opposed to the orthorhombic phase. A concomitant change in the electrical resistivity behavior, consistent with an alteration from nonadiabatic to adiabatic small polaron hopping regimes, was also observed at ${T}_{S}$. The simultaneous change in transport and magnetic properties are identified as a transition from a correlated polaron liquid for $T&lt;{T}_{S}$ to an uncorrelated polaron gas for $T&gt;{T}_{S}$, driven by the change in the crystal symmetry.