Thermal Expansion Anomalies Research Articles

The microstructure analysis of cerium-modified barium titanate having core-shell structured grains

The microstructure of chemical inhomogeneity having the core-shell structured grain of cerium-modified barium titanate was investigated using X-ray, TEM, dilatometer and DSC methods. Specific heat anomalies, thermal expansion anomalies and lattice parameter discontinuities were associated with the three ferroelectric transitions ( T c , T 1, and T 2) of the grain-core of BaTiO 3. The crystal symmetry of the chemically inhomogeneous grain was strongly dependent on the cerium concentration and temperature. In comparison to other physical quantities, the temperature dependence of the dielectric constant was easily influenced by stress.

Magnetic Frustration Induced Formation of the Spin-Peierls Phase in CuGeO3: Experimental Evidence.

Measurements of the magnetostriction of single crystalline CuGe${\mathrm{O}}_{3}$ at temperatures between 2 and 60 K and for magnetic fields up to 14 T are presented. At low temperatures the magnetostriction is dominated by changes of the spontaneous strains at the phase transitions characteristic for spin-Peierls systems. At higher temperatures a magnetoelastic coupling is found which is unexpectedly large and strongly anisotropic. A comparison of the magnetostriction well above the spin-Peierls transition temperature ${T}_{\mathrm{SP}}$ and the thermal expansion anomalies at ${T}_{\mathrm{SP}}$ yields a striking correlation between the uniaxial pressure dependencies of the spin-susceptibility and ${T}_{\mathrm{SP}}$. It is argued that this strongly supports electronic models of the spin-Peierls transition in CuGe${\mathrm{O}}_{3}$ which are based on competing antiferromagnetic intrachain interactions.

Crystal field phenomena in rare earth cuprates

We present some results pertaining to the crystal field (CF) interactions in high T c rare earth (RE) cuprates and related layered compounds and manifestations of their impact on magnetic properties. We show that the widely disputed specific-heat and susceptibility data in PrBa 2Cu 3O 7 can be interpreted consistently in terms of the standard CF theory for Pr 3+ based on the CF parameters extracted from the inelastic neutron scattering spectra via the superposition model. The CF-induced electric quadrupoles are shown to lead to thermal expansion anomalies in Sm 2CuO 4 and REBa 2Cu 3O 7 compounds. We examine general conditions for an Ising-like behaviour of the RE subsystem. We conclude that such a behaviour may possibly occur in cuprates containing Tb, Dy and Ho owing to the special nature of low-lying 4f states imprinted by the CF interaction.

Dilatometric study of the magnetic phase diagram of erbium

The magnetic phase diagram of erbium has been constructed from the observed anomalies in thermal expansion and longitudinal magnetostriction measurements. Using a capacitance dilatometer, single crystal measurements were made parallel to the crystallographic c-axis in the temperature range 2–100 K and in fields up to 7 T. Features of the phase diagram are compared with those of diagrams deduced from previous neutron scattering studies.

The magnetic order in Ho(Co1−Rh )2 alloys: A dilatometric study

Abstract Thermal expansion and magnetostriction measurements have been made on the cubic Laves-phase alloys Ho(Co1−xRhx)2 in the temperature range 2 to 110 K and in fields up to 7 T. The samples were polycrystalline, with x = 0.00, 0.02, 0.05, 0.08, 0.12, 0.16 and 0.25. Two thermal expansion anomalies are observed at a ferromagnetic transition and a spin-flop transition. The change in temperature and ordering nature of these transitions is reported as a function of Rh concentration. Magnetostriction measurements show a marked variance with Rh concentration and suggest the presence of three new distinct magnetic phases.

Magnetic calorimetry and thermal expansion of UPd 3

The results of calorimetry from 2 to 12 K on single crystals of localised-moment UPd 3 in magnetic fields of zero and 6 T applied along the crystallographic c-direction are presented. Three anomalies in the specific heat were observed which coincide with anomalies in magnetic thermal expansion. The different field-dependencies of these transitions are reported. Combining the two measurements we are able to map the changes in the zero-field Grüneisen parameter across the three transitions.

Specific heat of MnAlGe, MnGaGe and MnZnSb

The specific heat of MnAlGe, MnGaGe and MnZnSb are measured. The values of the electronic specific heat coefficient are 8.9 mJ/mol K 2 for MnAlGe, 8.7 mJ/mol K 2 for MnGage and 11.3 mJ/mol K 2 for MnZnSb. The magnetic anomalies of the specific heat are discussed in connection with the pressure dependence of T C and magnetic thermal expansion anomalies.

External field in the Landau theory of a weakly discontinuous phase transition: Pressure effect in the martensitic transitions.

The effect of hydrostatic as well as uniaxial pressure upon proper ferroelastic (martensitic) phase transitions in cubic crystals is considered. The volumetric strain is considered to be the secondary order parameter conjugated to the symmetrized shear strain that governs the reduction of point symmetry to a tetragonal one. Corresponding terms in the Ginzburg-Landau expansion of the Gibbs free energy are analyzed for transitions of the first as well as of the second order. The pressure effect on the transition temperature as well as on the anomalies of isothermal compressibility and the linear thermal-expansion coefficient are studied and recent experimental data on thermal expansion anomalies in ${\mathrm{V}}_{3}$Si, In-Tl, and Ni-Al are discussed. The nonlinearity of thermal expansion is found to imply a special relation between the shear strain and volume change that can lead to the transformation from a fcc lattice to a bcc one, as observed in the iron alloys.

Thermal expansion of a La 1.87Sr 0.13CuO 4 single crystal at T c in high magnetic fields

We present high-resolution dilatometry measurements of a La 1.87Sr 0.13CuO 4-single crystal parallel and perpendicular to the CuO 2-planes in magnetic fields up to 14 Tesla. We find a strong suppression of the thermal expansion anomalies for magnetic fields parallel to the c-axis. In contrast, a magnetic field perpendicular to the c-axis affects the anomalies only weakly.

Giant striction and thermal expansion anomalies in actinide systems with structural instabilities

A fluctuation theory is presented to discuss the phenomenon of giant striction, associated with a displacive phase transition, which takes account of both phonon and overdamped soft mode excitations - paraphonons. The expansion anomalies in uranium and plutonium below 50 K are attributed to the effects of giant striction and soft mode behaviour accompanying the transition to the incommensurate charge density wave state observed in neutron scattering experiments.

Structural and thermodynamic evidence of a change in thermal motion of hydrogen atoms in Ca(OH) 2 at low temperature

Accurate measurements of lattice parameters and heat capacity of Ca(OH 2) were performed in the temperature ranges 8–450 K and 12–300 K, respectively. No apparent phase transition was evident but thermal expansion and heat capacity anomalies were detected above about 180 K. Based on structural and NMR results, we have attributed these anomalies to a change in the thermal motion of hydrogen atoms; namely, that above 180 K OH-ions have a reorientational motion between three equivalent orientations.

Pressure effect on the Curie temperature and thermal expansion of CrTe

The pressure P derivative dTC/dP of the Curie temperature TC, the linear magnetostriction Delta l/l at 4.2 K, the lattice parameters a and c as functions of temperature T and the high-temperature paramagnetic susceptibility chi of CrTe have been measured. The value of dTC/dP is found to be -7.0+or-0.2 K kbar-1. Thermal expansion anomalies of both a and c below TC=350+or-2 K are observed upon cooling. The inverse of chi above TC shows a break at about 600 K. The field derivative d( Delta l/l)/dH in the interval 40 kOe<H<78 kOe at 4.2 K is found to be 1.2*10-9 Oe-1. The results obtained are discussed on the basis of an appropriate thermodynamic relation, together with the reported results on dTC/dP and the thermal expansions of Cr1- delta Te.

Low-temperature Thermal Expansion Anomalies in Indium and Its Solid Solution Alloys with Thallium

Low-temperature thermal expansion measurements have been made for fct, solid solution indium-thallium single-crystal alloys containing 6, 19, 24 and 29 at.% Tl. The expansion coefficients along the a and c axes for the 6, 19 and 24% alloys follow the same anomalous variations with temperature below about 15 K as for those of pure indium, with no change in magnitude. This contrasts with an increased magnitude for the anomalies previously reported for an In26�5at.%TI alloy and reported here for the a-axis expansion for In29at.%Tl. These observations are discussed in terms of the Fermi surface topology for indium and their implications for the fcc to fct transformation in In-TI alloys being electronically driven.

Thermal expansion anomalies of YCo 4B compound

Temperature dependence of YCo 4B compound has been studied by X-ray diffraction method in the temperature range from 78K to 700 K. Anomalous thermal expansions were observed at about 375 K and 150 K, which correspond to the magnetic order-disorder transition and spin reorientation transition respectively.

Thermal expansion anomalies and spontaneous magnetostriction in Tm 2Co 7

The temperature dependencies of the lattice parameters and magnetization have been studied in the intermetallic compound Tm 2Co 7. Magnetic ordering is accompanied by spontaneous magnetostriction in the basal plane and along the c axis (2.0 × 10 −3 and 2.7 × 10 −3 respectively at 5 K). This corresponds to a volume effect ω s = 6.6 × 10 −3. The Tm atoms on the quasi-cubic sites are considered to be responsible for the spin reorientation from the uniaxial type of magnetic anisotropy above 45 K to the cone of easy axes below this temperature, which is explained by magnetoelastic effects. In the cone range a large orthorhombic distortion ϵ (up to −3.5 × 10 −3 at 5 K) is observed. The additional volume anomaly δω s = −1 × 10 −3, which accompanies spin reorientation, can be explained by the anisotropy of the magnetic moments of the thulium and cobalt sublattices.

Thermal expansion and elastic anomalies in La(FexAl1-x)13 amorphous alloys

La(FexAl1-x)13 (0.80<or=x<or=0.95) amorphous alloys have been prepared by high-rate DC sputtering to investigate the thermal expansion and elastic properties. A large thermal expansion anomaly has been observed in a wide temperature range. This anomaly is retained, even at high temperatures well above Tc. Such a peculiar phenomenon is associated with a large spontaneous volume magnetostriction. A significant Delta E effect has also been found in the temperature and field dependences of Young's modulus. The anomalous behaviour of Young's modulus can be explained in terms of the softening of elastic modulus caused by the anomalous thermal expansion. From these results, it is concluded that the anomalies in the thermal expansion and in Young's modulus for the La(FexAl1-x)13 amorphous alloys are caused by a large spontaneous volume magnetostriction associated with the large temperature variation in amplitude of the local moment.

Thermal expansion anomalies at the magnetic transition temperatures of Au 4Mn and Au 2Mn

Measurements of the thermal expansion and specific heat are carried out for the ferromagnetic ordered alloy Au 4Mn and the antiferromagnetic one Au 2Mn at temperatures including the Curie temperature T c and Néel temperature T N. The pressure effect on T C and T N is also measured. It is found that both Au 4Mn and Au 2Mn have positive exchange strictions and d T C/d p and d T N/d p are positive. The results are discussed in terms of the MnMn distance dependence of the exchange interactions in AuMn alloys.

Effects of quadrupole interactions in rare-earth phosphates RPO 4 (R = Y, Tb-Yb)

The systematic experimental and theoretical investigations of rare-earth (RE) ion contribution to the temperature dependencies of the Young moduli and lattice parameters were performed for RE phosphates RPO 4 (R = Y, Tb-Yb). An anomalous behavior of the Young moduli and lattice parameters for the investigated phosphates in comparison with YPO 4 and a strong softening of Young's moduli for TbPO 4 and TmPO 4 were discovered. It is shown that anomalies of Young's moduli are well described by the temperature dependencies of strain susceptibilities of RE ions calculated utilizing available crystal-field parameters. The thermal expansion anomalies can be described in terms of temperature dependencies of quadrupole moments. Values of magnetoelastic coefficients were estimated. For Tb and Tm phosphates, the strong softening of Young's moduli is ascribed to the correlation enhancement of relevant strain susceptibilities.

Thermal expansion anomalies and spontaneous magnetostriction in R 2Fe 17C x intermetallic compounds

The influence of interstitial solution of carbon on the spontaneous magnetostriction of R 2 Fe 17 ( R = Y, Tb, Tm) has been investigated by means of X-ray dilatometry in the temperature range 5–800 K. The basal-plane linear deformation λ a at 5 K increases with increasing C content for R = Y and Tb, whereas the uniaxial deformation λ c remains practically the same. In the Tm compounds both deformations increase with increasing C content. The volume effect ω s increases from 1.1 × 10 -2 to 2.0 × 10 -2 in the Y series, reaching similar values as in R 2 Fe 14B compounds, despite the fact that the average magnetic moment of Fe atoms μ Fe does not change with increasing C content. Similar behaviour is found in the Tb and Tm compounds. For all compounds studied, the temperature dependence of ω s is in a good agreement with μ 2 Fe( T) in Y 2Fe 17, which points to a dominating contribution of the Fe sublattice to the spontaneous magnetostriction.

The Effect of Giant Striction and Thermal Expansion Anomalies in Systems with Structural Instabilities

AbstractA simple fluctuation theory analogous to the spin‐fluctuation theory of the magneto‐volume effect is presented to discuss the phenomenon of giant striction, associated with displacive phase transitions, and thermal expansion anomalies. It is argued that the giant striction accompanying the phase transition to the incommensurate charge density wave state is responsible for the low temperature anomalous expansion of α‐uranium and α‐plutonium. The spontaneous striction values in these materials are estimated to be 4.57 × 10−3 and 1.82 × 10−2 comparable with those known for systems with giant magnetostriction.