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Register a new userDamped Soft Phonons and Diffuse Scattering in 40PMN-60PT
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Using neutron elastic and inelastic scattering and high-energy x-ray diffraction, we present a comparison of 40% Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-60% PbTiO$_{3}$ (PMN-60PT) with pure Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) and PbTiO$_{3}$ (PT). We measure the structural properties of PMN-60PT to be identical to pure PT, however, the lattice dynamics are exactly that previously found in relaxors PMN and PZN. PMN-60PT displays a well-defined macroscopic structural transition from a cubic to tetragonal unit cell at 550 K. The diffuse scattering is shown to be weak indicating that the structural distortion is long-range in PMN-60PT and short-range polar correlations (polar nanoregions) are not present. Even though polar nanoregions are absent, the soft optic mode is short-lived for wavevectors near the zone-centre. Therefore, PMN-60PT displays the same waterfall effect as prototypical relaxors PMN and PZN. We conclude that it is random fields resulting from the intrinsic chemical disorder which is the reason for the broad transverse optic mode observed in PMN and PMN-60PT near the zone centre and not due to the formation of short-ranged polar correlations. Through our comparison of PMN, PMN-60PT, and pure PT, we interpret the dynamic and static properties of the PMN-xPT system in terms of a random field model in which the cubic anisotropy term dominates with increasing doping of PbTiO$_{3}$.
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Diffuse scattering is a rich source of information about disorder in crystalline materials, which can be modelled using atomistic techniques such as Monte Carlo and molecular dynamics simulations. Modern X-ray and neutron scattering instruments can rapidly measure large volumes of diffuse-scattering data. Unfortunately, current algorithms for atomistic diffuse-scattering calculations are too slow to model large data sets completely, because the fast Fourier transform (FFT) algorithm has long been considered unsuitable for such calculations [Butler & Welberry, J. Appl. Cryst. 25, 391 (1992)]. Here, a new approach is presented for ultrafast calculation of atomistic diffuse-scattering patterns. It is shown that the FFT can actually be used to perform such calculations rapidly, and that a fast method based on sampling theory can be used to reduce high-frequency noise in the calculations. These algorithms are benchmarked using realistic examples of compositional, magnetic and displacive disorder. They accelerate the calculations by a factor of at least 100, making refinement of atomistic models to large diffuse-scattering volumes practical.
Impulsive optical excitation can generate both coherent and squeezed phonons. The expectation value of the phonon displacement $<u_q>$ oscillates at the mode frequency for the coherent state but remains zero for a pure squeezed state. In contrast, both show oscillations in $<|u_q|^2>$ at twice the mode frequency. Therefore it can be difficult to distinguish them in a second-order measurement of the displacements, such as in first-order x-ray diffuse scattering. Here we demonstrate a simple method to distinguish squeezed from coherent atomic motion by measurement of the diffuse scattering following double impulsive excitation. We find that femtosecond optical excitation generates squeezed phonons spanning the Brillouin zone in Ge, GaAs and InSb. Our results confirm the mechanism suggested in [Nature Physics 9, 790 (2013)].
A quite unusual diffuse scattering phenomenology was observed in the single-crystal X-ray diffraction pattern of cubic perovskite BMT ($mathrm{BaMg}_{1/3}mathrm{Ta}_{2/3}mathrm{O}_3$). The intensity of the scattering is parametrized as a set of cube-like objects located at the centers of reciprocal space unit cells, resembling very broad and cubic-shaped (1/2,1/2,1/2)-satellites. BMT belongs to perovskites of formula AB$_{1/3}$B$_{2/3}$O$_{3}$ (A=Mg, B$=$Ta, B$=$Mg). The cubes of the intensity can be attributed to the partial correlations of the occupancies of the B site. The pair correlation function is the Fourier transform of the diffuse scattering intensity and the latters idealized form yields the unusual property of a power-law correlation decay with distance. Up to now this is observed only in a few exotic instances of magnetic order or nematic crystals. Therefore it cannot be classified as a short-range order phenomenon, as in most situations originating diffuse scattering. A Monte-Carlo search in configuration space yielded solutions that reproduce faithfully the observed diffuse scattering. Analysis of the results in terms of the electrostatic energy and the entropy point to this phase of BMT as a metastable state, kinetically locked, which could be the equilibrium state just below the melting point.
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V.A. Konstantinov
, E.S. Orel
, V.P. Revyakin (B.I. Verkin Institute forn Low Temperature Physics
2003
The temperature and volume dependences of the thermal conductivity of solid Kr(1-x)Xe(x)solution are analyzed within the model in which heat is transferred by mobile low-frequency phonons; above the phonon mobility edge this is done by localized modes migrating randomly from site to site. The phonon mobility edge (w0)is determined from the condition that the phonon mean -free path restricted by Umklapp processes and point defects scattering cannot be smaller than one-half the phonon wavelength. The Bridgman coefficient is the weighted - mean over these modes whose volume dependences differ widely. It is shown that the amount of heat transferred by the localized modes above 100 K is quite large even in pure Kr and it increases with rising temperature and impurity concentration.
The relaxor ferroelectric PbMg1/Nb2/3O3 was investigated by means of broad-band dielectric and Fourier Transform Infrared (FTIR) transmission spectroscopy in the frequency range from 1 MHz to 15 THz at temperatures between 20 and 900 K using PMN films on infrared transparent sapphire substrates. While thin film relaxors display reduced dielectric permittivity at low frequencies, their high frequency intrinsic or lattice response is shown to be the same as single crystal/ceramic specemins. It was observed that in contrast to the results of inelastic neutron scattering, the optic soft mode was underdamped at all temperatures. On heating, the TO1 soft phonon followed the Cochran law with an extrapolated critical temperature equal to the Burns temperature of 670 K and softened down to 50 cm-1. Above 450 K the soft mode frequency leveled off and slightly increased above the Burns temperature. A central mode, describing the dynamics of polar nanoclusters appeared below the Burns temperature at frequencies near the optic soft mode and dramatically slowed down below 1 MHz on cooling below room temperature. It broadened on cooling, giving rise to frequency independent losses in microwave and lower frequency range below the freezing temperature of 200 K. In addition, a new heavily damped mode appeared in the FTIR spectra below the soft mode frequency at room temperature and below. The origin of this mode as well as the discrepancy between the soft mode damping in neutron and infrared spectra is discussed.
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