Coherent Jahn-Teller Distortion Research Articles

Effects of rare-earth ion size on the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites

We present a theoretical study on the relation between the size of the rare earth ions, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ions. We analyze the appearance of the uniform shear distortion below the Jahn-Teller ordering temperature, estimate the Jahn-Teller ordering temperature and its variation between NdMnO3 and LaMnO3, and obtain the relations between distortions. We find good agreement between theoretical results and experimental data.

Nd3+ crystal-field study of weakly doped Nd1−xCaxMnO3

Nd3+ crystal-field excitations in Nd1−xCaxMnO3 (x=0.025, 0.05 and 0.1) single crystals are studied via infrared transmission as a function of temperature and external magnetic field. We report excitations associated with Nd3+ sites as detected in NdMnO3 and excitations due to Ca doping. The latter reveal phase separation between the usual A-type antiferromagnetic states and the insulating canted (ferromagnetic) spin states in the vicinity of doped Ca2+ ions. Both Nd3+ crystal-field levels could be described using calculated parameters for NdMnO3. Also, while oxygen stoichiometry and coherent Jahn–Teller distortions seem not to be affected by Ca doping, increased absorption bandwidths characterize the doped crystals.

Competition between Jahn-Teller coupling and orbital fluctuations inHoVO3

We have carried out a detailed study of the structural properties of ${\text{HoVO}}_{3}$ perovskite using a combination of single-crystal neutron diffraction and synchrotron x-ray and neutron powder diffraction. We focus on the competition between one-dimensional fluctuations of the occupied vanadium $d$ orbitals and coherent Jahn-Teller distortion due to long-range orbital ordering. At room temperature orbital fluctuations are dominant. Below 188 K a structural phase transition from orthorhombic $Pbnm$ to monoclinic $Pb11$ symmetry takes place, corresponding to a state where strong orbital fluctuations are superimposed on an underlying orbitally ordered state. However, the fluctuations are not strong enough to give rise to a long-range orbitally dimerized state as theoretically predicted. Ordering of the vanadium spins at 114 K has little effect on the orbital fluctuations, but the orbital ordering becomes coherent below a first-order transition to an orthorhombic $Pbnm$ phase at 40 K.

Neutron-diffraction study of structural transition and magnetic order in orthorhombic and rhombohedralLa7/8Sr1/8Mn1−γO3+δ

We report on a systematic neutron powder diffraction (NPD) study of polycrystallineLa7/8Sr1/8Mn1−γO3+δ compounds. We investigated the structural and magnetic phases and transitions in the temperaturerange between 10 and 900 K for two different samples: an Ar-annealed sample with an orthorhombicPbnm (Z = 4;O) structure at room temperature and an air-sintered sample with a rhombohedral (Z = 2;R) structure at room temperature. At higher temperatures, above 400 K, both samplesexhibit a rhombohedral structure. For the Ar-annealed sample, a Jahn–Teller (JT)transition occurs in the orthorhombic phase at about 298 K with very large variations in theMn–O2′ and Mn–O2 bond lengths and Mn–O1–Mn bond angle on cooling from 298 to 180 K.For this sample the ferromagnetic moment at 10 K in the magnetic space groupPb′n′m amountsto 3.22(5) μB/Mn.In contrast, the air-sintered sample undergoes on cooling a structural transition from rhombohedral(R) to orthorhombic (O) with a mixed phase of nearly equal R and O repartition at 120 K.Ferromagnetic order develops in this sample at about 240 K with a low-temperature moment of3.35(4) μB/Mn at 10 K. The coherent JT distortion in its orthorhombic phase occurs below some170 K. In addition, we have determined the coherent JT distortion parameterΔ, the tolerance factort and the one-electronbandwidth W of the eg-band.

Crystal structure and magnetic and transport properties ofSm0.5154Sr0.5MnO3:A-type antiferromagnetic phase and ferromagnetic polarons

Structural investigations (neutron diffraction and electron microscopy), as well as data on resistance, magnetization, and second harmonic of magnetization, are presented for $^{154}\mathrm{Sm}_{0.5}{\mathrm{Sr}}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ manganite. The neutron diffraction studies reveal a structural phase transition at ${T}_{\mathit{st}}\ensuremath{\approx}135\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ from a high-temperature $Pbnm$ space group to a mixture of two $Pbnm$ phases, which are coherently coupled by atomic positions but differ in cell parameters. Above ${T}_{\mathit{st}}$, this compound is a paramagnetic insulator. At room temperature, a strained and complex nanostructure state is observed in the high-resolution electron microscopy images. In the $Pbnm$ structure it is generated by the coexistence of twinning domains and a small deviation from orthorhombicity for a monoclinic structure. Below ${T}_{\mathit{st}}$, both structural phases begin to exhibit coherent Jahn-Teller distortions, which are different in the two phases. The structural transition is accompanied by the development of a magnetic ordering, so that the ground state is a mixture of ferromagnetism (F) and $A$-type antiferromagnetism (AF-$A$). The AF-$A$ order develops in the new low-temperature structural phase II, whereas the F moment is related to the high-temperature structure I. The several variants of the magnetic ordering are considered for phase I. A more physically justified scenario is that this phase does not possess an AF-$A$ component and exhibits F ordering with an F moment of $1.9(1)\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{Mn}$ at $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this case, the AF moment of phase II is found to be $4.1(1)\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{Mn}$ at $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This unexpected result is attributed to the formation of ferromagnetic polarons in connection with a new electronic structure proposed for the AF-$A$ state. A metalliclike behavior is only observed below $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and is related to the F phase. The AF-$A$ phase is expected to be insulating. It can exhibit a short-range charge ordering that is observed by electron diffraction at $92\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. A field hysteresis of the second harmonic is found above ${T}_{C}\ensuremath{\sim}{T}_{N}\ensuremath{\approx}{T}_{\mathit{st}}$, which is associated with AF-$A$ regions in the paramagnetic matrix which possess a weak ferromagnetism.

Neutron diffraction and ESR studies of pseudocubic Nd 0.75Ba 0.25MnO 3 and its unusual critical behavior above [formula omitted

Results of structural neutron diffraction study and electron spin resonance (ESR) measurements are presented for insulating Nd 1− x Ba x MnO 3 ( x = 0.25 ) with the Curie temperature T C ≈ 129 K . Its pseudocubic structure reveals the definite distortions to a lower symmetry. Detailed analysis of the data is performed in the frame of Pbnm space group in a temperature range 4.2–300 K. The compound is found to exhibit the Jahn-Teller (JT) transition at T JT≈250 K. Character of the coherent JT distortions and their temperature evolution differ from those of the x = 0.23 manganite. The ESR results correspond to behavior of a 3D isotropic ferromagnet above T * ≈ 143 K ( τ * ≈ 0.12 ⩽ τ < 1 , τ = ( T – T C ) / T C ) . It is shown that an anisotropic exchange coupling of the Mn and Nd magnetic moments may give a substantial contribution in ESR linewidth masking its critical enhancement. The different temperature treatments (slow/fast cooling/heating with/without external magnetic field) of the sample reveal a temperature hysteresis of the ESR spectra below T* indicating an anomalous response in the paramagnetic region. The study of the magnetic phase transition in the x = 0.23 and 0.25 NdBa manganites suggests a change in its character from the second to first order at T*. The conventional free energy including the magnetization and magnetic field failed to describe this first-order transition. The unconventional critical behavior is attributed to an orbital liquid metallic phase that begins to coexist with the initial orbital-ordered phases below T*.

Orbital-order-induced metal-insulator transition in La(1-x)Ca(x)MnO3.

We present evidence that the insulator-to-metal transition in La(1-x)Ca(x)MnO3 near x approximately 0.2 is driven by the suppression of coherent Jahn-Teller distortions, originating from d-type orbital ordering. The orbital-ordered state is characterized by large long-range Q2 distortions below T(O'- O*). Above T(O'- O*) we find evidence for coexistence between an orbital-ordered and an orbital-disordered state. This behavior is discussed in terms of electronic phase separation in an orbital-ordered insulating and an orbital-disordered metallic state.

Magic hole doped composition of 152Sm 1− xSr xMnO 3 manganite: crystal structure and unusual magnetic properties in paramagnetic phase at x=0.45

Neutron diffraction investigations and data on second harmonic of magnetization are presented for 152Sm 0.55Sr 0.45MnO 3 manganite. The coherent Jahn–Teller distortions are found to occur just below 300 K, collapse at T C∼125 K, and exhibit a rather complicated temperature behavior between these points. Temperature dependence of a spontaneous magnetization evidences the first-order transition at T C and a homogeneous ferromagnetic phase below T C with practically full ferromagnetic alignment at T∼1.5 K. A field hysteresis of the second harmonic is observed above T C that is associated with the CE-type antiferromagnetic regions possessing a weak ferromagnetism, as well as with the ferromagnetic domains. Interconnection of spin and Jahn–Teller phonons degrees of freedom is supposed to account for the inhomogeneous paramagnetic phase with the nontrivial properties.

Crystal structure and magnetic properties of the unique Jahn-Teller system154Sm0.6Sr0.4MnO3

Structural neutron diffraction investigations as well as data on resistance, linear susceptibility, and second harmonic of magnetization are presented for ${}^{154}{\mathrm{Sm}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_{3}$ manganite. The structural studies reveal the unique Jahn-Teller (JT) character of this compound which is established to exhibit a $\mathrm{Pbmn}$ space group. Unusual large coherent JT distortions are found to develop below ${T}_{\mathrm{JT}}\ensuremath{\approx}180 \mathrm{K}$ and remain even in a metallic ferromagnetic phase below the Curie point ${T}_{c}\ensuremath{\approx}120 \mathrm{K}$. In the paramagnetic region the resistance is consistent with polaron hopping. A field hysteresis of the second harmonic is observed above ${T}_{c}.$ This evidences the existence of the macroscopic ferromagnetic regions in the paramagnetic matrix. In contrast, temperature dependence of the linear susceptibility reveals a plateau at $Tg{T}_{c}.$ These conflicting peculiarities of the magnetic properties can be explained by assuming the ferromagnetic behavior to be associated with the antiferromagnetic regions possessing a weak ferromagnetism. The ordered regions appear as a result of first order transition at $T\ensuremath{\approx}160 \mathrm{K},$ destroy at ${T}_{*}\ensuremath{\approx}137.5 \mathrm{K}$ and reveal a characteristic three-critical behavior for $\stackrel{\ensuremath{\rightarrow}}{T}{T}_{*}+0$. Change in a balance between the ferromagnetic double exchange hopping of ${e}_{g}$ electrons and the antiferromagnetic interactions of ${t}_{2g}$ spins, which caused by the cooperative JT effect, and the charge ordering correlations are supposed to account for the anomalous magnetic behavior above ${T}_{c}.$

Correlation between coherent Jahn-Teller distortion and magnetic spin orientation inLa1−xSrxMnO3

The magnetic structure and its correlation with the Jahn-Teller (JT) distortion in lightly doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ $(0.11&lt;~x&lt;~0.185)$ have been studied as a function of temperature from 10--300 K using neutron powder diffraction. A correlation between the ferromagnetic spin orientation and the coherent JT distortion was observed, indicating a coupling between the structural and the magnetic properties. The arrangement of the spins varies from ferromagnetic ordered mainly along the b axis $(x=0.11)$ to ferromagnetic almost along the c axis $(x=0.185)$ in the Pbnm symmetry.

Structure-properties phase diagram forLa1−xSrxMnO3(0.1&lt;~x&lt;~0.2)

By combining the results of magnetization, resistivity, and neutron powder-diffraction data for stoichiometric ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3},$ we have constructed a phase diagram that describes the magnetic, transport, and structural properties and the relationships among them as a function of composition $(0.1l~xl~0.2)$ and temperature (10--340 K). We show that, with increasing Sr content, the Curie temperature increases linearly, while the temperature of an orbital ordering transition to a state with a large coherent Jahn-Teller (JT) distortion decreases. These two phase-transition lines cross at $x=0.145$ and $T=210\mathrm{K}.$ When the transition to the ferromagnetic state occurs in a phase that has a large coherent JT distortion $(xl0.145),$ a strong magnetolattice coupling is observed; the coherent JT distortion is dramatically reduced and the incoherent distortion is enhanced in the ferromagnetic phase. For $xg0.145,$ where the coherent JT distortion is small above Curie temperature, magnetolattice coupling reduces the incoherent distortion at ${T}_{C}$ and strongly suppresses the transition to a phase with a large coherent JT distortion. These observations are consistent with a competition between ferromagnetism and JT distortion that is mediated by a colossal spin-charge-lattice coupling. A metallic state occurs below the Curie temperature when both coherent and incoherent JT distortions are suppressed.