Low-temperature Tetragonal Phase Research Articles

Thermal conductivity and structural instability in La- and Cu-site-substituted La2CuO4.

The thermal conductivity \ensuremath{\kappa} and the sound velocity in ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$ are found to be significantly decreased by Nd, Zn, and Ni doping, reflecting the lattice instability of this compound. The rapid decrease of \ensuremath{\kappa} with x in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (M=Sr,Ba) in a small-x region that was previously reported by us is understood to be the result of an enhancement of the lattice instability induced by M doping, just as in the Nd-, Zn-, and Ni-doped insulating systems. The y dependence of the transition temperature from the LTO1 (low-temperature orthorhombic 1) to LTT (low-temperature tetragonal) phase in ${\mathrm{La}}_{1.875\mathrm{\ensuremath{-}}\mathit{y}}$${\mathit{R}}_{\mathit{y}}$${\mathrm{Sr}}_{0.125}$${\mathrm{CuO}}_{4}$ (R=Nd,Sm,Eu,Gd) indicates that the larger the difference of the ionic radii between ${\mathrm{La}}^{3+}$ and ${\mathit{R}}^{3+}$ ions, the more easily the transition to the LTT phase is induced. We propose that the LTT region exists around the ${\mathit{R}}^{3+}$ ion even in the LTO1 phase and extends with the decrease of temperature and the structural transition to the LTT phase is induced when the LTT region becomes connected in a whole area of the crystal. We also point out the importance of the local lattice distortion for the Cu-site-substitution effects on ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{M}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$.

Structural phase transitions and superconductivity in La-M-Cu-O compounds

Abstract The structural phase transitions D17 4h→D18 2h→D164h in La2-x M x CuO4 compounds and their influence on electronic spectrum for oxygen doped holes in CuO2 plane are considered on the basis of a micro-scopical anharmonic model. The free energy and equation for the two-component order parameter are calculated in the self-consistent phonon approximation. Taking into account the electron-phonon interaction with the soft tilting mode and other phonon branches the dependence of the superconducting temperature Tc on hole concentration n is obtained. A dip in Tc(n) dependence is found at the transition to the low temperature tetragonal phase (LTT) owing to splitting of two oxygen bands at half-filling.

Crystalline phases of sol-gel ZrO2 in the ZrO2-SiO2 system: Differential thermal analysis and electron microscopy studies

Zirconium oxide dispersed in silica was synthesized by the cogelation of tetraethoxysilane and zirconium acetate. For the gelation, acid and basic media were used. The final zirconia concentration was around 6.7 wt. %. Differential thermal analysis, thermogravimetric analysis, as well as electron microscope techniques were used for the characterization of the samples. It was found that the stabilization of the crystalline phases depends on the preparation conditions and thermal treatments. Tetragonal and monoclinic ZrO2 phases were identified in basic preparations. However, in acidic samples only low-temperature tetragonal phase was identified.

Hole doping in La 2− xBa xCuO 4− y using oxygen stoichiometry

The effects of the oxygen stoichiometry on the superconducting transition temperature T c in La 2− x Ba x CuO 4− y are investigated. The oxygen content was controlled (formula 0 ≤ y ≤ 0.03) by annealing in 0.123% oxygen at temperatures up to 1040°C. As shown previously the dip in T c tends to disappear with oxygen reduction. Neutron-diffraction studies confirm that samples remain in the low-temperature tetragonal phase. We find that the minimum in T c moves to higher x as oxygen is removed. We interpret this to be evidence that the minimum in T c is associated with a critical hole density h c ~ x − 2 y. We suggest that the gradual disappearance of the dip with reduction is related to the introduction of disorder into the structure.

Low temperature frustrated structural phase transition in single crystal La 1.85Sr 0.15CuO 4

We report the high resolution low temperature X-ray diffraction investigation on the structural phase transition (SPT) from a low temperature orthorhombic (LTO) to a low temperature tetragonal (LTT) phase for an optimally-doped La 2− x Sr x CuO 4 ( x = 0.15) high quality single crystal. Results show a frustrated SPT and the coexistence of both LTO and LTT phases at all temperatures below about 145 K with the LTO phase being predominant down to at least 5 K.

Calorimetric study of the structural and magnetic ordering in R2NiO4 (R = La, Pr and Nd)

Abstract

Effects of Diffusion of Co/Ni Cations on Sol-Gel Derived ZrO2 Polymorphic Transformations

ABSTRACTTransition metal incorporated ZrO2 gel matrices have been prepared by impregnation method and investigated with FTIR/DTA/XRD. The metastable tetragonal - monoclinic -tetragonal - cubic phase transformations are revealed in the DTA heating process up to 1400°C for the studied gels. High-temperature tetragonal to monoclinic transition is also observed in the cooling process. It is found that the diffusing metal cations stabilize the low-temperature tetragonal phase. However, for high-temperature (900°C) calcined gels, both as-prepared and metal-stabilized tetragonal phases are reduced substantially. Correlations between metal diffusion and gel polymorphic stabilities are also demonstrated.

Influence of oxygen on structural transitions in Pr 2NiO 4+δ

We have investigated structural transitions in Pr 2NiO 4+δ(δ≥0) using DSC, TGA and XRD measurements. This study reveals firstly that the so called LTT (Low Temperature Tetragonal) phase i.e. P4 2/ncm symmetry exists not only for δ=0 but also for slightly sur-stoichiometric compounds. The transition temperature from P4 2/ncm to Bmab is independent on δ in the small range of oxygen excess allowed. Moreover, an orthorhombic-tetragonal transition at about 720 K in the oxidized Pr 2NiO 4+δ was emphasized and extensively discussed. This transition occurs simultaneously with an oxygen loss. It is shown that the parameter controlling this transition is neither the temperature nor the oxygen partial pressure but the value of δ. This structural change occurs for the threshold value δ c=0.192. We suggest that this threshold reflects a frontier between an oxygen disordered HTT (High Temperature Tetragonal) phase and an orthorhombic phase LTO (Low Temperature Orthorhombic) in which excess oxygen and/or holes might be ordered. Finally, general considerations n the stability of this K 2NiF 4 type oxide and evolution of transition temperatures have been done as a function of the rare earth ion size.

Transport properties and low temperature structural phase transition around x=0.125 in La 2−x−yRE ySr xCuO 4 (RENd, Sm, Gd, Tb)

In La 1.9−itxGd 0.1Sr x CuO 4, a prominent change of the thermoelectric power S below ∼80K is observed in a narrow region of x around 0.12 although the structural phase transition to the tetragonal low-temperature (TLT) phase is observed at ∼ 115K in a wide region of x. These results suggest that the anomalous behaviors of the transport properties below ∼80K are responsible for the change of the electronic state itself. In La 1.875− y Gd y Sr 0.125CuO 4, T u ittd β, which is the structural phase transition temperature to the TLT pahse, increases with increasing y. The temperature where S begins to decrease rapidly is close to T u dβ in y≤0.04. In y≥0.05, a prominent change of S is observed below a lower temperature than T u dβ and an obvious sign reversal of S appears appears at ∼40K. These results indicate that the change of the electronic state is enhanced in the TLT phase.

Structure and superconductivity of rare earth doped La 2-xSr xCuO 4

We present results of a detailed investigation of the structural phase transitions and their influence on superconductivity in rare earth doped La 2−xSr xCuO 4. The structural transitions are predominantly determined by size differences of the atoms in alternating layers of the structure. In contrast to the findings in Ba-doped La 2 CuO 4 the influence of the low temperature transition on the electronic properties is not restricted to a narrow hole doping around x ⋍ 1 8 . Our data show a strong influence of the buckling of the planar (CuOCu) bonds on the electronic properties. In particular, superconductivity disappears in the low temperature tetragonal phase if the buckling exceeds a critical strength.

Observation of magnetic order of Cu-moment aroundx=0.12 in La2−x Ba x CuO4 and La2−x sr x CuO4 by μ+SR

Zero field positive muon spin rotation method (μ +SR) is applied on La2−x Sr x CuO4 around forx=0.12 at which the high-T c superconductivity (SC) is suppressed. The magnetically ordered state of Cu-moments, which is not a spin glass state but an antiferromagnetic like state, appears below 15 K for 0.105≤x≤0.120. The magnetic phase boundary is very similar to the one of the La2−x Ba x CuO4 in which the structural transition from the low temperature orthorhombic (LTO) phase to the low temperature tetragonal (LTT) phase is observed aroundx=0.12. The present study suggests that there is no big difference of the electronic state of the CuO2 plane between the La-Ba system and the La-Sr system and that the magnetic ordering of Cu-moments plays an important role for the suppression of the high-T c SC aroundx=0.12 in both of the systems, although the LTO-LTT structural transition has not been observed yet in the La-Sr system.

Buckling of the CuO 2 planes and the electronic properties of doped La 2CuO 4 superconductors

The electronic properties of rare earth (RE) doped La 2−xSr xCuO 4 (LSCO) depend sensitively on the tilt displacements of the CuO 6 octahedra. In particular, in the low temperature tetragonal (LTT) phase with increasing tilt angle Φ a crossover from a metallic, superconducting to a metallic, non-superconducting phase with probably (local) antiferromagnetic (AF) order is observed. We discuss the influence of the tilt displacements on the electronic properties in the three structural low temperature phases of RE-doped LSCO.

Effects of structural phase transitions on Superconductivity and magnetism in substituted derivatives of La 2CuO 4

Structural (neutron and x-ray diffraction) and magnetic (neutron diffraction and magnetic susceptibility) measurements of materials derived from La 2CuO 4 by isovalent and aliovalent substitution in the La 3+ site have been performed. Such substitutions permit the tilts of the CuO 6 octahedra (about the (110) and (1 10) axes of the undistorted 14/mmm structure), and the hole concentration in the CuO layers, to be independently varied. The results of these measurements, in combination with those from other research groups, demonstrate that all four tilt structures, HTT (high temperature tetragonal, 14/mmm), LTO1 (low temperature orthorhombic 1, Bmab), LTO2 (low temperature orthorhombic 2, Pccn) and LTT (low temperature tetragonal, P4 2/ncm), support bulk superconductivity, with T c ( HTT) > T c ( LTO1) > T c ( LTO2) > T c ( LTT) at the same hole concentration. Furthermore, there is a strong suppression of superconductivity when the doped carrier concentration is 1/8 hole per Cu 2+ in the LTT and LTO2 phases of these materials. The reason for this T c suppression is not established. Finally, the magnetic structures adopted by the Cu 2+ spins in the insulating phases, determined by neutron diffraction, are also influenced by the tilts of the octahedra. This may reflect a structural dependence of interlayer and intralayer magnetic anisotropies.

Theory of magnetism in low symmetry crystals: cuprates and nickelates

The effective spin Hamiltonians for the distorted NiO 2 and CuO 2 planes with low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) structures are derived by taking into account the spin-orbit coupling and structural distortion as perturbations. We explain why the weak ferromagnetism occurs in both LTO and LTT phases for cuprates whereas it does only in the LTT phase for nickelates. The spin-wave gaps observed by neutron scattering experiments are analyzed. We find that the gaps of the in-plane and out-of-plane modes in La 1.65Nd 0.35CuO 4 are caused, respectively, by anisotropic superexchange and direct-exchange interactions.

Critical buckling for the disappearance of superconductivity in rare-earth-doped La2-xSrxCuO4.

Analysis of the structural, transport, and superconducting properties of Nd-doped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ reveals a critical tilt angle of the Cu${\mathrm{O}}_{6}$ octahedra for the disappearance of superconductivity in the low temperature tetragonal phase. Our results indicate a strong influence of the tilt of the Cu${\mathrm{O}}_{6}$ octahedra on the electronic properties, suggesting the importance of spin-orbit coupling for the destruction of superconductivity and for the stabilization of a magnetic state.

Ion-size effects on the structure and superconductivity of La-214 compounds at [formula omitted] doping

A structural transition to a low-temperature tetragonal (TLT) phase occurs below T d2≈60 K in La 2− x Ba x CuO 4 only around x=0.125, accompanied by sharp suppression of superconductivity. In this study we investigated the effects of partial substitution of smaller divalent cations for Ba 2+ ions ( Ba 2+ > Sr 2+ > Ca 2+) on these anomalous low-temperature properties. The suppression of superconductivity is strongly affected by the appearance of the TLT phase for this carrier concentration. We also show that the variations of T d2 and T c are quantitatively scaled by the averaged size of cations at the La site. This dependence can be interpreted as chemical effects acting effectively as applied pressure. We discuss how our observation is related to recent reports which claim that the TLT structure itself does not necessarily destroy the superconductivity.

Superexchange anisotropy in the cuprates.

The single-bond superexchange anisotropy resulting from the 2${\mathit{p}}_{\mathrm{\ensuremath{\sigma}}}$ and the 2${\mathit{p}}_{\mathit{z}}$ orbitals on the ligand ion of the cuprates is analyzed for a general case in which the oxygen octahedra surrounding each copper rotate around a general axis in the ${\mathrm{CuO}}_{2}$ plane. An extension of Moriya's calculation shows that when the oxygen orbitals are degenerate, the single-bond spin interaction is rotationally invariant, contrary to previous claims. When the orbitals are nondegenerate the single-bond Hamiltonian is anisotropic, but the anisotropy is small. The ground state of the entire ${\mathrm{CuO}}_{2}$ plane is found to be completely antiferromagnetic in the low-temperature tetragonal phase. The weak ferromagnetism of the low-temperature orthorhombic phase is mainly due to the anisotropy resulting from frustration of the single-bond interactions. These frustration effects also generate an anisotropy which dominates that due to the nondegeneracy of the oxygen orbitals. We present explicit results for the weak ferromagnetic moment and for the spin-wave spectrum, as a function of the direction of the octahedra rotation axis.

Theory of Dzyaloshinski-Moriya antiferromagnetism in distorted CuO2 and NiO2 planes.

Moriya's perturbation theory is applied to the distorted ${\mathrm{NiO}}_{2}$ and ${\mathrm{CuO}}_{2}$ planes, and the effective spin Hamiltonians are derived. Spin-wave theory is explored for the Hamiltonians, and the spin-wave excitations are studied for nickelates and cuprates with low-temperature orthorhombic and low-temperature tetragonal (LTT) structures. It is shown that in the cuprates the origin of the anisotropy responsible for the spin-wave gaps is different for in-plane and out-of-plane modes: i.e., anisotropic superexchange for the in-plane mode, and anisotropic direct exchange for the out-of-plane mode. This difference explains the observed structural dependence of the spin-wave gaps in ${\mathrm{La}}_{1.65}$${\mathrm{Nd}}_{0.35}$${\mathrm{CuO}}_{4}$. In nickelates, the single-ion anisotropy is solely responsible for the spin-wave gaps, which are observed in ${\mathrm{La}}_{2}$${\mathrm{NiO}}_{4}$. We discuss the mechanism of the weak ferromagnetism; in the nickelates the Dzyaloshinski-Moriya (DM) interaction causes the weak ferromagnetism in the LTT phase where an appropriate spin configuration is prepared by the single-ion anisotropy. In the cuprates, where the single-ion anisotropy for preparing the spin configuration is absent, the DM and pseudodipolar interactions themselves determine the spin configuration: the weak ferromagnetism originates only from the competition between these two interactions. It is proposed that a multiorbital effect is essential to explain the observed weak ferromagnetism in the LTT phase of ${\mathrm{La}}_{1.65}$${\mathrm{Nd}}_{0.35}$${\mathrm{CuO}}_{4}$.

57Fe and 119Sn Mössbauer studies on La1.25Nd0.6Sr0.15CuO4: Evidence for local magnetic ordering below ≂32 K (abstract)

57Fe and 119Sn Mössbauer effect studies on 0.5 at. % 57Fe or 119Sn doped nonsuperconducting La1.25Nd0.6Sr0.15CuO4 in the temperature region 300 K≥T≥4.2 K reveal an onset of local magnetic ordering occurring at TM≂32 K for both 57Fe as well as 119Sn-doped samples. The local magnetic ordering shows up in the presence of a very large transferred hyperfine field of Bthf≂11.0(5) T at the 119Sn nuclei. Since such a large field is present in neither antiferromagnetic La2CuO4 superconducting La1.85Sr0.15CuO4 nor overdoped La2−xSrxCuO4 (x=0.4), the occurrence of such local spin correlations seems to be a signature of the nonsuperconducting low temperature tetragonal phase of La1.25Nd0.6Br0.15CuO4.

Low-temperature structural phase transition and electronic anomalies in La1.775R0.10Sr0.125CuO4 (R=Nd,Sm,Gd,Tb).

Sound velocity and powder x-ray diffraction in ${\mathrm{La}}_{1.9\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{R}}_{0.1}$${\mathrm{Sr}}_{\mathit{x}}$${\mathrm{CuO}}_{4}$ (R=Nd,Sm,Ge,Tb) reveal that the transition temperature to the tetragonal low-temperature (TLT) phase increases with decreasing the ionic radius of R up to 150 K (R=Tb around x\ensuremath{\sim}0.125). Prominent change of the resistivity \ensuremath{\rho} and the thermoelectric power S is observed in the narrow region of x around 0.125 and at almost the same temperature near 80 K in the Tb-, Gd-, and Sm-substituted samples whose crystal structure is the TLT phase. In the ND-substituted sample, less prominent but significant change of S is also observed below \ensuremath{\sim}50 K where the crystal structure is the orthorhombic midtemperature phase. These results suggest that the change of the transport properties is responsible for the change of the electronic state itself rather than for the structural phase transition and this modification of the electronic state is more pronounced in the TLT phase.