Crystallographic Phase Transition Research Articles

The Peierls transition of Perylene radical cation salts with 2:1 stoichiometry containing tetrahydrofuran

Crystal structure, microwave conductivity and static magnetic susceptibility were analyzed for the conducting perylene (PE) radical cation salts of nominal composition (PE)2(PF6)1−x(AsF6) x ·2/3 THF for 0≦x≦1. Crystallographic phase transitions in the high-temperature metal phase were also characterized by differential scanning calorimetry and electron spin resonance. The Peierls transition occuring at 100–120 K (depending onx) and the opening of the energy gap in the semiconductor phase were determined from the magnetic data. The dependence of these properties on the size of the complex anions and the packing of the PE molecules is discussed.

Theoretical investigation of the high-pressure crystal structures of Ce and Th.

The high-pressure crystal structures of Ce and Th are studied theoretically by means of a full potential linear muffin-tin orbital technique, where the generalized gradient correction to density-functional theory has been implemented. A crystallographic phase transition from fcc to bct is found for u-Ce at about 10 GPa, in good agreement with experiment. The calculated pressure dependence of the crystallographic c/a ratio (of the bct structure) is in good agreement with experiment. For n-Ce a subsequent transition to the hcp crystal structure is predicted at about 1600 GPa. For Th a continuous transformation from fcc to bct at 45 GPa is found, confirming recent experimental findings. Also for this material the calculated pressure dependence of the crystallographic c/a ratio (of the bct structure) is in good agreement with experiment. For Th we also predict a transformation to the hcp crystal structure at even higher pressures. The metals Ce (Z= 58) and Th (Z= 90) are located at the beginning of the 4f (lanthanides) and the Sf (actinides) transition-metal series, respectively. Ce is the first lanthanide metal that has an appreciable occupation of the 4f states, and is a known subject for controversy. Primarily the debate concerns the localized versus itinerant character of the 4f electron. The electronic structure of Ce has been reported earlier. As regards the 5f population at ambient conditions, calculations show that the actinide metal Th has a smaller f occupation than ct Ce Se-ver.al recent investigations of these two metals, both experimental as well as theoretical, have been motivated by their crystallographic phase transitions under compression. In the lanthanide series localized 4f states are progressively being filled and their inhuence on the chemical bonding is negligible. In the actinide series, however, the earlier metals have delocalized 5f electrons that contribute to the chemical bonding. The increasing 5f occupation as a function of atomic number results in increasingly distorted (complex) crystal structures. This is understood from the increased contribution to the bonding from the narrow 5f bands. ' Particularly interesting in the present context is the occurrence of the bct (c/a =0.825) structure at ambient conditions for Pa (Z=91). The increased 5f occupation in Pa, in comparison to Th, results in a bonding which favorizes the bct crystal structure over the fcc structure (which is found in Th at ambient pressure). The effect of pressure upon an itinerant 5f metal is first a lowering of the energy of the 5f bands relative to the other valence states and second a broadening of the Sf band as well as a broadening of the other bands. The relative lowering of the Sf states results in an increased Sf population. Therefore, in view of the known crystal structure of Pa (bct), it is not surprising to find a pressure-induced onset of a bct crystal structure in Th. Since u-Ce from certain aspects can be regarded as an analog element to Th, a similar behavior (stabilization of the bct structure) of Ce under pressure is not surprising either. However, for Ce, in contrast to Th, there is an intermediate bodycentered-monoclinic (bcm) phase found experimentally in the interval 6— 12 GPa. This bcm structure can be seen as a small distortion of the bct crystal structure, and can therefore be, approximately, described as a bct structure. In fact both the bcm and the bct phases of Ce at moderate compressions can be viewed as more or less distorted fcc structures. We note here that the experimental structure is not quite clear for Ce since it has been reported that an orthorhombic n-U type of structure is stabilized, instead of the bcm structure. In the present report we take the view point that the bcm and the u-U structures are very close in energy and that the u-U

Magnetic Studies on the 1-D Antiferromagnetic Chains of enH2Mn(Fe)F5

Measurements of the 57Fe Mössbauer effect and of magnetic susceptibilities have been performed on the 57Fe doped or pure quasi-1-d antiferromagnetic chain compound enH2MnF5 as a function of temperature. Particular attention was paid to the regions very near the Néel point at TN = 14.5(5) K. The Mössbauer spectra fitted by the Blume-Tjon model show definite relaxation effects, which are attributed to short-range order with temperature-dependent relaxation times. The soliton model of non-linear excitations was applied. Experimental data confirm the predicted exponential temperature dependence of the thermal excitation of moving domain walls. From the activation energy EA/k = 184(5) K and the 1-d exchange energy J/k = -13.6(2) K a local anisotropy energy D/k of -2.4 (3) K was obtained which is smaller than D/k = -3.5 K derived from our single crystal measurements of magnetic susceptibilities. Within the 3-d magnetic ordering the interchain interaction amounts to J′/k ≈ 0.22(4) K (J′/J = 15.8 10-3). A zero-spin reduction ⊿S/S equal to 17% was obtained. 1-d correlations are observed for TN<T< |J|S (S + 1)/k ≈ 78 K, and a crystallographic phase transition at 216 K was indicated in the magnetic properties as well as by DSC and X-ray powder diffraction measurements.

A model of displacive crystallographic phase transition for A15-type superconductor alloys and for icosahedral-type perovskites

A continuous polyhedral evolution of octahedron through icosahedron to cuboctahedron, previously described, is extended in the present paper to primitive and body-centered cubic crystal lattices. The evolution of these lattices describes displacive crystallographic phase transitions, with a monodimensional order parameter. It is shown that both the superconductor alloys of A15 type and some perovskite structures of icosahedral type can be described as phases in these transitions. Besides, some of these compounds present two or more phases, where the above evolution exists in between, for example, the observed quasi-continuous transition from Pm 3 m to Im 3 in the perovskite Na x WO 3. In other cases, such transition has not been observed, but we suggest that it could exist, as for instance between the phases Pm 3 m and Pm 3 observed in the A15-type superconductor HgTi 3.

Crystallographic data and magnetic properties of new CeFeSi-type RMnGe compounds (R = FaSm) studied by magnetization and neutron diffraction measurements

Investigations made by powder X-ray diffraction, susceptibility measurements and neutron diffraction experiments on new ternary RMnGe (R = LaNd) are reported. They crystallize in the well-known tetragonal CeFeSi-type structure (space group P4/nmm). This structure, which is closely related to the ThCr 2Si 2-type structure, can be described as isolated “ThCr 2Si 2 blocks” connected via R-R contacts. All these compounds are antiferromagnetic below T N ∼ 410 K (not detected in the bulk susceptibility measurements). At room temperature, the magnetic structures are characterized by a stacking of antiferromagnetic (001) Mn layers with Mn magnetic moments (about 3.6-3.2 μ B at 2 K) at 40° (CeMnGe) or along the c axis. In all cases, this antiferromagnetic arrangement of the Mn sublattice persists down to 2 K. At lower temperature, the R sublattice orders in ferromagnetic (001) layers, with various ordering schemes along the stacking axis. At 150 K, PrMnGe exhibits ferromagnetic behaviour followed by a ferro-antiferromagnetic transition below 80 K simultaneously with a crystallographic phase transition (tetragonal to orthorhombic symmetry). Below 200 K, NdMnGe exhibits a ferromagnetic ordering of the Nd sublattice which persists until 2 K. Within the complete ordering temperature range, the Pr and Nd magnetic moments are in the basal plane with a magnitude of 2.45(3) and 3(1) μ B for Pr and Nd, respectively, at 2K. The magnetic structures correspond to the sequence Mn(AF)R(+)R(+)Mn(AF)R(−or+). The Mn moments rotate at 50° from the c axis in PrMnGe, while they are in quadrature in NdMnGe. In CeMnGe, the Ce-sublattice orders antiferromagnetically below about 130 K (not detected in the magnetometric measurements), yielding the following ordering scheme: Mn(AF)Ce(+)Ce(+)Mn(AF)Ce(−)Ce(−); at 2 K the Ce moment value is 2.2(1) μ B. The Mn and Ce moment directions are almost parallel, and make an angle of about 35° with the c axis. The results are discussed and compared to those obtained previously on the corresponding ternary silicides and germanides of the CeFeSi- and ThCr 2Si 2-type structures.

Theoretical zero-temperature phase diagram for neptunium metal.

Electronic structure calculations, based on the density-functional theory with the generalized gradient approximation to the exchange and correlation energy, are used to study the crystallographic properties of neptunium metal under compression. Calculated ground-state properties, such as crystal structure and atomic volume, are found to be in excellent agreement with experiment. The calculated bulk modulus and the first pressure derivative of the bulk modulus are also in accordance with experiment. Our theory predicts that neptunium at low temperature undergoes two crystallographic phase transitions upon compression. First a transition \ensuremath{\alpha}-Np\ensuremath{\rightarrow}\ensuremath{\beta}-Np at 0.14 Mbar and second a \ensuremath{\beta}-Np\ensuremath{\rightarrow}bcc phase transition at 0.57 Mbar. These transitions are accompanied by small volume collapses of the order of 2\char21{}3 %. The high-pressure phase (bcc) is also investigated theoretically up to 10 Mbar. A canonical theory for the f-electron contribution to the hcp, fcc, and bcc structures is presented.

Mössbauer and energy-dispersive x-ray-diffraction studies of the pressure-induced crystallographic phase transition in C-type Yb2O3.

Mössbauer and energy-dispersive x-ray-diffraction studies of the pressure-induced crystallographic phase transition in C-type Yb2O3.

Ultrasonic velocity measurements of UNiSn

UNiSn in the MgAgAs-type crystal structure is characterised by a simultaneous magnetic, metallic and crystallographic phase transition occurring at 43 K. Results of measurement of both the transverse and longitudinal sound velocities in polycrystalline UNiSn through this transition are presented and compared with theoretical predictions.

Spontaneous magnetostriction of CrAs1−xSx compounds

Abstract Thermal expansion and magnetic susceptibility were measured for the compounds CrAs 1− x S x ( x ⩽ 0.20). The large spontaneous magnetostriction of CrAs, which occurs mainly along the B -axis of the orthorhombic MnP-type structure, decreases monotonously with the composition x and seems to disappear at x = 0.35. The susceptibility measurements for CrAs 0.84 S 0.16 show the crystallographic phase transition accompanied by a magnetic phase change at about 800 K.

Structural, superconducting, and magnetic properties of YNi2B2C and ErNi2B2C.

Discovery of a quaternary superconducting system Y-Ni-B-C has been reported recently. Our structural studies on ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}15.5 K) reveal large and anisotropic thermal vibrations of C atoms in the Y-C plane of the structure. No crystallographic phase transition is observed down to 50 K. Our specific-heat data suggest that ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C is a strong-coupling superconductor. Results of our resistivity, magnetic, and specific-heat data clearly suggest coexistence of superconductivity and magnetism (of Er spins) in ${\mathrm{ErNi}}_{2}$${\mathrm{B}}_{2}$C (${\mathit{T}}_{\mathit{c}}$\ensuremath{\approxeq}10.5 K) below \ensuremath{\approxeq}7 K.

Microdeformations in ferroelectrics

Ferroelectrics undergo one or more crystallographic phase transitions, which involve lattice distortions. The direction of spontaneous polarization in ferroelectrics can be reoriented by an applied electric field (or mechanical stress). There is a spontaneous strain accompanying spontaneous polarization. Phase transitions and domain reorientations thus result in microdeformations. Many devices such as actuators and transducers are based on this behaviour. The origin of microdeformations in ferroelectrics and their consequences are discussed here.

Crystallographic phase transitions in actinide metals as a function of pressure

We present first-principles calculations of the equilibrium volumes and crystal structures of the light actinides (ThPu). The calculated equilibrium volumes for f.c.c. Th, b.c.t. Pu, α-U, and β-Np are found to agree reasonably well with the experimental data, and when comparing the total energies of the b.c.c., f.c.c., b.c.t., α-U, and β-Np structures we obtain the correct crystal structures for all studied systems. Also, the calculated equilibrium volumes for ThPu, using a hypothetical f.c.c. structure, have been calculated and it is demonstrated that although spin—orbit coupling is included in these calculations the calculated equilibrium volume of Pu is smaller than for Np, in disagreement with experiment. Moreover, the calculated tetragonal elastic constant, C ′, is shown to be negative for b.c.c. U, b.c.c. Np, b.c.c. Pu and f.c.c. Pu. Thus, our zero temperature calculations suggest that the b.c.c. structure is unstable for these elements and that f.c.c. Pu is also unstable. This is in conflict with experiment and we are led to the conclusion that temperature effects must be of crucial importance for stabilizing cubic structures in U, Np, and Pu. Further, as a function of decreasing volume we predict a crystal structure sequence f. c. c. → b. c. t. → f. c. c. in Th, a sequence α- U → b. c. t. → b. c. c. in U, and a sequence β- Np → b. c. t. → b. c. c. in Np. Also, a sequence of transitions in Sc as a function of decreasing volume have been calculated, namely h. c. p. → f. c. c. → ω → β- Np → b. c. c.

The electrical resistance of PuSb under high pressure

A new experimental set-up with a Bridgman-type high pressure cell in a closed containment allows resistance measurements on highly radioactive materials. We present results of high pressure, low temperature studies on PuSb single crystals in the pressure range to 25 GPa and at temperatures between 1.3 K and 300 K. As pressure on PuSb is increased, its Néel temperature and the transition temperature to the ferromagnetic ground state are increased. In the pressure range from 10 to 15 GPa, we observed a strong decrease in the resistance associated with the crystallographic phase transition from the B1 (NaCl) to the B2 (CsCl) structure. The high pressure phase appears to be non-magnetic.

Synthesis, Structure, and Properties of the Organic Conductor (BEDT-TTF)2Br2SeCN

A new salt of the electron-donor molecule BEDT-TTF [bis(ethylenedithio)tetrathiafulvalence, or ET] with the unusual {open_quotes}T{close_quotes}-shaped anion Br{sub 2}SeCN{sup {minus}} was synthesized. Its crystal structure (space group P1, Z = 1; {alpha} = 5.9304-(6) {angstrom}, b = 8.8042(9) {angstrom}, C = 16.509(2) {angstrom}, {alpha} = 95.723(8){degree}, {beta} = 98.767(8){degrees}, {gamma} = 92.192(8){degrees}, V = 846.4(2) {angstrom}{sup 3} at 298 K; a = 5.8131(12) {angstrom}, b = 8.666(2){angstrom}, c = 16.626(4) {angstrom}, {alpha} = 93.75(2){degrees}, {beta} = 100.62(2){degrees}, {gamma} = 89.75(2){degrees}, V = 821.4(3) {angstrom}{sup 3} at 122 K) contains layers of ET molecules interspersed with anion layers. A crystallographic phase transition at {approximately}150 K is observed, accompanied by a discontinuity in unit cell angles. The band electronic structure calculation suggests the possibility of one-dimensional conduction, but electron localization renders the system semiconducting over the entire temperature range studied. Inductive techniques showed no sign of superconductivity at temperatures above 1.2 K and pressures below 5 kbar.

Magnetism from the Atom to the Bulk in Iron, Cobalt, and Nickel Clusters

Molecular beam deflection measurements of small iron, cobalt, and nickel clusters show how magnetism develops as the cluster size is increased from several tens to several hundreds of atoms for temperatures between 80 and 1000 K. Ferromagnetism occurs even for the smallest sizes: for clusters with fewer than about 30 atoms the magnetic moments are atomlike; as the size is increased up to 700 atoms, the magnetic moments approach the bulk limit, with oscillations probably caused by surface-induced spin-density waves. The trends are explained in a magnetic shell model. A crystallographic phase transition from high moment to low moment in iron clusters has also been identified.

A theoretical study of the crystallographic structures in neptunium

The total energy has been calculated as a function of volume for various crystal structures in neptunium metal. Around the equilibrium volume the experimentally observed low-temperature structure, alpha -Np, is found to have the lowest energy. As a function of compression we predict Np to undergo a crystallographic phase transition from alpha -Np to sec. The zero-temperature calculations also show that at volumes close to the alpha -Np to BCC transition the BCT structure is very close in energy, and high-pressure experiments performed at elevated temperatures on Np may allow observation of this structure as well.

High rare earth sublattice ordering temperatures in RMnSi compounds (R  LaSm, Gd) studied by susceptibility measurements and neutron diffraction

Investigations by bulk magnetization and neutron diffraction measurements are reported on the ternary silicides RMnSi (R  LaNd). All these compounds crystallize in the well-known tetragonal structure of the CeFeSi type (space group P4/ nmm). This structure, which is closely related to the ThCr 2Si 2- and TbFeSi 2-type structures, can be described as isolated “ThCr 2Si 2 blocks” connected via R-R contacts. The R, Mn and Si atoms are arranged in alternate layers stacked along the c axis in the sequence R Si(Mn 2) SiRR Si(Mn 2) SiR. All these compounds are antiferromagnetic below T N ≈ 310, 240, 265 and 280 K for La-, Ce-, Pr- and NdMnSi respectively. Their magnetic structures are characterized by a stacking of antiferromagnetic (001) Mn layers. The Mn magnetic moments (about 3 β b) are at 45° to c axis (LaMnSi) or in the (001) plane. At lower temperature PrMnSi and NdMnSi show additional transitions which correspond to an antiferromagnetic ordering of the rare earth sublattice ( T 1 = 130 and 185 K for Pr and Nd respectively) followed by a spin reorientation process simultaneously with a crystallographic phase transition (tetragonal to orthorhombic symmetry) at about 80 K for both compounds. At 2 K the antiferromagnetic (AF) structure of PrMnSi and NdMnSi consists of a stacking of ferromagnetic and antiferromagnetic (001) layers of R and Mn respectively in the sequence Mn(AF) R(+)R(+) Mn(AF) R(−)R(−); the moments are in the layers, collinear in PrMnSi and in quadrature in NdMnSi. In CeMnSi the ferromagnetic (001) cerium layers order antiferromagnetically below about 175 K and are coupled with the manganese sublattice according to the following scheme: Ce(+) Mn(AF) Ce(+)Ce(−) Mn(AF) Ce(+); at 2 K the Ce moment value is β Ce = 0.78(4) β B. The results are discussed and compared with those obtained previously for the corresponding ternary silicides and germanides of the CeFeSi-, TbFeSi 2- and ThCr 2Si 2-type structures.

Prediction of a bcc structure in compressed yttrium.

Crystallographic phase transitions between close-packed structures in Y have been studied theoretically, using a full-potential, linear muffin-tin orbitals method. At the equilibrium volume the hcp structure is correctly found to have the lowest energy, and at a calculated pressure of 60 kbar the fcc phase becomes stable. It is found that the so-called \ensuremath{\omega} phase is only marginally lower than the hcp and the fcc structures over a large volume range. At even higher pressures another crystal phase in yttrium is found, namely the bcc phase, which is predicted to become stable at a pressure of 2.8 Mbar.

High-pressure behavior of CmBi and UBi

We report here high-pressure X-ray diffraction studies of the two actinide compounds, UBi and CmBi. These compounds existed initially in the NaCl-type (B1) crystal structure and each was studied at room temperature using a diamond anvil cell and energy dispersive X-ray diffraction analysis. The U and Cm monobismuthides were studied up to pressures of 42 and 48 GPa, respectively. All materials exhibited first-order crystallographic phase transitions from their NaCl-type (B1) to a CsCl-type (B2) structure, which were accompanied by volume collapses: UBi at 5 GPa (11% volume change); CmBi at 12 GPa (16% volume change). With curium monobismuthide a subsequent second-order transition from the CsCl-type structure to a tetragonal phase (indexed in space group P4/mmm) occured at 19 GPa. The bulk moduli and their pressure derivatives were derived for each material from the experimental pressure-volume relationships for the B1 phases. These values were: for UBi, 91 GPa and -5.4; for CmBi, 53 GPa and 8.1.

Measurement of the heat capacity of BaEu 2Mn 2O 7

The heat capacities of non-stoichiometric BaEu 2Mn 2O 7 phase were measured from room temperature to 850 K using differential scanning calorimetry. In stoichiometric BaEu 2Mn 2O 7, a thermal anomaly caused by the crystallographic phase transition was observed. The transition temperature was 565 K and the enthalpy change was determined to be 2.28 kJ mol −1. It was found that the values of the heat capacity, except for in the vicinity of the transition temperature, were not greatly affected by non-stoichiometry and were larger than the maximum heat capacity which is predicted from the Dulong-Petit law.